Compression system

Abstract

To change the number of compressors used for compressing gas in a configuration having multiple serially connected compressors.SOLUTION: A BOG compression system 1 compresses gas supplied from a suction pipe L10 to a predetermined discharge pressure and discharges it to a discharge pipe L50. The BOG compression system 1 includes: a fourth-stage compressor 40; a fifth-stage compressor 50 for compressing another gas that is supplied from the fourth-stage compressor 40 via a fourth middle pipe L41; a bypass pipe L60 for connecting the fourth middle pipe L41 and the discharge pipe L50; a bypass valve B provided in a bypass pipe L60; and a controller 60. The controller 60 opens the bypass valve B when temperature of gas in the BOG compression system 1 becomes less than a predetermined temperature threshold, and closes the bypass valve B when temperature of gas becomes a temperature threshold or higher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a compression system for compressing a gas. [Background technology]

[0002] Liquefied gas is stored in a tank for storage or transportation. In general, the liquefaction temperature of gas is lower than the atmospheric temperature. Therefore, the liquefied gas stored in the tank is vaporized inside the tank due to heat input to the tank. This gas is called boil off gas (BOG). This gas (BOG) increases the internal pressure of the tank. Therefore, the internal pressure of the tank is kept at a predetermined value by compressing the vaporized gas.

[0003] Patent Document 1 discloses a system for compressing vaporized gas. This system controls the internal pressure of a tank that stores low-temperature liquefied gas. This system uses a compressor to compress the gas to a desired pressure. Furthermore, compressing the gas increases the temperature of the vaporized gas. For this reason, in such a system, multiple compressors are connected in series, and the compression ratio of each compressor is reduced to suppress the temperature rise while compressing the gas to the desired pressure. This suppresses the rise in temperature of the discharged vaporized gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-232351 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned system in which multiple compressors are connected in series, for example, there are cases where the temperature of the gas to be sucked in is low. In this case, even if not all of the multiple compressors are used, only some of the compressors may be able to compress the gas at a temperature that will not cause damage to the equipment. However, operating all the compressors despite this is a problem that requires improvement from the viewpoint of compressor life, etc.

[0006] Therefore, the present disclosure describes a compression system that is capable of changing the number of compressors used to compress gas in a configuration including multiple compressors connected in series. [Means for solving the problem]

[0007] One aspect of the present disclosure is a compression system that compresses gas supplied from an intake piping to a predetermined discharge pressure and discharges it to a discharge piping, the compression system including a first compressor that compresses the gas supplied from the intake piping, a second compressor to which the gas discharged from the first compressor is supplied via an intermediate piping and which compresses the supplied gas and discharges it to the discharge piping, a bypass piping that connects the intermediate piping and the discharge piping, a bypass valve provided in the bypass piping and switchable between an open state that allows gas to flow in the bypass piping and a closed state that restricts the flow of gas, and a controller that controls the state of the bypass valve, the controller opening the bypass valve when the temperature of the gas in the compression system becomes less than a predetermined temperature threshold, and closing the bypass valve when the temperature of the gas in the compression system becomes equal to or greater than the temperature threshold.

[0008] In this compression system, the controller controls the state of the bypass valve provided in the bypass piping based on the gas temperature. Here, when the gas temperature is below the temperature threshold, the controller opens the bypass valve. That is, in this case, the gas is allowed to flow through the bypass piping. This allows the compression system to compress the gas using the first compressor without using the second compressor. Also, when the gas temperature is equal to or higher than the temperature threshold, the controller closes the bypass valve. That is, in this case, the gas flow through the bypass piping is restricted. This allows the compression system to compress the gas using the first compressor and the second compressor while suppressing the compression ratio in each compressor to suppress a temperature rise. In this way, in a configuration including a plurality of compressors connected in series, the compression system can change the number of compressors used to compress the gas based on the gas temperature.

[0009] Here, as is well known, there is a strong correlation between the "temperature" and the "pressure" of the gas in the compression system. For this reason, the "temperature" of the gas can be expressed using the "pressure" of the gas. In the present disclosure, the compression system performing various controls based on the "temperature" of the gas has the same meaning as the compression system performing various controls based on the "pressure" of the gas. That is, in the present disclosure, the "temperature" of the gas may be expressed using the "pressure" of the gas. In the present disclosure, the "temperature" of the gas includes the meaning of the "pressure" of the gas. When the temperature of the gas is expressed using pressure, the compression system performs various controls using a pressure threshold value in which the temperature threshold value is expressed using pressure. In addition, in the present disclosure, the "pressure" of the gas includes the meaning of the "compression ratio" of the gas.

[0010] The compression system further includes a circulation pipe connecting the intermediate pipe and a discharge pipe, a supply valve provided in the intermediate pipe and switchable between an open state that allows gas to flow in the intermediate pipe and a closed state that restricts the flow of gas, and a discharge valve provided in the discharge pipe and switchable between an open state that allows gas to flow in the discharge pipe and a closed state that restricts the flow of gas, wherein a connection position between the intermediate pipe and the circulation pipe on the intermediate pipe is located closer to the second compressor than a connection position between the intermediate pipe and the bypass pipe, and a connection position between the discharge pipe and the circulation pipe on the discharge pipe is located closer to the second compressor than a connection position between the discharge pipe and the circulation pipe. the supply valve is provided in the intermediate pipe at a position between the connection position of the intermediate pipe and the bypass pipe and the connection position of the intermediate pipe and the circulation pipe, and the discharge valve is provided in the discharge pipe at a position between the connection position of the discharge pipe and the circulation pipe and the connection position of the discharge pipe and the bypass pipe, and the controller may further control states of the supply valve and the discharge valve, and when a temperature of gas in the compression system becomes less than a temperature threshold, the controller may close the supply valve and the discharge valve and open the bypass valve.

[0011] As a result, when the gas temperature falls below the temperature threshold, the compression system can restrict the flow of gas from the first compressor to the second compressor and direct the gas discharged from the first compressor to the discharge piping via the bypass piping.

