Method for controlling the operation of a multistage boil-off gas (BOG) compressor

Abstract

 Method for controlling the operation of a BOG multistage compressor (8) including plural stages of compression sections (9,10), which are connected with each other, for compressing boil-off gas (BOG) generated from liquefied natural gas. The method comprises, in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section (load ratio) smaller than load ratios in case of conditions other than the predetermined condition. The said predetermined condition is, either a condition where the suction temperature of the low pressure stage compression section (9) is not lower than a first preset temperature, or a condition where the discharge temperature of the low pressure stage compression section (9) is not lower than a second preset temperature. This control method prevents the temperature of the low pressure stage compression section (9) from exceeding a maximum allowable temperature.

Description

BACKGROUND OF THE INVENTION

Field of the Invention:

[0001] The present invention relates to a method for controlling the operation of a BOG multistage displacement compressor that compresses boil off gas (hereinafter referred to as "BOG"), which is vaporized naturally within a tank storing liquefied natural gas (LNG), and supplies the BOG to a plant. More particularly, the present invention relates to a method for controlling the operation of a BOG multistage displacement compressor, by which its low pressure stage-side discharge gas temperature is controlled not to exceed a temperature allowable for the operation of a low pressure stage compressor.

Description of the Related Art:

[0002] In an LNG plant, a BOG, generated by natural vaporization within an LNG storage tank, is pressurized to a pressure, which is suitable for delivery of natural gas to a power generation plant, city gas equipment or the like, by a BOG compressor, and then the BOG is joined together with a natural gas, which is discharged from an evaporator and main part of the gases, then the thus-joined gases are delivered to each of above equipment.

[0003] Because the BOG is adiabatically compressed in the BOG compressor, the temperature of BOG becomes higher at higher compression rate. Particularly at the time of start-up of the BOG compressor, the temperature of BOG discharged from the LNG storage tank rises to near a normal temperature, and the temperature of suction gas of the BOG compressor rises to a high temperature near the normal temperature (e.g., 30°C). If the compression of the BOG is continued without applying any process to the BOG, and if the discharge pressure is 0.9 MPa, for example, the temperature of the discharge gas from the BOG compressor rises to about 300°C that exceeds a temperature allowable for the operation of the compressor, e.g., 180°C, and thus the operation can not be continued.

[0004] A conventional technique for solving such a problem of the BOG compressor will be described below with reference to Fig. 6. Fig. 6 is a diagram showing the structure of LNG and BOG treatment equipment, to which a conventional operation controlling method is applied.

[0005] A BOG multistage compressor 38 related to this conventional technique includes a three-way valve 43, a cooler 45, temperature sensors 41, 42 and a control unit 46 for switching the three-way valve 43. In the BOG multistage compressor 38, two operation modes can be switched by switching the three-way valve 43; two operation modes include an operation mode 1 in which discharge gas from a low pressure stage-side compression section 39 is cooled by the cooler 45, the discharge gas having thus lowered temperature is supplied to a high pressure stage-side compression section 40, and an operation mode 2 in which discharge gas from the low pressure stage-side compression section 39 is supplied to the high pressure stage-side compression section 40 without passing through the cooler 45.

[0006] In the operation mode 1, the temperature of discharge gas from the high pressure stage-side compression section 40 can be made lower than that in the operation mode 2. The operation mode 1 is carried out in the case where, at the start-up of the BOG multistage compressor 38, the suction gas temperature of the lower pressure stage-side compression section 39 is higher (e.g., 30°C than that in a steady operation (e.g., -130°C), and there is a fear that the temperature of discharge gas from the high pressure stage-side compression section 40 exceeds a temperature allowable for the operation. On the other hand, the operation mode 2 is carried out in a steady operation of the BOG multistage compressor 38.

[0007] Switching between the two operation modes is performed in the following manner. In other words, when the temperature of high pressure stage-side discharge gas is detected to be higher than a preset temperature by the temperature sensor 41, the control unit 46 switches the three-way valve 43 to stop the operation in the operation mode 2 and start the operation in the operation mode 1. On the other hand, when conditions; which indicate that there is no fear of the high pressure stage-side discharge gas temperature exceeding the temperature allowable for the operation, are satisfied, the control unit 46 stops the operation in the operation mode 1 and starts the operation in the operation mode 2. By this operation controlling method, it is possible to prevent the discharge gas temperature from the high pressure stage-side compressor 40 from exceeding the temperature allowable for the operation (see Patent Document 1).

