CN109323127B

CN109323127B - Method for discharging lubricating oil and engine fuel supply method

Info

Publication number: CN109323127B
Application number: CN201810440546.7A
Authority: CN
Inventors: 朴青气
Original assignee: Daewoo Shipbuilding and Marine Engineering Co Ltd
Current assignee: Hanhua Ocean Co ltd
Priority date: 2017-07-31
Filing date: 2018-05-09
Publication date: 2021-06-01
Anticipated expiration: 2038-05-09
Also published as: JP6371930B1; KR101908569B1; JP2019027589A; CN109323127A

Abstract

A method of discharging lubricating oil from an boil-off gas reliquefaction system configured to reliquefy boil-off gas by compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by a heat exchanger via heat exchange with uncompressed boil-off gas, and reducing the pressure of fluid cooled via heat exchange by a pressure reducer, the compressor including at least one oil-lubricated cylinder, and an engine fuel supply method. The method comprises the following steps: sending the boil-off gas that is not used as the refrigerant in the heat exchanger to the compressor along a bypass line and then compressed by the compressor; and sending part or all of the boil-off gas compressed by the compressor to the heat exchanger, the condensed or solidified lubricating oil being discharged from the boil-off gas reliquefaction system through the boil-off gas whose temperature is increased during compression by the compressor after melting or viscosity is reduced.

Description

Method for discharging lubricating oil and engine fuel supply method

Technical Field

The present invention relates to a method of discharging lubricating oil and an engine fuel supply method, and more particularly, to a boil-off gas reliquefaction system in which, among boil-off gases generated in a storage tank of a Liquefied Natural Gas (LNG) ship to be supplied as fuel to an engine, surplus boil-off gas higher than a fuel requirement of the engine is reliquefied using the boil-off gas as a refrigerant.

Background

Recently, the consumption of liquefied gases such as Liquefied Natural Gas (LNG) has been rapidly increasing worldwide. Liquefied gases obtained by cooling natural gas to very low temperatures have a much smaller volume than natural gas and are therefore more suitable for storage and transport. Furthermore, since air pollutants in natural gas can be reduced or removed during the liquefaction process, liquefied gases such as LNG are eco-friendly fuels with low air pollutant emissions after combustion.

LNG is a colorless and transparent liquid obtained by cooling natural gas consisting mainly of methane to about-163 ℃ to liquefy the natural gas, and has a volume of about 1/600 of natural gas. Thus, liquefaction of natural gas enables extremely efficient transportation.

However, since natural gas is liquefied at an extremely low temperature of-163 ℃ under normal pressure, LNG may be easily vaporized due to a small change in temperature. Although the LNG storage tank is insulated, external heat may be continuously transferred to the storage tank, causing the LNG in transit to naturally vaporize, thereby generating Boil Off Gas (BOG).

The production of BOG means loss of LNG and thus has a great influence on the transport efficiency. Further, when BOG accumulates in the storage tank, there is a risk that the pressure inside the storage tank excessively increases, causing damage to the tank. Various studies have been conducted to process BOG produced in LNG storage tanks. Recently, in order to process BOG, a method of reliquefying BOG to return to an LNG storage tank, a method of using BOG as an energy source in a fuel consumption source such as a ship engine, and the like have been proposed.

Examples of methods for reliquefaction of BOG include a method using a refrigeration cycle having a separate refrigerant, in which BOG is allowed to exchange heat with the refrigerant for reliquefaction; and a method of re-liquefying BOG without any separate refrigerant using BOG as a refrigerant. Specifically, a system employing the latter method is called a partial re-liquefaction system (PRS).

Examples of marine engines that can be fueled by natural gas include gas engines, such as DFDE engines, X-DF engines, and ME-GI engines.

The DFDE engine has four strokes per cycle and uses an Otto cycle (Otto cycle) in which natural gas having a relatively low pressure of about 6.5 bar is injected into the combustion air inlet and then pushes the piston upwards to compress the gas.

The X-DF engine has two strokes per cycle and utilizes an otto cycle using natural gas as fuel, which has a pressure of about 16 bar.

ME-GI engines have two strokes per cycle and use a diesel cycle (diesel cycle) in which natural gas having a high pressure of about 300 bar is injected directly into the combustion chamber near the top dead center (top dead center) of the piston.

Disclosure of Invention

[ problem ] to

Thus, when boil-off gas (BOG) generated in a Liquefied Natural Gas (LNG) storage tank is compressed and reliquefied through heat exchange using the boil-off gas without a separate refrigerant, it is necessary to compress the BOG at a high pressure using an oil lubrication type cylinder for reliquefaction efficiency.

The boil-off gas compressed by the oil-lubricated cylinder compressor contains lubricating oil. The inventors of the present invention found that the lubricating oil contained in the compressed BOG is condensed or solidified before the BOG and blocks the fluid passages of the heat exchanger during cooling of the compressed BOG in the heat exchanger. In particular, Printed Circuit Heat Exchangers (PCHEs) having narrow fluid channels (e.g., microfluidic channel-type fluid channels) often have fluid channels that become plugged more frequently due to condensed or solidified lubricating oil.

Accordingly, the inventors of the present invention have developed various techniques for separating the lubricating oil from the compressed BOG in order to prevent the condensed or solidified lubricating oil from clogging the fluid passages of the heat exchanger.

Embodiments of the present invention provide a method and system for mitigating or preventing clogging of fluid channels of a heat exchanger by condensed or solidified lubricating oil, and which is capable of removing condensed or solidified lubricating oil clogging the fluid channels of the heat exchanger via a simple and economical process.

[ technical solution ]

According to an aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy a BOG by compressing the BOG by a compressor, cooling the compressed BOG by a heat exchanger via heat exchange with uncompressed BOG, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer, wherein a BOG to be used as a refrigerant in the heat exchanger is supplied to the heat exchanger along a first supply line, the BOG used as the refrigerant in the heat exchanger is supplied to the compressor along a second supply line, and a BOG not used as the refrigerant in the heat exchanger is supplied to the compressor along a bypass line bypassing the heat exchanger, and wherein a bypass valve for adjusting a flow rate of fluid and opening/closing of a corresponding supply line is disposed on the bypass line, a first valve for regulating the flow rate of fluid and the opening/closing of the corresponding supply line is disposed on the first supply line upstream of the heat exchanger, a second valve for regulating the flow rate of fluid and the opening/closing of the corresponding supply line is disposed on the second supply line downstream of the heat exchanger, and the compressor comprises at least one cylinder of the oil-lubricated type, the method comprising: 2) opening the bypass valve while closing the first valve and the second valve; 3) sending the BOG not used as the refrigerant in the heat exchanger along the bypass line to the compressor and then compressed by the compressor; and 4) sending part or all of the BOG compressed by the compressor to the heat exchanger, the condensed or solidified lubricant oil being discharged from the BOG reliquefaction system after melting or viscosity reduction through the BOG whose temperature increased during compression by the compressor.

The lubricating oil discharge method may further comprise: 1) it is determined whether it is time to remove the condensed or solidified lubricating oil before step 2.

After the reliquefaction of the BOG, the liquefied gas and the gaseous BOG generated by the reliquefaction may be separated from each other by a gas/liquid separator, the liquefied gas separated by the gas/liquid separator may be discharged from the gas/liquid separator along a fifth supply line, and the gaseous BOG separated by the gas/liquid separator may be discharged from the gas/liquid separator along a sixth supply line, the lube oil discharging method may further include: 5) sending the BOG having passed through the heat exchanger to a gas/liquid separator; and 6) discharging the lubricating oil accumulated in the gas/liquid separator.

The lubricant oil discharge method may further comprise: the BOG sent to the gas/liquid separator in step 5) is sent to the bypass line along the sixth supply line to be subjected to compression in step 3).

Steps 3) to 5) may be repeated until the temperature of the hot fluid path of the heat exchanger increases to the temperature of the BOG compressed by the compressor and sent to the heat exchanger.

In step 4), the BOG compressed by the compressor may be used by the engine as fuel, and excess BOG not used by the engine may be sent to the heat exchanger.

In step 1), it may be determined that it is time to discharge the condensed or solidified lubricating oil if at least one of the following conditions is satisfied: a condition in which a temperature difference between the BOG upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "cold-flow temperature difference") is a first preset value or more and lasts for a predetermined time period or more; a condition that a temperature difference between the BOG used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and sent to the heat exchanger (hereinafter, referred to as "temperature difference of heat flow") is a first preset value or more and lasts for a predetermined time period or more; and a condition that a pressure difference between the BOG compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the BOG cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter, referred to as "pressure difference of the hot fluid passage") is a second preset value or more and is continued for a predetermined time period or more.

In step 1), if a lower value between a temperature difference between the BOG upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "cold-flow temperature difference") and a temperature difference between the BOG used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and sent to the heat exchanger (hereinafter referred to as "hot-flow temperature difference") is a first preset value or more and lasts for a predetermined time period or more, or if a pressure difference between the BOG compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the BOG cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter, referred to as "pressure difference of a hot fluid passage") is a second preset value or more for a predetermined time period or more, it may be determined that it is time to discharge the condensed or solidified lubricating oil.

After the reliquefaction of the BOG, the liquefied gas separated by the gas/liquid separator may be sent to the storage tank along a fifth supply line, and an eighth valve for adjusting the flow rate of the fluid and the opening/closing of the corresponding supply line may be disposed on the fifth supply line, the eighth valve being closed during steps 2) to 6).

After determining that the heat exchanger is restored to normal, reliquefaction of the BOG may be performed after opening the first and second valves while closing the bypass valve.

According to another aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy a BOG by compressing the BOG by a compressor, cooling the compressed BOG by a heat exchanger via heat exchange with uncompressed BOG, and reducing the pressure of fluid cooled via heat exchange by a pressure reducer, wherein the compressor includes at least one oil-lubricated cylinder, the BOG is sent to the compressor via a bypass line bypassing the heat exchanger and compressed by the compressor, the BOG compressed by the compressor is supplied to the engine, and excess BOG not supplied to the engine is supplied to the heat exchanger to discharge condensed or solidified lubricating oil using BOG increased in temperature during compression by the compressor after melting or reducing the viscosity of the lubricating oil.

After the reliquefaction of the BOG, the liquefied gas and the gaseous BOG generated by the reliquefaction may be separated from each other by the gas/liquid separator, and the gaseous BOG separated by the gas/liquid separator may be discharged from the gas/liquid separator along the sixth supply line, and the lubricating oil in which the melting or viscosity is reduced and which is discharged through the BOG whose temperature is increased during compression by the compressor may be collected in the gas/liquid separator.

After filtering the lubricating oil from the BOG by at least one of the oil separator and the first oil filter, the BOG compressed by the compressor after having passed through the bypass line may be sent to the heat exchanger.

The first oil filter can separate the lubricating oil having a vapor phase or a mist phase.

The second oil filter may be disposed on at least one of: a fifth supply line, at a position between the pressure reducer and the gas/liquid separator, through which liquefied gas separated by the gas/liquid separator can be discharged, and a sixth supply line, the second oil filter being a low-temperature oil filter.

The second filter separates the lubricating oil having a solid phase.

BOG compressed by the compressor after having passed through the heat exchanger and sent to the gas/liquid separator may be subjected to repeated cycles of the cycle by being sent along the sixth supply line to the bypass line for compression by the compressor.

The cycle period may repeat until the temperature of the hot fluid channel of the heat exchanger reaches the temperature of the BOG compressed by the compressor and sent to the heat exchanger.

According to another aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy BOG using BOG as a refrigerant, wherein a heat exchanger cools the BOG compressed by a compressor via heat exchange after the BOG reliquefaction using the BOG discharged from a storage tank as the refrigerant; the compressor comprises at least one oil-lubricated cylinder; and the condensed or solidified lubricating oil is discharged by a bypass line disposed to bypass the heat exchanger and used in the process of servicing the heat exchanger after melting or viscosity reduction.

According to another aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy a BOG by compressing the BOG by a compressor, cooling the compressed BOG by a heat exchanger via heat exchange with uncompressed BOG, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer, wherein the compressor includes at least one oil-lubricated cylinder, and it is determined that it is time to discharge condensed or solidified lubricating oil if at least one of the following conditions is satisfied: a condition in which a temperature difference between the BOG upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "cold-flow temperature difference") is a first preset value or more and lasts for a predetermined time period or more; a condition that a temperature difference between the BOG used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and sent to the heat exchanger (hereinafter, referred to as "temperature difference of heat flow") is a first preset value or more and lasts for a predetermined time period or more; and a condition that a pressure difference between the BOG compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the BOG cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter, referred to as "pressure difference of the hot fluid passage") is a second preset value or more and is continued for a predetermined time period or more.

According to another aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy a BOG by compressing the BOG by a compressor, cooling the compressed BOG by a heat exchanger via heat exchange with uncompressed BOG, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer, wherein the compressor includes at least one oil-lubricated type cylinder, and if a lower value between a temperature difference between the BOG upstream of the heat exchanger to be used as a refrigerant in the heat exchanger and the BOG compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "temperature difference of cold flow") and a temperature difference between the BOG used as a refrigerant in the heat exchanger and the BOG compressed by the compressor and sent to the heat exchanger (hereinafter referred to as "temperature difference of hot flow") is a first preset value or higher and lasts for a predetermined time period or longer, or if a pressure difference between the BOG compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the BOG cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter, referred to as "pressure difference of a hot fluid passage") is a second preset value or more for a predetermined time period or more, it is determined that it is time to discharge the condensed or solidified lubricating oil.

According to another aspect of the present invention, there is provided a method of discharging lubricating oil from a BOG reliquefaction system configured to reliquefy BOG using BOG as a refrigerant, wherein a time point for discharging condensed or solidified lubricating oil is determined based on at least one of a temperature difference and a pressure difference of a device, and the time point for discharging condensed or solidified lubricating oil is indicated by a notification unit.

The apparatus may include a heat exchanger including microchannel-type fluid channels.

According to another aspect of the present invention, there is provided a method of discharging lube oil from a BOG reliquefaction system configured to reliquefy BOG using BOG as a refrigerant, wherein lube oil collected in a gas/liquid separator is discharged from the gas/liquid separator via a lube oil discharge line separated from a fifth supply line, and liquefied gas resulting from reliquefaction of BOG is discharged from the gas/liquid separator via the fifth supply line.