[0012] In the above compression system, the second compressor has a compression section that compresses gas sucked into a cylinder through a gas intake valve with a piston and discharges the compressed gas through a gas discharge valve, and an unloader that forcibly switches the gas intake valve to an open state, and the controller further controls the operation of the unloader, and when the temperature of the gas in the compression system becomes less than a temperature threshold, the controller may forcibly switch the gas intake valve to the open state using the unloader, and then close the supply valve and discharge valve and open the bypass valve.

[0013] In the second compressor, the gas is not compressed even if the piston is driven because the gas intake valve is forcibly switched to the open state. Therefore, after forcibly switching the gas intake valve to the open state, the controller closes the supply valve and discharge valve and opens the bypass valve. This allows the compression system to separate the second compressor from the gas compression system and compress the gas in the first compressor without stopping the drive of the piston of the second compressor.

[0014] The above compression system may further include a pressure equalizing valve that adjusts the gas pressure in a gas circulation path formed by the intermediate piping, the second compressor, the discharge piping, and the circulation piping, and the controller may further control a state of the pressure equalizing valve, and the controller may adjust the gas pressure in the circulation path by controlling the pressure equalizing valve after the supply valve and the discharge valve are closed and the bypass valve is opened.

[0015] In this case, the compression system can adjust the gas pressure in the closed circulation path using a pressure equalizing valve, which allows the compression system to, for example, reduce the gas pressure in the circulation path, thereby suppressing gas leakage compared to when the gas pressure is high.

[0016] The above compression system may further include a circulation valve provided in the circulation piping and switchable between an open state that allows gas to flow within the circulation piping and a closed state that restricts the flow of gas, and the controller may further control the state of the circulation valve, and when the temperature of the gas in the compression system becomes equal to or higher than a temperature threshold, the controller may open the supply valve and the discharge valve and close the bypass valve and the circulation valve.

[0017] In this case, even if the compression system includes a circulation pipe, the circulation valve can be used to restrict the flow of gas in the circulation pipe, and when the gas temperature reaches or exceeds a temperature threshold, the compression system can block the flow of gas in the circulation pipe, compress the gas discharged from the first compressor by the second compressor, and discharge the gas from the second compressor to the discharge pipe.

[0018] In the above compression system, when the temperature of the gas in the compression system becomes equal to or higher than a temperature threshold, the controller may open the supply valve, the discharge valve, and the circulation valve to increase the pressure in the second compressor with the gas, and then close the bypass valve and the circulation valve.

[0019] In this way, the controller increases the pressure in the second compressor before closing the bypass valve. This allows the compression system to reduce the pressure difference of the gas applied to the second compressor before and after closing the bypass valve. Therefore, the compression system can switch the gas flow path while suppressing excessive load caused by a high pressure difference being applied to the second compressor.

[0020] In the above compression system, the second compressor has a compression section that compresses gas sucked into a cylinder through the gas intake valve with a piston and discharges the compressed gas through a gas discharge valve, and an unloader that forcibly switches the gas intake valve to an open state, and the controller further controls the operation of the unloader, and the controller may be configured to: when the temperature of the gas in the compression system becomes equal to or higher than a temperature threshold, have the unloader forcibly switch the gas intake valve to the open state, then open the supply valve, discharge valve, and circulation valve to increase the pressure inside the second compressor with gas, then close the bypass valve and circulation valve, and then release the control of the unloader forcibly switching the gas intake valve to the open state; or, when the temperature of the gas in the compression system becomes equal to or higher than a temperature threshold, have the supply valve, discharge valve, and circulation valve open to increase the pressure inside the second compressor with gas, then forcibly switch the gas intake valve to the open state by the unloader, then close the bypass valve and circulation valve, and then release the control of the unloader forcibly switching the gas intake valve to the open state.

[0021] In this way, after the controller pressurizes the second compressor, it forcibly switches the gas intake valve to the open state. After that, the controller closes the bypass valve and the circulation valve, and releases the control that forcibly switches the gas intake valve to the open state. This allows the compression system to switch the gas flow path so that the gas can be compressed by the second compressor as well, without stopping the drive of the piston of the second compressor.

[0022] In the above compression system, the temperature of the gas in the compression system may be the temperature of the gas discharged from the first compressor. In this case, when the temperature of the gas discharged from the first compressor is high, the compression system can compress the gas to a predetermined discharge pressure using the first compressor and the second compressor while suppressing the compression ratio of each compressor. Also, when the temperature of the gas discharged from the first compressor is low, the compression system can compress the gas to a predetermined discharge pressure using the first compressor without using the second compressor. Effect of the Invention

[0023] According to one aspect of the present disclosure, in a configuration having multiple compressors connected in series, the number of compressors used to compress gas can be varied. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a BOG compression system according to an embodiment. [Diagram 2] FIG. 2 is a diagram showing a schematic configuration of a fifth stage compressor of the BOG compression system. [Diagram 3] FIG. 3 is a block diagram showing the configuration around the controller of the BOG compression system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numerals, and duplicated description will be omitted.

[0026] FIG. 1 shows a boil-off gas compression system (compression system) 100. In the following description, the boil-off gas compression system 100 is referred to as a "BOG compression system 1." The BOG compression system 1 is installed, for example, in a receiving terminal and a storage terminal for liquefied natural gas (LNG). The storage terminal includes a tank for storing the liquefied natural gas. Inside the tank, the liquefied natural gas is vaporized to produce natural gas. The BOG compression system 1 is used to compress this natural gas.

[0027] In the following description, the BOG compression system 1 will be described as being targeted for liquefied natural gas. However, the gas that the BOG compression system 1 compresses is not limited to liquefied natural gas. The BOG compression system 1 can also be applied to gaseous fuels such as propane gas and hydrogen gas. In other words, the BOG compression system 1 can be applied to systems that generate BOG. ​​Hereinafter, the gas, such as natural gas, that the BOG compression system 1 compresses will be referred to simply as "gas."

