JP5921160B2 - Pump control method and control apparatus therefor - Google Patents

Description

  The present invention relates to a pump control method and its control device for pouring and draining liquid to a tank storing liquid, and more particularly to a pump control method and control device for controlling so that cavitation does not occur when liquid is discharged from the tank. .

  Conventionally, in oil tankers such as oil tankers, multiple ballast tanks have been provided in order to ensure the depth of submersion of the propeller and safe navigation when empty, and what are the freight oil pipe systems such as freight oil pipes and freight oil pumps? It has a ballast pipe system dedicated to the separated ballast injection and drainage. Thereby, the ballast water (seawater) can be poured before leaving the port to form a draft, and the drainage of ballast water during cargo handling and the replacement of ballast water during voyage can be performed.

NPSH (Net Positive Suction Head) is used as a numerical value to evaluate the suction performance of the pump. This NPSH has an effective suction head that can be used in practice (NPSH Available) and the required suction that is specific to the performance of the pump itself. There is a head (NPSH Required).
Note: When the required suction head value exceeds the effective suction head value during operation, drainage pumps will cause cavitation in the impeller, resulting in a sudden drop in the total pump head, flow rate and pump efficiency, and noise and vibration. Furthermore, it is known that damage due to erosion occurs around the impeller. In particular, high-speed and large-capacity pumps such as the above-described coin oil pumps and ballast pumps are more likely to generate cavitation than small pumps, and countermeasures against cavitation damage are a serious issue.

  The pump operation control device of Patent Document 1 includes a suction tank provided on the suction side of a pump capable of controlling the number of revolutions, a replacement chamber for separating gas from the pumped liquid, and a suction tank for pumping liquid in the replacement chamber. A recirculation valve that recirculates the air and a controller that calculates the required suction head and the effective suction head and controls the pump rotation speed, the discharge valve opening, and the recirculation valve opening. By controlling the opening degree of the discharge valve to the throttle side based on the position, the discharge flow rate of the pump is adjusted so that the necessary suction head does not exceed the effective suction head.

  In addition, when a large-capacity pump is required as in an oil tanker, a drainage system equipped with a so-called rated rotational speed pump in which the rotational speed is kept constant is used. In such an injection / drainage system, a discharge valve capable of adjusting the flow rate connected to the discharge side of the pump for injecting / draining the tank, a suction pressure sensor for detecting the suction pressure on the pump suction side, and a pump discharge side And a controller for controlling the discharge valve based on the suction pressure and the discharge pressure detected by both sensors.

  FIG. 6 relates to the pouring / drainage control adopted by the applicant of the present application, and the controller utilizes that the tank liquid level (equivalent to an effective suction head) can be replaced by the suction pressure Pin on the pump suction side. Then, the suction pressure Pin and the pump discharge flow rate Q were set in advance as parameters, respectively, and the required suction head characteristic line Lh was set apart from the necessary suction head characteristic line Lh by the first allowable width and set to the flow rate reduction side. The cavitation determination characteristic line Lc and the return determination characteristic line Ls set on the flow rate decrease side by being separated from the cavitation determination characteristic line Lc by the second allowable width are stored. The pump operating state determined by the current pump flow rate Q and the suction pressure Pin calculated based on the detected suction pressure Pin and discharge pressure Pout is a cavitation determination characteristic line Lc according to the decrease in the suction pressure Pin. The controller adjusts the discharge valve to the throttle side until the operating state becomes the operating state on the return determination characteristic line Ls, and then repeatedly performs this adjustment control.

  The return determination characteristic line Ls used in the adjustment control is further determined when the tank returns to the horizontal state from the inclined state where the pressure is detected to be relatively higher than the suction pressure Pin detected when the tank is in the horizontal state. Is navigating so that the suction pressure Pin (effective suction head) on the pump suction side does not fall below the required suction head characteristic line Lh even when the tank is tilted to the opposite side due to the reaction at the time of hull return The second allowable width is set based on the design conditions considering the swing of the hull.

