Ship's evaporation gas reliquefaction system and evaporation gas reliquefaction method

Abstract

A vessel vapor re-liquefaction system and method are disclosed. [Solution] The ship's evaporative gas reliquefaction system of the present invention comprises a compressor that compresses evaporated gas generated from liquefied gas stored in a storage tank provided on the ship, a reliquefaction line that connects the compressor to the storage tank and reliquefies the evaporated gas and returns it to the storage tank, a heat exchanger provided in the reliquefaction line that cools the evaporated gas compressed by the compressor, a separator provided in the reliquefaction line that separates the evaporated gas cooled by the heat exchanger into gas and liquid and supplies the separated liquefied gas to the storage tank, a nitrogen blanket line that supplies nitrogen to the top of the separator, and a bypass line that branches off from the reliquefaction line downstream of the heat exchanger on the reliquefaction line, bypasses the separator and is connected to the storage tank.

Description

[Technical Field]

[0001] The present invention relates to a boil-off gas (BOG) reliquefaction system and method for cooling and reliquefying boil-off gas (BOG) generated from liquefied gas stored in a storage tank of a ship, and more particularly to a boil-off gas reliquefaction system and method for supplying nitrogen into a separator to maintain pressure within the separator, thereby smoothly supplying the liquefied gas separated by the separator to the storage tank, and, if excessive nitrogen dissolves in the liquefied gas, supplying supercooled liquefied gas to the storage tank by bypassing the separator. [Background technology]

[0002] Natural gas, primarily composed of methane, emits almost no environmental pollutants when burned, and is therefore gaining attention as an environmentally friendly fuel. Liquefied natural gas (LNG) is obtained by liquefying natural gas by cooling it to approximately -163°C at atmospheric pressure. Since liquefied natural gas has a volume that is approximately 1 / 600 of that of gaseous natural gas, it is highly suitable for long-distance transportation via sea routes. For this reason, natural gas is primarily stored and transported in the liquefied natural gas state, which is advantageous for storage and transportation.

[0003] Because the liquefaction point of natural gas is an extremely low temperature of approximately -163°C at atmospheric pressure, LNG storage tanks are usually insulated to maintain the LNG in a liquid state, but even with insulation, there is a limit to how well the LNG can be blocked from external heat.As a result, external heat is continuously transferred to the LNG storage tank, causing the LNG inside the LNG storage tank to naturally vaporize during the LNG transportation process, generating boil-off gas (BOG).

[0004] The continued generation of evaporative gases in LNG storage tanks causes the pressure inside the tank to rise. If the internal pressure inside the LNG storage tank exceeds the set safety pressure, there is a risk of an emergency such as tank rupture, so it is necessary to use a safety valve to release the evaporative gases outside the storage tank. However, since evaporative gases are one type of LNG loss and are a significant issue in terms of LNG transportation efficiency and fuel efficiency, various methods are used to deal with evaporative gases generated in storage tanks.

[0005] In recent years, methods have been developed and are being used that involve using evaporated gas at fuel demand sources such as ship engines, re-liquefying evaporated gas and recovering it in storage tanks, or combining these two methods. Summary of the Invention [Problem to be solved by the invention]

[0006] Methods for re-liquefying evaporated gas include a method in which the evaporated gas is re-liquefied by heat exchange with a refrigerant using a refrigeration cycle that uses another refrigerant, and a method in which the evaporated gas itself is used as a refrigerant without using another refrigerant.

[0007] As a method for re-liquefying evaporated gas using the evaporated gas itself as a refrigerant without using any other refrigerant, a partial re-liquefaction system (PRS) has been developed and is used on ships. This system cools compressed evaporated gas by heat exchange with the uncompressed evaporated gas, and then re-liquefies it by adiabatic expansion.

[0008] On the other hand, as a re-liquefaction method utilizing a refrigeration cycle using another refrigerant, a re-liquefaction method using a nitrogen refrigerant is known.

