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

Abstract

A reliquefaction system and a reliquefaction method for evaporated gas for a ship are disclosed. The reliquefaction system for evaporated gas for a ship of the present invention includes a storage tank installed on the ship for storing liquefied gas, a compressor for compressing evaporated gas generated from the liquefied gas, a reliquefaction device for cooling and reliquefying the compressed gas obtained by compressing the evaporated gas in the compressor by heat exchange with a refrigerant circulating in a refrigerant circulation section, and a reliquefaction capacity controller for controlling the reliquefaction capacity of the reliquefaction device, characterized in that when the pressure value of the evaporated gas detected in a vapor header through which the evaporated gas is discharged from the storage tank is lower than a preset low pressure setting value, the reliquefaction capacity controller reduces the reliquefaction capacity of the reliquefaction device to maintain the pressure in the storage tank.

Description

[Technical Field]

[0001] The present invention relates to a boil-off gas (BOG) re-liquefaction system and method for cooling and re-liquefying boil-off gas (BOG) generated from liquefied gas stored in a storage tank of a ship, adjusting the re-liquefaction volume according to the pressure in the storage tank, and maintaining a constant pressure in the storage tank. [Background technology]

[0002] Natural gas, which is primarily composed of methane and emits almost no environmental pollutants when burned, has been attracting 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. Compared to natural gas in its gaseous state, its volume is reduced to approximately 1 / 600, making it highly suitable for long-distance transportation via sea routes. For this reason, natural gas is mainly stored and transported in the liquid state of LNG, 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 normal pressure, LNG storage tanks are usually insulated to maintain the LNG in a liquid state. However, even with insulation, it is difficult to completely block external heat. Therefore, as external heat is continuously transferred 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] As evaporation gas continues to be generated inside an LNG storage tank, the pressure inside the tank increases. If the pressure inside the storage tank exceeds the set safety pressure, an emergency such as tank rupture may occur, so it is necessary to use a safety valve to release the evaporation gas outside the storage tank. However, since evaporation gas is one type of LNG loss and is a significant problem in terms of LNG transportation efficiency and fuel efficiency, various methods are used to deal with the evaporation gas generated in storage tanks.

[0005] In recent years, methods have been developed and are being used, such as using evaporated gas at fuel demand points 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] When applying a reliquefaction cycle to a ship, typical reliquefaction cycles that reliquefy evaporated gas include the SMR cycle and the C3MR cycle. The C3MR cycle (Propane-precooled Mixed Refrigerant Cycle) uses a single refrigerant, propane, to cool the evaporated gas, and then reliquefies it by cooling it with a mixed refrigerant. The SMR cycle (Single Mixed Refrigerant Cycle) reliquefies evaporated gas using a mixed refrigerant composed of multiple components.

[0007] These SMR and C3MR cycles use a mixed refrigerant, and as the liquefaction process progresses, the refrigerant leaks. This causes the composition ratio of the mixed refrigerant to change, reducing the liquefaction efficiency. Therefore, it is necessary to continuously measure the composition ratio of the mixed refrigerant and replenish any missing refrigerant components to maintain the refrigerant composition.

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

[0009] Although nitrogen refrigerant has a lower cooling efficiency than refrigeration cycles that use mixed refrigerants, it has the advantage of being an inert substance, highly safe, and not subject to phase changes, making it easy to apply to ships.

[0010] In this way, the evaporated gas generated during the operation of the ship is discharged from the storage tank, compressed by a compressor, and then supplied as fuel or supplied to a re-liquefaction cycle where it is re-liquefied and collected in the storage tank. In this case, the re-liquefaction capacity of the re-liquefaction system is adjusted by adjusting the amount of cold energy in the re-liquefaction cycle using a controller.

