BACKGROUND OF THE INVENTION
Field of the Invention:
[0001] The present invention relates to an operation control method for a BOG displacement
compressor configured to compress boil off gas (hereinafter referred to as BOG) generated
by natural vaporization within a tank storing liquefied natural gas (LNG) and supply
the compressed gas to a plant, and more specifically, relates to an operation control
method for a BOG multistage displacement compressor capable of suppressing increase
in load (gas load) due to a differential pressure between suction gas and discharge
gas in a high-pressure stage side compression unit in a multistage displacement compressor.
Description of the Related Art:
[0002] In an LNG station, the pressure of BOG generated by natural vaporization within an
LNG storage tank is raised by a BOG compressor to a pressure of natural gas delivered
to power generation plant, town gas facilities or the like, the BOG is merged with
natural gas that is released from an evaporator and is main part of the merged gas,
and the merged gas is delivered to each of the above facilities.
[0003] In the above-mentioned BOG compressor, since the BOG is adiabatically compressed,
higher compression ratio raises the temperature of BOG. Particularly at start-up of
the BOG compressor, the temperature of BOG derived from the LNG storage tank is raised
close to ordinary temperature, and suction gas temperature in the BOG compressor is
raised to a temperature close to the ordinary temperature (e.g., 30°C). If a state
where the BOG is compressed without any treatment continues, and when the discharge
pressure is 0.4 MPa, discharge gas temperature of the BOG compressor rises, for example,
to about 155°C, which exceeds a compressor allowable operation temperature, e.g.,
150°C, and the operation cannot be continued.
[0004] A conventional technique for solving such a problem of BOG compressors will be described
with reference to Fig. 6. Fig. 6 is a view showing a configuration of LNG and BOG
treatment equipment to which an operation control method according to the conventional
technique is applied.
[0005] A BOG multistage compressor 38 according to the conventional technique includes a
three-way valve 43, a cooler 45, temperature detectors 41, 42, and a control device
46 which governs the switching of the three-way valve 43. In the BOG multistage compressor
38, switching the three-way valve 43 can switch between an operation form (operation
form 1) in which discharge gas of a low-pressure stage side compression unit 39 is
cooled by the cooler 45 to reduce the temperature thereof, and then the discharge
gas is supplied to a high-pressure stage side compression unit 40, and an operation
form (operation form 2) in which the discharge gas of the low-pressure stage side
compression unit 39 does not pass through the cooler 45 and is supplied to the high-pressure
stage side compression unit 40.
[0006] In the above-mentioned operation form 1, the discharge gas temperature of the high-pressure
stage side compression unit 40 can be made lower than that in the operation form 2.
The operation form 1 is executed when, at the start-up of the BOG multistage compressor
38, there is fear that the suction gas temperature of the low-pressure stage side
compression unit 39 is higher than that (e.g., -130°C) in steady operation (e.g.,
the suction gas temperature is 30°C) and the discharge gas temperature of the high-pressure
stage side compression unit 40 exceeds an operation allowable temperature. In contrast,
the operation form 2 is executed in the steady operation of the BOG multistage compressor
38.
[0007] The switching between both the operation forms is performed as follows. Namely, when
it is detected by the temperature detector 41 that a high-pressure stage side discharge
gas temperature is higher than a set temperature, the control device 46 switches the
three-way valve 43 so that the operation by the operation form 2 is stopped and the
operation by the operation form 1 is started. On the other hand, when a condition
showing that there is no fear that the temperature of high-pressure stage side discharge
gas never exceeds the operation allowable temperature is ratified, the operation by
the operation form 1 is stopped, and the operation by the operation form 2 is started.
According to the above-mentioned operation control method, the discharge gas temperature
of the high-pressure stage side compressor 40 can be prevented from exceeding the
operation allowable temperature (refer to Japanese Patent Application Laid-Open No.
2002-213366).
[0008] Namely, according to the above-mentioned conventional technique, if there is the
fear that the temperature of high-pressure stage side discharge gas exceeds the operation
allowable temperature, the operation (operation form 1) such that the low-pressure
stage side discharge gas is cooled by the cooler 45 to reduce the temperature thereof,
and supplied to the high-pressure stage side compression unit 40 is performed, and
thereby the temperature of high-pressure stage side discharge gas is prevented from
exceeding the operation allowable temperature.
[0009] However, according to the BOG multistage compressor 38 of the conventional technique,
when the temperature of suction gas of the low-pressure stage side compression unit
39 is higher than that in steady operation, the pressure of discharge gas of the low-pressure
stage side compression unit 39 and the pressure of suction gas of the high-pressure
stage side compression unit 40 become lower than the pressures of those in steady
operation. Thus, the difference between the pressure of suction gas and the pressure
of discharge gas (differential pressure) in the high-pressure stage side compression
unit 40 is increased, resulting in an increased gas load in the high-pressure stage
side compression unit 40.