[0008] That is, according to the above conventional technique, when there is a fear that the temperature of the high pressure stage-side discharge gas may exceed the temperature allowable for the operation, the low pressure stage-side discharge gas is cooled by the cooler 45, and the discharge gas having thus lowered temperature is supplied to the high pressure stage-side compression section 40 (operation mode 1), thereby preventing the temperature of the high pressure stage-side discharge gas from exceeding the temperature allowable for the operation.

[0009] The BOG multistage compressor 38 related to the above conventional technique has a function of controlling the temperature of discharge gas from the high pressure stage-side compression section 40, but does not have a function of controlling the temperature of discharge gas from the low pressure stage-side compression section 39. Therefore, for preventing the low pressure stage-side discharge gas temperature from exceeding the temperature allowable for the operation of the low pressure stage side, it is necessary to reduce a low pressure stage-side compression ratio.

[0010] In this case, however, the number of stages in the compressor increases, resulting in increase of size and cost. In a steady operation, the low pressure stage-side compression section 39 in the BOG multistage compressor 38 sucks in gas at about -140°C to -130°C, for example, and discharges gas at about -50°C to -40°C, for example. Thus, the low pressure stage-side compression section 39 has such a character that it is not cooled with cooling water. Therefor, the temperature allowable for the operation is limited for the low pressure stage in the BOG multistage compressor 38, in comparison to the high pressure stage of the BOG multistage compressor 38 and a compressor that sucks in gas at normal temperature.

[Related Art Document]

[0011] [Patent Document 1]
Japanese Patent Laid-Open No. 2002-213366

SUMMARY OF THE INVENTION

[0012] Accordingly, it is an object of the present invention to provide a method for controlling the operation of a BOG multistage displacement compressor, capable of preventing the temperature of low pressure stage-side discharge gas from exceeding a temperature allowable for the operation, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation.

[0013] For achieving the above-mentioned object, according to a first aspect of the present invention, there is provided a method for controlling the operation of a BOG multistage displacement compressor including plural stages of displacement compression sections, which are connected with each other, for compressing BOG generated from liquefied natural gas, includes: in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section in the BOG multistage displacement compressor (load ratio) smaller than load ratios in case of conditions other than the predetermined condition.

[0014] The "load" as referred to herein indicates a nominal value of a ratio between "a gas treatment volume attained by a capacity adjusting device" and "a gas treatment volume in case of the capacity adjusting device being not operated" (the "gas treatment volume in case of the capacity adjusting device being not operated" corresponds to a treatment volume at 100% load).

[0015] According to a second aspect of the present invention, suction temperature of the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and the predetermined condition is a condition where a detected temperature of the suction temperature is not lower than a first preset temperature, in the method for controlling the operation of a BOG multistage displacement compressor according to the above first aspect.

[0016] According to a third aspect of the present invention, discharge temperature from the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and the predetermined condition is a condition from judgment that a detected temperature of the discharge temperature is not lower than a second preset temperature until judgment that the detected temperature has reached a third preset temperature lower than the second preset temperature, in the method for controlling the operation of a BOG multistage displacement compressor according to the above first aspect.

[0017] According to a first aspect of the present invention, a method for controlling the operation of a BOG multistage displacement compressor including plural stages of displacement compression sections, which are connected with each other, for compressing BOG generated from liquefied natural gas, includes: in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section in the BOG multistage displacement compressor (load ratio) smaller than load ratios in case of conditions other than the predetermined condition. Since the operation is performed at a low load ratio R, that is, the compression ratio of the low pressure stage compression section is made smaller, the temperature of low pressure stage-side discharge gas consequently drops. Thus, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the low pressure stage-side discharge gas temperature from exceeding the temperature allowable for the operation.

[0018] According to the method for controlling the operation of a BOG multistage displacement compressor according to a second aspect of the present invention, suction temperature of the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and the predetermined condition is a condition where a detected temperature of the suction temperature is not lower than a first preset temperature. Therefore, even when the temperature of the low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the temperature of the low pressure stage-side discharge gas from exceeding the temperature allowable for the operation, as similarly as in the above first aspect.