[0009] While nitrogen refrigerants have a lower cooling efficiency than refrigeration cycles that use mixed refrigerants, they have the advantage of being highly safe because nitrogen is an inert substance and does not undergo phase changes, making them easy to apply to ships.

[0010] The evaporated gas cooled by the cold energy of another refrigerant or the evaporated gas itself is separated into gas and liquid in a separator, and the separated re-liquefied gas is collected in a storage tank.

[0011] However, when the reliquefaction system is operating normally, the liquefied gas cooled by heat exchange may become supercooled and be supplied to the separator. In this case, almost no flash gas is generated from the liquefied gas supplied to the separator.

[0012] In such a situation, if a valve located downstream of the separator is opened to supply the liquefied gas in the separator to a storage tank, the pressure inside the separator will drop suddenly, making it difficult to adjust the pressure inside the separator.

[0013] The present invention provides an evaporated gas reliquefaction system and an evaporated gas reliquefaction method that can maintain the pressure inside the separator and smoothly supply the reliquefied gas to the storage tank even when the reliquefied liquefied gas is supplied from the separator to the storage tank as described above. [Means for solving the problem]

[0014] In order to solve the above problems, the present invention provides a reliquefaction system for evaporated gas on a ship, comprising: a compressor that compresses evaporated gas generated from liquefied gas stored in a storage tank installed on the ship; a reliquefaction line that connects the compressor to the storage tank and reliquefies the evaporated gas and returns it to the storage tank; a heat exchanger that is installed in the reliquefaction line and cools the evaporated gas compressed by the compressor; a separator that is installed in the reliquefaction line and separates the evaporated gas cooled by the heat exchanger into gas and liquid and supplies the separated liquefied gas to the storage tank; a nitrogen blanket line that supplies nitrogen to an upper part of the separator; and a bypass line that branches off from the reliquefaction line downstream of the heat exchanger on the reliquefaction line, bypasses the separator, and is connected to the storage tank.

[0015] Furthermore, in the present invention, it is preferable that the reliquefaction system further comprises a flow meter that detects the flow rate of nitrogen supplied to the separator through the nitrogen blanket line, wherein blanket nitrogen for maintaining the pressure inside the separator is supplied through the nitrogen blanket line, the nitrogen consumption amount inside the separator is monitored based on the nitrogen flow rate detected by the flow meter, and when the nitrogen consumption amount is greater than a predetermined value, the reliquefaction system is operated in a bypass operation mode in which the reliquefaction gas that has been supercooled in the heat exchanger is supplied to the storage tank by bypassing the separator through the bypass line.

[0016] In the present invention, it is preferable that the reliquefaction line further comprises a first control valve provided downstream of the branch point of the bypass line, and a second control valve provided in the bypass line.

[0017] Furthermore, in the present invention, it is preferable to open the second control valve to supply the supercooled reliquefied gas to the storage tank through the bypass line, control the opening of the first control valve so as to supply a portion of the supercooled reliquefied gas to the separator, and monitor the flow rate of nitrogen supplied to the separator to determine whether or not to resume operation of the reliquefaction system in normal operation mode in which the entire amount of reliquefied gas cooled in the heat exchanger is supplied to the separator.

[0018] Furthermore, in the present invention, it is preferable that the system further includes a pressure detector that detects the pressure inside the separator, a pressure compensation line that branches off from the reliquefaction line downstream of the compressor, bypasses the heat exchanger, merges with the nitrogen blanket line, and is connected to an upper part of the separator, and a pressure compensation valve that is provided downstream of the junction of the pressure compensation line with the nitrogen blanket line, and that the pressure compensation valve adjusts the pressure of the evaporated gas or nitrogen supplied to the separator in accordance with the pressure detected by the pressure detector.

[0019] In addition, in the present invention, it is preferable that the system further comprises a first shutoff valve provided upstream of a junction of the pressure compensation line with the nitrogen blanket line, a second shutoff valve provided in the nitrogen blanket line, and a check valve provided downstream of the second shutoff valve in the nitrogen blanket line to prevent backflow of nitrogen.

