Blowdown method for a shipboard reliquefaction system

Abstract

A blowdown method for a marine reliquefaction system is disclosed. In the blowdown method of the present invention, evaporated gas generated in a storage tank is compressed by a compressor, the compressed gas is supplied to a heat exchanger through a reliquefaction line to be cooled, and the cooled and reliquefied reliquefied gas is supplied to a gas-liquid separator to be separated and then recovered in a storage tank. A pressure compensation line is provided downstream of the compressor of the reliquefaction line, bypassing the heat exchanger and connected to an upper portion of the gas-liquid separator, and a nitrogen blanket line is provided to supply nitrogen gas to the pressure compensation line. When a trip occurs in the reliquefaction system, nitrogen gas is supplied to the pressure compensation line from the nitrogen blanket line, and the nitrogen gas supplied to the pressure compensation line is sent downstream of the compressor to supply nitrogen gas to the heat exchanger through the reliquefaction line, and the heat exchanger and the reliquefaction line are purged with nitrogen, and the compressed gas and the reliquefied gas remaining in the heat exchanger and the reliquefaction line are discharged.

Description

Technical Field

[0001] The present invention relates to a blow-down method for a shipboard re-liquefaction system. More specifically, the present invention relates to a re-liquefaction system for re-liquefying evaporation gas generated in a storage tank provided on a ship, and to a blow-down method for a shipboard re-liquefaction system that quickly discharges (blow-down) compressed gas and re-liquefied gas remaining in a heat exchanger and a re-liquefaction line provided in the re-liquefaction system when a trip occurs in the re-liquefaction system, thereby preventing damage to the heat exchanger.

Background Art

[0002] Natural gas, which mainly consists of methane and emits almost no environmental pollutants when burned, has attracted attention as an environmentally friendly fuel. Liquefied natural gas (LNG) is obtained by cooling natural gas to approximately -163°C at normal pressure (standard atmospheric pressure) to liquefy it. Since the volume of liquefied natural gas decreases to approximately 1 / 600 compared to gaseous natural gas, it is very suitable for long-distance transportation using sea routes. For these reasons, natural gas is mainly stored and transported in the liquid state of LNG, which is advantageous for storage and transportation.

[0003] Since the liquefaction point of natural gas is extremely low at approximately -163°C at normal pressure, LNG storage tanks are usually thermally insulated to maintain LNG in a liquid state. However, even with thermal insulation, it is difficult to completely block external heat. Therefore, due to the continuous transfer of external heat to the LNG storage tank, the LNG in the LNG storage tank naturally vaporizes during the LNG transportation process, generating evaporation gas (BOG; Boil-Off Gas).

[0004] If evaporation gas continuously occurs in the LNG storage tank, the pressure in the LNG storage tank will rise. When the pressure in the storage tank exceeds the set safety pressure, there is a risk of emergency situations such as tank rupture (Rupture). Therefore, it is necessary to use a safety valve to discharge the evaporation gas outside the storage tank. However, evaporation gas is one of the LNG losses, and since it is an important issue in terms of LNG transportation efficiency and fuel efficiency, various methods for treating the evaporation gas generated in the storage tank are used.

[0005] In recent years, methods such as using evaporation gas at fuel demand destinations such as ship engines, re-liquefying evaporation gas and recovering it into the storage tank, or using a combination of these two methods have been developed and used.

Summary of the Invention

Problems to be Solved by the Invention

[0006] When applying a re-liquefaction cycle for re-liquefying evaporation gas to a ship, typical re-liquefaction cycles such as the SMR cycle and the C3MR cycle are known. The C3MR cycle (Propane-precooled Mixed Refrigerant Cycle) cools evaporation gas using a single refrigerant of propane and then cools and re-liquefies it using a mixed refrigerant. The SMR cycle (Single Mixed Refrigerant Cycle) re-liquefies evaporation gas using a mixed refrigerant composed of multiple components.

