CN111219593B - BOG recovery system of LNG power ship and working method thereof - Google Patents

BOG recovery system of LNG power ship and working method thereof Download PDF

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Publication number
CN111219593B
CN111219593B CN202010185779.4A CN202010185779A CN111219593B CN 111219593 B CN111219593 B CN 111219593B CN 202010185779 A CN202010185779 A CN 202010185779A CN 111219593 B CN111219593 B CN 111219593B
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lng
valve
storage tank
bog
heat exchanger
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CN111219593A (en
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沈九兵
严思远
许津津
蒋庆峰
冯国增
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Jiangsu University of Science and Technology
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Jiangsu University of Science and Technology
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F25J1/0025—Boil-off gases "BOG" from storages
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C5/00—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures
    • F17C5/02—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures for filling with liquefied gases
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C13/00—Details of vessels or of the filling or discharging of vessels
    • F17C13/04—Arrangement or mounting of valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C5/00—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures
    • F17C5/02—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures for filling with liquefied gases
    • F17C5/04—Methods or apparatus for filling containers with liquefied, solidified, or compressed gases under pressures for filling with liquefied gases requiring the use of refrigeration, e.g. filling with helium or hydrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/0045—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by vaporising a liquid return stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0257—Construction and layout of liquefaction equipments, e.g. valves, machines
    • F25J1/0275—Construction and layout of liquefaction equipments, e.g. valves, machines adapted for special use of the liquefaction unit, e.g. portable or transportable devices
    • F25J1/0277—Offshore use, e.g. during shipping
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/01—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the phase
    • F17C2223/0146—Two-phase
    • F17C2223/0153—Liquefied gas, e.g. LPG, GPL
    • F17C2223/0161—Liquefied gas, e.g. LPG, GPL cryogenic, e.g. LNG, GNL, PLNG
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2223/00—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel
    • F17C2223/03—Handled fluid before transfer, i.e. state of fluid when stored in the vessel or before transfer from the vessel characterised by the pressure level
    • F17C2223/033—Small pressure, e.g. for liquefied gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2250/00—Accessories; Control means; Indicating, measuring or monitoring of parameters
    • F17C2250/03—Control means
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C2270/00—Applications
    • F17C2270/01—Applications for fluid transport or storage
    • F17C2270/0102—Applications for fluid transport or storage on or in the water
    • F17C2270/0105—Ships
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/30—Processes or apparatus using other separation and/or other processing means using a washing, e.g. "scrubbing" or bubble column for purification purposes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/90—Mixing of components
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/06—Splitting of the feed stream, e.g. for treating or cooling in different ways
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/02—Mixing or blending of fluids to yield a certain product
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/62—Separating low boiling components, e.g. He, H2, N2, Air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/30—Compression of the feed stream

Abstract

The invention discloses a BOG recovery system of an LNG power ship, which comprises an LNG storage tank, a heat exchanger and an LNG filling device, wherein the heat exchanger is internally provided with a filter screen, a spray pipe, a filler and an air inlet pipe; the third path is connected with a seventh valve, an LNG pump and a heat exchanger; LNG liquid level control liquid storage pot is connected to the LNG storage tank, and the BOG buffer tank is connected through the fifth valve with the LNG storage tank through the fourth valve jointly to the LNG liquid level control liquid storage pot, and the BOG compressor is connected to the BOG buffer tank, and the BOG compressor passes through the sixth valve and connects the heat exchanger, eighth valve, third governing valve to LNG storage tank are connected to the heat exchanger. The invention fully recovers BOG generated in the precooling and filling processes and the stopping process of the ship LNG storage tank, reduces the operation cost and lightens the environmental pollution.

Description

BOG recovery system of LNG power ship and working method thereof
Technical Field
The invention relates to reliquefaction of gas generated during precooling and filling processes and during stopping of an LNG (liquefied natural gas) storage tank of an LNG power ship, in particular to a BOG (boil off gas) recovery system of the LNG power ship and a working method thereof.
Background
The natural gas is a clean energy source, has high combustion heat value, no pollution and high energy utilization rate, occupies increasing market share, and is cooled to the temperature of minus 162.5 ℃ under normal pressure to become Liquid Natural Gas (LNG) in the storage and transportation process, and the volume of the LNG becomes 1/625 of the original volume.
With the aggravation of the atmospheric pollution condition in the global scope, the standard of ship tail gas emission at home and abroad is further improved, in order to reach the emission standard, and simultaneously, the addition of a desulfurization and denitrification device with high price and large volume is avoided, so that the ship taking LNG as power can be rapidly developed. When LNG in an LNG power ship storage tank is stopped, and in the process of pre-cooling and filling the LNG storage tank, the LNG is stored at the low temperature of-162.5 ℃ under normal pressure, due to heat leakage of the LNG storage tank, equipment heating and the like, when the LNG storage tank of the LNG power ship is pre-cooled and filled, the LNG can absorb heat and evaporate to generate a large amount of BOG (boil-off gas), when the LNG storage tank is stopped, the LNG in the LNG storage tank evaporates due to heat leakage of the storage tank to generate BOG, the pressure in a storage device can rise along with the increase of the BOG amount, the storage equipment and the system are adversely affected, potential safety hazards are caused, direct emptying not only can cause economic loss, but also can cause adverse effects on the environment, and therefore, timely re-liquefaction and recovery processing of the BOG generated by the LNG storage tank of the ship is necessary.
