NO339027B1

NO339027B1 - System and method for conditioning pressure in an LNG tank

Info

Publication number: NO339027B1
Application number: NO20150940A
Authority: NO
Inventors: Steinar Oppedal; Arne Mortensen; Ann Rigmor Nerheim
Original assignee: Rolls Royce Marine As
Priority date: 2015-07-15
Filing date: 2015-07-15
Publication date: 2016-11-07
Also published as: NO20150940A1

Description

The present invention relates generally to a method and a system for improving the pressure conditions in a LNG storage tank.

The method and system according to the present invention are particularly suitable for use on board a ship adapted for storage and transportation of liquefied natural gas (LNG) for the purpose of utilising a part or the total of the LNG to fuel the ship's engines and/or other consumers.

In the marine industry, a variety of gas fuelled systems exist. These systems include both LNG cargo ships, where part of the cargo is consumed as fuel, and other ships were gas is stored onboard in tanks for fuel consumption purposes only. LNG carrier solutions often use atmospheric LNG cargo tanks (max 0,7 barg), but solutions using pressure tanks (C - tanks) are also used.

In known gravity feed LNG fuel systems, the hydrostatic force of the liquid column in the LNG storage tank is the driving force in the system, feeding LNG from the tank to the pressure build-up unit (PBU). The gas pressure created by the PBU in the tank is the driving force to the main evaporator (VAP) and/or the super heater (SH) and at end to one or more consumers.

As LNG and/or natural gas from the tank flows to the VAP and/or SH, the pressure in the tank will be reduced. In order to maintain the pressure in the tank, some LNG is directed from the tank to the PBU, where the LNG is heated and evaporated before the gas is returned to the tank top. In this way, the pressure in the tank may be increased to a pressure above the LNG saturation pressure. The pressure in the tank is the sum of the LNG saturation pressure at the given temperature and the input to the tank top from the PBU, known as the "gas cushion".

The operating pressure in the tank is defined by the required inlet pressure to the one or more consumers and pressure losses in the heat exchangers, piping, valves etc. At low LNG temperatures the saturation pressure in the tank may be lower than the pressure required by the consumer. If the tank contents in such situations are subject to sloshing, a major part of the mass added to the vapour phase by the PBU may be condensed rapidly, resulting in a rapid pressure drop down to the saturation pressure. If the saturation pressure is lower than the required tank operating pressure, this will impact the consumer(s) condition. If sloshing continues, the PBU will not be able to mitigate the pressure loss fast enough to maintain the required gas pressure to the consumer.

US 2014/0190187 Al regards a cryogenic liquid conditioning system with flow driven by head pressure of liquid contained in a cryogenic storage tank, and a cryogenic liquid delivery system with flow driven by pressure in the vapor space of said cryogenic storage tank. A heat exchanger, coupled to the cryogenic storage tank located below the liquid level of said tank, operates as a portion of both the conditioning system and delivery system. A piping system moves cryogenic liquid to the heat exchanger where it is vaporized, and then moves vaporized liquid to the vapor space of the cryogenic tank and an application. The piping system includes a means for controlling flow through the system. A means for measuring the saturated pressure of cryogenic liquid is coupled to the storage tank or piping system, and is in communication with the means for controlling flow.

US 5.960.635 A regards an air conditioning apparatus using liquid nitrogen. A source of liquid nitrogen fills a pressure vessel as necessary. A release valve releases the liquid nitrogen from the pressure vessel into a housing to absorb latent heat and become nitrogen gas. A thermostat controls the release valve. A dehumidifying arrangement blows warmer air from outside the housing to mix with the nitrogen gas inside the housing to become a cooler air mixture. The dehumidifying arrangement further dehumidifies the cooler air mixture before directing the cooler air mixture to the atmosphere outside the housing.

GB 2.515.091 A regards a cryogenic tanker truck håving an inert medium vessel in addition to the High Value Cryogenic Liquid (HVCL) tank. The inert medium, which may be liquid nitrogen, liquid argon or any suitable inert gas, is used to the cool down delivery equipment prior to offloading of HVCL and thereby avoids loss of HVCL

An object of the present invention is to provide a system and a method for conditioning of LNG in fuel systems, such a system utilizing a part or the total of the LNG to fuel the ship's engine and/or to supply other consumers onboard the ship.

