Control system for ship reliquefaction systems

Abstract

A control system for a vessel reliquefaction system is disclosed. [Solution] A control system for a ship's reliquefaction system, which compresses evaporated gas generated from liquefied gas in a ship's storage tank using a compressor and reliquefies the compressed evaporated gas by cooling it through a heat exchanger, comprises: a reliquefaction line connecting the compressor to the storage tank; a control valve provided in the reliquefaction line downstream of the heat exchanger and adjusting the flow rate of evaporated gas flowing through the reliquefaction line; a flow rate adjustment unit controlling the opening of the control valve; and a load controller sending a signal to the flow rate adjustment unit to adjust the load of the reliquefaction system; when the load of the reliquefaction system changes, the load controller sends a pre-determined set point for the evaporated gas flow rate at that load to the flow rate adjustment unit to adjust the load of the reliquefaction system.

Description

[Technical Field]

[0001] The present invention relates to a control system for a ship's reliquefaction system that cools and reliquefies boil-off gas (BOG) generated from liquefied gas in the ship's storage tanks, and is capable of rapid control in response to changes in the load on the reliquefaction system. [Background technology]

[0002] Natural gas, which is primarily composed of methane, is gaining 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 at approximately -163°C under atmospheric pressure. Its volume is approximately 1 / 600 of that of gaseous natural gas, making it highly suitable for long-distance transportation by sea. Therefore, natural gas is mainly stored and transported in the liquefied natural gas state, which is easy to store and transport.

[0003] Because the liquefaction point of natural gas is an extremely low temperature of approximately -163°C at normal pressure, LNG storage tanks are generally insulated to maintain the LNG in a liquid state. However, even if an LNG storage tank is insulated, there is a limit to how much it can block external heat, and if external heat is continuously transferred to the LNG storage tank, the LNG will continuously vaporize naturally inside the LNG storage tank during the LNG transportation process, generating boil-off gas (BOG).

[0004] If evaporation gas continues to be generated in an LNG storage tank, it can cause an increase in the internal pressure of the LNG storage tank. If the internal pressure of the LNG storage tank exceeds a preset safety pressure, it could lead to an emergency situation such as the LNG storage tank rupturing. Therefore, a safety valve must be used to release the evaporation gas outside the LNG storage tank. However, evaporation gas is a type of LNG loss and is a significant issue in terms of LNG transportation efficiency and fuel efficiency. Therefore, various methods are used to treat the evaporation gas generated in storage tanks.

[0005] In recent years, methods have been developed and applied in which evaporated gas is used as fuel for ship engines and other applications, evaporated gas is re-liquefied and collected in storage tanks, or a combination of these two methods is used. Summary of the Invention [Problem to be solved by the invention]

[0006] Methods for re-liquefying evaporated gas include a method using a refrigeration cycle that uses a different refrigerant to re-liquefy the evaporated gas by heat exchange with the refrigerant, and a method using the evaporated gas itself as a refrigerant without using a different refrigerant to re-liquefy the evaporated gas.

[0007] As a method of re-liquefying evaporated gas without using a separate refrigerant, a system has been developed in which compressed evaporated gas is cooled and adiabatically expanded by heat exchange with uncompressed evaporated gas to re-liquefy the evaporated gas, and this system is used on ships.

[0008] Another type of system that uses a refrigeration cycle is a system that uses a re-liquefaction process using nitrogen refrigerant.

[0009] Nitrogen refrigerants are less efficient than cycles using mixed refrigerants, but they are safer because the refrigerant is inert, and they are easier to apply to ships because there is no phase change in the refrigerant.

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

[0011] An object of the present invention is to provide a reliquefaction system and an operating method thereof that can effectively adjust and operate the load of the reliquefaction system according to the amount of evaporated gas generated in a storage tank and reliquefied. [Means for solving the problem]

[0012] In order to solve the above problems, according to one embodiment of the present invention, a control system for a reliquefaction system for a ship, which compresses evaporated gas generated from liquefied gas in a storage tank on board a ship using a compressor and reliquefies the compressed evaporated gas by cooling it through a heat exchanger, comprises:

[0013] a reliquefaction line connecting the compressor to a storage tank; and

[0014] a control valve provided in the reliquefaction line downstream of the heat exchanger to adjust the flow rate of evaporated gas flowing through the reliquefaction line; and

[0015] a flow rate adjusting unit that controls the opening degree of the control valve; and

[0016] a load controller that sends a signal to the flow rate adjuster to adjust the load of the reliquefaction system;

[0017] When the load of the reliquefaction system changes, the load controller sends a pre-determined set point for the evaporated gas flow rate at the corresponding load to the flow rate adjusting unit to adjust the load of the reliquefaction system.