Japanese Patent No. 2716823

In the pump operation control device of Patent Document 1, it is possible to automatically control suppression of cavitation by integrally controlling the pump rotation speed, the discharge valve opening, and the recirculation valve opening.
However, since many control conditions are given to the controller and the number of measuring instruments for that increase, the structure becomes complicated, which may be disadvantageous in terms of maintenance such as manufacturing cost and failure handling. In addition, when the discharge flow rate of the pump is increased in order to improve the efficiency of the pouring / drainage work, the possibility of cavitation increases, and it is difficult to adjust the opening of the appropriate discharge valve.

In the pouring / drainage control shown in FIG. 6, there are few measuring devices that detect the control conditions, and adjustment control is performed only on the discharge valve, so that the occurrence of cavitation can be suppressed with a simple structure.
However, when the operating state reaches the cavitation determination characteristic line Lc according to the decrease in the suction pressure, the discharge valve is adjusted to the throttle side until the operating state on the return determination characteristic line Ls is reached. There is a risk that the drainage time will be prolonged. In particular, in an oil tanker such as an oil tanker, since it takes tens of hours to discharge ballast water, there is a concern that prolonged drainage time may affect the schedule and safe navigation.

  In order to increase the discharge flow rate of the pump, the return determination characteristic line can be set apart from the cavitation determination characteristic line Lc by an allowable width smaller than the second allowable width. However, since the navigating hull is swinging, cavitation may occur if the allowable width is reduced. That is, when the hull swing is large, if the suction pressure reaches the cavitation determination characteristic line Lc in the inclined state where the pressure is detected to be relatively higher than the suction pressure detected in the horizontal state, the return determination characteristic During the adjustment of the discharge valve to the throttle side until the operating state on the line Ls is reached, or after the adjustment, the hull may be inclined to the opposite side due to the reaction. At this time, when adjustment control using the return determination characteristic line Ls having a small allowable width separated from the cavitation determination characteristic line Lc is executed, the discharge valve is adjusted to the throttle side (operating state on the return determination characteristic line Ls). Nevertheless, the increase in the suction pressure on the pump suction side is insufficient, and as a result of the sudden decrease in the suction pressure, the suction pressure may fall below the required suction head characteristic line.

  An object of the present invention is to provide a pump control method and a control apparatus for the pump that can achieve both simplification of the structure and reduction of the drainage time of liquid and suppression of cavitation.

According to the pump control method of the first aspect of the present invention, the flow rate-adjustable discharge valve connected to the discharge side of the pump for pouring and draining the liquid is stored so that cavitation does not occur when the liquid is discharged from the tank. In the control method of the pump to be controlled, with the suction pressure and flow rate on the pump suction side as parameters, an NPSH characteristic line, and a cavitation determination characteristic line set on the flow rate reduction side separated from the NPSH characteristic line by a first allowable width; A return determination characteristic line set on the flow rate decrease side with a second allowable width away from the cavitation determination characteristic line is set in advance and stored in the characteristic storage unit, and the pump suction side detected by the suction pressure detection unit is stored. A first step of calculating the current flow rate using the suction pressure and the discharge pressure on the pump discharge side detected by the discharge pressure detecting means; A second step of determining whether or not the operating state determined by the sucked pressure has reached the cavitation determination characteristic line according to a decrease in the suction pressure, and the affirmative determination in the second step, the operation A third step of adjusting the discharge valve to the throttle side until the state reaches the return determination characteristic line; and after the third step, the operating state is in the middle of the return determination characteristic line and the cavitation determination characteristic line with respect to the flow rate. And a fourth step of adjusting the discharge valve to the open side until the operating state is reached.

