Microleak detection system for marine reliquefaction systems

Abstract

A microleak detection system for a reliquefaction system for a ship is disclosed. The microleak detection system for a reliquefaction system for a ship of the present invention, in which evaporated gas generated from liquefied gas stored in a storage tank of a ship is supplied to a heat exchanger to recover cold energy, compressed, and cooled by heat exchange with a refrigerant circulating in a refrigerant circulation line in the heat exchanger to reliquefy the gas, is characterized in that the reliquefaction system for a ship includes a heater that heats the evaporated gas supplied from the storage tank to the heat exchanger by heat exchange with a heat source, the heat source is connected to a residual liquid discharge port that discharges residual liquid from the heater, and includes a microleak detection device that detects microleakage of the heat source from the heater.

Description

[Technical field]

[0001] The present invention relates to a microleak detection system for a reliquefaction system for a ship, and more particularly to a microleak detection system that detects microleakage in a heater provided upstream of a heat exchanger in a reliquefaction system that reliquefies evaporated gas generated in a storage tank provided on a ship, and prevents foreign matter from entering the heat exchanger. [Background technology]

[0002] Natural gas, which is mainly composed of methane, is attracting attention as an environmentally friendly fuel because it emits almost no environmental pollutants when burned. Liquefied natural gas (LNG) is obtained by liquefying natural gas by cooling it to about -163°C at normal pressure, and since its volume is reduced to about 1 / 600 of that of gaseous natural gas, it is very suitable for long-distance transportation using 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] Since 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 keep LNG in a liquid state, but even if LNG storage tanks are insulated, 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] If evaporation gas continues to be generated in an LNG storage tank, the pressure in the LNG storage tank will rise. If the pressure in 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 to the outside of 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 treat the evaporation gas generated in the storage tank.

[0005] In recent years, methods have been developed and are being used that involve using evaporative gas at fuel demand sources such as ship engines, re-liquefying evaporative gas and recovering it in storage tanks, or a combination of 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 uses a mixed refrigerant to reliquefy it, while the SMR cycle (Single Mixed Refrigerant Cycle) uses a mixed refrigerant composed of multiple components to reliquefy the evaporated gas.

[0007] In these SMR and C3MR cycles, a mixed refrigerant is used, and as the liquefaction process progresses, the refrigerant leaks. This causes the composition ratio of the mixed refrigerant to change, decreasing the liquefaction efficiency. Therefore, it is necessary to maintain the refrigerant composition by continuously measuring the composition ratio of the mixed refrigerant and replenishing the missing refrigerant components.

[0008] As another reliquefaction method utilizing a reliquefaction cycle, a single-cycle reliquefaction method in which nitrogen refrigerant is used is known.

[0009] Nitrogen refrigerant has a lower cooling efficiency than refrigeration cycles that use mixed refrigerants. However, since nitrogen refrigerant is an inert substance, it is very safe. In addition, since the refrigerant does not undergo phase change, it has the advantage of being easily applicable to ships.

[0010] The reliquefaction system includes a compressor that receives and compresses the evaporated gas, a heat exchanger that cools the compressed gas obtained by compressing the evaporated gas in the compressor by heat exchange with a refrigerant, and a refrigerant circulation section in which the refrigerant used for heat exchange with the compressed gas in the heat exchanger circulates. In the case of a reliquefaction system that applies a refrigeration cycle using a nitrogen refrigerant, in the refrigerant circulation section, the nitrogen refrigerant discharged from the heat exchanger after heat exchange in the heat exchanger is compressed, supplied to the heat exchanger and cooled, further cooled by expansion, and then supplied to the heat exchanger again, thereby circulating the nitrogen refrigerant.

