Refrigerant cycle control system and control method for reliquefaction system for ship

Abstract

Disclosed herein are a system and method for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line. The system according to the present invention includes: a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas; a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and transmit a current suction pressure value (PVs); a load controller configured to transmit a suction pressure set point (SPs) for load adjustment of the reliquefaction system; and a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

Description

[Technical Field]

[0001] The present invention relates to a system and method for controlling a refrigerant cycle of a reliquefaction system for ships, which enables cooling and reliquefaction of boil-off gas (BOG) generated from liquefied gas stored in an onboard storage tank and can effectively adjust the amount of cold heat in the refrigerant cycle in response to changes in load of the reliquefaction system while ensuring stability of the reliquefaction process.[Background Art]

[0002] Natural gas contains methane as a main component and has been attracting attention as an eco-friendly fuel that emits little or no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by liquefying natural gas through cooling to about -163°C under normal pressure and is suited to long-distance transportation by sea since it has a volume of about 1 / 600 that of natural gas in a gaseous state. Accordingly, natural gas is stored and transported as liquefied natural gas, which is easy to store and transport.

[0003] Since natural gas is liquefied at a cryogenic temperature of -163°C under normal pressure, LNG storage tanks are typically insulated to maintain LNG in a liquid state. However, despite being insulated, such a storage tank is limited in ability to block external heat. Accordingly, due to external heat continuously transferred to the LNG storage tank, LNG stored in the LNG tank continues to evaporate naturally during transportation, causing generation of boil-off gas (BOG).

[0004] Continuous production of boil-off gas in the LNG storage tank increases the internal pressure of the LNG storage tank. If the internal pressure of the storage tank exceeds a predetermined safe pressure, this can cause an emergency situation such as rupture of the storage tank. Accordingly, there is a need to discharge boil-off gas from the storage tank using a safety valve. However, boil-off gas is a kind of LNG loss and is an important issue for transportation efficiency and fuel efficiency of LNG. Therefore, various methods are employed to handle boil-off gas generated in the LNG storage tank.

[0005] Recently, a method of using boil-off gas at a fuel demand site such as an engine of a ship, a method of reliquefying boil-off gas and returning the reliquefied boil-off gas to an LNG storage tank, and a method combining these two approaches have been developed and put into use.[Disclosure][Technical Problem]

[0006] Methods for reliquefying boil-off gas include a method that employs a refrigeration cycle using a separate refrigerant to reliquefy boil-off gas through heat exchange with the refrigerant, a method that reliquefies boil-off gas using the boil-off gas itself as a refrigerant without a separate refrigerant, and the like.

[0007] As a method of reliquefying boil-off gas using the boil-off gas itself as a refrigerant without a separate refrigerant, a reliquefaction system configured to cool compressed boil-off gas through heat exchange with uncompressed boil-off gas and reliquefy the cooled boil-off gas by adiabatic expansion has been developed and applied to ships.

[0008] Systems that utilize a separate refrigeration cycle include, for example, a system that employs a reliquefaction process using nitrogen as a refrigerant.

[0009] Despite relative inefficiency compared to a reliquefaction cycle using a mixed refrigerant, such a reliquefaction process using nitrogen refrigerant is safer due to inert properties of the nitrogen refrigerant and is easier to apply to ships since the nitrogen refrigerant does not undergo phase change.

[0010] As such, boil-off gas cooled through heat exchange with a separate refrigerant or the boil-off gas itself is introduced into a separator, in which the boil-off gas is separated into gas and liquid phases, and the resulting reliquefied gas is returned to a storage tank.

[0011] It is an aspect of the present invention to provide a solution to effectively adjust the amount of cold heat in a refrigerant cycle in response to changes in load of a reliquefaction system while ensuring stability of a reliquefaction process.[Technical Solution]

[0012] In accordance with one aspect of the present invention, there is provided a system for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, the refrigerant cycle control system including: a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas; a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and to transmit a current suction pressure value (PVs); a load controller configured to transmit a suction pressure set point (SPs) for load adjustment of the reliquefaction system; and a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0013] Preferably, the system further includes: a discharge pressure detector configured to detect a discharge pressure of the refrigerant discharged from the refrigerant compressor on the refrigerant circulation line and to transmit a current discharge pressure value (PVd) to the control unit, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β), the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

[0014] Preferably, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0015] Preferably, the system further includes: a refrigerant inventory system configured to supply the refrigerant to the refrigerant circulation line or receive the refrigerant discharged from the refrigerant circulation line; a refrigerant charging line connecting the refrigerant inventory system to an upstream side of the refrigerant compressor to supply the refrigerant to the refrigerant circulation line; and a refrigerant discharge line connecting a downstream side of the refrigerant compressor to the refrigerant inventory system to discharge the refrigerant from the refrigerant circulation line to the refrigerant inventory system.

[0016] Preferably, the refrigerant inventory system includes: an inventory tank storing the refrigerant to be supplied to the refrigerant circulation line; and a booster compressor configured to compress the refrigerant and supply the compressed refrigerant to the inventory tank, and the refrigerant in the refrigerant circulation line is nitrogen.

[0017] Preferably, the refrigerant charging line connects the inventory tank to the upstream side of the refrigerant compressor such that the refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure differential between the inventory tank and the refrigerant circulation line.

[0018] Preferably, the refrigerant discharge line includes: a first discharge line connecting the downstream side of the refrigerant compressor to the inventory tank; and a second discharge line branched off of the first discharge line upstream of the inventory tank and connected to the inventory tank through the booster compressor, wherein the refrigerant is discharged from the downstream side of the refrigerant compressor to the inventory tank along the first discharge line by a pressure differential between the refrigerant circulation line and the inventory tank and, in the event of pressure reversal, the refrigerant is discharged to the inventory tank through the booster compressor along the second discharge line.

[0019] Preferably, the system further includes: a refrigerant expander disposed on the refrigerant circulation line to expand and cool the refrigerant compressed by the refrigerant compressor and having been cooled by the heat exchanger and to supply the cooled refrigerant as a refrigerant to the heat exchanger.

[0020] In accordance with another aspect of the present invention, there is a provided a method for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, wherein, upon adjustment of a load of the reliquefaction system, a control unit receives a suction pressure set point (SPs) from a load controller and a current suction pressure value (PVs) of the refrigerant introduced into a refrigerant compressor on the refrigerant circulation line after undergoing heat exchange with the boil-off gas in the heat exchanger and charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point to adjust the amount of cold heat in the refrigerant circulation line, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0021] Preferably, the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value (PVd) is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β) by detecting a discharge pressure of the refrigerant discharged from the refrigerant compressor, the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

[0022] Preferably, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.[Advantageous Effects]

[0023] The refrigerant cycle control system and method according to the present invention can provide more effective cooling of boil-off gas to be reliquefied by utilizing both cold heat of the boil-off gas itself and cold heat in a refrigerant cycle, thereby ensuring an enhanced reliquefaction rate.

