Purge system and method for boil-off gas reliquefaction plant of a marine vessel

Abstract

Disclosed is a purge system and a purge method for a boil-off gas reliquefaction device of a ship. The purge system for a boil-off gas reliquefaction device of a ship includes a storage tank provided in a ship to store a cryogenic liquefied gas and having an insulation part including a vacuum insulation layer, a compressor compressing boil-off gas generated from the storage tank, a heat exchanger cooling the compressed gas compressed by the compressor in the heat exchanger, and a refrigerant circulation line circulating a refrigerant exchanging heat with the compressed gas in the heat exchanger via the refrigerant circulation line, wherein, when the reliquefaction device is purged with N2, a vacuum pump is provided to maintain an internal vacuum state in the vacuum insulation layer of the storage tank by sucking and discharging gas in the refrigerant circulation line and then supplying N2.

Description

Technical Field

[0001] This invention relates to a purging system and method for a reliquefaction equipment on a ship, and more specifically, to a purging system and method for a reliquefaction equipment on a ship in which, during purging of the reliquefaction equipment, N2 purging is performed after gas in the refrigerant circulation line is drawn out and discharged by a vacuum pump, the vacuum pump being configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank. Background Technology

[0002] Natural gas, containing methane as its main component, is favored as an eco-friendly fuel that releases little to no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by liquefying natural gas at atmospheric pressure by cooling it to approximately -163°C. Because LNG has a volume approximately 1 / 600th that of natural gas in its gaseous state, it is suitable for long-distance maritime transport. Therefore, natural gas is stored and transported in the easily stored and transportable form of LNG.

[0003] Because natural gas is liquefied at a low temperature of -163°C under normal pressure, LNG storage tanks are typically insulated to keep LNG in a liquid state. However, despite being insulated, these tanks still have limited ability to block external heat. Therefore, as external heat is continuously transferred to the LNG storage tank, the LNG stored in the tank continues to evaporate naturally during transportation, producing boil-off gas (BOG).

[0004] The continuous generation of vaporized gas in LNG storage tanks increases the internal pressure. If this pressure exceeds a predetermined safe level, it can lead to emergencies such as tank rupture. Therefore, safety valves are needed to release the vaporized gas from the tank. However, vaporized gas represents a loss of LNG and is a significant issue for both LNG transportation and fuel efficiency. Consequently, various methods are employed to manage the vaporized gas generated in LNG storage tanks.

[0005] Recently, a method for using evaporated gas at fuel demand sites (e.g., ship engines), a method for reliquefying evaporated gas and returning the reliquefied evaporated gas to LNG storage tanks, and a method combining the two have been developed and put into use. Summary of the Invention

[0006] Technical issues

[0007] In reliquefaction cycles used to reliquefy evaporated gases generated in ships, typical available liquefaction methods include processes using a single mixed refrigerant (SMR) cycle and a propane-precooled mixed refrigerant (C3MR) cycle. The C3MR cycle uses propane as the sole refrigerant to cool the natural gas, followed by liquefaction and subcooling using a mixed refrigerant, while the SMR cycle uses a mixed refrigerant composed of multiple components to liquefy the natural gas.

[0008] In this way, both the SMR and C3MR cycles use mixed refrigerants. However, if the composition of the mixed refrigerant changes due to refrigerant loss during the liquefaction of the evaporated gas, this can lead to poor liquefaction efficiency. Therefore, it is necessary to maintain a constant refrigerant composition by continuously measuring the composition of the mixed refrigerant and replenishing any missing refrigerant components.

[0009] An alternative reliquefaction cycle for reliquefying evaporated gas is a single-cycle liquefaction process using nitrogen refrigerant.

[0010] Although this reliquefaction cycle using nitrogen refrigerant is relatively less efficient than that using mixed refrigerants, it is safer due to the inert nature of nitrogen refrigerant and is easier to apply to ships because nitrogen refrigerant does not undergo a phase change.

[0011] All equipment and pipes on board are purged with N2 immediately after manufacture, or before the initial start-up after equipment maintenance, in order to reduce the oxygen concentration in the equipment and pipes while preventing moisture in the air from condensing at low temperatures.

[0012] Specifically, in reliquefaction cycles that use nitrogen refrigerant to process evaporated gases from cryogenic LNG, residual moisture in the equipment and piping can condense and freeze, potentially damaging key equipment including heat exchangers, measuring instruments, and tubing. Therefore, adjusting the dew point is crucial for reliquefaction cycles.