[0011] The temperature of the evaporated gas generated from LNG is about -100°C, and depending on the state of the storage tank, the temperature of the evaporated gas may be -130°C or lower. The temperature of the nitrogen refrigerant is lower than that of the evaporated gas, and the heat exchanger to which the evaporated gas and nitrogen refrigerant are supplied is subjected to thermal stress. In particular, when the temperature of the heat exchanger is about the same as room temperature, such as when the reliquefaction system is started, or when the heat exchanger is not sufficiently cooled down and extremely low temperature evaporated gas or the like is allowed to flow directly into the heat exchanger, or when the temperature of the evaporated gas changes due to a change in the state of the storage tank, the temperature difference between the heat exchanger and the evaporated gas becomes large, and the thermal stress applied to the heat exchanger increases, which may damage the heat exchanger.

[0012] The present invention proposes a reliquefaction system that solves these problems, reduces the thermal stress on the heat exchanger, and prevents problems such as micro-leaks that occur in additional equipment installed in addition to the above and the inflow of foreign matter into the heat exchanger due to micro-leaks. [Means for solving the problem]

[0013] In order to solve the above problems, an embodiment of the present invention provides a microleak detection system for a reliquefaction system for a ship, in which evaporated gas generated from liquefied gas stored in a storage tank of a ship is supplied to a heat exchanger to recover cold heat, compressed, and cooled by heat exchange with a refrigerant circulating in a refrigerant circulation line in the heat exchanger to reliquefy the gas, wherein the reliquefaction system for a ship is characterized in that it includes a heater that heats the evaporated gas supplied from the storage tank to the heat exchanger by heat exchange with a heat source, and a microleak detection device that is connected to a residual liquid discharge port that discharges residual liquid remaining in the heat source from the heater and detects microleakage of the heat source from the heater.

[0014] In addition, preferably, the microleak detection device includes a connection tube that is fastened to the residual liquid discharge outlet and extends downward from the residual liquid discharge outlet, and a leak detection unit that is provided on the connection tube and checks for the presence or absence of residual liquid being discharged from the residual liquid discharge outlet.

[0015] In addition, preferably, the microleak detection device further includes a first shutoff valve provided on the inlet side of the leak detection section of the connecting tube, a second shutoff valve provided on the outlet side of the leak detection section of the connecting tube, and a tube plug provided at the lower end of the connecting tube.

[0016] Also, preferably, in the microleak detection device, the first shutoff valve operates in a normally open state, and the second shutoff valve operates in a normally closed state, and during maintenance of the heater, the second shutoff valve is opened to drain the residual liquid from the heater.

[0017] Also, preferably, the leakage detection unit is a sight glass that enables visual confirmation of the presence or absence of the residual liquid being discharged from the residual liquid discharge port.

[0018] In addition, preferably, the leakage detection unit is a liquid level detector which detects the residual liquid discharged from the residual liquid discharge port.

[0019] Also preferably, the heat exchanger is a cryogenic heat exchanger, and the heater is a shell-and-tube heat exchanger. Effect of the Invention

[0020] In a re-liquefaction system equipped with the micro-leak detection system of the present invention, the temperature of the evaporated gas supplied to the heat exchanger is adjusted by a heater provided upstream of the heat exchanger, thereby reducing the thermal stress on the heat exchanger and preventing damage to the heat exchanger, even when the re-liquefaction system is started up or when the condition of the storage tank changes and the temperature of the evaporated gas changes.

[0021] In particular, the micro-leak detection system of the present invention can detect micro-leakage that is within the range of measurement error by devices such as pressure sensors. Therefore, even if the reliquefaction system is equipped with a heater that uses antifreeze or the like as a working fluid, it is possible to prevent the inflow of foreign matter into the heat exchanger caused by leakage of antifreeze or the like, thereby preventing corrosion inside the heat exchanger and shortened equipment life. [Brief description of the drawings]

[0022] [Figure 1] 1 is a schematic diagram showing the structure of a heater of a marine reliquefaction system provided with a microleak detection system according to an embodiment of the present invention; [Diagram 2] 1 is a schematic diagram showing a microleak detection system for a marine reliquefaction system according to an embodiment of the present invention. [Diagram 3] 3 shows in more detail the microleak detection device portion of the microleak detection system shown in FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The operational advantages and objectives attained by the embodiments of the present invention will now be described, by way of example only, with reference to the drawings and the contents depicted in the drawings.

