NO338906B1 - System and method for conditioning LNG in fuel system - Google Patents

Description

The present invention generally relates to a method and a system for conditioning LNG in a fuel system, where such a system also supplies natural gas to a consumer for heating or power generation purposes. The method and system according to the present invention are particularly suitable for use on board a vessel adapted for the storage and transport of liquefied natural gas (LNG) for the purpose of utilizing part or all of the LNG to supply fuel to the ship's machinery or other consumers.

There are a number of gas-powered systems within the marine industry. These systems include both LNG cargo ships, where part of the cargo is consumed as fuel, and other ships where gas is stored on board in tanks for the sole purpose of being consumed as fuel. Solutions for LNG ships often use atmospheric LNG cargo tanks (max 0.7 barg), but solutions that use pressure tanks (C tanks) are also used. The use of pressure tanks (C-tanks) has been the dominant solution in cargo ships, passenger ships and other ships, but the use of atmospheric tanks may increase in the future, especially for long-endurance vessels.

In LNG ships, a common solution is to run gas engines for propulsion on boil-off gas (BOG) from the cargo. However, the amount of BOG is often too low to maintain the supply of gas to the engines, and a forced BOG system is therefore installed. Such forced BOG is typically achieved by using a submerged cargo pump and a heat exchanger. Liquid LNG is taken out from the bottom of the tank through the LNG loading pump and fed to a heat exchanger, where the LNG is heated from approx. -160 °C to approximately -130 °C before the fluid is returned to the upper layer of LNG in the cargo tank. In this way, the temperature in the upper layer of LNG in the cargo tank increases, and thus a higher saturation pressure is achieved. A higher saturation pressure gives a higher evaporation rate and therefore an increased mass of gas for consumption by the engines. At the same time as this method aims to increase the temperature in the upper LNG layer, it is also an aim to keep the rest of the LNG at the lowest possible temperature in order to increase the loading capacity and holding time.

NO 335213 Bl relates to a gas supply system for combination or gas engines, where the system comprises at least one LNG storage tank, a first pump, a compressor, a cryogenic heat exchanger and a high-pressure pump that supplies gas to the engines. The system further comprises an ejector and a suction tank, where the ejector is arranged to receive LNG drive fluid from the first pump, draw condensate from the cryogenic heat exchanger and discharge a mixture of the drive fluid and condensate to the suction tank, where the suction tank is arranged to deliver the mixture of the LNG drive fluid and the condensate to the high-pressure pump.

WO 2014/091061 A1 relates to a method of starting gas delivery from a fuel system, in which system a fuel tank is filled with a liquefied gas through a filling method and the filling method results in the pressure in the tank being at least the operating pressure required by the engine, through which method the pressure in the tank is regulated by controlling a pressure build-up system comprising a pressure build-up evaporator which is arranged in a line leading from the bottom part of the tank to the top part of the tank. The pressure build-up system is regulated by controlling the flow rate of the vaporized gas, while a heat transfer medium is allowed to flow through the pressure-build-up evaporator.

The invention mainly relates to solutions based on pressure tanks (C-tanks), but can easily be adapted to suit atmospheric tank solutions. According to the invention, a combination of an external LNG pump and a heat exchanger is used to heat up the entire contents of the LNG tank, while the pressure in the tank is at the same time controlled and gas is supplied to the engines. The purpose of the process is to increase the temperature of the LNG as quickly as possible, in a controlled manner, while achieving a homogeneous temperature throughout the entire volume of LNG. Heating of LNG continues until a temperature corresponding to a given saturation pressure is reached, which is equal to or higher than the minimum pressure required by the gas engine (plus the pressure loss due to pipes, valves etc). As soon as this temperature and saturation pressure is reached, the pump is turned off and the gas fuel supply is maintained only by gravity mode. It follows from this that pump operating time is saved, the complexity of the system is reduced and the risk of cavitation in the pump at low liquid levels in the tank (or during violent flapping movements) is avoided.