[0007] In these SMR cycles and C3MR cycles, a mixed refrigerant is used, and the refrigerant leaks as the liquefaction process progresses. As a result, the liquefaction efficiency decreases due to the change in the composition ratio of the mixed refrigerant. Therefore, it is necessary to continuously measure the composition ratio of the mixed refrigerant and maintain the composition of the refrigerant by replenishing the insufficient refrigerant components.

[0008] Also, as another re-liquefaction method using a re-liquefaction cycle, a single-cycle re-liquefaction method using a nitrogen refrigerant is known.

[0009] Although the cooling efficiency of nitrogen refrigerant is lower than that of a refrigeration cycle using a mixed refrigerant, nitrogen refrigerant is an inert substance, so it has high safety. Also, since there is no phase change of the refrigerant, it has the advantage of being easily applicable to ships.

[0010] The reliquefaction system includes a compressor that is supplied with evaporation gas and compresses the supplied evaporation gas, a heat exchanger that cools the compressed gas compressed by the compressor through heat exchange with a refrigerant, and a refrigerant circulation section through which the refrigerant used for heat exchange with the compressed gas circulates in the heat exchanger. Further, in the case of a reliquefaction system to which a refrigeration cycle using nitrogen refrigerant is applied, in the refrigerant circulation section, after the nitrogen refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger is compressed, it is supplied to the heat exchanger and cooled, further cooled by expansion, and then supplied to the heat exchanger again, and the nitrogen refrigerant circulates.

[0011] By the way, during abnormal operation of the reliquefaction system, if the flow of the fluid flowing in and out of the heat exchanger cannot be quickly controlled, excessive thermal stress is applied to the heat exchanger that is sensitive to temperature changes, which may lead to damage to the heat exchanger due to fatigue failure or the like and shortening of the device life of the heat exchanger.

[0012] The present invention solves such problems and provides a method for quickly controlling the fluid flowing in and out of the heat exchanger during abnormal operation of the reliquefaction system to prevent damage to the heat exchanger.

Means for Solving the Problems

[0013] In order to solve the above problems, in an embodiment of the present invention, there is provided a blowdown method for a ship re-liquefaction system. The ship re-liquefaction system compresses evaporation gas generated from liquefied gas stored in a storage tank of a ship by a compressor, supplies the compressed gas with the evaporation gas compressed to a heat exchanger from a re-liquefaction line for cooling, supplies the re-liquefied gas cooled and re-liquefied to a gas-liquid separator for separation, and then recovers it to the storage tank. A pressure compensation line is provided in the ship re-liquefaction system, which bypasses the heat exchanger from the downstream side of the compressor in the re-liquefaction line and is connected to the upper part of the gas-liquid separator, and a nitrogen blanket line for supplying nitrogen gas to the pressure compensation line is provided. When a trip occurs in the re-liquefaction system, nitrogen gas is supplied from the nitrogen blanket line to the pressure compensation line, the nitrogen gas supplied to the pressure compensation line is sent to the downstream side of the compressor in the re-liquefaction line, nitrogen gas is supplied to the heat exchanger through the re-liquefaction line, nitrogen purge is performed on the heat exchanger and the re-liquefaction line, and compressed gas and re-liquefied gas staying in the heat exchanger and the re-liquefaction line are discharged. There is provided a blowdown method for a ship re-liquefaction system, which is characterized by the above.

[0014] Further, in the present invention, a bypass line is provided in the ship re-liquefaction system, which bypasses the gas-liquid separator from the downstream side of the heat exchanger in the re-liquefaction line and is connected to the storage tank, a first valve is provided downstream of the branch point of the bypass line in the re-liquefaction line, and a second valve is provided in the bypass line. It is preferable that the compressed gas, the re-liquefied gas, and the nitrogen gas discharged from the heat exchanger and the re-liquefaction line are discharged from the bypass line.

[0015] Further, in the present invention, it is preferable that a third valve is further provided in the ship re-liquefaction system upstream of the connection point of the pressure compensation line with the nitrogen blanket line, a fourth valve is provided in the nitrogen blanket line, and a fifth valve is provided downstream of the connection point of the pressure compensation line with the nitrogen blanket line.