[0024] Hereinafter, the configuration and operation of the embodiment of the present invention will be described with reference to the drawings. Note that, as far as possible, the same reference numerals are used for the same components in the different drawings.

[0025] The ship according to the embodiment of the present invention described later may be any type of ship provided with a storage tank for storing liquefied gas. Representative examples include ships having self-propelling capability, such as an LNG carrier, a liquid hydrogen carrier, and an LNG RV (regasification vessel), as well as floating offshore structures having no propulsion capability, such as an LNG FPSO (floating production storage offloading) and an LNG FSRU (floating storage regasification unit).

[0026] In addition, this embodiment can liquefy gas at a low temperature for transportation, and can be applied to the reliquefaction cycle of all kinds 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. In the embodiment described below, LNG, which is one of the representative liquefied gases, will be described as an example.

[0027] In a ship re-liquefaction system to which the microleak detection system of this embodiment is applied, evaporated gas generated from liquefied gas stored in a storage tank of the ship is discharged from a vapor header, supplied to a compressor for compression, and supplied as fuel to an onboard engine, etc., as necessary. The remaining evaporated gas that is not supplied as fuel is cooled and re-liquefied in a heat exchanger, and then returned to the storage tank.

[0028] In this reliquefaction system, evaporated gas generated from liquefied gas stored in the ship's storage tank is discharged from a vapor header and supplied to the compressor through a gas supply line. The gas supply line connects the storage tank and the compressor via a heat exchanger, and uncompressed evaporated gas discharged from the storage tank CH before being supplied to the compressor is supplied to the heat exchanger as cold heat.

[0029] The evaporated gas compressed by the compressor is supplied again to the heat exchanger and cooled by the cold energy of the uncompressed evaporated gas supplied through the gas supply line.

[0030] In addition to the non-compressed evaporative gas, other refrigerants circulating through a refrigerant circulation line (not shown) can also be supplied to the heat exchanger. For example, nitrogen refrigerant (N2) is used as the refrigerant circulating through the refrigerant circulation line. The refrigerant circulation line is provided with a refrigerant compressor that compresses the nitrogen refrigerant and a refrigerant expander that expands the nitrogen refrigerant compressed by the refrigerant compressor. The nitrogen refrigerant in the refrigerant circulation line is compressed by the refrigerant compressor, supplied to the heat exchanger and cooled, expanded and cooled by the refrigerant expander, and then supplied to the heat exchanger again as a refrigerant to circulate through the refrigerant circulation line. As a result, in the heat exchanger, heat is exchanged between four flows: the evaporative gas compressed by the compressor, the non-compressed 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.

[0031] The evaporated gas cooled in the heat exchanger is separated into gas and liquid, and the separated re-liquefied gas is collected in a storage tank.

[0032] However, when the evaporated gas is cooled and re-liquefied in a heat exchanger in this manner, when the re-liquefaction system is started up or when the condition of the storage tank changes and the temperature of the evaporated gas changes, if evaporated gas is supplied to the heat exchanger, the thermal stress on the heat exchanger may increase.

[0033] Usually, the temperature of the evaporated gas discharged from the storage tank and supplied to the heat exchanger is about -100°C, and depending on the state of the storage tank, it may be -130°C or lower. In order to handle the extremely low temperature evaporated gas generated from such LNG and the nitrogen refrigerant of the refrigeration cycle, a cryogenic heat exchanger such as a plate-fin type cryogenic heat exchanger (CRYOGENIC HEAT EXCHANGER, CHE) is provided as a heat exchanger, but depending on the temperature of the evaporated gas, excessive thermal stress is applied to the heat exchanger. In particular, when extremely low temperature evaporated gas is directly supplied to a heat exchanger immediately after the re-liquefaction system is restarted (i.e., a heat exchanger in a state in which the temperature of the heat exchanger is about the same as room temperature and is not sufficiently cooled), the larger the temperature difference between the temperature of the heat exchanger and the temperature of the evaporated gas, the larger the thermal stress applied to the heat exchanger, which may cause problems such as damage to the heat exchanger due to fatigue failure or shortening of the life of the heat exchanger.

