How does isolation valve induces vibration?

[ghensky]

We have two inlet sources for two pumps system (one being the spare pump) isolated by two ball valves. Both pumps performs perfectly separately when the respective isolated valve is 100% closed. However if the isolation valve is not closed 100%, both pumps exhibit vibration. What's the mechanism playing here for the vibration? Is it due to too much NPSHa? How does too much NPSHa negatively affect pump performance?

Sorry for the poor illustration of the pump systems made by using the doodle buddy.

Thanks.

 

[MFJewell]

Maybe cavitation across the partially open valve.

 

[adammal44]

Do you have any additional information? Is the pump running correctly (right flow/pressure)?

It could be swirling effects, due to odd/changing amounts of fluid entering your branch tee into the pumps.

It could be the pumps churning to find an operating point. As your pumps move up-and-down the flow-head operating curve, the flow conditions coming from the shared source line could be changing enough that the pumps are trying to adjust continuously. Rare to see that on the suction side, unless you have some feed forward logic.

 

[mk3223]

It seems that, as one pump running and the other one in standby, you may open up either one of the isolation valve to provide suction to the pump as needed. In the case, the isolation valve is in the fully open mode to ensure the adequate NPHAa to the pump.
Instead of one suction supply, you can also open up two isolation valves to provide two suctions to one pump. But, you should open them 100% to ensure the adequate flow.

 

[hydtools]

If the intent is to isolate each pump, the the isolation valves should be in each pumps suction branch and not in the common reservoir feed lines. Then each pump can draw freely from both reservoirs and its own isolation valve.

Ted