Control system and method for refrigerant cycle of reliquefaction system of ship

Abstract

A refrigerant cycle control system and method for a ship's reliquefaction system is disclosed. [Solution] A reliquefaction system compresses evaporated gas generated from liquefied gas in a storage tank on board a ship and reliquefies it by heat exchange with a refrigerant circulating in a refrigerant circulation line in a heat exchanger. The system comprises: a refrigerant compressor provided in the refrigerant circulation line that cools the evaporated gas in the heat exchanger and compresses the refrigerant discharged; a suction pressure detection unit that detects the suction pressure of the refrigerant flowing from the refrigerant circulation line to the refrigerant compressor and transmits a current value PVs of the suction pressure; a load controller that transmits a set value SPs of the suction pressure to adjust the load of the reliquefaction system; and a control unit that receives the set value and charges or discharges refrigerant from the refrigerant circulation line to adjust the amount of cold energy in the refrigerant circulation line. The control unit compares the current value of the suction pressure with the set value of the suction pressure and charges or discharges refrigerant from the refrigerant circulation line using PID control, and if the difference between the current value of the suction pressure and the set value of the suction pressure is less than or equal to a first correction value α (PVs≦SPs±α), prioritizes on / off control to stop refrigerant charging and discharging.

Description

[Technical Field]

[0001] The present invention relates to a control system and method for a refrigerant cycle of a ship's reliquefaction system, which cools and reliquefies boil-off gas (BOG) generated from liquefied gas in a ship's storage tank. More specifically, the present invention relates to a system and method for controlling the refrigerant cycle of a ship's reliquefaction system, which cools and reliquefies boil-off gas (BOG) generated from liquefied gas in a ship's storage tank. The system and method can effectively adjust the amount of cold energy in the refrigerant cycle when the load on the reliquefaction system changes, thereby ensuring stable operation of the reliquefaction process. [Background technology]

[0002] Natural gas, 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 primarily stored and transported in the liquefied form, 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 relatively less efficient than cycles using mixed refrigerants, but they have the advantage of being safer because the refrigerant is inert, and there is no phase change in the refrigerant, making them easier to apply to ships.

[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] The object of the present invention is to provide a control system and a control method for the refrigerant cycle of a ship's reliquefaction system that can effectively adjust the amount of cold energy in the refrigerant cycle in accordance with changes in the load of the reliquefaction system, thereby ensuring stability in the reliquefaction process. [Means for solving the problem]

[0012] According to one embodiment of the present invention, a refrigerant cycle control system for a ship re-liquefaction system that compresses evaporated gas generated from liquefied gas in a storage tank of a ship and re-liquefies the compressed evaporated gas by heat exchange with a refrigerant circulating in a refrigerant circulation line in a heat exchanger, includes:

[0013] a refrigerant compressor provided in the refrigerant circulation line and configured to compress the refrigerant discharged after cooling the evaporated gas in the heat exchanger; and

[0014] a suction pressure detection unit that detects the suction pressure of the refrigerant flowing from the refrigerant circulation line to the refrigerant compressor and transmits a current value (PV) of the suction pressure; and

[0015] a load controller transmitting suction pressure set points (SPs) for adjusting the load of the reliquefaction system; and

[0016] a control unit that receives the set value and charges or discharges refrigerant from the refrigerant circulation line to adjust the amount of cold in the refrigerant circulation line;

[0017] The control unit compares the current value of the suction pressure with the set value of the suction pressure and fills or discharges refrigerant in the refrigerant circulation line using PID control, and if the difference between the current value of the suction pressure and the set value of the suction pressure is less than or equal to a first correction value α (PVs≦SPs±α), it prioritizes on / off control to stop the filling and discharging of refrigerant.