[0028] The BOG compression system 1 includes a plurality of compressors for compressing gas. In the present disclosure, the compressors are reciprocating compressors as an example. The plurality of compressors are connected in series and sequentially compress the supplied gas to a predetermined pressure. In the present embodiment, the BOG compression system 1 is a five-stage system including five compressors. More specifically, the BOG compression system 1 includes five compressors, namely, a first-stage compressor (first compressor) 10, a second-stage compressor (first compressor) 20, a third-stage compressor (first compressor) 30, a fourth-stage compressor (first compressor) 40, and a fifth-stage compressor (second compressor) 50. In the BOG compression system 1, gas is compressed sequentially in the order of the first-stage compressor 10, the second-stage compressor 20, the third-stage compressor 30, the fourth-stage compressor 40, and the fifth-stage compressor 50.

[0029] Each stage compressor may be composed of a plurality of compressors. That is, for example, the first stage compressor 10 may be composed of two compressors. The number of compressors in the BOG compression system 1 is appropriately selected according to the performance required for the BOG compression system 1. For example, the BOG compression system 1 may be a three-stage system having three compressors, or a four-stage system having four compressors. Furthermore, for example, the BOG compression system 1 may be a three-stage system having four compressors.

[0030] The BOG compression system 1 includes, in addition to the first stage compressor 10 to the fifth stage compressor 50 described above, an intake pipe L10, a first intermediate pipe L11, a second intermediate pipe L21, a third intermediate pipe L31, a fourth intermediate pipe (intermediate pipe) L41, a discharge pipe L50, a first stage intercooler 11, a second stage intercooler 21, a third stage intercooler 31, a fourth stage intercooler 41, and an aftercooler 51. Vaporized gas is supplied from a tank to the intake pipe L10. The BOG compression system 1 compresses the gas supplied from the intake pipe L10 to a predetermined discharge pressure and discharges it to the discharge pipe L50. The gas discharged to the discharge pipe L50 is pressure-fed to a tank or a predetermined facility.

[0031] The downstream end of the suction pipe L10 in the gas flow direction is connected to the first stage compressor 10. The first intermediate pipe L11 connects the first stage compressor 10 and the second stage compressor 20. The first stage compressor 10 compresses the gas supplied from the suction pipe L10 and discharges it to the first intermediate pipe L11. The gas discharged to the first intermediate pipe L11 is sent to the second stage compressor 20. The first stage intercooler 11 is also provided in the first intermediate pipe L11. When the temperature of the gas discharged from the first stage compressor 10 is equal to or higher than a predetermined value, the first stage intercooler 11 cools the gas flowing through the first intermediate pipe L11. When the temperature of the gas discharged from the first stage compressor 10 is lower than a predetermined value, the first stage intercooler 11 does not cool the gas. For example, the cooling of the gas by the first stage intercooler 11 may be stopped by stopping the supply of refrigerant to the first stage intercooler 11. Alternatively, the gas flowing through the intake pipe L10 may be bypassed without passing through the first stage intercooler 11, so that the gas is not cooled by the first stage intercooler 11.

[0032] The second intermediate pipe L21 connects the second stage compressor 20 and the third stage compressor 30. The second stage compressor 20 compresses the gas supplied from the first intermediate pipe L11 and discharges it to the second intermediate pipe L21. The gas discharged to the second intermediate pipe L21 is sent to the third stage compressor 30. In addition, the second stage intercooler 21 is provided in the second intermediate pipe L21. Like the first stage intercooler 11, the second stage intercooler 21 can cool the gas flowing through the second intermediate pipe L21 depending on the temperature of the gas discharged from the second stage compressor 20.

[0033] The third intermediate pipe L31 connects the third stage compressor 30 and the fourth stage compressor 40. The third stage compressor 30 compresses the gas supplied from the second intermediate pipe L21 and discharges it to the third intermediate pipe L31. The gas discharged to the third intermediate pipe L31 is sent to the fourth stage compressor 40. In addition, the third stage intercooler 31 is provided in the third intermediate pipe L31. Like the first stage intercooler 11, the third stage intercooler 31 can cool the gas flowing through the third intermediate pipe L31 depending on the temperature of the gas discharged from the third stage compressor 30.

[0034] The fourth intermediate pipe L41 connects the fourth stage compressor 40 and the fifth stage compressor 50. The fourth stage compressor 40 compresses the gas supplied from the third intermediate pipe L31 and discharges it to the fourth intermediate pipe L41. The gas discharged to the fourth intermediate pipe L41 is sent to the fifth stage compressor 50. In addition, the fourth stage intercooler 41 is provided in the fourth intermediate pipe L41. The fourth stage intercooler 41 cools the gas flowing through the fourth intermediate pipe L41. The fourth stage intercooler 41 may cool the gas according to the temperature of the gas discharged from the fourth stage compressor 40, similar to the first stage intercooler 11.

[0035] An upstream end of the discharge pipe L50 in the gas flow direction is connected to a fifth-stage compressor 50. The fifth-stage compressor 50 compresses the gas supplied from the fourth intermediate pipe L41 and discharges the gas to the discharge pipe L50. The aftercooler 51 is provided in the discharge pipe L50. The aftercooler 51 cools the gas flowing through the discharge pipe L50.

[0036] Furthermore, the BOG compression system 1 includes a bypass pipe L60, a circulation pipe L70, a return pipe L80, a supply valve A, a bypass valve B, a discharge valve C, a circulation valve D, and a pressure equalizing valve E. In Fig. 1, the bypass pipe L60 and the circulation pipe L70 are shown with thicker lines than the other pipes in order to easily distinguish them from the other pipes.

[0037] The bypass pipe L60 connects the fourth intermediate pipe L41 and the discharge pipe L50. The bypass valve B is provided in the bypass pipe L60. The bypass valve B is switchable between an open state that allows gas to flow in the bypass pipe L60 and a closed state that restricts the gas flow. In the present disclosure, the closed state that restricts the gas flow is a state in which the gas flow is more restricted than in the open state that allows the gas flow. The closed state may be, for example, a state in which the gas flow is completely blocked.