[0020] In the present invention, it is preferable that the system further comprises a refrigerant circulation unit that circulates the refrigerant that exchanges heat with the evaporated gas in the heat exchanger, and that the refrigerant circulating in the refrigerant circulation unit is nitrogen.

[0021] In order to solve the above-mentioned problems, the present invention provides a method for re-liquefying evaporated gas on a ship, comprising the steps of compressing evaporated gas generated from liquefied gas stored in a storage tank installed on the ship using a compressor, cooling and re-liquefying the evaporated gas compressed by the compressor in a heat exchanger, separating the evaporated gas into gas and liquid in a separator, returning the separated liquefied gas to the storage tank, supplying nitrogen to an upper part of the separator through a nitrogen blanket line to maintain the pressure inside the separator, detecting the flow rate of nitrogen supplied to the separator, monitoring the nitrogen consumption in the separator, and when the nitrogen consumption becomes greater than a predetermined value, causing the re-liquefied gas that has been supercooled in the heat exchanger to bypass the separator through a bypass line and supply it to the storage tank.

[0022] In the present invention, it is also preferable that the reliquefied gas that has been supercooled in the heat exchanger is supplied to the storage tank via the bypass line, and that a portion of the supercooled reliquefied gas is supplied to the separator, and that the flow rate of nitrogen supplied to the separator is monitored to determine whether the entire amount of the reliquefied gas that has been cooled in the heat exchanger is supplied to the separator. [Effects of the Invention]

[0023] The present invention utilizes the cold energy of the evaporated gas itself and the cold energy of the refrigerant cycle, thereby making it possible to more effectively cool the evaporated gas to be reliquefied, thereby improving the reliquefaction rate of the evaporated gas.

[0024] In particular, even if the liquefied gas supplied to the separator is supercooled and almost no flash gas is generated within the separator, by supplying blanket nitrogen into the separator to maintain the pressure within the separator, the liquefied gas within the separator can be smoothly supplied to the storage tank, and the reliquefaction system can be operated stably.

[0025] Furthermore, if the reliquefied gas becomes supercooled and excessive blanket nitrogen dissolves in the reliquefied gas in the separator, the reliquefied gas can be bypassed by a bypass line and supplied directly to the storage tank, while a portion of the supercooled reliquefied gas is supplied to the separator, the flow rate of blanket nitrogen is monitored, and the reliquefaction system is operated in normal operation mode, thereby reducing nitrogen consumption. This reduces the capacity and operating costs of the equipment for supplying nitrogen on board, and also solves the problem of reduced calorific value and quality of the liquefied gas caused by dissolving a large amount of nitrogen in the reliquefied gas. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram illustrating a vessel vapor re-liquefaction system according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The operational advantages of the present invention and the objects achieved by the embodiments of the present invention will now be described by way of example with reference to the drawings and the contents depicted in the drawings.

[0028] The configuration and operation of the embodiment of the present invention will be described below with reference to the drawings. Note that, with respect to the reference numerals given to components in each drawing, the same reference numerals are used to designate the same components in other drawings as far as possible.

[0029] The vessels of the embodiments of the present invention described below may be any type of vessel equipped with storage tanks for storing liquefied gas. Typical examples include self-propelled vessels such as LNG carriers, liquid hydrogen carriers, and LNG regasification vessels (RVs), as well as floating offshore structures without propulsion capabilities, such as LNG floating production storage offloading systems (FPSOs) and LNG floating storage regasification units (FSRUs).

[0030] Furthermore, this embodiment can liquefy gas at low temperatures for transportation and can be applied to the re-liquefaction cycle of all types of liquefied gases that generate evaporated gas during storage. Examples of such liquefied gases include liquefied natural gas (LNG), liquefied ethanol gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas. Note that in the embodiments described below, LNG, one of the representative liquefied gases, will be used as an example.

[0031] FIG. 1 is a schematic diagram of a ship vapor reliquefaction system according to an embodiment of the present invention.