[0011] However, if the reliquefaction system continues to reliquefy a larger amount of evaporated gas than the amount of evaporated gas generated in the storage tank, particularly in a reliquefaction cycle using nitrogen refrigerant, and if the amount of evaporated gas generated in the storage tank is decreasing but the reliquefaction system supplies supercooled reliquefied gas reliquefied by the reliquefaction system to the storage tank in order to maintain the reliquefaction capacity of the reliquefaction system, the pressure inside the storage tank may drop excessively, which could lead to dangerous situations such as tank damage.

[0012] To solve such problems, the present invention provides an evaporated gas reliquefaction system and an evaporated gas reliquefaction method that can operate the reliquefaction system in accordance with the pressure in the storage tank and maintain a stable pressure in the storage tank. [Means for solving the problem]

[0013] In order to solve the above problem, an embodiment of the present invention provides an evaporated gas reliquefaction system for a ship, which is installed on a ship and includes: a storage tank for storing liquefied gas; a compressor for compressing evaporated gas generated from the liquefied gas; a reliquefaction device for cooling and reliquefying the compressed gas obtained by compressing the evaporated gas in the compressor by heat exchange with a refrigerant circulating in a refrigerant circulation section; and a reliquefaction capacity controller for controlling the reliquefaction capacity of the reliquefaction device, wherein when the pressure value of the evaporated gas detected in a vapor header through which the evaporated gas is discharged from the storage tank is lower than a predetermined low pressure setting value, the reliquefaction capacity controller reduces the reliquefaction capacity of the reliquefaction device to maintain the pressure in the storage tank.

[0014] Preferably, the storage tank further comprises a first pressure transmitter for detecting the absolute pressure of the evaporated gas in the vapor header, a second pressure transmitter for detecting the gauge pressure of the evaporated gas in the vapor header, a normal pressure control unit that receives the pressure value detected by the first pressure transmitter and adjusts the reliquefaction capacity of the reliquefaction device to maintain the pressure in the storage tank at a target value, and a low pressure control unit that controls the reliquefaction capacity controller to forcibly reduce the reliquefaction capacity of the reliquefaction device when the pressure value detected by the second pressure transmitter is lower than the low pressure set value.

[0015] Preferably, the normal pressure control unit includes a first normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in accordance with the pressure value detected by the first pressure transmitter, a second normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in accordance with the pressure value detected by the second pressure transmitter, and a selector that selects one of the operation signals from the first and second normal pressure controllers and outputs an operation signal to the reliquefaction capacity controller to adjust the reliquefaction capacity, and the normal pressure control unit and the reliquefaction capacity controller are connected in a cascade manner.

[0016] Preferably, a plurality of the reliquefaction apparatuses are provided on board the vessel, each of the reliquefaction apparatuses being installed as an independent train, and each of the reliquefaction apparatuses being provided with a reliquefaction capacity controller.

[0017] Preferably, each of the trains is provided with a train capacity controller for controlling the reliquefaction capacity controller of the reliquefaction device of each train.

[0018] Also preferably, each reliquefaction device in the train is operated in connection with the normal pressure control unit, or is operated independently of the normal pressure control unit by the train capacity controller.

[0019] In addition, an embodiment of the present invention provides a method for re-liquefying evaporated gas on a ship, in which evaporated gas generated from liquefied gas stored in a storage tank of the ship is compressed by a compressor, the compressed evaporated gas is cooled and re-liquefied in a re-liquefaction device by heat exchange with a refrigerant circulating in a refrigerant circulation section, and a re-liquefaction capacity controller is provided for controlling the re-liquefaction capacity of the re-liquefaction device, characterized in that when the pressure value of the evaporated gas detected in a vapor header from which the evaporated gas is discharged from the storage tank is lower than a predetermined low pressure setting value, the re-liquefaction capacity controller reduces the re-liquefaction capacity of the re-liquefaction device to maintain the pressure in the storage tank.