The use of natural gas as an energy source in ships is an advantage due to its efficient combustion and low emissions. Generally, gas is stored in liquid form because less space is required for storage in that way. However, the gas engine(s) require the fuel to be fed in gas phase. To supply gas to the engine(s), LNG is extracted from one or more LNG storage tanks through gravity and/or an LNG pump. The LNG then flows into at least one evaporator, where heat is added to the LNG to transfer the natural gas into a gas phase. The natural gas then flows to one or more super heaters, where the gas is heated to the temperature required by the gas engine(s). Due to the withdrawal of LNG for consumption purposes, the pressure in the LNG tank will drop. In order to maintain a pressure in the LNG tank that is above the minimum pressure required by the gas engine, some LNG is withdrawn from the LNG tank to be vaporized and heated in one or more pressure build-up units (PBU), before the natural gas from the pressure building units is returned back to the top of the tank. It follows from this that the mass is added to the gas jacket, thereby also increasing the pressure in the LNG tank. All or part of the production from the PBU can also be returned to the bottom of the LNG tank, where it adds heat to the LNG.

The pressure in the LNG tank is the sum of the saturation pressure of the LNG in the tank at the given temperature, and the pressure added by the pressure building units (PBUs). If sloshing or other movements of the liquid in the tank occur, the gas pressure can be reduced to the saturation pressure for the LNG at the given temperature. At low LNG temperatures, the saturation pressure is lower than the minimum pressure required by the engine. This can lead to a reduction in power from the engine. To avoid such situations, an LNG pump can be used. A pump ensures a sufficient supply of gas to the engine(s) regardless of low saturation pressure for the LNG in the tank. The capacity of the pump may be higher than that required for supply to engines, and a return line from the pump to the LNG storage tank is therefore installed. LNG from the pump return line can be injected through the gas phase, in this way to reduce the tank pressure, and/or it can be returned to the bottom of the LNG tank to thereby add heat to the LNG. Heating the LNG increases the saturation pressure of the LNG. The pump can also be used to feed the PBUs and to increase the pressure in the LNG tank and/or increase the temperature of the LNG in the tank.

The pressure and heat and mass flow in the system are controlled and regulated using various sensors, valves and regulators. The type and number of valves and regulators and sensors can vary from project to project, depending on the number of tanks, PBUs, pumps etc., which are easily prepared by a person with knowledge within process system design.

An object according to the present invention is to provide a system and a method for conditioning LNG in fuel systems, where such a system also supplies natural gas to a consumer, which minimizes and/or reduces one or more of the disadvantages of known techniques, or fat to provide a viable alternative.

It is also an object of the present invention to provide a system and a method for conditioning LNG in fuel systems, where such a system also supplies natural gas to a consumer, which minimizes the risk of damage, spillage of gas, etc.

These purposes are achieved with a system and a method for supplying natural gas according to the following independent claims, with further embodiments specified in the non-independent claims.

The present invention relates to a system for conditioning LNG in fuel systems, where such a system also supplies natural gas to a consumer, where

the system comprises at least one LNG tank, a pump and a pressure build-up unit, the pump being arranged to be in fluid communication with the at least one LNG tank through an outlet line, a pump inlet line and at least one pump return line, while the pressure build-up unit is arranged to be in fluid communication with the LNG tank through at least one inlet duct, one outlet line and/or one gravity feed line and one conduit. The pump and the pressure build-up unit are also, through a pump outlet line and a pressure build-up line, in fluid communication with each other.

The pump may be arranged as a stand-alone unit externally to the at least one LNG tank, or may be installed in the LNG tank.

According to one aspect of the present invention, the system for supplying natural gas can further comprise an evaporator and/or a superheater.

The vaporizer may be in fluid communication with the LNG tank through an outlet line and vaporizer inlet line. The evaporator can further be in fluid communication with the LNG tank through a pump, through a pump outlet line and an evaporator inlet line.

The evaporator may also be in fluid communication with the LNG tank through a gravity feed line.

In one aspect of the present invention, the superheater may be in fluid communication with the evaporator through a conduit.

The inlet pipe may be divided into two sub-pipes, a first sub-pipe may be arranged in such a way that the steam is delivered to a top of the at least one LNG tank, and a second sub-pipe may be arranged to extend to the bottom of the LNG tank.

The pump return line may be divided into two sub-lines, a first sub-line may be arranged in such a way that the liquid is delivered to a top of the at least one LNG tank, and a second sub-line may be arranged to extend to the bottom of the LNG tank.

According to one aspect of the present invention, one or more of the pressure building unit, the evaporator and the superheater can be combined in a common unit.

