JP4996987B2 - Reliquefaction device and reliquefaction method for BOG generated in LNG storage tank - Google Patents

Description

  The present invention relates to a re-liquefaction device and a re-liquefaction method for BOG generated in an LNG (liquefied natural gas) storage tank.

  LNG (liquefied natural gas) stored in the LNG storage tank is in a gas-liquid equilibrium state at normal pressure and -160 ° C, but this tank has a heat insulation structure, but it is not a little natural heat input. As a result, BOG (boil-off gas) is generated and the tank is filled. In the conventional LNG discharge mechanism from the LNG storage tank, in order to reuse this BOG, a BOG discharge line and an LNG discharge line are constructed from the storage tank, and both lines are passed through a heat exchanger to pass the BOG to the LNG. The BOG liquefied in the heat exchanger with cold heat (re-liquefaction) and then re-liquefied BOG is temporarily stored in the drum. This re-liquefied BOG is discharged from the drum, pressurized, sent to the vaporizer after joining the LNG discharge line, and vaporized with seawater etc. in the vaporizer to become city gas. In general, the BOG is reliquefied in such a flow.

  Here, the reason why BOG is once again liquefied is that the required energy of the pump at the time of delivery can be remarkably reduced to about 1/40 in comparison with the case where the BOG is delivered to the vaporizer in the gas state.

  By the way, the operating pressure of the apparatus described above is determined by the reliquefaction unit, that is, the value of the cooling LNG amount with respect to the BOG amount (cooling LNG amount / BOG amount). For example, if the load (load) increases and a BOG cooling heat source, that is, a large amount of cooling LNG can be secured, the operating pressure decreases and the power cost of the compressor or the like can be reduced, but the load decreases and the amount of cooling LNG If the sufficient pressure cannot be secured, the pressure in the drum rises and the operating pressure also rises, so that the power cost of the compressor and the like also increases.

  In Patent Document 1, the pressure and liquid temperature in the drum are constantly sensed, and the control valve is controlled to open when the drum pressure reaches a regression value containing several percent of nitrogen from the BOG reliquefaction liquid temperature or the drum pressure reaches an upper limit value. However, a device is disclosed in which the control valve is controlled to be closed when the drum pressure reaches a lower limit value or the like. According to this apparatus, even when the amount of LNG is large and the operating pressure is reduced and non-condensable gas is accumulated in the drum, it can be discharged out of the system. Driving in a state that suppresses the rise is possible.

JP 7-157782 A

  By the way, when the BOG treatment is less than the minimum flow of the reliquefied BOG booster pump, the reliquefied BOG drum cannot be returned and the liquid level is lowered, so that the liquid level cannot be controlled. Therefore, the BOG processing amount must be operated with a minimum flow amount of the pump. Here, since the BOG process below the minimum flow of the reliquefied BOG booster pump is extremely difficult, in this case, only the operation with a narrow operating range can be performed. In addition, while the minimum flow of the reliquefied BOG booster pump is about 30% to the capacity, the reliquefaction apparatus may be required to reduce its operation to about 25%. Therefore, the present situation is that such a request cannot be satisfied when the BOG processing below the minimum flow is difficult.

  Further, cavitation in the booster pump occurs when the BOG throughput decreases and the follow-up of the LNG flow rate is delayed and the liquefaction unit increases. For this reason, there are currently great restrictions on the operation of the apparatus, such as an operation in which fluctuation of the BOG processing amount is suppressed as much as possible or a constant load operation. For example, when the reliquefaction basic unit is 7, when the operation is reduced from 50% load to 25% load, the BOG amount is halved first, and the cooling LNG amount is controlled to follow thereafter. Initially, the liquefaction unit is doubled to 14. It is to be noted that if the amount of LNG for cooling is reduced in advance, the temperature of the reliquefied BOG increases and the pressure for reliquefying increases, so that the compressor with a large capacity is controlled. Is necessary. Here, the reliquefied BOG temperature decreases and the vapor pressure also decreases. As a result of the re-liquefied BOG in the low temperature and low vapor pressure state after the operation being lowered flowing in from the upper part of the drum that controls the drum pressure, the drum pressure is rapidly decreased. This decrease in drum pressure leads to a decrease in pressure in the reliquefied BOG boost pump suction. However, at the initial stage, the pump suction is filled with reliquefied BOG at a high temperature and high vapor pressure before the operation is reduced, and the pump suction pressure is likely to cause foaming of the liquid in the suction, which may cause cavitation. Get higher. Therefore, in order to avoid such restrictions, measures have been taken to prevent cavitation by placing a sufficient liquid head on the pump suction (suction side) so that the liquid in the suction does not foam even when the pump suction pressure is reduced. Yes.

