Why bypass Non-Return Valve (NRV) ?

Someone raised a question :

Non-Return Valve (NRV) is provided on centrifugal pump discharge to avoid pump damage due to backflow. In some special case, designer provide a manual block valve across the NRV. Why ? Isn't it defeat the purpose of NRV ?

Infact there are several reasons behind this application...

Art Montemayor :

" 1. Prime the pump with the discharge fluid that is on the downstream side of the check valve

2. Drain the discharge fluid that is on the downstream side of the check valve."

JoeWong :

"If the pump is started-up automatically whenever the duty pump failed, then the standby pump discharge isolation valve shall always open. The bypass gate shall always open to allow small amount of hot fluid from discharge manifold back to suction manifold. Allowing hot fluid passing the standby pump will always keep the pump hot. Whenever the standby pump kick in, it is ready to receive hot fluid avoid pump crack due to uneven thermal expansion. Sometime designer drill a hole at check valve to perform similar function..."

So got it ?

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