According to one aspect of the present invention, the system may further comprise one or more pumps and/or pressure building units.

According to one aspect of the present invention, the system can also include one or more evaporator(s) and/or superheater(s).

In one embodiment, the inlet line and the pump return line can be connected in a common line.

According to the present invention, a method for conditioning LNG in fuel systems, where such a method also supplies natural gas to a consumer, comprises the following steps: storing LNG in at least one LNG tank, transferring a quantity of LNG from the at least one LNG tank to a pump when a pressure in at least one LNG tank falls below a certain pressure, use the pump to increase the pressure of the LNG, transfer all or part of the LNG with increased pressure to a pressure building unit to heat and vaporize The LNG for at least one LNG tank.

According to one aspect, the method further comprises the steps of returning steam to a top of the LNG tank, thereby regulating the pressure of the LNG in the at least one LNG tank, and/or returning the steam to a bottom of the LNG tank, thereby to heat the LNG in at least one LNG tank.

According to another aspect of the present invention, the method further comprises the step of transferring all or part of the LNG with increased pressure to a vaporizer.

According to another aspect of the present invention, the method further comprises the step of fully or partially transferring the vaporized LNG to a superheater.

As LNG is extracted from the LNG tank and sent to the evaporator (VAP) and the superheater (SH), the pressure in the tank will be reduced. To maintain the pressure in the tank, some LNG is withdrawn from the tank, heated and vaporized in the pressure building unit (PBU), before the vapor is returned to the top of the tank. In this way, the pressure in the tank will be increased to a pressure above the LNG saturation pressure at the given temperature of the LNG in the tank. The pressure in the tank is the sum of the LNG saturation pressure at the given temperature, and the gas flow to the top of the tank from the PBU.

The operating pressure in the tank is defined by the required inlet pressure to the consumer and pressure loss in heat exchangers, pipelines, valves etc. At low LNG temperatures, the saturation pressure in the tank may be lower than the pressure required by the consumer. If the contents of the tank i

such situations are prone to sloshing, a large part of the supplied pressure formed by the steam from the PBU can be condensed quickly, which results in a rapid pressure drop down to the saturation pressure. If the saturation pressure is lower than the required tank operating pressure, this will affect the consumer condition. If the sloshing continues, the PBU will not be able to reduce the pressure drop fast enough to maintain the required gas pressure to the consumer.

In situations as described above, the supply to the consumer can be secured by using a pump to force LNG to the VAP, thereby maintaining the necessary gas supply and gas pressure to the consumer. In addition, the pump can be used to force LNG to the PBU to produce steam and increase tank pressure. At the same time, the PBU can be used to heat the LNG in the storage tank to a temperature that corresponds to a desired minimum saturation pressure.

The present invention is particularly relevant for marine fuel systems, but can also be used in other systems, both on land and at sea. The invention represents a compact, flexible and robust solution for gas pressure and gas supply control in marine LNG fuel systems.

These and other characteristics of the present invention will be clear from the following description of a preferred form of execution, given as non-limiting examples, with reference to the attached drawings where: Figure 1 illustrates an embodiment of a system for conditioning LNG in fuel systems, where the system also supplies natural gas to a consumer, according to the present invention, Figure 2 illustrates another embodiment of a system for conditioning LNG in fuel systems, where the system also supplies natural gas to a consumer, according to the present invention, and Figure 3 illustrates how two systems according to Figure 1 can be put together to form a common system. Figure 1 schematically shows how a system 1 for conditioning LNG according to the present invention can be used with a fuel system for a vessel. The system 1 for conditioning LNG in the fuel system, where the system also supplies natural gas to a consumer on board the marine vessel, is shown with only one LNG tank 2 for reasons of clarity, but it should be understood that the system may comprise more than one LNG tank 2 .

The LNG tank 2 is connected to a pump 11 through a tank outlet line 4 and a pump inlet line 6. The LNG tank 2 and the pump 11 can also be in fluid communication through an alternative or additional line 17. A pump return line 5 is also connected between the LNG tank 2 and the pump 11, whereby pressurized LNG can be returned to the LNG tank 2.

The pump return line 5 is divided into two partial lines 5a, 5b, where the first partial line 5a is arranged to be on top of or in an upper area of the LNG tank 2, while the second partial line 5b is arranged to extend towards the bottom of the LNG - the tank 2.

