Centrifugal pump behavior when the outlet is closed

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Discussion Overview

The discussion revolves around the behavior of a centrifugal pump when its outlet is closed while the pump continues to operate. Participants explore the implications for pressure, temperature, and mechanical integrity of the pump components, including the impeller and seals. The conversation includes theoretical considerations, practical experiences, and inquiries about pump specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that closing the outlet will lead to a small step increase in outlet pressure and a slight increase in the RPM of the motor due to reduced load.
  • There is a concern about the potential for water hammer and its effects on the pump's seals and overall system integrity.
  • One participant shares an experience of a pump boiling liquid in the casing when the discharge was inadvertently closed, noting that the seal remained intact due to an external seal flush.
  • Participants discuss the mechanics of how the impeller interacts with the trapped water, questioning why it does not break under pressure.
  • There are inquiries about the maximum pressure a centrifugal pump can generate and how it relates to the pressure experienced when the outlet is closed.
  • Some participants express confusion about the relationship between dead head pressure and actual operating pressure when a faucet is directly connected to the pump.
  • Discussions include the potential dangers of high-pressure water jets and the conditions under which they can cause injury.

Areas of Agreement / Disagreement

Participants express a range of views on the implications of closing the outlet, with no consensus on the exact outcomes or the mechanics involved. Some points are reiterated by multiple participants, but there remains uncertainty regarding the behavior of the pump and the effects on its components.

Contextual Notes

Limitations include varying definitions of pressure and flow conditions, as well as the absence of specific pump curve data for certain models discussed. The discussion also reflects differing levels of familiarity with pump mechanics and terminology.

Who May Find This Useful

This discussion may be useful for individuals interested in pump operation, mechanical engineering, and fluid dynamics, particularly those exploring the effects of system changes on pump performance.

  • #31
Baluncore said:
When the outlet is turned off there will be three things happen.

1. A small step increase in outlet pressure, as the flow stops.

2. A slight increase in RPM of the induction motor as pump load and slip is reduced.
The specified 3450 RPM will rise part way towards synchronous 3600 RPM.

3. The beginning of a gradual increase in temperature of water trapped in the pump.

Hi, I just read that cavitation can also occur when pump outlet is turned off. Wont this destroy the impeller too just like closed inlet? My plumber has not yet touched my systems because I am not sure he and his companion has Covid or not and awaiting vaccines. see:

https://www.google.com/amp/s/blog.craneengineering.net/what-is-pump-cavitation?hs_amp=true
"
When a pump's discharge pressure is extremely high or runs at less than 10% of its best efficiency point (BEP), discharge cavitation occurs. The high discharge pressure makes it difficult for the fluid to flow out of the pump, so it circulates inside the pump. Liquid flows between the impeller and the housing at very high velocity, causing a vacuum at the housing wall and the formation of bubbles.

As with suction cavitation, the implosion of those bubbles triggers intense shockwaves, causing premature wear of the impeller tips and pump housing. In extreme cases, discharge cavitation can cause the impeller shaft to break.

Possible causes of discharge cavitation:

  • Blockage in the pipe on discharge side
  • Clogged filters or strainers
  • Running too far left on the pump curve
  • Poor piping design"
 

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