[0020] Preferably, the absolute pressure of the evaporated gas in the vapor header is detected by a first pressure transmitter, the detected pressure value is transmitted to a normal pressure control unit, and the re-liquefaction capacity of the re-liquefaction device is adjusted so that the pressure in the storage tank is maintained at a target value; the gauge pressure of the evaporated gas in the vapor header is detected by a second pressure transmitter, and if the pressure value detected by the second pressure transmitter is lower than the low pressure set value, the low pressure control unit controls the re-liquefaction capacity controller so that the re-liquefaction capacity of the re-liquefaction device is forcibly reduced.

[0021] In addition, preferably, in a method for reliquefying evaporated gas on a ship, the normal pressure control unit is provided with a first normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in accordance with the pressure value detected by the first pressure transmitter, a second normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in accordance with the pressure value detected by the second pressure transmitter, and a selector that selects one of the operation signals from the first and second normal pressure controllers and outputs an operation signal to the reliquefaction capacity controller to adjust the reliquefaction capacity, and the normal pressure control unit and the reliquefaction capacity controller are connected in a cascade manner.

[0022] Also, preferably, in a method for reliquefying evaporated gas on a ship, a plurality of reliquefaction devices are provided, each reliquefaction device being provided as an independent train, and each reliquefaction device being provided with a reliquefaction capacity controller that controls the reliquefaction capacity of the reliquefaction device, the reliquefaction devices of each train are either connected to the normal pressure control unit and operated, or are operated independently from the normal pressure control unit by a train capacity controller that controls the reliquefaction capacity controller of the reliquefaction device provided in each train. [Effects of the Invention]

[0023] According to the present invention, the reliquefaction capacity of the reliquefaction device can be adjusted in accordance with the pressure inside the storage tank, thereby making it possible to maintain the pressure inside the storage tank constant.

[0024] Furthermore, even when multiple reliquefaction units using other refrigerants such as nitrogen refrigerant are installed, the reliquefaction capacity of the reliquefaction units can be adjusted according to the pressure inside the storage tank, allowing the reliquefaction system to operate efficiently and maintaining the pressure inside the storage tank. This prevents tank damage caused by excessive increases or decreases in pressure inside the storage tank, ensuring the safety of the ship. [Brief explanation of the drawings]

[0025] [Figure 1]1 is a schematic diagram illustrating a ship vapor reliquefaction system according to an embodiment of the present invention. [Figure 2] This is a graph showing the change in the total load of all reliquefaction devices due to the change in the output value output based on the pressure of evaporated gas detected by the vapor header when three reliquefaction devices are installed in a reliquefaction system of an embodiment of the present invention. [Figure 3] This is a graph showing the change in the total load of all reliquefaction devices due to the change in the output value output based on the pressure of evaporated gas detected by the vapor header when three reliquefaction devices are installed in a reliquefaction system of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] 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.

[0027] 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.

[0028] 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).

[0029] 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 ethane 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.

[0030] FIG. 1 is a schematic diagram showing a re-liquefaction system for evaporated gas on a ship according to an embodiment of the present invention.

[0031] As shown in Figure 1, the reliquefaction system of this embodiment is installed on a ship and includes a storage tank T for storing liquefied gas, a compressor for compressing evaporated gas generated from the liquefied gas, and a reliquefaction device NRS for cooling and re-liquefying the compressed gas compressed by the compressor through heat exchange with a refrigerant circulating in a refrigerant circulation section.

[0032] The evaporated gas generated from the liquefied gas stored in the storage tank T is discharged through the vapor header VH and supplied to a compressor (not shown). The evaporated gas is compressed by the compressor (not shown), for example, to a fuel supply pressure for the ship's main engine. For example, the evaporated gas 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 evaporated gas is supplied as fuel to the ship's main engine (not shown), and any remaining evaporated gas that is not supplied as fuel is re-liquefied.

[0033] According to ship regulations, the compressor that supplies fuel to the engine is required to have a redundant design (redundancy) in preparation for an emergency. Although the present embodiment will be described mainly using one compressor as an example, the compressor may be configured to include a main compressor and a standby compressor.