The pressure building unit (PBU) 12 is connected to the LNG tank 2 through the tank outlet pipe 4 and the gravity feed line 9, but LNG can also be transferred to the PBU 12 through an alternative or additional gravity feed line 16. The outlet of the gravity feed line 16 can be connected to the line 9, or directly to the dividing point between the lines 7, 8, 9 and 10. Furthermore, an inlet line 3 is arranged between the LNG tank 2 and the pressure build-up unit 12. The inlet line 3 is divided into two partial lines 3a, 3b, where the first partial line 3a is arranged to be on top of or in an upper area of the LNG tank 2, while the second part line 3b is arranged to extend towards the bottom of the LNG tank 2.

The pump 11 and the pressure build-up unit 12 are in fluid communication with each other through a pump outlet line 7 and a pressure build-up line 10.

The pump 11 is further connected to an evaporator (VAP) 13 and a superheater (SH) 14 through the pump outlet line 7, an evaporator inlet line 8, evaporator (VAP) 13 and superheater (SH) 14 which are in fluid communication with each other through a line 18. The superheater (SH) 14 will then deliver natural gas to an end user or consumer 15, through a line 19, where this end user or consumer 15 is, for example, one or more gas engines.

To supply gas to the engine(s), liquefied natural gas (LNG) is drawn from LNG tank 2 via gravity feed line 16 by gravity, or through lines 4, 6, 7 and pump 11, and directed to evaporator 13, after which heat, through the evaporator 13, is added to the LNG to transfer the natural gas to a gas phase. The gas is then fed to the superheater 14, where the gas is heated to a temperature required by one or more of the gas engines.

However, when LNG is extracted from LNG tank 2, the pressure in LNG tank 2 will be reduced. As it is desirable to maintain the pressure in the LNG tank 2 at least above a minimum pressure required by the one or more gas engines, some LNG is withdrawn from the LNG tank 2 and fed to the pressure build-up unit 12 to vaporize and heat up the LNG. This can either be done by using the gravity feed line 16 or by using the pump 11 to feed the LNG to the pressure building unit 12.

The vaporized and heated LNG will then be returned to the top of the LNG tank 2 through the inlet line 3 and sub-line 3a, to increase the pressure in the LNG tank 2. Alternatively, the vaporized and heated LNG can be returned to the bottom of the LNG - the tank 2, through the inlet line 3 and sub-line 3b, thereby adding heat to the LNG.

In order to avoid a situation where the gas pressure in the LNG tank 2 is reduced below the minimum consumer supply pressure, for example during sloshing or other movements of the liquid in the LNG tank 2, the pump 11 is used. The pump 11 will then ensure a sufficient supply of gas to one or several engines regardless of the low saturation pressure of the LNG in the LNG tank 2. The LNG will then be drawn from the LNG tank 2 through the outlet line 4 and the pump inlet line 6 and fed into the pump 11, after which the LNG is fed to the evaporator 13 and/ or the PBU 12 and back to the LNG tank 2 through the pump return line 5 and sub-line 5a, to deliver gas to the consumer and/or increase the pressure in the LNG tank 2. Alternatively, the LNG can be returned to the bottom of the LNG tank 2, through the pump return line 5 and the partial line 5b, thereby adding heat to the LNG. When the LNG has reached the saturation pressure and desired temperature, the pump 11 is switched off and the gas supply is maintained through the gravity feed line.

Figure 2 shows another embodiment of the system according to the present invention which can be used with a fuel system for a vessel: the LNG tank 2 is connected to a pressure build-up unit 12 through a gravity feed line 16 and an inlet line 3. The inlet line 3 is divided into two sub-lines 3a, 3b, where the partial conduit 3a is arranged to be on the top or in an upper area of the LNG tank 2, while the other partial conduit 3b is arranged to extend towards the bottom of the LNG tank 2.

A pump 11 is submerged in and arranged near the bottom of the LNG tank 2, and is connected to a vaporizer (VAP) 13 through an outlet line 20, a pump outlet line 7 and a vaporizer inlet line 8. The pump 11 is also connected to the LNG tank 2 through a pump return line 5, so that LNG can be fed back to the LNG tank 2. The pump return line 5 is further divided into two partial lines 5a, 5b, where the partial line 5a is arranged to be on top or in the upper area of the LNG tank 2, and the second part line 5b is arranged to extend towards the bottom of the LNG tank 2, so as to be able to return LNG either to the top of the LNG tank 2, to the bottom of the LNG tank 2, or both to the top and bottom of the LNG tank 2.