[0032] 1, the evaporated gas reliquefaction system of this embodiment is a system for reliquefying evaporated gas generated from liquefied gas stored in a storage tank CT provided on a ship and returning the evaporated gas to the storage tank CT. The evaporated gas reliquefaction system of this embodiment is provided with a reliquefaction line RL that supplies evaporated gas discharged from the storage tank CT to a compressor to compress it, cools the evaporated gas compressed by the compressor in a heat exchanger 100 to reliquefy it, and then returns it to the storage tank CT.

[0033] The evaporative gas discharged from the storage tank CT before being compressed is used as a refrigerant in the heat exchanger 100 and then supplied to the compressor. In this case, the evaporative gas is compressed in the compressor to, for example, the fuel supply pressure of the ship's main engine. For example, it is compressed to a pressure of 5.5 barg if a DF engine is installed, to a pressure of 15 barg if an X-DF engine is installed, and to a pressure of 300 barg if an ME-GI engine is installed. The compressed evaporative gas is supplied as fuel to the ship's main engine (not shown), and any remaining evaporative gas that is not supplied as fuel is re-liquefied.

[0034] In accordance with ship regulations, the compressor that supplies fuel to the engine is required to have a redundant design (redundancy) in preparation for an emergency. In this embodiment, the description will be mainly given using one compressor as an example, but the compressor is configured to include a main compressor and a standby compressor.

[0035] The evaporated gas compressed by the compressor is supplied to the heat exchanger 100 through a re-liquefaction line RL and cooled by heat exchange. In order to improve the re-liquefaction rate, the evaporated gas may be further compressed (additionally compressed), cooled by heat exchange, and re-liquefied.

[0036] The reliquefaction line RL is provided with a heat exchanger 100 that cools the evaporated gas compressed by the compressor, and a separator 200 that separates the evaporated gas cooled by the heat exchanger 100 into gas and liquid. The liquefied gas separated by the separator 200 is supplied to a storage tank CT. If necessary, a pressure reducing device (not shown) that reduces the pressure of the evaporated gas cooled by heat exchange is provided between the heat exchanger 100 and the separator 200.

[0037] The evaporated gas is cooled in the heat exchanger 100 by heat exchange with the refrigerant circulating in a refrigerant circulation section (not shown).

[0038] The refrigerant circulation unit includes a refrigerant circulation line (not shown) through which the refrigerant circulates. The refrigerant circulation line includes a compander expander (not shown) that expands and cools the refrigerant supplied to the heat exchanger 100, and a compander compressor (not shown) that receives the expansion energy of the refrigerant from the compander expander and compresses the refrigerant discharged from the heat exchanger 100. A motor (not shown) is also provided to drive the compander compressor. The compander compressor and compander expander are connected via a shaft, and the expansion energy of the refrigerant is used to compress the refrigerant, thereby reducing the power required to drive the refrigerant cycle.

[0039] Nitrogen (N2), for example, is used as the refrigerant that circulates through the refrigerant circulation line and is supplied to the heat exchanger 100. The nitrogen refrigerant compressed by the compander compressor is cooled in the heat exchanger 100, and then cooled by expansion in the compander expander, and is supplied to the heat exchanger 100 again as a refrigerant and circulates through the refrigerant circulation line.

[0040] A level control valve LV is provided downstream of the separator 200 on the reliquefaction line RL. The level control valve LV opens and closes the reliquefaction line RL to supply the reliquefied gas separated by the separator 200 to the storage tank CT. When the level control valve LV provided downstream of the separator 200 is opened to supply the reliquefied gas in the separator 200 to the storage tank CT, the pressure inside the separator 200 changes. As a result, flash gas is generated from the reliquefied gas supplied into the separator 200, thereby maintaining the pressure inside the separator 200.

[0041] However, depending on the composition of the evaporated gas generated in the storage tank CT (especially when the nitrogen content in the evaporated gas is low), even if the reliquefied gas is cooled by nitrogen refrigerant in the heat exchanger 100 and becomes supercooled, and is supplied to the separator 200, almost no flash gas is generated from the reliquefied gas supplied into the separator 200.