[0016] In the present invention, it is also preferable that the refrigerant circulating in the refrigerant circulation line is supplied as the refrigerant for cooling the compressed gas to the heat exchanger, and the uncompressed evaporation gas discharged from the storage tank and supplied to the compressor is supplied to the heat exchanger and then to the compressor, and the compressed gas is cooled by heat exchange with the refrigerant and the uncompressed evaporation gas in the heat exchanger.

[0017] In the present invention, it is also preferable that the refrigerant circulating in the refrigerant circulation line is nitrogen, and the heat exchanger is a brazed aluminum heat exchanger.

[0018] In the present invention, it is also preferable that the pressure in the gas-liquid separator is maintained by the flash gas generated from the re-liquefied gas supplied to the gas-liquid separator, and the re-liquefied gas in the gas-liquid separator is supplied to the storage tank. When the re-liquefied gas supplied into the gas-liquid separator is subcooled and the pressure in the gas-liquid separator cannot be maintained by the flash gas, it is preferable to supply evaporation gas or nitrogen gas to the gas-liquid separator from the pressure compensation line to maintain the pressure in the gas-liquid separator.

Advantages of the Invention

[0019] In the present invention, during abnormal operation such as when a trip occurs in the re-liquefaction system, nitrogen purging is performed on the heat exchanger and the re-liquefaction line, and the compressed gas and re-liquefied gas remaining in the heat exchanger and the re-liquefaction line are quickly discharged, thereby reducing the thermal stress applied to the heat exchanger, reducing the thermal fatigue of the heat exchanger, and preventing damage to the heat exchanger.

[0020] In addition, by using the existing equipment without adding and installing other equipment, etc., the installation cost and installation space of the re-liquefaction system can be reduced, and by preventing damage to the heat exchanger and the pipes connected to the heat exchanger, the maintenance cost can be reduced, and the safety of the ship can be improved.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] Hereinafter, with reference to the drawings and the content described in the drawings, the operational advantages of the present invention and the objectives achieved by the embodiments of the present invention will be described by taking the embodiments of the present invention as examples.

[0023] Hereinafter, with reference to the drawings, the configuration and operation of the embodiments of the present invention will be described. Regarding the reference numerals attached to the components in each drawing, the same components will be denoted by the same numerals as much as possible even if they are shown on other drawings.

[0024] As the ship of the embodiment of the present invention to be described later, it can be any type of ship provided with a storage tank for storing liquefied gas. Typical examples include ships with self-propulsion capabilities such as LNG carriers, liquid hydrogen carriers, and LNG regasification vessels, as well as offshore floating structures without propulsion capabilities such as LNG FPSOs (Floating Production Storage Offloading) and LNG FSRUs (Floating Storage Regasification Unit).

[0025] In addition, this embodiment can be applied to the reliquefaction cycle of all types of liquefied gases that can liquefy gas at a low temperature for transportation and generate evaporation gas during storage. Such liquefied gases include, for example, liquefied gases such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), Liquefied Ethylene Gas, and Liquefied Propylene Gas. In the embodiments described later, LNG, which is one of the representative liquefied gases, will be described as an example.

[0026] FIG. 1 schematically shows an operation example when nitrogen gas is supplied from a nitrogen blanket line to maintain the pressure in the gas-liquid separator of a shipboard reliquefaction system to which the blowdown method of the embodiment of the present invention is applied. FIG. 2 schematically shows an operation example when the present invention is applied for blowdown when a trip occurs in the reliquefaction system shown in FIG. 1.

[0027] First, in the shipboard reliquefaction system to which this embodiment is applied, evaporation gas generated from the liquefied gas stored in the storage tank CH of the ship is discharged from the vapor header, supplied to a compressor, compressed, and supplied as fuel to an in-ship engine or the like as necessary. Then, the evaporation gas remaining without being supplied as fuel is supplied from the reliquefaction line RL to the heat exchanger 100, cooled in the heat exchanger 100 to be reliquefied, and then the reliquefied gas is returned to the storage tank CH.