[0024] The refrigerant cycle control system and method according to the present invention can prevent destabilization of a reliquefaction process caused by frequent refrigerant charging and discharging by selectively performing PID control and on-off control depending on the conditions of the refrigerant cycle while appropriately charging / discharging a refrigerant into / from the refrigerant cycle depending on the load of the reliquefaction system.[Description of Drawings]

[0025] FIG. 1 and FIG. 2 are schematic diagrams of a control system used in a reliquefaction system to charge or discharge a refrigerant into or from a refrigerant circulation line using PID control depending on the suction pressure of a refrigerant compressor. FIG. 3 is a schematic illustration of refrigerant charging and discharging trends under PID control depending on the suction pressure of the refrigerant compressor. FIG. 4 illustrates different cases of the current suction pressure value and current discharge pressure value at the refrigerant compressor for controlling a refrigerant cycle of a reliquefaction system for ships using the refrigerant cycle control system according to one embodiment. FIG. 5 is a schematic illustration of refrigerant charging and discharging trends under the refrigerant cycle control system according to one embodiment. [Best Mode]

[0026] In order to fully appreciate the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings, which illustrate preferred embodiments of the present invention, and description thereof.

[0027] Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects thereof with reference to the accompanying drawings. It should be noted that like components will be denoted by like reference numerals throughout the specification and the accompanying drawings.

[0028] As used herein, the term "ship" may refer to any type of ship that is provided with a liquefied gas storage tank. For example, the ship may include self-propelled vessels, such as an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV), as well as non-self-propelled floating offshore structures, such as an LNG floating production storage and offloading (FPSO) unit and an LNG floating storage regasification unit (FSRU).

[0029] In addition, the embodiments of the present invention may be applied to a reliquefaction cycle for any type of liquefied gas that can be transported in a liquid state by liquefaction at cryogenic temperatures and can generate boil-off gas during storage. For example, such liquefied gas may include liquefied petrochemical gas, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas. In the following embodiments, the present invention will be described using LPG, which is a typical liquefied gas, as an example.

[0030] The control system according to this embodiment is used in a reliquefaction system for ships, in which boil-off gas generated from liquefied gas in an onboard storage tank (not shown) is compressed by a compressor, cooled and reliquefied by a heat exchanger, and returned to the storage tank, to control a refrigerant cycle of the reliquefaction system in response to changes in load of the reliquefaction system.

[0031] FIG. 1 and FIG. 2 are schematic diagrams of a reliquefaction system and a control system used in the reliquefaction system to charge or discharge a refrigerant into or from a refrigerant circulation line using PID control depending on the suction pressure of a refrigerant compressor.

[0032] The reliquefaction system includes a compressor (not shown) configured to receive and compress boil-off gas, a boil-off gas supply line (not shown) through which boil-off gas from the storage tank is supplied to the compressor, and a reliquefaction line (not shown) connecting the compressor to the storage tank and allowing boil-off gas to be reliquefied and returned to the storage tank therealong.

[0033] The boil-off gas supply line extends from the storage tank to the compressor through a heat exchanger 100 such that uncompressed boil-off gas from the storage tank is supplied to the compressor for compression after supplying cold heat to the heat exchanger.

[0034] The compressor (not shown) may compress the boil-off gas to a certain pressure, for example, to a fuel supply pressure required for a main engine of a ship. For example, the compressor may compress the boil-off gas to a pressure of about 5.5 barg for DF engines, about 15 barg for X-DF engines, and about 300 barg for ME-GI engines. The compressed boil-off gas may be supplied as fuel to a demand site, such as the main engine (not shown) of the ship, and surplus boil-off gas may be reliquefied through the reliquefaction system along the reliquefaction line.

[0035] The compressed boil-off gas from the compressor is introduced into the heat exchanger 100 along the reliquefaction line to be cooled through heat exchange, the cooled boil-off gas is subjected to gas-liquid separation in a separator (not shown), and the separated reliquefied gas is returned to the storage tank.

[0036] In the heat exchanger 100, the boil-off gas is cooled by both a refrigerant circulating through a refrigerant circulation line CL and cold heat of uncompressed boil-off gas to be introduced into the compressor.

[0037] The refrigerant circulation line CL, which constitutes a refrigerant cycle, is provided with a refrigerant expander (not shown) configured to expand and cool the refrigerant to be supplied to the heat exchanger, a refrigerant compressor 200 configured to compress the refrigerant discharged from the heat exchanger, and a motor configured to drive the refrigerant compressor. The refrigerant compressor and the refrigerant expander may be connected to each other through a common shaft to use expansion energy of the refrigerant for compression of the refrigerant in the refrigerant compressor, thereby reducing power consumption required to operate the refrigerant cycle.

[0038] The compressed refrigerant from the refrigerant compressor 200 is cooled by the heat exchanger 100, expanded and cooled by the refrigerant expander, and supplied as a refrigerant to the heat exchanger 100 while circulating through the refrigerant circulation line. Accordingly, in the heat exchanger 100, four streams including a stream of boil-off gas compressed by the compressor and to be reliquefied, a stream of uncompressed boil-off gas to be introduced into the compressor, a stream of refrigerant expanded and cooled by the refrigerant expander, and a stream of refrigerant compressed by the refrigerant compressor undergo heat exchange.

[0039] The cooled boil-off gas from the heat exchanger may be subjected to gas-liquid separation in the separator after passing through a control valve disposed downstream of the heat exchanger. The separated reliquefied gas from the separator (not shown) is supplied to the storage tank for re-storage and the resulting flash gas may be joined with the stream of uncompressed boil-off gas upstream of the heat exchanger on the boil-off gas supply line or may be delivered to a gas combustion unit (GCU).

[0040] For example, nitrogen (N 2 ) may be used as the refrigerant circulating through the refrigerant circulation line to supply cold heat to the heat exchanger.

[0041] When the amount of boil-off gas to be reliquefied changes, the amount of cold heat required for the reliquefaction system also changes. The amount of cold heat transferred to the heat exchanger may be adjusted by adding more nitrogen refrigerant to the refrigerant circulation line from a refrigerant inventory system 300 or discharging some nitrogen refrigerant from the refrigerant circulation line depending on changes in load of the reliquefaction cycle, thereby adjusting the load of the reliquefaction system.

[0042] The refrigerant inventory system 300 may include an inventory tank storing refrigerant to be supplied to the refrigerant circulation line CL and a booster compressor configured to compress refrigerant and supply the compressed refrigerant to the inventory tank. The refrigerant inventory system may be configured to receive nitrogen from an onboard nitrogen generator (N 2 generator) provided for supply of utility N 2 , reduce the dew point of the received nitrogen through a drying and filtration unit (not shown), compress the nitrogen through the booster compressor, store the compressed nitrogen in the inventory tank, and supply the stored nitrogen to the refrigerant circulation line of the reliquefaction system. Upon adjustment of the load of the reliquefaction system, supplementary refrigerant may be supplied from the inventory tank to the refrigerant circulation line or surplus refrigerant may be discharged from the refrigerant circulation to the inventory tank. To this end, the control system further includes: a refrigerant charging line FL connecting the inventory tank of the refrigerant inventory system to an upstream side of the refrigerant compressor; and a refrigerant discharge line DL connecting a downstream side of the refrigerant compressor to the refrigerant inventory system.

