Purge system and method for marine evaporative gas reliquefaction equipment

Abstract

A purge system and a purge method for a reliquefaction device for evaporated gas of a ship are disclosed. The purge system for the reliquefaction device for evaporated gas of a ship of the present invention includes a storage tank installed on the ship to store liquefied gas at a low temperature and equipped with an insulating part having a vacuum insulation layer, a compressor for compressing evaporated gas generated in the storage tank, a heat exchanger for cooling the compressed gas compressed by the compressor, and a refrigerant circulation line through which a refrigerant for cooling the compressed gas in the heat exchanger circulates. When performing nitrogen purging of the reliquefaction device, a vacuum pump installed to maintain a vacuum state in the vacuum insulation layer of the storage tank sucks and discharges gas in the refrigerant circulation line, and then supplies nitrogen gas into the refrigerant circulation line.

Description

Technical Field

[0001] The present invention relates to a purge system and a purge method for a re-liquefaction device of evaporation gas of a ship. More specifically, when purging the re-liquefaction device, in order to maintain the vacuum state in the vacuum insulation layer of the storage tank, a vacuum pump provided for this purpose sucks and discharges the gas in the refrigerant circulation line, and then relates to a purge system and a purge method for a ship re-liquefaction device that performs nitrogen purging (N2 purging).

Background Art

[0002] Natural gas, which mainly consists of methane and emits almost no environmental pollutants during combustion, has attracted attention as an environmentally friendly fuel. Liquefied natural gas (LNG) is obtained by cooling natural gas to about -163°C at normal pressure and liquefying it. Compared with gaseous natural gas, its volume decreases to about 1 / 600, making it 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 about -163°C at normal pressure, LNG storage tanks are usually heat-insulated to keep LNG in a liquid state. However, even with heat 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 boil-off gas (BOG).

[0004] If evaporative gases continue to be generated in an LNG storage tank, the pressure inside the tank will rise. If the pressure inside the storage tank exceeds the set safety pressure, there is a risk of emergencies such as tank rupture, so it is necessary to use a safety valve to discharge the evaporative gases to the outside of the storage tank. However, since evaporative gases are one of the LNG losses and are a significant issue in terms of LNG transport efficiency and fuel efficiency, various methods are used to treat the evaporative gases generated in storage tanks.

[0005] In recent years, methods have been developed and are being used at fuel demand sites such as ship engines, including methods for using evaporated gas, methods for reliquefying evaporated gas and recovering it in storage tanks, and methods that combine these two approaches.

[0006] When applying a reliquefaction cycle to a ship to re-liquefy evaporated gas, typical reliquefaction cycles include the SMR cycle and the C3MR cycle. The C3MR cycle (Propane-precooled Mixed Refrigerant Cycle) cools the evaporated gas using a single propane refrigerant, and then cools and re-liquefies it using a mixed refrigerant, while the SMR cycle (Single Mixed Refrigerant Cycle) re-liquefies the evaporated gas using a mixed refrigerant composed of multiple components.

[0007] These SMR and C3MR cycles use a mixed refrigerant, and refrigerant leakage occurs as the liquefaction process progresses. This changes the composition ratio of the mixed refrigerant, reducing liquefaction efficiency. Therefore, it is necessary to continuously measure the composition ratio of the mixed refrigerant and replenish any deficient refrigerant components to maintain the refrigerant's composition.

[0008] Another known reliquefaction method that utilizes a reliquefaction cycle is a single-cycle reliquefaction method that uses nitrogen as a refrigerant.

[0009] While nitrogen refrigerants have lower cooling efficiency compared to refrigeration cycles using mixed refrigerants, they offer advantages such as high safety due to their inert nature and ease of application to ships because they do not cause phase changes in the refrigerant.

[0010] All equipment and piping on a ship undergoes nitrogen purging (N2 purging) immediately after manufacture or before the initial start-up after equipment maintenance. This is done to reduce the oxygen concentration within the equipment and piping, and to prevent moisture in the air from cooling and condensing.

[0011] In particular, in the reliquefaction cycle, where evaporated gas generated from cryogenic LNG is cooled and reliquefied with a nitrogen refrigerant at a lower temperature than the evaporated gas, residual moisture in each device and piping can condense and freeze, potentially damaging major equipment including heat exchangers, measuring instruments, and piping. Therefore, adjusting the dew point of the gas in each device and piping is extremely important in the reliquefaction cycle. For example, see Patent Document 1. ).