[0010] The gas load means a quantity of force generated by the pressure of gas. The gas
load is loaded on a casing of the compressor. If the gas load is increased in the
high-pressure stage side compression unit 40, a facility that can allow for the increased
gas load is needed, generally resulting in an increased size of the compressor. The
conventional technique does not refer to countermeasures against such increase in
gas load.
SUMMARY OF THE INVENTION
[0011] The present invention thus has an object to provide an operation control method for
a BOG multistage displacement compressor that, even if the temperature of low-pressure
stage side suction gas is higher than that in steady operation in the BOG multistage
displacement compressor, is capable of suppressing increase in load (gas load) due
to a differential pressure between suction gas and discharge gas in a high-pressure
stage-side compression unit.
[0012] In order to attain the above-mentioned object, the operation control method for a
BOG multistage displacement compressor according to the present invention adopts the
following means. Namely, an operation control method for a BOG multistage displacement
compressor including connected multiple stages of displacement compression units for
compressing BOG generated from liquefied natural gas, includes: under a predetermined
state, performing operation control so that the ratio (load ratio) of a load of a
low-pressure stage compression unit to a load of a high-pressure stage compression
unit in the BOG multistage displacement compressor is larger than load ratios under
states other than the predetermined state. The "load" in the present application is
a nominal value of the ratio of "gas treatment quantity attained by a capacity adjusting
device" to "gas treatment quantity when the capacity adjusting device is not operated"
(the "gas treatment quantity when the capacity adjusting device is not operated" corresponds
to a treatment quantity with 100% load).
[0013] According to such an operation control method, the operation control method for a
BOG multistage displacement compressor including connected multiple stages of displacement
compression units for compressing BOG generated from liquefied natural gas, includes:
under a predetermined state, performing operation control so that the ratio (load
ratio) of a load of a low-pressure stage compression unit to a load of a high-pressure
stage compression unit in the BOG multistage displacement compressor is larger than
load ratios under states other than the predetermined state. Since the operation is
performed with a large load ratio R means that a compression ratio of the low-pressure
stage compression unit is increased and a compression ratio of the high-pressure stage
compression unit is decreased. Accordingly, the gas load on the high-pressure stage
side is also reduced. Consequently, even if the temperature of low-pressure stage
side suction gas is higher than that in steady operation, the load (gas load) due
to the differential pressure between suction gas and discharge gas in the high-pressure
stage compression unit can be suppressed from increasing, and the gas load on the
high-pressure stage side can be prevented from exceeding an allowable gas load.
[0014] In the above-mentioned operation control method for a BOG multistage displacement
compressor of the present invention, the BOG multistage displacement compressor may
be configured so that suction temperature in the low-pressure stage compression unit
can be detected, and the predetermined state may be set to a state where the detected
temperature of the suction temperature is equal to or higher than a set temperature
that is preset.
[0015] According to such an operation control method, since the BOG multistage displacement
compressor is configured so that suction temperature in the low-pressure stage compression
unit can be detected, and the predetermined state is set to a state where the detected
temperature of the suction temperature is equal to or higher than a set temperature
that is preset, the increase in load (gas load) due to the differential pressure between
suction gas and discharge gas in the high-pressure stage compression unit can be suppressed
even if the temperature of low-pressure stage side suction gas is higher than that
in steady operation, and the gas load on the high-pressure stage side can be prevented
from exceeding the allowable gas load, as described above.
[0016] In the above-mentioned operation control method for a BOG multistage displacement
compressor of the present invention, the BOG multistage displacement compressor may
be configured so that a differential pressure between suction pressure and discharge
pressure of the high-pressure stage compression unit can be detected, and the predetermined
state may be a state since it is determined that the differential pressure is equal
to or more than a first set differential pressure that is preset, until it is determined
that the differential pressure reaches a second set differential pressure that is
preset and smaller than the first set differential pressure.
[0017] According to such an operation control method, since the BOG multistage displacement
compressor is configured so that a differential pressure between suction pressure
and discharge pressure of the high-pressure stage compression unit can be detected,
and the predetermined state is a state since it is determined that the differential
pressure is equal to or more than a first set differential pressures that is preset,
until it is determined that the differential pressure reaches a second set differential
pressure that is preset and smaller than the first set differential pressure, the
increase in load (gas load) due to the differential pressure between suction gas and
discharge gas in the high-pressure stage compression unit can be suppressed similarly
to the above even if the temperature of low-pressure stage side suction gas is higher
than that in steady operation, and the gas load on the high-pressure stage side can
be prevented from exceeding the allowable gas load.
