JPS60145487A - Min. flow-rate control for pump - Google Patents

Abstract

PURPOSE:To reduce the energy loss by controlling the number of revolution of a motor according to the flow rate and the pressure of a pump driven by the motor and permitting the low flow-rate operation of the pump, keeping the pressure of the pump constant according to the min. flow-rate value. CONSTITUTION:A speed controller 6 for a motor 5 for driving a pump 1 is operated according to a detector 7 for the speed control information such as the flow rate and the pressure in a main flow-rate line 2 and controls the number of revolutions corresponding to the flow-rate value. When the flow rate reduces, a control instruction is transmitted into the speed controller 6, and the number of revolutions of the motor 5 is controlled to reduction, and the pressure can be controlled to a constant value even at the min. flow-rate point. Therefore, in a submerged pump, the temperature rise of the inhaled liquid is reduced by the reduction of the amount of the used electric power, and the necessary min. flow- rate for preventing caviation can be reduced further, and the pressure and the amount of used electric power for securing the min. flow rate can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling the minimum flow rate of a centrifugal pump that handles a liquid that is easily vaporized, in which control is performed to reduce the discharge pressure when the minimum flow rate is caused to flow.

In a centrifugal pump that handles liquids that easily vaporize, securing the minimum flow rate to prevent cavitation in the pump is as follows:
This is generally achieved by providing a minimum flow line using a bypass pipe. That is, as shown in Fig. 1, when liquid is being discharged through the main flow line 2 connected to the discharge side of the pump 1, the operating situation is as shown in Fig. 2, and as shown by the solid line ■. Then, the pump 1 is operated at high pressure from the steady operating point A to the minimum flow point B, and the electric power of the motor that drives the pump 1 changes as shown by the broken line (■).

In the above operation, when the liquid flow rate discharged through the main flow line 2 becomes to the left of the minimum flow point B in FIG. 2, that is, below the minimum flow value of the main flow line 2,
Pump cavitation occurs.

Therefore, in order to prevent cavitation from occurring even if the discharge flow rate of the liquid reaches the minimum flow rate, conventionally, a bypass pipe is used as the minimum flow line 3 that extends from the middle of the main flow line 2 to the tank (not shown). A bypass valve 4 is provided in the middle of the bypass pipe, and when the discharge flow rate of liquid reaches the minimum flow rate, the bypass valve 4 is automatically opened and the liquid is returned to the tank through the minimum flow line 3, thereby ensuring the minimum flow rate and preventing cavitation. We are trying to prevent this.

However, in the conventional system described above, the discharge pressure of the pump 1 when flowing liquid through the minimum flow line 3 when the flow becomes below the minimum flow m value remains high as is clear from FIG. The electric power required for the motor was also high, resulting in a large energy loss.

The present invention was made for the purpose of preventing the above-mentioned energy loss by controlling the discharge pressure of the pump to be lowered when the minimum flow rate is allowed to flow through the minimum flow line. By controlling the rotational speed of the motor according to the flow rate of the pump, the discharge pressure of the pump is kept constant and low flow rate operation is possible.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 shows an outline of a device for carrying out the method of the present invention, in which a rotation speed controller 6 is connected to a motor 5 that drives a pump 1, and a signal from the rotation speed controller 6 is used to control the motor. The rotation speed of the pump 5 is controlled to adjust the discharge pressure of the pump 1. The rotation speed controller 6 is configured to operate based on information from the rotation speed control information detector 7, such as the flow rate and pressure of the main flow line 2, so that the rotation speed is controlled according to the flow rate value. do.

Now, when liquid is being discharged through the main flow line 3, the pump 1 is operated at a high pressure. The relationship between the flow rate, discharge pressure, and electric energy during steady operation is as shown in Fig. 4. At the steady operation point A, as shown by the solid line ■, the pressure and flow rate are included, and the electric power is also shown by the broken line. It is large as shown by ■.

When the discharge flow rate starts to decrease from the above-mentioned steady operating state, a control command is given to the rotation speed controller 6 based on the rotation speed control information (discharge pressure and/or flow rate), and based on this, a control command is given to the rotation speed controller 6.
The rotation speed of the engine is controlled to decrease. When the rotation speed of the motor 5 is controlled to be low, the pump 1 is controlled accordingly.
Since the rotation of is also controlled, the pump discharge pressure at the minimum flow point falls to point B' in FIG. 4, and the amount of electric power can also be reduced from the level shown by the broken line ■ to the point shown by the broken line ■'. As is clear from FIG. 4, since the minimum flow point has been lowered from B to B' and is closer to the flow rate reduction side, the required minimum flow rate can also be reduced compared to the conventional system.

In the present invention, especially in a pump with a built-in motor or a pumped pump, the temperature rise of the suction liquid is reduced due to the reduction in the amount of power used, and the minimum flow rate required to prevent cavitation can be further reduced.

As described above, according to the present invention, since it is possible to operate a pump that requires continuous operation below the minimum flow rate by controlling the rotation speed of the motor, the discharge pressure of the pump when the minimum flow rate is secured, and the power consumption The present invention eliminates the energy loss that occurred when the minimum flow rate was returned to the tank with high pump pressure even when the minimum flow rate was secured, resulting in energy savings. can be achieved.

[Brief explanation of drawings]

Fig. 1 is an example of the conventional method for securing the minimum flow rate, Fig. 2 is a diagram that does not show the relationship between the flow rate, discharge pressure, and electric power in the case of Fig. 1, and Fig. 3 is a diagram of the method of the present invention. FIG. 4, which is a schematic diagram of the apparatus to be implemented, is a diagram showing the relationship between flow rate, discharge pressure, and electric power in the case of FIG. 3. 1 is a pump, 2 is a main flow line, 3 is a minimum flow line, 4 is a bypass valve, 5 is a motor, and 6 is a rotation speed controller. Patent Application Person Ishikawajima Harima Heavy Industries Co., Ltd. Patent Applicant Representative 6 - 4th Illustration Meeting

Claims (1)

[Claims] 1) A pump characterized in that the rotational speed of the motor is controlled using the value of the flow rate flowing through the discharge passage from the pump driven by the motor or the discharge pressure as information, and the pump is controlled according to the minimum flow value. Flow control method.