  A conventional BOG reliquefaction apparatus in which the above measures are taken is shown in FIGS. The reliquefaction apparatus 100 shown in FIG. 4 includes an LNG storage tank 1 in which liquefied natural gas L is accommodated and filled with boil-off gas G, a discharge pump 3 placed in the tank 1, and boil-off gas. The main discharge line of G, the BOG discharge line 4a for guiding the boil-off gas G slightly pressurized by the intermediate pressure compressor 5a to the heat exchanger 7A, and the LNG from the LNG storage tank 1 to the heat exchanger 7A as well. The payout line 2 includes a drum 8 that temporarily stores reliquefied BOG, and a booster pump 10 that pressurizes the reliquefied BOG discharged from the drum 8 and sends it to the vaporizer 12. . The BOG discharge line 4a is provided with a BOG flow meter 6 and the drum 8 is provided with a liquid level meter 9 for measuring the liquid level of the reliquefied BOG. The liquid level control valve 11 opens and closes so that the fluctuation is appropriately measured and the effective suction head on the pump suction side of the booster pump 10 is equal to or higher than the required NPSH (Net Positive Suction Head) required by the pump. It is like that. When the reliquefied BOG is less than the minimum flow of the booster pump 10, the intermediate pressure compressor 5a is stopped, the high pressure compressor 5b is operated to increase the pressure to a predetermined pressure, and the direct discharge is executed in a separate BOG discharge line 4b. It has become so.

  In the reliquefaction apparatus 100, the mounting height H1 of the drum 8 is set to a very high position in order to give a sufficient liquid head to the pump suction, and as a result, the apparatus scale must be increased. I don't get it.

  On the other hand, the reliquefaction apparatus 200 shown in FIG. 5 is configured to place the reliquefaction container 7B on the drum 8 instead of the heat exchanger 7A, mix BOG and LNG in the reliquefaction container 7B, LNG containing liquefied BOG is guided to the drum. In the reliquefaction apparatus 200 as well, as with the reliquefaction apparatus 100, the mounting height H2 of the drum 8 needs to be set to a very high position.

  The present invention has been made in view of the above-mentioned problems, and can eliminate the increase in the size of the apparatus or equipment caused by installing a drum for temporarily storing the reliquefied BOG at a high place, thereby increasing the pressure of the reliquefied BOG. The re-liquefaction device for BOG generated in the LNG storage tank, which can effectively suppress cavitation of the pump and can easily control the liquid level in the re-liquefied BOG drum even during the minimum flow of the re-liquefied BOG booster pump And to provide a reliquefaction method.

  In order to achieve the above-mentioned object, the re-liquefaction device for BOG generated in the LNG storage tank according to the present invention increases the pressure of BOG generated in the LNG storage tank by a compressor and then guides it to a heat exchanger or re-liquefaction container to store the LNG. The discharged LNG is guided from the tank to the heat exchanger or the reliquefaction container, and the BOG introduced into the heat exchanger or the reliquefaction container is cooled by the discharged LNG to be reliquefied, and this reliquefied BOG is put into the drum. A part of the discharge LNG led from the LNG storage tank to the heat exchanger or reliquefaction container in the re-liquefaction device for BOG generated in the LNG storage tank, which is temporarily stored and boosted by the booster pump and then sent to the vaporizer Is led to the pump suction of the booster pump through the control valve as supercooled LNG.