The present invention describes a system and a method to improve the pressure conditions in a LNG tank. The object is achieved with a system and a method for controlling and conditioning a pressure in a LNG tank according to the following independent claims, with additional embodiments set forth in the dependent claims.

The present invention relates to any system for controlling and conditioning of pressure in a LNG storage tank, where the LNG storage tank is provided with at least one inlet conduit for LNG and/or nitrogen and a gas (vapour) supply conduit for nitrogen and/or natural gas supply to a consumer, where the system comprises at least one LNG tank. In addition, the system may comprise one or several of the following components: pressure build-up unit, pump, compressor, evaporator, heater, nitrogen storage/bottle/generator, gas buffer tank.

According to the present invention a system for conditioning a pressure in a LNG storage tank may comprise at least one LNG storage tank, a pressure build-up unit (PBU) being provided with at least one inlet conduit and at least one return conduit, where the pressure build-up unit is in fluid communication with the at least one LNG storage tank through the at least one return conduit and at least one tank inlet conduit; at least one gravity feed conduit and the at least one inlet conduit, the at least one LNG storage tank being provided with at least one inlet conduit for LNG, Nitrogen and gas supply conduit.

According to one aspect of the present invention, the system for supplying natural gas may further comprise a pressure build-up unit, vaporizer and/or super heater.

The pressure build-up unit (PBU) may be in fluid communication with the LNG storage tank through a gravity feed conduit, at least one PBU inlet conduit and at least one outlet conduit, and at least one tank inlet conduit.

The vaporizer (VAP) may be in fluid communication with the LNG storage tank through a feed conduit and a vaporizer inlet conduit.

In one aspect of the present invention, the super heater (SH) may be in vapour communication with the vaporizer through a conduit.

According to one aspect of the present invention, one or more of the pressure build-up unit, vaporizer and super heater, may be combined in a common unit.

According to one aspect of the present invention, the system may comprise one or more pumps.

According to one aspect of the present invention, the system may also comprise one or more additional pressure build-up-unit(s), vaporizer(s) and/or super heater(s).

According to one aspect of the present invention, the system may include at least one gas buffer, thereby allowing gas to be vented from or supplied to the LNG storage tank.

A person skilled in the art would understand that the system according to the present invention also may comprise various sensors, valves and/or regulators, in order to be able to control the pressure, heat and mass flow in the system.

According to the present invention a method for controlling a pressure in a LNG storage tank may comprise the following steps: bunkering LNG in at least one LNG tank, injecting nitrogen (vapour) in top of the at least one LNG storage tank until a minimum operating pressure is achieved in the LNG storage tank, and feeding an amount of LNG from the at least one LNG storage tank to at least one pressure build-up-unit (PBU) when a pressure in the at least one LNG tank fall below a certain pressure, using the at least one pressure build up unit to raise the pressure of the LNG.

The nitrogen may be delivered from one or more nitrogen bottles, one or more nitrogen generators, and/or from one or more gas buffers, where the nitrogen bottle(s), nitrogen generator(s) and/or gas buffer(s) may be arranged on an onshore LNG storage and filling plant and/or on board the ship.

The nitrogen (vapour) may be supplied to the LNG storage tank after bunkering or re-bunkering and during purging, or also during the voyage when a certain pressure drop occurs in the LNG storage tank.

Pure nitrogen may be vented to the atmosphere.

A person skilled in the art would understand that additional nitrogen may be injected directly to the LNG storage tank after purging is completed, until a minimum acceptable operating pressure is achieved in the LNG storage tank.

According to one other aspect of the present invention, the method may further comprise the step of transferring wholly or in part the raised pressure LNG to a vaporizer (VAP).

According to one aspect of the present invention, the method may further comprise the step of transferring wholly or in part the vaporized LNG to a super heater (SH).

As LNG and/or natural gas is withdrawn from the LNG tank and sent to the vaporizer (VAP) and/or super heater (SH), the pressure in the LNG tank will be reduced. In order to maintain the pressure in the LNG tank, some LNG is withdrawn from the LNG tank, heated and evaporated in the pressure build-up unit (PBU), before the vapour is returned to the top of the LNG tank. In this way, the pressure in the LNG tank may be kept above the LNG saturation pressure.

