Why Discharge Valve Closed During Starting of Centrifugal Pump ?

 >By closing the discharge valve, we can reduce the starting current

>As you know,the current will be high during the starting of any motor. If you start the pump with discharge valve open, the discharge head will act on the pump ie, more resistance, so the motor has to give more starting torque to the pump which means more current is drawn by the motor. 

>If you start the pump discharge valve closed, it means no discharge head minimum resistance to the pump and minimum current drawn. Even though current taken during startup is more than normal, by closing discharge valve you can avoid extra load on the pump

>If you have a check valve, then the discharge valve can be opened. Without a check valve, you need to have the discharge valve closed if there is pressure on the discharge side at the time of startup.

>Thinking on a different way , If there is pressure on the discharge side of the pump,prior to start up, it can flow back  through the pump, causing a backward spin and may draw more current, thereby causing damage to  the pump. It can also be prevented by keeping the discharge valve closed.

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Explanation Using Pump Characteristics

>The discharge valve of the pump has to be closed during startup. This is maintained so, inorder  to prevent sudden load on the power system, if the valve is open. If you look at the characteristics of the pump, the power required ,increases with flow rate. If the valve is open and the pump is switched on, there is a sudden power load corresponding to the flow rate. If the valve is closed (zero flow) the power load is minimum ie minimum current.