[0018] Preferably, a discharge pressure detection unit is further provided for detecting the discharge pressure of the refrigerant discharged from the refrigerant compressor in the refrigerant circulation line and transmitting the current value PVd of the discharge pressure to the control unit. When the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the current value of the discharge pressure is greater than the value obtained by subtracting the second correction value β from the set value of the discharge pressure (PVd>SPd-β), the refrigerant in the refrigerant circulation line is discharged. When the current value of the suction pressure is less than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is less than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd<SPd+β), the control unit fills or discharges the refrigerant in the refrigerant circulation line by PID control.

[0019] Preferably, when the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the current value of the discharge pressure is less than the value obtained by subtracting the second correction value from the set value of the discharge pressure (PVd<SPd-β); and when the current value of the suction pressure is less than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is greater than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd>SPd+β), the control unit preferentially executes on / off control to stop the filling and discharging of the refrigerant.

[0020] Preferably, a refrigerant inventory unit for filling or discharging the refrigerant in the refrigerant circulation line; a refrigerant filling line connected from the refrigerant inventory unit to the upstream side of the refrigerant compressor for supplying the refrigerant to the refrigerant circulation line; and a refrigerant discharge line connected from the upstream side of the refrigerant compressor to the refrigerant inventory unit for discharging the refrigerant in the refrigerant circulation line to the refrigerant inventory unit are further provided.

[0021] Preferably, the refrigerant inventory section includes an inventory tank that stores the refrigerant to be supplied to the refrigerant circulation line; and a boost compressor that compresses the refrigerant and supplies it to the inventory tank; and the refrigerant in the refrigerant circulation line is nitrogen.

[0022] Preferably, the refrigerant charging line is connected from the inventory tank to the upstream side of the refrigerant compressor, and refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure difference between the inventory tank and the refrigerant circulation line.

[0023] Preferably, the refrigerant discharge line comprises: a first discharge line connected from the downstream side of the refrigerant compressor to the inventory tank; and a second discharge line branched from the first discharge line upstream of the inventory tank and connected to the inventory tank via the boost compressor; and the refrigerant is discharged from the downstream side of the refrigerant compressor to the inventory tank via the first discharge line due to the pressure difference between the refrigerant circulation line and the inventory tank, and when pressure reverses, the refrigerant is discharged via the second discharge line via the boost compressor to the inventory tank.

[0024] Preferably, the system further comprises a refrigerant expander that is provided in the refrigerant circulation line and expands and cools the refrigerant that has been compressed by the refrigerant compressor and then cooled through a heat exchanger, and supplies the refrigerant as a refrigerant for the heat exchanger.

[0025] According to another embodiment of the present invention, a method for controlling a refrigerant cycle of a reliquefaction system of a ship, which compresses evaporated gas generated from liquefied gas in a storage tank of the ship, and re-liquefies the compressed evaporated gas by heat exchange with a refrigerant circulating in a refrigerant circulation line in a heat exchanger, includes:

[0026] When adjusting the load of the re-liquefaction system, the control unit receives the set value (SPs) of the suction pressure received from the load controller and the current value (PVs) of the suction pressure of the refrigerant flowing into the refrigerant compressor of the refrigerant circulation line after heat-exchanging with the evaporation gas in the heat exchanger, compares the current value of the suction pressure with the set value of the suction pressure, and fills or discharges the refrigerant in the refrigerant circulation line by PID control to adjust the cooling capacity of the refrigerant circulation line.

[0027] If the difference between the current value of the suction pressure and the set value of the suction pressure is less than or equal to the first correction value α (PVs≦SPs±α), the on-off control for stopping the refrigerant filling and discharging by the control unit is preferentially executed.

[0028] Preferably, when the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the discharge pressure of the refrigerant discharged from the refrigerant compressor is detected, and the current value of the discharge pressure PVd is greater than the value obtained by subtracting the second correction value β from the set value of the discharge pressure SPd (PVd>SPd-β), the refrigerant in the refrigerant circulation line is discharged. When the difference between the current value of the suction pressure and the set value of the suction pressure is less than the value obtained by subtracting the first correction value (PVs<SPs-α), and the current value of the discharge pressure is less than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd<SPd+β), the refrigerant circulation line is filled with refrigerant.