[0034] Of the gas compressed by the compressor, the remaining gas that is not supplied as fuel is supplied to the reliquefaction unit NRS and reliquefied.

[0035] The reliquefaction unit NRS includes a heat exchanger that cools, by heat exchange, the compressed gas obtained by compressing the evaporated gas in the compressor, and a gas-liquid separator that is provided downstream of the heat exchanger and separates the reliquefied gas obtained by reliquefying the evaporated gas into gas and liquid. Furthermore, if necessary, a pressure reducing valve that reduces the pressure of the compressed gas cooled by the heat exchanger to adjust the amount of reliquefaction is provided upstream of the gas-liquid separator on the reliquefaction line that reliquefies the evaporated gas and recovers it in the storage tank T.

[0036] In the heat exchanger, the compressed gas is cooled and re-liquefied using the refrigerant circulating in the refrigerant circulation section as a cold source. Also, by supplying the evaporated gas discharged from the storage tank T to the heat exchanger, where the cold is recovered, and then supplied to the compressor, the cold of the uncompressed evaporated gas before it is supplied to the compressor can also be used in the heat exchanger.

[0037] After being cooled in the heat exchanger, the reliquefied gas separated in the gas-liquid separator is supplied to the storage tank T and stored again. Meanwhile, the flash gas separated in the gas-liquid separator is supplied to the flow of evaporative gas before compression upstream of the heat exchanger in the evaporative gas supply line that supplies the evaporative gas discharged from the vapor header VH to a compressor (not shown), or is sent to the gas combustion unit (GCU).

[0038] In the refrigerant circulation section (not shown) of the reliquefaction unit NRS, a refrigerant circulates in a refrigerant circulation line and cools the compressed gas by heat exchange in a heat exchanger. The refrigerant circulating in the refrigerant circulation line is, for example, a nitrogen refrigerant.

[0039] The refrigerant circulation unit includes a refrigerant expander that expands and cools the refrigerant supplied to the heat exchanger, and a refrigerant compressor that is connected to the refrigerant expander and receives the refrigerant's expansion energy to compress the refrigerant discharged from the heat exchanger after heat exchange. A motor is also provided to drive the refrigerant compressor, and the refrigerant compressor and refrigerant expander are connected via a shaft, utilizing the refrigerant's expansion energy to compress the refrigerant. This reduces the power required to operate the refrigeration cycle.

[0040] The refrigerant cooled by expansion in the refrigerant expander is supplied to a heat exchanger to provide cold, where it exchanges heat, is discharged from the heat exchanger, and is compressed in a refrigerant compressor. The refrigerant compressed in the refrigerant compressor is supplied to the heat exchanger and cooled, then supplied to a refrigerant expander and cooled by expansion, and is supplied to the heat exchanger again, whereby the refrigerant circulates through the refrigerant circulation line.

[0041] Therefore, in the heat exchanger, heat is exchanged between four flows: the evaporative gas compressed by the compressor, the uncompressed evaporative gas before being supplied to the compressor, the refrigerant cooled by expansion in the refrigerant expander, and the refrigerant compressed by the refrigerant compressor. That is, in the heat exchanger, the compressed gas compressed by the compressor and the refrigerant compressed by the refrigerant compressor are cooled by heat exchange with the uncompressed evaporative gas before being supplied to the compressor and the refrigerant cooled by expansion in the refrigerant expander.

[0042] Such a reliquefaction unit NRS is provided with reliquefaction capacity controllers NCC1, NCC2, and NCC3 that control the reliquefaction capacity.

