NO20111201A1 - System and method for increasing the pressure in boiling gas in an LNG fuel system - Google Patents

Abstract

A gas supply system for combination or gas engines, comprising at least one LNG cargo tank (1), a first pump (2), a compressor (30), a cryogenic heat exchanger (16) and a high pressure pump (10) supplying gas to the engines. According to the present invention, the system further comprises an ejector (4) and a suction tank (6); the ejector (4) is arranged to receive LNG propellant fluid from the first pump, draw condensate from the cryogenic heat exchanger (16) and discharge a mixture of the propellant fluid and condensate to the suction tank (6); and the suction tank (6) is arranged to deliver the mixture of the LNG propellant fluid and the condensate to the high pressure pump (10).

Description

System and method for increasing the pressure in decoction gas in an LNG d r i vstoff sy ste m

The present invention relates to a system and a method for increasing the pressure in boil-off gas ("boil-off gas"; BOG) in an LNG (condensed natural gas) fuel system, more specifically to increase the pressure in the boil-off gas with an ejector.

Several methods have been proposed for how to operate an LNG ship with gas instead of marine diesel or similar. Proposed methods distinguish between mainly two types of main engine:

- medium-speed engines, which require a relatively low propellant gas pressure of approx.

500 kPa.

- low-speed diesel engines, which require between 20,000 and 30,000 kPa propellant gas pressure.

One such method is described in EP 1 990 272 Bl, where LNG is pumped from the cargo tanks, heat exchanged with compressed decoction gas, and then further compressed to injection pressure. The compressed decoction gas is condensed against the LNG and sent back to the cargo tanks. When the main engine is running on marine diesel or at low speed, especially on cargo voyages, there is little or no LNG available to remove heat flowing into the I a stop p preservation system.

NO20082158 and NO20093562 describe systems and methods for gas supply to gas or combination engines, where the gas supply system is integrated with a recondensation system to remove heat that flows into the I a stop p preservation system.

GB1440318A describes a system for a system that uses an ejector to control vapor pressure in LNG cargo tanks by pumping LNG, vaporizing it and directing the resulting gas as the driving fluid to an ejector that sucks from the vapor space, thereby eliminating the need for compressor operations to maintain the vapor pressure. The system in GB1440318A is intended for combination boilers used for main propulsion machinery and does not discuss the technical problems encountered if the main propulsion machinery is low speed diesel engines. Combi boilers typically require a propellant gas pressure of less than 200 kPa, while a low-speed diesel engine requires propellant gas pressure of between 20,000 and 30,000 kPa.

Since low-speed diesel engines require high propellant gas pressure, a high-pressure pump is usually provided in the system to provide the necessary gas pressure. In order to avoid pump cavitation, it is important to ensure that the high-pressure pump has the required NPSH (Net Positive Suction Height) during all operating modes, and especially during ballast travel, where it is common to have a free-flowing circulation of the decoction gas to avoid recirculation through the decoction gas compressor so as not to enter the compressor pumping area. Boil-off rates are significantly lower on ballast voyages than cargo voyages, so it may be most practical to leave the compressor off and direct the boil-off gas directly to the recondenser using free flow (natural circulation) principles. Especially in harsh sea conditions in combination with ballast travel, experience shows that pump cavitation is a challenge on known installations as a result of insufficient NPSH.

This invention describes a system and a method for ensuring that the high pressure pump has required NPSH (Net Positive Suction Height) under all operating modes, and in particular during ballast travel, where it is common to have a free-flowing circulation of the decoction gas to avoid recirculation through the compressor. Especially in harsh sea conditions in combination with ballast travel, experience shows that pump cavitation is a challenge on similar installations as a result of insufficient NPSH.

It is therefore an object of the present invention to provide a system and a method to ensure that the high-pressure pump has required NPSH ("Net Positive Suction Height") under all operating modes.

To solve the above-mentioned problems, the present invention describes a gas supply system for combination or gas engines, comprising at least one LNG cargo tank 1, a first pump 2, a compressor 30, a cryogenic heat exchanger 16 and a high-pressure pump 10 which supplies gas to the engines, where the system further comprises an ejector 4 and a suction tank 6; the ejector 4 is arranged to receive LNG drive fluid from the first pump, draw condensate from the cryogenic heat exchanger 16 and discharge a mixture of the drive fluid and condensate to the suction tank 6; and the suction tank 6 is arranged to supply the mixture of the LNG drive fluid and the condensate to the high pressure pump 10.