[0029] Preferably, when the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the current value of the discharge pressure is less than the value obtained by subtracting the second correction value from the set value of the discharge pressure (PVd<SPd-β); and when the current value of the suction pressure is less than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is greater than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd>SPd+β), the control unit preferentially executes on-off control to stop the filling and discharging of the refrigerant.

Advantages of the Invention

[0031] The refrigeration cycle control system and method of the present invention effectively adjusts the amount of cold energy by appropriately charging or discharging the refrigerant in the refrigeration cycle based on the load of the reliquefaction system, while selectively performing PID control and on-off control based on the conditions of the refrigeration cycle, thereby preventing instability in the reliquefaction process caused by frequent charging and discharging of the refrigerant. [Brief explanation of the drawings]

[0032] [Figure 1] Schematic diagram of a control system used in a reliquefaction system, which uses PID control to charge or discharge refrigerant from the refrigerant circulation line depending on the suction pressure of the refrigerant compressor. [Figure 2] Similar to Figure 1, this is a schematic diagram of a control system used in a reliquefaction system, which uses PID control to charge or discharge refrigerant from the refrigerant circulation line depending on the suction pressure of the refrigerant compressor.

[0033] [Figure 3] Schematic diagram showing the progress of refrigerant charging and discharging during PID control according to the suction pressure of the refrigerant compressor.

[0034] [Figure 4] 1 is a conceptual diagram showing a case where the current value of the suction pressure and the current value of the discharge pressure in the refrigerant compressor are different when controlling the refrigerant cycle with the refrigerant cycle control system of the ship reliquefaction system according to one embodiment of the present invention.

[0035] [Figure 5] 2 is a schematic diagram showing the progress of charging and discharging of refrigerant in a refrigerant cycle control system according to an embodiment of the present invention; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0036] 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.

[0037] 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.

[0038]

[0039] 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).

[0040] 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.

[0041]

[0042] The control system of this embodiment is used in a ship's re-liquefaction system, which compresses evaporated gas generated from liquefied gas in a ship's storage tank (not shown) using a compressor, cools and re-liquefies this compressed evaporated gas through a heat exchanger using a refrigerant cycle, and returns it to the storage tank, and controls the refrigerant cycle in accordance with changes in the load on the re-liquefaction system.

[0043] 1 and 2 are schematic diagrams showing a reliquefaction system for a ship and a control system for charging or discharging refrigerant in a refrigerant circulation line by PID control according to the suction pressure of a refrigerant compressor.

[0044] The reliquefaction system includes a compressor (not shown) that receives evaporated gas and compresses it, an evaporated gas supply line (not shown) that supplies evaporated gas generated in the storage tank to the compressor, and a reliquefaction line (not shown) that connects the compressor to the storage tank, reliquefies the evaporated gas, and returns it to the storage tank.

[0045] 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 heat to the heat exchanger, and then is supplied to the compressor and compressed.

[0046] 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 for a DF engine, 15 barg for an X-DF engine, and approximately 300 barg for an ME-GI engine. The compressed evaporated gas is supplied as fuel to consumers such as the ship's main engine (not shown), and evaporated gas that is not supplied as fuel is passed through a reliquefaction line and reliquefied in a reliquefaction system.

[0047] The evaporated gas compressed by the compressor flows into the heat exchanger 100 along the re-liquefaction line and 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.

[0048] In the heat exchanger 100, the evaporated gas is cooled by the cold energy of the refrigerant circulating along the refrigerant circulation line CL and the uncompressed evaporated gas input into the compressor.

[0049] The refrigerant circulation line CL that constitutes the refrigeration cycle is provided with a refrigerant expander (not shown) that expands and cools the refrigerant supplied to the heat exchanger, a refrigerant compressor 200 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.