[0013] Traditionally, purging operations are performed by repeating the dew point check process several times to ensure that the N2 dew point is met after the purging operation. However, this method has a problem: if the equipment and pipes constituting the reliquefaction cycle are large, the N2 purging flow rate used to adjust the N2 dew point becomes large and the execution time becomes long.

[0014] One aspect of the present invention is to provide a system and method that achieves rapid reliquefaction cycle by reducing nitrogen consumption for N2 purging while reducing the time spent on purging operations.

[0015] Technical solutions

[0016] According to one aspect of the present invention, a purging system for a ship's evaporative gas reliquefaction equipment is provided, the purging system comprising: a storage tank disposed in a ship for storing cryogenic liquefied gas and having an insulating portion including a vacuum insulation layer;

[0017] A compressor compresses the evaporated gas generated from the storage tank;

[0018] A heat exchanger for cooling the compressed gas compressed in the compressor; and

[0019] A refrigerant circulation line allows the refrigerant that will exchange heat with the compressed gas in the heat exchanger to circulate within the refrigerant circulation line.

[0020] When purging the reliquefaction equipment with N2, the purging system uses a vacuum pump to draw gas from the refrigerant circulation line and discharge the gas before supplying N2. The vacuum pump is configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank.

[0021] Preferably, the refrigerant circulation line includes: a refrigerant expander for expanding and cooling the refrigerant to be supplied to the heat exchanger; and a refrigerant compressor for compressing the refrigerant discharged after heat exchange in the heat exchanger, wherein the refrigerant circulating in the refrigerant circulation line is nitrogen, and the refrigerant compressor is driven by receiving the expansion energy of the refrigerant in the refrigerant expander.

[0022] Preferably, the purging system further includes: a rigid tube extending from the vacuum pump to the reliquefaction device; and a flexible tube connecting the rigid tube to the various tubes and devices of the reliquefaction device.

[0023] Preferably, the N2 purging of the reliquefaction equipment includes: 1) a suction step, in which gas is suctioned and discharged from the pipes and apparatus of the reliquefaction equipment by the vacuum pump; 2) a purging step, in which nitrogen is supplied to the pipes and apparatus and discharged from the pipes and apparatus; and 3) an inspection step, in which the dew point of the pipes and apparatus is inspected.

[0024] Preferably, step 3) is performed after steps 1) and 2) have been performed two or more times in sequence, and if the dew point has not yet dropped to the preset value in step 3), the N2 purging returns to step 1), and then steps 1) to 3 are performed in sequence.

[0025] Preferably, step 1) is performed for 1 hour by operating the vacuum pump at a vacuum pressure of about 7 millibars, and in step 2), the tubes and devices are purged by supplying and purging nitrogen until the internal pressure reaches about 5 bar.

[0026] According to another aspect of the present invention, a purging method is provided for an evaporative gas reliquefaction device for a ship, wherein evaporative gas generated from cryogenic liquefied gas stored in a storage tank on board is compressed in a compressor and cooled and reliquefied by exchanging heat in a heat exchanger with refrigerant circulating along a refrigerant circulation line.

[0027] The storage tank is equipped with an insulation section including a vacuum insulation layer to keep the cryogenic liquefied gas in a cold state.

[0028] During the N2 purging of the reliquefaction equipment, N2 is supplied after the gas is drawn from the refrigerant circulation line and discharged using a vacuum pump. The vacuum pump is configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank.

[0029] Preferably, the N2 purging of the reliquefaction equipment includes: 1) a suction step, in which gas is suctioned and discharged from the pipes and apparatus of the reliquefaction equipment by the vacuum pump; 2) a purging step, in which nitrogen is supplied to the pipes and apparatus and discharged from the pipes and apparatus; and 3) an inspection step, in which the dew point in the pipes and apparatus is inspected.

[0030] Preferably, step 3) is performed after steps 1) and 2) have been performed two or more times in sequence, and if the dew point has not yet dropped to the preset value in step 3), the N2 purging returns to step 1), and then steps 1) to 3 are performed in sequence.

[0031] Preferably, the suction step is performed by connecting a rigid tube from the vacuum pump to the reliquefaction device; and by using flexible tubes to connect the rigid tube to the various tubes and devices of the reliquefaction device.

[0032] Beneficial effects

[0033] In this invention, when purging the reliquefaction equipment, purging is performed by supplying N2 after drawing and discharging gas from the pipes and each device of the reliquefaction equipment using a vacuum pump configured to evacuate the vacuum insulation layer of the storage tank.