At present, the main reliquefaction modes of BOG include a reliquefaction mode taking a cryogenic refrigerator as a cold source and a reliquefaction mode taking a cryogenic working medium (such as liquid nitrogen) as the cold source, BOG reliquefaction recovery of LNG power ships is realized, if the cryogenic refrigerator is used, the defect of high power consumption is existed, if BOG reliquefaction recovery taking liquid nitrogen as the cold source, a liquid nitrogen storage tank is additionally added, and regular supplement is needed, the recovery equipment is complex, and the benefit is low. In order to fully utilize the continuously supplied supercooled LNG in the filling process, the BOG reliquefaction method using the supercooled LNG as the cold source has the advantages of less equipment, simple flow and high yield, so that a set of BOG reliquefaction system using the supercooled LNG as the cold source is designed for the BOG generated by the LNG power ship LNG storage tank.
Disclosure of Invention
The invention aims to provide a BOG reliquefaction system and a BOG reliquefaction method of an LNG power ship, which utilize a contact type heat exchanger and supercooled LNG as a cold source to directly contact and liquefy BOG aiming at reliquefaction of BOG generated during precooling and filling processes and during stopping of an LNG power ship LNG storage tank.
The invention can overcome the problem that the prior BOG reliquefaction system is complex and unstable, reduces the heat exchange loss during the reliquefaction of the BOG, and simultaneously has good load regulation capacity.
In order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows:
a BOG recovery system of an LNG power ship comprises an LNG storage tank, a heat exchanger and an LNG filling device, wherein a filter screen, a spray pipe, a filler and an air inlet pipe are sequentially arranged in the heat exchanger in an isolated mode from top to bottom; the third path is sequentially connected with a seventh valve, the LNG pump and a spray pipe inlet in the heat exchanger;
export is passed through pipe connection to LNG liquid level control liquid storage tank entry on the LNG storage tank, and export is passed through the fifth valve and is connected to jointly through fourth valve and LNG storage tank on the LNG liquid level control liquid storage tank export BOG buffer tank entry, BOG buffer tank exit linkage to BOG compressor entry, BOG compressor export is connected to the intake pipe entry in the heat exchanger through the sixth valve, export connects gradually eighth valve, third governing valve extremely under the heat exchanger the entry under the LNG storage tank.
Further preferably, an outlet of the LNG filling device is provided with a first valve respectively communicated with the second valve, the third valve and the seventh valve.
Further preferably, an outlet at the top of the heat exchanger is provided with a noncondensable gas emptying regulating valve communicated with the atmosphere, and a safety valve is arranged at the outlet.
Further preferably, the BOG compressor is a variable frequency screw compressor or a piston compressor with an adjustable volume.
Preferably, the third regulating valve is a thermal liquid level regulating valve which can simultaneously regulate the liquid level and start the throttling action.
Further preferably, the heat exchanger is a contact heat exchanger with red copper or aluminum alloy filler inside.
Further preferably, the LNG pump is an LNG variable-frequency cryogenic pump.