[0043] The refrigerant discharge line DL includes a first discharge line DLa connecting the downstream side of the refrigerant compressor to the inventory tank and a second discharge line DLb branched off of the first discharge line DLa upstream of the inventory tank and connected to the inventory tank through the booster compressor.

[0044] Upon adjustment of the load of the reliquefaction system, a pressure differential between the refrigerant circulation line and the inventory tank may be utilized to add more nitrogen refrigerant to the refrigerant circulation line from the inventory tank or to discharge some nitrogen refrigerant from the refrigerant circulation line.

[0045] The refrigerant charging line connects the inventory tank to the upstream side of the refrigerant compressor such that supplementary refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure differential between the inventory tank and the refrigerant circulation line.

[0046] Upon reducing the load of the reliquefaction system, a pressure differential between the downstream side of the refrigerant compressor on the refrigerant circulation line and the inventory tank may be utilized to discharge the refrigerant from the downstream side of the refrigerant compressor to the inventory tank along the first discharge line. However, when pressure reversal occurs, making it difficult to discharge refrigerant to the inventory tank using such a pressure differential, the nitrogen refrigerant discharged from the downstream side of the refrigerant compressor may be compressed by the booster compressor along the second discharge line DLb and then delivered to the inventory tank.

[0047] On the refrigerant circulation line, a suction pressure detector P1 is disposed upstream of the refrigerant compressor to detect a suction pressure of the refrigerant introduced into the refrigerant compressor and a discharge pressure detector P2 is disposed downstream of the refrigerant compressor to detect a discharge pressure of the refrigerant compressed by the refrigerant compressor.

[0048] A control system for adjusting the load of the reliquefaction system includes: a load controller LC configured to adjust the load of the reliquefaction system; and a control unit PIC configured to receive a set point from the load controller and charge or discharge the refrigerant into or from the refrigerant circulation line.

[0049] The control unit may receive a refrigerant suction pressure detected upstream of the refrigerant compressor by the suction pressure detector and a suction pressure set point transmitted from the load controller and may change a mass flow rate of refrigerant in the refrigerant circulation line by adding more refrigerant to the refrigerant circulation line or discharging some refrigerant to the refrigerant inventory system using PID control, thereby adjusting the amount of cold heat in the refrigerant circulation line and the load of the reliquefaction system.

[0050] FIG. 1 shows the case where refrigerant is charged into the refrigerant circulation line using PID control based on the refrigerant suction pressure detected upstream of the refrigerant compressor and FIG. 2 shows the case where some refrigerant is discharged from the refrigerant circulation line to the refrigerant inventory system using PID control based on the detected refrigerant suction pressure.

[0051] When the refrigerant suction pressure at the upstream side of the refrigerant compressor is less than the suction pressure set point transmitted from the load controller, supplementary nitrogen refrigerant may be supplied from the inventory tank to the upstream side of the refrigerant compressor along the refrigerant charging line, as shown in FIG. 1(a), thereby increasing the amount of cold heat in the refrigerant circulation line. When the refrigerant suction pressure at the upstream side of the refrigerant compressor is greater than the suction pressure set point transmitted from the load controller, surplus nitrogen refrigerant may be discharged from the downstream side of the refrigerant compressor to the inventory tank along the refrigerant discharge line, as shown in FIG. 2(a), thereby reducing the amount of cold heat in the refrigerant circulation line. As described above, in the event of a pressure reversal where the pressure of the inventory tank exceeds the pressure at the downstream side of the refrigerant compressor, the refrigerant discharged from the downstream side of the refrigerant compressor may be delivered to the inventory tank through the boosting compressor along the second discharge line.

[0052] In the graphs of FIG. 1(b) and FIG. 2(b), the horizontal axis represents changes in the set point transmitted from the load controller, the vertical axis represents a pressure value, line IT represents the pressure of the inventory tank, line CLl represents the refrigerant suction pressure at the upstream side of the refrigerant compressor, and line CLh represents the refrigerant discharge pressure at the downstream side of the refrigerant compressor. As shown in FIG. 1(b), when the set point transmitted from the load controller is increased to increase the load of the reliquefaction system, refrigerant is added to the refrigerant circulation line along the refrigerant charging line by a difference between the pressure of the inventory tank and the refrigerant suction pressure at the upstream side of the refrigerant compressor (①). Conversely, as shown in FIG. 2(b), when the set point transmitted from the load controller is decreased to reduce the load of the reliquefaction system, the refrigerant is discharged from the refrigerant circulation line along the refrigerant discharge line by a difference between the pressure of the inventory tank and the refrigerant discharge pressure at the downstream side of the refrigerant compressor. Specifically, when the pressure at the downstream side of the refrigerant compressor is greater than the pressure of the inventory tank, the refrigerant is discharged to the inventory tank along the first discharge line by a pressure differential (②) and, in the event of a pressure reversal where the pressure of the inventory tank exceeds the pressure at the downstream side of the refrigerant compressor, the refrigerant is discharged from the downstream side of the refrigerant compressor by a pressure differential and then delivered to the inventory tank through the booster compressor along the second discharge line (③).

[0053] FIG. 3 is a schematic illustration of the refrigerant charging and discharging trends under PID control depending on the suction pressure of the refrigerant compressor. When refrigerant charging and discharging are performed using PID control solely based on the suction pressure PVs of the refrigerant compressor, refrigerant injection NF and refrigerant discharge NR are repeatedly alternated with respect to the set point SP in response to changes in suction pressure of the refrigerant compressor, as shown in FIG. 3, resulting in inefficient system operation. In particular, if the suction pressure at the upstream side of the refrigerant compressor fluctuates, this triggers corresponding refrigerant charging and discharging processes, causing significant fluctuations in discharge pressure at the downstream side of the refrigerant compressor and thus destabilization of the refrigerant cycle, which can lead to process instability of the reliquefaction system, deviations from a designed equipment operating point, emergency shutdown of the reliquefaction system, and the like.

[0054] To solve this problem, in this embodiment, the control unit is configured to control the refrigerant cycle by performing on / off control along with PID control. In particular, in consideration of both the suction pressure and discharge pressure at the refrigerant compressor, the control unit PIC compares a current suction pressure value PVs at the upstream side of the refrigerant compressor, as detected by the suction pressure detector P1, with a suction pressure set point SPs transmitted from the load controller LC for load adjustment of the reliquefaction system and charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on the comparison results. However, when specific conditions are met, the control unit PIC may preferentially perform on / off control over PID control to stop refrigerant injection or discharge, thereby preventing inefficient system operation due to repeated alternation between refrigerant injection and refrigerant discharge and thus ensuring process stability of the reliquefaction system.

[0055] FIG. 4 illustrates different cases of the current suction pressure value and current discharge pressure value (indicated as dots in each diagram) at the refrigerant compressor for controlling a refrigerant cycle of a reliquefaction system for ships using the refrigerant cycle control system according to this embodiment.