The pump control device according to claim 2 is configured such that a discharge valve capable of adjusting a flow rate connected to a discharge side of a pump for pouring and draining a liquid storing tank does not generate cavitation when the liquid is discharged from the tank. In the control device of the pump to be controlled, the suction pressure detecting means for detecting the suction pressure on the pump suction side, the discharge pressure detecting means for detecting the discharge pressure on the pump discharge side, and the suction pressure detected by the both detecting means And a control means for controlling the discharge valve based on the discharge pressure, the control means comprising: a flow rate calculating means for calculating a current flow rate based on the detected suction pressure and discharge pressure; and a pump suction side The NPSH characteristic line set in advance with the suction pressure and the flow rate as parameters, respectively, and the cavitation determination characteristic set on the flow rate decreasing side by separating the first allowable width from the NPSH characteristic line. And a characteristic storage means for storing a return determination characteristic line set on the flow rate reduction side with a second allowable width away from the cavitation determination characteristic line, a current flow rate calculated by the flow rate calculation means, and a detected suction When the operating state determined by the pressure reaches the cavitation determination characteristic line according to the decrease in the suction pressure, the discharge valve is adjusted to the throttle side until the operating state on the return determination characteristic line is reached, and then the return with respect to the flow rate is performed. Discharge valve adjustment control means for adjusting the discharge valve to the open side until an operating state near the middle between the determination characteristic line and the cavitation determination characteristic line is provided.

According to the first aspect of the present invention, since there are few control conditions for controlling the pump and there are few measuring instruments therefor, the structure of the facility can be simplified, which is advantageous in terms of maintenance such as manufacturing cost and failure handling. When the pump operating state reaches the cavitation determination characteristic line as the suction pressure decreases, the discharge valve is throttled until the operating state (target stable flow rate) is in the middle of the return determination characteristic line and the cavitation determination characteristic line with respect to the flow rate. Therefore, the amount of decrease in the pump flow rate can be suppressed compared to the pump flow rate when the discharge valve is adjusted to the throttle side until the operating state on the return determination characteristic line is reached, and the drainage time can be shortened.

  Also, before adjusting the discharge valve to the operating state in the vicinity of the middle between the return determination characteristic line and the cavitation determination characteristic line, the discharge valve is temporarily adjusted to the throttle side operating state rather than the operating state corresponding to the target stable flow rate. Can do. Therefore, the suction pressure reaches the cavitation characteristic line in the inclined state where the pressure is detected to be relatively higher than the suction pressure detected in the horizontal state, and the hull is inclined to the opposite side due to the reaction. Even in this case, since it is possible to form an adjustment period on the throttle side with respect to the operating state and on the throttle side with respect to the target stable flow rate, it is possible to suppress a decrease in the suction pressure on the pump suction side, and the suction pressure is required. Occurrence of cavitation due to falling below the head characteristic line can be suppressed. In addition, since the discharge valve is adjusted to the operating state on the return determination characteristic line in the adjustment period on the throttle side, the adjustment control of the discharge valve can be simplified and the occurrence of cavitation can be further suppressed.

  According to the second aspect of the present invention, it is possible to obtain a pump control device that basically exhibits the same effect as the first aspect of the present invention.

It is a whole block diagram of the control apparatus of the pump which concerns on Example 1 of this invention. The characteristic line figure of the pump which concerns on Example 1 is shown, (a) is a relational expression of a total head and pump flow volume, (b) is a figure which shows the relational expression of a required suction head and pump flow volume. It is a flowchart of discharge valve adjustment control. It is a figure which shows the operation state of the adjustment control of the pump which concerns on Example 1. FIG. It is a principal part enlarged view of FIG. It is a figure which shows the operating state of the adjustment control of the conventional pump.

  DESCRIPTION OF EMBODIMENTS Hereinafter, modes for carrying out the present invention will be described based on an embodiment applied to a pump control device of a tanker ballast pipe system facility. The following description includes a description of a pump control method.