  The BOG re-liquefaction apparatus of the present invention relates to an apparatus that facilitates liquid level control at the time of low load (during low operation) in a drum that stores re-liquefied BOG, and can suppress cavitation of a booster pump of re-liquefied BOG. For this purpose, the supercooling LNG is directly guided to the suction (suction side) of the booster pump as necessary. In addition, the supercooling LNG means LNG having a temperature lower than the saturation temperature at an arbitrary pressure in a graph showing a relationship between an arbitrary saturation temperature and a saturation pressure. In addition, as described above, this apparatus re-liquefies the BOG without mixing the BOG and the cooling LNG in the heat exchanger, and mixes the BOG and the discharge LNG in the re-liquefaction container. There are two forms of re-liquefying and introducing the total LNG containing re-liquefied BOG to the drum, both of which are included.

Here, as one embodiment of the apparatus of the present invention, effective suction head calculation means for calculating the effective suction head of the boost pump as needed, storage means for storing the reference value of the effective suction head and the minimum flow amount of the boost pump And a deficient amount calculating means for calculating a deficient amount that is deficient in the minimum flow amount of the booster pump, and the control valve is controlled to open and close the control valve. A first control method for controlling the discharge amount of the supercooling LNG so that a calculation result by the head calculation means is equal to or greater than a reference value of the effective suction head; and the undercooling amount calculated by the shortage amount calculation means There is also a form in which the payout amount is controlled by selecting either one of the second control methods for the LNG payout amount.

  In the first control method, for example, when the load is reduced from 100% to 75% or the like, the pressure in the drum also decreases, and the supercooling LNG required to suppress the occurrence of cavitation that may occur in the booster pump when the load decreases. The amount of discharge is calculated and forcibly introduced into the pump suction of the booster pump. When a relatively low-temperature reliquefied BOG enters the drum, a predetermined amount of supercooled LNG is supplied to the suction side, causing fluctuations in the effective suction head due to fluctuations in the saturation pressure in the drum. Even in this case, the occurrence of cavitation that may occur in the booster pump is effectively suppressed. In particular, when the load is equal to or lower than the minimum flow of the booster pump, the second control method uses the second amount of control to reduce the amount of deficiency calculated by the deficiency calculation means to the overcooling LNG. This is also forcibly introduced into the pump suction of the booster pump.

  Here, in the first control, the pressure drop due to the reliquefied BOG is offset by the vapor pressure drop, and the effective suction head is set to the reference value of the effective suction head, that is, the required NPSH (Net Positive Suction Head required by the pump). ) Flow control is performed so that the above is achieved.

  The booster pump used in the LNG dispenser has a minimum flow of 35% load, for example. If the load condition is less than this value (for example, 25% load), the difference amount is There is also a measure to ensure a minimum flow while assuring with circulating fluid (a part of the reliquefied BOG that has passed through the pump is returned to the pump), but there is a problem that new BOG is generated in this circulation process. Therefore, as described above, by adopting a configuration in which the supercooling LNG corresponding to the differential amount is guided to the suction of the booster pump, the generation of the new BOG is suppressed, and the load condition below the minimum flow of the booster pump is also included. The booster pump can be operated at minimum flow or higher.

  Specifically, a part of the discharge LNG led from the LNG storage tank to the heat exchanger or the reliquefaction vessel is led as supercooled LNG to the pump suction of the booster pump via the control valve. For example, after boosting the BOG generated in the LNG storage tank with a compressor, the BOG flow rate in the vaporized state is measured with a sensor on the line leading to the heat exchanger or reliquefaction vessel as needed, and the liquid temperature and level in the drum are Measured from time to time with a temperature sensor and a liquid level meter. From these measured values, the effective suction head calculation means calculates the amount of change in the head corresponding to the drum pressure, and the vapor-liquid equilibrium with respect to the drum liquid level head and pump suction liquid temperature. The change amount of the head corresponding to the pressure and the change amount of the head corresponding to various pressure losses are calculated, and based on these, the discharge amount of the supercooling LNG necessary for the effective suction head to become the required NPSH or more is calculated. . Next, a predetermined amount of supercooling LNG can be supplied to the suction of the booster pump by opening and closing the control valve at an appropriate opening degree in order to pay out the calculated amount of supercooling LNG.

