Why are there still bubbles after the pump?

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Discussion Overview

The discussion revolves around the persistence of bubbles in a pipe system with a centrifugal pump, specifically addressing the phenomenon of cavitation and the behavior of vapor bubbles as they pass through the pump. The context includes experimental observations and potential causes for the observed bubble formation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that cavitation occurs when pressure falls below the water vapor pressure, leading to bubble formation before the pump.
  • Another participant suggests that bubbles may persist due to temperature increases in the pump caused by low flow conditions, or due to the impeller tip passing too close to the cutwater.
  • It is proposed that dissolved gases might be coming out of solution due to pressure drops or temperature increases within the pump, contributing to bubble formation.
  • A later reply emphasizes that throttling the flow can lead to heating of the fluid in the pump, which may explain the presence of bubbles downstream from the pump.

Areas of Agreement / Disagreement

Participants present multiple competing views on the causes of the bubbles, and the discussion remains unresolved regarding the definitive explanation for the phenomenon.

Contextual Notes

The discussion does not clarify the specific conditions under which the bubbles are observed, nor does it resolve the interplay between pressure, temperature, and flow conditions in the pump system.

Guybob
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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!
 
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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.
 
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.
 
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.
 

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