[0018] Since the operation form is decided based on the differential pressure on the high-pressure
stage side directly involved in the gas load on the high-pressure stage side, the
increase in load (gas load) due to the differential pressure between suction gas and
discharge gas in the high-pressure stage compression unit can be surely suppressed
even if the pressure of discharge gas on the high-pressure stage side is fluctuated
due to fluctuation in gas demand quantity at a supply destination or the like, and
the gas load can be prevented from exceeding the allowable gas load.
[0019] In the above-mentioned operation control method for a BOG multistage displacement
compressor of the present invention, when the operation control is performed so that
the load ratio under the predetermined state is larger than load ratios under states
other than the predetermined state, discharge gas of the low pressure compression
unit may pass through a cooler, and be supplied to the high-pressure stage compression
unit.
[0020] According to such an operation control method, since, when performing the operation
control so that the load ratio under the predetermined state is larger than load ratios
under states other than the predetermined state, discharge gas of the low pressure
compression unit passes through a cooler, and is supplied to the high-pressure stage
compression unit, the increase in load (gas load) due to the differential pressure
between suction gas and discharge gas in the high-pressure stage compression unit
can be suppressed similarly to the above, even if the temperature of low-pressure
stage-side suction gas is higher than that in steady operation, and the gas load on
the high-pressure stage side from is prevented from exceeding the allowable gas load.
In addition, the temperature rise of discharge gas on the low-pressure stage side
can be also suppressed and the temperature of discharge gas on the high-pressure stage
side is prevented from exceeding an allowable upper limit temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
Fig. 1 is a systematic diagram of LNG and BOG treatment equipment to which an operation
control method for a BOG multistage displacement compressor according to Embodiment
1 of the present invention is applied;
Fig. 2 is a view for illustrating temporal change in BOG temperature just after start-up
in the operation control method for a BOG multistage displacement compressor according
to Embodiment 1 of the present invention;
Fig. 3 is a systematic diagram of LNG and BOG treatment equipment to which an operation
control method for a BOG multistage displacement compressor according to Embodiment
2 of the present invention is applied;
Fig. 4 is a systematic diagram of LNG and BOG treatment equipment to which an operation
control method for a BOG multistage displacement compressor according to Embodiment
3 of the present invention is applied;
Fig. 5 is a systematic diagram of LNG and BOG treatment equipment to which an operation
control method for a BOG multistage displacement compressor according to Embodiment
4 of the present invention is applied; and
Fig. 6 is a view for illustrating a configuration of LNG and BOG treatment equipment
to which an operation control method according to a related art is applied.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] In order to explain an operation control method for a BOG multistage displacement
compressor according to Embodiment 1 of the present invention, a case in which a reciprocating
compressor is applied to the BOG multistage displacement compressor will be given
as an example with reference to Figs. 1 and 2. Fig. 1 is a systematic diagram of LNG
and BOG treatment equipment to which the operation control method for a BOG multistage
displacement compressor according to Embodiment 1 of the present invention is applied,
and Fig. 2 is a view for illustrating temporal change in BOG temperature just after
start-up in the operation control method for a BOG multistage displacement compressor
according to Embodiment 1 of the present invention.
[0023] The BOG multistage displacement compressor according to Embodiment 1 of the present
invention is a BOG multistage compressor 8 for compressing BOG generated by natural
vaporization within an LNG storage tank 2 storing liquefied natural gas (LNG) 1 and
supplying the compressed BOG to a plant not shown, which is composed of a reciprocating
compressor including a low-pressure stage compression unit 9 and a high-pressure stage
compression unit 10. The low-pressure stage compression unit 9 is configured so as
to be capacity-adjustable by a low-pressure stage capacity adjusting device 21 including
a suction valve unloader 9a and a head end unloader 9b, and the high-pressure stage
compression unit 10 is configured so as to be capacity-adjustable 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 extraction line 3 is connected to the LNG storage tank
2 of liquefied natural gas (LNG) 1, an LNG pump 4 and an evaporator 5 for vaporizing
LNG with seawater or the like are connected to the line 3, and a gas transfer line
6 for the vaporized gas is connected to, for example, a gas turbine of a power generation
plant not shown. A BOG delivery line 7 is connected to a top portion 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 to simultaneously drive the low-pressure
stage compression unit 9 and the high-pressure stage compression unit 10 by one drive
motor 11. The BOG delivery line 7 is connected to the suction side of the low-pressure
stage compression unit 9, and the discharge side of the low-pressure stage compression
unit 9 is connected to the suction side of the high-pressure stage compression unit
10 by an intermediate line 12. A discharge line 17 on the discharge side of the high-pressure
stage compression unit 10 of the BOG multistage compressor 8 is connected to the gas
transfer line 6 through a junction portion 23.
