Why we need to close the(discharge) isolation valve of a centrifugal pump during start up.

Why we need to close the(discharge) isolation valve of a centrifugal pump during startup⁉️ 

# Answer(1) 

✳️ It is general practice to close the delivery valve of the centrifugal pump during start-up and it is highly recommended for the critical services where the liquid is at boiling/bubble point (Ex. Reflux pump of distillation column). The procedure is to close the valve and start the pump. After this open discharge valve slowly till 100% this stabilizes the complete system. If this procedure is not followed and the valve is kept 100% open and the pump is started, NPSHa for this will be minimum because of losses due to acceleration during startup. This will result in cavitation of the pump. 

#️ Answer(2)

 1️⃣ To minimize the starting torque on the motor.

 2️⃣ To load the delivery pipe network gradually if it is the only pump that feeds the network.

 3️⃣ If it is a parallel-connected pump, this method will insure against any reverse flow due to malfunctioning of the upstream side check valve. 

# There are some centrifugal pumps, called axial or propeller pumps, for which the delivery valve has to be kept open at a min. % as determined by the pump characteristics so, the reason for keeping the delivery valve in the minimum open condition is to minimize the starting torque imposed on the motor.