The speed at which the lube oil is discharged from the gas/liquid separator can be increased by supplying nitrogen gas to the gas/liquid separator.

After reliquefaction of the BOG, the compressed BOG may be cooled in a heat exchanger using the BOG as a refrigerant, and after discharging the lube oil, nitrogen may be supplied to the gas/liquid separator along the hot fluid passage, and the compressed BOG is supplied to the heat exchanger via the hot fluid passage.

The nitrogen supplied to the gas/liquid separator may have a pressure of 5 bar to 7 bar.

After the reliquefaction of the BOG, the liquefied gas separated by the gas/liquid separator may be sent to the storage tank along a fifth supply line, and an eighth valve may be disposed on the fifth supply line to adjust a flow rate of the fluid and opening/closing of the fifth supply line, the eighth valve being closed during the discharge of the lubricating oil.

The engine may be driven during the discharge of lubricating oil.

After the discharge of the lubricating oil, the BOG to be supplied to the cold fluid channel of the heat exchanger may be compressed and sent to the hot fluid channel of the heat exchanger after bypassing the heat exchanger.

According to another aspect of the present invention, there is provided a fuel supply method for an engine, in which fuel is supplied to the engine by melting or reducing the viscosity of condensed or solidified lubricating oil during discharge of the condensed or solidified lubricating oil.

The point in time for discharging the condensed or solidified lubricating oil may be indicated by the notification unit.

If the performance of the heat exchanger is reduced to 60% to 80% of its normal performance, it is determined that it is time to discharge the condensed or solidified lubricating oil.

The first preset value may be 35 deg.c.

The second preset value may be twice the preset value for normal operation.

The second preset value may be 2 bar (200 kPa).

The predetermined time period may be 1 hour.

The cold fluid temperature differential may be detected by a first temperature sensor disposed upstream of the cold fluid passage of the heat exchanger and a fourth temperature sensor disposed downstream of the hot fluid passage of the heat exchanger.

The temperature differential of the hot fluid may be detected by a second temperature sensor disposed downstream of the cold fluid passage of the heat exchanger and a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger.

The pressure difference of the hot fluid passage may be detected by a first pressure sensor disposed upstream of the hot fluid passage of the heat exchanger and a second pressure sensor disposed downstream of the hot fluid passage of the heat exchanger.

The pressure difference of the hot fluid channel may be detected by a pressure difference sensor measuring a pressure difference between upstream of the hot fluid channel of the heat exchanger and downstream of the hot fluid channel of the heat exchanger.

The inventive compressor can compress BOG to a pressure of 150 bar to 350 bar.

The compressor may compress BOG to a pressure of 80 bar to 250 bar.

The heat exchanger may comprise microchannel-type fluid channels.

The heat exchanger may be a Printed Circuit Heat Exchanger (PCHE).

[ advantageous effects ]

According to the embodiments of the present invention, it is possible to remove condensed or solidified lubricating oil inside a heat exchanger through a simple and economical process using existing equipment without installing separate equipment or supplying separate fluid to remove the lubricating oil.

According to an embodiment of the present invention, it is possible to service the heat exchanger while the engine is continuously operating by driving the engine during removal of the condensed or solidified lubricating oil. Furthermore, it is possible to remove the condensed or solidified lubricating oil using excess BOG that is not used by the engine. Furthermore, it is possible to use the engine to burn lubricating oil mixed with BOG.

According to the embodiments of the present invention, it is possible to effectively discharge the melted or viscosity-reduced lubricating oil using the improved gas/liquid separator when the lubricating oil is collected in the gas/liquid separator.

According to the embodiment of the present invention, the low-temperature oil filter is disposed at least one of the fifth supply line through which the liquefied gas is discharged from the gas/liquid separator, and the sixth supply line through which the BOG is discharged from the gas/liquid separator, at a position downstream of the decompressor, whereby effective removal of the lubricating oil mixed with the BOG is achieved.

According to the embodiments of the present invention, it is possible to satisfy the intake pressure condition of the compressor and the engine fuel requirement of the engine while maintaining the reliquefaction performance through a simple and economical process even with existing equipment without separate equipment.

Drawings

FIG. 1 is a schematic diagram of a BOG reliquefaction system according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram of a BOG reliquefaction system according to a second embodiment of the present invention.

FIG. 3 is a schematic diagram of a BOG reliquefaction system according to a third embodiment of the present invention.

FIG. 4 is an enlarged view of a gas/liquid separator according to one embodiment of the invention.

Fig. 5(a) and 5(b) are enlarged views of a second oil filter according to an embodiment of the present invention.

Fig. 6(a) and 6(b) are enlarged views of a second oil filter according to another embodiment of the present invention.

FIG. 7 is a schematic diagram of a BOG reliquefaction system according to a fourth embodiment of the present invention.

Fig. 8 is an enlarged view of a stress-reducer according to an embodiment of the invention.

Fig. 9 is an enlarged view of a stress-reducer according to another embodiment of the present invention.

FIG. 10 is an enlarged view of a heat exchanger and gas/liquid separator according to one embodiment of the invention.

Fig. 11 and 12 are graphs depicting the amount of reliquefaction depending on the BOG pressure in a Partial Reliquefaction System (PRS).

Fig. 13 is a plan view of the filter element shown in fig. 5(a) and 5(b) and fig. 6(a) and 6 (b).

Description of the reference numerals

100: a heat exchanger;

200: a compressor;

201: a compressor;

210: a cylinder;

211: a cooler;

220: a cylinder;

221: a cooler;

230: a cylinder;

231: a cooler;

240: a cylinder;

241: a cooler;

250: a cylinder;

251: a cooler;

300: an oil separator;

410: a first oil filter;

420: a second oil filter;

421: a filter element;

422: an inflow pipe;

423: a discharge pipe;

424: an oil discharge pipe;

425: a fixing plate;

510: a first valve;

520: a second valve;

530: a third valve;

541: a recirculation valve;

542: a recirculation valve;

543: a recirculation valve;

544: a recirculation valve;

550: a check valve;

560: a sixth valve;

570: a seventh valve;

571: a tenth valve;

581: an eighth valve;

582: a ninth valve;

583: a nitrogen gas valve;

590: a bypass valve;

600: a pressure reducer;

610: a pressure reducer;

620: an isolation valve;

700: a gas/liquid separator;

810: a first temperature sensor;

820: a second temperature sensor;

830: a third temperature sensor;

840: a fourth temperature sensor;

910: a first pressure sensor;

920: a second pressure sensor;

930: a pressure difference sensor;

940: a fluid level sensor;

a: a location;

b: a location;

c: a location;

l1: a first supply line;

l2: a second supply line;

l3: a third supply line;

l4: a fourth supply line;

l5: a fifth supply line;

l6: a sixth supply line;

l22: a seventh supply line;

l33: an eighth supply line;

BL: a bypass line;

NL: a nitrogen supply line;

OL: a lube oil drain line;

SL: a fuel supply line;

rc 1: a first recycle line;

rc 2: a second recycle line;

rc 3: a third recycle line;

rc 4: a fourth recycle line;

t: a storage tank;

x: a space;

z: a space.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The BOG reliquefaction system according to the present invention is applicable to various ships such as ships equipped with engines fueled by natural gas, ships including liquefied gas storage tanks, ship structures, and the like. It will be appreciated that the following examples may be modified in different ways and do not limit the scope of the invention.

Furthermore, the fluid in each fluid supply line of the system according to the present invention may have a liquid phase, a mixed vapor-liquid phase, a vapor phase, and a supercritical fluid phase, depending on the operating conditions of the system.

Referring to fig. 1, the BOG reliquefaction system according to this embodiment includes a compressor 200, a heat exchanger 100, a decompressor 600, a bypass line BL, and a bypass valve 590.

The compressor 200 compresses BOG discharged from the storage tank T, and may include a plurality of

cylinders

210, 220, 230, 240, 250 and a plurality of

coolers

211, 221, 231, 241, 251. The BOG compressed by the compressor 200 may have a pressure of about 150 to 350 bar.

Some of the BOG compressed by the compressor 200 may be supplied to the main engine of the ship along the fuel supply line SL, and other BOG, which will not be used by the main engine, may be supplied to the heat exchanger 100 along the third supply line L3 so as to be subjected to the reliquefaction process. The primary engine may be a ME-GI engine using high pressure natural gas having a pressure of about 300 bar as fuel.

Some of the BOG passing through some of the

cylinders

210, 220 of the compressor 200 is distributed and supplied to the generator. The generator according to this embodiment may be a DF engine using low pressure natural gas with a pressure of about 6.5 bar as fuel.

The heat exchanger 100 cools the BOG compressed by the compressor 200 and supplied along the third supply line L3 via heat exchange using the BOG discharged from the storage tank T and supplied along the first supply line L1 as a refrigerant. The BOG used as the refrigerant in the heat exchanger 100 is sent to the compressor 200 along the second supply line L2, and the fluid cooled by the heat exchanger 100 is supplied to the pressure reducer 600 along the fourth supply line L4.

The decompressor 600 reduces the pressure of the BOG compressed by the compressor 200 and then cooled by the heat exchanger 100. Part or all of the BOG gas is reliquefied via compression by the compressor 200, cooling by the heat exchanger 100, and decompression by the decompressor 600. The pressure reducer 600 may be an expansion valve, such as a Joule-Thomson valve, or may be an inflator.

The BOG reliquefaction system according to this embodiment may further include a gas/liquid separator 700 disposed after the decompressor 600 to separate the BOG remaining in the vapor phase from the liquefied natural gas generated by reliquefaction of the BOG gas through the compressor 200, the heat exchanger 100, and the decompressor 600.

The liquefied gas separated by the gas/liquid separator 700 is supplied to the storage tank T along the fifth supply line L5, and the BOG separated by the gas/liquid separator 700 may be combined with the BOG discharged from the storage tank T and supplied to the heat exchanger 100.

A ninth valve 582 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the sixth supply line L6 through which BOG having a vapor phase is discharged from the gas/liquid separator 700.

If the heat exchanger 100 is not available, for example, after service or failure of the heat exchanger 100, BOG discharged from the storage tank T may be allowed to bypass the heat exchanger 100 via the bypass line BL. The bypass line BL is provided with a bypass valve 590 that opens and closes the bypass line BL.

Referring to fig. 2, the BOG reliquefaction system according to this embodiment includes a heat exchanger 100, a first valve 510, a second valve 520, a first temperature sensor 810, a second temperature sensor 820, a compressor 200, a third temperature sensor 830, a fourth temperature sensor 840, a first pressure sensor 910, a second pressure sensor 920, a pressure reducer 600, a bypass line BL, and a bypass valve 590.

The heat exchanger 100 cools the BOG compressed by the compressor 200 via heat exchange using the BOG discharged from the storage tank T as a refrigerant. The BOG discharged from the storage tank T and used as the refrigerant in the heat exchanger 100 is sent to the compressor 200, and the BOG compressed by the compressor 200 is cooled by the heat exchanger 100 using the BOG discharged from the storage tank T as the refrigerant.

The BOG discharged from the storage tank T is supplied to the heat exchanger 100 along the first supply line L1 and used as a refrigerant, and the BOG used as the refrigerant in the heat exchanger 100 is sent to the compressor 200 along the second supply line L2. Part or all of the BOG compressed by the compressor 200 is supplied to the heat exchanger 100 along the third supply line L3 for cooling, and the fluid cooled by the heat exchanger 100 is supplied to the pressure reducer 600 along the fourth supply line L4.

The first valve 510 is disposed on the first supply line L1 to regulate the flow rate and the opening/closing of the corresponding supply line, and the second valve 520 is disposed on the second supply line L2 to regulate the flow rate and the opening/closing of the corresponding supply line.

The first temperature sensor 810 is disposed in front of the heat exchanger 100 on the first supply line L1 to measure the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger 100. Preferably, the first temperature sensor 810 is disposed immediately in front of the heat exchanger 100 to measure the temperature of the BOG immediately before being supplied to the heat exchanger 100.

Herein, the term "forward" means upstream, and the term "rearward" means downstream.

The second temperature sensor 820 is disposed downstream of the heat exchanger 100 on the second supply line L2 to measure the temperature of the BOG used as the refrigerant in the heat exchanger 100 after being discharged from the storage tank T. Preferably, the second temperature sensor 820 is disposed immediately behind the heat exchanger 100 to measure the temperature of the BOG immediately after being used as the refrigerant in the heat exchanger 100.

The compressor 200 compresses BOG used as refrigerant in the heat exchanger 100 after being discharged from the storage tank T. The BOG compressed by the compressor 200 may be supplied into the high-pressure engine to be used as fuel, and the BOG remaining after being supplied into the high-pressure engine may be supplied to the heat exchanger 100 to achieve reliquefaction.

A sixth valve 560 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the fuel supply line SL through which the BOG compressed by the compressor 200 is supplied to the high pressure engine.

The sixth valve 560 acts as a safety device to shut off the supply of BOG to the high pressure engine upon interruption of gas mode operation of the high pressure engine. Gas mode means a mode in which the engine operates using natural gas as fuel. When the BOG to be used as fuel is insufficient, the engine is switched to a fuel oil mode to allow the fuel oil to be used as fuel for the engine.

A seventh valve 570 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the supply line through which an excess BOG higher than the fuel requirement of the high pressure engine among the BOGs compressed by the compressor 200 is supplied to the heat exchanger 100.

When the BOG compressed by the compressor 200 is supplied to the high-pressure engine, the compressor 200 may compress the BOG to a pressure required by the high-pressure engine. The high pressure engine may be a ME-GI engine that uses high pressure BOG as fuel.

ME-GI engines are known to use natural gas as fuel having a pressure of about 150 to 400 bar, preferably about 150 to about 350 bar, more preferably about 300 bar. The compressor 200 may compress the BOG to a pressure of about 150 bar to about 350 bar in order to supply the compressed BOG to the ME-GI engine.

Instead of an ME-GI engine as the primary engine, an X-DF engine or DF engine using BOG as fuel at a pressure of about 6 bar to about 20 bar may be used. In this case, because the compressed BOG for supply to the main engine has a low pressure, the compressed BOG to be supplied to the main engine may be further compressed to re-liquefy the BOG. The further compressed BOG for reliquefaction may have a pressure of about 80 to 250 bar.