The pump 11 and the pressure build-up unit 12 are also in fluid communication with each other through the outlet line 20, the pump outlet line 7 and a pressure build-up line 10.

A line 18 will put the evaporator (VAP) 13 and a superheater (SH) in fluid communication with each other, whereby a gas from the superheater 14 can be delivered to an end user or consumer 15 through a line 19.

To supply gas to the engine(s), according to this embodiment, liquefied natural gas (LNG) is drawn from the LNG tank 2 via the gravity feed line 16 by gravity and directed to the evaporator 13, after which heat is applied to the LNG to transfer natural gas to a gas phase. The gas is then fed to the superheater 14, where the gas is heated to a temperature required by one or more gas engines.

However, when LNG is extracted from the LNG tank 2, the pressure in the LNG tank 2 will be reduced. As it is desirable to maintain the pressure in the LNG tank 2 at least above a minimum pressure required by the one or more gas engines, some LNG is extracted from the LNG tank 2 and fed to the pressure building unit 12 to vaporize and heat the LNG 'one. This can either be done by using the gravity feed line 16 or by using the pump 11 to feed the LNG to the pressure build-up unit 12.1 in this case the LNG will be fed through the outlet line 20, the pump outlet line 7 and the pressure build-up line 10.

The vaporized and heated LNG will then be returned to the top of the LNG tank 2 through the inlet line 3 and sub-line 3a, to increase the pressure in the LNG tank 2. Alternatively, the vaporized and heated LNG can be returned to the bottom of the LNG- the tank 2, through the inlet line 3 and sub-line 3b, thereby adding heat to the LNG. It should be understood that the LNG can be returned to both the top and the bottom of the LNG tank 2, where both partial lines 3a, 3b are used.

In order to avoid a situation where the gas pressure in LNG tank 2 is reduced to below the minimum consumer supply pressure, for example during sloshing or other movements of the liquid in LNG tank 2, pump 11 is used. Pump 11 will then ensure a sufficient supply of gas to one or more engines regardless of low saturation pressure in the LNG in LNG tank 2. The LNG will then be drawn from the LNG tank 2 through the outlet line 4 and the pump inlet line 6 and fed into the pump 11, after which the LNG is fed to the evaporator and /or the PBU and back to the LNG tank 2 through the pump return line 5 and sub-line 5a, to deliver gas to the consumer and/or increase the pressure in the LNG tank 2. Alternatively, the LNG can be returned to the bottom of the tank 2, through the pump return line 5 and sub-line 5b, thereby adding heat to the LNG.

When the LNG has reached the saturation pressure and the desired temperature, the pump 11 is switched off, and the gas supply is maintained only by gravity feed mode.

Figure 3 shows how two systems as described according to Figure 1 can be connected to form a common system. Each of the systems can then function independently of each other, i.e. to deliver gas to its own end user or consumer 15, or to cooperate with each other, or to be a reserve system for each other, thereby also providing a redundancy in the common system .

The joint system comprises a first system IA and a second system IB, where the first and second systems IA, IB are described according to figure 1.

As can be seen, the gravity feed lines 16 of the first and second systems IA, IB are connected via a connecting line 16A. Furthermore, the dividing point between the lines 7, 8, 9 and 10 in the first and second system IA, IB is also connected through another connection line 20A. A line 18A is connected to the line lines 18 in the first and second systems IA, IB, whereby LNG from the second system IB, after being transferred to the gas in the vaporizer 13, can be fed to the line 18 in the first system IA. Correspondingly, gas heated in the superheater 14 in the second system IB will be fed from a line 19 in the second system IB, to a line 19 in the first system IA, thereby feeding heated gas to an end user or consumer 15.

The connecting line 20a can also be connected to either the line 18A or the line 19, to be able to feed the gas either after the evaporator 13 or the superheater 14.

