Why close the outlet valve when the centrifugal pump starts?

When the centrifugal pump starts up, there is no water in the outlet line of the pump, so there is no pipeline resistance and lift resistance. After the centrifugal pump is started, the centrifugal pump has a very low lift and a large flow rate. At this time, the pump motor (shaft power) output is very large (according to the pump performance curve), it is easy to overload, will cause the pump motor and circuit damage, so when starting to close the outlet valve, can make the pump normal operation.
If the axial flow pump is used on the contrary, it must be started by the open valve. In this case, the power of the motor will run at a high flow rate in a minimum short period of time, resulting in pump vibration, noise, and even overload operation of the motor, burn down the motor.
Closing the outlet valve is equivalent to artificially setting the resistance pressure of the pipe, starting the valve slowly with the normal operation of the pump, allowing the pump to work normally according to the law of its performance curve step by step.
Before the centrifugal pump starts, two things must be guaranteed:
(1)Filling the pump shell with water for the purpose of forming a vacuum;
(2)The gate valve on the outlet pipe must be closed, so that the pump does not form the flow rate, which can reduce the starting current of the motor and is conducive to the smooth start of the pump. With the smooth start of the pump, the gate valve should be opened slowly and in time.
The centrifugal pump lifts the water by the suction force that forms the vacuum by the centrifugal force of the impeller. Therefore, when the centrifugal pump is started, the outlet valve must be closed and watered first. When the water level exceeds the impeller position and the air in the centrifugal pump is discharged, it can be started. After starting, a vacuum is formed around the impeller to suck up the water, which can be automatically opened to lift the water. Therefore, the outlet valve must be closed first.