[0018] Preferably, the system further comprises a temperature adjustment unit that detects the temperature of the compressed gas downstream of the heat exchanger in the reliquefaction line and transmits a correction value for fine-tuning the load of the reliquefaction system; and a calculation unit that receives the default setting value of the load controller and the correction value in the temperature adjustment unit and transmits a correction value for adjusting the opening of the control valve to the flow rate adjustment unit.

[0019] Preferably, the system further comprises: a flow meter provided upstream of the heat exchanger in the reliquefaction line, which detects the flow rate of the evaporated gas flowing into the heat exchanger and transmits the detected flow rate to the flow rate adjustment unit; and a temperature detection unit which detects the temperature of the compressed gas downstream of the heat exchanger in the reliquefaction line and transmits the detected temperature to the temperature adjustment unit.

[0020] Preferably, when the load of the reliquefaction system changes, the flow rate adjusting unit and the temperature adjusting unit operate in a master-slave manner to improve the responsiveness of the load adjustment.

[0021] Preferably, the reliquefaction system further includes a refrigerant circulation section provided with: a refrigerant circulation line through which a refrigerant that exchanges heat with the compressed gas in the heat exchanger circulates; a refrigerant expander that expands and cools the refrigerant supplied to the heat exchanger; and a refrigerant compressor that compresses the refrigerant after heat exchange in the heat exchanger.

[0022] Preferably, the refrigerant circulating through the refrigerant circulation section is nitrogen, and when the load on the reliquefaction system changes, a portion of the refrigerant in the refrigerant circulation section is discharged or replenished to adjust the mass flow rate of the refrigerant, thereby adjusting the amount of cold supplied to the reliquefaction system.

[0023] Preferably, the evaporated gas cooled through the heat exchanger is separated into gas and liquid in a separator, and the separated liquefied gas is supplied to the storage tank, and the flash gas separated in the separator is combined with the evaporated gas flowing into the compressor, recovers cold energy in the heat exchanger, and is then supplied to the compressor. [Effects of the Invention]

[0024] The present invention utilizes the cold energy of the evaporated gas itself and the cold energy of the refrigerant cycle to more effectively cool the evaporated gas to be reliquefied, thereby increasing the reliquefaction rate.

[0025] When the amount of evaporated gas generated in the storage tank and reliquefied changes, causing the load on the reliquefaction system to change, the flow rate of evaporated gas flowing into the heat exchanger is adjusted to quickly adjust the load on the reliquefaction system. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram illustrating a control system for a reliquefaction system of a ship in accordance with an embodiment of the present invention;

[0027] [Figure 2] FIG. 10 is a diagram showing a change in the evaporated gas flow rate due to a change in the load of the reliquefaction system of the control system of the present invention.

[0028] [Figure 3] FIG. 2 is a detailed diagram more specifically illustrating the control system of the reliquefaction system shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0029] For a full understanding of the operating advantages and objects attained by the embodiments of the present invention, reference should be made to the accompanying drawings and the contents thereof, which illustrate the embodiments of the present invention.

[0030] The configuration and operation of an embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Note that when assigning reference numerals to components in each drawing, the same components are denoted by the same numerals whenever possible, even if they appear in different drawings.

[0031]

[0032] In the following, the term "vessel" refers to any vessel equipped with storage tanks for storing liquefied gas. This includes, for example, self-propelled vessels such as LNG carriers, liquid hydrogen carriers, and LNG RVs (regasification vessels), as well as floating offshore structures that do not have self-propelled capabilities, such as LNG FPSOs (floating production storage offloading systems) and LNG FSRUs (floating storage regasification units).