[0038] The circulation pipe L70 connects the fourth intermediate pipe L41 and the discharge pipe L50. The circulation valve D is provided in the circulation pipe L70. The circulation valve D is switchable between an open state that allows gas to flow through the circulation pipe L70 and a closed state that restricts the flow of gas.

[0039] On the fourth intermediate pipe L41, a connection position P1 between the fourth intermediate pipe L41 and the circulation pipe L70 is located closer to the fifth stage compressor 50 than a connection position P2 between the fourth intermediate pipe L41 and the bypass pipe L60. On the discharge pipe L50, a connection position P3 between the discharge pipe L50 and the circulation pipe L70 is located closer to the fifth stage compressor 50 than a connection position P4 between the discharge pipe L50 and the bypass pipe L60.

[0040] The supply valve A is provided in the fourth intermediate pipe L41 at a position between a connection position P2 between the fourth intermediate pipe L41 and the bypass pipe L60 and a connection position P1 between the fourth intermediate pipe L41 and the circulation pipe L70. The supply valve A is switchable between an open state that allows gas to flow in the fourth intermediate pipe L41 and a closed state that restricts the flow of gas.

[0041] The discharge valve C is provided in the discharge pipe L50 at a position between a connection position P3 between the discharge pipe L50 and the circulation pipe L70 and a connection position P4 between the discharge pipe L50 and the bypass pipe L60. The discharge valve C is switchable between an open state that allows gas to flow through the discharge pipe L50 and a closed state that restricts the flow of gas.

[0042] The return pipe L80 connects the circulation pipe L70 and the suction pipe L10. On the circulation pipe L70, a connection position P5 between the return pipe L80 and the circulation pipe L70 is located between a connection position P3 between the circulation pipe L70 and the discharge pipe L50 and the circulation valve D.

[0043] The pressure equalizing valve E is provided in the return pipe L80. The pressure equalizing valve E adjusts the gas pressure in the gas circulation path formed by the fourth intermediate pipe L41, the fifth stage compressor 50, the discharge pipe L50, and the circulation pipe L70. The gas discharged from the circulation path by adjusting the gas pressure with the pressure equalizing valve E is returned to the suction pipe L10 via the return pipe L80.

[0044] Next, the configurations of the first stage compressor 10 to the fifth stage compressor 50, which are reciprocating compressors, will be described. The first stage compressor 10 to the fifth stage compressor 50 each have the same configuration. For this reason, the configuration of the fifth stage compressor 50 will be described, and a description of the configurations of the first stage compressor 10 to the fourth stage compressor 40 will be omitted. As shown in FIG. 2, the fifth stage compressor 50 includes a compression section 110, a piston drive section 120, and an unloader 130.

[0045] The compression section 110 has a cylinder 111, a piston 112, a piston rod 113, a gas suction valve 114, and a gas discharge valve 115. The cylinder 111 and the piston 112 form compression spaces S1 and S2 that compress gas. The gas suction valve 114 and the gas discharge valve 115 are provided so as to be able to suction and discharge gas into and from the compression spaces S1 and S2, respectively. An end of the piston rod 113 is connected to the piston 112. The other end of the piston rod 113 is connected to the piston drive section 120.

[0046] The piston driving unit 120 reciprocates the piston rod 113. For example, the piston driving unit 120 can convert the rotational motion of the driving source into reciprocating motion using a crank and a connecting rod connected to the crank.

[0047] The gas intake valve 114 switches the state of gas flow from the fourth intermediate pipe L41 into the cylinder 111 according to the internal pressure of the cylinder 111. The gas intake valve 114 is switchable between an open state that allows gas to flow in and out of the cylinder 111 and a closed state that prohibits gas from flowing in and out. The open state and the closed state are switched in accordance with the internal pressure of the compression spaces S1, S2. For example, when the internal pressure of the compression spaces S1, S2 decreases (intake), the gas intake valve 114 takes the open state that allows gas to flow in and out. On the other hand, when the internal pressure of the compression spaces S1, S2 increases (compression), the gas intake valve 114 takes the closed state that prohibits gas from flowing in and out.

[0048] Like the gas suction valve 114, the gas discharge valve 115 switches the gas flow state from the cylinder 111 to the discharge pipe L50 depending on the internal pressure of the cylinder 111. However, the gas discharge valve 115 and the gas suction valve 114 have different relationships between the internal pressure of the compression spaces S1, S2 and the open and closed modes. That is, the gas discharge valve 115 is in the closed mode when the internal pressure of the compression spaces S1, S2 decreases (suction). On the other hand, the gas discharge valve 115 is in the open mode when the internal pressure of the compression spaces S1, S2 increases (compression).

[0049] In this manner, the compression section 110 compresses the gas sucked into the cylinder 111 via the gas intake valve 114 by the piston 112 , and discharges the compressed gas via the gas discharge valve 115 .

[0050] The unloader 130 functions as a capacity adjustment mechanism for the fifth-stage compressor 50. The unloader 130 is attached to the gas intake valve 114. It has been described that the gas intake valve 114 closes when the internal pressure of the compression spaces S1 and S2 increases (compression). When the internal pressure of the compression spaces S1 and S2 increases, the unloader 130 forcibly switches the closed state of the gas intake valve 114 to an open state. For example, when capacity control is required, the unloader 130 presses the gas intake valve 114 to open the gas intake valve 114. When the gas intake valve 114 is in the open state, gas is no longer compressed in the cylinder 111, and the internal pressure does not increase. As a result, the gas discharge valve 115, which is opened by the increase in the internal pressure of the compression spaces S1 and S2, is not opened, and compressed gas is not provided. Therefore, by using the unloader 130, it is possible to adjust the compression capacity of the fifth-stage compressor 50.

[0051] Like the fifth-stage compressor 50, the first-stage compressor 10 to the fourth-stage compressor 40 each include a compression section 110, a piston driving section 120, and an unloader .

[0052] The BOG compression system 1 shown in Figure 1 can switch between compressing gas using four compressors, a first stage compressor 10 to a fourth stage compressor 40, and discharging it into a discharge pipe L50, and compressing gas using five compressors, a first stage compressor 10 to a fifth stage compressor 50, and discharging it into a discharge pipe L50.