The common system is operated in the same way as described for the system according to figure 1, i.e. that both the first and second systems IA, IB when they are run independently of each other. The first and second systems IA, IB can also cooperate with each other, so that gas from the second system IB can be transferred to the first system IA through one or more of the connection lines 16A, the connection line 20A, the line 18A and/or the line 19, depending on where the gas is to be transferred to in the first system IA.

The common system may include one or more of the described connecting pipes, and in other combinations, depending on the customer's requirements.

The present invention has now been explained with reference to embodiments, but a person skilled in the field will understand that changes and modifications can be made to these embodiments that lie within the scope of the invention as defined in the following claims.

Claims (18)

1. A system (1) for conditioning LNG in a fuel system, where the system also supplies natural gas (NG), where the system comprises at least an LNG tank (2), a pump (11) and a pressure building unit (12), where the pump (11) is in fluid communication with the LNG tank (2) through a tank outlet line (4), a pump inlet line (6, 17) and at least one pump return line (5), where the pressure building unit (12) is in fluid communication with the LNG the tank (2) through at least one tank inlet line (3), tank outlet line (4), gravity feed lines (9, 10) and/or gravity feed line (16, 9, 10), where the pump (11) and the pressure build-up unit (12) are further in fluid communication through a pump outlet line (7) and a pressure build-up line (10). 2. System according to claim 1, characterized in that the pump (11) is submerged in the LNG tank (2). 3. System according to any of the preceding claims 1-2, characterized in that the system further comprises an evaporator (13) and/or a superheater (14). 4. System according to claims 1-2, characterized in that the evaporator (13) is in fluid communication with the LNG tank (2) through an outlet line (4), a gravity feed line (9) or (16) and the evaporator inlet line (8), where the evaporator further is in fluid communication with a pump (11) through a pump outlet line (7) and the evaporator inlet line (8). 5. System according to claims 1-3, characterized in that the superheater (14) is in fluid communication with the evaporator (13) through a line (18). 6. System according to claims 1-4, characterized in that the inlet line (3) is divided into two partial lines (3a, 3b), where a first partial line (3a) is connected to a top of the LNG tank (2), and a second partial line (3b) extends to a bottom of the LNG tank (2). 7. System according to claims 1-5, characterized in that the pump return line (5) is divided into two partial lines (5a, 5b), where a first partial line (5a) is connected to a top of the LNG tank (2), and a second partial line (5b) extends to a bottom of the LNG tank (2). 8. System according to any one of claims 1-6, characterized in that one or more of the pressure building unit (12), the evaporator (13) and the superheater (14) are combined in a common unit. 9. System according to claims 1-7, characterized in that the system comprises one or more additional pump(s) (11). 10. System according to claim 1, characterized in that the system comprises one or more additional pressure build-up unit(s) (12). 11. System according to claims 1-8, characterized in that the system comprises one or more additional evaporator(s) (13) and/or superheater(s) (14). 12. System according to any one of claims 1-10, characterized in that the inlet line (3) and the pump return line (5) are connected in a common line. 13. System according to any of the preceding claims 1-11, characterized in that one or more internal return lines arranged in the LNG tank (2) are connected to a spray line, where the spray line comprises a horizontal pipe provided with a number of holes along the at least part of the length of the pipe, where the number of holes is arranged in one or more radial directions. 14. A combined system comprising two or more of the system according to claims 1-13. 15. A method for conditioning LNG in a fuel system, where the method also supplies natural gas, characterized in that the method comprises the following steps: storing LNG in at least one LNG tank (2), transferring a quantity of LNG from the at least one The LNG tank (2) to a pump (11) when a pressure in at least one LNG tank (2) falls below a certain pressure, to use the pump (11) to raise the pressure in the LNG, to transfer all or parts of the pressurized LNG to a pressure building unit (12) to heat the LNG and to transfer the fluid to the at least one LNG tank (2). 16. Method according to claim 14, characterized in that the method further comprises the steps: returning steam to a top of the LNG tank (2) to thereby regulate the pressure in the at least one LNG tank (2), and/or returning the fluid to a bottom of the LNG tank (2), thereby heating the LNG in the at least one LNG tank (2). 17. Method according to claims 14-15, characterized in that the method further comprises the step: transferring all or parts of the pressurized LNG to a vaporizer (13). 18. Method according to claims 14-16, characterized in that the method further comprises the step: transferring all or parts of the pressurized LNG to a superheater (14).