  As shown in FIG. 1, the control device of the pump 1 detects a suction pressure sensor 2 (suction pressure detection means) that detects a suction pressure Pin on the suction side of the pump 1 and a discharge pressure Pout on the discharge side of the pump 1. The discharge side of the pump 1 is provided with a discharge pressure sensor 3 (discharge pressure detection means) and a controller 10 (control means) for pouring and draining a ballast tank 4 (tank) for storing seawater (liquid). The discharge valve 5 that is adjustable in flow rate is connected to the ballast tank 4 so that cavitation does not occur when the seawater is discharged from the ballast tank 4. One or a plurality of ballast tanks 4 are provided, for example, on the left and right sides of the hull, and each ballast tank 4 is connected to the pump 1 in parallel.

  The pump 1 is provided in the middle of a connecting pipe 7 that connects a discharge port 6 disposed so as to be always located below the water surface and a position near the lower portion of the ballast tank 4. The pump 1 is a rated speed pump that is rotated at a constant speed, and is configured to discharge only in one direction. Therefore, the pump 1 is connected to the connecting pipe 7 via a discharge direction switching circuit (not shown), and after discharging from the ballast tank 4, when the seawater is sucked and poured into the ballast tank 4, the discharge direction is switched. The direction of seawater flow is changed to the opposite direction by operating the circuit.

The controller 10 is electrically connected to the suction pressure sensor 2, the discharge pressure sensor 3, and the discharge valve 5. The discharge valve 5 is based on the suction pressure Pin and the discharge pressure Pout detected by the sensors 2 and 3. The degree of opening is adjustable. The controller 10 includes a microcomputer, and includes a flow rate calculation means 11, a characteristic storage means 12, and a discharge valve adjustment control means 13.
The flow rate calculation means 11 calculates the current discharge flow rate Q of the pump 1 based on the detected suction pressure Pin and discharge pressure Pout. Since the function f (H) of the total head H and the pump flow rate Q is stored in advance in the characteristic storage unit 12 (see FIG. 2A), the flow rate calculation unit 11 determines the suction pressure Pin and the discharge pressure Pout. The total head H is calculated based on the pressure difference, and the current flow rate Q is calculated from the total head H using a function f (H).

  As shown in FIGS. 2, 4, and 5, the characteristic storage means 12 has a pump flow rate Q that is set in advance from the function f (H) of the total head H and the pump flow rate Q described above and the performance of the pump 1. And a function f (Q) of the required suction head h, an NPSH characteristic line Lh, a cavitation determination characteristic line Lc, and a return determination characteristic line Ls are stored in advance.

  The effective suction head is expressed by using the head (water head) how much the total pressure of the liquid is higher than the saturated vapor pressure of the liquid at the temperature on the reference surface of the pump 1, and in this embodiment, The suction pressure Pin on the pump suction side corresponds to the effective suction head. Thus, the NPSH characteristic line Lh corresponding to the necessary suction head h can be set in advance based on the function f (Q) with the suction pressure Pin on the pump suction side and the discharge flow rate Q of the pump 1 as parameters. Therefore, the cavitation determination characteristic line Lc is set to be separated from the NPSH characteristic line Lh by a first allowable width, for example, a flow rate decrease side corresponding to 0.5 m in terms of the head, and the return determination characteristic line Ls is the cavitation determination characteristic line. It is set apart from Lc to a second allowable width, for example, a flow rate decreasing side equivalent to 0.5 m in terms of head. Note that the second allowable width is an inclination in which the tank is inclined to the opposite side due to a reaction when the hull returns from an inclined state where the pressure is detected to be relatively higher than the suction pressure Pin detected when the ballast tank 4 is in a horizontal state. Even in the state, the suction pressure Pin on the pump suction side is set in advance based on experimental values so as not to fall below the NPSH characteristic line Lh.