The pressure in the LNG tank is the sum of the LNG saturation pressure at the given temperature, the added nitrogen partial pressure and the pressure added from the PBU. In situations with sloshing, the minimum pressure in the LNG tank is given by the LNG saturation pressure and the partial pressure of the added nitrogen. If the LNG saturation pressure is lower than the required LNG tank operating pressure, the added nitrogen provides a stable pressure in situations of sloshing. Pressure loss in the LNG tank due to sloshing is mitigated by the added nitrogen. Nitrogen may be injected to the gas phase during and/or after the bunkering process, or whenever pressure drop occur in the tank.

The pressure and heat and mass flow in the system is controlled by various sensors, valves and regulators. The type and number of valves and regulators and sensors may vary from project to project, depending on the number of tanks, PBUs, pumps etc, as easily worked out by someone skilled in process system design.

These and other characteristics of the present invention will be clear from the following description of a preferential form of embodiments, given as non-restrictive examples, with reference to the attached drawings wherein: Figure 1 illustrates an embodiment of a system for controlling and/or conditioning the pressure in a LNG storage tank according to the present invention, Figures 2 to 4 illustrates how a method for controlling and/or conditioning a pressure in a LNG storage tank can be used to control and/or condition the system shown in figures 1 and/or 5, and Figure 5 illustrates another embodiment of a system for controlling and/or conditioning the pressure in a LNG storage tank according to the present invention. Figure 1 shows schematically how a system 1 for conditioning the pressure in a LNG storage tank according to the present invention can be utilized with a fuel system for a ship. The system 1 for conditioning of LNG in the fuel system, the system 1 also supplying natural gas to one or more consumers on board the marine vessel is shown with only one LNG storage tank 2 for clarity reasons, but it should be understood that the system may comprise more than one LNG storage tank 2.

The LNG storage tank 2 is connected to a pressure build-up unit 3 (PBU) through feed conduit 7 and a pressure build-up unit inlet conduit 8, such that LNG can be transferred to the pressure build-up unit 3. LNG may also be transferred to the pressure build-up unit 3 through an alternative or additional (gravity) feed conduit 20. A pressure build-up return conduit 9 and a tank inlet conduit 10 allow vapour from the pressure build-up unit 3 to be returned to the top of the LNG storage tank 2.

The feed conduit 7 is also connected to a vaporizer 4 (VAP) through a VAP inlet conduit 11.

The feed conduit 7, pressure build-up unit inlet conduit 8 and VAP inlet conduit 11 are connected via a branching control point, such that LNG can be supplied directly to the pressure build-up unit 3, directly to the vaporizer 4 or to both the pressure build-up unit 3 and the vaporizer 4.

The vaporizer 4 is connected to a super heater 5 (SH) through a SH inlet conduit 12, the super heater 5 (SH) further being connected to one or more end users or consumers 6 through a conduit 13. The end user or consumers 6 can for instance be one or more engines.

A gas (vapour) supply conduit 17 connects the LNG storage tank 2 and super heater 5, such that nitrogen and/or natural gas directly can be supplied to the super heater 5.

To supply gas to the one or more end users or consumers 6, LNG is withdrawn from the LNG storage tank 2 through the feed conduit 7 and directed to the vaporizer 4, whereafter heat, through the vaporizer 4, is added to the LNG in order to transfer the LNG to gaseous phase. Thereafter the gas is fed to the super heater 5, where the gas is heated to a temperature required by the one or more engines.

However, when LNG is withdrawn from the LNG storage tank 2, the pressure in the LNG storage tank 2 will decrease. As it is desired to maintain the pressure in the LNG storage tank 2 at least above a minimum pressure required by the one or more engines, some LNG is withdrawn from the LNG storage tank 2 and is fed to the pressure build-up unit 3 in order to heat up and vaporize LNG. This can either be done by using the gravity feed conduit 7 or (gravity) feed conduit 20.

The vaporized and heated LNG will thereafter be returned to the top of the LNG storage tank 2 through the pressure build-up return conduit 9 and the tank inlet conduit 10.