[0019] According to the method for controlling the operation of the BOG multistage displacement compressor according to a third aspect of the present invention, discharge temperature from the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and the predetermined condition is a condition from judgment that a detected temperature of the discharge temperature is not lower than a second preset temperature until judgment that the detected temperature has reached a third preset temperature lower than the second preset temperature. Therefore, even when the temperature of the low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the temperature of the low pressure stage-side discharge gas from exceeding the temperature allowable for the operation, as similarly as in the above first aspect.

[0020] Since the temperature of discharge gas from the low pressure stage compression section, which is an object of the control, is directly detected, and the detected temperature is used as an input for the control, it is possible to surely prevent the temperature of low pressure stage-side discharge gas from exceeding the temperature allowable for the operation, even when the low pressure stage-side discharge gas temperature changes greatly as a result of a change in the amount of gas the supply destination demands.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] 

Fig. 1 is a system diagram of LNG and BOG treatment equipment, to which a method for controlling the operation of a BOG multistage displacement compressor according to a first embodiment of the present invention, is applied;

Fig. 2 is a diagram showing changes in BOG temperature with time just after start-up in case the method for controlling the operation of a BOG multistage displacement compressor according to a first embodiment of the present invention is used;

Fig. 3 is a system diagram of LNG and BOG treatment equipment, to which a method for controlling the operation of a BOG multistage displacement compressor according to a second embodiment of the present invention, is applied;

Fig. 4 is a system diagram of LNG and BOG treatment equipment, to which a method for controlling the operation of a BOG multistage displacement compressor according to a third embodiment of the present invention, is applied;

Fig. 5 is a diagram showing changes in BOG temperature with time just after start-up in case the method for controlling the operation of a BOG multistage displacement compressor according to a third embodiment of the present invention is used; and

Fig. 6 is a diagram showing the structure of LNG and BOG treatment equipment, to which a conventional operation controlling method is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] First, a method for controlling the operation of a BOG multistage displacement compressor according to a first embodiment of the present invention will be described by exemplifying a case, in which a reciprocating compressor is applied to the BOG multistage compressor, with reference to Figs. 1 and 2. Fig. 1 is a system diagram of LNG and BOG treatment equipment, to which the method for controlling the operation of a BOG multistage displacement compressor of the first embodiment of the present invention is applied. Fig. 2 is a diagram showing changes of BOG temperature with time just after a start-up in case the method for controlling the operation of a BOG multistage displacement compressor of the first embodiment of the present invention is used.

[0023] The BOG multistage displacement compressor according to the first embodiment of the present invention is a BOG multistage compressor 8 that is for compressing BOG generated by natural vaporization within an LNG storage tank 2 that stores liquefied natural gas (LNG) 1 and for supplying the thus-compressed BOG to a plant (not shown). The BOG multistage compressor 8 is a reciprocating compressor including a low pressure stage compression section 9 and a high pressure stage compression section 10. The capacity of the low pressure stage compression section 9 can be adjusted by a low pressure stage capacity adjusting device 21 including a suction valve unloader 9a and a head end unloader 9b. The capacity of the high pressure stage compression section 10 can be adjusted by a high pressure stage capacity adjusting device 22 including a suction valve unloader 10a and a head end unloader 10b.

[0024] On the other hand, an LNG conducting line 3 is connected to the LNG storage tank 2 for liquefied natural gas (LNG) 1. An LNG pump 4 and an evaporator 5 using sea water, for example, for evaporating LNG are connected to the line 3. Further, a gas conveying line 6 for conveying vaporized gas is connected to, for example, a gas turbine in a power generation plant (not shown). A BOG delivery line 7 is connected to the top of the LNG storage tank 2 and the BOG multistage compressor 8 is connected to the BOG delivery line 7.

[0025] The BOG multistage compressor 8 is configured such that both low pressure stage compression section 9 and high pressure stage compression section 10 are simultaneously driven by a single drive motor 11. The BOG delivery line 7 is connected to a suction side of the low pressure stage compression section 9. An intermediate line 12 connects a discharge side of the low pressure stage compression section 9 with a suction side of a high pressure stage compression section 10. A discharge line 17 disposed on a discharge side of the high pressure stage compression section 10 in the BOG multistage compressor 8 is connected to the gas conveying line 6 through a confluence section 23.