[0012] Conventional purging methods involved repeating the purging process and subsequent dew point checks multiple times until the dew point reached the required set value. However, this method has drawbacks: when the capacity of the equipment and piping constituting the reliquefaction cycle is large, the amount of nitrogen gas required for nitrogen purging increases, and the time required for adjusting the nitrogen gas dew point (N2Dew Point) also increases. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Korean Patent Publication No. 10-2020-0022873 [Overview of the project] [Problems that the invention aims to solve]

[0014] The present invention aims to solve these problems and provide a purging system and purging method that can reduce the amount of nitrogen gas consumed for nitrogen purging, shorten the time required for purging, and enable rapid operation of the reliquefaction cycle. [Means for solving the problem]

[0015] To solve the above problems, an embodiment of the present invention provides: a storage tank installed on a ship for storing liquefied gas at low temperatures and equipped with an insulating section having a vacuum insulating layer; a compressor for compressing the evaporated gas generated in the storage tank; a heat exchanger for cooling the compressed gas compressed by the compressor; and a refrigerant circulation line through which a refrigerant circulates to cool the compressed gas in the heat exchanger. In a purge system for a reliquefaction device comprising: Inside the vacuum insulation layer of the storage tank of vacuum exhaust vacuum pump Furthermore, To supply nitrogen gas into the refrigerant circulation line. in The nitrogen purging of the aforementioned reliquefaction device is performed. Prior to that, by a vacuum pump The gas in the refrigerant circulation line is drawn in and discharged. The system comprises: a hard pipe connecting the vacuum pump and the reliquefaction device; and a flexible pipe connecting the hard pipe to the piping and each component of the reliquefaction device. A purge system for a marine evaporative gas reliquefaction unit is provided, characterized by the following:

[0016] Preferably, the system includes a refrigerant expander provided in the refrigerant circulation line for expanding and cooling the refrigerant supplied to the heat exchanger, and a refrigerant compressor provided in the refrigerant circulation line for compressing the refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger, wherein the refrigerant circulating in the refrigerant circulation line is nitrogen refrigerant, and the refrigerant compressor is driven by the expansion energy of the refrigerant in the refrigerant expander.

[0017] To solve the above problems, the present invention has embodiments The evaporated gas generated from the liquefied gas stored in the ship's storage tanks is compressed, and the compressed gas is cooled and reliquefied by heat exchange with the refrigerant circulating in the refrigerant circulation line. In a purging method for a reliquefaction apparatus, which involves purging the piping and each component of the reliquefaction apparatus with nitrogen, Nitrogen gas is supplied to the refrigerant circulation line. Prior to that, In the aforementioned storage tank to be established A vacuum pump that maintains a vacuum state within a vacuum insulation layer to keep the liquefied gas at a low temperature. Using including the step of sucking and discharging the gas in the refrigerant circulation line nothing There is provided a purging method for a marine evaporation gas re-liquefaction device, which is characterized in this way.

[0018] Also preferably, the nitrogen purge of the re-liquefaction device includes: 1) a vacuum exhaust step of sucking and discharging the gas in the piping and each device of the re-liquefaction device with the vacuum pump; 2) a purge step of supplying nitrogen gas to the piping and each device to discharge the gas in the piping and each device; and 3) a confirmation step of checking the dew point of the gas in the piping and each device, and the nitrogen purge is performed.

[0019] Also preferably, after sequentially performing the steps 1) and 2) two or more times, the step 3) is performed. If the dew point does not decrease to the required set value in the step 3), return to the step 1) and sequentially perform the steps 1) to 3).

[0020] Also preferably, the vacuum pump and the re-liquefaction device are connected by a hard pipe, and when performing the nitrogen purge, the piping and each device of the re-liquefaction device and the hard pipe are connected by a flexible pipe to perform the vacuum exhaust step. Furthermore, the vacuum evacuation in step 1) is performed by operating a vacuum pump to bring the vacuum pressure inside the piping and each device to 7 mbara, and in step 2), nitrogen gas is supplied to bring the pressure inside the piping and each device to 5 barg to discharge the gas inside the piping and each device and purge it. 。

Effect of the Invention

[0021] When performing the purge (Purging) of the re-liquefaction device, the present invention uses a vacuum pump provided for evacuating the vacuum insulation layer of the storage tank to suck and discharge the gas in the piping and the device of the re-liquefaction device, and then supplies nitrogen gas (N2) to perform the purge.

[0022] By using an existing vacuum pump on the ship and evacuating the inside of the piping and the device of the re-liquefaction device prior to the implementation of the nitrogen purge, the amount of nitrogen gas required for the nitrogen purge can be reduced, the dew point of the re-liquefaction device can be adjusted to the target value, the time required for the nitrogen purge can be shortened, and the re-liquefaction device can be quickly operated.