  Further, there is an embodiment in which a large value is selected from among the payout amounts of the respective subcooling LNG by the first and second control methods, and the payout amount is controlled. This is a control method for realizing both prevention of cavitation of the re-liquefied BOG boost pump and BOG processing operation below the minimum flow, particularly when the boost pump is below the minimum flow.

  According to the BOG reliquefaction apparatus of the present invention, the suction level of the booster pump can be reduced without the need to place the drum at a high place in addition to the control of the liquid level in the drum at the time of low load and the suppression of cavitation of the booster pump. Since the effective suction head can be secured above the required NPSH, the scale can be reduced as compared with the conventional apparatus, and the manufacturing cost can be greatly reduced.

  In another embodiment of the re-liquefaction apparatus for BOG generated in the LNG storage tank according to the present invention, the amount of discharge of the supercooled LNG required at the maximum load of the effective suction head of the booster pump is set. The first payout amount and the second payout amount when the payout amount of the supercooling LNG corresponding to the shortage amount deficient in the minimum flow amount of the booster pump is set as the second payout amount. A large value is selected from among the amounts, and is preliminarily defined as a fixed value of the payout amount, and the control valve performs discharge control of the supercooled LNG with the fixed amount of payout. Is.

  In the embodiment of the reliquefaction apparatus of the present invention, the required amount of supercooled LNG discharged is calculated in advance and set as a fixed value, and when necessary, the supercooled LNG corresponding to this fixed value amount is pumped to the booster pump. It is set so as to be supplied to the suction, and by doing so, it becomes unnecessary to control the supply amount of the supercooling LNG according to the load fluctuation, and a simpler system can be constructed.

  The method for reliquefying BOG generated in the LNG storage tank according to the present invention includes a step of guiding the BOG generated in the LNG storage tank to a heat exchanger or a reliquefaction container, and a heat exchanger or reliquefaction from the LNG storage tank. A step of guiding the discharge LNG to the container and cooling the BOG introduced into the heat exchanger or the reliquefaction container by the discharge LNG, and a step of temporarily storing the reliquefied BOG in the drum; And a step of sending the pressure to the vaporizer after being boosted by a booster pump, and a method for re-liquefying BOG generated in the LNG storage tank, comprising a part of the discharge LNG led from the LNG storage tank to the heat exchanger or re-liquefaction container Further comprising a step of leading to pump suction of the booster pump as supercooled LNG Is shall.

  According to the BOG reliquefaction method of the present invention, the liquid level in the drum can be very easily controlled even if the load fluctuates, and cavitation that can occur in the booster pump can be effectively suppressed.

  Here, one embodiment of the step of leading the supercooling LNG to the pump suction of the boost pump calculates the effective suction head of the boost pump, compares the effective suction head with its reference value, and the effective suction head is the reference. When the amount of subcooled LNG satisfying at least the reference value when it is less than the first value is led to the pump suction of the booster pump, or when the re-liquefied BOG amount does not satisfy the minimum flow amount of the booster pump One of the second steps for leading the insufficient amount of supercooled LNG to the pump suction of the booster pump is selected.

  Furthermore, the method further includes a third step of comparing the payout amounts of the respective subcooling LNGs in the first and second steps, and selecting a large value to determine the payout amount. It may be.

  In addition, the supercooling LNG delivery amount required at the maximum load of the effective suction head of the booster pump is set as a first delivery amount, and the supercooling corresponding to the shortage amount insufficient for the minimum flow amount of the booster pump. When the LNG payout amount is the second payout amount, a large value is selected from the first payout amount and the second payout amount, and the fixed value of the payout amount is defined in advance. There is also an embodiment in which the supercooled LNG with a value delivery amount is led to the pump suction of the booster pump.

  According to the present embodiment, there is no need to obtain the amount of supercooled LNG dispensed according to the load fluctuation, and this method can be executed with a simple system.

  As can be understood from the above description, according to the re-liquefaction device and re-liquefaction method for BOG generated in the LNG storage tank according to the present invention, the cavitation of the re-liquefaction BOG booster pump can be performed without increasing the size of the device or equipment. In addition, the liquid level in the reliquefied BOG drum can be easily controlled even during the minimum flow of the reliquefied BOG booster pump.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an embodiment of a BOG reliquefaction apparatus of the present invention, and FIG. 2 is an enlarged configuration diagram of a vaporizer and a drum. FIG. 3 is a configuration diagram of another embodiment of the BOG reliquefaction apparatus of the present invention. In addition, the same code | symbol shall be attached to the structure same as FIG. 4, 5 which showed the conventional BOG reliquefaction apparatus.