[0026] A pressure sensor 24 is provided in the LNG storage tank 2 for detecting the pressure of BOG therein. Its detected value is inputted to a control unit 25. The control unit 25 controls the operation of the BOG multistage compressor 8 in such a manner that when the pressure of BOG in the tank 2 detected by the pressure sensor 24 exceeds a preset pressure, the compressor 8 is turned on, while when the pressure of BOG in the tank 2 has lowered to a predetermined value, the compressor 8 is turned off.

[0027] Next, the method for controlling the operation according to the first embodiment of the present invention, carried out in this equipment, will be described with reference to Fig. 2 also. In Fig. 2, a temperature curve A represents a low pressure stage-side suction gas temperature, while a temperature curve B represents a low pressure stage-side discharge gas temperature at position P1 on the intermediate line 12 shown in Fig. 1.

<Operation Controlling Method (No. 1)>

[0028] Upon start-up of the BOG multistage compressor 8, the suction gas temperature of the low pressure stage compression section 9 (temperature of BOG delivered from the LNG storage tank 2) keeps the substantially same value as a value at the start-up for an extremely short time, and thereafter drops gradually as indicated by the temperature curve A in Fig. 2. In the case where a load ratio R of the BOG multistage compressor 8 (a ratio of a load on the low pressure stage compression section 9 to a load on the high pressure stage compression section 10 in the BOG multistage compressor 8) is the same as in a steady operation, the low pressure stage discharge gas temperature starts from a value at the start-up and once rises to a peak temperature, as indicated by a two-dot chain line in the temperature curve B shown in Fig. 2, and then gradually drops according to the change of the temperature curve A.

[0029] Accordingly, in case the gas treatment volume ratio R is the same as in a steady operation, the low pressure stage-side discharge gas temperature starts from the value at the start-up, exceeds a temperature Th1 allowable for the operation, once rises to the peak temperature and then drops gradually, as indicated by the two-dot chain line in the temperature curve B shown in Fig. 2.

[0030] Upon start-up of the BOG multistage compressor 8, the control unit 25 compares temperature of low pressure stage-side suction gas, which is detected by a temperature sensor 26, with a first preset temperature T1 preset in the control unit 25. Then, if the detected temperature of the low pressure stage-side suction gas is not lower than the first preset temperature T1, the control unit 25 controls the operation of the low pressure stage capacity adjusting device 21 and that of the high pressure stage capacity adjusting device 22 so that the load ratio R becomes lower than that in a steady operation. The control unit 25 controls so as to set a low pressure stage-side load and a high pressure stage-side load of to 25% and 50%, respectively, for example.

[0031] As a result of such an operation controlling method, the low pressure stage-side discharge pressure is reduced, and the low pressure stage-side discharge gas temperature is consequently inhibited from rising and suppressed to a temperature not higher than the temperature Th1 allowable for the operation, as indicated by the temperature curve B in Fig. 2. The low pressure stage-side suction gas temperature gradually reduces as mentioned above.

[0032] The control unit 25 compares the temperature of low pressure stage-side suction gas, which is detected by the temperature sensor 26, with the first preset temperature T1. As a result of the comparison, if the detected temperature of the low pressure stage-side suction gas reaches (or becomes lower than) the first preset temperature T1, the control unit 25 controls the operation of the low pressure stage capacity adjusting device 21 and that of the high pressure stage capacity adjusting device 22 so as to make the load ratio R the same as that in a steady operation.

[0033] For example, the control unit 25 controls so as to set both low and high pressure stage-side loads to 100%. As a result, the temperature of the low pressure stage-side discharge gas rises temporarily, but does not reach Th1. Thereafter, the low pressure stage-side suction gas temperature and the low pressure stage-side discharge gas temperature drop gradually and reach a stable state where each of them is maintained within a certain range.

[0034] Thus, according to the method for controlling the operation of a BOG multistage displacement compressor according to the first embodiment of the present invention, the suction temperature of the low pressure stage compression section 9 of the multistage displacement compressor 8 is made detectable, and the above predetermined condition is a condition where the detected suction temperature is not lower than the first preset temperature T1. Therefore, as described above, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the temperature of low pressure stage-side discharge gas from exceeding the temperature allowable for operation.