[0042] In such a situation, if the liquid level adjustment valve LV provided downstream of the separator 200 is opened, the pressure inside the separator 200 will drop suddenly, making it difficult to adjust the pressure inside the separator 200. Therefore, in this embodiment, even in such a case, the device is configured to compensate for the pressure inside the separator 200 and maintain the pressure inside the separator 200.

[0043] In this embodiment, a pressure compensation line PL is provided downstream of the compressor, branching off from the reliquefaction line RL and connected to the top of the separator 200. A nitrogen blanket line NBL, which supplies blanket nitrogen, is connected to the pressure compensation line PL. When the liquefied gas in the separator 200 is supplied to the storage tank CT, the pressure in the separator 200 can be maintained by supplying compressed gas or blanket nitrogen to the separator 200 through the pressure compensation line PL.

[0044] In this embodiment, a pressure detector PT that detects the pressure inside the separator 200 and a liquid level detector LT that detects the liquid level of the liquefied gas inside the separator 200 are also provided. A pressure compensation valve PV is provided downstream of the junction of the pressure compensation line PL with the nitrogen blanket line NBL. Meanwhile, a first shutoff valve SV1 is provided upstream of the junction of the pressure compensation line PL with the nitrogen blanket line NBL. The nitrogen blanket line NBL is also provided with a second shutoff valve SV2 and a check valve CHV that is provided downstream of the second shutoff valve SV2 and prevents backflow of nitrogen.

[0045] In response to the pressure inside the separator 200 detected by the pressure detector PT, the pressure compensation valve PV adjusts the pressure of the evaporated gas or nitrogen supplied to the top of the separator 200 through the pressure compensation line PL. In this case, one of the first shutoff valve SV1 and the second shutoff valve SV2 is opened and the other is closed, and the evaporated gas or nitrogen is supplied to the separator 200.

[0046] The nitrogen supplied to the separator 200 through the nitrogen blanket line NBL is supplied from a nitrogen buffer tank (N2Buffer Tank) of the ship's nitrogen supply system (N2Supply System) or a nitrogen inventory system (N2Inventory System) that supplies and replenishes nitrogen refrigerant to the refrigerant circulation section where the nitrogen refrigerant circulates.

[0047] When evaporated gas is supplied to the top of the separator 200 through the pressure compensation line PL, the evaporated gas dissolves in the liquefied gas in the separator 200 until it becomes saturated, and the temperature of the liquefied gas gradually rises, breaking the supercooling of the liquefied gas. When such liquefied gas is supplied from the separator 200 to the storage tank CT, a large amount of flash gas is generated due to the pressure difference between the pressure inside the separator 200 and the pressure inside the storage tank CT.

[0048] Furthermore, when blanket nitrogen is supplied to the separator 200 through the nitrogen blanket line NBL, the liquefaction temperature of nitrogen is lower than that of methane, so nitrogen is less likely to dissolve in the liquefied gas. This allows pressure compensation and continuous supercooling operation, and reduces the amount of flash gas generated in the storage tank CT.

[0049] In the above-described embodiment, it is not necessary to constantly supply evaporative gas or nitrogen to the separator 200 through the pressure compensation line PL. Furthermore, as described above, the liquefied gas supplied to the separator 200 is supercooled, and therefore flash gas is not generated within the separator 200. Furthermore, if the pressure within the separator 200 cannot be maintained by flash gas alone even when the liquid level adjustment valve LV is opened, the pressure within the separator 200 can be maintained by supplying evaporative gas or nitrogen to the separator 200 through the pressure compensation line PL.

[0050] However, the inventors have confirmed through simulations and demonstration experiments that in such a reliquefaction system, when blanket nitrogen is supplied to the top of the separator 200 through the nitrogen blanket line NBL, more nitrogen than expected dissolves in the reliquefied gas that has been supercooled in the heat exchanger 100, resulting in a large amount of blanket nitrogen being consumed and making it difficult to smoothly supply the liquefied gas to the storage tank CT.