[0034] By using a shipboard vacuum pump to purge all pipes and devices of the reliquefaction unit before N2 purging, the amount of nitrogen required for N2 purging and to meet the dew point requirements of the reliquefaction unit, as well as the time required for N2 purging, can be reduced, thereby enabling the reliquefaction unit to start up quickly.

[0035] Specifically, in a reliquefaction facility that uses cryogenic nitrogen refrigerant to treat the evaporated gas generated from LNG, the purging system and method according to the present invention can prevent damage to the main equipment, measuring instruments and pipes of the reliquefaction facility due to condensation and freezing of residual moisture in the pipes and devices of the reliquefaction facility. Attached Figure Description

[0036] Figure 1 This is a schematic block diagram of an evaporative gas reliquefaction device for a ship to which the purging system according to the present invention can be applied. Detailed Implementation

[0037] To fully understand the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the invention and their description.

[0038] In the following description, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings in terms of their features and effects. It should be noted that throughout the specification and in all the drawings, the same components will be indicated by the same reference numerals.

[0039] As used herein, the term "ship" can refer to any type of vessel equipped with liquefied gas storage tanks. For example, a ship can include: self-propelled vessels, such as LNG carriers, liquid hydrogen carriers, and LNG regasification vessels (RVs); and non-self-propelled floating offshore structures, such as floating production storage and offloading (FPSO) units and floating storage regasification units (FSRUs).

[0040] Furthermore, embodiments of the present invention can be applied to the reliquefaction cycle of any type of liquefied gas that can be transported in a liquid state by liquefaction at cryogenic temperatures and that may generate evaporating gas during storage. For example, such liquefied gases may include liquefied petrochemical gases, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas. In the following embodiments, LNG, as a typical liquefied gas, will be used as an example to illustrate the present invention.

[0041] Figure 1 This is a schematic block diagram of an evaporative gas reliquefaction device for a ship to which the purging system according to the present invention can be applied.

[0042] Reference Figure 1 The evaporative gas reliquefaction device includes: a compressor 100 for receiving and compressing evaporative gas from a storage tank T on board; a heat exchanger 200 for cooling the compressed gas from the compressor in the heat exchanger 200; and a refrigerant circulation component 300 for circulating refrigerant that will exchange heat with the compressed gas in the heat exchanger in the refrigerant circulation component 300.

[0043] After recovering heat in a heat exchanger, the evaporated gas generated from the cryogenic liquefied gas stored in storage tank T is supplied to compressor 100 via the heat exchanger. Compressor 100 compresses the evaporated gas to, for example, the fuel supply pressure required by the main engine of a ship. For example, for a DF engine, the compressor can compress the evaporated gas to a pressure of 5.5 bar; for an X-DF engine, the compressor can compress the evaporated gas to a pressure of 15 bar; and for a ME-GI engine, the compressor can compress the evaporated gas to a pressure of 300 bar. The compressed evaporated gas can be supplied as fuel to the main engine of a ship (not shown), and excess compressed evaporated gas can be reliquefied.

[0044] Classification societies require that compressors used to supply fuel to engines have redundant designs for emergency situations. Therefore, although... Figure 1 A compressor is shown, but the compressor may include a main compressor and additional compressors.

[0045] A reliquefaction line RL is connected downstream of the compressor to reliquefy the evaporated gas and return the reliquefied evaporated gas to the storage tank T. The evaporated gas compressed in the compressor is introduced into the heat exchanger 200 along the reliquefaction line RL and cooled by heat exchange.

[0046] A gas-liquid separator is installed downstream of the heat exchanger along the reliquefaction line RL to separate the reliquefied gas into gas and liquid phases. If necessary, a pressure-reducing valve can be installed upstream of the gas-liquid separator in the reliquefaction line to reduce the pressure of the compressed gas cooled in the heat exchanger and adjust the reliquefaction rate.

[0047] The reliquefied gas separated by the gas-liquid separator 400 can be supplied to a storage tank for further storage, and the flash gas can be supplied to an uncompressed evaporative gas stream upstream of a heat exchanger in the evaporative gas supply line or can be delivered to the GCU.

[0048] In the refrigerant circulation component 300, the refrigerant circulates along the refrigerant circulation line CL and is cooled by heat exchange in the heat exchanger 200.

[0049] The refrigerant cycle component 300 includes: a refrigerant expander 320 for expanding and cooling refrigerant to be supplied to a heat exchanger; and a refrigerant compressor 310 connected to the refrigerant expander to receive the expansion energy of the refrigerant and compress the refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger. A motor (not shown) may be provided to drive the refrigerant compressor, and the refrigerant compressor and refrigerant expander may be coaxially connected to use the expansion energy of the refrigerant to compress the refrigerant, thereby reducing the power required to drive the refrigerant cycle.