[0053] When gas is compressed using the four compressors, the first stage compressor 10 to the fourth stage compressor 40, the BOG compression system 1 passes gas through the bypass piping L60 and does not supply gas to the fifth stage compressor 50. When gas is compressed using the five compressors, the first stage compressor 10 to the fifth stage compressor 50, the BOG compression system 1 blocks the flow of gas through the bypass piping L60 and supplies gas to the fifth stage compressor 50. In this way, the BOG compression system 1 can change the number of compressors used for compression by switching between passing and blocking the flow of gas in the bypass piping L60.

[0054] Here, the BOG compression system 1 compresses the gas supplied from the intake pipe L10 to a predetermined discharge pressure and discharges it to the discharge pipe L50. The BOG compression system 1 compresses the gas to a predetermined discharge pressure whether the gas is compressed using four compressors or five compressors. The five compressors, the first stage compressor 10 to the fifth stage compressor 50, are positive displacement compressors. Therefore, in the BOG compression system 1, the discharge pressure is the same whether the gas is compressed using four compressors or five compressors. In other words, when five compressors are used, the compression ratio of the gas in each compressor can be made smaller than when four compressors are used. In this way, by making the compression ratio of the gas in the compressors smaller, the heat generation of the gas due to compression is suppressed.

[0055] 3, the BOG compression system 1 further includes a controller 60 that controls switching of the number of compressors used when compressing the gas supplied from the suction pipe L10. The controller 60 controls the states of the supply valve A, the bypass valve B, the discharge valve C, the circulation valve D, and the pressure equalizing valve E. Furthermore, the controller 60 controls the operation of the unloader 130 provided in each of the first stage compressor 10 to the fifth stage compressor 50.

[0056] The controller 60 is an electronic control unit having, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory), etc. The controller 60, for example, loads a program recorded in the ROM into the RAM and executes the program loaded into the RAM by the CPU, thereby realizing various functions.

[0057] The controller 60 switches between compressing the gas using four compressors and compressing the gas using five compressors based on whether the temperature of the gas in the BOG compression system 1 is below a predetermined temperature threshold or above a predetermined temperature threshold.

[0058] In this embodiment, the temperature of the gas in the BOG compression system 1 may be the temperature of the gas before being compressed by the fourth stage compressor 40 and supplied to the fourth stage intercooler 41. However, the temperature of this gas is not limited to the temperature of the gas compressed by the fourth stage compressor 40, but may be the temperature of the gas before being compressed by any one of the first stage compressor 10 to the third stage compressor 30 and supplied to the intercooler of each stage. It may also be the temperature of the gas after being cooled by the intercooler of each stage. Furthermore, the temperature of this gas may be the temperature of the gas supplied to the BOG compression system 1 (the temperature of the gas supplied to the suction pipe L10) or the temperature of the gas discharged to the discharge pipe L50. In this way, the temperature of the gas in the BOG compression system 1 may be the temperature of the gas at a predetermined position on the gas flow path in the BOG compression system 1.

[0059] The temperature of the gas in the BOG compression system 1 varies depending on various factors such as the operating conditions of the BOG compression system 1. For example, when the BOG compression system 1 starts operating after being stopped for a long time, the temperature of the gas supplied to the BOG compression system 1 may rise to atmospheric temperature. When such gas is compressed, the temperature of the gas in the BOG compression system 1 increases. Furthermore, when the operation of the BOG compression system 1 continues, the temperature of the gas supplied to the BOG compression system 1 decreases. In this case, the temperature of the gas in the BOG compression system 1 decreases. The controller 60 switches the number of compressors to be used based on such gas temperatures.

[0060] In addition, for example, it is considered that the gas heated by compression may damage the components of the equipment constituting the BOG compression system 1. For this reason, the "temperature threshold" used in the judgment made by the controller 60 may be determined based on the heat resistance temperature of the equipment constituting the BOG compression system 1. For example, when gas is compressed using four compressors, the first stage compressor 10 to the fourth stage compressor 40, the temperature of the gas becomes high in the fourth stage compressor 40, which is the last stage. For this reason, for example, the "temperature threshold" may be determined based on the heat resistance temperature of the fourth stage compressor 40. The heat resistance temperature of this fourth stage compressor 40 may be the heat resistance temperature of a piston ring of the fourth stage compressor 40. For example, the "temperature threshold" may be set to a value lower than the heat resistance temperature of the equipment constituting the BOG compression system 1.

[0061] The controller 60 opens the bypass valve B when the temperature of the gas in the BOG compression system 1 becomes lower than a predetermined temperature threshold. In other words, when the temperature of the gas is lower than the predetermined temperature threshold, the controller 60 controls each part so that the gas is compressed using the four compressors, the first stage compressor 10 to the fourth stage compressor 40. On the other hand, when the temperature of the gas in the BOG compression system 1 becomes equal to or higher than the temperature threshold, the controller 60 closes the bypass valve B. In other words, when the temperature of the gas is equal to or higher than the predetermined temperature threshold, the controller 60 controls each part so that the gas is compressed using the five compressors, the first stage compressor 10 to the fifth stage compressor 50.

[0062] First, details of the control performed by the controller 60 when the temperature of the gas in the BOG compression system 1 falls below a predetermined temperature threshold will be described. That is, when the temperature of the gas falls below the predetermined temperature threshold, the bypass pipe L60 is put into a circulating state, and the gas is compressed using four compressors instead of the five compressors. When the gas is compressed using the five compressors, the supply valve A and the discharge valve C are open, and the bypass valve B, the circulation valve D, and the pressure equalizing valve E are closed.

[0063] When the gas temperature falls below the temperature threshold, the controller 60 performs the following steps A1 to A5 in this order. (Step A1) The controller 60 changes the loads of the five compressors, the first stage compressor 10 to the fifth stage compressor 50, to 0%. A compressor load of 0% means that gas is not compressed in the compressor. Here, the controller 60 controls the operation of each unloader 130 of the first stage compressor 10 to the fifth stage compressor 50 to forcibly open the gas intake valve 114 of each compressor. This allows the controller 60 to set the compression capacity of each compressor to 0. In other words, the controller 60 can set the load of each compressor to 0%.