  FIG. 1 is a configuration diagram showing an embodiment of a BOG reliquefaction apparatus of the present invention. This BOG reliquefaction apparatus 300 is a reliquefaction apparatus having a heat exchanger 7A. Specifically, an LNG storage tank 1 in which liquefied natural gas L is stored and boil-off gas G is filled above, a discharge pump 3 placed in the tank 1, and a boil-off gas G discharge line The BOG discharge line 4a for introducing the boil-off gas G slightly pressurized by the intermediate pressure compressor 5a to the heat exchanger 7A, the LNG discharge line 2 that similarly leads from the LNG storage tank 1 to the heat exchanger 7A, A drum 8 that temporarily stores the liquefied BOGL 1 and a booster pump 10 that pressurizes the re-liquefied BOG discharged from the drum 8 and sends it to the vaporizer 12. The BOG discharge line 4a is provided with a BOG flow meter 6, and the drum 8 is provided with a liquid level meter 9a for measuring the liquid level of the reliquefied boil-off gas L1, and a thermometer 9b for measuring the liquid temperature. The fluctuation of the liquid level of the reliquefied boil-off gas L1 in the drum 8 and the fluctuation of the liquid temperature are appropriately measured, and an effective suction head on the pump suction side of the booster pump 10 is required according to these fluctuations. The supercooling LNG introduction valve 11a is configured to be openable and closable so as to be equal to or higher than NPSH.

  Here, opening / closing control of the supercooling LNG introduction valve 11a is executed by the control unit 13. Here, measurement data on the liquid level, liquid temperature, BOG flow rate, etc., load change command signal of the apparatus, etc. Is calculated by the built-in CPU, and the opening / closing control of the valve 11a is executed in order to directly supply the required amount of supercooled LNG to the pump suction of the booster pump 10.

  Next, the internal configuration of the control unit 13 will be described. In the control unit 13, an effective suction head calculation unit for calculating the effective suction head of the booster pump 10, a required NPSH serving as a reference value for the effective suction head, and a minimum flow amount (for example, 35% load) of the booster pump 10 A storage unit for storing each of the input data and the like, and an insufficient amount calculation unit for calculating an insufficient amount that is insufficient for the minimum flow amount of the booster pump 10. Furthermore, the first setting unit that sets the discharge amount of the supercooling LNG so that the calculation result by the effective suction head calculation unit is equal to or greater than the required NPSH, and the minimum flow, the shortage amount calculated by the shortage amount calculation unit is excessive. A second setting unit that sets the amount of cooling LNG to be dispensed is incorporated. Further, a third setting unit for comparing the amount of the supercooling LNG payout amount set by the first setting unit and the amount of supercooling LNG payout set by the second setting unit is further incorporated. Are connected to the CPU via a bus, and each part is executed.

  When the apparatus is operating under conditions of the minimum flow or less due to the load condition, the amount of supercooled LNG dispensed set by the third setting unit is provided to the pump suction of the booster pump 10. On the other hand, when the apparatus is operating at the minimum flow or more and the load is changed (usually, the load is lowered), the amount of discharge of the supercooled LNG set in the first setting unit is the pump of the booster pump 10. By providing the suction, cavitation in the booster pump 10 is effectively suppressed.

  In FIG. 1, the amount of supercooled LNG dispensed (for example, Q ′) required to prevent cavitation in the booster pump 10 due to a change in load is an excess required for liquid level control in the drum 8. If it is larger than the cooling LNG payout amount (for example, Q ″), the payout amount required to prevent cavitation is selected. In this case, the supercooling LNG introduction valve 11a is set to the payout amount. The supercooled LNG is supplied to the pump suction, and after passing through the booster pump 10, the liquid level control valve 11b removes the difference (Q′−Q ″), so that the liquid level in the drum is also controlled to be constant. .