  The discharge valve adjustment control unit 13 determines that the operating state X of the pump 1 determined from the current flow rate Q calculated by the flow rate calculation unit 11 and the detected suction pressure Pin is in accordance with a decrease in the suction pressure Pin (effective suction head). When the discharge valve 5 is adjusted to the throttle side until reaching the operating state on the return determination characteristic line Ls when the cavitation determination characteristic line Lc is reached, the target stability near the middle of the return determination characteristic line Ls and the cavitation determination characteristic line Lc is reached. The discharge valve 5 is adjusted to the open side until an operating state corresponding to the flow rate Qtn (where n = 1, 2,...) Is obtained. Thereby, before adjusting the pump 1 to the target stable flow rate Qtn, it is possible to ensure an adjustment period in which the pump flow rate is narrower than the target stable flow rate Qtn.

  Next, a processing procedure for adjusting the flow rate of the pump 1 will be specifically described based on the flowchart of the discharge valve adjustment control in FIG. 3 and the explanatory diagrams in FIGS. 4 and 5. Si (i = 1, 2,...) Indicates each step.

First, in S1, simultaneously with the start of drainage from the ballast tank 4, the suction pressure Pin detected by the suction pressure sensor 2 and the discharge pressure Pout detected by the discharge pressure sensor 3 are read, respectively, and the process proceeds to S2. In S2, the total head H is calculated from the suction pressure Pin and the discharge pressure Pout, and the current pump flow rate Q is calculated using the function f (H).
As shown in FIG. 5, the operating state X can be determined from the current pump flow rate Q and the suction pressure Pin. At the start of processing, the operating state X (Q, Pin) is a state in which the initial flow rate Q1 is maintained. The suction pressure Pin gradually decreases.

In S3, it is determined whether or not the operating state X has reached the cavitation determination characteristic line Lc in accordance with the decrease in the suction pressure Pin. As a result of the determination in S3, when the operating state X reaches the cavitation determination characteristic line Lc, the discharge valve 5 is closed at a predetermined speed (S4), and when the operating state X does not reach the cavitation determination characteristic line Lc, Return to S1.
As shown in FIG. 5, when the operating state X reaches a point X1 (Q1, P1) on the cavitation determination characteristic line Lc due to a decrease in the suction pressure Pin, the discharge valve 5 is closed to the throttle side. When the operating state X has not reached the point X1 (Q1, P1) on the cavitation determination characteristic line Lc, the drainage is continued while the current flow rate Q1 is maintained.

  In S5, it is determined whether or not the operating state X has reached the return determination characteristic line Ls according to the closing operation of the discharge valve 5. As a result of the determination in S5, when the operating state X reaches the return determination characteristic line Ls, the target stable flow rate Qtn is determined (S6), and when the operating state X does not reach the return determination characteristic line Ls, the process returns to S4. . As shown in FIG. 5, when the operating state X reaches a point X2 (Q2, P1) on the return determination characteristic line Ls by closing the discharge valve 5, the closing operation of the discharge valve 5 is stopped and the target stable flow rate is reached. Qtn is calculated. When the operating state X has not reached the point X2 (Q2, P1) on the return determination characteristic line LS, the closing operation of the discharge valve 5 is continued, and the flow rate reduction width of the pump is increased.

In S6, a target stable flow rate Qtn is calculated.
In this embodiment, the target stable flow rate Qtn is calculated by the following equation (1).
Qtn = (Qn + Q (n + 1)) / 2 (1)
(N = 1, 2, 3 ...)
As shown in FIG. 5, the target stable flow rate Qt1 is set to (Q1 + Q2) / 2, and the discharge valve 5 is opened so as to be the target stable flow rate Qt1 (S7).

In S8, it is determined whether or not the operating state X has reached the operating state X3 (Qt1, P2) corresponding to the target stable flow rate Qt1 by opening the discharge valve 5.
As shown in FIG. 5, when the operating state X reaches the operating state X3 (Qt1, P2) corresponding to the target stable flow rate Qt1 as a result of the determination in S8, the opening operation of the discharge valve 5 is stopped. As a result of the determination in S8, when the operating state X has not reached the operating state X3 (Qt1, P2) corresponding to the target stable flow rate Qt1, the process returns to S7 and the opening operation of the discharge valve 5 is continued.