Fig. 11 and 12 are graphs depicting the amount of reliquefaction depending on the BOG pressure in a Partial Reliquefaction System (PRS). Reliquefying the target BOG means that the BOG to be reliquefied via cooling is distinguished from the BOG used as refrigerant.

Referring to fig. 11 and 12, it can be seen that the reliquefaction reaches a maximum value when the pressure of the BOG is in the range of 150 to 170 bar, and the reliquefaction is substantially unchanged when the pressure of the BOG is in the range of 150 to 300 bar. Accordingly, as a high pressure engine, a ME-GI engine using BOG having a pressure of about 150 bar to about 350 bar (mostly 300 bar) as fuel can easily control a reliquefaction system to supply fuel to the high pressure engine while maintaining a high liquefaction amount.

The compressor 200 may include a plurality of

cylinders

210, 220, 230, 240, 250, and a plurality of

coolers

211, 221, 231, 241, 251 disposed downstream of the plurality of

cylinders

210, 220, 230, 240, 250, respectively. The

coolers

211, 221, 231, 241, 251 cool the BOG compressed by the

cylinders

210, 220, 230, 240, 250 and having high pressure and temperature.

In the structure in which the compressor 200 includes the plurality of

cylinders

210, 220, 230, 240, 250, the BOG sent to the compressor 200 is compressed by the plurality of

cylinders

210, 220, 230, 240, 250 through a plurality of stages. Each of the

cylinders

210, 220, 230, 240, 250 may serve as a compression terminal for each of the compressors 200.

The compressor 200 may include: a first recirculation line Rc1 through which part or all of the BOG having passed through the first cylinder 210 and the first cooler 211 is supplied to the front end of the first cylinder 210; a second recirculation line Rc2 through which part or all of the BOG, which has passed through the second cylinder 220 and the second cooler 221, is supplied to the front end of the second cylinder 220; a third recirculation line Rc3 through which some or all of the BOG having passed through the third cylinder 230 and the third cooler 231 is supplied to the front end of the third cylinder 230; and a fourth recirculation line 244 through which some or all of the BOG, which has passed through the fourth cylinder 240, the fourth cooler 241, the fifth cylinder 250, and the fifth cooler 251, is supplied to the front end of the fourth cylinder 240.

Further, a first recirculation valve 541 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the first recirculation line Rc1, a second recirculation valve 542 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the second recirculation line Rc2, a third recirculation valve 543 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the third recirculation line Rc3, and a fourth recirculation valve 544 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the fourth recirculation line Rc 4.

The recirculation lines Rc1, Rc2, Rc3, Rc4 protect the compressor 200 by recirculating part or all of the BOG when the reserve tank T has a low pressure to satisfy the intake pressure condition required for the compressor 200.

Recirculation valves

541, 542, 543, 544 are closed when recirculation lines Rc1, Rc2, Rc3, Rc4 are not in use, and

recirculation valves

541, 542, 543, 544 are open when the required intake pressure conditions for compressor 200 are not met and recirculation lines Rc1, Rc2, Rc3, Rc4 need to be in use.

Although fig. 2 illustrates a structure in which the BOG having passed through all of the plurality of

cylinders

210, 220, 230, 240, 250 of the compressor 200 is supplied to the heat exchanger 100, the BOG having passed through some of the

cylinders

210, 220, 230, 240, 250 may be distributed in the compressor 200 to be supplied to the heat exchanger 100.

Further, the BOG that has passed through some of the

cylinders

210, 220, 230, 240, 250 may be distributed in the compressor 200 to be supplied to a low-pressure engine for use as fuel, and the surplus BOG may be supplied to a Gas Combustion Unit (GCU) to be combusted.

The low pressure engine may be a DF engine (e.g. DFDE) using BOG having a pressure of about 6 to 10 bar as fuel.

Some of the

cylinders

210, 220, 230, 240, 250 included in the compressor 200 may be operated in an oil-free lubrication manner, and other cylinders may be operated in an oil-lubrication manner. Specifically, when the BOG is compressed to 80 bar or more, preferably 100 bar or more, in order to use the BOG compressed by the compressor 200 as fuel of a high-pressure engine or to achieve reliquefaction efficiency, the compressor 200 includes an oil lubrication type cylinder in order to compress the BOG to a high pressure.

In the related art, lubricating oil for lubrication and cooling is supplied to the reciprocating type compressor 200 (e.g., piston seal parts thereof) so as to compress BOG to 100 bar or more.

Since lubricating oil is supplied to the oil-lubricated type cylinder, some lubricating oil is mixed with the BOG that has passed through the oil-lubricated type cylinder in the related art. The inventors of the present invention found that the lubricating oil mixed with the compressed BOG condenses or solidifies in the heat exchanger 100 before the BOG to block the fluid passages of the heat exchanger 100.

The BOG reliquefaction system according to this embodiment may further include an oil separator 300 and a first oil filter 410 disposed between the compressor 200 and the heat exchanger 100 to separate oil from BOG.

The oil separator 300 typically separates the lubricating oil in the liquid phase, and the first oil filter 410 separates the lubricating oil in the vapor or mist phase. Because the oil separator 300 separates the lubricating oil having a larger particle size than the lubricating oil separated by the first oil filter 410, the oil separator 300 is disposed upstream of the first oil filter 410 so that the BOG compressed by the compressor 200 can be supplied to the heat exchanger 100 after sequentially passing through the oil separator 300 and the first oil filter 410.

Although fig. 2 shows a structure in which the BOG reliquefaction system includes both the oil separator 300 and the first oil filter 410, the BOG reliquefaction system according to this embodiment may include one of the oil separator 300 and the first oil filter 410. Preferably, both the oil separator 300 and the first oil filter 410 are used.

Further, although fig. 2 shows a structure in which the first oil filter 410 is provided to the second supply line L2 downstream of the compressor 200, the first oil filter 410 may also be provided to the third supply line L3 upstream of the heat exchanger 100, and may be provided in plurality so as to be arranged in parallel.

In the structure in which the BOG reliquefaction system includes one of the oil separator 300 and the first oil filter 410 and the compressor 200 includes the oil-free cylinder and the oil-lubricated cylinder, the BOG having passed through the oil-lubricated cylinder may be supplied to the oil separator 300 and/or the first oil filter 410, and only the BOG having passed through the oil-free cylinder may be directly supplied to the heat exchanger 100 without passing through the oil separator 300 or the oil filter 410.

By way of example, the compressor 200 according to this embodiment includes five

cylinders

210, 220, 230, 240, 250, wherein the first three

cylinders

210, 220, 230 may be oil-lubricated type cylinders and the last two

cylinders

240, 250 may be oil-lubricated type cylinders. Here, in the BOG reliquefaction system according to this embodiment, the BOG may be directly supplied to the heat exchanger 100 without passing through the oil separator 300 or the first oil filter 410 after the BOG is distributed in three stages or less, and may be supplied to the first heat exchanger 100 after passing through the oil separator 300 and/or the first oil filter 410 after the BOG is distributed in four stages or more.

The first filter 410 may be a coalescer oil filter.

A check valve 550 may be disposed on the fuel supply line SL between the compressor 200 and the high pressure engine. The check valve 550 serves to prevent the BOG from returning to and damaging the compressor when the high pressure engine is stopped.

In configurations where the BOG reliquefaction system includes the oil separator 300 and/or the first oil filter 410, a check valve 550 may be disposed downstream of the oil separator 300 and/or the first oil filter 410 to prevent BOG from flowing back to the oil separator 300 and/or the first oil filter 410.

Further, since the BOG may flow back to the compressor 200 and damage the compressor 200 when the pressure reducer 600 is suddenly closed, the check valve 550 may be disposed upstream of the branch point of the third supply line L3, which branches from the fuel supply line SL.

The third temperature sensor 830 is disposed upstream of the heat exchanger 100 on the third supply line L3 to measure the temperature of the BOG compressed by the compressor 200 and then supplied to the heat exchanger 100. Preferably, the third temperature sensor 830 is disposed immediately in front of the heat exchanger 100 to measure the temperature of the BOG immediately before being supplied to the heat exchanger 100.

A fourth temperature sensor 840 is disposed downstream of the heat exchanger 100 on the fourth supply line L4 to measure the temperature of the BOG compressed by the compressor 200 and then cooled by the heat exchanger 100. Preferably, the fourth temperature sensor 840 is disposed immediately behind the heat exchanger 100 to measure the temperature of the BOG immediately after cooling by the heat exchanger 100.

The first pressure sensor 910 is disposed upstream of the heat exchanger 100 on the third supply line L3 to measure the pressure of the BOG compressed by the compressor 200 and then supplied to the heat exchanger 100. Preferably, the first pressure sensor 910 is disposed immediately in front of the heat exchanger 100 to measure the pressure of the BOG immediately before being supplied to the heat exchanger 100.

A second pressure sensor 920 is disposed downstream of the heat exchanger 100 on the fourth supply line L4 to measure the pressure of the BOG compressed by the compressor 200 and then cooled by the heat exchanger 100. Preferably, the second pressure sensor 920 is positioned immediately behind the heat exchanger 100 to measure the pressure of the BOG immediately after cooling by the heat exchanger 100.

As shown in fig. 2, although it is preferable that all of the first to fourth temperature sensors 810 to 840, the first pressure sensor 910 and the second pressure sensor 920 are provided to the reliquefaction system, it is to be understood that the present invention is not limited thereto. Alternatively, the reliquefaction system may have only the first temperature sensor 810 and the fourth temperature sensor 840 ("first pair"), only the second temperature sensor 820 and the third temperature sensor 830 ("second pair"), only the first pressure sensor 910 and the second pressure sensor 920 ("third pair"), or two of the first to third pairs.

The decompressor 600 is disposed downstream of the heat exchanger 100 to decompress the BOG compressed by the compressor 200 and then cooled by the heat exchanger 100. Part or all of the BOG gas is reliquefied via compression by the compressor 200, cooling by the heat exchanger 100, and decompression by the decompressor 600. The pressure reducer 600 may be an expansion valve, such as a joule-thomson valve, or may be an inflator.

The BOG reliquefaction system according to this embodiment may further include a gas/liquid separator 700 disposed downstream of the decompressor 600 to separate the BOG remaining in the vapor phase from the liquefied natural gas produced by reliquefaction of the BOG gas through the compressor 200, the heat exchanger 100, and the decompressor 600.

The liquefied gas separated by the gas/liquid separator 700 is supplied to the storage tank T along the fifth supply line L5, and the BOG separated by the gas/liquid separator 700 may be combined with the BOG discharged from the storage tank T along the sixth supply line L6 and supplied to the heat exchanger 100.

Although fig. 2 shows a structure in which the BOG separated by the gas/liquid separator 700 is combined with the BOG discharged from the storage tank T and then supplied to the heat exchanger 100, it is to be understood that the present invention is not limited thereto. By way of example, the heat exchanger 100 may consist of three fluid passages, and the BOG separated by the gas/liquid separator 700 may be supplied to the heat exchanger 100 along separate fluid passages for use as refrigerant therein.

Alternatively, the gas/liquid separator 700 may be omitted and the BOG reliquefaction system may be configured to allow partial or complete reliquefaction of the fluid via depressurization of the pressure reducer 600 to be supplied directly to the storage tank T.

An eighth valve 581 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the fifth supply line L5. The level of the liquefied gas in the gas/liquid separator 700 is regulated by an eighth valve 581.

A ninth valve 592 for adjusting the flow rate and the opening/closing of the corresponding supply line may be disposed on the sixth supply line L6. The internal pressure of the gas/liquid separator 700 can be regulated by the ninth valve 592.

FIG. 4 is an enlarged view of a gas/liquid separator according to one embodiment of the invention. Referring to FIG. 4, the gas/liquid separator 700 may have a fluid level sensor 940 that measures the level of the natural gas in the gas/liquid separator 700.

The BOG reliquefaction system according to this embodiment may include a second oil filter 420 disposed between the pressure reducer 600 and the gas/liquid separator 700 to filter the lubricating oil mixed with the fluid subjected to the pressure reduction of the pressure reducer 600.

Referring to fig. 2 and 4, the second oil filter 420 may be disposed on the fourth supply line L4 between the pressure reducer 600 and the gas/liquid separator 700 (in fig. 4, position a), on the fifth supply line L5 through which the liquefied gas is discharged from the gas/liquid separator 700 (in fig. 4, position B), or on the sixth supply line L6 through which the gaseous BOG is discharged from the gas/liquid separator 700 (in fig. 4, position C). Fig. 2 shows a structure in which the second oil filter 420 is disposed at position a in fig. 4.

The BOG separated by the gas/liquid separator 700 may be combined with the BOG discharged from the storage tank T and supplied to the cold fluid passage of the heat exchanger 100. Here, since the lubrication oil is collected in the gas/liquid separator 700, there is a possibility that even a small amount of lubrication oil may be mixed with the gaseous BOG separated by the gas/liquid separator 700.

The inventors of the present invention found that, when the gaseous BOG separated by the gas/liquid separator 700 is mixed with the lubricating oil and sent to the cold fluid passage of the heat exchanger 100, a more difficult situation may occur than in the case where the lubricating oil mixed with the BOG compressed by the compressor 200 is supplied to the hot fluid passage of the heat exchanger 100.

Because the fluid to be used as the refrigerant in the heat exchanger 100 is sent to the cold fluid channel of the heat exchanger 100, the operation of the reliquefaction system is such that low temperature BOG is continuously supplied to the reliquefaction system and has a temperature high enough so that the fluid on which the condensed or solidified oil is melted is not supplied thereto. Therefore, it is extremely difficult to remove the condensed or solidified oil accumulated in the cryogenic fluid passage of the heat exchanger 100.

In order to reduce the possibility of supplying a mixture of the lubricating oil and the gaseous BOG separated by the gas/liquid separator 700 to the cold fluid passage of the heat exchanger 100 as low as possible, the second oil filter 420 may be disposed at a position a or C in fig. 4.

In the structure in which the second oil filter 420 is disposed at the position C in fig. 4, since most of the melted or viscosity-reduced lubricating oil is collected in the gas/liquid separator 700 in the liquid phase and the amount of the gaseous lubricating oil discharged along the sixth supply line L6 is small, the reliquefaction system has an advantage of high filtering efficiency and does not require frequent replacement of the second oil filter 420.

