Why Close The Valve When Starting The Centrifugal Pump?

When the centrifugal pump is started, there is no water in the outlet pipeline of the pump, so there is no pipeline resistance and lifting height resistance. After the centrifugal pump is started, the head of the centrifugal pump is very low and the flow is very large. At this time, the output of the pump motor (shaft power) is very large (according to the pump performance curve), which is easy to overload, which will damage the pump motor and circuit. Therefore, close the outlet valve when starting, In order to make the pump operate normally. Closing the outlet valve is equal to artificially setting the pipe resistance pressure. After the pump operates normally, slowly start the valve to make the pump work normally step by step along the law of its performance curve.

Two points must be ensured before starting the centrifugal pump:

1. Fill the pump casing with water to form a vacuum;

2. The valve on the water outlet pipe must be closed so that the water pump does not form a flow, which can reduce the motor starting current and facilitate the smooth start of the water pump. With the smooth start of the water pump, the gate valve should be opened slowly and timely.

Centrifugal pump relies on the suction of vacuum formed by the centrifugal force of impeller to lift the water. Therefore, when starting the centrifugal pump, you must first close the outlet valve and fill water. When the water level exceeds the position of the impeller, the centrifugal pump can be started only after the air in the centrifugal pump is discharged. After starting, a vacuum is formed around the impeller to suck up the water, which can be opened automatically and lift the water. Therefore, the outlet valve must be closed first.

About centrifugal pump:

Centrifugal pump is a vane pump, which relies on the rotating impeller. In the process of rotation, due to the interaction between the blade and the liquid, the blade transmits mechanical energy to the liquid, so that the pressure of the liquid can be increased to achieve the purpose of conveying the liquid. The centrifugal pump has the following characteristics:

1. There is a limit value for the head generated by the centrifugal pump at a certain speed. The operating point flow and shaft power depend on the condition of the device system connected to the pump (level difference, pressure difference and pipeline loss). The head varies with the flow.

2. Stable operation, continuous transportation, and no pulsation of flow and pressure.

3. Generally, it has no self-priming capacity. It is necessary to fill the pump with liquid or vacuum the pipeline before starting to work.

4. The centrifugal pump is started when the discharge pipeline valve is closed, and the vortex pump and axial flow pump are started when the valve is fully open to reduce the starting power.


Before the pump is started, the pump shell is filled with transported liquid; After startup, the impeller rotates at a high speed driven by the shaft, and the liquid between the blades must also rotate with it. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge and obtains energy, leaving the outer edge of the impeller at high speed and entering the volute pump housing.

In the volute, the liquid decelerates due to the gradual expansion of the flow channel, converts part of the kinetic energy into static pressure energy, and finally flows into the discharge pipe at a higher pressure and is sent to the place where it is needed. When the liquid flows from the center of the impeller to the outer edge, a certain vacuum is formed in the center of the impeller. Since the pressure above the liquid level of the storage tank is greater than the pressure at the inlet of the pump, the liquid is continuously pressed into the impeller. It can be seen that as long as the impeller rotates continuously, the liquid will be sucked in and discharged continuously.


Start up of other centrifugal pumps:

The above mentioned are centrifugal pumps. For other types of pumps, the situation is as follows:

Large flow starting characteristics of:

01 axial flow pump

When the full open valve starts the axial flow pump, the shaft power is the maximum under zero flow condition, which is 140% ~ 200% of the rated shaft power, and the power is the minimum at maximum flow. Therefore, in order to minimize the starting current, the starting characteristic of shaft power should be large flow starting (i.e. full open valve starting).

02 starting characteristics of mixed flow pump

When the mixed flow pump is started with full open valve, the shaft power is between the above two pumps under zero flow condition, which is 100% ~ 130% of the rated power. Therefore, the starting characteristics of the mixed flow pump should also be between the above two pumps, and it is best to start with full open valve.

03 startup characteristics of vortex pump

Full open valve start vortex pump has the maximum shaft power under zero flow condition, which is 130% ~ 190% of the rated shaft power. Therefore, similar to axial flow pump, the starting characteristic of vortex pump should be large flow start (i.e. full open valve start).

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