  As shown in FIG. 5, after reaching the operating state X3 (Qt1, P2) corresponding to the target stable flow rate Qt1, the target stable flow rate Qt1 of the pump 1 is maintained (S9), and the process returns. As a result, the discharge valve 5 is adjusted to the operating state X3 (Qt1, P2) corresponding to the target stable flow rate Qt1 via the operating state X2 (Q2, P1). Can be formed.

  As shown in FIG. 5, after S9, in the operating state X, the suction pressure Pin gradually decreases while maintaining the target stable flow rate Qt1, and therefore the operating state X (Q, Pin) is on the cavitation determination characteristic line Lc. When the point X4 (Qt1, P3) is reached, the discharge valve 5 is closed to the throttle side. Thereafter, the aforementioned adjustment control is repeated, and the target stable flow rate Qt2 is adjusted to adjust the discharge valve 5 to the operating state X6 (Qt2, P4) corresponding to the target stable flow rate Qt2 via the operating state X5 (Q3, P3). An adjustment period closer to the aperture can be formed. These adjustment controls are repeated until the ballast tank 4 reaches a predetermined storage amount.

Next, the operation and effect of the pump control device will be described.
According to this pump control device, since there are few control conditions for controlling the pump 1 and there are few measuring instruments therefor, the structure of the equipment can be simplified, which is advantageous in terms of maintenance such as manufacturing cost and failure handling. When the operating state of the pump 1 reaches the cavitation determination characteristic line Lc in accordance with the decrease in the suction pressure Pin, the discharge valve 5 until the operating state X3, X6 near the middle of the return determination characteristic line Ls and the cavitation determination characteristic line Lc. , The amount of decrease in the pump flow rate Q can be suppressed compared to the pump flow rate Q when the discharge valve 5 is adjusted to the throttle side until the operating state X2, X5 on the return determination characteristic line Ls is reached. The drainage time can be shortened.

  In addition, as shown in FIGS. 4 and 5, before the discharge valve 5 is adjusted to the operation states X3 and X6 near the middle of the return determination characteristic line Ls and the cavitation determination characteristic line Lc, the discharge valve 5 is set to the operation state X3. , X6 can be adjusted to the operating states X2 and X5 on the aperture side. Therefore, the suction pressure Pin reaches the cavitation determination characteristic line Lc in the inclined state where the pressure is detected to be relatively higher than the suction pressure Pin detected in the horizontal state, and the return determination characteristic line Ls and the cavitation determination Even after the discharge valve 5 is adjusted to the throttle side until the operating state X3, X6 near the middle of the characteristic line Lc, even if the hull is inclined to the opposite side due to the reaction, the operating state X3, X6 In addition, since it is possible to form an adjustment period on the throttle side and on the throttle side with respect to the target stable flow rates Qt1 and Qt2, it is possible to suppress a decrease in the suction pressure Pin on the pump suction side, and the suction pressure Pin is necessary for the suction head characteristic line. Occurrence of cavitation due to falling below Lh can be suppressed. Moreover, since the discharge valve 2 is adjusted to the operating states X2 and X5 on the return determination characteristic line Ls in the adjustment period on the throttle side, the adjustment control of the discharge valve 5 can be simplified and the occurrence of cavitation is further suppressed. can do.

Next, a modification in which the above embodiment is partially changed will be described.
1] Although the example of the ballast pouring / draining pump has been described in the above embodiment, the present invention can be applied to a coin oil pump and the like. Further, the present invention may be applied to pump control in other fields without being limited to the ship field.

2) In the above-described embodiment, the suction pressure is kept constant from the time when the operating state reaches the cavitation determination characteristic line due to the reduction of the suction pressure to the time when the operating state reaches the return determination characteristic line. However, it is sufficient that at least the adjustment period on the throttle side can be formed, and the closing operation may be such that the suction pressure increases with the closing operation of the discharge valve.