In the structure in which the second oil filter 420 is disposed at the position B in fig. 4, since the lubricating oil can be prevented from flowing into the storage tank T, it is possible to prevent the contamination of the liquefied gas stored in the storage tank T.

Because the first oil filter 410 is disposed downstream of the compressor 200 and the BOG compressed by the compressor 200 has a temperature of about 40 ℃ to about 45 ℃, a low temperature oil filter need not be used. However, because the fluid whose pressure is reduced by pressure reducer 600 has a temperature of about-160 ℃ to about-150 ℃ to allow re-liquefaction of at least a portion of the BOG, and because the liquefied gas and BOG separated by gas/liquid separator 700 have a temperature of about-160 ℃ to about-150 ℃, second oil filter 420 must be designed for cryogenic temperatures regardless of the location of second oil filter 420 among locations A, B, C and D in fig. 4.

Furthermore, because most of the lube oil mixed with the BOG compressed by the compressor 200 and having a temperature of about 40 ℃ to 45 ℃ has a liquid phase or a mist phase, the oil separator 300 is designed to be suitable for separating lube oil of the liquid phase, and the first oil filter 410 is designed to be suitable for separating lube oil of the mist phase (which may include some lube oil in the vapor phase).

In contrast, the second oil filter 420 is designed to be suitable for separating the lubricating oil in the solid phase (or in the solidified state) below the flow point as the fluid of low temperature fluid and reduced in pressure by the pressure reducer 600, the BOG separated by the gas/liquid separator 700, and the lubricating oil in the solid phase (or in the solidified state) mixed with the liquefied gas separated by the gas/liquid separator 700.

Fig. 5(a) and 5(b) are enlarged views of a second oil filter according to one embodiment of the present invention, and fig. 6(a) and 6(b) are enlarged views of a second oil filter according to another embodiment of the present invention.

Referring to fig. 5(a) and 5(b) and fig. 6(a) and 6(b), the second oil filter 420 may have a structure as shown in fig. 5(a) and 5(b) (hereinafter, "downward drain type") or a structure as shown in fig. 6(a) and 6(b) (hereinafter, "upward drain type"). In fig. 5(a) and 5(b) and fig. 6(a) and 6(b), the dotted lines indicate the fluid flow direction.

Referring to fig. 5(a) and 5(b) and fig. 6(a) and 6(b), the second oil filter 420 includes a fixing plate 425 and a filter element 421, and is connected to an inflow pipe 422, a discharge pipe 423, and an oil discharge pipe 424.

The filter element 421 is provided to the fixing plate 425 to separate the lubricating oil from the fluid flowing through the inflow pipe 422.

Fig. 13 is a plan view of the filter element 421 shown in fig. 5(a) and 5(b) and fig. 6(a) and 6 (b). Referring to fig. 13, the filter element 421 may have a hollow (Z space in fig. 13) cylindrical shape in which a plurality of layers having different meshes are stacked on each other. The lubricating oil is filtered from the fluid flowing into the second oil filter 420 via the inflow pipe 422 while the fluid passes through the layers of the filter element 421. The filter element 421 may separate the lube oil by a physical adsorption method.

The fluid (BOG, liquefied gas, or fluid of vapor-liquid mixture) filtered by the filter element 421 is discharged via the discharge pipe 423, and the lubricating oil filtered by the filter element 421 is discharged via the oil discharge pipe 424.

The components of second oil filter 420 are formed of materials capable of withstanding cryogenic conditions in order to separate the lubricating oil from the fluids having very low temperatures. The filter element 421 may be formed of a metal capable of withstanding low temperature conditions, specifically, Special Use Stainless (SUS).

Referring to fig. 5(a) and 5(b), in the drain-down type oil filter, fluid supplied through an inflow pipe 422 connected to an upper portion of the oil filter passes through a space (X of fig. 5(a) and 5 (b)) defined below a filter element 421 and a fixing plate 425, and is then drained through a drain pipe 423 connected to a lower portion of the oil filter.

In the downward drain type oil filter, a fixing plate 425 is connected to a lower portion of the oil filter, a filter element 421 is disposed on an upper surface of the fixing plate 425, and a drain pipe 423 is connected to a side of the oil filter opposite to the filter element 421 with respect to the fixing plate 425.

Further, in the downward drain type oil filter, the inflow pipe 422 is preferably connected to the oil filter to be disposed above the upper end of the filter element 421 so as to allow the fluid flowing into the oil filter via the inflow pipe 422 to be filtered even by the upper portion of the filter element 421 (that is, so as to use the filter element as much as possible).

It is preferable that the inflow pipe 422 and the discharge pipe 423 are disposed on opposite sides (on the left and right sides with respect to the filter element 421 in fig. 5(a) and 5 (b)) in terms of fluid flow, and it is preferable that the oil discharge pipe 424 is connected to a lower portion of the filter element 421 because the lubrication oil filtered by the filter element 421 is collected at a lower side of the oil filter.

In the downward drain type oil filter, the oil drain pipe 424 may be connected to an oil filter to be disposed immediately above the fixing plate 425.

As shown in fig. 5(a), when a fluid mainly composed of a liquid component (for example, 90% by volume of liquid and 10% by volume of gas) is supplied to the downward-drain type oil filter, a downward flow of the fluid is generated due to the high density of the liquid component, thereby maintaining a good filtering effect.

On the other hand, as shown in fig. 5(b), when a fluid composed of gaseous components (for example, 10% by volume of liquid and 90% by volume of gas) is supplied to the downward-drain type oil filter, the gaseous components having a small density remain in the upper portion of the oil filter, thereby deteriorating the fluid flow and the filtering effect.

Referring to fig. 6(a) and 6(b), in the upward-drain type oil filter, fluid supplied via an inflow pipe 422 connected to an upper portion of the oil filter passes through a space (Y of fig. 6(a) and 6 (b)) defined above a filter element 421 and a fixing plate 425, and is then drained via a drain pipe 423 connected to a lower portion of the oil filter.

In the upward-drain type oil filter, a fixing plate 425 is connected to an upper portion of the oil filter, a filter element 421 is disposed on a lower surface of the fixing plate 425, and a drain pipe 423 is connected to a side of the oil filter opposite to the filter element 421 with respect to the fixing plate 425.

Further, in the upward-drain type oil filter, the inflow pipe 422 is preferably connected to the oil filter to be disposed below the lower end of the filter element 421 so as to allow the fluid flowing into the oil filter via the inflow pipe 422 to be filtered even by the lower portion of the filter element 421 (that is, so as to use the filter element as much as possible).

It is preferable that the inflow pipe 422 and the discharge pipe 423 are disposed on opposite sides with respect to fluid flow (on left and right sides with respect to the filter element 421 in fig. 6(a) and 6 (b)), and it is preferable that the oil discharge pipe 424 is connected to a lower portion of the filter element 421 because the lubrication oil filtered by the filter element 421 is collected at a lower side of the oil filter.

Referring to fig. 6(a) and 6(b), in the upward-drain type oil filter, fluid supplied to the oil filter through an inflow pipe 422 connected to a lower portion of the oil filter passes through a filter element 421, and is drained through a drain pipe 423 connected to an upper portion of the oil filter. The lubricating oil filtered by the filter element 421 is discharged through the oil discharge pipe 424.

As shown in fig. 6(a), when a fluid mainly composed of gaseous components (for example, 10% by volume of liquid and 90% by volume of gas) is supplied to the upward-drain type oil filter, an upward flow of the fluid is generated due to the low density of the gaseous components, thereby providing a suitable upward flow while maintaining a good filtering effect.

On the other hand, as shown in fig. 6(b), when a fluid composed of liquid components (for example, 90% by volume of liquid and 10% by volume of gas) is supplied to the upward-drain type oil filter, the liquid components having a high density remain in the lower portion of the oil filter, thereby deteriorating the fluid flow and the filtering effect.

Accordingly, in a structure in which the second oil filter 420 is disposed at the position B of fig. 4, it is preferable that a downward-drain type oil filter as shown in fig. 5(a) and 5(B) is used as the second oil filter 420, and when the second oil filter 420 is disposed at the position C of fig. 4, it is preferable that an upward-drain type oil filter as shown in fig. 6(a) and 6(B) is used as the second oil filter 420.

In the structure in which the second oil filter 420 is disposed at the position a in fig. 4, the fluid whose pressure is reduced by the pressure reducer 600 is a vapor-liquid mixture (theoretically, 100% re-liquefaction is possible) in which the volume ratio of the gaseous component is higher than that of the liquid component. Therefore, it is preferable that an upward-drain type oil filter as shown in fig. 6(a) and 6(b) be used as the second oil filter 420.

According to an embodiment, the bypass line BL branches from the first supply line L1 upstream of the heat exchanger 100 to bypass the heat exchanger 100 and joins to the second supply line L2 downstream of the heat exchanger 100.

Typically, a bypass line that bypasses the heat exchanger is disposed inside the heat exchanger to be integrated with the heat exchanger. In the structure in which the bypass line is disposed inside the heat exchanger, when the valve disposed upstream and/or downstream of the heat exchanger is closed, the fluid cannot be supplied to the heat exchanger and the bypass line.

In an embodiment of the present invention, the bypass line BL is disposed outside the heat exchanger 100 to be separated from the heat exchanger 100, and is branched from the first supply line L1 upstream of the first valve 510 and joined to the second supply line L2 downstream of the second valve 520, so that the BOG can be sent to the bypass line BL even when the first valve 510 upstream of the heat exchanger 100 and/or the second valve 520 downstream of the heat exchanger 100 are closed.

The bypass valve 590 is disposed on the bypass line BL and is opened when the use of the bypass line BL is required.

Basically, the bypass line BL will be used when the heat exchanger 100 cannot be used, for example, when the heat exchanger 100 fails or is repaired. For example, if the heat exchanger 100 cannot be used when the BOG reliquefaction system according to this embodiment sends part or all of the BOG compressed by the compressor 200 to the high-pressure engine, the BOG discharged from the storage tank T is directly sent to the compressor 200 along the bypass line BL bypassing the heat exchanger 100 without reliquefying the surplus BOG that is not used by the high-pressure engine, and the BOG compressed by the compressor 200 is supplied to the high-pressure engine while the surplus BOG is sent to the GCU to combust the surplus BOG.

When the bypass line BL is used to service the heat exchanger 100, for example, when the fluid passage of the heat exchanger 100 is blocked by condensed or solidified lubricating oil, the condensed or solidified lubricating oil may be removed via the bypass line BL.

Furthermore, if no further liquefaction of BOG is required because there is little excess BOG (as in the ballast condition of the ship), all BOG discharged from the storage tank T may be sent to the bypass line BL so as to allow all BOG to be sent directly to the compressor 200 while bypassing the heat exchanger 100. The BOG compressed by the compressor 200 is used as fuel for a high-pressure engine. If it is determined that no further liquefaction of BOG is required due to little excess BOG, bypass valve 590 may be controlled to open automatically.

The inventors of the present invention have found that when a BOG is supplied to an engine via a heat exchanger having a narrow fluid passage according to an embodiment, the BOG is subjected to a severe pressure drop due to the heat exchanger. If no reliquefaction of the BOG is required, the fuel can be smoothly supplied to the engine by compressing the BOG while bypassing the heat exchanger, as described above.

Furthermore, because of the increased amount of BOG that is not reliquefied, a bypass line BL may also be used to reliquefy BOG.

When it is necessary to reliquefy the BOG due to the increase in the amount of the BOG (i.e., after starting or restarting the BOG reliquefaction), all of the BOG discharged from the storage tank T may be sent to the bypass line BL so as to allow all of the BOG to be directly sent to the compressor 200 while bypassing the heat exchanger 100, and the BOG compressed by the compressor 200 may be sent to the hot fluid channel of the heat exchanger 100. Some of the BOG compressed by the compressor 200 may be supplied to the high pressure engine.

When the temperature of the hot fluid path of the heat exchanger 100 is increased by the above-described process after the BOG reliquefaction is started or restarted, the BOG reliquefaction may advantageously begin after removing any condensed or solidified lube oil, other residue, or impurities that may remain in the heat exchanger 100, other equipment, piping, etc. during the previous BOG reliquefaction process.

The residue may include BOG compressed by the compressor 200 in a previous BOG liquefaction and then supplied to the heat exchanger, and lubricating oil mixed with the BOG compressed by the compressor 200.

If the cold BOG discharged from the storage tank T is directly supplied to the heat exchanger 100 without increasing the temperature of the heat exchanger 100 via the bypass line BL after the BOG reliquefaction is started or restarted, the cold BOG discharged from the storage tank T is sent to the cold fluid channel of the heat exchanger 100 in a state where the hot BOG is not sent to the hot fluid channel of the heat exchanger 100. Thus, the lubricating oil remaining in the heat exchanger 100 in an uncondensed or unset state may also condense or solidify as the temperature of the heat exchanger 100 decreases.

When the bypass line BL is used to increase the temperature of the heat exchanger 100 for a particular period of time (which may be determined by one skilled in the art if it is determined that the condensed or solidified lube oil or other impurities are almost completely removed, and may be from about 1 minute to about 30 minutes, preferably from about 3 minutes to about 10 minutes, and more preferably from about 2 minutes to about 5 minutes), BOG reliquefaction is initiated by slowly opening the first valve 510 and the second valve 520 while slowly closing the bypass valve 590. As time further passes, the first valve 510 and the second valve 520 are fully opened, and the bypass valve 590 is fully closed to allow all of the BOG discharged from the storage tank T to be used as refrigerant for re-liquefying the BOG in the heat exchanger 100.

In addition, when the internal pressure of the storage tank T is low, the bypass line BL may be used to satisfy the intake pressure condition of the compressor 200.

Further, if it is required to control the internal pressure of the storage tank T to a low pressure, the bypass line BL may be used to satisfy the intake pressure condition of the compressor 200 even if the internal pressure of the storage tank T is reduced.

The following description will focus on the case where the condensed or solidified lubricating oil is removed using the bypass line BL, and the case where the intake pressure condition of the compressor 200 is satisfied using the bypass line BL when the internal pressure of the storage tank T is low.