[0043] A plurality of reliquefaction units NRS may be provided on board the ship. When a plurality of reliquefaction units NRS are provided, each reliquefaction unit NRS is installed on board the ship as an independent train, and each reliquefaction unit train TR1, TR2, TR3 is provided with a reliquefaction capacity controller NCC1, NCC2, NCC3, respectively. Each train TR1, TR2, TR3 provided with a reliquefaction unit NRS is provided with a train capacity controller TLC1, TLC2, TLC3 that controls the reliquefaction capacity controller NCC1, NCC2, NCC3 of the reliquefaction unit NRS provided on each train TR1, TR2, TR3.

[0044] According to this embodiment, the reliquefaction capacity of the reliquefaction device NRS is adjusted according to the pressure inside the storage tank T.

[0045] For this reason, the re-liquefaction system of this embodiment is provided with a first pressure transmitter PT1 that detects the absolute pressure of the evaporated gas in the vapor header VH, and a second pressure transmitter PT2 that detects the gauge pressure of the evaporated gas in the vapor header VH.

[0046] The reliquefaction system of this embodiment is equipped with a normal pressure control unit that adjusts the reliquefaction capacity of the reliquefaction device NRS in accordance with the pressure values ​​detected by the first and second pressure transmitters PT1 and PT2 so that the pressure in the storage tank T is maintained at a target value within a predetermined range.

[0047] The reliquefaction system of this embodiment is further equipped with a low pressure control unit LPC that forcibly reduces the reliquefaction capacity of the reliquefaction device NRS, particularly to prevent the pressure in the storage tank T from dropping excessively.

[0048] If the pressure value of the vapor header VH detected by the second pressure transmitter PT2 is lower than the preset low pressure value, the low pressure control unit LPC controls the reliquefaction capacity controllers NCC1, NCC2, and NCC3 to forcibly reduce the reliquefaction capacity of the reliquefaction device NRS, thereby preventing the pressure in the storage tank T from dropping too much and thus preventing damage to the tank.

[0049] The normal pressure control unit includes a first normal pressure controller NPC1 that outputs an operation signal to adjust the reliquefaction capacity of the reliquefaction unit NRS in response to the pressure value detected by the first pressure transmitter PT1, a second normal pressure controller NPC2 that outputs an operation signal to adjust the reliquefaction capacity of the reliquefaction unit NRS in response to the pressure value detected by the second pressure transmitter PT2, and a selector SS that selects one of the operation signals from the first and second normal pressure controllers NPC1 and NPC2 and outputs an operation signal to adjust the reliquefaction capacity to the reliquefaction capacity controllers NCC1, NCC2, and NCC3 of each train TR1, TR2, and TR3. The normal pressure control unit and each reliquefaction capacity controller NCC1, NCC2, and NCC3 are connected in a cascade configuration, automatically adjusting the reliquefaction capacity of each reliquefaction unit NRS to maintain the pressure in the storage tank T at a preset target value.

[0050] When multiple reliquefaction device trains TR1, TR2, TR3 are provided, the reliquefaction device NRS of each train TR1, TR2, TR3 may be connected to the normal pressure control unit and operated individually, or the reliquefaction device NRS may be operated independently of the normal pressure control unit by each train capacity controller TLC1, TLC2, TLC3.

[0051] Figures 2 and 3 are graphs showing the change in the total load of all reliquefaction devices due to the change in the output value based on the pressure of evaporated gas detected by the vapor header VH when three reliquefaction device trains TR1, TR2, and TR3 are installed.

[0052] First, the graph in Figure 2 shows the change in the total load of all reliquefaction units NRS due to changes in the output value when three reliquefaction unit trains TR1, TR2, and TR3 are connected to a normal pressure control unit and the reliquefaction capacity controllers NCC1, NCC2, and NCC3 of each train TR1, TR2, and TR3 share the reliquefaction load according to the output value of the normal pressure control unit based on the pressure in the storage tank T.