[0050] The refrigerant compressed by the refrigerant compressor 200 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.

[0051] The evaporated gas cooled by passing through the heat exchanger passes through a control valve 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).

[0052] Nitrogen (N2) is an example of a refrigerant that circulates through the refrigerant circulation line to supply cold to the heat exchanger.

[0053] When the amount of evaporated gas to be reliquefied changes, the amount of cold required by the reliquefaction system changes accordingly. The amount of cold transferred to the heat exchanger can be adjusted by adding nitrogen refrigerant from the refrigerant inventory section 300 to the refrigerant circulation line or by discharging part of the refrigerant from the refrigerant circulation line in accordance with changes in the load of the reliquefaction cycle, thereby adjusting the load of the reliquefaction system.

[0054] The refrigerant inventory section 300 includes an inventory tank that stores the refrigerant supplied to the refrigerant circulation line CL and a boost compressor that compresses the refrigerant and supplies it to the inventory tank. The refrigerant inventory section receives nitrogen generated by a nitrogen generator (N2 generator) installed onboard to supply utility nitrogen (N2). The nitrogen passes through a drying section and a filtration section (not shown) to lower the dew point, then compresses it in the boost compressor, stores it in the inventory tank, and supplies it to the refrigerant circulation line of the reliquefaction system. When adjusting the load of the reliquefaction system, refrigerant may be replenished or discharged from the inventory tank to the refrigerant circulation line. To this end, the control system further includes a refrigerant charging line FL that connects the inventory tank of the refrigerant inventory section to the upstream side of the refrigerant compressor and a refrigerant discharge line DL that connects the downstream side of the refrigerant compressor to the refrigerant inventory section.

[0055] The refrigerant discharge line DL includes a first discharge line DLa that connects the downstream side of the refrigerant compressor to the inventory tank, and a second discharge line DLb that branches off from the first discharge line DLa upstream of the inventory tank, passes through the boost compressor, and connects to the inventory tank.

[0056] When adjusting the load, the pressure difference between the refrigerant circulation line and the inventory tank is used to replenish or drain nitrogen refrigerant from the inventory tank to the refrigerant circulation line.

[0057] The refrigerant charge line connects the inventory tank to the upstream side of the refrigerant compressor, and a pressure difference between the inventory tank and the refrigerant circulation line supplies supplemental refrigerant from the inventory tank to the refrigerant circulation line.

[0058] When the load on the reliquefaction system is reduced, the refrigerant is discharged from the downstream side of the refrigerant compressor to the inventory tank via the first discharge line using the pressure difference between the downstream side of the refrigerant compressor in the refrigerant circulation line and the inventory tank. However, if a pressure reversal occurs and it becomes difficult to discharge the refrigerant to the inventory tank using this pressure difference, the nitrogen refrigerant discharged from the downstream side of the refrigerant compressor is compressed by the boost compressor via the second discharge line DLb and transported to the inventory tank.

[0059] In the refrigerant circulation line, a suction pressure detection unit P1 is provided upstream of the refrigerant compressor to detect the suction pressure of the refrigerant flowing into the refrigerant compressor, and a discharge pressure detection unit P2 is provided downstream of the refrigerant compressor to detect the discharge pressure of the refrigerant compressed by the refrigerant compressor.

[0060] The control system for adjusting the load of the reliquefaction system includes a load controller LC that adjusts the load of the reliquefaction system, and a control unit PIC that receives a set value sent by the load controller and controls the charging or discharging of refrigerant in the refrigerant circulation line.

[0061] The control unit receives the suction pressure of the refrigerant detected at the front end of the refrigerant compressor using the suction pressure detection unit and the set value of the suction pressure sent by the load controller, and adjusts the amount of cold energy in the refrigerant circulation line and the load on the reliquefaction system by replenishing the refrigerant in the refrigerant circulation line using PID control or discharging a portion of the refrigerant to the refrigerant inventory unit to change the mass flow rate of the refrigerant.