[0051] To solve this problem, it is conceivable to change the cooling temperature of the evaporated gas compressed by the heat exchanger 100, but in that case, the cold power of the reliquefaction cycle cannot be fully utilized, and the cooling efficiency decreases, which may cause other problems. Also, even if the pressure inside the separator 200 decreases due to the dissolution of blanket nitrogen into the liquefied gas inside the separator 200, it is conceivable to install an additional device such as a supply pump so that the liquefied gas inside the separator 200 can be smoothly supplied to the storage tank CT, but in that case, there is a problem that CAPEX (Capital Expenditure) increases.

[0052] In this embodiment, to solve this problem, a bypass line BL is provided that connects the heat exchanger 100 and the storage tank CT, bypassing the separator 200. As a result, even if the reliquefied gas in the separator 200 cannot be smoothly supplied to the storage tank CT due to excessive dissolution of blanket nitrogen in the reliquefied gas in the separator 200, or if the amount of nitrogen consumed as blanket nitrogen becomes excessive, the supercooled reliquefied gas can be directly supplied to the storage tank CT through the bypass line BL.

[0053] 1, a bypass line BL is provided that branches off from the reliquefaction line RL downstream of the heat exchanger 100, bypasses the separator 200, and is connected to the storage tank CT. A first control valve CV1 is provided downstream of the branch point of the reliquefaction line RL with the bypass line BL. A second control valve CV2 is provided in the bypass line BL.

[0054] Under the control of the control unit XC, the second shutoff valve SV2 is opened, and blanket nitrogen is supplied to the separator 200 through the nitrogen blanket line NBL so as to maintain the pressure inside the separator 200. In addition, the flow rate of the nitrogen supplied to the separator 200 through the nitrogen blanket line NBL is detected by a flow meter (not shown). Based on the flow rate of nitrogen detected by the flow meter, the amount of nitrogen consumed by dissolving in the liquefied gas inside the separator 200 is monitored.

[0055] When the nitrogen consumption in the separator 200 exceeds a predetermined value, the reliquefaction system is switched to bypass operation mode, the second control valve CV2 is opened, and the reliquefaction gas that has been supercooled in the heat exchanger 100 is bypassed around the separator 200 via the bypass line BL and supplied directly to the storage tank CT.

[0056] However, if operation in the bypass operation mode continues, the reliquefied gas in a gas-liquid mixed state will be supplied to the storage tank CT, causing the pressure inside the storage tank CT to rise. Therefore, when reliquefied gas is supplied to the storage tank CT from the heat exchanger 100 in the bypass operation mode, the aperture of the first control valve CV1 is adjusted to supply a small amount of reliquefied gas cooled in the heat exchanger 100 to the separator 200. In addition, the flow rate of the nitrogen supplied to the separator 200 is monitored using the flow meter, the amount of blanket nitrogen is confirmed, and a decision is made as to whether to resume operation of the reliquefaction system in the normal operation mode in which the entire amount of reliquefied gas cooled in the heat exchanger 100 is supplied to the separator 200.

[0057] In the system of the above embodiment, even if a predetermined amount or more of blanket nitrogen dissolves in the reliquefied gas in the separator 200, resulting in an excessive amount of blanket nitrogen being required to maintain the pressure inside the separator 200, the reliquefied gas can be supplied directly to the storage tank CT through the bypass line BL, thereby smoothly supplying the reliquefied gas to the storage tank CT and reducing the amount of nitrogen consumed for blanketing (N2 blanketing). This reduces the capacity and operating costs of the equipment for supplying nitrogen to the ship and also solves the problem of a decrease in the calorific value and quality of the liquefied gas caused by dissolving a large amount of nitrogen in the reliquefied gas.

[0058] The present invention is not limited to the above-described embodiments, and it will be obvious to those skilled in the art to which the present invention pertains that various changes or modifications can be made without departing from the technical gist of the present invention.