<Example>

[0035] An example of the method for controlling the operation of a BOG multistage displacement compressor according to the first embodiment of the present invention will be described below also with reference to Fig. 1.

[0036] In a steady operation of the multistage displacement compressor, both first stage-side compression section 9 and second stage-side compression section 10 are operated at 100% load (Comparative Example-1 in Table 1). According to the BOG multistage displacement compressor according to the first embodiment of the present invention, in case of the above predetermined condition, such as a start-up, the first stage-side compression section 9 is operated at 25% load and the second stage-side compression section 10 is operated at 50% load by the capacity adjusting devices 21 (the suction valve unloader 9a, the head end unloader 9b and the like) and 22 (the suction valve unloader 10a, the head end unloader 10b and the like) in the first and second stage-side compression sections 9 and 10. That is, load ratio at the start-up is smaller than load ratio in a steady operation (see Example in Table 1).

[0037] In Table 1, states at the start-up of conventional BOG multistage displacement compressors are listed in Table 1 to show the effect of the present invention. One is an operation state in which both first stage-side compression section 9 and second stage-side compression section 10 operate at 25% load (Comparative Example-3 in Table 1). The other is an operation state in which both first and second stage-side compression sections 9 and 10 operate at 50% load (Comparative Example-2 in Table 1). In both Comparative Examples-2 and -3 the load ratio is the same as in a steady operation.

[0038] Table 2 shows, for each of the operation examples in Fig. 1, suction and discharge temperatures of the first stage-side compression section 9, and suction and discharge gas pressures of the first stage-side compression section 9 and the second stage-side compression section 10. As shown in Comparative Example-1 in Table 2, in a steady operation of the BOG multistage displacement compressor, the first stage-side discharge temperature is about -50°C. In operations at the start-up of the conventional BOG multistage displacement compressor, however, the first stage-side discharge temperature reaches as high as about 155°C (more particularly, 153°C and 156°C in Comparative Examples-2 and -3, respectively), as shown in Comparative Examples-2 and -3 in Table 2.

[0039] On the other hand, in the operation at the start-up of the BOG multistage displacement compressor according to the first embodiment of the present invention, the first stage-side discharge temperature is suppressed to about 135°C (more particularly, 136°C in Example), as shown in Example in Table 2. This is because the first stage-side discharge pressure is suppressed to about 335 kPa in Example, in comparison to about 405 kPa (more particularly, 404 kPa) in Comparative Examples-2, 3, by making the load ratio smaller than that in a steady operation,.

[0040] Next, a method for controlling the operation of a BOG multistage displacement compressor according to a second embodiment of the present invention will be described by exemplifying a case, in which a screw compressor is applied to the BOG multistage compressor, with reference to Fig. 3. Fig. 3 is a system diagram of LNG and BOG treatment equipment, to which the method for controlling the operation of a BOG multistage displacement compressor according to a second embodiment of the present invention is applied.

[0041] This second embodiment of the present invention differs from the first embodiment in the type of a BOG multistage displacement compressor and the structure of both low and high pressure stage capacity adjusting devices. Since the second embodiment has the completely same structure as in the first embodiment except the above differences, the same components as in the first embodiment are given the same reference numerals, and the differences will be described below.

[0042] In the BOG multistage compressor 8, to which the method for controlling the operation of a BOG multistage displacement compressor according to the first embodiment is applied and which includes a reciprocating compressor, the capacity of the low pressure stage compression section 9 is adjusted by the low pressure stage capacity adjusting device 21 that includes the suction valve unloader 9a and the head end unloader 9b, while the capacity of the high pressure stage compression section 10 is adjusted by the high pressure stage capacity adjusting device 22 that includes the suction valve unloader 10a and the head end unloader 10b.

[0043] On the other hand, in a BOG multistage compressor 18, to which the method for controlling the operation of a BOG multistage displacement compressor according to the second embodiment shown in Fig. 3 is applied and which includes a screw type compressor, the capacity of a low pressure stage compression section 19 is adjusted by a low pressure stage capacity adjusting device that includes a slide valve 19a, while the capacity of a high pressure stage compression section 20 is adjusted by a high pressure stage capacity adjusting device that includes a slide valve 20a.