[0026] The LNG storage tank 2 includes a pressure detector 24 for detecting gas pressure
of BOG in the tank 2, and a detection value thereof is input to a controller 25. The
controller 25 performs operation control so as to start and operate the BOG multistage
compressor 8 when the gas pressure of BOG in the tank 2 detected by the pressure detector
24 exceeds a set pressure, and to stop the compressor 8 when the gas pressure of BOG
in the tank 2 is reduced to a predetermined value.
[0027] Next, the operation control method according to Embodiment 1 of then present invention
to be executed in this equipment is described with reference to Fig. 2. In Fig. 2,
a temperature curve A indicates the temperature of suction gas on the low-pressure
stage side, a pressure curve B indicates the pressure of discharge gas on the high-pressure
stage side at a position P2 on the discharge line 17 of Fig. 1, a pressure curve C
indicates the pressure of discharge gas on the low-pressure stage side (the pressure
of suction gas on the high-pressure stage side) at a position P1 on the intermediate
line 12 of Fig. 1, and a gas load curve D indicates the gas load (compression load)
on the high-pressure stage side.
<Operation Control Method (1)>
[0028] When the BOG multistage compressor 8 is started, the temperature of suction gas of
the low-pressure stage compression unit 9 (the temperature of BOG derived from the
LNG storage tank 2) maintains substantially the same level as that at start-up for
an extremely short time and gradually drops, as shown by the temperature curve A of
Fig. 2.
[0029] When the load ratio (or the ratio of the load of the low-pressure stage compression
unit to the load of the high-pressure stage compression unit) is equal to that in
steady operation, the pressure of suction gas on the high-pressure stage side starts
from the level obtained at start-up, and reaches a level of P_11 which is lower than
a pressure P_13 of suction gas in steady operation within an extremely short time
(time sl), as shown by a dashed-dotted line in the pressure curve C of Fig. 2. Thereafter,
the pressure of suction gas on the high-pressure stage side gradually increases according
to the change of the pressure curve C, and stabilizes at the pressure P_13.
[0030] On the other hand, the pressure of discharge gas on the high-pressure stage side
starts from the level obtained at start-up, reaches a pressure P_hl, which is obtained
in steady operation, in an extremely short time (time sl), and gets into a stabilized
state, as shown by the pressure curve B of Fig. 2. Accordingly, the gas load on the
high-pressure stage side with the same load ratio as in steady operation starts from
zero, exceeds an allowable gas load GL_h on the high-pressure stage side in an extremely
short time, and reaches a high level of GL_4, as shown by a dashed-dotted line in
the gas load curve D of Fig. 2. Thereafter, it gradually decreases and stabilizes
near a gas load Gel_2.
[0031] In the operation control method according to Embodiment 1 of the present invention,
when the BOG multistage compressor 8 is started, the controller 25 performs operation
control on the low-pressure stage capacity adjusting device 21 and the high-pressure
stage capacity adjusting device 22 during the time from the start-up to time s2 described
below so that the load ratio R is larger than that in steady operation. For example,
the control is performed so that the low-pressure stage side load is 100% and the
high-pressure stage side load is 75%.
[0032] By such an operation control method, the pressure of discharge gas on the low-pressure
stage side (the pressure of suction gas on the high-pressure stage side) rises and
reaches a pressure P_14 at time s2 as shown by a solid line of the pressure curve
C in Fig. 2. On the other hand, the pressure of discharge gas on the high-pressure
stage side starts from the level obtained at start-up, reaches the pressure P_h1,
which is obtained in steady operation, and, after an extremely short time (time s1),
gets into a stabilized state, similarly to the case in which the load ratio is the
same as in steady operation. The gas load increases during the time from the start-up
to time s1 and reaches a GL_3 at the time s1 as shown by a solid line of the pressure
curve D of Fig. 2. Note that this GL_3 is suppressed to a level lower than the allowable
gas load GL_h. After the time s1, the gas load decreases and reaches GL_1 at time
s2.
[0033] The controller 25 compares a temperature of low-pressure stage-side suction gas detected
by the temperature detector 26 with a set temperature T1 (e.g., -45°C) that is preset.
As a result of the comparison, when the detected temperature of low-pressure stage-side
suction gas reaches the set temperature T1 (or when the detected temperature of low-pressure
stage-side suction gas becomes lower than the set temperature T1) that is, at the
time s2 in Fig. 2, the controller 25 controls the operation of the low-pressure stage
capacity adjusting device 21 and the high-pressuro stage capacity adjusting device
22, so that the load ratio R is equal to that in steady operation. For example, the
control is performed so that the low-pressure stage-side load is 100%, and the high-pressure
stage-side load is 100%.
[0034] Due to above, after the time s2, the pressure of discharge gas on the lower pressure
stage side (the pressure of suction gas on the high-pressure stage side) temporally
decreases. To the contrary, the gas load on discharge stage side accordingly increases.