The working method of the BOG recovery system of the LNG power ship is characterized by comprising the following specific contents and steps:
when the LNG storage tank needs to be precooled, the second valve and the first regulating valve are opened, the third valve, the second regulating valve and the seventh regulating valve are closed, the supercooled LNG flows out of the LNG filling device and is sent to an inlet of a spraying pipe of the LNG storage tank in a pressurized mode, the LNG storage tank is precooled by the spraying pipe in the LNG storage tank, and the first regulating valve is used for controlling the flow of the supercooled LNG in the precooling process;
after the pre-cooling of the LNG storage tank is finished, opening a third valve and a second regulating valve at the moment, closing the second valve and a first regulating valve, filling the LNG storage tank with the supercooled LNG from an inlet at the left side of the bottom of the LNG storage tank through the third valve and the second regulating valve, opening the second regulating valve for controlling the flow of the filled supercooled LNG, generating a large amount of BOG due to heat leakage of the storage tank and flash evaporation of the LNG, gradually increasing the pressure in the LNG storage tank along with the increase of the BOG amount, opening a fifth valve, a sixth valve, a seventh valve and an eighth valve when the pressure of the storage tank reaches 0.2Mpa, starting a BOG compressor and an LNG pump, enabling BOG gas to enter a BOG buffer tank through the fifth valve, then being sucked by the BOG compressor and compressed to 0.5-0.6 Mpa, sending the BOG gas into a heat exchanger from an inlet of the heat exchanger through the sixth valve, enabling the supercooled gas inlet pipe to pass through the seventh valve, pumping the LNG pump into an inlet of the heat exchanger, and achieving direct contact with the BOG through a spray pipe in the heat exchanger to reliquefaction of the BOG and then liquefying the BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger together, at the moment, the third regulating valve is opened to automatically regulate the liquid level in the heat exchanger, then the liquefied BOG stored at the bottom of the heat exchanger and the LNG which releases cold energy are filled into the LNG storage tank from the right inlet at the lower part of the LNG storage tank through the eighth valve and the third regulating valve, at the moment, the LNG filling device and the heat exchanger are filled into the LNG storage tank at the same time, when the liquid level height in the LNG storage tank reaches a set value, if the LNG storage tank is continuously filled, the LNG which is higher than the liquid level flows into the LNG liquid level control storage tank to ensure that the liquid level of the LNG storage tank is in a set range, and if the pressure in the LNG liquid level control storage tank exceeds the set value, the fourth valve is opened to discharge the redundant BOG and carry out reliquefaction treatment together with the BOG in the LNG storage tank;
when the LNG storage tank of the ship is stopped, the LNG storage tank can also generate BOG due to heat leakage, equipment heating and other reasons, when the pressure in the LNG storage tank reaches 0.2Mpa, the third valve, the fifth valve, the sixth valve, the seventh valve, the eighth valve and the second regulating valve are opened, the BOG compressor and the LNG pump are started, the BOG in the LNG storage tank enters the BOG buffer tank through the fifth valve, is compressed to 0.5-0.6 Mpa by the BOG compressor, and then is sent into the heat exchanger through the sixth valve and the air inlet pipe inlet, at the moment, subcooled LNG flows out from the left outlet at the bottom of the LNG storage tank, enters the LNG pump through the second regulating valve, the third valve and the seventh valve, is pumped into the heat exchanger through the spray pipe inlet after being pressurized by the LNG pump, is directly contacted with the BOG for reliquefaction, the liquefied BOG and the LNG after releasing the cold energy are stored at the bottom of the heat exchanger, at the moment, the third regulating valve is opened for automatically regulating the heat exchanger, and then the BOG and the LNG stored at the bottom of the heat exchanger after being liquefied at the bottom of the heat exchanger and the LNG releasing the cold energy is released through the eighth valve and the third regulating valve And after liquefaction is finished, closing the third valve, the fifth valve, the sixth valve, the seventh valve, the eighth valve and the second regulating valve, closing the LNG pump and the BOG compressor, and closing the third regulating valve.
Further preferably, the load of the BOG compressor is adjusted in direct proportion to the pressure in the LNG storage tank; the frequency of the LNG pump is in a direct proportional adjustment relationship with the temperature of the LNG stored at the bottom of the heat exchanger; the opening degree of the third regulating valve is in direct proportion regulating relation with the liquid level in the heat exchanger; the opening degree of the non-condensable gas emptying regulating valve is in direct proportion regulation relation with the pressure in the heat exchanger.
According to the invention, sufficient supercooled LNG and a contact type heat exchanger in the filling process are utilized, so that the supercooled LNG and BOG are directly contacted to reliquefy the BOG. The method comprises the following steps that the LNG storage tank needs to be pre-cooled before filling, a spray pipe is arranged in the LNG storage tank, and a first regulating valve is connected with the spray pipe and used for controlling the cooling rate of the LNG storage tank; an upper outlet of the LNG storage tank is connected with an LNG liquid level control liquid storage tank, the liquid level control requirement of the LNG storage tank in the filling process is guaranteed, the filling amount is controlled to be 90% -95% of the volume of the LNG storage tank, the upper outlet of the LNG storage tank is connected with a BOG buffer tank, and the generated BOG firstly enters the BOG buffer tank to play roles of buffering and pressure stabilization; in the BOG reliquefaction process, the BOG compressor and the LNG pump in the system can flexibly adjust the rotating speed according to the load, the cold quantity of liquefied BOG is matched with the cold quantity of supercooled LNG to be released, and meanwhile, a noncondensable gas emptying adjusting valve and a safety valve are arranged above the heat exchanger and used for ensuring the safe use of the heat exchanger.
Compared with the prior art, the technical scheme of the invention has the following technical effects:
the BOG generated by the ship LNG storage tank is subjected to reliquefaction treatment by using the supercooled LNG, and compared with a traditional BOG reliquefaction system which takes liquid nitrogen as a cold source and a Stirling refrigerator as the cold source, the BOG reliquefaction system does not need to add a specific low-temperature substance storage tank and has no problem of high energy consumption, so that the equipment investment cost and the operation cost are reduced, and the system is simpler and more stable; when BOG is liquefied again, the supercooled LNG is directly contacted with the BOG by using the contact type heat exchanger, so that the heat exchange loss is reduced, the system load is reduced, and meanwhile, the system has excellent load regulation capacity; no matter the system is at precooling, filling or in-process of stopping, can both make the BOG that produces in time liquefy, and the at utmost reduces the energy waste, and heat exchanger upper portion is equipped with noncondensable gas air-release governing valve and relief valve simultaneously for system and equipment have good safety characteristic.