[0064] (Step A2) The controller 60 switches the supply valve A and the discharge valve C to a closed state. The controller 60 also switches the bypass valve B to an open state. In this manner, the controller 60 closes the supply valve A and the discharge valve C and opens the bypass valve B after each unloader 130 opens the gas intake valve 114 of each compressor. The timing for opening the supply valve A, the timing for closing the discharge valve C, and the timing for opening the bypass valve B may be simultaneous, or at least two of them may be different from each other.

[0065] (Step A3) The controller 60 changes the loads of the four compressors, the first stage compressor 10 to the fourth stage compressor 40, to a load state in which gas is compressed by the four compressors. The controller 60 can switch the load state by controlling each unloader 130 of the first stage compressor 10 to the fourth stage compressor 40. At that time, the controller 60 may maintain a state in which the load of the fifth stage compressor 50 is 0%.

[0066] (Step A4) The controller 60 switches the circulation valve D to an open state. Then, the controller 60 opens the pressure equalizing valve E (slightly open state). As a result, the gas in the circulation path formed by the fourth intermediate pipe L41, the fifth stage compressor 50, the discharge pipe L50, and the circulation pipe L70 is returned to the suction pipe L10. By returning the gas to the suction pipe L10, the gas pressure in the closed circulation path around the fifth stage compressor 50 is reduced. As a result, the BOG compression system 1 can suppress gas leakage around the fifth stage compressor 50 compared to when the gas pressure is high. In this way, the controller 60 adjusts the gas pressure in the circulation path around the fifth stage compressor 50 by controlling the pressure equalizing valve E after the supply valve A and the discharge valve C are closed and the bypass valve B is opened. It is preferable that the load of the fifth stage compressor 50 is 0% when the gas pressure in the circulation path is reduced. Therefore, it is preferable that the controller 60 controls the load of the fifth stage compressor 50 to 0% before reducing the gas pressure in the circulation path.

[0067] (Step A5) After that, the controller 60 switches the pressure equalizing valve E and the circulation valve D to the closed state. For example, even if the load of the fifth-stage compressor 50 is 0%, the gas may be heated in the fifth-stage compressor 50 when the piston 112 is operating. For this reason, the controller 60 switches the open / close state of the circulation valve D as necessary. Specifically, when the gas is heated by the fifth-stage compressor 50 and reaches a predetermined temperature or higher, the controller 60 switches the circulation valve D to the open state and releases the control of the unloader 130 of the fifth-stage compressor 50 to forcibly open the gas intake valve 114 of the fifth-stage compressor 50. In other words, the controller 60 causes the fifth-stage compressor 50 to perform a compression operation to circulate the gas in the circulation path. Here, an aftercooler 51 is provided in the discharge pipe L50. For this reason, the gas circulating in the circulation path by the compression operation of the fifth-stage compressor 50 is cooled by the aftercooler 51. This allows the BOG compression system 1 to protect the piston rings of the fifth stage compressor 50 and the like.

[0068] Next, details of the control performed by the controller 60 when the gas temperature in the BOG compression system 1 becomes equal to or higher than a predetermined temperature threshold will be described. That is, when the gas temperature becomes equal to or higher than a predetermined temperature threshold, the flow of gas in the bypass pipe L60 is restricted, and gas is compressed using five compressors instead of four compressors. When gas is compressed using four compressors, the supply valve A and discharge valve C are closed, and the bypass valve B is open.

[0069] When the gas temperature reaches or exceeds the temperature threshold, the controller 60 performs the following steps B1 to B5 in this order. (Step B1) The controller 60 switches the supply valve A, the discharge valve C, and the circulation valve D to an open state, thereby increasing the gas pressure in the gas circulation path formed by the fourth intermediate pipe L41, the fifth stage compressor 50, the discharge pipe L50, and the circulation pipe L70.

[0070] (Step B2) After the pressure in the circulation path around the fifth-stage compressor 50 is increased, the controller 60 changes the loads of the five compressors, the first-stage compressor 10 to the fifth-stage compressor 50, to 0%. Note that the controller 60 may change the load of the fifth-stage compressor 50 to 0% before increasing the gas pressure in the circulation path in step B1. (Step B3) The controller 60 changes the bypass valve B and the circulation valve D to a closed state.

[0071] Thereafter, the controller 60 changes the loads of the five compressors, the first stage compressor 10 to the fifth stage compressor 50, to a load state in which the gas is compressed by the five compressors. The controller 60 can switch the load state by controlling each unloader 130 of the first stage compressor 10 to the fifth stage compressor 50.

[0072] In this way, when the temperature of the gas becomes equal to or higher than the temperature threshold, the controller 60 opens the supply valve A, the discharge valve C, and the circulation valve D, thereby increasing the pressure in the fifth stage compressor 50 by the gas. After that, the controller 60 forces the gas intake valve 114 of each compressor to be in the open state by each unloader 130. After that, the controller 60 closes the bypass valve B and the circulation valve D. After that, the controller 60 releases the control that forces the gas intake valve 114 to be in the open state by the unloader 130. By releasing the control that forces the gas intake valve 114 to be in the open state, the gas intake valve 114 of each compressor switches between the open state and the closed state depending on the internal pressure of the compression spaces S1 and S2. This allows the first stage compressor 10 to the fifth stage compressor 50 to compress the gas.

[0073] As described above, the controller 60 controls the state of the bypass valve B provided in the bypass pipe L60 based on the temperature of the gas in the BOG compression system 1. Here, the controller 60 opens the bypass valve B when the gas temperature is below the temperature threshold. That is, in this case, the flow of gas through the bypass pipe L60 is permitted. This allows the BOG compression system 1 to compress the gas to a predetermined discharge pressure using the four compressors, the first stage compressor 10 to the fourth stage compressor 40, without using the fifth stage compressor 50.