[0056] In the following, a refrigerant cycle control method will be described with reference to different cases of the current suction pressure value at the upstream side of the refrigerant compressor and the current discharge pressure value at the downstream side of the refrigerant compressor. First, when the current suction pressure value at the upstream side of the refrigerant compressor is greater than a value obtained by adding a first adjustment value α to the suction pressure set point transmitted from the load controller and the current discharge pressure value at the downstream side of the refrigerant compressor is greater than a value obtained by subtracting a second adjustment value β from the discharge pressure set point SPd transmitted from the load controller, as shown in FIG. 4(a), the control unit PIC performs control such that the refrigerant is discharged from the refrigerant circulation line. This process control condition may be expressed mathematically as follows: PVs > SPs + α , and PVd > SPd - β

[0057] The first adjustment value α is an adjustable value used to control the system such that refrigerant injection and discharge are not performed when the current suction pressure value is within a certain range with respect to the suction pressure set point. The second adjustment value β is an adjustable value used to control the system such that refrigerant injection and discharge are not performed when the current discharge pressure value is within a certain range with respect to the discharge pressure set point. Here, α and β may be set to any value depending on operating conditions; for example, α may be 0.03 bara and β may be 0.04 bara.

[0058] Under this process control condition, the control unit performs control to prevent refrigerant charging into the refrigerant circulation line while allowing some refrigerant to be discharged from the downstream side of the refrigerant compressor to the inventory tank along the refrigerant discharge line by a pressure differential therebetween using PID control.

[0059] Next, when the current suction pressure value at the upstream side of the refrigerant compressor is less than a value obtained by subtracting the first adjustment value from the suction pressure set point and the current discharge pressure value at the downstream side of the refrigerant compressor is less than a value obtained by adding the second adjustment value to the discharge pressure set point, as shown in FIG. 4(b), the control unit performs control such that the refrigerant is charged into the refrigerant circulation line. This process control condition may be expressed mathematically as follows: PVs < SPs - α , and PVd < SPd + β

[0060] In this case, the control unit performs control to prevent refrigerant discharge from the refrigerant circulation line while allowing more refrigerant to be added to the refrigerant circulation line from the inventory tank along the refrigerant charging line using PID control.

[0061] When a difference between the current suction pressure value and the suction pressure set point is less than or equal to the first adjustment value α (PVs ≤ SPs ± α), as shown in FIG. 4(c), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0062] When the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point, as shown in FIG. 4(d), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant. This process control condition may be expressed mathematically as follows: PVs > SPs + α , and PVd < SPd - β

[0063] Under this condition, where the current suction pressure value exceeds a certain range while the current discharge pressure value is less than the value obtained by subtracting the second adjustment value from the discharge pressure set point, on / off control is performed to prevent refrigerant discharge, given that the suction pressure can also decrease after the discharge pressure stabilizes.

[0064] Lastly, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point, as shown in FIG. 4(e), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant. This process control condition may be expressed mathematically as follows: PVs < SPs − α , andPVd > SPd + β

[0065] In this case, where the current suction pressure value is less than a certain range while the current discharge pressure value is greater than a value adding the second adjustment value to the discharge pressure set point, on / off control is performed to temporarily prevent supply of supplementary refrigerant, given that the suction pressure can also increase after the process proceeds for a certain period of time and the discharge pressure stabilizes.

[0066] FIG. 5 is a schematic illustration of refrigerant charging and discharging trends under the refrigerant cycle control system according to this embodiment.

[0067] Referring to FIG. 5, in the control system according to this embodiment, on / off control is preferentially performed to stop refrigerant injection and discharge when the suction pressure at the upstream side of the refrigerant compressor is within a predetermined range (SPs±α) defined by the first adjustment value α relative to the suction pressure set point SPs, whereby, when the current suction pressure value PVs is less than the predetermined range, refrigerant charging NR is performed and, when the current suction pressure value PVs reaches the predetermined certain range (A), on / off control is performed to stop refrigerant injection and discharge. When the suction pressure at the upstream side of the refrigerant compressor stabilizes through this control, the discharge pressure at the downstream side of the refrigerant compressor also stabilizes, thereby preventing inefficient system operation due to repeated alternation between refrigerant injection and refrigerant discharge and thus ensuring process stability of the reliquefaction system.

[0068] Although some embodiments have been described, it will be apparent to those skilled in the art that these embodiments are given by way of illustration only, and that various modifications, changes, alterations, and equivalent embodiments can be made without departing from the spirit and scope of the invention.

Examples

Embodiment Construction

[0026]In order to fully appreciate the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings, which illustrate preferred embodiments of the present invention, and description thereof.

[0027]Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects thereof with reference to the accompanying drawings. It should be noted that like components will be denoted by like reference numerals throughout the specification and the accompanying drawings.

[0028]As used herein, the term "ship" may refer to any type of ship that is provided with a liquefied gas storage tank. For example, the ship may include self-propelled vessels, such as an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV), as well as non-self-propelled floating offshore structures, such as an LNG floating production storage and ...

[Technical Field]

[0001] The present invention relates to a system and method for controlling a refrigerant cycle of a reliquefaction system for ships, which enables cooling and reliquefaction of boil-off gas (BOG) generated from liquefied gas stored in an onboard storage tank and can effectively adjust the amount of cold heat in the refrigerant cycle in response to changes in load of the reliquefaction system while ensuring stability of the reliquefaction process.[Background Art]

[0002] Natural gas contains methane as a main component and has been attracting attention as an eco-friendly fuel that emits little or no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by liquefying natural gas through cooling to about -163°C under normal pressure and is suited to long-distance transportation by sea since it has a volume of about 1 / 600 that of natural gas in a gaseous state. Accordingly, natural gas is stored and transported as liquefied natural gas, which is easy to store and transport.

[0003] Since natural gas is liquefied at a cryogenic temperature of -163°C under normal pressure, LNG storage tanks are typically insulated to maintain LNG in a liquid state. However, despite being insulated, such a storage tank is limited in ability to block external heat. Accordingly, due to external heat continuously transferred to the LNG storage tank, LNG stored in the LNG tank continues to evaporate naturally during transportation, causing generation of boil-off gas (BOG).

[0004] Continuous production of boil-off gas in the LNG storage tank increases the internal pressure of the LNG storage tank. If the internal pressure of the storage tank exceeds a predetermined safe pressure, this can cause an emergency situation such as rupture of the storage tank. Accordingly, there is a need to discharge boil-off gas from the storage tank using a safety valve. However, boil-off gas is a kind of LNG loss and is an important issue for transportation efficiency and fuel efficiency of LNG. Therefore, various methods are employed to handle boil-off gas generated in the LNG storage tank.

[0005] Recently, a method of using boil-off gas at a fuel demand site such as an engine of a ship, a method of reliquefying boil-off gas and returning the reliquefied boil-off gas to an LNG storage tank, and a method combining these two approaches have been developed and put into use.[Disclosure][Technical Problem]

[0006] Methods for reliquefying boil-off gas include a method that employs a refrigeration cycle using a separate refrigerant to reliquefy boil-off gas through heat exchange with the refrigerant, a method that reliquefies boil-off gas using the boil-off gas itself as a refrigerant without a separate refrigerant, and the like.