[0053] Point A in the graph of Figure 2 corresponds to the case where the output value of the normal pressure control unit is 0%, and the load of the reliquefaction unit (NRS) of each train TR1, TR2, TR3 at this time is approximately 11%, and the total load of all reliquefaction unit (NRS) of the three reliquefaction unit trains TR1, TR2, TR3 at this time is approximately 33% of the minimum value. Point B in the graph corresponds to the case where the output value of the normal pressure control unit is 53%, and the load of all reliquefaction unit (NRS) of each train TR1, TR2, TR3 at this time is approximately 58%, and the total load of all reliquefaction unit (NRS) of the three reliquefaction unit trains TR1, TR2, TR3 at this time is approximately 173%. Point C on the graph represents the case where the output value of the normal pressure control unit is 85% and two trains TR1 and TR2 are operating, with the load on the reliquefaction units of each train TR1 and TR2 at approximately 87%, and the total load on both reliquefaction unit NRS of the two reliquefaction unit trains TR1 and TR2 at this time is approximately 180%. Point D on the graph represents the case where the output value of the normal pressure control unit is 100%, with the load on the reliquefaction unit NRS of each train TR1, TR2, and TR3 at this time also being 100%, and the total load on all reliquefaction unit NRS of the three reliquefaction unit trains TR1, TR2, and TR3 at this time being 300% of the maximum value.

[0054] Next, the graph in Figure 3 shows the change in the total load of all reliquefaction units NRS due to changes in output value when one train TR1 is operated individually by the train capacity controller TLC1 and two trains TR2 and TR3 are connected to the normal pressure control unit and the reliquefaction load is shared according to their output values.

[0055] The first train TR1 is operated individually with a fixed re-liquefaction load of 58% by the train capacity controller TLC1, and the second and third trains TR2 and TR3 are connected to a normal pressure control unit, and the re-liquefaction load is shared according to the output value of the normal pressure control unit based on the pressure of the evaporated gas in the storage tank T.

[0056] At point A in the graph of Figure 3, the reliquefaction load of the first train TR1 is 58%, and the output value of the normal pressure control unit of the second and third trains TR2 and TR3 is 0%. At this time, the reliquefaction unit NRS of each train TR2 and TR3 operates at a minimum load of approximately 11%, and the total load of all reliquefaction units NRS at this time is approximately 80%. At point B in the graph, the reliquefaction load of the first train TR1 is 58%, and the output value of the normal pressure control unit of the second and third trains TR2 and TR3 is 53%. At this time, the reliquefaction unit NRS of each train TR2 and TR3 operates at a load of approximately 58%, and the total load of all reliquefaction units NRS at this time is approximately 173%. At point C in the graph, the reliquefaction load of the first train TR1 is 58%, and the output value of the normal pressure control unit of the second and third trains TR2 and TR3 is the maximum value of 100%. At this time, the reliquefaction units NRS of each train TR2 and TR3 operate at a load of 100%, and the total load of all the reliquefaction units NRS at this time is 258%.

[0057] As described above, in the reliquefaction system of this embodiment, the reliquefaction device trains TR1, TR2, TR3 can be operated as needed by connecting the reliquefaction device NRS of each train TR1, TR2, TR3 to a normal pressure control unit and operating it according to its output value, and the load on the reliquefaction device NRS can be adjusted by operating them individually using the train capacity controllers TLC1, TLC2, TLC3 provided in each train TR1, TR2, TR3.

[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 storage tank mounted on the ship for storing liquefied gas; and a compressor that compresses evaporated gas generated from the liquefied gas; and a re-liquefaction device that cools and re-liquefies the compressed gas obtained by compressing the evaporated gas in the compressor through heat exchange with a refrigerant circulating in a refrigerant circulation section; and a reliquefaction capacity controller for controlling the reliquefaction capacity of the reliquefaction device; When the pressure value of the evaporated gas detected in the vapor header through which the evaporated gas is discharged from the storage tank is lower than a preset low pressure setting value, the re-liquefaction capacity controller reduces the re-liquefaction capacity of the re-liquefaction device to maintain the pressure in the storage tank. Ship's vapor reliquefaction system.

