JP4437575B2 - Equipment and method for intermittent operation of low boiling point liquid pump - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an intermittent operation facility for intermittently operating a low boiling point liquid pump and a method therefor.
[0002]
[Prior art]
In storage facilities and chemical plants that handle LNG, LPG, etc. (hereinafter referred to as low-boiling liquid), a small amount of low-boiling liquid may be allowed to flow constantly for the purpose of keeping pipes cold. In this case, an airtight container (drum) as illustrated in FIG. 4A is provided, and the low-boiling liquid used for cold storage or the like is always allowed to flow into the drum 1 from the low-boiling liquid line 2 to be temporarily stored. The low boiling liquid pump 3 is started when the liquid level in the inside reaches a predetermined high liquid level H (level H), and the pump is stopped when the liquid level drops to a predetermined low liquid level L (level L). There is a case of driving. In this case, the levels H and L are detected using the liquid level sensor 4, for example.
[0003]
[Problems to be solved by the invention]
In the intermittent operation of the low-boiling liquid pump described above, if the time from the pump stop to restart is long, the entire low-boiling liquid is close to saturation due to heat input during that time, so cavitation occurs when the pump is started. There was a problem that the pump trip occurred easily.
[0004]
That is, since the impeller rotation always causes a low pressure portion around the impeller (for example, a portion lower by about 100 mmAq than the surroundings), in order to start the pump reliably, the low boiling point liquid is not vaporized at this low pressure portion. It is necessary that a static pressure larger than the vaporization pressure (saturation pressure) of the part is acting. The fluid pressure or fluid depth corresponding to this static pressure is called NPSH (Net Positive Suction Head).
[0005]
However, when a low boiling point liquid such as LNG or LPG is held in the drum for a long time, as shown in FIG. 4B, even if the low boiling point liquid is in a deep position, the liquid pressure at that position (solid line in the figure) Since the entire low boiling point liquid is in a state close to saturation, and the impeller rotates and a low pressure part is generated around the impeller, that part is It always becomes lower than the saturation pressure of the part, and cavitation occurs.
[0006]
Therefore, in the intermittent operation of the conventional low boiling liquid pump, the longer the time from the pump stop to the restart, the more likely the pump trip occurs, and the trip occurred at a certain frequency.
[0007]
Therefore, in order to essentially avoid the pump trip, it is necessary to select a pump with a discharge amount that balances the flow rate of the low-boiling liquid constantly flowing from the line 2 and to continuously operate the pump. However, in this case, not only the continuous operation cost is required, but when the flow rate of the low boiling liquid flowing in is too small, it is necessary to dare to supply the low boiling liquid according to the capacity of the pump.
[0008]
The present invention has been made to solve such problems. That is, an object of the present invention is to intermittently operate a low-boiling-point liquid pump that allows a low-boiling point liquid to always flow in and temporarily store it and reliably discharge the pump without causing a pump trip. It is to provide facilities and methods.
[0009]
[Means for Solving the Problems]
According to the present invention, an airtight container (1) for temporarily storing a low boiling point liquid that always flows from a predetermined low boiling point liquid line (2), and a low pressure point for discharging the low boiling point liquid in the airtight container. A boiling-point liquid pump (3) and a liquid level sensor (4) for detecting and discharging the high and low liquid levels in the hermetic container are provided, and the liquid level is low when the liquid level reaches a predetermined high liquid level H. In the intermittent operation equipment of the low boiling liquid pump that starts the boiling liquid pump and stops the pump when it has fallen to a predetermined low liquid level L,
A replenishment line (12) having an on-off valve (11) for supplying a low boiling point liquid to the hermetic container (1), an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H, And a control device (16) for controlling the on-off valve (11). The control device detects the intermediate liquid level M and shortens the low-boiling point liquid from the replenishment line while the low-boiling point liquid pump is stopped. Supply at the time, stop the supply when the liquid level reaches the high liquid level H ,
The intermediate liquid level M is set such that the hydraulic pressure difference from the high liquid level H is set to be larger than the negative pressure generated by the rotation of the impeller, and intermittent operation equipment for a low boiling liquid pump is provided.