1. Case of removing condensed or solidified lubricating oil using bypass line BL

The inventors of the present invention found that, because a certain amount of lubricating oil is mixed with the BOG having passed through the oil-lubricated cylinder of the compressor 200, and the lubricating oil contained in the BOG is condensed or solidified before the BOG in the heat exchanger 100 and accumulated in the heat exchanger 100, it is necessary to remove the condensed or solidified lubricating oil from the heat exchanger 100 after a predetermined period of time because the amount of condensed or solidified lubricating oil accumulated in the heat exchanger 100 increases over time.

In particular, while the heat exchanger 100 according to this embodiment is ideal for a printed circuit heat exchanger (PCHE, also referred to as DCHE) in view of the pressure and/or flow rate of the BOG to be reliquefied, reliquefaction efficiency, and the like, the PCHE has narrow spiral-shaped fluid channels (microchannel-type fluid channels) and thus has problems such as easy clogging of the fluid channels with condensed or solidified lubricating oil, easy accumulation of condensed or solidified lubricating oil at the spiral-shaped portions of the fluid channels, and the like. PCHE (DCHE is manufactured by Kobelko Co., Ltd., Alfallaval Co., Ltd., or the like).

The condensed or solidified lubricating oil may be removed via the following steps:

1) determining whether it is time to remove the condensed or solidified lubricant;

2) opening the bypass valve 590 while closing the first valve 510 and the second valve 520;

3) the BOG discharged from the storage tank T and having passed through the bypass line BL is compressed by the compressor 200;

4) sending part or all of the hot BOG compressed by the compressor 200 to the heat exchanger 100;

5) the BOG having passed through the heat exchanger 100 is sent to the gas/liquid separator 700;

6) discharging the lube oil from the gas/liquid separator 700; and

7) determining whether the heat exchanger 100 is restored to normal

1) A step of determining whether it is time to remove the condensed or solidified lubricating oil

When the fluid passage of the heat exchanger 100 is clogged with the condensed or solidified lubricating oil, the cooling efficiency of the heat exchanger 100 may be reduced. Accordingly, if the performance of the heat exchanger 100 falls below a preset value of normal performance, it can be estimated that the condensed or solidified lubricating oil accumulates in the heat exchanger 100 in a certain amount or more. By way of example, if the performance of the heat exchanger 100 drops to about 50% to about 90%, preferably about 60% to about 80%, more preferably about 70% or less than about 70% of normal performance, then the time to remove the condensed or solidified lubricating oil from the heat exchanger 100 may be determined.

As used herein, the range of "about 50% to about 90%" of normal performance includes all values of about 50% or less than 50%, about 60% or less than 60%, about 70% or less than 70%, about 80% or less than 80%, and about 90% or less than 90%, and the range of "about 60% to about 80%" of normal performance includes all values of about 60% or less than 60%, about 70% or less than 70%, and about 80% or less than 80%.

When the performance of the heat exchanger 100 is deteriorated, the temperature difference between the cold BOG (L1) supplied to the heat exchanger 100 and the cold BOG (L4) discharged from the heat exchanger 100 is increased, and the temperature difference between the hot BOG (L2) discharged from the heat exchanger 100 and the hot BOG (L3) supplied to the heat exchanger 100 is also increased. Further, when the fluid passage of the heat exchanger 100 is clogged with the condensed or solidified lubricating oil, the fluid passage of the heat exchanger 100 is narrowed, thereby increasing the pressure difference between the front end (L3) and the rear end (L4) of the heat exchanger 100.

Accordingly, it is possible to determine whether it is time to remove the condensed or solidified lubricating oil based on the temperature difference of the cold fluid supplied to the heat exchanger 100 or discharged from the heat exchanger 100, the temperature difference of the hot fluid supplied to the heat exchanger 100 or discharged from the heat exchanger 100, and the pressure difference of the hot fluid passage of the heat exchanger 100.

Specifically, if the temperature difference (representing an absolute value, hereinafter referred to as "cold flow temperature difference") between the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger 100 as measured by the first temperature sensor 810 and the temperature of the BOG compressed by the compressor 200 and cooled by the heat exchanger 100 as measured by the fourth temperature sensor 840 is higher than a normal temperature difference for a certain period of time or longer, it may be determined that the heat exchange is normally performed in the heat exchanger 100.

By way of example, when a state in which the temperature difference of the cold flow is 20 ℃ to 50 ℃ or more, preferably 30 ℃ to 40 ℃ or more, more preferably about 35 ℃ or more, lasts for a period of 1 hour or longer, the time for discharging the condensed or solidified lubricating oil can be determined.

When the heat exchanger 100 is normally operated, the BOG compressed to about 300 bar by the compressor 200 has a temperature of about 40 ℃ to about 45 ℃, and the BOG discharged from the storage tank T and having a temperature of about-160 ℃ to about-140 ℃ is supplied to the heat exchanger 100. Here, the temperature of the BOG discharged from the storage tank T is increased to about-150 ℃ to about-110 ℃, preferably about-120 ℃ during delivery to the heat exchanger 100.

In the BOG reliquefaction system according to this embodiment including the gas/liquid separator 700, when the gaseous BOG separated by the gas/liquid separator 700 is combined with the BOG discharged from the storage tank T and then supplied to the heat exchanger 100, the temperature of the BOG finally supplied to the heat exchanger 100 is lower than the temperature of the BOG discharged from the storage tank T to the heat exchanger 100, and the temperature of the BOG supplied to the heat exchanger 100 may be further decreased as the amount of the gaseous BOG separated by the gas/liquid separator 700 increases.

The BOG supplied to the heat exchanger 100 along the third supply line L3 and having a temperature of about 40 ℃ to 45 ℃ is cooled by the heat exchanger 100 to about-130 ℃ to about-110 ℃, and the temperature difference of the cold stream is preferably about 2 ℃ to about 3 ℃ in a normal state.

Further, if the temperature difference (representing an absolute value, hereinafter, referred to as "temperature difference of heat flow") between the temperature of the BOG discharged from the storage tank T and used as the refrigerant by the heat exchanger 100 as measured by the second temperature sensor 820 and the temperature of the BOG compressed by the compressor 200 and supplied to the heat exchanger 100 as measured by the third temperature sensor 830 is higher than a normal temperature difference for a certain period of time or longer, it may be determined that the heat exchange is normally performed in the heat exchanger 100.

When the state in which the temperature difference of the heat flow is 20 ℃ to 50 ℃ or more, preferably 30 ℃ to 40 ℃ or more, more preferably about 35 ℃ or more is continued for 1 hour or more, the time for discharging the condensed or solidified lubricating oil can be determined.

When the heat exchanger 100 is normally operated, the BOG discharged from the storage tank T and having a slightly increased temperature of about-150 ℃ to about-110 ℃ (preferably about-120 ℃) during delivery to the heat exchanger 100 may have a temperature of about-80 ℃ to 40 ℃, depending on the speed of the ship after being used as a refrigerant in the heat exchanger 100, and the BOG used as a refrigerant in the heat exchanger 100 and having a temperature of about-80 ℃ to 40 ℃ is compressed by the compressor 200 to have a temperature of about 40 ℃ to about 45 ℃.

Further, if a pressure difference between the pressure of the BOG compressed by the compressor 200 and supplied to the heat exchanger 100 as measured by the first pressure sensor 910 and the temperature of the BOG cooled by the heat exchanger 100 as measured by the second pressure sensor 920 (hereinafter, referred to as "pressure difference of the hot fluid passage") is higher than a normal pressure difference for a certain period of time or longer, it may be determined that the heat exchanger 100 is abnormally operated.

Since the BOG discharged from the storage tank T is not mixed with oil or has a trace amount of oil and the point of time when the lube oil is mixed with the BOG is when the BOG is compressed by the compressor 200, the condensed or solidified lube oil is not substantially accumulated in the cold fluid channel of the heat exchanger 100 (which uses the BOG discharged from the storage tank T as a refrigerant and then supplies the BOG to the compressor 200) and in the hot fluid channel of the heat exchanger 100, where the BOG compressed by the compressor 200 is cooled and supplied to the decompressor 600.

Accordingly, since the pressure difference between the front and rear ends of the heat exchanger 100 is rapidly increased in the hot fluid channel due to the fluid channel being blocked by the condensed or solidified lubricating oil, it is determined whether it is time to remove the condensed or solidified lubricating oil by measuring the pressure of the hot fluid channel of the heat exchanger 100.

Considering that a PCHE having a narrow and spiral fluid passage may be used as the heat exchanger according to this embodiment, it may be advantageous to use information about whether it is time to remove condensed or solidified lubricating oil based on a pressure difference between the front and rear ends of the heat exchanger 100.

By way of example, when the pressure difference of the hot fluid channel is two times or more than its normal pressure difference and lasts for 1 hour or longer than 1 hour, it may be determined that it is time to discharge the condensed or solidified lubricating oil.

When the heat exchanger 100 is operating normally, the BOG compressed by the compressor 200 experiences a pressure drop of about 0.5 bar to about 2.5 bar, preferably about 0.7 bar to about 1.5 bar, more preferably about 1 bar, without suffering a significant pressure drop even when the BOG is cooled while passing through the heat exchanger 100. The time to discharge the condensed or solidified lubricant may be determined when the pressure difference of the hot fluid passage therein is at least a predetermined pressure or more, for example, 1 bar to 5 bar or more, preferably 1.5 bar to 3 bar or more, more preferably about 2 bar (200kPa) or more.

Although the time point for removing the condensed or solidified lubricating oil may be determined based on any one of the temperature difference of the cold fluid, the temperature difference of the hot fluid, and the pressure difference of the hot fluid channel as described above, the time point for removing the condensed or solidified lubricating oil may be determined based on at least two of the temperature difference of the cold fluid, the temperature difference of the hot fluid, and the pressure difference of the hot fluid channel in order to improve reliability.

By way of example, when a lower value between the temperature difference of the cold stream and the temperature difference of the hot stream is maintained at a temperature of 35 ℃ or more for a time of 1 hour or more than 1 hour, or when the pressure difference of the hot fluid channel is twice or more than its normal pressure difference or 200kPa or more and continues for a time of 1 hour or more than 1 hour, it is possible to determine that the time to remove the condensed or solidified lubricating oil is reached.

The first temperature sensor 810, the second temperature sensor 820, the third temperature sensor 830, the fourth temperature sensor 840, the first pressure sensor 910, and the second pressure sensor 920 may be regarded as a detection member for detecting whether the heat exchanger 100 is clogged with the lubricating oil.

Furthermore, the BOG reliquefaction system according to an embodiment of the present invention may further include a controller (not shown) to determine whether the heat exchanger 100 is clogged with the lubricating oil based on a detection result obtained by at least one of the first temperature sensor 810, the second temperature sensor 820, the third temperature sensor 830, the fourth temperature sensor 840, the first pressure sensor 910, and the second pressure sensor 920. The controller may be regarded as a determination member for determining whether the heat exchanger 100 is clogged with the lubricating oil.

2) Step of opening the bypass valve 590 while closing the first valve 510 and the second valve 520

If it is determined in step 1 that it is time to remove the condensed or solidified lubricating oil from the heat exchanger 100, the bypass valve 590 disposed on the bypass line BL is opened, and the first valve 510 disposed on the first supply line L1 and the second valve 520 disposed on the second supply line L2 are closed.

When the bypass valve 590 is opened while the first valve 510 and the second valve 520 are closed, the BOG discharged from the storage tank T is sent to the compressor 200 via the bypass line BL, and is prevented from being supplied to the heat exchanger 100. Therefore, the refrigerant is not supplied to the heat exchanger 100.

3) Step of compressing the BOG discharged from the storage tank T and having passed through the bypass line BL by the compressor 200

The BOG discharged from the storage tank T bypasses the heat exchanger 100 via the bypass line BL and is then sent to the compressor 200. The BOG sent to the compressor 200 undergoes increases in temperature and pressure while being compressed by the compressor 200. The BOG compressed to about 300 bar by the compressor 200 has a temperature of about 40 ℃ to about 45 ℃.

4) A step of sending part or all of the hot BOG compressed by the compressor 200 to the heat exchanger 100

While the BOG compressed by the multi-stage compressor 200 is continuously supplied to the heat exchanger 100, the cold BOG, which is used as the refrigerant in the heat exchanger 100 and discharged from the storage tank T, is not supplied to the heat exchanger 100, and the hot BOG is continuously supplied to the heat exchanger 100, thereby gradually increasing the temperature of the hot fluid passage of the heat exchanger 100, through which the BOG compressed by the compressor 200 is transferred.

When the temperature of the hot fluid passages of the heat exchanger 100 exceeds the condensation or freezing point of the lubricating oil, the condensed or frozen lubricating oil accumulated in the heat exchanger 100 gradually melts or has a reduced viscosity, and then the molten or low-viscosity lubricating oil mixes with the BOG and exits the heat exchanger 100.

When the condensed or solidified lubricant oil is removed using the bypass line BL, the BOG circulates through the bypass line BL, the compressor 200, the hot fluid passage of the heat exchanger 100, the pressure reducer 600, and the gas/liquid separator 700 until the heat exchanger 100 is restored to normal.

Further, when the condensed or solidified lubricating oil is removed using the bypass line BL, the BOG discharged from the storage tank T and passing through the bypass line BL, the compressor 200, the hot fluid passage of the heat exchanger 100, and the pressure reducer 600 may be sent to a separate tank or another collection mechanism separate from the storage tank T, wherein the BOG is mixed with the molten or reduced-viscosity lubricating oil. BOG stored in a separate tank or another collection mechanism is sent to the bypass line BL to continue the process of removing condensed or solidified lube oil.

Even in the structure in which the gas/liquid separator 700 is disposed downstream of the pressure reducer 600, when the fluid composed of the BOG mixed with the molten or reduced-viscosity lubricating oil is sent to a separate tank or other collecting mechanism, the gas/liquid separator 700 provides the same function as that of the typical BOG reliquefaction system, and the molten or reduced-viscosity lubricating oil is not collected in the gas/liquid separator 700 (the molten or reduced-viscosity lubricating oil is collected by a separate tank or other collecting mechanism separate from the storage tank T). Therefore, the BOG reliquefaction system according to this embodiment may omit a gas/liquid separator configured to discharge the lubricating oil, thereby achieving a reduction in cost.