[0023] In particular, in reliquefaction equipment that reliquefies evaporated gas generated from cryogenic LNG by cooling it with a nitrogen refrigerant at a lower temperature than the evaporated gas, residual moisture inside the equipment and piping can condense and freeze, potentially causing malfunctions or damage to key equipment, measuring instruments, and piping. However, this technology can prevent such malfunctions and ensure the safety of the liquefaction equipment and the vessel. [Brief explanation of the drawing]

[0024] [Figure 1] A schematic diagram shows a reliquefaction device for evaporated gases in a ship to which the purge system of the present invention is applied. [Modes for carrying out the invention]

[0025] The operational advantages of the present invention and the objectives achieved by embodiments of the present invention will be described below with reference to the drawings and the contents described in the drawings, using embodiments of the present invention as examples.

[0026] The configuration and operation of embodiments of the present invention will be described below with reference to the drawings. Note that, as far as possible, the same reference numerals will be used for the same components shown in other drawings.

[0027] The vessels of the embodiments of the present invention, described later, can be any type of vessel equipped with a storage tank for storing liquefied gas. Typical examples include self-propelled vessels such as LNG carriers, liquid hydrogen carriers, and LNG RVs (Regasification Vessels), as well as floating offshore structures without propulsion capabilities, such as LNG FPSOs (Floating Production Storage Offloading) and LNG FSRUs (Floating Storage Regasification Units).

[0028] Furthermore, this embodiment allows for the liquefaction and transport of gases at low temperatures and can be applied to the reliquefaction cycle of all types of liquefied gases that generate evaporated gas during storage. Examples of such liquefied gases include 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, one of the representative liquefied gases, will be used as an example.

[0029] Figure 1 schematically shows a reliquefaction device for evaporated gases in a ship to which the purge system of the present invention is applied.

[0030] The evaporative gas reliquefaction apparatus shown in Figure 1 comprises a compressor 100 that compresses evaporative gas supplied from an evaporative gas supply line to a storage tank T installed on a ship, a heat exchanger 200 that cools the compressed gas obtained by the compressor 100, and a refrigerant circulation unit 300 through which a refrigerant used for heat exchange with the compressed gas in the heat exchanger 200 circulates.

[0031] The evaporated gas generated from the liquefied gas stored in storage tank T is supplied to heat exchanger 200, where the cooling energy is recovered, and then supplied to compressor 100. In compressor 100, the evaporated gas is compressed to, for example, the fuel supply pressure of the ship's main engine. For example, if a DF engine is installed, it is compressed to a pressure of 5.5 barg; if an X-DF engine is installed, it is compressed to a pressure of 15 barg; and if an ME-GI engine is installed, it is compressed to a pressure of approximately 300 barg. The compressed evaporated gas is supplied as fuel to the ship's main engine (not shown), and any evaporated gas that is not supplied as fuel is reliquefied.

[0032] Incidentally, according to regulations concerning ships, the compressor that supplies fuel to the engine is required to be designed with redundancy in case of emergencies. In this embodiment, we will mainly explain using a single compressor as an example, but the compressor may be configured to include a main compressor and a backup compressor.

[0033] Downstream of the compressor 100, a reliquefaction line RL is connected to reliquefy the evaporated gas and recover it in a storage tank T. The evaporated gas compressed by the compressor 100 is supplied to the heat exchanger 200 via the reliquefaction line RL and cooled.

[0034] Downstream of the heat exchanger 200 in the reliquefaction line RL, a gas-liquid separator 400 is provided to separate the reliquefied gas into gas and liquid phases. Furthermore, if necessary, a pressure reducing valve is provided upstream of the gas-liquid separator 400 in the reliquefaction line RL to reduce the pressure of the compressed gas cooled by the heat exchanger 200 and adjust the reliquefaction rate.

[0035] The reliquefied gas separated in the gas-liquid separator 400 is supplied to the storage tank T for re-storage. Meanwhile, the flash gas separated in the gas-liquid separator 400 is supplied to the uncompressed evaporative gas flow upstream of the heat exchanger 200 in the evaporative gas supply line, or sent to the gas combustion unit (GCU).

[0036] In the refrigerant circulation section 300, the refrigerant circulates through the refrigerant circulation line CL, and the compressed gas is cooled by heat exchange in the heat exchanger 200.

[0037] The refrigerant circulation unit 300 includes a refrigerant expander 320 that expands and cools the refrigerant supplied to the heat exchanger 200, and a refrigerant compressor 310 connected to the refrigerant expander 320, to which the expansion energy of the refrigerant is transmitted and which compresses the refrigerant discharged from the heat exchanger 200 after heat exchange. A motor (not shown) is also provided to drive the refrigerant compressor 310, and the refrigerant compressor 310 and the refrigerant expander 320 are connected via a shaft, so that the expansion energy of the refrigerant is used to compress the refrigerant. This reduces the power required to drive the refrigeration cycle.

