Vapor/bubbles after pump

[Guybob]

Hi everyone,
Got a question I'm trying to solve here. It is for an experiment I'm running at achool.

I got a pipe system with a centrifugal pump. There is cavitation in front of the pump due to the pressure falling below the water Vapor pressure. When the water flow enters to the backwards fan centrifugal pump, the Vapor/bubbles should collapse due to the high pressure at that location. But there are a few bubbles that passes through the pump. Why is this the case and what is happening?

Thanks for viewing!

 

[OldYat47]

Best guesses: Temperature increase in the pump caused by low flow conditions (throttled discharge). Vane pass (impeller tip passing too close to the cutwater). Aspiration, basically leaking gaskets and/or shaft seals.

Describe the "backward fan" please.

 

[Mech_Engineer]

It might be possible that some dissolved gases are coming out of solution due to the pressure drop (and/or temperature increase) within the pump. This would then show up as bubbles in the flow.

 

[OldYat47]

On the downstream side that occurs if the flow is throttled so much that the fluid heats up in the pump. First in my list above. Note that the original post says the bubbles are downstream from the pump.