Why Close The Valve When The Centrifugal Pump Starts?

Mar 06, 2021

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 lift 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), and it is easy to overload, which will damage the motor and circuit of the pump. Therefore, the outlet valve should be closed when the pump is started, so that the pump can be started The pump operates normally. Closing the outlet valve is equal to manually setting the pipe resistance pressure. With the normal operation of the pump, slowly start the valve to make the pump work normally along its performance curve.

Two points must be ensured before starting centrifugal pump

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

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

Centrifugal pump is to rely on the centrifugal force of the impeller to form a vacuum suction to lift the water, therefore, when the centrifugal pump starts, the outlet valve must be closed first to fill water. When the water level exceeds 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 automatically open and lift up the water. Therefore, the outlet valve must be closed first.

About centrifugal pump

Centrifugal pump is a kind of vane pump, relying on the rotating impeller, in the process of rotation, due to the interaction between the blade and liquid, the blade will transfer the mechanical energy to the liquid, so that the pressure of the liquid can be increased, so as to achieve the purpose of conveying liquid. Centrifugal pump has the following characteristics:

1. There is a limited value for the lift of centrifugal pump at a certain speed. The flow rate and shaft power at the working point depend on the condition of the device system connected with the pump (position difference, pressure difference and pipeline loss). The head changes with the flow rate.

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

3. Generally, there is no self-priming capacity, so the pump needs to be filled with liquid or the pipeline is vacuumized before it can start to work.

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


Before the start of the pump, the pump shell is filled with the transported liquid; after the start, the impeller is driven by the shaft to rotate at high speed, 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 shell.

In the spiral case, the liquid slows down due to the gradual expansion of the flow channel, and then transforms 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 needs to be. 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. Because 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. Visible, as long as the impeller continues to rotate, the liquid will continue to be inhaled and discharged.


Start up of other centrifugal pumps

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


Large flow starting characteristics of axial flow pump

When the axial-flow pump is started with full open valve, the maximum shaft power is 140% ~ 200% of the rated shaft power under zero flow condition, and the minimum power is under the maximum flow condition. 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).


Start up characteristics of mixed flow pump

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


Start up characteristics of vortex pump

The maximum shaft power is 130% ~ 190% of the rated shaft power under zero flow condition. Therefore, similar to the axial flow pump, the start-up characteristic of the vortex pump should be high flow (i.e. full open valve start-up).