5) A step of sending the BOG having passed through the heat exchanger 100 to the gas/liquid separator 700

As the temperature of the hot fluid passage of the heat exchanger 100 increases, the condensed or solidified lubricating oil accumulated in the heat exchanger 100 is gradually melted or reduced in viscosity, and then sent to the gas/liquid separator 700 after being mixed with BOG. In the process of removing the condensed or solidified lube oil in the heat exchanger 100 via the bypass line BL, since the BOG is not re-liquefied, the re-liquefied gas is not collected by the gas/liquid separator 700, and the BOG and the melted or low-viscosity lube oil are collected.

The gaseous BOG collected in the gas/liquid separator 700 is discharged from the gas/liquid separator 700 along the sixth supply line L6 and sent to the compressor 200 along the bypass line BL. Because the first valve 510 is closed in step 2, the gaseous BOG separated by the gas/liquid separator 700 is combined with the BOG discharged from the storage tank T and sent to the compressor 200 along the bypass line BL, without being sent to the cold fluid passage of the heat exchanger 100.

Supplying the gaseous BOG separated by the gas/liquid separator 700 to the bypass line BL with the first valve 510 in the closed state can prevent the lubricating oil contained in the BOG from being supplied to the heat exchanger 100, thereby preventing blocking of the cold fluid passage of the heat exchanger 100.

The cycle process in which the gaseous BOG collected in the gas/liquid separator 700 is discharged from the gas/liquid separator 700 along the sixth supply line L6 and then sent back to the compressor 200 along the bypass line BL continues until it is determined that the temperature of the hot fluid channel of the heat exchanger 100 is increased to the temperature of the BOG compressed by the compressor 200 and sent to the hot fluid channel of the heat exchanger 100. However, the looping process may continue until it is empirically determined that sufficient time has elapsed.

During the removal of the condensed or solidified lubricating oil from the heat exchanger 100 using the bypass line BL, the eighth valve 581 is closed to prevent the lubricating oil collected in the gas/liquid separator 700 from flowing to the reserve tank T along the fifth supply line L5. If the lubricating oil is introduced into the storage tank T, the purity of the liquefied gas stored in the storage tank T may be deteriorated, thereby deteriorating the value of the liquefied gas.

6) Step of discharging the lubricating oil from the gas/liquid separator 700

The melted or reduced viscosity lubricating oil discharged from the heat exchanger 100 is collected in the gas/liquid separator 700. In order to treat the lubricating oil collected in the gas/liquid separator 700, the BOG reliquefaction system according to this embodiment may employ the gas/liquid separator 700 obtained by modifying a typical gas/liquid separator.

FIG. 10 is an enlarged view of a heat exchanger and gas/liquid separator according to one embodiment of the invention. In fig. 10, some components are omitted for convenience of description.

Referring to fig. 10, a gas/liquid separator 700 includes: a lube oil discharge line OL through which lube oil collected in the gas/liquid separator 700 is discharged; and a fifth supply line L5 through which the liquefied gas separated by the gas/liquid separator 700 is sent to the storage tank T. In order to allow efficient discharge of the lube oil collected at the lower portion of the gas/liquid separator 700, a lube oil discharge line OL is connected to the lower end of the gas/liquid separator 700, and one end of a fifth supply line L5 is disposed above the lower end of the gas/liquid separator 700 connected to the lube oil discharge line OL in the gas/liquid separator 700. In order to prevent the fifth supply line L5 from being clogged with the lubricating oil, it is preferable that the end of the fifth supply line L5 be positioned above the level of the lubricating oil when the amount of the lubricating oil collected in the gas/liquid separator 700 reaches a maximum value.

A third valve 530 for adjusting the flow rate of the fluid and the opening/closing of the corresponding line may be disposed on the lube oil discharge line OL, and may be provided in plurality.

Since the lubrication oil collected in the gas/liquid separator 700 may be naturally drained or may take a long time to be drained, the lubrication oil in the gas/liquid separator 700 may be drained via a nitrogen purge. When nitrogen is supplied to the gas/liquid separator 700 at a pressure of about 5 to 7 bar, the internal pressure of the gas/liquid separator 700 increases and allows rapid discharge of the lubricating oil.

To discharge lube oil from gas/liquid separator 700 via a nitrogen flush, nitrogen supply line NL may be disposed so as to join to third supply line L3 upstream of heat exchanger 100. Several nitrogen feed lines may be disposed at different locations as desired.

A nitrogen valve 583 for regulating the flow rate of the fluid and the opening/closing of the corresponding lines may be disposed on the nitrogen supply line NL, and normally maintained in a closed state when the nitrogen supply line NL is not used. Subsequently, when nitrogen needs to be supplied to the gas/liquid separator 700 using the nitrogen line NL for nitrogen flushing, the nitrogen valve 583 is opened. There may be multiple nitrogen valves 583.

Although the discharge of the lube oil may be performed via nitrogen flushing by injecting nitrogen directly into the gas/liquid separator 700, if a nitrogen supply line for other purposes has been installed, it is preferable to discharge the lube oil from the gas/liquid separator 700 using another installed nitrogen supply line that may have been previously installed for other purposes.

After the process of sending the whole of the BOG discharged from the storage tank T to the bypass line BL to be compressed by the compressor 200, sending the BOG compressed by the compressor 200 to the hot fluid passage of the heat exchanger 100, sending the BOG passing through the exchanger 100 and reduced in pressure in the pressure reducer 600 to the gas/liquid separator 700, and sending the BOG discharged from the gas/liquid separator 700 to the bypass line BL, if it is determined that most of the condensed or solidified lubricating oil in the heat exchanger 100 is collected in the gas/liquid separator 700 (that is, if it is determined that the heat exchanger 100 is returned to normal), nitrogen flushing is performed by blocking the flow of the BOG compressed by the compressor 200 into the heat exchanger 100 and opening the nitrogen gas valve 583.

7) Step of determining whether the heat exchanger 100 is restored to normal

If it is determined that the heat exchanger 100 is again restored to normal via the draining of the condensed or solidified lubricating oil from the heat exchanger 100, and when the process of draining the lubricating oil from the gas/liquid separator 700 is completed, the BOG reliquefaction system is again normally operated by opening the first valve 510 and the second valve 520 while closing the bypass valve 590. When the BOG reliquefaction system normally operates, the BOG discharged from the storage tank T is used as the refrigerant in the heat exchanger 100, and part or all of the BOG used as the refrigerant in the heat exchanger 100 is reliquefied via compression by the compressor 200, cooling by the heat exchanger 100, and pressure reduction by the pressure reducer 600.

The determination of whether the heat exchanger 100 is again restored to normal is based on at least one of a temperature difference of the cold fluid, a temperature difference of the hot fluid, and a pressure difference of the hot fluid passage, as well as the determination of whether it is time to remove the condensed or solidified lubricating oil.

In addition to condensed or solidified lubricating oil inside the heat exchanger 100, condensed or solidified lubricating oil accumulated in pipes, valves, instruments, and other equipment may be removed via the above-described process.

Conventionally, during the step of removing the condensed or solidified lubricating oil inside the heat exchanger 100 using the bypass line BL, a high-pressure engine and/or a low-pressure engine (hereinafter referred to as "engine") may be driven. After servicing a portion of the equipment contained in the fuel supply system or the reliquefaction system, the engine is usually in an undriven state because fuel cannot be supplied to the engine or excess BOG cannot be reliquefied.

In contrast, if the engine can be driven during removal of condensed or solidified lubricating oil from the heat exchanger 100 as in the present invention, there are the following advantages because it is possible to service the heat exchanger 100 during operation of the engine: it is possible to propel the ship and generate power and remove condensed or solidified lubricating oil using excess BOG during overhaul of the heat exchanger 100.

Further, when the engine is driven during the removal of the condensed or solidified lubricating oil from the heat exchanger 100, there are the following advantages: it is possible to burn the lubricating oil mixed with the BOG during compression by the compressor 200. That is, the engine is used not only for the purpose of propelling the boat or generating power, but also for removing oil mixed with the BOG.

On the other hand, the process of the alarm determination based on whether it is time to remove the condensed or solidified lubricating oil may include (r) an alarm activation step, and/or (r) an alarm generation step. In the embodiment of the present invention, the alarm is explained as an example of the notification unit.

Fig. 7 is a schematic view of a BOG reliquefaction system according to a fourth embodiment of the present invention, fig. 8 is an enlarged view of a pressure reducer according to one embodiment of the present invention, and fig. 9 is an enlarged view of a pressure reducer according to another embodiment of the present invention.

Referring to fig. 7, two

compressors

200, 201 may be arranged in parallel in the present invention. The two

compressors

200, 201 may have the same specifications and may serve as a backup to provide for the prevention of failure of either of the compressors. The description of the other devices is omitted for convenience of description.

Referring to fig. 7, in the structure in which the

compressors

200, 201 are arranged in parallel, the BOG discharged from the storage tank T is sent to the second compressor 201 via the seventh supply line L22, and the BOG compressed by the second compressor 201 is partially discharged to the high pressure engine via the fuel supply line SL, while the surplus BOG is sent to the heat exchanger 100 via the eighth supply line L33 to undergo the reliquefaction process. A tenth valve 571 for adjusting the flow rate and the opening/closing of the corresponding lines may be disposed on the eighth supply line L33.

In other embodiments, the two

pressure reducers

600, 610 may be arranged in parallel as shown in fig. 8, and the two pairs of

pressure reducers

600, 610 arranged in series may be arranged in parallel as shown in fig. 9.

Referring to fig. 8, two

pressure reducers

600, 610 arranged in parallel may serve as a backup to prepare for preventing a failure of any one of the compressors, and each of the

pressure reducers

600, 610 may be provided in such a manner that an isolation valve 620 is provided at front and rear ends thereof.

Referring to fig. 9, two pairs of

reducers

600, 610 connected in series are arranged in parallel. Depending on the manufacturer, two pressure reducers 600 are connected in series to achieve pressure reduction stability. The two pairs of

pressure reducers

600, 610 connected in parallel can serve as back-ups to provide for the prevention of any failure of the pressure reducers.

Each of the

pressure reducers

600, 610 connected in parallel may be provided in such a manner that an isolation valve 620 is provided at front and rear ends thereof. The isolation valve 620 shown in fig. 8 and 9 isolates the pressure reducer 600 upon maintenance or service of the pressure reducer 600 due to a failure, etc., of the

pressure reducers

600, 610.

Step of alarm activation

In a configuration in which the BOG reliquefaction system includes one compressor 200 and one decompressor 600 as shown in fig. 2, an alarm is activated under the following conditions: the degree to which the pressure reducer 600 is opened is a preset value or more, the seventh valve 570 and the second valve 520 are opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal liquid level.

In a configuration in which the BOG reliquefaction system includes one compressor 200 as shown in fig. 2 and two

pressure reducers

600, 610 connected in parallel as shown in fig. 8, an alarm is activated under the following conditions (hereinafter referred to as "first alarm activation conditions"): the degree to which the first pressure reducer 600 or the second pressure reducer 610 is opened is a preset value or more, the seventh valve 570 and the second valve 520 are opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal liquid level.

In a configuration in which the BOG reliquefaction system includes one compressor 200 as shown in fig. 2 and two pairs of

pressure reducers

600, 610 connected in parallel as shown in fig. 9, an alarm is activated under the following conditions (hereinafter referred to as "second alarm activation conditions"): one of the two first pressure reducers 600 arranged in series or one of the two second pressure reducers 610 connected in series is opened to a preset value or more, the seventh valve 570 and the second valve 520 are opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal liquid level.

In the structure in which the BOG reliquefaction system includes two

compressors

200, 201 connected in parallel as shown in fig. 7 and one pressure reducer 600 as shown in fig. 2, an alarm is activated under the following conditions (hereinafter referred to as "third alarm activation condition"): the degree to which the pressure reducer 600 is opened is a preset value or more, the seventh valve 570 or the tenth valve 571 is opened, the second valve 520 is opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal level.

In a structure in which the BOG reliquefaction system includes two

compressors

200, 201 connected in parallel as shown in fig. 7 and two

pressure reducers

600, 610 connected in parallel as shown in fig. 8, an alarm is activated under the following conditions (hereinafter referred to as "fourth alarm activation conditions"): the degree to which the first pressure reducer 600 or the second pressure reducer 610 is opened is a preset value or more, the seventh valve 570 or the tenth valve 571 is opened, the second valve 520 is opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal level.

In a configuration in which the BOG reliquefaction system includes two

compressors

200, 201 connected in parallel as shown in fig. 7 and two pairs of

pressure reducers

600, 610 connected in parallel as shown in fig. 9, an alarm is activated under the following conditions (hereinafter referred to as "fifth alarm activation conditions"): one of the two first pressure reducers 600 arranged in series or one of the two second pressure reducers 610 connected in series is opened to a preset value or more, the seventh valve 570 or the tenth valve 571 is opened, the second valve 520 is opened, and the liquid level of the liquefied gas in the gas/liquid separator 700 is a normal liquid level.

In the above-described first to fifth alarm activation conditions, the predetermined opening degree of the first pressure reducer 600 or the second pressure reducer 610 may be 2%, and the normal level of the liquefied gas in the gas/liquid separator 700 means a case where the reliquefaction process can be normally performed by confirming the reliquefied gas in the gas/liquid separator 700.

② step of alarm generation

An alarm may be generated to indicate a point in time for removing condensed or solidified lubricant if any of the following conditions are met: a condition that a temperature difference of the cold fluid is a preset value or more for a predetermined time period, a condition that a temperature difference of the hot fluid is a preset value or more for a predetermined time period, and a condition that a pressure difference of the hot fluid passage is a preset value or more for a predetermined time period.

To improve reliability, an alarm may be generated to indicate a point in time for removing condensed or solidified lubricant if at least two of the following conditions are met: a condition that a temperature difference of the cold fluid is a preset value or more for a predetermined time period, a condition that a temperature difference of the hot fluid is a preset value or more for a predetermined time period, and a condition that a pressure difference of the hot fluid passage is a preset value or more for a predetermined time period.

Further, if a lower value of the temperature difference of the cold flow and the temperature difference of the hot flow is a preset value or more for a predetermined time period (or condition), or if the pressure difference of the hot flow channel is a preset value or more for a predetermined time period, an alarm may be generated to indicate a time point for removing the condensed or solidified lubricating oil.