2. a first pressure transmitter for detecting the absolute pressure of the vaporized gas in the vapor header; and a second pressure transmitter for detecting a gauge pressure of the vapor gas in the vapor header; and a normal pressure control unit that receives the pressure value detected by the first pressure transmitter and adjusts the reliquefaction capacity of the reliquefaction device so as to maintain the pressure in the storage tank at a target value; and a low pressure control unit that controls the reliquefaction capacity controller to forcibly reduce the reliquefaction capacity of the reliquefaction device when the pressure value detected by the second pressure transmitter is lower than the low pressure setting value. The ship vapor reliquefaction system according to claim 1.

3. The normal pressure control unit is a first normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in response to the pressure value detected by the first pressure transmitter; and a second normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in response to the pressure value detected by the second pressure transmitter; and a selector that selects one of the operation signals from the first and second normal pressure controllers and outputs an operation signal to the reliquefaction capacity controller to adjust the reliquefaction capacity; The normal pressure control unit and the reliquefaction capacity controller are connected in a cascade system.

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

4. A plurality of the reliquefaction units are provided on the ship, each reliquefaction unit being installed as an independent train; Each reliquefaction device is provided with a reliquefaction capacity controller.

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

5. Each of the trains is provided with a train capacity controller that controls the reliquefaction capacity controller of the reliquefaction device of each train.

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

6. Each reliquefaction device in the train is operated by being connected to the normal pressure control unit, or is operated by the train capacity controller independently of the normal pressure control unit. The ship vapor reliquefaction system according to claim 5.

7. The evaporated gas generated from the liquefied gas stored in the ship's storage tank is compressed by a compressor, The compressed evaporated gas is cooled and re-liquefied in the re-liquefaction device by heat exchange with the refrigerant circulating in the refrigerant circulation section, A method for reliquefying evaporated gas on a ship, comprising: When the pressure value of the evaporated gas detected in the vapor header through which the evaporated gas is discharged from the storage tank is lower than a preset low pressure setting value, the re-liquefaction capacity controller reduces the re-liquefaction capacity of the re-liquefaction device to maintain the pressure in the storage tank. A method for re-liquefying ship's vapors.

8. Detecting the absolute pressure of the evaporated gas in the vapor header with a first pressure transmitter, transmitting the detected pressure value to a normal pressure control unit, and adjusting the re-liquefaction capacity of the re-liquefaction device so that the pressure in the storage tank is maintained at a target value; The gauge pressure of the evaporated gas in the vapor header is detected by a second pressure transmitter, and when the pressure value detected by the second pressure transmitter is lower than the low pressure setting value, a low pressure control unit controls the reliquefaction capacity controller so that the reliquefaction capacity of the reliquefaction device is forcibly reduced. The method for reliquefying evaporated gas from a ship according to claim 7.

9. The normal pressure control section a first normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in response to the pressure value detected by the first pressure transmitter; and a second normal pressure controller that outputs an operation signal for adjusting the reliquefaction capacity of the reliquefaction device in response to the pressure value detected by the second pressure transmitter; and a selector that selects one of the operation signals from the first and second normal pressure controllers and outputs an operation signal to the reliquefaction capacity controller to adjust the reliquefaction capacity, The normal pressure control unit and the reliquefaction capacity controller are connected in a cascade system. The method for reliquefying evaporated gas from a ship according to claim 8.

10. A plurality of the reliquefaction devices are provided, and each reliquefaction device is provided as an independent train, A method for reliquefying evaporated gas on a ship, in which each reliquefaction device is provided with a reliquefaction capacity controller, The reliquefaction device of each train is operated by being connected to the normal pressure control unit, or is operated independently of the normal pressure control unit by a train capacity controller that controls the reliquefaction capacity controller of the reliquefaction device provided in each train. The method for reliquefying evaporated gas from a ship according to claim 9.

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