The present invention also describes a method for supplying gas to combination or gas engines in a system comprising at least one LNG cargo tank 1, a first pump 2, a compressor 30, a cryogenic heat exchanger 16 and a high-pressure pump 10 which supplies gas to the engines, wherein the method comprises providing a suction tank (6) upstream of the high pressure pump (10); providing an ejector (4) between the first pump (2) and the suction tank (6); pump, by means of the first pump (2), LNG drive fluid to the ejector (4); drawing, by means of LNG drive fluid received by the ejector (4), condensate from the cryogenic heat exchanger (16); discharging, with the ejector (4), a mixture of the driving fluid and the condensate to the suction tank (6); and deliver from the suction tank (6) the mixture of the LNG drive fluid and the condensate to the high-pressure pump (10).

Other advantageous embodiments of the present invention are to be understood from the independent patent claims and the detailed description which follows, with reference to the modified figures; where: Fig. 1 is a schematic overview of a prior art recondensation unit; Fig. 2 is a schematic overview of one embodiment of a system according to the present invention; and Fig. 3 is a schematic overview of another embodiment of a system according to the present invention. Figure 1 illustrates a typical known cargo storage system in an LNG ship with at least one cargo tank 1. Cargo lines are not shown. As a result of natural heat leakage into the cargo storage system, a certain amount of the cargo evaporates, known as boil-off gas. The decoction gas flows via line 14 to a decoction gas compressor 30. The decoction gas compressor 30 can be of the single-stage or multi-stage type. Compressed decoction gas leaves the decoction gas compressor via line 15 and enters cooler 31. Selection

of refrigerant may depend on the compressor discharge temperature. Various sketches are known to the person skilled in the art and will not be further described here. Cooled and compressed decoction gas leaves the decoction gas compressor after cooler 31 via line 32 and enters a cryogenic heat exchanger 16. The cryogenic heat exchanger is typically a multi-flow exchanger where the flows are connected to a loop of circulating refrigerant. This is not further illustrated here, since such systems are known to the person skilled in the art.

One operating mode for the LNG ship is for the propulsion machinery and auxiliary engines (not shown) to run on marine diesel oil or equivalent. This mode illustrates the current state of the art and is illustrated in Figure 1. In this mode, the condensate leaves the cryogenic heat exchanger 16 via line 17 to separator 18. Valve 25 is completely open, and pump 8 in combination with valve 34 controls the liquid level in separator 18 at known control principles, and the liquid is pumped back to the at least one cargo tank 1 via line 33. If non-condensable gases enter separator 18, they are discharged via line 21 to a safe location.

Condensate is to be understood as condensed boil-off gas, where boil-off gas is steam that flows out from the cargo as a result of a constant heat leak into the cargo tanks.

Fig. 2 is a schematic overview of a system according to the present invention where the LNG loading system in fig. 1 is integrated with a supply system for LNG propellant gas, where the supply system for LNG propellant gas is provided with a suction tank 6, an ejector 4, a high-pressure pump 10 and further valves 12, 19, 26, 39, 40. The pump 2, hereafter referred to as a drive fluid pump, pumps LNG via line 3 into the ejector 4, where LNG as drive fluid draws the condensate via valve 19 into the ejector 4, after which the ejector 4 releases a mixture of drive fluid (LNG) and condensate at a pressure above the condensing pressure in the heat exchanger 16 and sent via line 5 to the suction tank 6. Since the pressure in 5 is above the condensation pressure in the heat exchanger 16, the suction tank 6 can be placed higher up than the separator 18, so that an increased pressure height is provided between the liquid surface and the inlet flange of the high-pressure pump 10, whereby the ensure that a sufficient NPSH is maintained at all times for the high-pressure pump 10.

This device thus eliminates operation of pump 8, and the problem of pump cavitation is eliminated.

The high-pressure pump 10 then sucks from suction tank 6 via line 7 and lifts the liquid under pressure (the liquid is the mixture of LNG and condensate) and compresses it to typically 30,000 kPa. Liquid flowing from the high-pressure pump 10 then typically enters a series of heat exchangers to be heated and transferred to the propulsion machinery and/or the auxiliary engines. This principle is well documented in e.g. NO20093562 and is consequently omitted from this description.

In the exemplary operating mode of the LNG ship described with reference to FIG. 1, the propulsion machinery and auxiliary engines run on marine diesel oil or equivalent. When the same operating mode is applied to the system in Figure 2, the following applies: a three-way valve 26 is open to the separator 18. Liquid flows from the separator 18 via line 20, through valve 25, then lifted in pressure by pump 8, then via line 33 back to at least one cargo tank 1.