[0010]
Further, according to the present invention, the air-tight container (1) for temporarily storing the low-boiling liquid that always flows from the predetermined low-boiling liquid line (2), and the low-boiling liquid in the air-tight container are discharged. When the liquid level reaches a predetermined high liquid level H, the low boiling liquid pump (3) and the liquid level sensor (4) for detecting and discharging the high and low liquid levels in the hermetic container In the intermittent operation method of the low-boiling-point liquid pump that starts the low-boiling-point liquid pump and stops the pump when the pump reaches a predetermined low liquid level L,
A replenishment line (12) having an on-off valve (11) for supplying a low boiling point liquid to the hermetic container (1), an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H, The low boiling point liquid is supplied from the replenishment line in a short time while the intermediate liquid level M is detected and the pump for the low boiling point liquid is stopped, and the supply is stopped when the liquid level reaches the high liquid level H. And
The intermediate liquid level M is set such that the hydraulic pressure difference from the high liquid level H is set to be larger than the negative pressure generated by the rotation of the impeller, and the intermittent operation method of the low boiling liquid pump is provided.
[0011]
According to a preferred embodiment of the present invention, the flow rate of the supply line (12) is set from sufficiently large flow of low-boiling fluid line (2).
[0012]
According to the apparatus and method of the present invention, the intermediate liquid level lower than the high liquid level H by the intermediate liquid level sensor (14) before the liquid level in the airtight container (1) reaches the predetermined high liquid level H. M is detected, and the control device (16) opens the on-off valve (11) to supply (supplement) the low boiling point liquid from the replenishment line (12) to the high liquid level H in a short time. By this replenishment, the liquid level in the airtight container (1) increases by the difference between the high liquid level H and the intermediate liquid level M, so the pressure of the low boiling point liquid before replenishment rises by that amount, and before replenishment. Even when the entire low-boiling liquid is in a state close to saturation, the whole becomes high by the rising liquid level, and is in a supercooled state. Therefore, even if the impeller rotates and a low pressure portion is generated around the impeller, the portion can be maintained higher than the saturation pressure, and cavitation can be prevented.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.
[0014]
FIG. 1 is an overall configuration diagram of intermittent operation equipment for a low boiling point liquid pump according to the present invention. As shown in this figure, the intermittent operation equipment 10 of the present invention includes an airtight container 1 (drum), a low boiling point liquid pump 3 and a liquid level sensor 4. The airtight container 1 is a sufficiently cooled container, and temporarily stores a low-boiling liquid that always flows from the low-boiling liquid line 2. The low boiling point liquid is, for example, LNG, LPG or the like, and is the remaining amount (unvaporized portion) that is always supplied in a small amount for keeping the pipe cold. Therefore, at the time of flowing into the airtight container 1, a part of the low boiling point liquid is already vaporized and supplied to the BOG processing facility from a gas line (not shown).
[0015]
The low-boiling liquid pump 3 has a discharge amount sufficiently larger than the flow rate of the low-boiling liquid that always flows from the low-boiling liquid line 2, and the low-boiling liquid in the hermetic container 1 is discharged to a predetermined high liquid level H. To a low liquid level L in a short time.
[0016]
Further, the liquid level sensor 4 is a sensor that detects the high and low H and L liquid levels in the airtight container 1. The liquid level sensor 4 is, for example, a float type, a capacitance type, a differential pressure type or the like, and can output a control signal at a high liquid level H and a low liquid level L. With this configuration, when the liquid level in the hermetic container 1 reaches a predetermined high liquid level H by the control device described later or another independent control device, the low boiling point liquid pump 3 is started, and the predetermined low liquid The pump 3 is stopped at the time when it has lowered to the position L.