Drawings
FIG. 1 is a schematic diagram of the system configuration of the present invention:
the reference numbers in the figures illustrate: 1 is the LNG storage tank, 2 is the heat exchanger, 3 is the BOG compressor, 4 is the BOG buffer tank, 5 is the LNG pump, 6 is LNG liquid level control liquid storage pot, 7 is the LNG filling device, 8 is noncondensable gas unloading governing valve, 9 is the relief valve, 10 is first valve, 11 is the second valve, 12 is first governing valve, 13 is the third valve, 14 is the second governing valve, 15 is the fourth valve, 16 is the fifth valve, 17 is the sixth valve, 18 is the seventh valve, 19 is the eighth valve, 20 is the third governing valve.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
As shown in fig. 1: the BOG recovery system of the LNG power ship comprises an LNG storage tank 1, a heat exchanger 2 and an LNG filling device 7, wherein a filter screen 2a, a spray pipe 2b, a filler 2c and an air inlet pipe 2d are sequentially arranged in the heat exchanger 2 in an isolated manner from top to bottom, and the spray pipe 2b is beneficial to full heat exchange between supercooled LNG and BOG, so that the heat exchange efficiency is improved;
one path of the LNG filling device 7 is sequentially connected with the second valve 11, the first regulating valve 12 and the inlet c of the spray pipe 2b of the LNG storage tank 1 through pipelines, and the other path of the LNG filling device is sequentially connected with the third valve 13, the second regulating valve 14 and the lower inlet d of the LNG storage tank 1; the third path is sequentially connected with a seventh valve 18, the LNG pump 5 and an inlet f of a spray pipe 2b in the heat exchanger 2, wherein the first regulating valve 12 controls the flow of the supercooled LNG in the precooling process so as to control the precooling rate, and the second regulating valve 14 controls the flow of the supercooled LNG in the filling process so as to control the filling time;
an upper outlet b of the LNG storage tank 1 is connected to an inlet of the LNG liquid level control liquid storage tank 6 through a pipeline, an outlet of the LNG liquid level control liquid storage tank 6 is connected to an inlet of the BOG buffer tank 4 together with an upper outlet a of the LNG storage tank 1 through a fourth valve 15 and a fifth valve 16, wherein the LNG liquid level control liquid storage tank 6 can control the liquid level of the LNG storage tank 1 to be stable, and potential safety hazards caused by the fact that the pressure exceeds a set value due to excessive liquid filling of the storage tank are avoided; an outlet of the BOG buffer tank 4 is connected to an inlet of a BOG compressor 3, and an outlet of the BOG compressor 3 is connected to an inlet g of an air inlet pipe 2d in the heat exchanger 2 through a sixth valve 17;
the lower outlet h of the heat exchanger 2 is sequentially connected with an eighth valve 19 and a third regulating valve 20 through pipelines to the lower inlet j of the LNG storage tank 1, the flow entering the LNG storage tank 1 can be regulated through the third regulating valve 20, and meanwhile, the liquid level in the heat exchanger 2 is guaranteed to be within a set range.
The outlet of the LNG filling device 7 is provided with a first valve 10 which is respectively communicated with a second valve 11, a third valve 13 and a seventh valve 18, the outlet e at the top of the heat exchanger 2 is provided with a noncondensable gas emptying regulating valve 8 communicated with the atmosphere so as to timely regulate the pressure in the heat exchanger 2 to stabilize the condensation temperature of BOG, and simultaneously ensure that the LNG pump 5 can smoothly pump the supercooled LNG into the heat exchanger 2, and the outlet i is provided with a safety valve 9 which plays a role of pressure relief and protection equipment;
the BOG compressor 3 is a variable-frequency screw compressor or a piston compressor with adjustable volume.
The third regulating valve 20 is a thermal liquid level regulating valve which can simultaneously regulate the liquid level and start the throttling action.
The heat exchanger 2 is a contact heat exchanger with red copper or aluminum alloy filler 2c inside.
The LNG pump 5 is an LNG variable-frequency cryogenic pump.