The pressure of discharge gas on the low-pressure stage side (the pressure of suction
gas on the high-pressure stage side) decreases to the P_12 until time s3, and starts
to increase again. Accordingly, the gas load on the discharge stage side increases
until the time s3 and then starts to decrease. Thereafter (after the time s3), the
pressure of low-pressure stage-side discharge gas (the pressure of high-pressure stage-side
suction gas) further gradually increases and gets into a stabilized state at the pressure
P_13, and the gas load on the discharge stage side gradually decreases and gets into
a stabilized state at the gas load GL_2.
[0035] According to the operation control method for a BOG multistage displacement compressor
of Embodiment 1 of the present invention, the increase in load (gas load) due to the
differential pressure between suction gas and discharge gas in the high-pressure stage
compression unit can be suppressed to prevent the gas load from exceeding the allowable
gas load GL_h.
<Example>
[0036] Example of the operation control method for a BOG multistage displacement compressor
according to Embodiment 1 of the present invention is then described with reference
to Fig. 1. In the steady operation of the multistage displacement compressor, both
the primary side compression unit 9 and the secondary side compression unit 10 are
operated with 100% load (Comparative Example-1 of Table 1). According to the BOG multistage
displacement compressor of Embodiment 1 of the present invention, when it is under
the predetermined state, for example, at the start-up, the primary side compression
unit 9 and the secondary compression unit 10 are operated with 100% load and with
75% load, respectively, by the capacity adjusting devices 21 (the suction valve unloader
9a, the head end unloader 9b, etc.) and 22 (the suction valve unloader 10a, the head
end unloader 10b, etc.) of the primary side compression unit 9 and the secondary compression
unit 10. Namely, the load ratio in start-up is larger than the load ratio in steady
operation (refer to Example of Table 1).
[0037] In Table 1, start-up operation states of the BOG multistage displacement compressor
according to a related art. are also given for showing the effects of the present
invention. One operation state is an operation form (Comparative Example-2 of Table
1) in which both the primary side compression unit 9 and the secondary side compression
unit 10 are operated with 100% load. The other operation state is an operation form
(Comparative Example-3) in which both the primary side compression unit 9 and the
secondary side compression unit 10 are operated with 75% load. In both Comparative
Example-2 and Comparative Example-3, the load ratio is equal to the load ratio in
steady operation.

[0038] Table 2 shows suction/discharge gas pressures of the primary side compression unit
9 and the secondary side compression unit 10, and compression loads and tension loads
of the primary side compression unit 9 and the secondary side compression unit 10,
in each operation form of Table 1. As shown in Comparative Example-1 of Table 2, in
the steady operation state of the BOG multistage displacement compressor, the compression
load on the secondary side is about 8,990 kgf. However, in the start-up operation
state of the BOG multistage displacement compressor according to a related art, the
compression load on the secondary side reaches a level exceeding the allowable gas
load of 9,000 kgf, concretely, 9,040 kgf in Comparative Example-2 and 9,020 kgf in
Comparative Example-3, as shown in Comparative Examples-2 and 3 of Table 2.
[0039] On the other hand, in the start-up operation state of the BOG multistage displacement
compressor according to Embodiment 1 of the present invention, the compression load
on the secondary side is suppressed to a level not exceeding 9,000 kgf of the allowable
gas load, concretely, to 8,400 kgf, as shown in Example of Table 2. This effect of
Example is obtained by setting the load ratio in the start-up operation larger than
that in the steady operation.

[0040] Next, in order to explain an operation control method for a BOG multistage displacement
compressor according to Embodiment 2 of the present invention, a case in which a screw
compressor is applied to the BOG multistage compressor is given as an example with
reference to Fig. 3. Fig. 3 is a systematic diagram of LNG and BOG treatment equipment
to which the operation control method for a BOG multistage displacement compressor
of Embodiment 2 of the present invention is applied.
[0041] Embodiment 2 of the present invention has the same structure as the above-mentioned
Embodiment 1 except that the type of the BOG multistage displacement compressor and
the structures of the low-pressure stage capacity adjusting device and the high-pressure
stage capacity adjusting device are differed from those in Embodiment 1. Therefore,
about the differences from the above-mentioned Embodiment 1 are described below while
assigning the same reference numbers to the same ones as Embodiment 1.
[0042] Namely, according to the operation control method for a BOG multistage displacement
compressor of Embodiment 1, in the BOG multistage compressor 8 composed of a reciprocating
compressor, the capacity of the low-pressure stage compression unit 9 is adjusted
by the low-pressure stage capacity adjusting device 21 including the suction valve
unloader 9a and the head end unloader 9b, and the capacity of the high-pressure stage
compression unit 10 is adjusted by the high pressure capacity adjusting device 22
including the suction valve unloader 10a and the head end unloader 10b.