[0033] Furthermore, as an embodiment, the present invention can be applied to a reliquefaction cycle for any liquefied gas that liquefies a gas at a low temperature, transports the gas, and generates evaporated gas when stored. Examples of liquefied gases include LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), liquefied ethylene gas, and liquefied propylene gas. However, in the embodiment described below, LNG, a typical liquefied gas, will be used as an example.

[0034]

[0035] FIG. 1 is a schematic diagram illustrating a control system for a reliquefaction system for a ship according to one embodiment of the present invention.

[0036] Referring to Figure 1, the control system of this embodiment is a re-liquefaction system in which evaporated gas generated from liquefied gas in a storage tank (not shown) of a ship is compressed by a compressor, and the compressed evaporated gas is cooled and re-liquefied via a heat exchanger and returned to the storage tank.The control system quickly adjusts the load of the re-liquefaction system in response to changes in the load of the re-liquefaction system.

[0037] The reliquefaction system includes a compressor (not shown) that receives and compresses the evaporated gas, an evaporated gas supply line (not shown) that supplies the evaporated gas generated in the storage tank to the compressor, and a reliquefaction line RL that connects the compressor to the storage tank, reliquefies the evaporated gas, and returns it to the storage tank.

[0038] The evaporative gas supply line extends from the storage tank to the compressor via the heat exchanger 100, and the uncompressed evaporative gas generated in the storage tank supplies cold energy to the heat exchanger, and then is supplied to the compressor and compressed.

[0039] The compressor (not shown) compresses the evaporated gas, for example, to a fuel supply pressure required by the ship's main engine. For example, it is compressed to 5.5 barg if a DF engine is installed, 15 barg if an X-DF engine is installed, and approximately 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 other consumers, and evaporated gas that is not supplied as fuel is passed through a re-liquefaction line RL and re-liquefied in a re-liquefaction system.

[0040] The evaporated gas compressed by the compressor flows into the heat exchanger 100 along the re-liquefaction line RL, where it is cooled by heat exchange. The cooled evaporated gas is separated into gas and liquid in a separator (not shown), and the separated re-liquefied gas is collected in a storage tank.

[0041] In the heat exchanger 100, the evaporated gas is cooled by the cold energy of the refrigerant circulating in the refrigerant circulation section and the uncompressed evaporated gas flowing into the compressor.

[0042] A refrigerant circulation system (not shown) includes a refrigerant circulation line (not shown) through which a refrigerant circulates. The refrigerant circulation line is provided with a refrigerant expander (not shown) that expands and cools the refrigerant supplied to a heat exchanger, a refrigerant compressor (not shown) that compresses the refrigerant discharged from the heat exchanger, and a motor that drives the refrigerant compressor. The refrigerant compressor and refrigerant expander are connected via a common shaft, and the expansion energy of the refrigerant is used to compress the refrigerant in the refrigerant compressor, thereby reducing the power required to operate the refrigerant cycle.

[0043] An example of the refrigerant that circulates through the refrigerant circulation line and supplies cold to the heat exchanger is nitrogen (N2). The amount of cold transferred to the heat exchanger can be adjusted by replenishing the nitrogen refrigerant circulating through the refrigerant circulation line from a refrigerant inventory system (not shown) or by discharging part of the refrigerant from the refrigerant circulation line.

[0044] The refrigerant compressed by the refrigerant compressor is cooled in the heat exchanger 100, expanded and cooled in the refrigerant expander, and supplied as refrigerant to the heat exchanger 100, where it circulates through the refrigerant circulation line. Therefore, in the heat exchanger 100, four refrigerants flowing through the heat exchanger exchange heat: the evaporated gas compressed and re-liquefied in the compressor, the uncompressed evaporated gas flowing into the compressor, the refrigerant expanded and cooled in the refrigerant expander, and the refrigerant compressed in the refrigerant compressor.

[0045] The evaporated gas cooled by passing through the heat exchanger passes through a control valve 200 installed downstream of the heat exchanger and is then separated into gas and liquid in a separator. The re-liquefied gas separated in the separator (not shown) is supplied to a storage tank and stored again, and the flash gas is either combined with the flow of uncompressed evaporated gas upstream of the heat exchanger in the evaporated gas supply line or supplied to a GCU (Gas Combustion Unit).