[0017]
The intermittent operation equipment 10 for the low boiling point liquid pump of the present invention further includes a replenishment line 12, an intermediate liquid level sensor 14, and a control device 16. The replenishment line 12 supplies a low boiling point liquid independently from a low boiling point liquid line (not shown). The low boiling point liquid supplied from this line is preferably sufficiently cooled and in a supercooled state lower than the saturation temperature.
The replenishment line 12 is provided with an on-off valve 11 (for example, a solenoid valve) that can be operated remotely. Further, when the on-off valve 11 is opened, the flow rate of the replenishment line 12 is set to be sufficiently larger than the flow rate of the low boiling liquid line 2. The flow rate of the replenishment line 12 is set such that the low-boiling point liquid in the hermetic container 1 before reaching the high liquid level H by replenishment to be described later does not become saturated again. .
[0018]
The intermediate liquid level sensor 14 is configured to detect the intermediate liquid level M that is higher than the low liquid level L and lower than the high liquid level H. This intermediate liquid level M is set such that the hydraulic pressure difference ΔP with respect to the high liquid level H is sufficiently larger than the negative pressure generated by the rotation of the impeller (for example, 300 mmAq or more). The intermediate liquid level sensor 14 is, for example, a float type, a capacitance type, a differential pressure type, etc., like the liquid level sensor 4, and can output a control signal at the intermediate liquid level H.
[0019]
The control device 16 receives at least a control signal from the intermediate liquid level sensor 14 and controls opening / closing of the on-off valve 11. That is, when the intermediate liquid level M is detected by the intermediate liquid level sensor 14 while the low boiling point liquid pump 3 is stopped by the control device 16, the control valve 16 receives the control signal and fully opens the on-off valve 11. The boiling liquid is supplied, and when the liquid level reaches the high liquid level H in a short time, the on-off valve 11 is fully closed to stop the supply. During the operation of the low-boiling liquid pump 3, even if the intermediate liquid level M is detected by the intermediate liquid level sensor 14, this control is not performed and the on-off valve 11 is kept fully closed. Interlock is provided.
Further, in this example, the control device 16 receives the control signal of the liquid level sensor 4 described above, and controls the start / stop of the low boiling point liquid pump 3.
[0020]
FIG. 2 is a flowchart of the intermittent operation method of the present invention. As shown in this figure, the intermittent operation method of the low-boiling liquid pump of the present invention is as follows. (1) The low-boiling liquid is detected from the replenishment line 12 while the intermediate liquid level M is detected and the low-boiling liquid pump 3 is stopped. Is supplied in a short time, and the supply step S1 is stopped when the liquid level reaches the high liquid level H, and (2) the pump for the low boiling point liquid when the liquid level reaches the predetermined high liquid level H. 3 and a discharge step S2 in which the pump 3 is stopped at the time when the pressure drops to a predetermined low liquid level L.
In a normal operation state, after a “standby state” in which a small amount of low-boiling liquid always flows into the airtight container 1 from the predetermined low-boiling liquid line 2 continues for a long time, the replenishment step S1 starts, and then the discharge step S2 is performed. . Thereafter, the process returns to the initial standby state, and this is repeated.
[0021]
FIG. 3 is an explanatory view showing a change in the liquid pressure of the low boiling point liquid in the intermittent operation method of the present invention. When a low boiling point liquid such as LNG or LPG is held in the drum for a long time, as shown in FIG. 3, even a low boiling point liquid at a deep position corresponds to the liquid pressure at that position (shown by a solid line A in the figure). Since the pressure is close to the saturation pressure (shown by the broken line in the figure), the entire low-boiling point liquid is in a state close to saturation. Therefore, when the impeller rotates in this state and a low pressure portion is generated around the impeller, the portion always becomes lower than the saturation pressure of the portion, and cavitation occurs.