Nitrogen and/or LNG can be delivered to the LNG storage tank 2 through at least one inlet conduit 15 for LNG and/or nitrogen and conduit 14, 16.

According to one aspect of the present invention, the method may further comprise the step of injecting nitrogen (vapour) gas to the gas phase in the LNG storage tank 2 top at end of bunkering, or whenever pressure drop occur in the LNG storage tank 2. The nitrogen may be added at end of bunkering by discharging the nitrogen used for purging the bunkering pipes to the LNG storage tank 2 top. In this way, the amount of venting of natural gas and nitrogen to the atmosphere in connection to the bunkering process is reduced to a minimum. When purging is completed, additional nitrogen may be injected directly to the LNG storage tank 2 top using the same pipe connections, or by using custom made arrangements dedicated for the nitrogen tank injection purpose on-board the ship. Nitrogen is injected until a minimum acceptable operating pressure is achieved in the LNG storage tank 2. The process system is illustrated in Figure 1.

The temperature of the bunkered LNG may be down to approximately -164 °C. At this temperature the saturation pressure in the LNG storage tank 2 may be lower than the required pressure to the engine and/or other consumer(s).

Nitrogen has a dew point of approximately -196°C, which is lower than the minimum LNG temperature. Therefore, if sloshing occurs in the LNG storage tank 2, the nitrogen will not be condensed but will remain in vapour phase. Independent of sloshing movements, the pressure in the LNG storage tank 2 is the sum of the saturation pressure of the LNG at the given temperature and the partial pressure of the injected nitrogen. In this way, the pressure in the LNG storage tank 2 can be kept above the minimum acceptable operating pressure. The minimum acceptable operating pressure may vary from system to system. The amount of nitrogen injected must be adjusted according to the operating pressure, tank volume, consumption profile and bunkering schedule. The method of the present invention is illustrated in Figures 2, 3 and 4. The illustrations are not in scale, and are only meant to illustrate various realistic implementations of the invention.

During voyage, the temperature of the stored LNG will slowly increase due to heat leak from the surroundings and from heat exchangers. An increased LNG temperature gives higher LNG saturation pressure and reduced density. As LNG is consumed by the engine and/or by the other consumer(s), the tank gas (cushion) volume increase s and the gas pressure decreases. As mentioned earlier, dependent on the severity of sloshing movements in the LNG tank and the saturation pressure of the LNG (temperature), the pressure may be maintained by using the PBU.

In reasonable time before the next bunkering the tank pressure should be reduced to the minimum acceptable operating pressure. This may be executed by either consuming from the liquid phase or from the gas phase, or both. At start of bunkering, the upper spray line of the LNG storage tank 2 should be used, to further reduce the tank pressure. The minimum achievable tank pressure is the sum of the LNG saturation pressure at the given temperature and the partial pressure of formerly injected nitrogen. If gas has been consumed from the tank gas phase, the nitrogen content in the tank gas phase may have been reduced. Hence, after bunkering it may be necessary to compensate this nitrogen loss by injecting more nitrogen.

If gas is never consumed from the gas phase during voyage, a situation may occur where the gas phase contains more nitrogen than recommended with respect to consumer composition requirements and/or tank pressure control purposes. In such cases, the pressure and nitrogen content of the gas phase may be reduced by consuming from the gas phase while the ship is in harbour. The engines may run until they stop due to either low pressure or low methane / high nitrogen content. Thereafter, bunkering may proceed as illustrated in Figure 2. Pure nitrogen may be vented to the atmosphere.

The present invention is in particular relevant for marine fuel gas systems, but is also applicable in other systems, both on land and sea. The invention represents a cost efficient, compact, flexible and robust solution for gas pressure and gas supply conditioning in marine LNG fuel systems.

Figures 2 to 4 illustrates some of the different conditions that may occur in the LNG storage tank 2, as a function of LNG volume and pressure in the LNG storage tank 2. The tank volume will vary between a bunkering start volume (the lowest volume in the tank) and a bunkering end volume (the highest volume in the tank) in the tank. The different parameters, such as LNG saturation pressure during bunkering, minimum tank pressure during re-bunkering, natural gas (NG) partial pressure before initial bunkering, tank pressure before re-bunkering and total tank pressure after re-bunkering are shown as vertical broken lines along the (X axis) total pressure axis.