[0007] As a method of reliquefying boil-off gas using the boil-off gas itself as a refrigerant without a separate refrigerant, a reliquefaction system configured to cool compressed boil-off gas through heat exchange with uncompressed boil-off gas and reliquefy the cooled boil-off gas by adiabatic expansion has been developed and applied to ships.

[0008] Systems that utilize a separate refrigeration cycle include, for example, a system that employs a reliquefaction process using nitrogen as a refrigerant.

[0009] Despite relative inefficiency compared to a reliquefaction cycle using a mixed refrigerant, such a reliquefaction process using nitrogen refrigerant is safer due to inert properties of the nitrogen refrigerant and is easier to apply to ships since the nitrogen refrigerant does not undergo phase change.

[0010] As such, boil-off gas cooled through heat exchange with a separate refrigerant or the boil-off gas itself is introduced into a separator, in which the boil-off gas is separated into gas and liquid phases, and the resulting reliquefied gas is returned to a storage tank.

[0011] It is an aspect of the present invention to provide a solution to effectively adjust the amount of cold heat in a refrigerant cycle in response to changes in load of a reliquefaction system while ensuring stability of a reliquefaction process.[Technical Solution]

[0012] In accordance with one aspect of the present invention, there is provided a system for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, the refrigerant cycle control system including: a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas; a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and to transmit a current suction pressure value (PVs); a load controller configured to transmit a suction pressure set point (SPs) for load adjustment of the reliquefaction system; and a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0013] Preferably, the system further includes: a discharge pressure detector configured to detect a discharge pressure of the refrigerant discharged from the refrigerant compressor on the refrigerant circulation line and to transmit a current discharge pressure value (PVd) to the control unit, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β), the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

[0014] Preferably, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0015] Preferably, the system further includes: a refrigerant inventory system configured to supply the refrigerant to the refrigerant circulation line or receive the refrigerant discharged from the refrigerant circulation line; a refrigerant charging line connecting the refrigerant inventory system to an upstream side of the refrigerant compressor to supply the refrigerant to the refrigerant circulation line; and a refrigerant discharge line connecting a downstream side of the refrigerant compressor to the refrigerant inventory system to discharge the refrigerant from the refrigerant circulation line to the refrigerant inventory system.

[0016] Preferably, the refrigerant inventory system includes: an inventory tank storing the refrigerant to be supplied to the refrigerant circulation line; and a booster compressor configured to compress the refrigerant and supply the compressed refrigerant to the inventory tank, and the refrigerant in the refrigerant circulation line is nitrogen.

[0017] Preferably, the refrigerant charging line connects the inventory tank to the upstream side of the refrigerant compressor such that the refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure differential between the inventory tank and the refrigerant circulation line.

[0018] Preferably, the refrigerant discharge line includes: a first discharge line connecting the downstream side of the refrigerant compressor to the inventory tank; and a second discharge line branched off of the first discharge line upstream of the inventory tank and connected to the inventory tank through the booster compressor, wherein the refrigerant is discharged from the downstream side of the refrigerant compressor to the inventory tank along the first discharge line by a pressure differential between the refrigerant circulation line and the inventory tank and, in the event of pressure reversal, the refrigerant is discharged to the inventory tank through the booster compressor along the second discharge line.

[0019] Preferably, the system further includes: a refrigerant expander disposed on the refrigerant circulation line to expand and cool the refrigerant compressed by the refrigerant compressor and having been cooled by the heat exchanger and to supply the cooled refrigerant as a refrigerant to the heat exchanger.

[0020] In accordance with another aspect of the present invention, there is a provided a method for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, wherein, upon adjustment of a load of the reliquefaction system, a control unit receives a suction pressure set point (SPs) from a load controller and a current suction pressure value (PVs) of the refrigerant introduced into a refrigerant compressor on the refrigerant circulation line after undergoing heat exchange with the boil-off gas in the heat exchanger and charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point to adjust the amount of cold heat in the refrigerant circulation line, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0021] Preferably, the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value (PVd) is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β) by detecting a discharge pressure of the refrigerant discharged from the refrigerant compressor, the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

[0022] Preferably, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.[Advantageous Effects]

[0023] The refrigerant cycle control system and method according to the present invention can provide more effective cooling of boil-off gas to be reliquefied by utilizing both cold heat of the boil-off gas itself and cold heat in a refrigerant cycle, thereby ensuring an enhanced reliquefaction rate.

[0024] The refrigerant cycle control system and method according to the present invention can prevent destabilization of a reliquefaction process caused by frequent refrigerant charging and discharging by selectively performing PID control and on-off control depending on the conditions of the refrigerant cycle while appropriately charging / discharging a refrigerant into / from the refrigerant cycle depending on the load of the reliquefaction system.[Description of Drawings]

[0025] FIG. 1 and FIG. 2 are schematic diagrams of a control system used in a reliquefaction system to charge or discharge a refrigerant into or from a refrigerant circulation line using PID control depending on the suction pressure of a refrigerant compressor. FIG. 3 is a schematic illustration of refrigerant charging and discharging trends under PID control depending on the suction pressure of the refrigerant compressor. FIG. 4 illustrates different cases of the current suction pressure value and current discharge pressure value at the refrigerant compressor for controlling a refrigerant cycle of a reliquefaction system for ships using the refrigerant cycle control system according to one embodiment. FIG. 5 is a schematic illustration of refrigerant charging and discharging trends under the refrigerant cycle control system according to one embodiment. [Best Mode]

[0026] In order to fully appreciate the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings, which illustrate preferred embodiments of the present invention, and description thereof.

[0027] Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects thereof with reference to the accompanying drawings. It should be noted that like components will be denoted by like reference numerals throughout the specification and the accompanying drawings.

[0028] As used herein, the term "ship" may refer to any type of ship that is provided with a liquefied gas storage tank. For example, the ship may include self-propelled vessels, such as an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV), as well as non-self-propelled floating offshore structures, such as an LNG floating production storage and offloading (FPSO) unit and an LNG floating storage regasification unit (FSRU).

[0029] In addition, the embodiments of the present invention may be applied to a reliquefaction cycle for any type of liquefied gas that can be transported in a liquid state by liquefaction at cryogenic temperatures and can generate boil-off gas during storage. For example, such liquefied gas may include liquefied petrochemical gas, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas. In the following embodiments, the present invention will be described using LPG, which is a typical liquefied gas, as an example.

[0030] The control system according to this embodiment is used in a reliquefaction system for ships, in which boil-off gas generated from liquefied gas in an onboard storage tank (not shown) is compressed by a compressor, cooled and reliquefied by a heat exchanger, and returned to the storage tank, to control a refrigerant cycle of the reliquefaction system in response to changes in load of the reliquefaction system.

[0031] FIG. 1 and FIG. 2 are schematic diagrams of a reliquefaction system and a control system used in the reliquefaction system to charge or discharge a refrigerant into or from a refrigerant circulation line using PID control depending on the suction pressure of a refrigerant compressor.

[0032] The reliquefaction system includes a compressor (not shown) configured to receive and compress boil-off gas, a boil-off gas supply line (not shown) through which boil-off gas from the storage tank is supplied to the compressor, and a reliquefaction line (not shown) connecting the compressor to the storage tank and allowing boil-off gas to be reliquefied and returned to the storage tank therealong.