[0062] Figure 1 shows how refrigerant is charged into the refrigerant circulation line by PID control based on the refrigerant suction pressure detected upstream of the refrigerant compressor. Figure 2 shows how a portion of the refrigerant is discharged from the refrigerant circulation line to the refrigerant inventory section by PID control based on the detected refrigerant suction pressure.

[0063] When the suction pressure upstream of the refrigerant compressor is lower than the set suction pressure value sent by the load controller, auxiliary nitrogen refrigerant is supplied from the inventory tank to the upstream of the refrigerant compressor via the refrigerant charging line, as shown in Figure 1(a), increasing the amount of cold energy in the refrigerant circulation line. When the suction pressure upstream of the refrigerant compressor is higher than the set suction pressure value sent by the load controller, excess nitrogen refrigerant is discharged from the downstream of the refrigerant compressor to the inventory tank via the refrigerant discharge line, as shown in Figure 2(a), decreasing the amount of cold energy in the refrigerant circulation line. As mentioned above, when the pressure with the inventory tank is reversed, the refrigerant discharged from the downstream of the refrigerant compressor is transported to the inventory tank via the second discharge line and the boost compressor.

[0064] In the graphs shown in Figures 1(b) and 2(b), the horizontal axis represents the change in the setpoint transmitted from the load controller, the vertical axis represents the pressure value, the line labeled IT represents the inventory tank pressure, the line labeled CLl represents the refrigerant suction pressure upstream of the refrigerant compressor, and the line labeled CLh represents the refrigerant discharge pressure downstream of the refrigerant compressor. As shown in Figure 1(b), when the setpoint transmitted from the load controller is increased to increase the load on the reliquefaction system, refrigerant is replenished into the refrigerant circulation line through the refrigerant charge line by the pressure difference between the inventory tank and the refrigerant suction pressure upstream of the refrigerant compressor (1). Conversely, as shown in Figure 2(b), when the setpoint transmitted from the load controller is decreased to decrease the load on the reliquefaction system, refrigerant is discharged into the refrigerant circulation line through the refrigerant discharge line by the pressure difference between the inventory tank and the refrigerant discharge pressure downstream of the refrigerant compressor. If the pressure downstream of the refrigerant compressor is higher, refrigerant is discharged into the inventory tank (2). When the pressure reverses and the pressure in the inventory tank is higher, refrigerant is discharged in an amount equal to the pressure difference. The refrigerant discharged from the downstream side of the refrigerant compressor is transported to the inventory tank via the second discharge line and the boost compressor (3).

[0065] Figure 3 is a schematic diagram showing the progress of refrigerant charging and discharging under PID control based on the refrigerant compressor suction pressure. When refrigerant is charged and discharged using PID control based solely on the refrigerant compressor suction pressure (PV), as shown in Figure 3, the refrigerant injection (NF) and discharge (NR) are continuously repeated based on the setpoint (SP) in response to changes in the refrigerant compressor suction pressure, resulting in inefficient system operation. In particular, if the suction pressure upstream of the refrigerant compressor is unstable, the refrigerant charging and discharging will occur accordingly, leading to significant instability in both the discharge pressure downstream of the refrigerant compressor and the refrigerant cycle, which can lead to process instability in the re-liquefaction system, deviations from the designed operating point, and emergency shutdowns of the re-liquefaction system.

[0066]

[0067] To improve this, in this embodiment, the control unit performs on / off control together with PID control to control the refrigerant cycle. In particular, the suction pressure and discharge pressure of the refrigerant compressor are simultaneously considered. The control unit PIC compares the current value PVs of the suction pressure upstream of the refrigerant compressor detected by the suction pressure detection unit P1 with the set value SPs of the suction pressure for adjusting the load of the reliquefaction system transmitted from the load controller LC, and uses PID control to charge or discharge refrigerant from the refrigerant circulation line. When certain conditions are met, the control unit PIC prioritizes on / off control over PID control to stop the injection or discharge of refrigerant, preventing inefficient operation due to continuous repetition of refrigerant injection and discharge and ensuring process stability of the reliquefaction system.