[0043] In contrast, according to the operation control method for a BOG multistage displacement
compressor of Embodiment 2 shown in Fig. 3, in a BOG multistage compressor 18 composed
of a screw compressor, the capacity of a low-pressure stage compression unit 19 is
adjusted by a low-pressure stage capacity adjusting device including a slide valve
19a and the capacity of a high-pressure stage compression unit 20 is adjusted by a
high-pressure stage capacity adjusting device including a slide valve 20a.
[0044] By the operation control method for a BOG multistage displacement compressor according
to Embodiment 2 of the present invention, also, the increase in load (gas load) due
to the differential pressure between suction gas and discharge gas in the high-pressure
stage compression unit 20 can be suppressed similarly to the above-mentioned Embodiment
1, and the gas load can be prevented from exceeding the allowable gas load GL_h.
[0045] In order to explain an operation control method for a BOG multistage displacement
compressor according to Embodiment 3 of the present invention, a case in which a reciprocating
compressor is applied to the BOG multistage compressor will be given as an example
with reference to Fig .4. Fig. 4 is a systematic diagram of LNG and BOG treatment
equipment to which an operation control method for a BOG multistage displacement compressor
according to Embodiment 3 of the present invention is applied.
[0046] Embodiment 3 of the present invention is in common with the above-mentioned Embodiment
1 in many structural paints. Therefore, in Fig. 4, the same reference numbers are
assigned to the same ones as in Embodiment 1 (Fig. 1) to omit the description thereof.
A difference of Embodiment 3 of the present invention from Embodiment 1 is that the
former includes a differential pressure gauge 27 so that a differential pressure ΔP
between the pressure of discharge gas on the low-pressure stage side located at a
position P1 on the intermediate line 12 (the pressure of high-pressure stage-side
suction gas) and the pressure of discharge gas on the high-pressure stage side located
at a position P2 on the discharge line 17 can be detected, while the latter includes
the temperature detector 26 provided in the BOG delivery line 7 so that the temperature
of suction gas on the low-pressure stage side can be detected. The difference is mainly
described below.
[0047] Namely, according to the operation control method of the BOG multistage displacement
compressor 8 of Embodiment 1, the temperature of low-pressure stage-side suction gas
is detected by the temperature detector 26 interposed in the BOG delivery line 7,
and the operation control is performed by the operation control method (1). In contrast,
according to the operation control method of the BOG multistage displacement compressor
8 of Embodiment 3 shown in Fig. 4, the differential pressure ΔP between the pressure
of low-pressure stage side discharge gas (the pressure of high-pressure stage-side
suction gas) and the pressure of high-pressure stage side discharge gas is detected
by the differential pressure gauge 27, and the operation control is performed by an
operation control method (2) which will be described below.
<Operation Control Method (2)>
[0048] According to this operation control method (2), when the BOG multistage compressor
8 is started, the controller 25 compares the differential pressure ΔP detected by
the differential pressure gauge 27 with a firs set differential pressure ΔP1 that
is preset within the controller 25. When the differential pressure ΔP detected by
the differential pressure gauge 27 is larger than the first set differential pressure
ΔP1, the controller 25 controls the operation of the low-pressure stage capacity adjusting
device 21 and the high-pressure stage capacity adjusting device 22 so that the load
ratio R is larger than that in steady operation.
[0049] For example, the control is performed so that the low-pressure stage-side load is
100% and the high-pressure stage-side load is 75%. Accordingly, the gas load on the
high-pressure stage side is suppressed to a level lower than the allowable gas load
GL_h.
[0050] The controller 25 further compares the differential pressure ΔP detected by the differential
pressure gauge 27 with a second set differential pressure ΔP2. As a result of the
comparison, when the differential pressure ΔP detected by the differential pressure
gauge 27 is smaller than the second differential pressure ΔP2, the controller 25 controls
the operation of the low-pressure stage capacity adjusting device 21 and the high-pressure
stage capacity adjusting device 22 so that the load ratio R is equal to that in steady
operation. For example, the low-pressure stage-side load is set to 100%, and the high-pressure
stage-side load is set to100%. The second set differential pressure ΔP2 is preset
to a value smaller than the first set differential pressure ΔP1.
[0051] In this way, according to the operation control method for a BOG multistage displacement
compressor of Embodiment 3 of the present invention, the operation form is decided
based on the differential pressure ΔP on the high-pressure stage side that is directly
related to the gas load on the high-pressure stage side. Thus, even if the pressure
of discharge gas on the high-pressure stage side is fluctuated due to fluctuation
of the quantity of gas demanded by supply destination or the like, the increase in
load (gas load) due to the differential pressure between suction gas and discharge
gas in the high-pressure stage compression unit 10 can be surely suppressed and the
gas load can be prevented from exceeding the allowable gas load GL_h.