According to the present invention, abnormality of the heat exchanger, alarm generation, and the like can be determined by an appropriate controller. As the controller for determining abnormality, alarm generation, and the like of the heat exchanger, a controller used by the BOG reliquefaction system according to the present invention may be used, preferably a controller used by a ship or an offshore structure to which the BOG reliquefaction system according to the present invention is applied, and a separate controller for determining abnormality, alarm generation, and the like of the heat exchanger may also be used.

Furthermore, the use of bypass lines, the draining of lubricating oil, the supply of fuel to the engine, the starting or restarting of the BOG reliquefaction system, and the opening or closing of various valves for these components may be controlled automatically or manually by the controller.

2. Case of satisfying the intake pressure condition of the compressor 200 using the bypass line BL when the internal pressure of the storage tank T is low

In the case where the storage tank T has a low internal pressure, the compressor 200 does not normally satisfy the intake pressure condition upstream of the compressor 200, for example, when the amount of BOG generated is small because of a small amount of liquefied gas in the storage tank T or when the amount of BOG supplied to the engine for propelling the ship is large because of a high speed of the ship.

Specifically, in the PCHE (dche) used as the heat exchanger 100, when the BOG discharged from the storage tank T passes through the PCHE, a pressure drop is large due to its narrow fluid passage.

Conventionally, when the compressor 200 fails to satisfy the intake pressure condition, the

recirculation valves

541, 542, 543, 544 are opened to protect the compressor 200 by recirculating part or all of the BOG through the recirculation lines Rc1, Rc2, Rc3, Rc 4.

However, if the intake pressure condition of the compressor 200 is satisfied by the recycled BOG, the amount of BOG compressed by the compressor 200 is reduced, thereby causing the reliquefaction performance to deteriorate and failing to satisfy the fuel consumption requirement of the engine. In particular, if the engine does not meet the fuel consumption requirements, the operation of the ship may be significantly disturbed. Therefore, a BOG reliquefaction method capable of satisfying the intake pressure condition of the compressor and the fuel consumption requirement of the engine even when the internal pressure of the reserve tank T is low is required.

According to the present invention, the bypass line BL provided for maintenance and repair of the heat exchanger 100 can be used to satisfy the intake pressure condition of the compressor 200 even when the internal pressure of the storage tank T is low, without reducing the amount of BOG compressed by the compressor 200, without providing additional equipment. It is possible to satisfy the suction pressure condition required for the compressor 200 without reducing the amount of BOG.

According to the present invention, when the internal pressure of the storage tank T is reduced to the preset value or less, the bypass valve 590 is opened to allow part or all of the BOG discharged from the storage tank T to be directly sent to the compressor 200 via the bypass line BL bypassing the heat exchanger 100.

The amount of BOG sent to the bypass line BL may be adjusted depending on the pressure of the storage tank T compared to the required intake pressure conditions of the compressor 200. That is, all of the BOG discharged from the storage tank T may be sent to the bypass line BL by opening the bypass valve 590 while closing the first and

second valves

510 and 520, or only some of the BOG discharged from the storage tank T may be sent to the bypass line BL, and the remaining BOG may be sent to the heat exchanger 100 by partially opening the bypass valve 590, the first and

second valves

510 and 520. That is, all of the BOG discharged from the storage tank T may be sent to the bypass line BL by opening the bypass valve 590 while closing the first and

second valves

510 and 520. The pressure drop of the BOG decreases as the amount of BOG bypassing the heat exchanger 100 via the bypass line BL increases.

Although there is an advantage in minimizing the pressure drop when the BOG discharged from the storage tank T bypasses the heat exchanger 100 and is directly sent to the compressor 200, the cold heat of the BOG cannot be used to re-liquefy the BOG. Therefore, the amount of BOG to be sent to the bypass line BL, among the amount of BOG discharged from the storage tank T and the amount of BOG used to reduce the pressure drop using the bypass line BL, is determined based on the internal pressure of the storage tank T, the fuel consumption requirement of the engine, the amount of BOG to be reliquefied, and the like.

By way of example, it may be determined that it is advantageous to do so: the pressure drop is reduced using the bypass line BL when the internal pressure of the storage tank T is a preset value or less and the ship is operating at a predetermined speed or more. In particular, it may be determined that it is advantageous to do so: the bypass line BL is used to reduce the pressure drop when the internal pressure of the storage tank T is 1.09 bar or less and the speed of the ship is 17 knots or more.

Further, even when all the BOG discharged from the storage tank T is sent to the compressor 200 via the bypass line BL, the intake pressure condition of the compressor 200 is often not satisfied. In this case, the intake pressure condition is satisfied using the recirculation lines Rc1, Rc2, Rc3, Rc 4.

That is, the recirculation lines Rc1, Rc2, Rc3, Rc4 are used in the related art to protect the compressor 200 when the intake pressure condition of the compressor 200 cannot be satisfied because the pressure of the reserve tank T is lowered, however, according to the present invention, the bypass line BL is mainly used in order to satisfy the intake pressure condition of the compressor 200, and the recirculation lines Rc1, Rc2, Rc3, Rc4 are secondarily used when the intake pressure condition of the compressor 200 cannot be satisfied even by sending all BOG discharged from the reserve tank T to the compressor via the bypass line BL.

In order to satisfy the intake pressure condition of the compressor 200, the bypass line BL is mainly used and the recirculation lines Rc1, Rc2, Rc3, and Rc4 are secondarily used, and the pressure condition under which the bypass valve 590 is opened is set to a value higher than the pressure condition under which the

recirculation valves

541, 542, 543, and 544 are opened.

The conditions under which the

recirculation valves

541, 542, 543, 544 are open and the conditions under which the bypass valve 590 is open are preferably determined based on the pressure upstream of the compressor 200. Alternatively, these conditions may be determined based on the internal pressure of the storage tank T.

The pressure upstream of the compressor 200 may be measured by a third pressure sensor (not shown) disposed upstream of the compressor 200, and the internal pressure of the storage tank T may be measured by a fourth pressure sensor (not shown).

On the other hand, in the structure in which the sixth supply line L6 for discharging the gaseous BOG separated by the gas/liquid separator 700 is joined to the first supply line L1 at a position downstream of the branch point of the bypass line BL branching from the first supply line L1, some BOG discharged from the reserve tank T while preventing a pressure drop may be used as the refrigerant in the heat exchanger 100 by: the gaseous BOG separated by the gas/liquid separator 700 is sent directly to the bypass line BL, where all of the bypass valve 590, the first valve 510, and the second valve 520 are opened in the operation of the system.

Since the temperature of the gaseous BOG separated by the gas/liquid separator 700 is lower than the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger 100, and the cooling efficiency of the heat exchanger 100 may be deteriorated when the gaseous BOG separated by the gas/liquid separator 700 is directly sent to the bypass line BL, it is preferable that at least some of the gaseous BOG separated by the gas/liquid separator 700 is sent to the heat exchanger 100.

Here, if the amount of BOG generated in the storage tank T is smaller than the amount of BOG that is fuel required for the engine, it may not be necessary to liquefy the BOG any more. However, when it is not necessary to re-liquefy the BOG, all of the gaseous BOG separated by the gas/liquid separator 700 may be sent to the bypass line BL because it is not necessary to supply the refrigerant to the heat exchanger 100.

Accordingly, in the present invention, the sixth supply line L6 is joined to the first supply line L1 at a position upstream of the branch point of the bypass line BL branching from the first supply line L1. In the structure in which the sixth supply line L6 is joined to the first supply line L1 upstream of the branch point of the bypass line, the BOG discharged from the storage tank T and the gaseous BOG separated by the gas/liquid separator 700 are combined with each other at a position upstream of the branch point of the bypass line BL, and then the amount of BOG to be sent to the bypass line BL and the heat exchanger 100 is determined depending on the degree to which the bypass valve 590 and the first valve 510 are opened, whereby it is easy to control the system, and the gaseous BOG separated by the gas/liquid separator 700 is prevented from being sent directly to the bypass line BL.

Preferably, the bypass valve 590 is a valve providing a higher response than a typical valve, so as to allow rapid adjustment of the degree of opening depending on pressure changes of the storage tank T.

Referring to fig. 3, a BOG reliquefaction system according to a third embodiment of the present invention is different from the BOG reliquefaction system according to the first embodiment shown in fig. 1 in that: the BOG reliquefaction system according to the third embodiment includes a pressure difference sensor 930 instead of the first pressure sensor 910 and the second pressure sensor 920, and the following description will focus on different features of the BOG reliquefaction system according to the third embodiment. The description of the same components as those of the BOG reliquefaction system according to the first embodiment will be omitted.

Unlike the first embodiment, the BOG reliquefaction system according to the third embodiment includes a pressure difference sensor 930 that measures a pressure difference between the third supply line L3 upstream of the heat exchanger 100 and the fourth supply line L4 downstream of the heat exchanger 100, instead of the first pressure sensor 910 and the second pressure sensor 920.

The pressure difference of the hot fluid passage may be obtained by the pressure difference sensor 930, and whether it is time to remove the condensed or solidified lubricating oil may be determined based on at least one of the pressure difference of the hot fluid passage, the temperature difference of the cold flow, and the temperature difference of the hot flow, as in the first embodiment.

It will be apparent to those skilled in the art that the present invention is not limited to the above-described embodiments and various modifications, changes, alterations, and equivalent embodiments can be made without departing from the spirit and scope of the invention.

Claims

1. A method of discharging lubricating oil from an boil-off gas reliquefaction system, characterized in that the boil-off gas reliquefaction system is configured to reliquefy the boil-off gas by compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by a heat exchanger via heat exchange with the uncompressed boil-off gas, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer,

wherein an evaporation gas to be used as refrigerant in the heat exchanger is supplied to the heat exchanger along a first supply line,

the evaporation gas used as the refrigerant in the heat exchanger is supplied to the compressor along a second supply line, an

The evaporation gas not used as the refrigerant in the heat exchanger is supplied to the compressor along a bypass line bypassing the heat exchanger, an

Wherein a bypass valve for regulating the flow rate of the fluid and the opening/closing of the corresponding supply line is disposed on the bypass line,

a first valve for regulating the flow rate of fluid and the opening/closing of the corresponding supply line is disposed on the first supply line upstream of the heat exchanger,

a second valve for regulating the flow rate of the fluid and the opening/closing of the corresponding supply line is disposed on the second supply line downstream of the heat exchanger, an

The compressor includes at least one cylinder of an oil lubrication type,

the method comprises the following steps:

2) opening the bypass valve while closing the first valve and the second valve;

3) sending the boil-off gas that is not used as the refrigerant in the heat exchanger along the bypass line to the compressor and then compressed by the compressor; and

4) sending part or all of the boil-off gas compressed by the compressor to the heat exchanger,

the condensed or solidified lubricating oil is discharged from the boil-off gas reliquefaction system through the boil-off gas whose temperature is increased during compression by the compressor after being melted or reduced in viscosity.

2. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 1, further comprising:

1) it is determined whether it is time to remove the condensed or solidified lubricating oil before step 2.

3. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 1, wherein after reliquefaction of the boil-off gas, liquefied gas and gaseous boil-off gas generated by the reliquefaction are separated from each other by a gas/liquid separator, the liquefied gas separated by the gas/liquid separator is discharged from the gas/liquid separator along a fifth supply line, and the gaseous boil-off gas separated by the gas/liquid separator is discharged from the gas/liquid separator along a sixth supply line,

the method further comprises:

5) sending the boil-off gas having passed through the heat exchanger to the gas/liquid separator; and

6) discharging the lubricating oil accumulated in the gas/liquid separator.

4. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 3, wherein the boil-off gas sent to the gas/liquid separator in step 5) is sent to the bypass line along the sixth supply line to be subjected to compression in step 3).

5. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 4, wherein the steps 3) through 5) are repeated until the temperature of the hot fluid passage of the heat exchanger increases to the temperature of the boil-off gas compressed by the compressor and sent to the heat exchanger.

6. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 1, wherein in step 4), the boil-off gas compressed by the compressor is used as fuel by an engine, and surplus boil-off gas not used by the engine is sent to the heat exchanger.

7. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 2, wherein in step 1), it is determined that it is time to drain the condensed or solidified lubricating oil if at least one of the following conditions is satisfied:

a condition that a temperature difference of a cold flow, which is a temperature difference between the evaporation gas upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and cooled by the heat exchanger, is a first preset value or more for a predetermined time period or more;

a condition that a temperature difference of a hot flow is the first preset value or more and continues for a predetermined time period or more, wherein the temperature difference of the hot flow is a temperature difference between the evaporation gas used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and sent to the heat exchanger; and

a condition that a pressure difference of a hot fluid passage is a second preset value or more for a predetermined time period or more, wherein the pressure difference of the hot fluid passage is a pressure difference between the evaporation gas compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the evaporation gas cooled by the heat exchanger at a position downstream of the heat exchanger.

8. The method of discharging lubricating oil from an evaporation gas reliquefaction system according to claim 2, wherein in step 1), if a lower value between a temperature difference of a cold flow, which is a temperature difference between the evaporation gas upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and cooled by the heat exchanger, and a temperature difference of a hot flow, which is the evaporation gas used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and sent to the heat exchanger, is a first preset value or more for a predetermined time period or more, or if a pressure difference of a hot flow passage is a second preset value or more for a predetermined time period or more, it is determined that it is time to discharge the condensed or solidified lubricating oil, wherein the temperature difference of the hot flow is a temperature difference between the evaporation gas used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and sent to the heat exchanger A temperature difference between the boil-off gases, wherein the pressure difference of the hot fluid passage is a pressure difference between the boil-off gas compressed by the compressor and sent to the heat exchanger at a location upstream of the heat exchanger and the boil-off gas cooled by the heat exchanger at a location downstream of the heat exchanger.

9. The method of draining lubricating oil from boil-off gas reliquefaction system according to any one of claims 3 to 5, wherein the liquefied gas separated by the gas/liquid separator is sent to a storage tank along the fifth supply line after reliquefaction of the boil-off gas, and

an eighth valve for regulating the flow rate of fluid and the opening/closing of the corresponding supply line is disposed on the fifth supply line,

the eighth valve is closed during step 2) to step 6).

10. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to any one of claims 1 to 8, wherein the reliquefaction of the boil-off gas is performed after opening the first valve and the second valve while closing the bypass valve after determining that the heat exchanger is returned to normal.

11. The method of draining lubricating oil from boil-off gas reliquefaction systems according to claim 7 or 8, wherein a point in time for draining the condensed or solidified lubricating oil is indicated by a notification unit.

12. The method of draining lubricating oil from boil-off gas reliquefaction system according to claim 7 or 8, wherein if the performance of the heat exchanger is reduced to 60% to 80% of its normal performance, it is determined that it is time to drain the condensed or solidified lubricating oil.

13. The method of draining lubricating oil from boil-off gas reliquefaction system according to claim 7 or 8, wherein the first preset value is 35 ℃.

14. The method of draining lubricating oil from boil-off gas reliquefaction system according to claim 7 or 8, wherein the second preset value is twice the preset value for normal operation.

15. The method of draining lubricating oil from boil-off gas reliquefaction system according to claim 7 or 8, wherein the second preset value is 2 bar (200 kPa).

16. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 7 or 8, wherein the predetermined time period is 1 hour.

17. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 7 or 8, wherein the temperature difference of the cold flow is detected by a first temperature sensor disposed upstream of a cold fluid passage of the heat exchanger and a fourth temperature sensor disposed downstream of the hot fluid passage of the heat exchanger.

18. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 7 or 8, wherein the temperature difference of the hot stream is detected by a second temperature sensor disposed downstream of a cold fluid channel of the heat exchanger and a third temperature sensor disposed upstream of the hot fluid channel of the heat exchanger.

19. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 7 or 8, wherein the pressure difference of the hot fluid passage is detected by a first pressure sensor disposed upstream of the hot fluid passage of the heat exchanger and a second pressure sensor disposed downstream of the hot fluid passage of the heat exchanger.

20. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 7 or 8, wherein the pressure difference of the hot fluid passage is detected by a pressure difference sensor measuring a pressure difference between upstream of the hot fluid passage of the heat exchanger and downstream of the hot fluid passage of the heat exchanger.

21. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 1 to 8, wherein the compressor compresses the boil-off gas to a pressure of 150 bar to 350 bar.

22. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 1 to 8, wherein the compressor compresses the boil-off gas to a pressure of 80 bar to 250 bar.

23. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 1 to 8, wherein the heat exchanger includes a microchannel-type fluid channel.

24. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 23, wherein the heat exchanger is a printed circuit heat exchanger.

25. A method of discharging lubricating oil from an boil-off gas reliquefaction system, characterized in that the boil-off gas reliquefaction system is configured to reliquefy the boil-off gas by compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by a heat exchanger via heat exchange with the uncompressed boil-off gas, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer,

wherein the compressor includes at least one oil-lubricated cylinder,

the boil-off gas is sent to and compressed by the compressor via a bypass line that bypasses the heat exchanger,

the evaporation gas compressed by the compressor is supplied to an engine, an

Surplus boil-off gas not supplied to the engine is supplied to the heat exchanger to discharge the condensed or solidified lubricating oil using the boil-off gas whose temperature is increased during compression by the compressor after the lubricating oil is melted or its viscosity is reduced.

26. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 25, wherein after reliquefaction of the boil-off gas, liquefied gas and gaseous boil-off gas generated by the reliquefaction are separated from each other by a gas/liquid separator, and the gaseous boil-off gas separated by the gas/liquid separator is discharged from the gas/liquid separator along a sixth supply line, and

wherein the lubricating oil, which is melted or reduced in viscosity and discharged by the boil-off gas increased in temperature during compression by the compressor, is collected in the gas/liquid separator.

27. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 25, wherein the boil-off gas compressed by the compressor after having passed through the bypass line is sent to the heat exchanger after the lubricating oil is filtered from the boil-off gas by at least one of an oil separator and a first oil filter.

28. The method of draining lube oil from a boil-off gas reliquefaction system according to claim 27, wherein the first oil filter separates lube oil having a vapor phase or a mist phase.

29. The method of draining lube oil from a boil-off gas reliquefaction system according to claim 26, wherein a second oil filter is disposed on at least one of: a position between the pressure reducer and the gas/liquid separator, a fifth supply line through which the liquefied gas separated by the gas/liquid separator is discharged, and the sixth supply line, the second oil filter being a low-temperature oil filter.

30. The method of draining lube oil from a boil-off gas reliquefaction system according to claim 29, wherein the second oil filter separates the lube oil having a solid phase.

31. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 26, wherein the boil-off gas compressed by the compressor after having passed through the heat exchanger and sent to the gas/liquid separator is subjected to repeated cycle periods by being sent along the sixth supply line to the bypass line for compression by the compressor.

32. The method of discharging lubricating oil from a boil-off gas reliquefaction system according to claim 31, wherein the cycle is repeated until a temperature of a hot fluid passage of the heat exchanger reaches a temperature of the boil-off gas compressed by the compressor and sent to the heat exchanger.

33. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 25 to 32, wherein the compressor compresses the boil-off gas to a pressure of 150 bar to 350 bar.

34. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 25 to 32, wherein the compressor compresses the boil-off gas to a pressure of 80 bar to 250 bar.

35. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 25 to 32, wherein the heat exchanger includes microchannel-type fluid channels.

36. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 35, wherein the heat exchanger is a printed circuit heat exchanger.

37. A method of discharging lubricating oil from an boil-off gas reliquefaction system, the boil-off gas reliquefaction system configured to reliquefy the boil-off gas using the boil-off gas as a refrigerant,

wherein the heat exchanger cools the evaporation gas compressed by the compressor via heat exchange using the evaporation gas discharged from the storage tank as the refrigerant after the evaporation gas is reliquefied;

the compressor includes at least one oil-lubricated cylinder; and

the condensed or solidified lubricating oil is discharged by a bypass line disposed to bypass the heat exchanger and used in servicing the heat exchanger after melting or viscosity reduction.

38. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 37, wherein the compressor compresses the boil-off gas to a pressure of 150 bar to 350 bar.

39. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 37, wherein the compressor compresses the boil-off gas to a pressure of 80 bar to 250 bar.

40. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 37, wherein the heat exchanger includes microchannel-type fluid channels.

41. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 40, wherein the heat exchanger is a printed circuit heat exchanger.

42. A method of discharging lubricating oil from an boil-off gas reliquefaction system, characterized in that the boil-off gas reliquefaction system is configured to reliquefy the boil-off gas by compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by a heat exchanger via heat exchange with uncompressed boil-off gas, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer,

wherein the compressor includes at least one oil-lubricated cylinder, and it is determined that it is time to discharge the condensed or solidified lubricating oil if at least one of the following conditions is satisfied:

a condition that a temperature difference of a cold flow, which is a temperature difference between the evaporation gas upstream of the heat exchanger to be used as a refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and cooled by the heat exchanger, is a first preset value or more for a predetermined time period or more;

43. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein a point in time for draining the condensed or solidified lubricating oil is indicated by a notification unit.

44. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein if the performance of the heat exchanger is reduced to 60% to 80% of its normal performance, a time to drain the condensed or solidified lubricating oil is determined.

45. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the first predetermined value is 35 ℃.

46. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the second predetermined value is twice the predetermined value for normal operation.

47. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the second preset value is 2 bar (200 kPa).

48. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the predetermined period of time is 1 hour.

49. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the temperature difference of the cold stream is detected by a first temperature sensor disposed upstream of a cold fluid passage of the heat exchanger and a fourth temperature sensor disposed downstream of the hot fluid passage of the heat exchanger.

50. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the temperature differential of the hot stream is detected by a second temperature sensor disposed downstream of a cold fluid passage of the heat exchanger and a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger.

51. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the pressure difference of the hot fluid passage is detected by a first pressure sensor disposed upstream of the hot fluid passage of the heat exchanger and a second pressure sensor disposed downstream of the hot fluid passage of the heat exchanger.

52. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the pressure difference of the hot fluid channel is detected by a pressure difference sensor measuring a pressure difference between upstream of the hot fluid channel of the heat exchanger and downstream of the hot fluid channel of the heat exchanger.

53. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the compressor compresses the boil-off gas to a pressure of 150 bar to 350 bar.

54. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the compressor compresses the boil-off gas to a pressure of 80 bar to 250 bar.

55. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 42, wherein the heat exchanger includes microchannel-type fluid channels.

56. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 55, wherein the heat exchanger is a printed circuit heat exchanger.

57. A method of discharging lubricating oil from an boil-off gas reliquefaction system, characterized in that the boil-off gas reliquefaction system is configured to reliquefy the boil-off gas by compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by a heat exchanger via heat exchange with uncompressed boil-off gas, and reducing the pressure of a fluid cooled via heat exchange by a pressure reducer,

wherein the compressor includes at least one oil-lubricated type cylinder, and if a lower value between a temperature difference of a cold flow, which is a temperature difference between the evaporation gas upstream of the heat exchanger to be used as a refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and cooled by the heat exchanger, and a temperature difference of a hot flow, which is a temperature difference between the evaporation gas used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and sent to the heat exchanger, is a first preset value or more and is for a predetermined time period or more, or if a pressure difference of a hot flow passage is a second preset value or more and is for a predetermined time period or more, it is determined that it is time to discharge the condensed or solidified lubricating oil, wherein the pressure difference of the hot fluid channel is a pressure difference between the boil-off gas compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the boil-off gas cooled by the heat exchanger at a position downstream of the heat exchanger.

58. The method of draining lubricating oil from boil-off gas reliquefaction systems according to claim 57, wherein a point in time for draining the condensed or solidified lubricating oil is indicated by a notification unit.

59. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein if the performance of the heat exchanger is reduced to 60% to 80% of its normal performance, a time to drain the condensed or solidified lubricating oil is determined.

60. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the first predetermined value is 35 ℃.

61. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the second predetermined value is twice the predetermined value for normal operation.

62. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the second predetermined value is 2 bar (200 kPa).

63. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the predetermined period of time is 1 hour.

64. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the temperature differential of the cold stream is detected by a first temperature sensor disposed upstream of a cold fluid passage of the heat exchanger and a fourth temperature sensor disposed downstream of the hot fluid passage of the heat exchanger.

65. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the temperature differential of the hot stream is detected by a second temperature sensor disposed downstream of a cold fluid passage of the heat exchanger and a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger.

66. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the pressure differential of the hot fluid passage is detected by a first pressure sensor disposed upstream of the hot fluid passage of the heat exchanger and a second pressure sensor disposed downstream of the hot fluid passage of the heat exchanger.

67. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the pressure difference of the hot fluid channel is detected by a pressure difference sensor measuring a pressure difference between upstream of the hot fluid channel of the heat exchanger and downstream of the hot fluid channel of the heat exchanger.

68. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the compressor compresses the boil-off gas to a pressure of 150 bar to 350 bar.

69. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the compressor compresses the boil-off gas to a pressure of 80 bar to 250 bar.

70. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 57, wherein the heat exchanger includes microchannel-type fluid channels.

71. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 70, wherein the heat exchanger is a printed circuit heat exchanger.

72. A method of discharging lubricating oil from an boil-off gas reliquefaction system, the boil-off gas reliquefaction system configured to reliquefy the boil-off gas using the boil-off gas as a refrigerant,

wherein a point in time for discharging the condensed or solidified lubricating oil is determined based on at least one of a temperature difference and a pressure difference of the device, and the point in time for discharging the condensed or solidified lubricating oil is indicated by a notification unit.

73. The method of draining lubricating oil from a boil-off gas reliquefaction system according to claim 72, wherein the apparatus includes a heat exchanger including microchannel-type fluid channels.

74. A method of discharging lubricating oil from an boil-off gas reliquefaction system, the boil-off gas reliquefaction system configured to reliquefy the boil-off gas using the boil-off gas as a refrigerant,

wherein the lube oil collected in the gas/liquid separator is discharged from the gas/liquid separator via a lube oil discharge line separated from a fifth supply line, and liquefied gas produced by reliquefaction of the boil-off gas is discharged from the gas/liquid separator via the fifth supply line.

75. The method of draining lube oil from a boil-off gas reliquefaction system according to claim 74, wherein a speed of draining the lube oil from the gas/liquid separator is increased by supplying nitrogen gas into the gas/liquid separator.

76. The method of discharging a lubricating oil from a boil-off gas reliquefaction system according to claim 75, wherein after reliquefaction of the boil-off gas, the compressed boil-off gas is cooled in a heat exchanger using the boil-off gas as a refrigerant, and after discharging the lubricating oil, nitrogen is supplied to the gas/liquid separator along a hot fluid passage through which the compressed boil-off gas is supplied to the heat exchanger.

77. The method of draining lube oil from a boil-off gas reliquefaction system according to claim 75 or 76, wherein the nitrogen supplied to the gas/liquid separator has a pressure of 5 to 7 bar.

78. The method of draining lube oil from boil-off gas reliquefaction system according to any one of claims 74 to 76, wherein after reliquefaction of the boil-off gas, the liquefied gas separated by the gas/liquid separator is sent to a storage tank along the fifth supply line, and an eighth valve is disposed on the fifth supply line to regulate a flow rate of fluid and opening/closing of the fifth supply line, the eighth valve being closed during draining of the lube oil.

79. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 74 to 76, wherein an engine is driven during the draining of the lubricating oil.

80. The method of draining lubricating oil from a boil-off gas reliquefaction system according to any one of claims 74 to 76, wherein after the draining of the lubricating oil, the boil-off gas to be supplied to a cold fluid channel of a heat exchanger is compressed and sent to a hot fluid channel of the heat exchanger after bypassing the heat exchanger.

81. A fuel supply method for an engine is characterized in that,

compressing the boil-off gas by a compressor;

supplying part of the compressed boil-off gas to an engine as fuel;

exchanging heat of the compressed boil-off gas, which is not supplied to the engine, with the uncompressed boil-off gas using a heat exchanger to cool the compressed boil-off gas;

reducing the pressure of the cooled boil-off gas by using a pressure reducer;

wherein the fuel is supplied to the engine during discharge of the condensed or solidified lubricating oil by melting or reducing viscosity of the condensed or solidified lubricating oil by increasing temperature of a hot fluid passage of the heat exchanger by the evaporation gas increased in temperature during compression by the compressor.