[0050] Refrigerant, which has already been expanded and cooled in the refrigerant expander 320, is introduced into the heat exchanger 200 to provide heating or cooling. The refrigerant discharged from the heat exchanger after heat exchange is compressed in the refrigerant compressor 310. The refrigerant compressed in the refrigerant compressor 310 is cooled by the heat exchanger 200 and supplied to the refrigerant expander 320 for further expansion and cooling, and then supplied back to the heat exchanger 200, thus circulating along the refrigerant circulation line CL.

[0051] Therefore, in the heat exchanger 200, the four streams of gas that is compressed in the compressor, the uncompressed gas that will be introduced into the compressor, the refrigerant that is expanded and cooled in the refrigerant expander, and the refrigerant that is compressed in the refrigerant compressor undergo heat exchange. The compressed gas that is compressed in the compressor and the refrigerant that is compressed in the refrigerant compressor are cooled by exchanging heat with the uncompressed gas that will be introduced into the compressor and the refrigerant that is expanded and cooled in the refrigerant expander.

[0052] Each pipe and device in this reliquefaction equipment is purged with N2 after manufacturing or before initial startup after equipment maintenance to reduce the oxygen concentration in each pipe and device and prevent moisture in the air from condensing at lower temperatures.

[0053] The purging system according to this embodiment is adapted to enable N2 purging of such reliquefaction equipment.

[0054] This embodiment proposes the use of a vacuum pump, which is configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank during N2 purging of the reliquefaction equipment.

[0055] Storage tanks for storing cryogenic liquefied gases (specifically, cryogenic LNG) are equipped with insulation to keep the liquefied gases cold and prevent external heat from entering, and the insulation may include a vacuum insulation layer.

[0056] In order to maintain the internal vacuum state of the vacuum insulation layer, a vacuum pump is installed in the ship to operate under vacuum pressure.

[0057] In this embodiment, since the vacuum pump is not always operational, it is used to purge the reliquefaction equipment. That is, before N2 purging, the vacuum pump draws in and discharges gas from each device and pipe in the refrigerant circulation line including the reliquefaction equipment to empty the pipes and devices of the reliquefaction equipment, and then supplies N2 for purging. Therefore, the amount of nitrogen used for N2 purging of the reliquefaction equipment and the time spent on N2 purging can be reduced by meeting the N2 dew point requirements.

[0058] The purging system according to this embodiment can be used to purge the reliquefaction equipment as follows.

[0059] Rigid tubing extends from the vacuum pump to the location where the reliquefaction equipment is installed, connecting the vacuum pump to the reliquefaction equipment. When purging of the pipes and equipment of the reliquefaction equipment is required, the rigid tubing is connected to the pipes and devices of the reliquefaction equipment via flexible tubing.

[0060] The N2 purging of the reliquefaction equipment can be carried out by the following steps: 1) a suction step, in which gas is suctioned and discharged from the pipes and devices of the reliquefaction equipment by the vacuum pump after connecting the flexible tubes; 2) a purging step, in which nitrogen is supplied to the pipes and devices of the reliquefaction equipment and discharged from the pipes and devices; and 3) an inspection step, in which the dew point of the pipes and devices is checked.

[0061] Step 1) is performed for 1 hour by operating the vacuum pump at a vacuum pressure of about 7 mbar, and in step 2), the tubes and devices are purged by supplying and purging nitrogen until the internal pressure reaches about 5 bar.

[0062] Before performing the suction step, check whether each of the devices, pipes and instruments in the reliquefaction equipment can withstand vacuum pressure, and once it is determined that each of the devices, pipes and instruments in the reliquefaction equipment can withstand vacuum pressure, the purging system according to this embodiment can be applied.

[0063] After performing steps 1) and 2) two or more times in sequence, check whether the N2 dew point in each pipe and device has dropped to the preset level.

[0064] If the dew point has not yet dropped to the preset level, the purging process returns to step 1) to proceed sequentially from step 1) to step 3).

[0065] As described above, the various pipes and devices of the reliquefaction equipment are evaluated using the ship's vacuum pump before N2 purging to reduce the amount of nitrogen required for N2 purging of the reliquefaction equipment and to reduce the time required for N2 purging until the preset dew point is reached, thereby enabling the reliquefaction equipment to be started up quickly.

[0066] Specifically, using pre-installed shipboard equipment eliminates the need for additional installations, thereby increasing equipment utilization while reducing installation costs.