A working method of a BOG recovery system of an LNG power ship comprises the following specific contents and steps:
when the LNG storage tank 1 needs to be precooled, the second valve 11 and the first regulating valve 12 are opened, the third valve 13, the second regulating valve 14 and the seventh valve 18 are closed, the supercooled LNG flows out of the LNG filling device 7 and is sent to an inlet c of a spray pipe 2b of the LNG storage tank 1 in a pressurizing mode, the LNG storage tank 1 is precooled by the spray pipe 2b in the LNG storage tank 1, the phenomenon that the steel plate has large temperature difference stress and bending in the filling process to cause cracks and leakage is prevented, and the flow of the supercooled LNG is regulated by the first regulating valve 12 according to the cooling rate of the LNG storage tank 1 in the precooling process;
after the pre-cooling of the LNG storage tank 1 is completed, the third valve 13 and the second regulating valve 14 are opened at the moment, the second valve 11 and the first regulating valve 12 are closed, the subcooled LNG is filled into the LNG storage tank 1 from the inlet d at the left side of the bottom of the LNG storage tank 1 through the third valve 13 and the second regulating valve 14, in order to control the filling time and avoid safety accidents caused by the tumbling of the LNG in the LNG storage tank 1 due to uneven filling rate, the second regulating valve 14 is used for controlling the flow rate of the filled subcooled LNG, a large amount of BOG is generated due to heat leakage of the storage tank and flash evaporation of the LNG, the pressure in the LNG storage tank 1 gradually rises along with the increase of the BOG amount, when the pressure of the storage tank reaches 0.2Mpa, the fifth valve 16, the sixth valve 17, the seventh valve 18 and the eighth valve 19 are opened, the BOG compressor 3 and the LNG pump 5 are started, BOG gas enters the BOG buffer tank 4 through the fifth valve 16 and then is sucked by the BOG compressor 3 and compressed to 0.5-0.6 Mpa, the subcooled LNG is sent into the heat exchanger 2 from an inlet g of an air inlet pipe 2d of the heat exchanger 2 through a sixth valve 17, the subcooled LNG passes through a seventh valve 18, is pumped into an inlet f of a spray pipe 2b of the heat exchanger 2 through an LNG pump 5, is sprayed through the spray pipe 2b in the heat exchanger 2 to be directly contacted with the BOG for reliquefying the BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger 2 together, at the moment, a third regulating valve 20 is opened to automatically regulate the liquid level in the heat exchanger 2, then the liquefied BOG stored at the bottom of the heat exchanger 2 and the LNG which releases cold energy are filled into the LNG storage tank 1 from an inlet j at the right side of the lower part of the LNG storage tank 1 through an eighth valve 19 and the third regulating valve 20, at the moment, the LNG filling device 7 and the heat exchanger 2 are filled into the LNG storage tank 1 at the same time, when the liquid level in the LNG storage tank 1 reaches a set value, if the LNG storage tank 1 is continuously filled, the LNG higher part flows into the LNG liquid level control liquid storage tank 6 to ensure that the liquid level of the LNG storage tank is within a set range, if the pressure in the LNG liquid level control liquid storage tank 6 exceeds a set value, the fourth valve 15 is opened, redundant BOG is discharged and reliquefied together with the BOG in the LNG storage tank 1, a noncondensable gas emptying adjusting valve 8 is arranged at an outlet e at the left side of the top of the heat exchanger 2, the functions of emptying noncondensable gas and adjusting the pressure in the heat exchanger 2 are achieved, a safety valve 9 is arranged at an outlet i at the right side of the top of the heat exchanger 2, the purpose is to discharge excessive gas in time when the pressure in the heat exchanger 2 exceeds a critical safety pressure, the pressure relief effect is achieved, and the safety of a system and equipment is guaranteed;
when the LNG storage tank 1 of the ship is stopped, the LNG storage tank 1 can also generate BOG due to heat leakage, equipment heating and the like, when the pressure in the LNG storage tank 1 reaches 0.2Mpa, a third valve 13, a fifth valve 16, a sixth valve 17, a seventh valve 18, an eighth valve 19 and a second regulating valve 14 are opened, a BOG compressor 3 and an LNG pump 5 are started, the BOG in the LNG storage tank 1 enters a BOG buffer tank 4 through the fifth valve 16, is compressed to 0.5-0.6 Mpa by the BOG compressor 3, then is sent into the heat exchanger 2 through an air inlet pipe 2d inlet g through the sixth valve 17, at the moment, subcooled LNG flows out from an outlet d at the left side of the bottom of the LNG storage tank 1, enters the LNG pump 5 through the second regulating valve 14, the third valve 13 and the seventh valve 18, is pressurized by the LNG pump 5, is pumped into the heat exchanger 2 through an inlet f of a spray pipe 2b, is in direct contact with the BOG to reliquefy the BOG, and the liquefied BOG and the LNG after the released cold energy is stored at the bottom of the heat exchanger 2, at this time, the third regulating valve 20 is opened to automatically regulate the liquid level in the heat exchanger 2, then the liquefied BOG stored at the bottom of the heat exchanger 2 and the LNG with the released cold are sent back to the LNG storage tank 1 again from the right inlet j at the lower part of the LNG storage tank 1 through the eighth valve 19 and the third regulating valve 20, and after the liquefaction is completed, the third valve 13, the fifth valve 16, the sixth valve 17, the seventh valve 18, the eighth valve 19 and the second regulating valve 14 are closed, the LNG pump 5 and the BOG compressor 3 are closed, and the third regulating valve 20 is also closed.
