Why Close the Discharge Valve Before Starting the Centrifugal Pump?

Reasons for closing the discharge valve

The starting current can be reduced by closing the discharge valve. This is the core reason for the standard operating procedure to close the discharge valve of a centrifugal pump during startup.

The current will be very high when any motor starts. When the pump is started with an open discharge valve, the discharge head will have greater resistance to the pump. Therefore, the motor must give the pump a larger starting torque, which means that the motor needs to draw more current.

If the discharge valves are closed when the pump is started, it means that there is no discharge head and minimum resistance to the pump, and therefore the output current is the smallest. Although the current is greater than the normal value when the motor starts, closing the discharge valve of the centrifugal pump can avoid additional load on the pump.

Situations when the discharge valve can be opened

The discharge valves can be opened when there is a check valve. If there is pressure on the discharge side of the centrifugal pump without a check valve when starting, the discharge valve should be closed. With a transparent plastic pipe, leakage can be easily found. It should be used with clear PVC fittings to perform well. It's good to work with clear PVC pipe manufacturer to solve problems together.

Risks of the unclosed discharge valve

Thinking from another aspect, if there are pressures at the discharge end of the pump, it may flow back into the pump before starting. And it will cause backward rotation, which can attract more current and cause damage to the pump. This scenario highlights a critical operational risk and reinforces why we close the discharge valve of centrifugal pump before starting—to prevent mechanical damage from reverse rotation. Therefore, it can be prevented by closing the discharge valve of the centrifugal pump when it starts.