[0067] Furthermore, in reliquefaction equipment that uses cryogenic nitrogen refrigerant to treat the evaporated gas generated from LNG, the purging system and method according to the present invention can prevent damage to the main equipment, measuring instruments and pipes of the reliquefaction equipment due to condensation and freezing of residual moisture in the various devices and pipes of the reliquefaction equipment.

[0068] Although some embodiments have been described herein, it will be apparent to those skilled in the art that the invention is not limited thereto and can be implemented with various modifications or variations without departing from the spirit of the invention.

Claims

1. A purging system for a ship's evaporative gas reliquefaction equipment, characterized in that, The purging system includes: A storage tank, installed in a ship to store cryogenic liquefied gas and equipped with an insulating portion including a vacuum insulation layer; A compressor compresses the evaporated gas generated from the storage tank; A heat exchanger for cooling the compressed gas compressed in the compressor; and A refrigerant circulation line allows the refrigerant that will exchange heat with the compressed gas in the heat exchanger to circulate within the refrigerant circulation line. In the process of purging the evaporating gas reliquefaction equipment with N2, the purging system uses a vacuum pump to draw gas from the refrigerant circulation pipeline and discharge the gas before supplying N2. The vacuum pump is configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank.

2. The purging system according to claim 1, wherein the refrigerant circulation line comprises: A refrigerant expander expands and cools the refrigerant to be supplied to the heat exchanger; as well as A refrigerant compressor compresses the refrigerant that has been discharged after heat exchange in the heat exchanger. The refrigerant circulating in the refrigerant circulation line is nitrogen, and The refrigerant compressor is driven by receiving the expansion energy of the refrigerant in the refrigerant expander.

3. The purging system according to claim 2 further includes: A rigid tube extends from the vacuum pump to the evaporative gas reliquefaction device; as well as Flexible tubes connect the rigid tubes to the various tubes and devices of the evaporative gas reliquefaction equipment.

4. The purging system according to claim 3, wherein purging the evaporated gas reliquefaction equipment with N2 comprises: 1) Suction step, in which the gas in each pipe and each device of the evaporating gas reliquefaction equipment is suctioned and discharged by the vacuum pump; 2) A purging step, in which nitrogen is supplied to and discharged from each of the pipes and devices; and 3) Inspection steps, in which the dew point in each pipe and each device is checked.

5. The purging system according to claim 4, wherein step 3) is performed after steps 1) and 2) are performed sequentially two or more times, and when the dew point in step 3) has not yet dropped to a preset value, N2 purging returns to step 1), and then steps 1) to 3) are performed sequentially.

6. The purging system according to claim 4, wherein step 1) is performed for 1 hour by operating the vacuum pump at a vacuum pressure of 7 mbar, and in step 2), the tubes and devices are purged by supplying and venting nitrogen until the internal pressure reaches 5 bar.

7. A purging method for an evaporative gas reliquefaction device used in a ship, characterized in that, In the purging method, the evaporated gas generated from the cryogenic liquefied gas stored in the storage tank on the ship is compressed in a compressor and cooled and reliquefied by exchanging heat with the refrigerant circulating along the refrigerant circulation line in a heat exchanger. The storage tank is equipped with an insulation section including a vacuum insulation layer to keep the cryogenic liquefied gas in a cold state. Specifically, during N2 purging of the evaporative gas reliquefaction equipment, N2 is supplied after the gas is drawn from the refrigerant circulation line and discharged using a vacuum pump. The vacuum pump is configured to maintain the internal vacuum state of the vacuum insulation layer of the storage tank.

8. The purging method according to claim 7, wherein purging the evaporated gas reliquefaction equipment with N2 comprises: 1) Suction step, in which the gas in each pipe and each device of the evaporative gas reliquefaction equipment is suctioned and discharged by the vacuum pump; 2) A purging step, in which nitrogen is supplied to and discharged from each of the pipes and devices; and 3) Inspection steps, in which the dew point in each pipe and each device is checked.

9. The purging method according to claim 8, wherein step 3) is performed after steps 1) and 2) are performed sequentially two or more times, and when the dew point in step 3) has not yet dropped to a preset value, N2 purging returns to step 1), and then steps 1) to 3) are performed sequentially.

10. The purging method according to claim 8 or 9, wherein the suction step is performed by connecting a rigid tube from the vacuum pump to the evaporative gas reliquefaction device; and connecting the rigid tube to the tubes and devices of the evaporative gas reliquefaction device using a flexible tube.

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