As illustrated in Figure 2, the method for controlling and conditioning a pressure in a LNG storage tank 2 can be used to control and/or condition the system shown in figures 1 and/or 5 where: Bunkering of a LNG storage tank 2 starts at a given low LNG volume in the LNG storage tank 2. During bunkering (step 1) the volume of LNG in the LNG storage tank 2 increases, while the pressure is reduced. Bunkering ends at a given LNG volume, where the pressure in the tank is equal to, or close to, the LNG saturation pressure at the given temperature. After end of bunkering, and during purging, nitrogen is discharged from the pipe line to the tank gas phase (step 2). The amount of nitrogen injected is dependent on the minimum partial pressure of nitrogen desired in the LNG storage tank 2. When nitrogen injection is complete the LNG fuel system is operated as normal (step 3) with consumption from the liquid LNG phase and pressure build-up using the PBU. The temperature of the LNG increases with time, due to heat transfer from the surroundings and from the PBU. As the LNG temperature increases the density of the LNG decreases, and the saturation pressure increases. When the LNG has heated to a temperature corresponding to the desired saturation pressure the system pressure may be held constant by adjusting the PBU activity (step 4) for the rest of the voyage until preparation for re-bunkering starts. As a preparation for re-bunkering the tank pressure can be reduced by consumption from the liquid and/or vapour phase of the tank (step 5). Before gas is consumed from the gas phase the operator must confirm that the composition of the gas phase is adequate according to the consumer/engine specifications, since the composition of the gas phase is different from the liquid phase.

In figure 3 is shown another condition for the system 1 shown in figures 1 and 5, where the system 1 has been bunkered and pressurized similar to the procedures shown in Figure 2, but where the subsequent pressure reduction is started at a higher filling level (step 9) and is ended at a higher pressure and higher filling level of the tank. At this pressure and filling level the LNG storage tank 2 is re-bunkered. The conditions of the LNG storage tank 2 before bunkering is dependent on the ships gas consumption during voyage and the pressure reduction method in the period before bunkering. By regulating the consumption from the gas phase and liquid phase of the tank the pressure may be decreased down to the saturation pressure of the LNG at the given temperature. Hence, the minimum achievable tank pressure before bunkering is a function of temperature of the LNG in the LNG storage tank 2 and remaining nitrogen. As bunkering is started, the LNG in the LNG storage tank 2 decreases in temperature, leading to a reduction in pressure. When the minimum pressure is reached, the pressure may be kept constant by regulating top and bottom filling, until the upper space of the LNG storage tank 2 is reached, where pressure increases rapidly (the rate of pressure increase in the upper part of the tank is dependent on the tank geometry). If a significant amount of nitrogen has been consumed from the gas phase (during the pressure reduction phase (step 9)) additional nitrogen may be injected after end of bunkering. If the major pressure reduction before bunkering is performed by consuming from the liquid phase, the nitrogen will mainly remain in the gas phase and additional injection will not be necessary.

Figure 4 shows how this method of pressure conditioning can be used in a system where the tank pressure is kept at a lower pressure than in Figures 2 and 3. The objective is to show that this method of conditioning the pressure in the LNG storage tank 2 is applicable to a wide range of operating conditions and operating philosophies, i.e. to a variety of ship and gas engine applications.

When the desired tank pressure is reached, re-bunkering is started. At start of re-bunkering, top filling of the tank may be used to reduce the pressure in the tank down to the minimum pressure (step 1), limited by the LNG temperature and the remaining injected nitrogen. The minimum pressure during re-bunkering will be the sum of the LNG saturation pressure at the given temperature and the partial pressure of the remaining formerly added nitrogen. Some of the added nitrogen may have been consumed from the gas phase, as illustrated in Figure 3 and 4. This nitrogen amount can be compensated for by injecting the equivalent nitrogen amount after end of re-bunkering (step 2).

The enclosed figures illustrate only a few of the possible combinations of the pressure conditioning process. In one aspect of the invention, the pressure during the first period after re-bunkering reduces, due to consumption from the liquid phase and no or minor use or effect of the PBU.