Figure 2 is an enlarged view of the vaporizer 7A and the drum 8, a system which combines the lower receiving reliquefaction BOG drum H _D change suppressing type. Here, the change in H _D is to be governed by the liquid surface temperature of the drum 8, by accepting reliquefaction BOG conduits 14 from the carburetor 7A in liquid in the lower drum 8, the liquid surface temperature change suppressing can, it is possible to suppress a change in H _D at reduced load.

  Returning to FIG. 1, the BOG reliquefaction apparatus 300 is a system that effectively suppresses cavitation by directly providing the pump suction with the necessary amount of supercooled LNG against the load fluctuation. There is no need to secure the effective head by placing the drum at a high place. Therefore, both the height to the drum 8: h1 and the height to the upper portion of the heat exchanger 7A above it: h2 can be set much lower than those of the conventional BOG reliquefaction apparatus. In particular, regarding the height: h1, there is no problem in theory even if the height is set to zero or a height close thereto. Therefore, the scale of the entire apparatus can be made as small as possible, and the manufacturing cost of the entire apparatus can be reduced.

  FIG. 3 is a configuration diagram showing another embodiment of the BOG reliquefaction apparatus of the present invention. This BOG reliquefaction device 400 is a reliquefaction device having a reliquefaction container 7B. The difference from the BOG reliquefaction apparatus 300 is that the heat exchanger 7A has been replaced with the reliquefaction container 7B. In this apparatus form, the discharge BOG and the discharge LNG are mixed in the reliquefaction container 7B to generate a reliquefied BOG. All of the LNG including the reliquefied BOG is directly stored in the drum 8. Therefore, the scale of the drum 8 becomes larger than that of the BOG reliquefaction apparatus 300, and the booster pump 10 having higher performance and higher output is used. However, the configuration in which the necessary amount of supercooling LNG is directly provided to the pump suction of the booster pump 10 and the opening / closing adjustment of the supercooling LNG introduction valve by the control unit 13 are the same as in the BOG reliquefaction apparatus 300.

  Also in the BOG reliquefaction device 400, the height to the drum 8: h3 and the height above the reliquefaction container 7B: h4 are both set much lower than the conventional BOG reliquefaction device. can do.

  Next, an outline of a method for controlling the discharge amount of the supercooled LNG will be described.

  The supercooling LNG to prevent cavitation of the reliquefied BOG boost pump at the time of load reduction cancels the reliquefied pressure drop by lowering the vapor pressure, and the effective suction head of the pump exceeds a certain value (NPSH required by the pump) The amount of payout required at this time is defined as a flow rate Q1.

[Formula 1]
_{H sv = H D + h s} - h v - h l ≧ (NPSH of the pump needs)
Here, H _sv : Effective suction head (NPSH) (m), H _D : Head (m) corresponding to drum pressure, h _s : Drum liquid level head (drum liquid level height from pump suction) (m) , H _v : head (m) corresponding to gas-liquid equilibrium pressure with respect to pump suction liquid temperature, h _l : head (m) corresponding to pressure loss of suction pipe.

Note that the above equation is re-liquefaction pressure decrease amount when lowering the load the change amount [Delta] H _D of H _D, in order to hold the net positive suction head Hsv above a predetermined value (NPSH pump needs) is subcooled LNG is introduced, -hv is increased, and flow control of the supercooled LNG is executed so that ΔH _D + Δ (−hv−hl) ≧ 0. If Δhl is small, ΔH _D + Δ (−hv) ≧ 0 may be satisfied.

  After the load is lowered, the liquid temperature of the entire drum 8 is lowered, the pump suction temperature of the booster pump 10 is also lowered, and when there is no possibility of cavitation, the flow rate Q1 is set to 0 (the provision of the supercooling LNG is stopped). ).

  When the reliquefaction device is operating below the minimum flow of the booster pump 10, an amount of supercooling LNG that is equal to the amount that is insufficient for the minimum flow amount is introduced into the pump suction (this flow rate is referred to as flow rate Q2). While operating the pump, the liquid level in the drum 8 is controlled to a constant value.