[0033] The boil-off gas supply line extends from the storage tank to the compressor through a heat exchanger 100 such that uncompressed boil-off gas from the storage tank is supplied to the compressor for compression after supplying cold heat to the heat exchanger.

[0034] The compressor (not shown) may compress the boil-off gas to a certain pressure, for example, to a fuel supply pressure required for a main engine of a ship. For example, the compressor may compress the boil-off gas to a pressure of about 5.5 barg for DF engines, about 15 barg for X-DF engines, and about 300 barg for ME-GI engines. The compressed boil-off gas may be supplied as fuel to a demand site, such as the main engine (not shown) of the ship, and surplus boil-off gas may be reliquefied through the reliquefaction system along the reliquefaction line.

[0035] The compressed boil-off gas from the compressor is introduced into the heat exchanger 100 along the reliquefaction line to be cooled through heat exchange, the cooled boil-off gas is subjected to gas-liquid separation in a separator (not shown), and the separated reliquefied gas is returned to the storage tank.

[0036] In the heat exchanger 100, the boil-off gas is cooled by both a refrigerant circulating through a refrigerant circulation line CL and cold heat of uncompressed boil-off gas to be introduced into the compressor.

[0037] The refrigerant circulation line CL, which constitutes a refrigerant cycle, is provided with a refrigerant expander (not shown) configured to expand and cool the refrigerant to be supplied to the heat exchanger, a refrigerant compressor 200 configured to compress the refrigerant discharged from the heat exchanger, and a motor configured to drive the refrigerant compressor. The refrigerant compressor and the refrigerant expander may be connected to each other through a common shaft to use expansion energy of the refrigerant for compression of the refrigerant in the refrigerant compressor, thereby reducing power consumption required to operate the refrigerant cycle.

[0038] The compressed refrigerant from the refrigerant compressor 200 is cooled by the heat exchanger 100, expanded and cooled by the refrigerant expander, and supplied as a refrigerant to the heat exchanger 100 while circulating through the refrigerant circulation line. Accordingly, in the heat exchanger 100, four streams including a stream of boil-off gas compressed by the compressor and to be reliquefied, a stream of uncompressed boil-off gas to be introduced into the compressor, a stream of refrigerant expanded and cooled by the refrigerant expander, and a stream of refrigerant compressed by the refrigerant compressor undergo heat exchange.

[0039] The cooled boil-off gas from the heat exchanger may be subjected to gas-liquid separation in the separator after passing through a control valve disposed downstream of the heat exchanger. The separated reliquefied gas from the separator (not shown) is supplied to the storage tank for re-storage and the resulting flash gas may be joined with the stream of uncompressed boil-off gas upstream of the heat exchanger on the boil-off gas supply line or may be delivered to a gas combustion unit (GCU).

[0040] For example, nitrogen (N 2 ) may be used as the refrigerant circulating through the refrigerant circulation line to supply cold heat to the heat exchanger.

[0041] When the amount of boil-off gas to be reliquefied changes, the amount of cold heat required for the reliquefaction system also changes. The amount of cold heat transferred to the heat exchanger may be adjusted by adding more nitrogen refrigerant to the refrigerant circulation line from a refrigerant inventory system 300 or discharging some nitrogen refrigerant from the refrigerant circulation line depending on changes in load of the reliquefaction cycle, thereby adjusting the load of the reliquefaction system.

[0042] The refrigerant inventory system 300 may include an inventory tank storing refrigerant to be supplied to the refrigerant circulation line CL and a booster compressor configured to compress refrigerant and supply the compressed refrigerant to the inventory tank. The refrigerant inventory system may be configured to receive nitrogen from an onboard nitrogen generator (N 2 generator) provided for supply of utility N 2 , reduce the dew point of the received nitrogen through a drying and filtration unit (not shown), compress the nitrogen through the booster compressor, store the compressed nitrogen in the inventory tank, and supply the stored nitrogen to the refrigerant circulation line of the reliquefaction system. Upon adjustment of the load of the reliquefaction system, supplementary refrigerant may be supplied from the inventory tank to the refrigerant circulation line or surplus refrigerant may be discharged from the refrigerant circulation to the inventory tank. To this end, the control system further includes: a refrigerant charging line FL connecting the inventory tank of the refrigerant inventory system to an upstream side of the refrigerant compressor; and a refrigerant discharge line DL connecting a downstream side of the refrigerant compressor to the refrigerant inventory system.

[0043] The refrigerant discharge line DL includes a first discharge line DLa connecting the downstream side of the refrigerant compressor to the inventory tank and a second discharge line DLb branched off of the first discharge line DLa upstream of the inventory tank and connected to the inventory tank through the booster compressor.

[0044] Upon adjustment of the load of the reliquefaction system, a pressure differential between the refrigerant circulation line and the inventory tank may be utilized to add more nitrogen refrigerant to the refrigerant circulation line from the inventory tank or to discharge some nitrogen refrigerant from the refrigerant circulation line.

[0045] The refrigerant charging line connects the inventory tank to the upstream side of the refrigerant compressor such that supplementary refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure differential between the inventory tank and the refrigerant circulation line.

[0046] Upon reducing the load of the reliquefaction system, a pressure differential between the downstream side of the refrigerant compressor on the refrigerant circulation line and the inventory tank may be utilized to discharge the refrigerant from the downstream side of the refrigerant compressor to the inventory tank along the first discharge line. However, when pressure reversal occurs, making it difficult to discharge refrigerant to the inventory tank using such a pressure differential, the nitrogen refrigerant discharged from the downstream side of the refrigerant compressor may be compressed by the booster compressor along the second discharge line DLb and then delivered to the inventory tank.

[0047] On the refrigerant circulation line, a suction pressure detector P1 is disposed upstream of the refrigerant compressor to detect a suction pressure of the refrigerant introduced into the refrigerant compressor and a discharge pressure detector P2 is disposed downstream of the refrigerant compressor to detect a discharge pressure of the refrigerant compressed by the refrigerant compressor.

[0048] A control system for adjusting the load of the reliquefaction system includes: a load controller LC configured to adjust the load of the reliquefaction system; and a control unit PIC configured to receive a set point from the load controller and charge or discharge the refrigerant into or from the refrigerant circulation line.

[0049] The control unit may receive a refrigerant suction pressure detected upstream of the refrigerant compressor by the suction pressure detector and a suction pressure set point transmitted from the load controller and may change a mass flow rate of refrigerant in the refrigerant circulation line by adding more refrigerant to the refrigerant circulation line or discharging some refrigerant to the refrigerant inventory system using PID control, thereby adjusting the amount of cold heat in the refrigerant circulation line and the load of the reliquefaction system.

[0050] FIG. 1 shows the case where refrigerant is charged into the refrigerant circulation line using PID control based on the refrigerant suction pressure detected upstream of the refrigerant compressor and FIG. 2 shows the case where some refrigerant is discharged from the refrigerant circulation line to the refrigerant inventory system using PID control based on the detected refrigerant suction pressure.