Wherein, the load of the BOG compressor 3 is in a direct proportional regulation relation with the pressure in the LNG storage tank 1, when the BOG pressure in the LNG storage tank 1 is higher than 0.2Mpa and continuously rises, the load of the BOG compressor 3 is increased therewith, and when the BOG pressure in the LNG storage tank 1 begins to fall, the load of the BOG compressor 3 is reduced therewith;
the frequency of the LNG pump 5 is in a proportional regulation relation with the temperature of the LNG stored at the bottom of the heat exchanger 2, when the temperature of the liquefied BOG stored at the bottom of the heat exchanger 2 and the temperature of the LNG after the cold energy release are higher than a set value, the frequency of the LNG pump 5 is increased, and when the temperature of the liquefied BOG and the LNG after the cold energy release is lower than the set value, the frequency of the LNG pump 5 is decreased;
the opening degree of the third regulating valve 20 is in a direct proportional regulating relation with the liquid level in the heat exchanger 2, when the liquid level in the heat exchanger 2 rises, the opening degree of the third regulating valve 20 is increased, and when the liquid level in the heat exchanger 2 falls, the opening degree of the third regulating valve 20 is reduced;
the opening of the non-condensable gas emptying regulating valve 8 is in a proportional regulating relation with the pressure in the heat exchanger 2, when the pressure in the heat exchanger 2 exceeds a set value and continuously rises, the opening of the non-condensable gas emptying regulating valve 8 is increased, and when the pressure in the heat exchanger 2 begins to decrease, the opening of the non-condensable gas emptying regulating valve 8 is reduced.
Although the present invention is disclosed above, the present invention is not limited thereto. Various changes and modifications may be effected by one skilled in the art without departing from the spirit and scope of the invention, and it is intended that the scope of the invention be limited only by the claims appended hereto.

Claims (6)

1. The BOG recovery system of the LNG power ship is characterized by comprising an LNG storage tank (1), a heat exchanger (2) and an LNG filling device (7), wherein a filter screen (2a), a spray pipe (2b), a filler (2c) and an air inlet pipe (2d) are sequentially arranged in the heat exchanger (2) from top to bottom in an isolated mode, one path of the LNG filling device (7) is sequentially connected with a second valve (11), a first regulating valve (12) and an inlet (c) of the spray pipe (2b) of the LNG storage tank (1) through a pipeline, and the other path of the LNG filling device is sequentially connected with a third valve (13), a second regulating valve (14) and an inlet (d) at the left side of the bottom of the LNG storage tank (1); the third path is sequentially connected with a seventh valve (18), an LNG pump (5) and an inlet (f) of a spray pipe (2b) in the heat exchanger (2);
an upper outlet (b) of the LNG storage tank (1) is connected to an inlet of the LNG liquid level control liquid storage tank (6) through a pipeline, an outlet of the LNG liquid level control liquid storage tank (6) is connected to an inlet of a BOG buffer tank (4) through a fourth valve (15) and another upper outlet (a) of the LNG storage tank (1) through a fifth valve (16), an outlet of the BOG buffer tank (4) is connected to an inlet of a BOG compressor (3), an outlet of the BOG compressor (3) is connected to an inlet (g) of an air inlet pipe (2d) in a heat exchanger (2) through a sixth valve (17), a lower outlet (h) of the heat exchanger (2) is sequentially connected with an eighth valve (19) and a third regulating valve (20) to a right inlet (j) at the lower part of the LNG storage tank (1) through pipelines, wherein the third regulating valve (20) is a thermal liquid level regulating valve capable of simultaneously regulating liquid level and starting throttling;
the outlet of the LNG filling device (7) is provided with a first valve (10) which is respectively communicated with a second valve (11), a third valve (13) and a seventh valve (18);
an outlet (e) at the top of the heat exchanger (2) is provided with a noncondensable gas emptying regulating valve (8) communicated with the atmosphere, and the other outlet (i) at the top is provided with a safety valve (9);
the working method of the BOG recovery system of the LNG power ship comprises the following steps:
when the LNG storage tank (1) needs to be precooled, the second valve (11) and the first regulating valve (12) are opened, the third valve (13), the second regulating valve (14) and the seventh valve (18) are closed, the supercooled LNG flows out from the LNG filling device (7), the supercooled LNG is pressurized and sent to an inlet (c) of a spray pipe (2b) of the LNG storage tank (1), the LNG storage tank (1) is precooled by using the spray pipe (2b) in the LNG storage tank (1), and the first regulating valve (12) is used for controlling the flow of the supercooled LNG in the precooling process;
