I have a customer using a Gorman Rupp 4" self-priming centrifugal trash handling pump in a hot water application, 170=BAF max. There is 1ft of postive suction lift, so the pump really does not have to prime. Based upon the NPSHr of the pump and what is available, the customer should have plenty of NPSHA, even when considering the higher vapor pressure of the hot water. He is pulling from a tank open to atmosphere. There is an automatic air vent on the pump discharge to purge air from the pump. This goes back to the tank, emptying just above the fluid level.
The problem: Periodically, the pump will quit putting out flow, except for a very small amount. It is not getting clogged up, and there are no restrictions in the suction pipe or the discharge pipe. The rpm is a constant 1170, as expected. I did notice; however, that there is a 6x4 concentric reducer on the pump suction line, which, according to everything I've read is a "no-no". An eccentric reducer is preferred with the flat side installed at the top. The entire pump and suction line was drained, and refilled. After that, the pump ran like a champ again.
My question: Could a concentric reducer form enough of an air bubble to drastically reduce the flow? I know they are not supposed to be used in this manner, but I can't see how this could affect the flow output so much.=20
Thanks,
Greg