[0074] Moreover, when the temperature of the gas becomes equal to or higher than the temperature threshold, the controller 60 closes the bypass valve B. That is, in this case, the flow of the gas through the bypass pipe L60 is restricted. As a result, the BOG compression system 1 can compress the gas to a predetermined discharge pressure using five compressors, the first stage compressor 10 to the fifth stage compressor 50. When compressing the gas to a predetermined discharge pressure using five compressors, the compression ratio of each compressor can be reduced compared to when compressing the gas to a predetermined discharge pressure using four compressors. As a result, when using five compressors, the gas can be compressed to a predetermined discharge pressure while suppressing the temperature rise compared to when using four compressors.

[0075] In this way, in a configuration including a plurality of compressors, the first stage compressor 10 to the fifth stage compressor 50, connected in series, the BOG compression system 1 can change the number of compressors used to compress the gas based on the gas temperature. This allows the BOG compression system 1 to compress the gas to a predetermined discharge pressure using an optimal number of compressors required for gas compression. Furthermore, when the gas temperature is low, the BOG compression system 1 does not use the fifth stage compressor 50 to compress the gas. This allows the BOG compression system 1 to extend the life of the fifth stage compressor 50.

[0076] When the temperature of the gas in the BOG compression system 1 falls below the temperature threshold, the controller 60 closes the supply valve A and the discharge valve C and opens the bypass valve B. As a result, when the temperature of the gas falls below the temperature threshold, the BOG compression system 1 can restrict the inflow of gas from the fourth stage compressor 40 to the fifth stage compressor 50 and guide the gas discharged from the fourth stage compressor 40 to the discharge pipe L50 via the bypass pipe L60.

[0077] When the temperature of the gas in the BOG compression system 1 becomes lower than the temperature threshold value, the controller 60 forcibly switches the gas intake valve 114 of each compressor such as the fifth stage compressor 50 to the open state by the unloader 130, and then closes the supply valve A and the discharge valve C and opens the bypass valve B. Here, in each compressor such as the fifth stage compressor 50, the gas is not compressed even if the piston 112 is driven because the gas intake valve 114 is forcibly switched to the open state. Therefore, after forcibly switching the gas intake valve 114 to the open state, the controller 60 closes the supply valve A and the discharge valve C and opens the bypass valve B. As a result, the BOG compression system 1 can separate the fifth stage compressor 50 from the gas compression system and compress the gas with the four compressors, the first stage compressor 10 to the fourth stage compressor 40, without stopping the drive of the piston 112 of each compressor such as the fifth stage compressor 50.

[0078] After the supply valve A and the discharge valve C are closed and the bypass valve B is opened, the controller 60 adjusts the gas pressure in the circulation path around the fifth stage compressor 50 by controlling the pressure equalizing valve E. In this case, the BOG compression system 1 can adjust the gas pressure in the closed circulation path around the fifth stage compressor 50 using the pressure equalizing valve E. Thereby, the BOG compression system 1 can suppress gas leakage around the fifth stage compressor 50, for example, by lowering the gas pressure in the circulation path, compared to when the gas pressure is high.

[0079] When the temperature of the gas in the BOG compression system 1 becomes equal to or higher than the temperature threshold, the controller 60 opens the supply valve A and the discharge valve C and closes the bypass valve B and the circulation valve D. In this case, even if the BOG compression system 1 includes the circulation pipe L70, the BOG compression system 1 can restrict the flow of gas in the circulation pipe L70 using the circulation valve D. As a result, when the temperature of the gas becomes equal to or higher than the temperature threshold, the BOG compression system 1 can block the flow of gas in the circulation pipe L70, compress the gas discharged from the fourth stage compressor 40 in the fifth stage compressor 50, and discharge the gas from the fifth stage compressor 50 to the discharge pipe L50.

[0080] When the temperature of the gas in the BOG compression system 1 becomes equal to or higher than the temperature threshold, the controller 60 opens the supply valve A, the discharge valve C, and the circulation valve D to increase the pressure in the fifth-stage compressor 50 by gas, and then closes the bypass valve B and the circulation valve D. In this manner, the controller 60 increases the pressure in the fifth-stage compressor 50 before closing the bypass valve B. This allows the BOG compression system 1 to reduce the pressure difference of the gas applied to the fifth-stage compressor 50 before and after closing the bypass valve B. Therefore, the BOG compression system 1 can switch the gas flow path while suppressing an excessive load caused by a high pressure difference being applied to the fifth-stage compressor 50.

[0081] When the temperature of the gas in the BOG compression system 1 becomes equal to or higher than the temperature threshold value, the controller 60 increases the pressure in the fifth stage compressor 50, and then forcibly switches the gas intake valves 114 of each compressor, such as the fifth stage compressor 50, to the open state. After that, the controller 60 closes the bypass valve and the circulation valve, and releases the control that forcibly switches the gas intake valve 114 to the open state. The controller 60 may forcibly switch the gas intake valve 114 of the fifth-stage compressor 50 to the open state before increasing the pressure inside the fifth-stage compressor 50. In this way, the BOG compression system 1 can switch the gas flow path so that the gas can be compressed by the fifth-stage compressor 50 as well, without stopping the driving of the piston 112 of each compressor such as the fifth-stage compressor 50.

[0082] The BOG compression system 1 switches the gas flow path based on whether the temperature of the gas discharged from the fourth-stage compressor 40 is equal to or higher than the temperature threshold. In this case, when the temperature of the gas discharged from the fourth-stage compressor 40 is high, the BOG compression system 1 can compress the gas to a predetermined discharge pressure using the first-stage compressor 10 to the fifth-stage compressor 50 while suppressing the compression ratio of each compressor. Also, when the temperature of the gas discharged from the fourth-stage compressor 40 is low, the BOG compression system 1 can compress the gas to a predetermined discharge pressure using the first-stage compressor 10 to the fourth-stage compressor 40 without using the fifth-stage compressor 50.

