JP2001123964A - Intermittent operating facility and method for low boiling point liquid pump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide intermittent operating facilities and a method for low boiling point liquid pump, capable of allowing low boiling point liquid to always flow, temporarily storing it, reliably and intermittently operating the liquid pump without pump trip, and discharging low boiling point liquid. SOLUTION: This intermittent operating facility is provided with a replenishing line 12 having an opening and closing valve 11 and for feeding low boiling point liquid to an airtight container 1, an intermediate level sensor 14 for detecting the intermediate level M lower than the high level H, and a controller 16 for controlling the opening and closing valve 11. The controller detects the intermediate level M, feeds the low boiling point liquid by the replenishing line in a short time during the stop of a low boiling point liquid pump, and stops feeding at the time that the level reaches the high level H.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermittent operation facility for intermittently operating a low boiling point liquid pump and a method thereof.

[0002]

2. Description of the Related Art In a storage facility or a chemical plant which handles LNG, LPG, etc. (hereinafter, referred to as low boiling point liquid), a small amount of low boiling point liquid may be constantly flowed in order to keep pipes cool. In this case, an airtight container (drum) as shown in FIG. 4 (A) is provided, and the low boiling liquid used for cooling or the like is constantly flowed into the drum 1 from the low boiling liquid line 2 to temporarily store the drum. The low boiling point 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). You may drive. In this case, the levels H and L are detected using, for example, the liquid level sensor 4.

[0003]

In the above-described intermittent operation of the pump for low boiling point liquid, if the time from stop to restart of the pump is long, the heat input during that time will bring the entire low boiling point liquid into a state close to saturation. Therefore, there is a problem that cavitation occurs when the pump is started and a pump trip is likely to occur.

[0004] That is, since a low-pressure portion (for example, a portion about 100 mmAq lower than the circumference) is always generated around the impeller due to the rotation of the impeller, in order to reliably start the pump, the low-boiling-point liquid is not vaporized at the low-pressure portion. In addition, it is necessary that a static pressure higher than the vaporization pressure (saturation pressure) of that portion acts. The liquid pressure or liquid depth corresponding to this static pressure is called NPSH (Net Positive Suction Head).

However, when low boiling liquids such as LNG and LPG are held in the drum for a long time, as shown in FIG.
Even if the low boiling point liquid is at a deep position, it is close to the saturation pressure (broken line in the figure) corresponding to the liquid pressure at that position (solid line in the figure), so the entire low boiling point liquid is close to the saturated state When the impeller rotates 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.

Therefore, in the conventional intermittent operation of the low boiling point liquid pump, the longer the time from pump stop to restart is, the more likely the pump trip occurs, and the trip occurs at a certain frequency.

[0007] Therefore, in order to essentially avoid the pump trip, it is necessary to select a pump having a discharge amount balanced with the flow rate of the low boiling point 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 also when the flow rate of the low boiling point liquid flowing in is too small, it is necessary to supply the low boiling point liquid according to the capacity of the pump.

The present invention has been made to solve such a problem. That is, an object of the present invention is to provide an intermittent operation of a low-boiling-point liquid pump in which a low-boiling-point liquid can be constantly introduced and temporarily stored, and the pump can be reliably discharged intermittently without causing a pump trip. To provide facilities and methods.

[0009]

According to the present invention, an airtight container (1) for temporarily storing a low-boiling liquid constantly flowing from a predetermined low-boiling liquid line (2); A pump for low-boiling liquid (3) for discharging the low-boiling liquid, and a liquid level sensor (4) for detecting and discharging the high and low liquid levels in the airtight container, the liquid level being a predetermined high level The intermittent operation equipment for the low boiling point liquid pump which starts the low boiling point liquid pump when the liquid level reaches H and stops the pump when the liquid level falls to the predetermined low level L has an open / close valve (11). A supply line (12) for supplying a low boiling liquid to the airtight container (1), an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H, and an on-off valve (11). And a control unit (16) for controlling the control unit, which detects the intermediate liquid level M and controls the low boiling point liquid. Intermittent operation of the low boiling point liquid pump, wherein the low boiling point liquid is supplied from the supply line in a short time while the pump is stopped, and the supply is stopped when the liquid level reaches the high liquid level H. Facilities are provided.

According to the present invention, an airtight container (1) for temporarily storing a low-boiling liquid constantly flowing from a predetermined low-boiling liquid line (2), and a low-boiling liquid in the airtight container (1) are stored. A low-boiling liquid pump (3) for discharging, and a liquid level sensor (4) for detecting and discharging high and low liquid levels in an airtight container, so that the liquid level reaches a predetermined high liquid level H. In the intermittent operation method of the low-boiling-point liquid pump in which the low-boiling-point liquid pump is started at the time when the temperature has reached and the pump is stopped when the low-boiling-point liquid has dropped to the predetermined low level L, the airtight container having the on-off valve (11) is provided. 1) a replenishing line (12) for supplying a low boiling point liquid; and an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H. While the low boiling liquid pump is stopped, supply the low boiling liquid from the replenishment line in a short time, and Stopping the supply when it reaches H, intermittent operation method of the low-boiling liquid pump is provided, characterized in that.