[0044] According to such a method for controlling the operation of a BOG multistage displacement compressor according to a second embodiment of the present invention, a suction temperature of the low pressure stage compression section 19 of the BOG multistage displacement compressor 18 is made detectable, and the above predetermined condition is a condition in which the detected suction temperature is not lower than a first preset temperature T1, as similarly as the first embodiment. Therefore, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the temperature of low pressure stage-side discharge gas from exceeding a temperature allowable for the operation.

[0045] Next, a method for controlling the operation of a BOG multistage displacement compressor according to a third embodiment of the present invention will be described by exemplifying a case where a reciprocating compressor is applied to the BOG multistage compressor with reference to Figs. 4 and 5. Fig. 4 is a system diagram of LNG and BOG treatment equipment, to which the method for controlling the operation of a BOG multistage displacement compressor according to a third embodiment of the present invention is applied. Fig. 5 is a diagram showing changes of BOG temperature with time just after a start-up in case the method for controlling the operation of a BOG multistage displacement compressor of a third embodiment of the present invention is used.

[0046] This third embodiment of the present invention differs from the above first embodiment in a mounting position of a temperature sensor and a method for controlling the operation. Since the third embodiment has the completely same structure as in the first embodiment except the differences, the same components as in the first embodiment are given the same reference numerals, and the differences will be described below.

[0047] In other words, according to the method for controlling the operation of the BOG multistage displacement compressor 8 according to the above first embodiment, the temperature of low pressure stage-side suction gas is detected by the temperature sensor 26 disposed in the BOG delivery line 7, and the operation is controlled, by the operation controlling method (No.1). On the other hand, according to the method for controlling the operation of the BOG multistage displacement compressor 8 according to this third embodiment shown in Figs. 4 and 5, the temperature of low pressure stage-side discharge gas is detected by a temperature sensor 27 disposed in the intermediate line 12, and the operation of is controlled by an operation controlling method (No. 2) that will be described below.

<Operation Controlling Method (No. 2)>

[0048] According to this operation controlling method (No. 2), upon start-up of the BOG multistage compressor 8, the control unit 25 compares the temperature of low pressure stage-side discharge gas, which is detected by the temperature detector 27, with a second preset temperature T2, which is preset in the control unit 25. As shown in Fig. 5, in an extremely short period just after start-up, the detected temperature of low pressure stage-side discharge gas is lower than the second preset temperature T2. Thus, during this period, the load ratio R is the same as that in a steady operation. If the detected temperature of low pressure stage-side discharge gas is not lower than the second preset temperature T2, the control unit 25 controls the operation of the low pressure stage capacity adjusting device 21 and that of the high pressure stage capacity adjusting device 22 so as to make the load ratio R lower than that in a steady operation.

[0049] For example, the control unit 25 controls so as to set a low pressure stage-side load and a high pressure stage-side load to 25% and 50%, respectively. As a result, the low pressure stage-side discharge pressure is reduced, and the low pressure stage-side discharge gas temperature is consequently inhibited from rising and suppressed to a temperature not higher than the temperature Th1 allowable for the operation, as indicated by the temperature curve B in Fig. 5. The low pressure stage-side suction gas temperature gradually reduces as mentioned above.

[0050] Further, if the temperature of low pressure stage-side discharge gas which is detected by the temperature sensor 27 reaches a third preset temperature T3 (lower than the second preset temperature T2), the control unit 25 controls the operation of the low pressure stage capacity adjusting device 21 and that of the high pressure stage capacity adjusting device 22 so as to make the load ratio R to be the same as that in a steady operation. For example, the control unit 25 sets both the low pressure stage-side load and the high pressure stage-side load to 25%. As a result, the temperature of low pressure stage-side discharge gas rises temporarily, but does not reach Th1. Thereafter, the low pressure stage-side suction gas temperature and the low pressure stage-side discharge gas temperature drop gradually and reach a stable state where each of them is maintained within a certain range.

