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Why the outlet valve need to be closed when the first time of centrifugal pump started

Why the outlet valve need to be closed when the first time of centrifugal pump started.

Water pump Start closing pressure means when you start the centrifugal water pump at first time if there is no water in the pipeline,you ‘d better close the outlet pipe valve.
Why？because at the first time ,there is no water in the pipe,in the case of zero flow, the electric pump head is the highest,the power is minimum,With the opening of the outlet valve increased,the flow rate increases, the head decreases, and the power increases, efficiency increases maximum,If the outlet valve is fully opened at firt time , the flow of the pump may deviate from the rated flow , the water pump efficiency is also reduced a lot,
As a result, the shaft power is greater than the motor power, causing tripping or even burning the motor.Therefore, the operation rules of multistage centrifugal pumps require that the inlet valve be opened before starting, and the outlet valve shall be closed.However, the outlet valve closed time can not be too long, because at this time the liquid in the pump cycling, the liquid temperature gradually increased, easy to cause mechanical seals, due to lack of lubrication friction and wear or damage.
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