[0038] The refrigerant, cooled by expansion in the refrigerant expander 320, is supplied to the heat exchanger 200 to provide cooling. After heat exchange in the heat exchanger 200, it is discharged from the heat exchanger 200 and compressed in the refrigerant compressor 310. The refrigerant compressed in the refrigerant compressor 310 is supplied to the heat exchanger 200 for cooling, then supplied to the refrigerant expander 320 for further cooling by expansion, and supplied back to the heat exchanger 200, thereby circulating the refrigerant through the refrigerant circulation line CL.

[0039] Therefore, in the heat exchanger 200, heat exchange occurs between four flows: the evaporated gas compressed by the compressor 100, the uncompressed evaporated gas before being supplied to the compressor 100, the refrigerant cooled by expansion in the refrigerant expander 320, and the refrigerant compressed by the refrigerant compressor 310. In other words, in the heat exchanger 200, the compressed gas compressed by the compressor 100 and the refrigerant compressed by the refrigerant compressor 310 are cooled by heat exchange between the uncompressed evaporated gas before being supplied to the compressor 100 and the refrigerant cooled by expansion in the refrigerant expander 320.

[0040] Before the initial start-up of such a reliquefaction unit, nitrogen purging (N2 purging) is performed on each device and piping to reduce the oxygen concentration inside each device and piping, and to prevent moisture in the air from being cooled and condensed.

[0041] The purging system of this embodiment can perform nitrogen purging using such a reliquefaction device.

[0042] In this embodiment, when purging nitrogen from the reliquefaction device, a method is proposed that utilizes a vacuum pump provided to maintain a vacuum state within the vacuum insulation layer of the storage tank T.

[0043] Storage tanks T, which store low-temperature liquefied gases, especially cryogenic LNG, are equipped with an insulation section to maintain the temperature of the liquefied gas at a low level and prevent the intrusion of external heat. This insulation section includes a vacuum insulation layer.

[0044] To maintain the vacuum state within this vacuum insulation layer, a vacuum pump that operates using vacuum pressure is installed on the ship.

[0045] The vacuum pump is not a device that operates continuously; in this embodiment, such a vacuum pump is used for purging the reliquefaction device. Specifically, prior to performing nitrogen purging, the vacuum pump is used to suck out and discharge gas from each pipe, including the refrigerant circulation line CL of the reliquefaction device, and from each device, creating a vacuum (Vacuum) in each pipe and each device, after which nitrogen gas (N2) is supplied to perform the purge. This reduces the amount of nitrogen gas required for nitrogen purging of the reliquefaction device, allows the dew point of the nitrogen gas to be adjusted to the target value, and shortens the time required for nitrogen purging.

[0046] With the purging system of this embodiment, purging of the reliquefaction device is performed as follows.

[0047] A rigid pipe extends from the vacuum pump to the location of the reliquefaction unit, connecting the vacuum pump and the reliquefaction unit. Furthermore, when purging the piping and components of the reliquefaction unit, the rigid pipe is connected to the piping and components of the reliquefaction unit using a flexible pipe.

[0048] Nitrogen purging of a reliquefaction unit involves first connecting the rigid pipe to the piping and equipment of the reliquefaction unit with a flexible pipe, then using a vacuum pump to suck out and remove the gas from the piping and equipment of the reliquefaction unit (vacuum evacuation step). Next, nitrogen is supplied to the piping and equipment of the reliquefaction unit to vent the gas from the piping and equipment (purge step). Finally, the dew point of the nitrogen gas in the piping and equipment is checked (checking step).

[0049] In the vacuum evacuation step, the vacuum pump is operated for approximately one hour until the pressure inside the piping and equipment reaches a vacuum pressure of about 7 mbar. Then, in the purging step, nitrogen gas is supplied to the piping and equipment to reduce the pressure to about 5 bar, thereby purging the gas inside the piping and equipment.

[0050] Prior to the vacuuming step, each device, piping, and measuring instrument installed in the reliquefaction unit is inspected to determine whether it can withstand the vacuum pressure. If each device, piping, and measuring instrument can withstand the vacuum pressure, the purge system of this embodiment is applied.

[0051] After performing the vacuuming and purging steps at least twice, confirm whether the dew point of the nitrogen gas in the piping and equipment has decreased to the required set value.

[0052] If the dew point has not decreased to the required set value (target value), return to the vacuum evacuation step and repeat the steps from the vacuum evacuation step to the verification step in order.