[0022]
According to the apparatus and method of the present invention described above, when the low boiling point liquid gradually increases in this state and the liquid level in the airtight container 1 reaches the intermediate liquid level M lower than the high liquid level H, the control device 16 Thus, the on-off valve 11 is opened, and the low boiling point liquid is supplied (supplemented) from the replenishment line 12 to the high liquid level H in a short time. By this replenishment, the liquid level in the hermetic container 1 is increased by the difference ΔP between the high liquid level H and the intermediate liquid level M. Therefore, as shown by the solid line B in the figure, the pressure of the low boiling point liquid before replenishment Rises by that amount and enters a supercooled state. Therefore, even if the impeller rotates and a low pressure portion is generated around the impeller, the portion can be maintained higher than the saturation pressure, and cavitation can be prevented.
[0023]
The present invention is not limited to the embodiments described above, and various modifications can be made without departing from the scope of the invention.
[0024]
【The invention's effect】
As described above, according to the present invention, stable startup of the intermittent operation pump can be obtained, and the operation cost can be reduced as compared with continuous operation. That is, the low boiling liquid pump intermittent operation equipment and method according to the present invention allows the low boiling liquid to always flow in and temporarily store it, and reliably discharge the pump without causing a pump trip. And so on.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of intermittent operation equipment of a pump for low boiling point liquid according to the present invention.
FIG. 2 is a flowchart of the intermittent operation method of the present invention.
FIG. 3 is an explanatory diagram showing a change in hydraulic pressure of a low boiling point liquid in the present invention.
FIG. 4 is an explanatory diagram of conventional intermittent operation means.
[Explanation of symbols]
1 Airtight container (drum)
2 Low-boiling point liquid line 3 Low-boiling point liquid pump 4 Liquid level sensor 10 Intermittent operation equipment 11 On-off valve 12 Supply line 14 Intermediate liquid level sensor 16 Controller

Claims (3)

An airtight container (1) for temporarily storing a low boiling liquid that always flows from a predetermined low boiling liquid line (2), and a low boiling liquid pump (3 for discharging the low boiling liquid in the airtight container) ) And a liquid level sensor (4) for detecting and discharging the high and low liquid levels in the hermetic container, and when the liquid level reaches a predetermined high liquid level H, the low boiling liquid pump is activated. In the intermittent operation equipment of the low-boiling-point liquid pump that stops the pump when it falls to a predetermined low liquid level L,
A replenishment line (12) having an on-off valve (11) for supplying a low boiling point liquid to the hermetic container (1), an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H, And a control device (16) for controlling the on-off valve (11). The control device detects the intermediate liquid level M and shortens the low-boiling point liquid from the replenishment line while the low-boiling point liquid pump is stopped. Supply at the time, stop the supply when the liquid level reaches the high liquid level H ,
The intermediate liquid level M is set to have a higher hydraulic pressure difference from the high liquid level H than a negative pressure generated by the rotation of the impeller . The flow rate of the supply line (12) is sufficiently larger than the flow rate of the low-boiling fluid line (2), the intermittent operation equipment of low boiling liquid pump according to claim 1, characterized in that. An airtight container (1) for temporarily storing a low boiling liquid that always flows from a predetermined low boiling liquid line (2), and a low boiling liquid pump (3 for discharging the low boiling liquid in the airtight container) ) And a liquid level sensor (4) for detecting and discharging the high and low liquid levels in the hermetic container, and when the liquid level reaches a predetermined high liquid level H, the low boiling liquid pump is activated. In the intermittent operation method of the low-boiling-point liquid pump that stops the pump when the liquid level drops to a predetermined low liquid level L,
A replenishment line (12) having an on-off valve (11) for supplying a low boiling point liquid to the hermetic container (1), an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H, The low boiling point liquid is supplied from the replenishment line in a short time while the intermediate liquid level M is detected and the pump for the low boiling point liquid is stopped, and the supply is stopped when the liquid level reaches the high liquid level H. And
The intermediate liquid level M is set to have a higher hydraulic pressure difference from the high liquid level H than a negative pressure generated by the rotation of the impeller .

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