[0051] When the refrigerant suction pressure at the upstream side of the refrigerant compressor is less than the suction pressure set point transmitted from the load controller, supplementary nitrogen refrigerant may be supplied from the inventory tank to the upstream side of the refrigerant compressor along the refrigerant charging line, as shown in FIG. 1(a), thereby increasing the amount of cold heat in the refrigerant circulation line. When the refrigerant suction pressure at the upstream side of the refrigerant compressor is greater than the suction pressure set point transmitted from the load controller, surplus nitrogen refrigerant may be discharged from the downstream side of the refrigerant compressor to the inventory tank along the refrigerant discharge line, as shown in FIG. 2(a), thereby reducing the amount of cold heat in the refrigerant circulation line. As described above, in the event of a pressure reversal where the pressure of the inventory tank exceeds the pressure at the downstream side of the refrigerant compressor, the refrigerant discharged from the downstream side of the refrigerant compressor may be delivered to the inventory tank through the boosting compressor along the second discharge line.

[0052] In the graphs of FIG. 1(b) and FIG. 2(b), the horizontal axis represents changes in the set point transmitted from the load controller, the vertical axis represents a pressure value, line IT represents the pressure of the inventory tank, line CLl represents the refrigerant suction pressure at the upstream side of the refrigerant compressor, and line CLh represents the refrigerant discharge pressure at the downstream side of the refrigerant compressor. As shown in FIG. 1(b), when the set point transmitted from the load controller is increased to increase the load of the reliquefaction system, refrigerant is added to the refrigerant circulation line along the refrigerant charging line by a difference between the pressure of the inventory tank and the refrigerant suction pressure at the upstream side of the refrigerant compressor (①). Conversely, as shown in FIG. 2(b), when the set point transmitted from the load controller is decreased to reduce the load of the reliquefaction system, the refrigerant is discharged from the refrigerant circulation line along the refrigerant discharge line by a difference between the pressure of the inventory tank and the refrigerant discharge pressure at the downstream side of the refrigerant compressor. Specifically, when the pressure at the downstream side of the refrigerant compressor is greater than the pressure of the inventory tank, the refrigerant is discharged to the inventory tank along the first discharge line by a pressure differential (②) and, in the event of a pressure reversal where the pressure of the inventory tank exceeds the pressure at the downstream side of the refrigerant compressor, the refrigerant is discharged from the downstream side of the refrigerant compressor by a pressure differential and then delivered to the inventory tank through the booster compressor along the second discharge line (③).

[0053] FIG. 3 is a schematic illustration of the refrigerant charging and discharging trends under PID control depending on the suction pressure of the refrigerant compressor. When refrigerant charging and discharging are performed using PID control solely based on the suction pressure PVs of the refrigerant compressor, refrigerant injection NF and refrigerant discharge NR are repeatedly alternated with respect to the set point SP in response to changes in suction pressure of the refrigerant compressor, as shown in FIG. 3, resulting in inefficient system operation. In particular, if the suction pressure at the upstream side of the refrigerant compressor fluctuates, this triggers corresponding refrigerant charging and discharging processes, causing significant fluctuations in discharge pressure at the downstream side of the refrigerant compressor and thus destabilization of the refrigerant cycle, which can lead to process instability of the reliquefaction system, deviations from a designed equipment operating point, emergency shutdown of the reliquefaction system, and the like.

[0054] To solve this problem, in this embodiment, the control unit is configured to control the refrigerant cycle by performing on / off control along with PID control. In particular, in consideration of both the suction pressure and discharge pressure at the refrigerant compressor, the control unit PIC compares a current suction pressure value PVs at the upstream side of the refrigerant compressor, as detected by the suction pressure detector P1, with a suction pressure set point SPs transmitted from the load controller LC for load adjustment of the reliquefaction system and charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on the comparison results. However, when specific conditions are met, the control unit PIC may preferentially perform on / off control over PID control to stop refrigerant injection or discharge, thereby preventing inefficient system operation due to repeated alternation between refrigerant injection and refrigerant discharge and thus ensuring process stability of the reliquefaction system.

[0055] FIG. 4 illustrates different cases of the current suction pressure value and current discharge pressure value (indicated as dots in each diagram) at the refrigerant compressor for controlling a refrigerant cycle of a reliquefaction system for ships using the refrigerant cycle control system according to this embodiment.

[0056] In the following, a refrigerant cycle control method will be described with reference to different cases of the current suction pressure value at the upstream side of the refrigerant compressor and the current discharge pressure value at the downstream side of the refrigerant compressor. First, when the current suction pressure value at the upstream side of the refrigerant compressor is greater than a value obtained by adding a first adjustment value α to the suction pressure set point transmitted from the load controller and the current discharge pressure value at the downstream side of the refrigerant compressor is greater than a value obtained by subtracting a second adjustment value β from the discharge pressure set point SPd transmitted from the load controller, as shown in FIG. 4(a), the control unit PIC performs control such that the refrigerant is discharged from the refrigerant circulation line. This process control condition may be expressed mathematically as follows: PVs > SPs + α , and PVd > SPd - β

[0057] The first adjustment value α is an adjustable value used to control the system such that refrigerant injection and discharge are not performed when the current suction pressure value is within a certain range with respect to the suction pressure set point. The second adjustment value β is an adjustable value used to control the system such that refrigerant injection and discharge are not performed when the current discharge pressure value is within a certain range with respect to the discharge pressure set point. Here, α and β may be set to any value depending on operating conditions; for example, α may be 0.03 bara and β may be 0.04 bara.

[0058] Under this process control condition, the control unit performs control to prevent refrigerant charging into the refrigerant circulation line while allowing some refrigerant to be discharged from the downstream side of the refrigerant compressor to the inventory tank along the refrigerant discharge line by a pressure differential therebetween using PID control.

[0059] Next, when the current suction pressure value at the upstream side of the refrigerant compressor is less than a value obtained by subtracting the first adjustment value from the suction pressure set point and the current discharge pressure value at the downstream side of the refrigerant compressor is less than a value obtained by adding the second adjustment value to the discharge pressure set point, as shown in FIG. 4(b), the control unit performs control such that the refrigerant is charged into the refrigerant circulation line. This process control condition may be expressed mathematically as follows: PVs < SPs - α , and PVd < SPd + β

[0060] In this case, the control unit performs control to prevent refrigerant discharge from the refrigerant circulation line while allowing more refrigerant to be added to the refrigerant circulation line from the inventory tank along the refrigerant charging line using PID control.

[0061] When a difference between the current suction pressure value and the suction pressure set point is less than or equal to the first adjustment value α (PVs ≤ SPs ± α), as shown in FIG. 4(c), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

[0062] When the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point, as shown in FIG. 4(d), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant. This process control condition may be expressed mathematically as follows: PVs > SPs + α , and PVd < SPd - β

[0063] Under this condition, where the current suction pressure value exceeds a certain range while the current discharge pressure value is less than the value obtained by subtracting the second adjustment value from the discharge pressure set point, on / off control is performed to prevent refrigerant discharge, given that the suction pressure can also decrease after the discharge pressure stabilizes.