3) In the above embodiment, an example has been described in which the target stable flow rate is set to 50% between the pump flow rate when reaching the cavitation determination characteristic line and the pump flow rate when reaching the return determination characteristic line. The effect of the present invention can be achieved by setting at least a range of 40 to 60% between the pump flow rate when reaching the cavitation determination characteristic line and the pump flow rate when reaching the return determination characteristic line. .

4) In addition, those skilled in the art can implement the present invention in various forms added with various modifications without departing from the spirit of the present invention, and the present invention includes such modifications. is there.

  The present invention relates to a pump control method and a control device for controlling so that cavitation does not occur when liquid is discharged from a tank. When the suction pressure reaches the cavitation determination characteristic line, the target stabilization is performed before the throttle is adjusted to the target stable flow rate. By forming the adjustment period on the throttle side with respect to the flow rate, it is possible to achieve both simplification of the structure and reduction of the liquid drain time and suppression of cavitation.

Reference Signs List 1 pump 2 suction pressure sensor 3 discharge pressure sensor 4 ballast tank 5 discharge valve 10 controller 11 flow rate calculation means 12 characteristic storage means 13 discharge valve adjustment control means Lh NPSH characteristic line Lc cavitation determination characteristic line Ls return determination characteristic line X operating state

Claims (2)

In a pump control method for controlling a discharge valve capable of adjusting a flow rate connected to a discharge side of a pump for pouring and draining a liquid storing tank so that cavitation does not occur when the liquid is discharged from the tank,
Using the suction pressure and flow rate on the pump suction side as parameters, the NPSH characteristic line, the cavitation determination characteristic line set on the flow rate reduction side by separating the first allowable width from the NPSH characteristic line, and the cavitation determination characteristic line (2) A return determination characteristic line set on the flow rate reduction side with a separation of the allowable width is set in advance and stored in the characteristic storage means,
A first step of calculating the current flow rate using the suction pressure on the pump suction side detected by the suction pressure detection means and the discharge pressure on the pump discharge side detected by the discharge pressure detection means;
A second step of determining whether an operating state determined by the current flow rate and the detected suction pressure has reached the cavitation determination characteristic line according to a decrease in the suction pressure;
A third step of adjusting the discharge valve to the throttle side until the operating state reaches the return determination characteristic line when an affirmative determination is made in the second step;
After the third step, the fourth step of adjusting the discharge valve to the open side until the operating state becomes an operating state near the middle of the return determination characteristic line and the cavitation determination characteristic line with respect to the flow rate, A pump control method characterized by the above. In the control device of the pump that controls the discharge valve, which is connected to the discharge side of the pump for pouring and draining the liquid storing tank, so that cavitation does not occur when the liquid is discharged from the tank,
A suction pressure detecting means for detecting a suction pressure on the pump suction side;
A discharge pressure detecting means for detecting a discharge pressure on the pump discharge side;
Control means for controlling the discharge valve based on the suction pressure and the discharge pressure detected by the both detection means,
The control means includes
A flow rate calculation means for calculating a current flow rate based on the detected suction pressure and discharge pressure;
The suction pressure and flow rate on the pump suction side are set in advance as parameters, the NPSH characteristic line, the cavitation determination characteristic line set on the flow rate reduction side with a first allowable width distance from the NPSH characteristic line, and the cavitation determination characteristic Characteristic storage means for storing a return determination characteristic line set on the flow rate reduction side with a second allowable width away from the line;
When the operating state determined by the current flow rate calculated by the flow rate calculation means and the detected suction pressure reaches the cavitation determination characteristic line according to the reduction of the suction pressure, the operating state is set on the return determination characteristic line. And a discharge valve adjustment control means for adjusting the discharge valve to the open side until the operating state is in the middle of the return determination characteristic line and the cavitation determination characteristic line with respect to the flow rate after the discharge valve is adjusted to the throttle side. A control device for a pump.

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