after the pre-cooling of the LNG storage tank (1) is completed, opening a third valve (13) and a second regulating valve (14) at the moment, closing a second valve (11) and a first regulating valve (12), filling the LNG storage tank (1) from an inlet (d) at the left side of the bottom of the LNG storage tank (1) by the super-cooled LNG through the third valve (13) and the second regulating valve (14), controlling the flow of the filled super-cooled LNG by the second regulating valve (14), generating a large amount of BOG due to heat leakage of the storage tank and flash evaporation of the LNG, gradually increasing the pressure in the LNG storage tank (1) along with the increase of the BOG amount, opening a fifth valve (16), a sixth valve (17), a seventh valve (18) and an eighth valve (19) when the pressure of the storage tank reaches 0.2MPa, starting a BOG compressor (3) and an LNG pump (5), allowing BOG gas to enter a BOG buffer tank (4) through the fifth valve (16), and then being sucked by the BOG compressor (3) and compressed to 0.5-0.6 MPa, the subcooled LNG passes through a seventh valve (18), is pumped into an inlet (f) of a spray pipe (2b) of the heat exchanger (2) from an air inlet pipe (2d) of the heat exchanger (2) through an LNG pump (5), is sprayed through the spray pipe (2b) in the heat exchanger (2) to be directly contacted with the BOG for reliquefying the BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger (2), at the moment, a third regulating valve (20) is opened to automatically regulate the liquid level in the heat exchanger (2), then the liquefied BOG stored at the bottom of the heat exchanger (2) and the LNG which releases cold energy are filled into the LNG storage tank (1) from an inlet (j) at the right side of the lower part of the LNG storage tank (1) through an eighth valve (19) and the third regulating valve (20), at the moment, the LNG filling device (7) and the heat exchanger (2) simultaneously fill the LNG storage tank (1), when the height of the liquid level in the LNG storage tank (1) reaches a set value, if the LNG storage tank (1) is continuously filled, LNG higher than the liquid level flows into the LNG liquid level control liquid storage tank (6) to ensure that the liquid level of the LNG storage tank (1) is within a set range, and if the pressure in the LNG liquid level control liquid storage tank (6) exceeds the set value, the fourth valve (15) is opened to discharge redundant BOG and carry out reliquefaction treatment together with the BOG in the LNG storage tank (1);
when the LNG storage tank (1) of the ship is stopped, the LNG storage tank (1) can also generate BOG due to heat leakage and equipment heating, when the pressure in the LNG storage tank (1) reaches 0.2Mpa, a third valve (13), a fifth valve (16), a sixth valve (17), a seventh valve (18), an eighth valve (19) and a second regulating valve (14) are opened, a BOG compressor (3) and an LNG pump (5) are started, the BOG in the LNG storage tank (1) enters a BOG buffer tank (4) through the fifth valve (16), then is compressed to 0.5-0.6 Mpa by the BOG compressor (3), then is sent into a heat exchanger (2) through an air inlet pipe (2d) and an inlet (g) through the sixth valve (17), at the moment, the LNG is subcooled and flows out from a left inlet (d) at the bottom of the LNG storage tank (1), and enters the LNG pump (5) through a second regulating valve (14), the third valve (13) and the seventh valve (18), and then is pumped into the LNG pump (5) through a spray pipe (2) and a heat exchanger (f), BOG is reliquefied by directly contacting with BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger (2), at the moment, the third regulating valve (20) is opened to automatically regulate the liquid level in the heat exchanger (2), then the liquefied BOG stored at the bottom of the heat exchanger (2) and the LNG which releases cold energy are sent back to the LNG storage tank (1) again from an inlet (j) at the right side of the lower part of the LNG storage tank (1) through the eighth valve (19) and the third regulating valve (20), after liquefaction is completed, the third valve (13), the fifth valve (16), the sixth valve (17), the seventh valve (18), the eighth valve (19) and the second regulating valve (14) are closed, the LNG pump (5) and the BOG compressor (3) are closed, and the third regulating valve (20) is also closed.
2. BOG recovery system for an LNG powered vessel according to claim 1, characterised in that the BOG compressor (3) is a variable frequency screw compressor or a piston compressor with adjustable volume.
3. BOG recovery system for LNG powered ships according to claim 1, characterized in that the heat exchanger (2) is a contact heat exchanger with built-in red copper or aluminum alloy packing.
4. BOG recovery system for an LNG powered vessel according to claim 1, characterized in that the LNG pump (5) is an LNG variable frequency cryogenic pump.