[0046]

[0047] On the other hand, if the amount of evaporative gas generated in the storage tank changes, if the amount of evaporative gas consumed by the engine or other fuel changes, or if the composition of the evaporative gas changes, the amount of cold energy required by the reliquefaction system will change, and it will be necessary to adjust the load on the reliquefaction system to accommodate these changes. Figure 1 shows a schematic diagram of a control system for a ship's reliquefaction system according to this embodiment, and Figure 3 shows the control system shown in Figure 1 in more detail.

[0048] The control system of this embodiment is for adjusting the load of the reliquefaction system and includes the following: As shown in Figures 1 and 3, it includes a control valve 200 that is provided downstream of the heat exchanger 100 and adjusts the flow rate of the evaporated gas flowing through the reliquefaction line, a flow rate adjustment unit FIC that controls the opening of the control valve, and a load controller LC that sends a signal to the flow rate adjustment unit to adjust the load of the reliquefaction system.

[0049] It also includes a temperature adjustment unit TIC that detects the temperature of the compressed gas downstream of the heat exchanger in the reliquefaction line and transmits a correction value (adjustment) for fine-tuning the load on the reliquefaction system, and a calculation unit FX that receives the predetermined setting value of the load controller and the correction value in the temperature adjustment unit and transmits a correction value to the flow rate adjustment unit for adjusting the opening of the control valve.

[0050] The reliquefaction line RL is provided with a flow meter FM located upstream of the heat exchanger 100 to detect the flow rate of evaporated gas flowing into the heat exchanger and send it to the flow rate adjustment unit, and a temperature detection unit T1 located upstream of the heat exchanger to detect the temperature of the compressed gas and send it to the temperature adjustment unit.

[0051] In this embodiment, when the load of the reliquefaction system changes, the load controller LC sends a pre-determined set point for the evaporated gas flow rate at that load to the flow rate adjuster FIC to adjust the load of the reliquefaction system.

[0052] Load adjustment in a reliquefaction system that uses nitrogen as a refrigerant is primarily achieved by adjusting the amount of cold energy in the nitrogen (N2) refrigerant cycle, and secondarily by controlling the temperature of the evaporated gas being reliquefied, achieving load balance in the system. However, a single method based on temperature control has limitations in the responsiveness of load adjustment, leading to energy imbalances and potentially causing the reliquefaction process to become unstable, necessitating an emergency shutdown.

[0053] To solve this problem, in this embodiment, when the load of the reliquefaction system changes, the flow rate of the evaporated gas is controlled to adjust the load, and the flow rate adjustment unit and temperature adjustment unit are operated in a master-slave manner, thereby improving the responsiveness of the load adjustment and the accuracy of control.

[0054] That is, when the load of the reliquefaction system changes, the load is adjusted by first adjusting the mass flow rate of the refrigerant by discharging a portion of the refrigerant from the refrigerant circulation section or by replenishing a portion of the refrigerant in the refrigerant circulation line, thereby adjusting the amount of cold supplied to the reliquefaction system. Secondly, the load controller LC transmits a predetermined set point (PSP) derived from actual load test data for the evaporative gas flow rate at that load to the flow rate adjuster FIC. The flow rate adjuster preferentially adjusts the opening of the control valve 200 according to the predetermined set point, thereby adjusting the flow rate of the evaporative gas flowing into the heat exchanger along the reliquefaction line. While the temperature detector T1 detects the temperature of the compressed gas downstream of the heat exchanger, the temperature adjuster TIC transmits a correction value AJ for fine-tuning the load according to the detected temperature of the compressed gas downstream of the heat exchanger. The calculation unit FX applies the correction value received from the temperature adjuster to the default set point received from the load controller to calculate a corrected correction value ASP, and transmits the calculated correction value to the flow rate adjuster. The flow rate adjusting unit adjusts the opening of the control valve based on the received correction value to adjust the flow rate of evaporated gas flowing into the heat exchanger, thereby adjusting the load balance of the reliquefaction system.