[0068] Figure 4 shows cases where the current values ​​of the suction pressure and discharge pressure (shown as dots in each figure) in the refrigerant compressor are different when the refrigerant cycle of a ship's reliquefaction system is controlled by the refrigerant cycle control system of this embodiment.

[0069] 4(a), when the current suction pressure on the upstream side of the refrigerant compressor is greater than the set value of the suction pressure transmitted from the load controller plus a first correction value α, and the current discharge pressure on the downstream side of the refrigerant compressor is greater than the set value of the discharge pressure SPd transmitted from the load controller minus a second correction value β, the refrigerant in the refrigerant circulation line is discharged. This process control condition can be expressed as follows:

[0070] PVs>SPs+α, PVd>SPd-β

[0071] The first correction value α is an adjustable value for controlling the refrigerant supply and discharge so that the refrigerant supply and discharge are not performed if the current suction pressure value is within a predetermined range based on the set value of the suction pressure. The second correction value β is an adjustable value for controlling the refrigerant supply and discharge so that the refrigerant supply and discharge are not performed if the current discharge pressure value is within a predetermined range based on the set value of the discharge pressure. α and β are arbitrary values ​​depending on the operating conditions, for example, α is 0.03 bara and β is 0.04 bara.

[0072] Under process control conditions, the control unit uses PID control to discharge a portion of the refrigerant from the downstream side of the refrigerant compressor through the refrigerant discharge line to the inventory tank using differential pressure, while preventing the refrigerant from being charged into the refrigerant circulation line.

[0073] Next, as shown in Figure 4(b), when the current value of the suction pressure on the upstream side of the refrigerant compressor is smaller than the value obtained by subtracting the first correction value from the suction pressure setting, and the current value of the discharge pressure on the downstream side of the refrigerant compressor is smaller than the value obtained by adding the second correction value to the discharge pressure setting, the refrigerant circulation line is filled with refrigerant. This process control condition can be expressed by the following equation.

[0074] PVs <SPs-α、PVd<SPd+β

[0075] In this case, the control unit uses PID control to charge the refrigerant from the inventory tank into the refrigerant circulation line via the refrigerant charging line, while preventing the refrigerant from being discharged from the refrigerant circulation line.

[0076] As shown in Figure 4(c), if the difference between the current value of the suction pressure and the set value of the suction pressure is less than or equal to the first correction value α (PVs≦SPs±α), on / off control is preferentially performed to stop the charging and discharging of the refrigerant.

[0077] As shown in Figure 4(d), when the current value of the suction pressure is greater than the sum of the set value of the suction pressure and the first correction value, and the current value of the discharge pressure is less than the difference between the set value of the discharge pressure and the second correction value, the control unit performs on / off control with priority and stops charging and discharging the refrigerant. This process control condition can be expressed as follows:

[0078] PVs>SPs+α, PVd <SPd-β

[0079] Under these conditions, if the current value of the suction pressure is higher than the specified range but the current value of the discharge pressure is smaller than the value obtained by subtracting the second correction value from the set value of the discharge pressure, the suction pressure may also decrease after the discharge pressure stabilizes, so on / off control is performed to prevent the refrigerant from being discharged.

[0080] Finally, as shown in Figure 4(e), if the current suction pressure is smaller than the set suction pressure minus the first correction value, and the current discharge pressure is greater than the set discharge pressure plus the second correction value, the control unit will prioritize on-off control and stop the charging and discharging of refrigerant. This process control condition can be expressed as follows:

[0081] PVs<SPs-α、PVd> SPd+β

[0082] Under this condition, the current value of the suction pressure is lower than the specified range, but if the current value of the discharge pressure is higher than the set value of the discharge pressure plus the second correction value, the discharge pressure may stabilize after a certain period of time has passed, so on / off control is performed to temporarily stop the supply of auxiliary refrigerant.