[0053] In this embodiment, prior to performing nitrogen purging, the piping and equipment of the reliquefaction unit are evacuated using a vacuum pump already installed on the ship. This reduces the amount of nitrogen gas required for nitrogen purging of the reliquefaction unit, shortens the time required for the dew point to reach the target value through dew point adjustment by nitrogen purging, and allows for rapid restart of the reliquefaction unit.

[0054] In particular, by utilizing existing equipment on the ship without the need to install additional devices, the efficiency of equipment utilization can be improved, and the cost of installing reliquefaction equipment can be reduced.

[0055] Furthermore, in a reliquefaction device that reliquefies evaporated gas generated from cryogenic LNG using a nitrogen refrigerant at a lower temperature than the evaporated gas, residual moisture inside the device and piping could freeze, potentially damaging key equipment, measuring instruments, and piping, but this can be prevented.

[0056] The present invention is not limited to the embodiments described above, and it will be obvious to those skilled in the art that various modifications or variations can be made without exceeding the technical essence of the invention.

Claims

1. A storage tank installed on a ship for storing liquefied gas at low temperatures and equipped with an insulated section having a vacuum insulation layer; A compressor for compressing the evaporated gas generated in the aforementioned storage tank; and A heat exchanger for cooling the compressed gas compressed by the aforementioned compressor; A purge system for a reliquefaction apparatus comprising: a refrigerant circulation line through which a refrigerant circulates to cool the compressed gas in the heat exchanger; The storage tank is further equipped with a vacuum pump for evacuating the vacuum insulation layer, A purge system for a reliquefaction apparatus, comprising: a hard pipe extending from a vacuum pump to the installation location of the reliquefaction apparatus, which is used to suck and discharge gas from the refrigerant circulation line by a vacuum pump prior to performing nitrogen purging of the reliquefaction apparatus by supplying nitrogen gas into the refrigerant circulation line; and a flexible pipe connecting the hard pipe to the piping and each device of the reliquefaction apparatus.

2. A refrigerant expander provided in the refrigerant circulation line for expanding and cooling the refrigerant supplied to the heat exchanger; The refrigerant circulation line includes a refrigerant compressor that compresses the refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger: The refrigerant circulating in the refrigerant circulation line is nitrogen refrigerant, and the refrigerant compressor is driven by the expansion energy of the refrigerant in the refrigerant expander. A purge system for a ship's evaporative gas reliquefaction apparatus as described in claim 1.

3. In a reliquefaction device that compresses evaporated gas generated from liquefied gas stored in a ship's storage tank, and then cools the compressed gas (the resulting compressed evaporated gas) through heat exchange with a refrigerant circulating in a refrigerant circulation line to reliquefy it, a method for purging nitrogen into the piping and each component of this reliquefaction device, A purging method for a ship's evaporative gas reliquefaction apparatus, characterized by including the step of using a vacuum pump that maintains a vacuum state in a vacuum insulation layer for maintaining the liquefied gas in the storage tank at a low temperature, to suck out and discharge the gas in the refrigerant circulation line, prior to supplying nitrogen gas to the refrigerant circulation line.

4. The nitrogen purging of the aforementioned reliquefaction device is 1) A vacuum evacuation step in which the vacuum pump is used to suck out and discharge gas from the piping and each device of the reliquefaction apparatus; 2) A purging step in which nitrogen gas is supplied to the piping and each device to discharge the gas inside the piping and each device; 3) A confirmation step to check the dew point of the gas inside the piping and each device; and a nitrogen purge, A purging method for a ship's evaporative gas reliquefaction apparatus according to claim 3.

5. After performing steps 1) and 2) in order at least twice, perform step 3), In step 3), if the dew point has not decreased to the required set value, the process returns to step 1), and steps 1) through 3) are performed sequentially. A purging method for a ship's evaporative gas reliquefaction apparatus according to claim 4.

6. The vacuum evacuation step is characterized by extending a hard pipe from the vacuum pump to the installation location of the reliquefaction device, and when performing nitrogen purging, connecting the piping and each device of the reliquefaction device to the hard pipe with a flexible pipe, thereby performing the vacuum evacuation step. A purging method for a ship's evaporative gas reliquefaction apparatus according to claim 4 or claim 5.

7. Step 1) is characterized by operating a vacuum pump to achieve a vacuum pressure of 7 mbar in the piping and each device, and step 2) is characterized by supplying nitrogen gas to reduce the pressure in the piping and each device to 5 barg, thereby purging the gas from the piping and each device. A purging method for a reliquefaction apparatus according to claim 4.

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