[0034] In order to solve such problems, the reliquefaction system provided with the microleak detection system of this embodiment is provided with a heating line that branches off from the gas supply line upstream of the heat exchanger and heats all or a part of the evaporated gas supplied to the heat exchanger. The evaporated gas heated in the heating line is supplied to the upstream side of the heat exchanger (i.e., between the heat exchanger and the branch point of the heating line that branches off from the gas supply line upstream of the heat exchanger). In addition, the heating line is provided with a heater that heats the evaporated gas. A shell-tube heat exchanger can be used as the heater, and antifreeze or glycol water can be used as the heat source (working fluid) of the heater.

[0035] FIG. 1 is a schematic diagram showing the structure of a heater provided in a marine reliquefaction system to which the microleak detection system of this embodiment is applied.

[0036] As shown in Fig. 1, the evaporative gas BOG is heated by passing through the heater 100 and then discharged from the heater 100. Antifreeze liquid GW is supplied to the heater 100 as a heat source for heating the evaporative gas. The antifreeze liquid GW cooled by supplying heat to the evaporative gas by the heater 100 is discharged to the outside of the heater 100. A residual liquid discharge port 110 is provided at the bottom of the heater 100 for discharging the antifreeze liquid GW remaining in the heater 100 to the outside of the heater 100 during maintenance of the heater 100.

[0037] In this way, a part (or all) of the evaporated gas supplied from the storage tank is heated by the heater 100, and the flow of the evaporated gas heated by the heater 100 is merged with the flow of the evaporated gas that has not passed through the heater 100 and is not heated, and the resulting mixture is supplied to the heat exchanger, thereby adjusting the temperature of the evaporated gas supplied to the heat exchanger. This reduces the thermal stress on the heat exchanger, reduces the thermal fatigue of the heat exchanger, and prevents damage to the heat exchanger.

[0038] On the other hand, if such a heater 100 is installed upstream of a heat exchanger, if a leak of antifreeze or the like occurs at the connection with the heater 100 or within the piping of the heater 100, the antifreeze or the like may mix with the evaporative gas and flow into the heat exchanger, which may cause corrosion inside the heat exchanger, shorten the life of the device, or damage to the heat exchanger.

[0039] To prevent this, pressure sensors are installed upstream and downstream of the heat exchanger to detect changes in the state and flow rate of the evaporated gas. If the pressure sensor detects an abnormality, the control unit of the reliquefaction system will issue an alarm or automatically perform control such as an emergency stop.

[0040] However, if the leakage of antifreeze or the like is minute (small leakage, fine leakage) that is within the range of the measurement error of the pressure sensor, the pressure sensor will not be able to detect the leakage, and foreign matter such as antifreeze will continue to be mixed into the evaporative gas, which may ultimately cause corrosion inside the heat exchanger and shorten the life of the equipment, and may also reduce the re-fluid performance.

[0041] The microleak detection system of this embodiment is configured to be able to detect such microleakage in the heater 100.

[0042] Fig. 2 shows a microleak detection system for a marine reliquefaction system according to an embodiment of the present invention, and Fig. 3 shows in more detail a microleak detection device portion of the microleak detection system shown in Fig. 2.

[0043] Referring to Figures 2 and 3, a residual liquid drain outlet 110, which drains the residual liquid (antifreeze liquid GW) remaining in the heater 100, is connected to a micro-leak detection device 200 for detecting micro-leakage within the heater 100.

[0044] The microleak detection device 200 is provided with a connection tube DT that is fastened to the residual liquid discharge port 110 and extends downward from the residual liquid discharge port 110. The connection tube DT is also provided with a leak detection unit 220 that detects the presence or absence of residual liquid discharged from the residual liquid discharge port 110, a first shutoff valve 210 provided on the inlet side of the leak detection unit 220 of the connection tube DT (above the leak detection unit 220), a second shutoff valve 230 provided on the outlet side of the leak detection unit 220 of the connection tube DT (below the leak detection unit 220), and a tube plug 240 provided at the lower end of the connection tube DT.