[0064] Lastly, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point, as shown in FIG. 4(e), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant. This process control condition may be expressed mathematically as follows: PVs < SPs − α , andPVd > SPd + β

[0065] In this case, where the current suction pressure value is less than a certain range while the current discharge pressure value is greater than a value adding the second adjustment value to the discharge pressure set point, on / off control is performed to temporarily prevent supply of supplementary refrigerant, given that the suction pressure can also increase after the process proceeds for a certain period of time and the discharge pressure stabilizes.

[0066] FIG. 5 is a schematic illustration of refrigerant charging and discharging trends under the refrigerant cycle control system according to this embodiment.

[0067] Referring to FIG. 5, in the control system according to this embodiment, on / off control is preferentially performed to stop refrigerant injection and discharge when the suction pressure at the upstream side of the refrigerant compressor is within a predetermined range (SPs±α) defined by the first adjustment value α relative to the suction pressure set point SPs, whereby, when the current suction pressure value PVs is less than the predetermined range, refrigerant charging NR is performed and, when the current suction pressure value PVs reaches the predetermined certain range (A), on / off control is performed to stop refrigerant injection and discharge. When the suction pressure at the upstream side of the refrigerant compressor stabilizes through this control, the discharge pressure at the downstream side of the refrigerant compressor also stabilizes, thereby preventing inefficient system operation due to repeated alternation between refrigerant injection and refrigerant discharge and thus ensuring process stability of the reliquefaction system.

[0068] Although some embodiments have been described, it will be apparent to those skilled in the art that these embodiments are given by way of illustration only, and that various modifications, changes, alterations, and equivalent embodiments can be made without departing from the spirit and scope of the invention.

Examples

Embodiment Construction

[0026]In order to fully appreciate the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings, which illustrate preferred embodiments of the present invention, and description thereof.

[0027]Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects thereof with reference to the accompanying drawings. It should be noted that like components will be denoted by like reference numerals throughout the specification and the accompanying drawings.

[0028]As used herein, the term "ship" may refer to any type of ship that is provided with a liquefied gas storage tank. For example, the ship may include self-propelled vessels, such as an LNG carrier, a liquid hydrogen carrier, and an LNG regasification vessel (RV), as well as non-self-propelled floating offshore structures, such as an LNG floating production storage and ...

Claims

1. A system for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, the refrigerant cycle control system comprising: a refrigerant compressor disposed on the refrigerant circulation line to compress the refrigerant discharged from the heat exchanger after cooling the boil-off gas; a suction pressure detector configured to detect a suction pressure of the refrigerant introduced into the refrigerant compressor on the refrigerant circulation line and to transmit a current suction pressure value (PVs); a load controller configured to transmit a suction pressure set point (SPs) for load adjustment of the reliquefaction system; and a control unit configured to receive the suction pressure set point and adjust the amount of cold heat in the refrigerant circulation line by charging the refrigerant into the refrigerant circulation line or discharging the refrigerant from the refrigerant circulation line, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

2. The system according to claim 1, further comprising: a discharge pressure detector configured to detect a discharge pressure of the refrigerant discharged from the refrigerant compressor on the refrigerant circulation line and transmit a current discharge pressure value (PVd) to the control unit, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β), the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

3. The system according to claim 2, wherein, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

4. The system according to claim 2, further comprising: a refrigerant inventory system configured to supply the refrigerant to the refrigerant circulation line or receive the refrigerant discharged from the refrigerant circulation line; a refrigerant charging line connecting the refrigerant inventory system to an upstream side of the refrigerant compressor to supply the refrigerant to the refrigerant circulation line; and a refrigerant discharge line connecting a downstream side of the refrigerant compressor to the refrigerant inventory system to discharge the refrigerant from the refrigerant circulation line to the refrigerant inventory system.

5. The system according to claim 4, wherein the refrigerant inventory system comprises: an inventory tank storing the refrigerant to be supplied to the refrigerant circulation line; and a booster compressor configured to compress the refrigerant and supply the compressed refrigerant to the inventory tank, and the refrigerant in the refrigerant circulation line is nitrogen.

6. The system according to claim 5, wherein the refrigerant charging line connects the inventory tank to the upstream side of the refrigerant compressor such that the refrigerant is supplied from the inventory tank to the refrigerant circulation line by a pressure differential between the inventory tank and the refrigerant circulation line.

7. The system according to claim 5, wherein the refrigerant discharge line comprises: a first discharge line connecting the downstream side of the refrigerant compressor to the inventory tank; and a second discharge line branched off of the first discharge line upstream of the inventory tank and connected to the inventory tank through the booster compressor, and the refrigerant is discharged from the downstream side of the refrigerant compressor to the inventory tank along the first discharge line by a pressure differential between the refrigerant circulation line and the inventory tank and, in the event of pressure reversal, the refrigerant is discharged to the inventory tank through the booster compressor along the second discharge line.

8. The system according to claim 7, further comprising: a refrigerant expander disposed on the refrigerant circulation line to expand and cool the refrigerant compressed by the refrigerant compressor and having been cooled by the heat exchanger and to supply the cooled refrigerant as a refrigerant to the heat exchanger.

9. A method for controlling a refrigerant cycle of a reliquefaction system for ships, in which boil-off gas generated from liquefied gas stored in an onboard storage tank is compressed and is reliquefied by a heat exchanger through heat exchange with a refrigerant circulating through a refrigerant circulation line, wherein, upon adjustment of a load of the reliquefaction system, a control unit receives a suction pressure set point (SPs) from a load controller and a current suction pressure value (PVs) of the refrigerant introduced into a refrigerant compressor on the refrigerant circulation line after undergoing heat exchange with the boil-off gas in the heat exchanger and charges or discharges the refrigerant into or from the refrigerant circulation line using PID control based on comparison between the current suction pressure value and the suction pressure set point to adjust the amount of cold heat in the refrigerant circulation line, and, when a difference between the current suction pressure value and the suction pressure set point is less than or equal to a first adjustment value (α) (PVs ≤ SPs ± α), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

10. The method according to claim 9, wherein the control unit charges or discharges the refrigerant into or from the refrigerant circulation line using PID control such that, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value (PVd) is greater than a value obtained by subtracting a second adjustment value (β) from a discharge pressure set point (SPd) (PVd > SPd - β) by detecting a discharge pressure of the refrigerant discharged from the refrigerant compressor, the refrigerant is discharged from the refrigerant circulation line and, when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is less than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd < SPd + β), the refrigerant is charged into the refrigerant circulation line.

11. The method according to claim 10, wherein, when the current suction pressure value is greater than a value obtained by adding the first adjustment value to the suction pressure set point (PVs > SPs + α) and the current discharge pressure value is less than a value obtained by subtracting the second adjustment value from the discharge pressure set point (PVd < SPd - β) and when the current suction pressure value is less than a value obtained by subtracting the first adjustment value from the suction pressure set point (PVs < SPs - α) and the current discharge pressure value is greater than a value obtained by adding the second adjustment value to the discharge pressure set point (PVd > SPd + β), the control unit preferentially performs on / off control to stop charging and discharging of the refrigerant.

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