Could you define NRV?

Non-return valves (NRV) are generally needed when operating centrifugal pumps in parallel to prevent reverse flow through non-running pumps. I don't believe their use in mandated, however, and their use might be avoided under certain conditions, namely:
  1. If the flow diverted through the idle pump(s) does not compromise the required performance of the fluid system. This may rarely be true for critical heat transfer fluid systems like boiler feed or reactor coolant systems although it might be accomodated in initial design so as to avoid non-return valve problems like waterhammer piping loading and NR valve failures usually caused by fatigue of movable components.
  2. The idle pumps can withstand the reverse flow through the pump that also drives the motor shaft in reverse. If the bearings can operate in both rotational directions and the reverse flowrates are relatively low, the rotor "runaway" speeds might be no higher than forward rotation. This is usually true for low specific speed centrifugal pumps but not for axial flow pumps of high specific speed. If the runaway speeds are too high, then anti-rotation pawls can be used on the motor shaft to prevent reverse rotation.

Use of NRVs in pump discharge is not mandatory but very useful in the cases already told above. Reverse flow will damage, mainly, mechanical seals and also in parallel installations (with standby pumps) flow gets bypassed.

Regards,

If you are designing for the oil and gas industry and API-14C is applicable, then from Section A.7 of the API, A check valve should be provided in the pump discharge line to minimize backflow.

no doubt mucour deserves the star for this posting.
a.

On pumps that have threaded on impellers, reverse flow can unscrew the impeller which is not good.  In addition reverse flow from an adjacent pump is wasted horsepower.  A non return valve will prevent reverse flow from the other pump or from its own operation after pump shutdown.

If you want to eliminate non return valves for two 100% capacity pumps you need to have one dedicated disharge line per pump.  To tolerate the reverse flow when the pump is shutdown make sure the impeller is a keyed on design.  For sumps with a lot of trash and debris that will clog a non return valve a good design basis will eliminate the non return valve.

OK, dumb question here.  Why doesn't a check valve in the discharge line do this job?  Why do I necessarily need a MOV?  Seems to me you size and select the check valve appropriate for the job at hand (swing, soft-seated, delayed-closing, etc.) - no?

Thanks!
Pete