According to a preferred embodiment of the present invention, the flow rate of the replenishment line (12) is set to be sufficiently higher than the flow rate of the low boiling point liquid line (2), and the intermediate liquid level M is high. The liquid pressure difference from the liquid level H is set to be sufficiently larger than the negative pressure generated by the rotation of the impeller.

According to the apparatus and method of the present invention, before the liquid level in the airtight container (1) reaches a predetermined high liquid level H,
The intermediate liquid level sensor (14) detects an intermediate liquid level M lower than the high liquid level H, and the control device (16) controls the on-off valve (1).
By opening 1), the low boiling point liquid can be supplied (supplied) to the high liquid level H in a short time from the supply line (12). By this replenishment, the liquid level in the airtight container (1) rises by the difference between the high liquid level H and the intermediate liquid level M, so that the pressure of the low-boiling liquid before replenishment increases by that much, and Even when the low boiling point liquid as a whole is in a state close to saturation, the whole liquid becomes higher by the rising liquid level,
It becomes a supercooled state. Therefore, even if the impeller rotates and a low-pressure portion is generated around the impeller, the portion can be maintained at a higher pressure than the saturation pressure, and cavitation can be prevented.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. In addition, the same reference numerals are given to the common parts in the respective drawings, and the duplicate description will be omitted.

FIG. 1 is an overall block diagram of an intermittent operation facility 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 liquid pump 3 and a liquid level sensor 4. The airtight container 1 is a container that is sufficiently kept cool, and temporarily stores a low-boiling liquid constantly flowing from the low-boiling liquid line 2. This low boiling point liquid is, for example, LN
G, LPG, etc., which are the remaining amounts (unvaporized components) constantly flowing in small amounts for the purpose of keeping the pipe cool. Therefore, the airtight container 1
At the time when the liquid flows into the BOG, a part of the low-boiling liquid is already vaporized and supplied to a BOG processing facility from a gas line (not shown).

The low-boiling liquid pump 3 has a discharge rate sufficiently larger than the flow rate of the low-boiling liquid constantly flowing from the low-boiling liquid line 2 to discharge the low-boiling liquid in the airtight container 1 to a predetermined high pressure. The liquid can be discharged from the liquid level H to the low liquid level L in a short time.

Further, the liquid level sensor 4 is a sensor for detecting the liquid level of the high and low H and L 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 airtight container 1 reaches a predetermined high liquid level H, the low boiling liquid pump 3 is controlled by a control device described later or another independent control device.
Is started, and when the liquid level falls to a predetermined low level L, the pump 3
To stop.

The low-boiling liquid pump intermittent operation equipment 10 of the present invention further includes a replenishment line 12, an intermediate liquid level sensor 14
And a control device 16. The supply line 12 is configured to independently supply a low-boiling liquid from a low-boiling liquid line (not shown). The low boiling point liquid supplied from this line is
Preferably, it is sufficiently cooled and is in a supercooled state lower than the saturation temperature. The supply line 12 is provided with an on-off valve 11 (for example, a solenoid valve) that can be remotely operated.
Further, when the on-off valve 11 is opened, the supply line 12
Is set to be sufficiently larger than the flow rate of the low boiling point liquid line 2. The flow rate of this supply line 12 is
The flow rate is set such that the low-boiling-point liquid in the airtight container 1 before reaching the high liquid level H from the intermediate liquid level M by the replenishment described below does not become saturated again.

The intermediate liquid level sensor 14 detects the intermediate liquid level M higher than the low liquid level L and lower than the high liquid level H. The intermediate liquid level M is set so that the hydraulic pressure difference ΔP from 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, or the like, like the liquid level sensor 4, and can output a control signal at the intermediate liquid level H.

The control device 16 receives at least a control signal from the intermediate liquid level sensor 14, and controls the opening and 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 liquid pump 3 is stopped by the control device 16, the control signal is received and the on-off valve 11 is fully opened to lower the liquid level below the supply line 12. The boiling point 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 operation of the low boiling point liquid pump 3, even if the intermediate liquid level sensor 14 detects the intermediate liquid level M, this control is not performed and the on-off valve 11 is kept fully closed. , An interlock is provided. In this example, the control device 16
In response to the control signal from the liquid level sensor 4, the start and stop of the low boiling point liquid pump 3 are also controlled.