[0052] Next, in order to explain an operation control method for a BOG multistage displacement
compressor according to Embodiment 4 of the present invention, a case in which a reciprocating
compressor is applied to the BOG multistage compressor is given as an example with
reference to Fig. 5. Fig. 5 is a systematic diagram of LNG and BOG treatment equipment
to which the operation control method for a BOG multistage displacement compressor
of Embodiment 4 of the present invention is applied.
[0053] Embodiment 4 of the present invention is in common with the above-mentioned Embodiment
1 in many structural points. Therefore, in Fig. 5, the same reference numbers are
assigned to the same members as in Embodiment 1 (Fig. 1) to omit the description thereof.
Differences between Embodiment 4 of the present invention and Embodiment 1 are mainly
described below.
[0054] In the BOG multistage displacement compressor according to Embodiment 4 of the present
invention, a three-way valve 13 is connected to the middle of the intermediate line
12. The three-way valve 13 includes, at its outlet side, a selector port 13a connected
to the intermediate line 12, and a selector port 13b connected to a bypass line 14.
A cooler 15 is connected to the bypass line 14, and its downstream side is connected
to the intermediate line 12. The cooler 15 includes a cooling pipe 16 through which
cooling water such as seawater is passed. The selector ports 13a, 13b of the three-way
valve 13 are switched by a control valve 28, and the control valve 28 is controlled
by the controller 25 for the opening/closing.
<Operation Control Method (3)>
[0055] In this operation control method (3), when the BOG multistage compressor 8 is started,
the controller 25 compares a temperature of low-pressure stage-side suction gas detected
by the temperature detector 26 with a set temperature T1 (e.g., -45°C) that is preset.
As a result of the comparison, when the temperature is higher than the set temperature
T1, the controller 25 controls the control valve 28 for the opening/closing so that
an outlet port of the three-way valve 13 is switched to the selector port 13b. Simultaneously,
the controller 25 controls the operation of the low-pressure stage capacity adjusting
device 21 and the high-pressure stage capacity adjusting device 22 so that the load
ratio R is larger than that in steady operation. For example, the control is performed
so that the low-pressure stage side load is 100%, and the high-pressure stage side
load is 75%.
[0056] BOG from the BOG delivery line 7 is compressed by the low-pressure stage compression
unit 9, branched from the intermediate line 12 at the selector port 13b of the three-way
valve 13, introduced into the bypass line 14, and cooled by the cooler 15. The BOG
is then merged into the intermediate line 12 to enter the high-pressure stage compression
unit 10, compressed therein, passes the discharge line 17 and the junction portion
23, and then is supplied together with natural gas in the gas transfer line 6 to the
plant.
[0057] Thereafter, when the temperature of discharge gas from the low-pressure stage compression
unit 9 drops to the set temperature T1 or lower, the controller 25 controls the control
valve 28 for the opening/closing so that the outlet port of the three-way valve 13
is switched to the selector port 13a, and thereby the discharge gas from the low-pressure
stage compression unit 9 is directly supplied from the intermediate line 12 to the
high-pressure stage compression unit 10. Simultaneously, the controller 25 controls
the operation of the low-pressure stage capacity adjusting device 21 and the high-pressure
stage capacity adjusting device 22 so that the load ratio R is equal to that in steady
operation. For example, the operation is performed so that the low-pressure stage-side
load is 100%, and the high-pressure stage-side load is 100%.
[0058] Also according to the operation control method for a BOG multistage displacement
compressor of Embodiment 4 of the present invention, the increase in load (gas load)
due to the differential pressure between suction gas and discharge gas in the high-pressure
stage compression unit 10 can be suppressed to prevent the gas load from exceeding
the allowable gas load GL_h. When the operation of the low-pressure stage capacity
adjusting device 21 and the high-pressure stage capacity adjusting device 22 is controlled
so that the load ratio R is larger than that in steady operation, the gas load on
the low-pressure stage side is increased in stead of reduction of the gas load on
the high-pressure stage side, and the temperature of discharge gas on the low-pressure
stage side rises.
[0059] However, since the operation of the low-pressure stage capacity adjusting device
21 and the high-pressure stage capacity adjusting device 22 is controlled so that
the load ratio R is larger than that in steady operation, and since the other operation
is simultaneously controlled so that the BOG compressed by the low-pressure stage
compression unit 9 passes the bypass line 14, is cooled by the cooler 15, and then
merges into the intermediate line 12 to enter the high-pressure stage compression
unit 10, the temperature of discharge gas on the low-pressure stage side is suppressed
from rising. Therefore, the temperature of discharge gas on the high-pressure stage
side never exceeds an allowable upper limit temperature.