Again with reference to fig. 2, according to another exemplary embodiment of the present invention, the suction tank 6, since the rate of supply of condensate or LNG may change due to operational problems, typically has a long enough retention time to ensure stable calorific value of the combined mixture of LNG and condensed decoction gas. This means that the high-pressure pump 10 can feed the gas engine for a while without the drive fluid pump 2 being in operation.

The boiling rate during cargo travel depends on several factors, such as thermal insulation thickness, ambient temperatures, cargo composition and sea conditions. For situations where the boil-off rate is higher than the fuel requirement, excess condensate is sent back to loading tank 1. This mode is described in NO20093562. For modes where the fuel demand is higher than the boil-off rate, additional fuel is supplied from the cargo tank via drive fluid pump 2 through line 3 to ejector 4. The condensate flows either via line 20 or 27 to ejector 4. The amount of drive fluid supplied via line 3 is balanced to ensure that the resulting the fluid released from the ejector 4 has sufficient pressure to enter suction tank 6. High-pressure pump 10 sucks from this tank via line 7, while any excess liquid that enters 6 flows via line 33 back to loading tank 1.

During ballast travel, the quantity of decoction gas is significantly smaller than during cargo travel, and in order not to waste energy through recycling part of the decoction gas through the decoction gas compressor 30, provision has been made for the free flow of decoction gas by opening the recirculation valve 37 completely, so that the decoction gas can flow freely from the at least one cargo tank 1 to the cryogenic heat exchanger 16.

Free flow is not easy to initiate and its rate of growth is determined by the rate of condensation at any given time in the cryogenic heat exchanger 16. Common experience suggests that free flow may take several hours to reach the required flow rate.

One possible method of initiating the free flow would be to open valve 22 and allow the steam from the tank to flow through the system and then to an oxidizer or to an air tower. This is not a good solution, since both the oxidizer and the air mast involve the release of greenhouse gases into the atmosphere.

According to an exemplary embodiment of the present invention, the free flow is started by pumping LNG with drive fluid pump 2 and sending it via line 3 to the ejector 4 as the drive fluid. The ejector 4 generates a suction via lines 27, 17 or via line 20 to receive inlet fluid from the separator 18 or the cold box 16 respectively. In both cases, valve 25 is closed.

The separator 18 is typically used when there are large quantities of non-condensable gases (nitrogen) present which must be vented from the condensate. The flow line from the cryogenic heat exchanger 16 will then be on line 17 via the three-way valve 26, now open to separator 18. Condensate is drawn from separator 18 via line 20 to the ejector 4. Valve 19 regulates the flow on the basis of known principles. Before and during start-up, the control algorithm for level regulation of 18 is bypassed.

Bypassing the separator is a typical solution when the amount of nitrogen in the boil-off gas is low, so that complete condensation is achieved. Valve 26 now opens to line 27 and directs condensate from the cryogenic heat exchanger 16 via lines 17, 27 and 20 to the ejector 4. The flow is controlled by valve 19. Bypassing the separator is a preferred start-up mode.

When the system has not reached sufficiently low temperatures during start-up, a certain degree of boiling will occur. This steam is recycled via line 11 back to the inlet side of the decoction gas compressor 30.

Another alternative embodiment of the present invention is described with reference to Figure 3. When traveling with cargo, it is from time to time desirable to use LNG pumped from the cargo tank 1 instead of decoction gas as fuel, e.g. if the LNG has a calorific value that is higher than acceptable values for the pipeline network on the receiving side, such as at an onshore plant. It is common to inject nitrogen into the vaporized LNG on the receiving side to reduce the calorific value. In order to minimize the amount of required injected nitrogen on the shore side, it is thus desirable to recycle as much as possible of the evaporated nitrogen back to at least one cargo tank 1. Furthermore, the installed recondensation unit is not necessarily able to condense the boil-off gas completely, so it is not unusual that a certain amount of nitrogen must be released via line 21. There is also an unwanted methane loss associated with the nitrogen release via line 21.

In order to capture, recycle and utilize parts of the nitrogen/methane losses discharged in 21, in the system in figure 2 a line 49 is also provided with a control valve 50, a second ejector 46, and a three-way valve 47. Alternatively, the functionality of the three-way valve can 47 is provided by other means, such as two common valves. Drive fluid for the second ejector 46 is supplied by the drive fluid pump 2 via line 3 through valve 47, then via line 48 to the ejector 46. Non-condensed gases separated in 18 flow via line 21, valve 22 is closed, and the gases flow via line 49 to the ejector 46. Valve 50 ensures the correct pressure in separator 18. The resulting mixture of propellant fluid (LNG) and gas from line 49 is discharged from the ejector via line 45 and enters separator 6. Gases (if any) separated in separator 6 flow via line 51 and mixes with the condensed decoction gas in stream 33 before returning to the at least one loading tank 1.