[0028] In this reliquefaction system, evaporation gas generated from the liquefied gas stored in the storage tank CH of the ship is discharged from a vapor header (not shown) and supplied to a compressor (not shown) from a gas supply line (not shown). The gas supply line connects the storage tank CH and the compressor via a heat exchanger, and the uncompressed evaporation gas discharged from the storage tank CH and before being supplied to the compressor is supplied to the heat exchanger as a refrigerant.

[0029] The evaporated gas compressed by the compressor is supplied back to the heat exchanger 100 and cooled by the cold heat of the uncompressed evaporated gas supplied from the gas supply line.

[0030] In addition to the uncompressed evaporated gas, other refrigerants circulating in a refrigerant circulation line (not shown) can also be supplied to the heat exchanger 100. As the refrigerant circulating in such a refrigerant circulation line, for example, nitrogen (N2) is used. Further, a refrigerant compressor for compressing the nitrogen refrigerant and a refrigerant expander for expanding the nitrogen refrigerant compressed by the refrigerant compressor are provided in the refrigerant circulation line. The nitrogen refrigerant in the refrigerant circulation line is compressed by the refrigerant compressor, supplied to the heat exchanger 100, cooled, then expanded and cooled by the refrigerant expander, and then supplied back to the heat exchanger 100 as a refrigerant to circulate in the refrigerant circulation line. Thereby, in the heat exchanger 100, four flows, namely, the evaporated gas compressed by the compressor, the uncompressed evaporated gas before being supplied to the compressor, the refrigerant cooled by expansion by the refrigerant expander, and the refrigerant compressed by the refrigerant compressor, are heat-exchanged.

[0031] Further, the heat exchanger 100 uses a cryogenic heat exchanger according to the refrigeration cycle of the evaporated gas generated from the extremely low temperature LNG and the nitrogen refrigerant. For example, a brazed aluminium heat exchanger (BAHE) is used.

[0032] The evaporated gas cooled by the heat exchanger 100 is supplied to the gas-liquid separator 200 for gas-liquid separation, and the separated reliquefied gas is recovered in the storage tank CH. Further, a bypass line BL is provided which branches from the re-liquefaction line RL downstream of the heat exchanger 100, bypasses the gas-liquid separator 200, and is connected to the storage tank CH. By means of the bypass line BL, the reliquefied gas cooled by the heat exchanger 100 can also be directly supplied to the storage tank CH. Further, a first valve V1 is provided downstream of the branch point of the bypass line BL of the re-liquefaction line RL. Also, a second valve V2 for opening and closing the bypass line BL is provided in the bypass line BL.

[0033] By the way, when the valve LV provided downstream of the gas-liquid separator 200 is opened to supply the reliquefied gas separated by the gas-liquid separator 200 to the storage tank CH, the pressure inside the gas-liquid separator 200 changes. Even in such a case, the pressure inside the gas-liquid separator 200 is maintained by the flash gas (i.e., off-gas) generated from the reliquefied gas supplied into the gas-liquid separator 200, so that the reliquefied gas inside the gas-liquid separator 200 can be supplied to the storage tank CH.

[0034] However, when the liquefied gas cooled by heat exchange with the nitrogen refrigerant is supercooled and supplied into the gas-liquid separator 200, almost no off-gas is generated. When the valve LV downstream of the gas-liquid separator 200 is opened, the pressure inside the gas-liquid separator 200 may rapidly decrease. In the reliquefaction system to which this embodiment is applied, in order to compensate for the pressure inside the gas-liquid separator 200 and maintain the pressure inside the gas-liquid separator 200, a pressure compensation line PL that branches from the reliquefaction line RL downstream of the compressor of the reliquefaction line RL and is connected to the upper part of the gas-liquid separator 200, and a nitrogen blanket line NBL that supplies nitrogen gas to the pressure compensation line PL are provided. Thereby, even when the reliquefied gas inside the gas-liquid separator 200 is supplied to the storage tank CH, the pressure inside the gas-liquid separator 200 can be maintained by supplying evaporation gas or nitrogen gas to the gas-liquid separator 200 from the pressure compensation line PL.