However, for all the pressure conditioning processes the principle of the process is unchanged. The final pressure at end of re-bunkering is a measure for remaining injected nitrogen, and a measure for the need of additional nitrogen injection.

The method is applicable for all tank sizes and can be used independent of LNG volumes in the tank before and after bunkering. Compared to other LNG fuel systems, this process requires an adequate injection connection for nitrogen, and a gas consumption pipe line from the top of the tank to the heater and/or consumer. As can be seen from figure 5, the system according to the present invention may also include a gas buffer 19, for storage of a portion of the gas cap at high pressure as compared to the normal operating pressure.

The rest of the system 1 shown in figure 5 is the same as explained according to figure 1.

Claims

1. A system (1) for conditioning a pressure in a LNG storage tank (2), the system comprising at least one LNG tank (2), a pressure build-up unit (PBU) (3) being provided with an inlet conduit (8) and one return conduit (9), the pressure build up unit (3) further being in fluid communication with the LNG tank (2) through the return conduit (9) and at least one tank inlet conduit (10); at least one gravity feed conduit (7, 20) and the inlet conduit (8), the LNG storage tank (2) being provided with at least one inlet conduit for LNG and/or Nitrogen (15, 16) and a gas (vapour) supply conduit (17).

2. A system according to claim 1,characterized in thatthe system further comprises at least one vaporizer (VAP) (4) and/or a super heater (SH) (5).

3. A system according to claim 1-2,characterized in thatthe VAP (4) is in fluid communication with the LNG storage tank (2) through the feed conduit (7) and VAP inlet conduit (11).

4. A system according to claim 1-2,characterized in thatthe SH (5) is in fluid communication with the VAP (4) through a conduit (12).

5. A system according to claim 1-4,characterized in thatthe gas (vapour) supply conduit (17) for nitrogen and/or natural gas from the LNG tank is in fluid communication with the conduit (12) or conduit (13).

6. A system according to any one of the claims 1-6,characterized in thatone or more of the PBU (3), VAP (4) and SH (5) is/are combined in a common unit.

7. A system according to claim 1-6,characterized in thatthe system comprises one or more additional vaporizer(s) (4) and/or super heater(s) (5).

8. A combined system comprising two or more of the systems according to claims 1-7.

9. A method for conditioning a pressure in a LNG storage tank (2),characterized inthat the method comprises the following steps: bunkering LNG in at least one LNG tank (2), injecting nitrogen (vapour) to the LNG storage tank (2) until a minimum operating pressure is achieved in the LNG storage tank (2), and feeding an amount of LNG from the at least one LNG storage tank (2) to a pressure build up unit (PBU) (3) when a pressure in the at least one LNG tank (2) fall below a certain pressure, using the pressure build up unit (3) to raise the pressure in the LNG tank.

10. A method according to claim 9,characterized in thatthe method further comprises the step of bunkering the LNG storage tank to a given volume and a pressure equal to, or close to, a LNG saturation pressure at a given temperature.

11. A method according to claim 10, where the method further comprises, after bunkering and during purging, supplying nitrogen to the tank gas phase until a minimum partial pressure of nitrogen is obtained in the LNG storage tank.

12. A method according to any one of the claims 9-11,characterized in thatthe method comprises the step of supplying nitrogen directly to the tank gas phase, independent of bunkering and purging.

13. A method according to any one of the claims 9-12,characterized in thatthe method comprises the step of, when the gas phase contains more nitrogen than recommended, consuming from the gas phase.

14. A method according to any one of the claims 9-13,characterized in thatpure nitrogen is vented to the atmosphere.

15. A method according to any one of the claims 9-14,characterized in thatthe method comprises the step of heating the LNG to a temperature corresponding to a desired saturation pressure.

16. A method according to any one of claims 9-15,characterized in thatthe method comprises the step of decreasing the pressure to a saturation pressure of the LNG at a given temperature by consuming from the gas phase and liquid phase of the LNG storage tank.

17. A method according to any one of claims 9-16,characterized in thatthe method comprises the step of supplying additional nitrogen when a significant amount of nitrogen has been consumed from the gas phase.