[0083] FIG. 5 is a schematic diagram showing the progress of charging and discharging of refrigerant by the refrigerant cycle control system of this embodiment.

[0084] As shown in Figure 5, the control system of this embodiment prioritizes on-off control to stop refrigerant charging and discharging within a predetermined range (SPs ± α) based on the suction pressure setpoint SPs in response to changes in the suction pressure of the refrigerant compressor. Therefore, when the current suction pressure PVs is lower than the predetermined range, charging NR is performed. When the current suction pressure PVs reaches the predetermined range (A), on-off control is performed to stop refrigerant charging and discharging. If this control stabilizes the suction pressure upstream of the refrigerant compressor, the discharge pressure downstream of the refrigerant compressor will also stabilize. This prevents inefficient repeated refrigerant charging and discharging and ensures process stability in the reliquefaction system.

[0085]

[0086] 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. In a refrigerant cycle control system for a ship's re-liquefaction system, evaporated gas generated from liquefied gas in a storage tank of the ship is compressed and re-liquefied by heat exchange with the refrigerant circulating in the refrigerant circulation line in a heat exchanger. a refrigerant compressor provided in the refrigerant circulation line and configured to compress the refrigerant discharged after cooling the evaporated gas in the heat exchanger; and a suction pressure detection unit that detects the suction pressure of the refrigerant flowing from the refrigerant circulation line to the refrigerant compressor and transmits a current value (PVs) of the suction pressure; and a load controller that transmits suction pressure set points (SPs) to regulate the load on the reliquefaction system; and a control unit that receives the set value and charges or discharges refrigerant from the refrigerant circulation line to adjust the amount of cold in the refrigerant circulation line, the control unit compares the current value of the suction pressure with the set value of the suction pressure and fills or discharges the refrigerant in the refrigerant circulation line by PID control, and if the difference between the current value of the suction pressure and the set value of the suction pressure is equal to or less than a first correction value α (PVs≦SPs±α), it preferentially executes on / off control to stop the filling or discharging of the refrigerant. Refrigerant cycle control system for ship reliquefaction systems.

2. a discharge pressure detection unit that detects a discharge pressure of the refrigerant discharged from the refrigerant compressor through the refrigerant circulation line and transmits a current value PVd of the discharge pressure to the control unit, When the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α) and the current value of the discharge pressure is greater than the value obtained by subtracting the second correction value β from the set value of the discharge pressure SPd (PVd>SPd-β), the refrigerant in the refrigerant circulation line is discharged; When the current value of the suction pressure is smaller than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is smaller than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd<SPd+β), the control unit fills or discharges the refrigerant in the refrigerant circulation line by PID control in order to fill the refrigerant in the refrigerant circulation line. The refrigerant cycle control system for a ship reliquefaction system according to claim 1.

3. the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the current value of the discharge pressure is less than the value obtained by subtracting the second correction value from the set value of the discharge pressure (PVd<SPd-β); If the current value of the suction pressure is smaller than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is larger than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd>SPd+β), The control unit prioritizes on / off control to stop charging and discharging of the refrigerant. The refrigerant cycle control system for a ship reliquefaction system according to claim 2.

4. a refrigerant inventory unit for charging or discharging the refrigerant in the refrigerant circulation line; and a refrigerant charging line connected from the refrigerant inventory section to an upstream side of the refrigerant compressor, for supplying refrigerant to the refrigerant circulation line; and a refrigerant discharge line connected to the refrigerant inventory section from the upstream side of the refrigerant compressor to discharge the refrigerant in the refrigerant circulation line into the refrigerant inventory section. The refrigerant cycle control system for a ship reliquefaction system according to claim 2.