[0083] Although the embodiment of the present disclosure has been described above, the present disclosure is not limited to the above embodiment. For example, the first stage compressor 10 to the fourth stage compressor 40 and the fifth stage compressor 50 may not all be provided. Each compressor such as the fifth stage compressor 50 may have a snapper tank that suppresses pulsation of the intake gas and a snapper tank that suppresses pulsation of the discharged gas. The drive source that drives the piston 112 of the compressor such as the fifth stage compressor 50 may be common to the compressors. For example, the piston 112 of the third stage compressor 30 and the piston 112 of the fifth stage compressor 50 may be driven by a common drive source. The compression system according to the present disclosure is not limited to being applied to the BOG compression system 1. The compression system according to the present disclosure may be applied to other compression systems other than the BOG compression system 1. [Explanation of symbols]

[0084] 1 BOG Compression System (Compression System) 10 1st stage compressor (first compressor) 20 Second stage compressor (first compressor) 30 3rd stage compressor (1st compressor) 40 4th stage compressor (1st compressor) 50 5th stage compressor (2nd compressor) 60 Controller 110 Compression section 111 Cylinder 112 Piston 114 Gas intake valve 115 Gas discharge valve 130 Unloader A Supply Valve B Bypass valve C Discharge valve D Circulation valve E Equalizing valve L10 Suction pipe L41 4th intermediate pipe (intermediate pipe) L50 discharge piping L60 Bypass piping L70 circulation piping

Claims

1. A compression system that compresses gas supplied from a suction pipe to a predetermined discharge pressure and discharges the gas to a discharge pipe, a first compressor that compresses the gas supplied from the suction pipe; a second compressor to which the gas discharged from the first compressor is supplied via an intermediate pipe, and which compresses the supplied gas and discharges it to the discharge pipe; a bypass pipe connecting the intermediate pipe and the discharge pipe; a bypass valve provided in the bypass pipe and switchable between an open state that allows the gas to flow through the bypass pipe and a closed state that restricts the gas flow; A controller for controlling a state of the bypass valve; Equipped with The controller opens the bypass valve when a temperature of the gas in the compression system is less than a predetermined temperature threshold, and closes the bypass valve when a temperature of the gas in the compression system is equal to or greater than a predetermined temperature threshold.

2. a circulation pipe connecting the intermediate pipe and the discharge pipe; a supply valve provided in the intermediate pipe and switchable between an open state that allows the gas to flow through the intermediate pipe and a closed state that restricts the gas flow; a discharge valve provided in the discharge pipe and switchable between an open state that allows the gas to flow through the discharge pipe and a closed state that restricts the gas flow; Further comprising: a connection position between the intermediate pipe and the circulation pipe on the intermediate pipe is located closer to the second compressor than a connection position between the intermediate pipe and the bypass pipe, On the discharge pipe, a connection position between the discharge pipe and the circulation pipe is located closer to the second compressor than a connection position between the discharge pipe and the bypass pipe, the supply valve is provided in the intermediate pipe at a position between a connection position between the intermediate pipe and the bypass pipe and a connection position between the intermediate pipe and the circulation pipe, the discharge valve is provided in the discharge piping at a position between a connection position of the discharge piping and the circulation piping and a connection position of the discharge piping and the bypass piping; the controller further controls the state of the supply valve and the discharge valve; 2. The compression system of claim 1, wherein the controller causes the supply valve and the discharge valve to the closed state and the bypass valve to the open state when a temperature of the gas in the compression system is below the temperature threshold.

3. The second compressor is a compression section that compresses the gas sucked into a cylinder via a gas intake valve by a piston and discharges the compressed gas via a gas discharge valve; an unloader for forcibly switching the gas intake valve to an open state; having The controller further controls the operation of the unloader; 3. The compression system of claim 2, wherein when a temperature of the gas in the compression system becomes less than the temperature threshold, the controller causes the unloader to forcibly switch the gas intake valve to the open configuration, and then switches the supply valve and the discharge valve to the closed state and the bypass valve to the open state.

4. a pressure equalizing valve that adjusts a gas pressure in a gas circulation path formed by the intermediate piping, the second compressor, the discharge piping, and the circulation piping; The controller further controls a state of the pressure equalization valve; 4. The compression system of claim 3, wherein the controller adjusts the gas pressure in the circulation path by controlling the pressure equalizing valve after the supply valve and the discharge valve are in the closed state and the bypass valve is in the open state.

5. The circulation valve is provided in the circulation pipe and can be switched between an open state that allows the gas to flow through the circulation pipe and a closed state that restricts the gas to flow through the circulation pipe. The controller further controls a state of the circulation valve; 3. The compression system of claim 2, wherein the controller causes the supply valve and the discharge valve to be in the open state and the bypass valve and the circulation valve to be in the closed state when a temperature of the gas in the compression system is equal to or greater than the temperature threshold.

6. 6. The compression system according to claim 5, wherein, when a temperature of the gas in the compression system becomes equal to or higher than the temperature threshold, the controller opens the supply valve, the discharge valve, and the circulation valve to increase pressure in the second compressor with the gas, and then closes the bypass valve and the circulation valve.

7. The second compressor is a compression section that compresses the gas sucked into a cylinder via a gas intake valve by a piston and discharges the compressed gas via a gas discharge valve; an unloader for forcibly switching the gas intake valve to an open state; having The controller further controls the operation of the unloader; The controller: when the temperature of the gas in the compression system becomes equal to or higher than the temperature threshold value, the unloader forcibly switches the gas intake valve to the open state, then the supply valve, the discharge valve, and the circulation valve are opened to increase the pressure inside the second compressor by the gas, then the bypass valve and the circulation valve are closed, and then the control of the unloader forcibly switching the gas intake valve to the open state is released. Alternatively, when a temperature of the gas in the compression system becomes equal to or higher than the temperature threshold value, the supply valve, the discharge valve, and the circulation valve are placed in the open state to increase pressure in the second compressor by the gas, and then the unloader is used to forcibly switch the gas intake valve to the open state, and then the bypass valve and the circulation valve are placed in the closed state, and then the control of the unloader forcibly switching the gas intake valve to the open state is released.

8. The compression system according to any one of claims 1 to 7, wherein the temperature of the gas in the compression system is the temperature of the gas discharged from the first compressor.

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