[0045] 3, the leakage detection unit 220 may be, for example, a sight glass that can visually check the presence or absence of residual liquid discharged from the residual liquid discharge port 110. Also, the leakage detection unit 220 may be a liquid level detector that detects or measures the liquid discharged from the residual liquid discharge port 110. Note that the leakage detection unit 220 is not limited to these, and any device that can detect minute leakage may be used as appropriate.

[0046] In the microleak detection device 200 provided in the microleak detection system of this embodiment, the first shutoff valve 210 operates in a normally open state so as to continuously monitor the presence or absence of antifreeze liquid discharged from the residual liquid discharge outlet 110 due to microleakage from the heater 100.

[0047] Furthermore, in the event that a minute leak occurs in the heater 100, the second shutoff valve 230 is normally closed so that the leak detection unit 220 is filled with antifreeze liquid GW. However, when all the residual liquid remaining in the heater 100 is to be drained from the residual liquid drain port 110 of the heater 100 during maintenance of the heater 100 or the like, the second shutoff valve 230 and the tube plug 240 are opened to drain the residual liquid from the connection tube DT via the residual liquid drain port 110 of the heater 100.

[0048] As described above, the microleak detection system of this embodiment continuously detects microleakage that is within the measurement error range of pressure sensors installed upstream and downstream of the heat exchanger, thereby preventing foreign matter such as antifreeze from entering the heat exchanger, and preventing corrosion inside the heat exchanger and shortening of the device's lifespan. This makes it possible to stably maintain the performance of the heat exchanger in the reliquefaction process, and reduce the frequency of maintenance of the entire reliquefaction system due to device abnormalities and deterioration of reliquefaction performance.

[0049] The present invention is not limited to the above-described embodiment, and it will be apparent 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 microleak detection system for a ship re-liquefaction system in which evaporated gas generated from liquefied gas stored in a storage tank of a ship is supplied to a heat exchanger to recover cold energy, compressed, and cooled by heat exchange with a refrigerant circulating in a refrigerant circulation line in the heat exchanger to re-liquefy the evaporated gas, The marine reliquefaction system includes a heater that heats the evaporated gas supplied from the storage tank to the heat exchanger by heat exchange with a heat source; a microleakage detection device that is connected to a residual liquid discharge port that discharges residual liquid remaining in the heater from the heater and detects microleakage of the heat source from the heater; A microleak detection system for ship reliquefaction systems.

2. The microleak detection device comprises: a connection tube that is fastened to the residual liquid discharge port and extends downward from the residual liquid discharge port; and a leakage detection unit provided in the connection tube and configured to check the presence or absence of the residual liquid discharged from the residual liquid discharge port.

2. A microleak detection system for a marine reliquefaction system according to claim 1.

3. The microleak detection device comprises: a first shutoff valve provided on the inlet side of the connection tube relative to the leak detection unit; and a second shutoff valve provided on the outlet side of the leak detection unit of the connecting tube; and a tube plug provided at the lower end of the connection tube; 3. A microleak detection system for a marine reliquefaction system according to claim 2.

4. In the microleak detection device, the first shutoff valve operates in a normally open state, and the second shutoff valve operates in a normally closed state; During maintenance of the heater, the second shutoff valve is opened to drain the residual liquid from the heater.

4. A microleak detection system for a marine reliquefaction system according to claim 3.

5. The leakage detection unit is a sight glass that allows visual confirmation of the presence or absence of the residual liquid discharged from the residual liquid discharge port.

4. A microleak detection system for a marine reliquefaction system according to claim 3.

6. The leakage detection unit is a liquid level detector that detects the residual liquid discharged from the residual liquid discharge port.

4. A microleak detection system for a marine reliquefaction system according to claim 3.

7. The heat exchanger is a cryogenic heat exchanger, and the heater is a multi-tube heat exchanger. A microleak detection system for a marine reliquefaction system according to any one of claims 1 to 6.

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