[0035] A third valve V3 is provided upstream of the connection point of the pressure compensation line PL with the nitrogen blanket line NBL. Further, a fourth valve V4 is provided in the nitrogen blanket line NBL. Further, a fifth valve V5 is provided downstream of the connection point of the pressure compensation line PL with the nitrogen blanket line NBL.

[0036] As shown in FIG. 1, in the re-liquefaction system to which the present embodiment is applied, in an operation example where nitrogen gas is supplied from the nitrogen blanket line NBL to maintain the pressure in the gas-liquid separator 200, the third valve V3 is closed, the fourth valve V4 and the fifth valve V5 are opened, and nitrogen gas is supplied from the nitrogen blanket line NBL to the upper part of the gas-liquid separator 200. By supplying nitrogen gas to the upper part of the gas-liquid separator 200 in this way and maintaining the pressure in the gas-liquid separator 200, the re-liquefied gas in the gas-liquid separator 200 can be smoothly supplied to the storage tank CH.

[0037] Here, usually, the temperature of the evaporation gas discharged from the storage tank CH and supplied to the heat exchanger 100 is about -100°C, and depending on the state of the storage tank CH, it may be -130°C or lower. For example, when the temperature of the evaporation gas changes rapidly, such as at the initial startup of the re-liquefaction system, or when the temperature difference between the temperature of the heat exchanger 100 and the temperature of the evaporation gas is large, excessive thermal stress may be applied to the heat exchanger 100. In particular, when the flow of the fluid flowing in and out of the heat exchanger 100 cannot be quickly controlled during abnormal operation of the re-liquefaction system, excessive thermal stress is applied to the heat exchanger 100, which may lead to damage to the heat exchanger 100 due to fatigue failure or the like and shortening of the device life of the heat exchanger 100. In order to prevent such damage to the heat exchanger 100, in the present embodiment, during abnormal operation of the re-liquefaction system, the compressed gas and the re-liquefied gas remaining in the heat exchanger 100 and the pipes connected to the heat exchanger 100 are quickly discharged (blow-down).

[0038] FIG. 2 schematically shows an operation example when blow-down is performed during abnormal operation of the re-liquefaction system, particularly when a trip occurs, according to the blow-down method of the present embodiment.

[0039] In the present embodiment, nitrogen purging is performed on the heat exchanger 100 and the re-liquefaction line RL by using the pressure compensation line PL and the nitrogen blanket line NBL provided to maintain the pressure in the gas-liquid separator 200, and the compressed gas and the re-liquefied gas remaining in the heat exchanger 100 and the re-liquefaction line RL are discharged.

[0040] That is, when a trip occurs in the reliquefaction system, the fourth valve V4 and the third valve V3 are opened to supply nitrogen gas from the nitrogen blanket line NBL to the pressure compensation line PL, and the nitrogen gas supplied to the pressure compensation line PL is sent to the downstream side of the compressor in the reliquefaction line RL, and nitrogen gas is supplied to the heat exchanger 100 through the reliquefaction line RL. Further, nitrogen purging is performed on the heat exchanger 100 and the reliquefaction line RL, the first valve V1 provided upstream of the gas-liquid separator 200 is closed, the second valve V2 provided in the bypass line BL is opened, and the compressed gas and the reliquefied gas staying in the heat exchanger 100 and the reliquefaction line RL are discharged from the bypass line BL (blow-down).

[0041] As described above, when the reliquefaction system is operating abnormally, nitrogen purging is performed on the heat exchanger 100 and the reliquefaction line RL, and the compressed gas and the reliquefied gas staying in the heat exchanger 100 and the reliquefaction line RL are quickly discharged, thereby reducing the thermal stress applied to the heat exchanger 100, reducing the thermal fatigue of the heat exchanger 100, and preventing damage to the heat exchanger 100.