  On the other hand, when the re-liquefaction apparatus is operated below the minimum flow of the booster pump 10, the larger one of the flow rates Q1 and Q2 is used in order to simultaneously realize the cavitation prevention of the booster pump 10 and the BOG processing operation below the minimum flow. A value is selected and introduced into the pump suction of the booster pump 10.

Here, there is also a method of introducing the flow rate Q1 with a fixed value. In this case, by obtaining the maximum value of [Delta] H _D, if securing a fixed value supercooling LNG flow rate commensurate with the delta (-hv) corresponding thereto, fine control of the supercooling LNG flow rate is not necessary. In this case, since the occurrence of cavitation can be avoided, only the flow rate Q2 may be controlled.

  The control flow rate value is automatically calculated on the system as described above, and the flow rate Q1 is calculated from the flow rate change amount, the heat exchanger performance (UA), the liquid temperature in the drum, the physical property value, etc. Q2 is calculated from the flow rate change amount and the minimum flow amount.

  A control method in the case of the BOG reliquefaction apparatus 300 will be described using measured values while comparing with a comparative example.

  At 100% load, BOG throughput is 20 t / h, re-liquefaction basic unit is 7 and minimum flow is 35%. Here, the flow rate: Q2 = 20 (t / h) × (35-25) /100=2.0 t / h.

Assuming a case where the load fluctuation is reduced from 50% to 25%, it is equivalent to a liquid head of ΔH _D = 0.437 MPa = 119 (m) ≧ Δhv.

That is, if it is not the device configuration (when there is no, or equivalent [Delta] H _D suppression system), it is necessary to 119m loading the liquid head at the maximum. This makes it necessary to make the equipment very expensive and it is practically difficult to realize the equipment. In this system, the supercooling LNG is introduced so as to satisfy ΔH _D ≧ Δhv, and the liquid head can be suppressed to an appropriate value in construction.

  Here, the supercooled LNG introduction amount (flow rate Q1) is 6.16 t / h. The relationship between the load change and the amount of introduced supercooled LNG (flow rate Q1) is shown in Table 1 below. In addition, there is a method to control the optimum amount by calculating the introduction amount according to the load change, and to introduce the maximum introduction amount as a fixed value in advance in the load change. May be adopted.

  From Table 1, when the load change is 100% → 75% → 50% → 25%, the amount of supercooled LNG introduced as a fixed value is introduced in advance through a fixed orifice of 6.16 t / h or more. It is not necessary to control the flow rate according to the fluctuation, and a simple system can be constructed.

  When the reliquefaction apparatus is operating below the minimum flow of the booster pump 10, since the liquid level control of the drum 8 and the cavitation prevention temperature control of the booster pump 10 must be satisfied at the same time, the larger flow rate Q1, Q2 is adopted. However, when the load change is 50% → 25%, both the above control methods always satisfy Q1> Q2, and therefore, the flow rate Q1 for preventing cavitation is introduced.

  A control method in the case of the BOG reliquefaction apparatus 400 will be described using measured values while comparing with a comparative example.

  At 100% load, BOG throughput is 20 t / h, re-liquefaction basic unit is 7 and minimum flow is 35%. Here, the flow rate is Q2 = 160 (t / h) × (35−25) /100=16.0 t / h.

  The relationship between the load change and the amount of introduced supercooled LNG (flow rate Q1) is shown in Table 2 below.

  Note that the amount of supercooled LNG introduced in the case of the BOG reliquefaction apparatus 400 is easily calculated by the following equation.

[formula]
X = kB (ab) / 100
Where X is the required amount of supercooled LNG introduced (t / h), K is the reliquefaction unit, B is the 100% BOG throughput (t / h), a is the load before load change (%), and b is the load change After load (%).