<Example>
[0060] Example of the operation control method for a BOG multistage displacement compressor
according to Embodiment 4 of the present invention is described with reference to
Fig. 5. In the steady operation of the multistage displacement compressor, both the
primary side compression unit 9 and the secondary side compression unit 10 are operated
with 100% load (Comparative Example-4 of Table 3). According to the BOG multistage
displacement compressor of Embodiment 4 of the present invention, under the predetermined
state, for example at the start-up, the primary side compression unit 9 and the secondary
side compression unit 10 are operated respectively with 100% load and with 75% load
by the capacity adjusting devices 21 (the suction valve unloader 9a, the head end
unloader 9b, etc.) and 22 (the suction valve unloader 10a, the head end unloader 10b,
etc.) of the primary side compression unit 9 and the secondary side compression unit
10. Namely, the load ratio in start-up is larger than the load ratio in steady operation
(refer to Example of Table 3).
[0061] In Table 3, start-up operation states of the BOG multistage displacement compressor
according to a related art are given for showing the effects of the present invention.
One operation state is an operation form (Comparative Example-5 of Table 3) where
both the primary side compression unit 9 and the secondary side compression unit 10
are operated with 100% load. The other is an operation form (Comparative Example-6
of Table 3) where both the primary side compression unit 9 and the secondary side
compression unit 10 are operated with 75% load. In both Comparative Example-5 and
Comparative Example-6, the load ratio is equal to the load ratio in steady operation.

[0062] Table 4 shows suction/discharge gas pressures of the primary side compression unit
9 and the secondary side compression unit 10 in each operation form of Table 1, and
compression loads and tension loads of the primary side compression unit 9 and the
secondary side compression unit 10. As shown in Comparative Example-4 of Table 3,
when the BOG multistage displacement compressor is in the steady operation state,
the compression load on the secondary side is about 8,490 kfg. However, when the load
ratio in start-up is set to the same value as the load ratio in steady operation,
the compression load on the secondary side reaches 9,230 kgf (1.09 times that in Comparative
Example 4) and 8,970 kgf (1.06 times that in Comparative Example-4) as shown in Comparative
Examples-5, 6 in Table 4.
[0063] On the other hand, in the start-up operation state of the BOG multistage displacement
compressor according to Embodiment 4 of the present invention, as shown in Example
of Table 4, the compression load on the secondary side is suppressed to 8,540 kgf
(1.01 times that in Comparative Example-4). In Example, the effect is obtained by
setting the load ratio to a value larger than that in steady operation and performing
the operation control so that the BOG compressed by the low-pressure stage compression
unit 9 passes the bypass line 14, is cooled by the cooler 15, and then merges into
the intermediate line 12 to enter the high-pressure stage compression unit 10.

[0064] As described so far, according to the operation control method for a BOG multistage
displacement compressor of the present invention, under a predetermined state, operation
control is performed so that the ratio (road ratio) of the load R of the low-pressure
stage compression unit to the load of the high-pressure stage compression unit in
the BOG multistage displacement compressor is larger than those under states other
than the predetermined state. The operation performed with a large load ratio R means
that compression ratio of the low-pressure stage compression unit is increased, and
compression ratio of the high-pressure stage compression unit is decreased. Accordingly,
the gas load on the high-pressure stage side is reduced. Thus, even if the low-pressure
stage-side suction gas has a temperature higher than that in steady operation, the
gas load on the high-pressure stage side can be prevented from exceeding an allowable
gas load.
[0065] In the above-mentioned embodiments, the operation control method for a BOG multistage
displacement compressor according to the present invention has been described while
taking a reciprocating compressor and a screw compressor as examples, and the capacity
adjusting device is explained while taking a suction valve unloader, a head end unloader
and a slide valve as examples. However, the present invention is never limited to
them, but the operation control method for a BOG multistage displacement compressor
according to the present invention can be applied to various types of displacement
compressors and BOG displacement compressors including capacity adjusting devices
of various configurations.
[0066] Provided is an operation control method for a BOG multistage displacement compressor
including connected multiple displacement compression units for compressing boil off
gas (BOG) generated from liquefied natural gas, including: under a predetermined state,
performing operation control so that the ratio (load ratio) of a load of a low-pressure
stage compression unit to a load of a high-pressure stage compression unit in the
BOG multistage displacement compressor is larger than load ratios under states other
than the predetermined state. Namely, the BOG multistage displacement compressor is
configured so that suction temperature in the low-pressure stage compression unit
can be detected, and the predetermined state is set to a state where the detected
temperature of the suction temperature is equal to or higher than a set temperature
that is preset. According to such a method, even if the low-pressure stage side suction
gas has a temperature higher than that in steady operation, the load (gas load) due
to a differential pressure between suction gas and discharge gas on the high-pressure
stage side can be prevented from exceeding an allowable gas load.