FIG. 2 is a flowchart of the intermittent operation method of the present invention. As shown in this drawing, the intermittent operation method of the low boiling point liquid pump according to the present invention is as follows. (1) When the intermediate liquid level M is detected and the low boiling point liquid pump 3 is stopped, the low boiling point liquid Is supplied in a short time, and the supply is stopped when the liquid level reaches the high liquid level H, and (2) the low boiling point liquid pump when the liquid level reaches the predetermined high liquid level H 3 is started and the pump 3 is stopped when the liquid level drops to a predetermined low liquid level L. In a normal operating state, after a long “standby state” in which a small amount of low-boiling liquid always flows into the hermetic container 1 from the predetermined low-boiling liquid line 2, a replenishing step S1 starts and then a discharging step S2 is performed. . Thereafter, the process returns to the initial standby state, and this is repeated.

FIG. 3 is an explanatory diagram showing a change in the liquid pressure of the low boiling point liquid in the intermittent operation method of the present invention. LNG, LPG
When a low-boiling liquid such as is held in the drum for a long time, as shown in FIG. 3, even if the low-boiling liquid is at a deep position, the saturation pressure corresponding to the liquid pressure at that position (indicated by a solid line A in the figure) (Shown by a broken line in the figure), the entire low-boiling liquid is almost in a saturated state. Therefore, when the impeller rotates in this state and a low pressure portion is generated around the impeller,
That portion always becomes lower than the saturation pressure of that portion, and cavitation occurs.

According to the above-described apparatus and method of the present invention,
In this state, when the low-boiling liquid gradually increases and the liquid level in the airtight container 1 reaches an intermediate liquid level M lower than the high liquid level H, the control device 16 opens the on-off valve 11 to open the low boiling point liquid from the replenishment line 12. The liquid is supplied (supplied) to the high liquid level H in a short time. By this replenishment, the liquid level in the airtight container 1 rises by the difference ΔP between the high liquid level H and the intermediate liquid level M, and as shown by the solid line B in FIG. 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 at a higher pressure than the saturation pressure, and cavitation can be prevented.

The present invention is not limited to the embodiment described above, and various changes can be made without departing from the gist of the invention.

[0024]

As described above, according to the present invention, the intermittent operation pump can be started stably, and the operation cost can be reduced as compared with the continuous operation. In other words, the low-boiling liquid pump intermittent operation equipment and method according to the present invention always allows the low-boiling liquid to flow in temporarily, temporarily stores the low-boiling liquid, and discharges the low-boiling liquid by intermittently operating the pump without causing a pump trip. And other excellent effects.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a low-boiling liquid pump intermittent operation facility according to the present invention.

FIG. 2 is a flowchart of an intermittent operation method of the present invention.

FIG. 3 is an explanatory diagram showing a change in liquid pressure of a low boiling point liquid in the present invention.

FIG. 4 is an explanatory view of a conventional intermittent operation means.

[Explanation of symbols]

 DESCRIPTION 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 Open / close valve 12 Supply line 14 Intermediate liquid level sensor 16 Control device

Claims (3)

[Claims] An airtight container (1) for temporarily storing a low-boiling liquid constantly flowing from a predetermined low-boiling liquid line (2).
A low-boiling liquid pump (3) for discharging the low-boiling liquid in the hermetic container, and a liquid level sensor (4) for detecting and discharging the high and low liquid levels in the hermetic container. In the intermittent operation equipment for a low-boiling-point liquid pump that starts the low-boiling-point liquid pump when the liquid level reaches a predetermined high liquid level H and stops the pump when the liquid level drops to a predetermined low-liquid level L, A supply 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 replenishing line while the low boiling point liquid pump is stopped. Low-boiling liquid, wherein the liquid is supplied at a predetermined time and the supply is stopped when the liquid level reaches a high liquid level H. Intermittent operation equipment for water pumps. 2. The flow rate of the supply line (12) is set to be sufficiently larger than the flow rate of the low boiling point liquid line (2), and the intermediate liquid level M has a liquid pressure difference from the high liquid level H. The intermittent operation equipment according to claim 1, wherein the pump is set to be sufficiently higher than a negative pressure generated by rotation of the impeller. 3. An airtight container (1) for temporarily storing a low-boiling liquid constantly flowing from a predetermined low-boiling liquid line (2).
A low-boiling liquid pump (3) for discharging the low-boiling liquid in the hermetic container, and a 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 liquid pump, which starts the low-boiling liquid pump when the liquid level reaches a predetermined high liquid level H and stops the pump when the liquid level falls to a predetermined low liquid level L, A supply line (12) having an on-off valve (11) and supplying a low boiling point liquid to the airtight container (1); an intermediate liquid level sensor (14) for detecting an intermediate liquid level M lower than the high liquid level H; When the intermediate liquid level M is detected and the low boiling liquid pump is stopped, the low boiling liquid is supplied from the replenishment line in a short time, and the supply is stopped when the liquid level reaches the high liquid level H. An intermittent operation method of a pump for a low-boiling liquid.