Although the invention has been illustrated and described in detail in the drawings and in the foregoing description, these drawings and this description are to be regarded as illustrative or exemplary and not limiting, and they are not intended to limit the invention to the described embodiments. The mere fact that certain features are mentioned in mutually different non-independent patent claims does not mean that a combination of these features cannot be advantageously used. Any reference sign in the claims shall not be interpreted as a limitation of the scope of the invention.

Claims (14)

1. Gas supply system for combination or gas engines, comprising at least one LNG cargo tank (1), a first pump (2), a compressor (30), a cryogenic heat exchanger (16) and a high pressure pump (10) which supplies gas to the engines, characterized in that the system further comprises an ejector (4) and a suction tank (6); the ejector (4) is arranged to receive LNG drive fluid from the first pump, draw condensate from the cryogenic heat exchanger (16) and discharge a mixture of the drive fluid and condensate to the suction tank (6); and the suction tank (6) is arranged to supply the mixture of the LNG propellant fluid and the condensate to the high-pressure pump (10). 2. System according to claim 1, in which the pressure in the suction tank (6) is higher than the condensation pressure in the heat exchanger (16). 3. System according to claim 1, in which the suction tank (6) is arranged to have a long enough retention time to ensure stable calorific value of the combined mixture of LNG and condensed decoction gas. 4. System according to claim 1, in which free flow of decoction gas from the at least one cargo tank (1) to the cryogenic heat exchanger (16) is provided by opening a valve (37) bypassing the compressor (30). 5. System according to claim 1, in which a separator (18) is arranged between the cryogenic heat exchanger (16) and the ejector (4). 6. System according to claim 5, wherein the system further comprises a second injector (46) arranged to receive drive fluid from the first pump (2), draw non-condensed gas from the separator (18) and discharge a mixture of LNG- drive fluid and the non-condensed gas to the suction tank (6); and a line (51) arranged to lead separated gas from the suction tank (6) to the at least one loading tank (1). 7. System according to claim 1, in which the first pump (2) is a drive fluid pump. 8. Method for supplying gas to combination or gas engines in a system comprising at least one LNG cargo tank (1), a first pump (2), a compressor (30), a cryogenic heat exchanger (16) and a high-pressure pump (10) which supplies gas to the engines, characterized in that the method comprises: providing a suction tank (6) upstream of the high pressure pump (10); providing an ejector (4) between the first pump (2) and the suction tank (6); pumping, by means of the first pump (2), LNG propellant fluid to the ejector (4); withdrawing, by means of LNG propellant fluid received by the ejector (4), condensate from the cryogenic heat exchanger (16); to discharge, by means of the ejector (4), a mixture of the driving fluid and the condensate to the suction tank (6); and to deliver from the suction tank (6) the mixture of the LNG drive fluid and the condensate to the high-pressure pump (10). 9. Method according to claim 8, in which the ejector (4) releases the mixture of the driving fluid and the condensate at a higher pressure than the condensation pressure in the heat exchanger (16). 10. Method according to claim 8, in which free flow of decoction gas from the at least one cargo tank (1) to the cryogenic heat exchanger (16) is initiated by opening a valve (37) which bypasses the compressor (30); pump LNG with drive fluid the pump (2) to the ejector (4) as the drive fluid; and generating a suction with the ejector (4) to receive fluid from the separator (18) or the cryogenic heat exchanger (16). 11. Method according to claim 8, in which the high-pressure pump (10) feeds the gas engine for a while without the first pump (2) being in operation, by means of the combined mixture of LNG and condensed decoction gas retained in the suction tank (6). 12. Method according to claim 8, wherein the method further comprises providing a separator (18) between the cryogenic heat exchanger (16) and the ejector (4). 13. Method according to claim 12, wherein the method further comprises providing a second injector (46) between the first pump (2) and the suction tank (2), where the second injector (46) is arranged to receive drive fluid from the first the pump (2), drawing non-condensed gas from the separator (18) and discharging a mixture of LNG propellant fluid and the non-condensed gas to the suction tank (6); and providing a line (51) between the suction tank (6) and the at least one cargo tank (1) arranged so that it conducts separated gas from the suction tank (6) to the at least one cargo tank. 14. Method according to claim 8, in which the first pump (2) is a drive fluid pump.