  As in the first embodiment, it is not necessary to control the flow rate in accordance with the load variation by introducing the supercooled LNG introduction amount as a fixed value through a fixed orifice of 35 t / h or more in advance.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

It is a block diagram of one Embodiment of the BOG reliquefaction apparatus of this invention. It is the block diagram which expanded the vaporizer and the drum. It is a block diagram of other embodiment of the BOG reliquefaction apparatus of this invention. It is a block diagram of one Embodiment of the conventional BOG reliquefaction apparatus. It is a block diagram of other embodiment of the conventional BOG reliquefaction apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Storage tank, 2, 2a ... LNG discharge line, 3 ... Discharge pump, 4a, 4b ... BOG discharge line, 5a ... Medium pressure compressor, 5b ... High pressure compressor, 6 ... BOG flowmeter, 7A ... Heat exchanger, 7B Reliquefaction container, 8 ... Drum, 9, 9a ... Liquid level meter, 9b ... Liquid thermometer, 10 ... Booster pump, 11, 11b ... Liquid level control valve, 11a ... Supercooled LNG introduction valve, 12 ... Evaporation , 13 ... control unit, 14 ... pipeline, 100, 200, 300, 400 ... BOG reliquefaction device, L ... liquefied natural gas (LNG), L1 ... reliquefied boil-off gas (re-liquefied BOG), G ... boil-off gas (BOG)

Claims (4)

After boosting the BOG generated in the LNG storage tank with a compressor, the BOG is guided to a heat exchanger or a reliquefaction vessel, discharged from the LNG storage tank to the heat exchanger or reliquefaction vessel, and led to the heat exchanger or reliquefaction vessel. BOG introduced into the liquefaction container is cooled and re-liquefied, and this re-liquefied BOG is temporarily stored in the drum, and is generated in the LNG storage tank that is pressurized by the booster pump and then sent to the vaporizer. In the BOG reliquefaction device,
A part of the discharge LNG led from the LNG storage tank to the heat exchanger or the reliquefaction container is led as supercooled LNG to the pump suction of the booster pump via the control valve ,
Effective suction head calculating means for calculating the effective suction head of the boost pump, storage means for storing the reference value of the effective suction head and the minimum flow amount of the boost pump, and a shortage amount insufficient for the minimum flow amount of the boost pump An opening / closing control of the control valve is performed by a control mechanism comprising at least a deficient amount calculating means.
The control mechanism includes a first control method for controlling a discharge amount of the supercooling LNG such that a calculation result by the effective suction head calculation unit is equal to or greater than a reference value of the effective suction head, and a shortage calculated by the shortage amount calculation unit. A re-liquefaction device for BOG generated in an LNG storage tank, wherein either one of the second control methods in which the amount corresponding to the amount of supercooled LNG discharged is selected to control the amount discharged. In the re-liquefaction device for BOG generated in the LNG storage tank according to claim 1 , when the load is below the minimum flow of the booster pump , both the first and second control methods are selected, A reliquefaction device for BOG generated in an LNG storage tank, wherein a large value is selected from among the amount of each subcooled LNG discharged by the control method, and the amount discharged is controlled. A step of guiding BOG generated in the LNG storage tank to a heat exchanger or a reliquefaction container; and discharging the LNG from the LNG storage tank to the heat exchanger or the reliquefaction container; guiding the LNG; LNG storage comprising the steps of: cooling the BOG led to re-liquefaction and re-liquefying; temporarily storing the re-liquefied BOG in the drum; In the re-liquefaction method of BOG generated in the tank,
A part of the discharge LNG led from the LNG storage tank to the heat exchanger or the reliquefaction container is further provided as a supercooled LNG and led to the pump suction of the booster pump ;
The step of leading the supercooled LNG to the pump suction of the booster pump includes:
The effective suction head of the boost pump is calculated, the effective suction head is compared with its reference value, and when the effective suction head is less than the reference value, an amount of supercooling LNG that satisfies at least the reference value is used as the pump suction of the boost pump. A first step of guiding;
In the LNG storage tank, which is one of the second step of leading the undercooled LNG to the pump suction of the booster pump when the reliquefied BOG amount does not satisfy the minimum flow amount of the booster pump Of liquefaction of BOG generated in the tank. 4. The method for reliquefying BOG generated in an LNG storage tank according to claim 3 , wherein when the load is equal to or lower than the minimum flow of the booster pump , both the first and second steps are selected, and each step is selected. A method for reliquefying BOG generated in an LNG storage tank, further comprising a third step of comparing the discharge amount of each of the subcooled LNGs according to the above and selecting a large value to determine the discharge amount.

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