[0051] Thus, according to the method for controlling the operation of a BOG multistage displacement compressor according to a third embodiment of the present invention, the discharge gas temperature of the low pressure stage compression section 9 of the BOG multistage displacement compressor 8 is made detectable, and a predetermined condition is a condition from judgment that the detected temperature of the discharge temperature is not lower than the second preset temperature T2 until judgment that the detected temperature has reached the third preset temperature T3 lower than the second preset temperature T2. Therefore, even if the temperature of low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the temperature of low pressure stage-side discharge gas from exceeding the temperature allowable for the operation.

[0052] As described above, according to the present invention, the method for controlling the operation of a BOG multistage displacement compressor, includes: in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section in the BOG multistage displacement compressor (load ratio) R smaller than load ratios in case of conditions other than the predetermined condition. Since the operation is consequently performed at a low load ratio R, that is, the compression ratio of the low pressure stage compression section is made smaller, the temperature of low pressure stage-side discharge gas consequently drops. Thus, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation, it is possible to prevent the low pressure stage-side discharge gas temperature from exceeding the temperature allowable for the operation.

[0053] In the above embodiments, the methods for controlling the operation of a BOG multistage displacement compressor according to the present invention have been described by exemplifying a reciprocating compressor and a screw type compressor, and the capacity adjusting devices are described by exemplifying suction valve unloaders, head end unloaders and slide valves. The present invention is, however, not limited thereto, but the method for controlling the operation of a BOG multistage displacement compressor according to the present invention is also applicable to various types of displacement compressors and BOG displacement compressors having capacity adjusting devices of various structures.

Explanation of Reference Numerals

[0054] 

1: Liquefied natural gas (LNG),	2: LNG storage tank,
3: LNG conducting line,	4: LNG pump,
5: Evaporator,	6: Gas conveying line,
7: BOG delivery line,
8: BOG multistage (displacement) compressor (Reciprocating compressor),
9: Low pressure stage compression section (First stage-side compression section),
9a: Suction valve unloader,	9b: Head end unloader,
10: High pressure stage compression section (Second stage-side compression section),
10a: Suction valve unloader,	10b: Head end unloader,
11: Drive motor,	12: Intermediate line,
17: Discharge line,
18: BOG multistage (displacement) compressor (Screw type compressor),
19: Low pressure stage compression section, 19a: Slide valve,
20: High pressure stage compression section, 20a: Slide valve,
21: Low pressure stage capacity adjusting device,
22: High pressure stage capacity adjusting device,
23: Confluence section,	24: Pressure sensor,
25: Control unit,	26, 27: Temperature sensor

[0055] It is an object of the present invention to provide a method for Controlling the operation of a BOG multistage displacement compressor, capable of preventing the temperature of low pressure stage-side discharge gas from exceeding a temperature allowable for the operation, even when the temperature of low pressure stage-side suction gas is higher than that in a steady operation.

[0056] A method for controlling the operation of a BOG multistage displacement compressor including plural stages of displacement compression sections, which are connected with each other, for compressing BOG generated from liquefied natural gas, includes: in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section in the BOG multistage displacement compressor (load ratio) smaller than load ratios in case of conditions other than the predetermined condition. In other words, suction temperature of the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and the predetermined condition is a condition where a detected temperature of the suction temperature is not lower than a first preset temperature.

Claims

1. A method for controlling the operation of a BOG multistage displacement compressor including plural stages of displacement compression sections, which are connected with each other, for compressing BOG generated from liquefied natural gas, comprising:

in case of a predetermined condition, making a ratio of a load on a low pressure stage compression section to a load on a high pressure stage compression section in the BOG multistage displacement compressor (load ratio) smaller than load ratios in case of conditions other than the predetermined condition.

2. The method for controlling the operation of a BOG multistage displacement compressor according to claim 1, wherein
suction temperature of the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and
the predetermined condition is a condition where a detected temperature of the suction temperature is not lower than a first preset temperature.

3. The method for controlling the operation of a BOG multistage displacement compressor according to claim 1, wherein
discharge temperature from the low pressure stage compression section of the BOG multistage displacement compressor is made detectable, and
the predetermined condition is a condition from judgment that a detected temperature of the discharge temperature is not lower than a second preset temperature until judgment that the detected temperature has reached a third preset temperature lower than the second preset temperature.

Drawing