[0042] In particular, it is not necessary to add and install other equipment, etc., and by using the existing equipment, the installation cost and installation space of the reliquefaction system can be reduced, and by preventing damage to the heat exchanger 100 and the piping connected to the heat exchanger 100, the maintenance cost can be reduced, and the safety of the ship can be improved.

[0043] The present invention is not limited to the above-described embodiments, and it is obvious to those skilled in the technical field to which the present invention pertains that various changes or modifications can be made within the scope not exceeding the technical gist of the present invention.

Claims

1. A blowdown method for a ship re-liquefaction system, comprising: The ship re-liquefaction system compresses evaporation gas generated from liquefied gas stored in a storage tank of a ship by a compressor, supplies the compressed gas with the evaporation gas compressed to a heat exchanger through a re-liquefaction line for cooling, supplies the re-liquefied gas cooled and re-liquefied to a gas-liquid separator for separation, and then recovers it to the storage tank. In the ship re-liquefaction system, from the downstream side of the compressor in the re-liquefaction line, a pressure compensation line that bypasses the heat exchanger and is connected to the upper part of the gas-liquid separator, a nitrogen blanket line that supplies nitrogen gas to the pressure compensation line, and from the downstream side of the heat exchanger in the re-liquefaction line, a bypass line that bypasses the gas-liquid separator and is connected to the storage tank; a first valve provided downstream of the branch point of the bypass line in the re-liquefaction line; and a second valve provided in the bypass line are provided. When a trip occurs in the re-liquefaction system, nitrogen gas is supplied from the nitrogen blanket line to the pressure compensation line, the nitrogen gas supplied to the pressure compensation line is sent to the downstream side of the compressor in the re-liquefaction line, nitrogen gas is supplied to the heat exchanger through the re-liquefaction line, nitrogen purging is performed on the heat exchanger and the re-liquefaction line, and the compressed gas and re-liquefied gas staying in the heat exchanger and the re-liquefaction line are discharged. The compressed gas, the re-liquefied gas, and the nitrogen gas discharged from the heat exchanger and the re-liquefaction line are discharged from the bypass line. A blowdown method for a ship re-liquefaction system.

2. In the ship re-liquefaction system, a third valve provided upstream of the connection point of the pressure compensation line with the nitrogen blanket line; a fourth valve provided in the nitrogen blanket line; and a fifth valve provided downstream of the connection point of the pressure compensation line with the nitrogen blanket line are further provided. The blowdown method for a ship re-liquefaction system according to Claim 1.

3. Refrigerant circulating in a refrigerant circulation line is supplied to the heat exchanger as a refrigerant for cooling the compressed gas. The uncompressed evaporation gas discharged from the storage tank and before being supplied to the compressor is supplied to the heat exchanger and then supplied to the compressor. In the heat exchanger, cooling the compressed gas by heat exchange with the refrigerant and the uncompressed evaporation gas, characterized in that The blowdown method of the ship re-liquefaction system according to claim 2.

4. The refrigerant circulating in the refrigerant circulation line is nitrogen, The heat exchanger is an aluminum brazed heat exchanger, characterized in that The blowdown method of the ship re-liquefaction system according to claim 3.

5. Maintaining the pressure in the gas-liquid separator with the flash gas generated from the re-liquefied gas supplied to the gas-liquid separator, and supplying the re-liquefied gas in the gas-liquid separator to the storage tank, When the re-liquefied gas supplied into the gas-liquid separator is subcooled and the pressure in the gas-liquid separator cannot be maintained by the flash gas, supplying evaporation gas or nitrogen gas to the gas-liquid separator from the pressure compensation line to maintain the pressure in the gas-liquid separator, characterized in that The blowdown method of the ship re-liquefaction system according to any one of claims 1 to 4.

Images