[0055] FIG. 2 is a graph showing the change in the evaporative gas flow rate (tons / h) due to changes in the load (%) of the reliquefaction system in the control system of this embodiment. In this graph, the top dotted line UB is the upper boundary of the evaporative gas flow rate according to the load, and the bottom dotted line LB is the lower boundary. The bold line PSP shows the change in the evaporative gas flow rate due to changes in the load of the reliquefaction system, derived using actual load test data. In the bold line graph, the evaporative gas flow rate at that load is sent from the load controller to the flow rate adjustment unit as a predetermined set value. The shaded area AJ is a correction value adjusted by the temperature adjustment unit according to the temperature of the evaporative gas downstream of the heat exchanger.

[0056] It was confirmed that adjusting the load using the master-slave method as in this embodiment increases the control speed by more than 35% compared to controlling the temperature using a single method, and that responsiveness is greatly improved.

[0057] Furthermore, even if the amount of cold energy supplied to the reliquefaction system is the same, the amount of reliquefaction will vary depending on the composition of the evaporated gas.However, in this embodiment, after adjusting the load based on the evaporated gas flow rate, correction is made by the temperature adjustment unit, thereby improving control accuracy and enabling stable operation of the reliquefaction system.

[0058]

[0059] 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 modifications and changes can be made without departing from the technical gist of the present invention.

Claims

1. A control system for a ship's reliquefaction system, which compresses evaporated gas generated from liquefied gas in a storage tank of a ship using a compressor and reliquefies the compressed evaporated gas by cooling it through a heat exchanger, a reliquefaction line connecting the compressor to a storage tank; and a control valve provided in the reliquefaction line downstream of the heat exchanger for adjusting the flow rate of evaporated gas flowing through the reliquefaction line; and a flow rate adjusting unit that controls the opening degree of the control valve; and a load controller that sends a signal to the flow rate adjuster to adjust the load of the reliquefaction system; A control system for a ship's reliquefaction system, characterized in that when the load of the reliquefaction system changes, the load controller sends a predetermined set value for the evaporated gas flow rate at that load to the flow rate adjustment unit to adjust the load of the reliquefaction system.

2. a temperature adjustment unit that senses the temperature of the compressed gas downstream of the heat exchanger in the reliquefaction line and sends a correction value to fine-tune the load on the reliquefaction system; and 2. The control system for a ship reliquefaction system as described in claim 1, further comprising: a calculation unit that receives a predetermined setting value of the load controller and a correction value of the temperature adjustment unit, and transmits a correction value for adjusting the opening of the control valve to the flow rate adjustment unit.

3. a flow meter provided on the reliquefaction line upstream of a heat exchanger, for detecting the flow rate of evaporated gas flowing into the heat exchanger and transmitting the detected flow rate to the flow rate adjusting unit; and 3. The control system for a ship reliquefaction system according to claim 2, further comprising: a temperature detection unit that detects the temperature of the compressed gas downstream of the heat exchanger in the reliquefaction line and transmits the temperature to the temperature adjustment unit.

4. 4. A control system for a ship's reliquefaction system as described in claim 3, characterized in that when the load of the reliquefaction system changes, the flow rate adjustment unit and the temperature adjustment unit operate in a master-slave manner to improve the responsiveness of load adjustment.

5. 4. A control system for a ship reliquefaction system as described in claim 3, wherein the reliquefaction system further comprises a refrigerant circulation section provided with: a refrigerant circulation line through which a refrigerant that exchanges heat with the compressed gas in the heat exchanger circulates; a refrigerant expander that expands and cools the refrigerant supplied to the heat exchanger; and a refrigerant compressor that compresses the refrigerant after heat exchange in the heat exchanger.

6. A control system for a ship's reliquefaction system as described in claim 5, characterized in that the refrigerant circulating through the refrigerant circulation section is nitrogen, and when the load on the reliquefaction system changes, a portion of the refrigerant in the refrigerant circulation section is discharged or replenished to adjust the mass flow rate of the refrigerant and adjust the amount of cold supplied to the reliquefaction system.

7. A control system for a ship reliquefaction system described in any one of claims 1 to 6, characterized in that the evaporated gas cooled through the heat exchanger is separated into gas and liquid in a separator, and the separated liquefied gas is supplied to the storage tank, and the flash gas separated by the separator is combined with the evaporated gas flowing into the compressor, recovers cold energy in the heat exchanger, and then is supplied to the compressor.

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