Claims

1. a compressor for compressing evaporated gas generated from liquefied gas stored in a storage tank provided on the ship; and a re-liquefaction line connecting the compressor and the storage tank, for re-liquefying the evaporated gas and returning it to the storage tank; and a heat exchanger provided in the reliquefaction line for cooling the evaporated gas compressed by the compressor; and a separator provided in the reliquefaction line, which separates the evaporated gas cooled by the heat exchanger into gas and liquid and supplies the separated liquefied gas to the storage tank; and a nitrogen blanket line supplying nitrogen to the top of the separator; and a bypass line branching off from the reliquefaction line downstream of the heat exchanger, bypassing the separator and connecting to the storage tank; and a flow meter that detects the flow rate of nitrogen supplied to the separator through the nitrogen blanket line; the nitrogen blanket line supplies blanket nitrogen to maintain pressure within the separator; the reliquefaction system is operated in a bypass operation mode in which the nitrogen consumption amount in the separator is monitored based on the nitrogen flow rate detected by the flow meter, and when the nitrogen consumption amount is greater than a predetermined value, the reliquefaction gas that has been supercooled in the heat exchanger is supplied to the storage tank by bypassing the separator through the bypass line. Ship's vapor reliquefaction system.

2. a first control valve provided in the reliquefaction line downstream of a branch point of the bypass line; and a second control valve provided in the bypass line; The ship vapor reliquefaction system according to claim 1.

3. the second control valve is opened to supply the supercooled reliquefied gas to the storage tank through the bypass line, and the opening of the first control valve is controlled so as to supply a portion of the supercooled reliquefied gas to the separator; and the flow rate of nitrogen supplied to the separator is monitored to determine whether or not to resume operation of the reliquefaction system in a normal operation mode in which the entire amount of the reliquefied gas cooled in the heat exchanger is supplied to the separator.

3. A ship vapor reliquefaction system according to claim 2.

4. a pressure detector that detects the pressure in the separator; and a pressure compensation line branching off from the reliquefaction line downstream of the compressor, bypassing the heat exchanger, joining the nitrogen blanketing line, and connected to an upper portion of the separator; and a pressure compensation valve disposed downstream of the junction of the pressure compensation line with the nitrogen blanketing line; The pressure compensation valve adjusts the pressure of the evaporated gas or nitrogen supplied to the separator in accordance with the pressure detected by the pressure detector.

3. A ship vapor reliquefaction system according to claim 2.

5. a first shutoff valve located upstream of the pressure compensation line's junction with the nitrogen blanket line; and a second shutoff valve in the nitrogen blanket line; and a check valve provided in the nitrogen blanketing line downstream of the second shutoff valve to prevent backflow of nitrogen.

5. A ship vapor reliquefaction system according to claim 4.

6. a refrigerant circulation unit that circulates the refrigerant that is heat exchanged with the evaporated gas in the heat exchanger; The refrigerant circulating in the refrigerant circulation section is nitrogen. The ship vapor reliquefaction system according to any one of claims 1 to 5.

7. The evaporated gas generated from the liquefied gas stored in the storage tank installed on the ship is compressed by a compressor, The evaporated gas compressed by the compressor is cooled and re-liquefied in a heat exchanger, and the gas and liquid are separated in a separator, and the separated liquefied gas is returned to the storage tank. supplying nitrogen to the top of the separator through a nitrogen blanket line to maintain pressure within the separator; the reliquefaction system is operated in a bypass operation mode in which a flow rate of nitrogen supplied to the separator through the nitrogen blanket line is detected by a flow meter, a nitrogen consumption amount in the separator is monitored based on the nitrogen flow rate detected by the flow meter, and when the nitrogen consumption amount becomes greater than a predetermined value, the reliquefaction gas that has been supercooled in the heat exchanger is caused to bypass the separator through a bypass line and is supplied to the storage tank. A method for re-liquefying ship's vapors.

8. the reliquefied gas that has been supercooled in the heat exchanger is supplied to the storage tank via the bypass line, and a portion of the supercooled reliquefied gas is supplied to the separator; and the flow rate of nitrogen supplied to the separator is monitored to determine whether or not the entire amount of the reliquefied gas that has been cooled in the heat exchanger is to be supplied to the separator. The method for reliquefying evaporated gas from a ship according to claim 7.

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