5. A method of operating a BOG recovery system of an LNG powered vessel according to any of claims 1 to 4, characterized in that the specific content and steps are as follows:
when the LNG storage tank (1) needs to be precooled, the second valve (11) and the first regulating valve (12) are opened, the third valve (13), the second regulating valve (14) and the seventh valve (18) are closed, the supercooled LNG flows out from the LNG filling device (7), the supercooled LNG is pressurized and sent to an inlet (c) of a spray pipe (2b) of the LNG storage tank (1), the LNG storage tank (1) is precooled by using the spray pipe (2b) in the LNG storage tank (1), and the first regulating valve (12) is used for controlling the flow of the supercooled LNG in the precooling process;
after the pre-cooling of the LNG storage tank (1) is completed, opening a third valve (13) and a second regulating valve (14) at the moment, closing a second valve (11) and a first regulating valve (12), filling the LNG storage tank (1) from an inlet (d) at the left side of the bottom of the LNG storage tank (1) by the super-cooled LNG through the third valve (13) and the second regulating valve (14), controlling the flow of the filled super-cooled LNG by the second regulating valve (14), generating a large amount of BOG due to heat leakage of the storage tank and flash evaporation of the LNG, gradually increasing the pressure in the LNG storage tank (1) along with the increase of the BOG amount, opening a fifth valve (16), a sixth valve (17), a seventh valve (18) and an eighth valve (19) when the pressure of the storage tank reaches 0.2MPa, starting a BOG compressor (3) and an LNG pump (5), allowing BOG gas to enter a BOG buffer tank (4) through the fifth valve (16), and then being sucked by the BOG compressor (3) and compressed to 0.5-0.6 MPa, the subcooled LNG passes through a seventh valve (18), is pumped into an inlet (f) of a spray pipe (2b) of the heat exchanger (2) from an air inlet pipe (2d) of the heat exchanger (2) through an LNG pump (5), is sprayed through the spray pipe (2b) in the heat exchanger (2) to be directly contacted with the BOG for reliquefying the BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger (2), at the moment, a third regulating valve (20) is opened to automatically regulate the liquid level in the heat exchanger (2), then the liquefied BOG stored at the bottom of the heat exchanger (2) and the LNG which releases cold energy are filled into the LNG storage tank (1) from an inlet (j) at the right side of the lower part of the LNG storage tank (1) through an eighth valve (19) and the third regulating valve (20), at the moment, the LNG filling device (7) and the heat exchanger (2) simultaneously fill the LNG storage tank (1), when the height of the liquid level in the LNG storage tank (1) reaches a set value, if the LNG storage tank (1) is continuously filled, LNG higher than the liquid level flows into the LNG liquid level control liquid storage tank (6) to ensure that the liquid level of the LNG storage tank (1) is within a set range, and if the pressure in the LNG liquid level control liquid storage tank (6) exceeds the set value, the fourth valve (15) is opened to discharge redundant BOG and carry out reliquefaction treatment together with the BOG in the LNG storage tank (1);
when the LNG storage tank (1) of the ship is stopped, the LNG storage tank (1) can also generate BOG due to heat leakage and equipment heating, when the pressure in the LNG storage tank (1) reaches 0.2Mpa, a third valve (13), a fifth valve (16), a sixth valve (17), a seventh valve (18), an eighth valve (19) and a second regulating valve (14) are opened, a BOG compressor (3) and an LNG pump (5) are started, the BOG in the LNG storage tank (1) enters a BOG buffer tank (4) through the fifth valve (16), then is compressed to 0.5-0.6 Mpa by the BOG compressor (3), then is sent into a heat exchanger (2) through an air inlet pipe (2d) and an inlet (g) through the sixth valve (17), at the moment, the LNG is subcooled and flows out from a left inlet (d) at the bottom of the LNG storage tank (1), and enters the LNG pump (5) through a second regulating valve (14), the third valve (13) and the seventh valve (18), and then is pumped into the LNG pump (5) through a spray pipe (2) and a heat exchanger (f), BOG is reliquefied by directly contacting with BOG, the liquefied BOG and the LNG which releases cold energy are stored at the bottom of the heat exchanger (2), at the moment, the third regulating valve (20) is opened to automatically regulate the liquid level in the heat exchanger (2), then the liquefied BOG stored at the bottom of the heat exchanger (2) and the LNG which releases cold energy are sent back to the LNG storage tank (1) again from an inlet (j) at the right side of the lower part of the LNG storage tank (1) through the eighth valve (19) and the third regulating valve (20), after liquefaction is completed, the third valve (13), the fifth valve (16), the sixth valve (17), the seventh valve (18), the eighth valve (19) and the second regulating valve (14) are closed, the LNG pump (5) and the BOG compressor (3) are closed, and the third regulating valve (20) is also closed.
6. Working method according to claim 5, characterized in that the load of the BOG compressor (3) is adjusted in direct proportion to the pressure in the LNG storage tank (1); the frequency of the LNG pump (5) is in direct proportion regulation relation with the temperature of the LNG stored at the bottom of the heat exchanger (2); the opening degree of the third regulating valve (20) is in direct proportion regulating relation with the liquid level in the heat exchanger (2); the opening degree of the non-condensable gas air-release regulating valve (8) is in direct proportion regulation relation with the pressure in the heat exchanger (2).
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