5. The refrigerant inventory unit an inventory tank for storing the refrigerant to be supplied to the refrigerant circulation line; and a boost compressor that compresses refrigerant and supplies it to the inventory tank; The refrigerant in the refrigerant circulation line is nitrogen. The refrigerant cycle control system for a ship reliquefaction system according to claim 4.

6. the refrigerant filling line is connected from the inventory tank to the upstream side of the refrigerant compressor, and the refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure difference between the inventory tank and the refrigerant circulation line. The refrigerant cycle control system for a ship reliquefaction system according to claim 5.

7. The refrigerant discharge line is a first discharge line connected downstream of the refrigerant compressor to the inventory tank; and a second discharge line branched from the first discharge line upstream of the inventory tank and connected to the inventory tank via the boost compressor; Discharging the refrigerant from the downstream side of the refrigerant compressor into the inventory tank through the first discharge line due to a pressure difference between the refrigerant circulation line and the inventory tank; When a pressure reversal occurs, the refrigerant is discharged through the second discharge line to the inventory tank via the boost compressor. The refrigerant cycle control system for a ship reliquefaction system according to claim 5.

8. a refrigerant expander that is provided in the refrigerant circulation line and expands and cools the refrigerant that has been compressed by the refrigerant compressor and then cooled through the heat exchanger, and supplies the refrigerant as a refrigerant for the heat exchanger. The refrigerant cycle control system for a ship reliquefaction system according to claim 7.

9. A refrigerant cycle control method for a ship's reliquefaction system, in which evaporated gas generated from liquefied gas in a storage tank of a ship is compressed and reliquefied by heat exchange with a refrigerant circulating in a refrigerant circulation line in a heat exchanger, comprising: When adjusting the load of the reliquefaction system, a control unit receives a set value (SPs) of a suction pressure received from a load controller and a current value (PVs) of a suction pressure of the refrigerant flowing into the refrigerant compressor of the refrigerant circulation line after heat exchange with the evaporated gas in the heat exchanger, compares the current value of the suction pressure with the set value of the suction pressure, and charges or discharges the refrigerant in the refrigerant circulation line through PID control to adjust the amount of cold energy in the refrigerant circulation line; If the difference between the current value of the suction pressure and the set value of the suction pressure is equal to or less than a first correction value α (PVs≦SPs±α), the control unit preferentially executes on-off control to stop refrigerant charging and discharging. A method for controlling the refrigerant cycle of a ship's reliquefaction system.

10. a current value of the suction pressure is greater than the sum of the set value of the suction pressure and the first correction value (PVs>SPs+α), and a discharge pressure of the refrigerant discharged from the refrigerant compressor is detected, and if the current value of the discharge pressure PVd is greater than the set value of the discharge pressure SPd minus the second correction value β (PVd>SPd-β), the refrigerant in the refrigerant circulation line is discharged; When the difference between the current value of the suction pressure and the set value of the suction pressure is smaller than the value obtained by subtracting the first correction value (PVs<SPs-α), and the current value of the discharge pressure is smaller than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd<SPd+β), the refrigerant in the refrigerant circulation line is filled. The method for controlling a refrigerant cycle of a ship reliquefaction system according to claim 9.

11. the current value of the suction pressure is greater than the value obtained by adding the first correction value to the set value of the suction pressure (PVs>SPs+α), and the current value of the discharge pressure is less than the value obtained by subtracting the second correction value from the set value of the discharge pressure (PVd<SPd-β); If the current value of the suction pressure is smaller than the value obtained by subtracting the first correction value from the set value of the suction pressure (PVs<SPs-α), and the current value of the discharge pressure is larger than the value obtained by adding the second correction value to the set value of the discharge pressure (PVd>SPd+β), The control unit performs on / off control with priority to stop charging and discharging of the refrigerant. The method for controlling a refrigerant cycle of a ship reliquefaction system according to claim 10.

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