Centrifugal Pump: Water Pressure at 50m & 4 Bar Gauge

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Discussion Overview

The discussion revolves around the behavior of water in a centrifugal pump system when a valve is opened to atmospheric pressure. Participants explore the implications of pressure measurements, the effects of gravity, and the role of compressed air in the tank.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the 4 bar gauge pressure is measured at the valve/pump or in the tank.
  • Another participant clarifies that the 4 bar is measured at the tank, suggesting that opening the valve will cause the water level to fall.
  • It is proposed that if there is compressed air at the top of the tank, the water level may drop about 10 m, while if there is no air, the drop will be minimal, and the tank pressure will increase to 5 bar gauge.
  • A participant raises a question about the necessity of the pump, suggesting that gravity could fill the tank instead.
  • In response, another participant explains that the pump is needed for control, as the vessel must be flooded at a controlled rate.
  • It is mentioned that further compressed air may be used to push water and induce flow if dewatering is required.

Areas of Agreement / Disagreement

Participants express differing views on the implications of pressure changes and the necessity of the pump. The discussion remains unresolved regarding the exact behavior of the water level and the role of the pump in this context.

Contextual Notes

Participants do not fully agree on the effects of pressure and gravity on the water level, and there are assumptions regarding the presence of compressed air that are not fully explored.

Fellps
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I have a centrifugal pump 50 m above a vessel, I’m pumping water in at 4 bar gauge into this vessel. I then close a valve to cease pumping ensuring no air enters the system.​
My question is, if I now open that same valve to atmospheric air this time, will the water exit from the pipe through the valve at a 4 bar pressure differential. Or will it, the water, go down the pipe to 40 m from its originally closed valve position of 50 m.
 
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Fellps said:
I have a centrifugal pump 50 m above a vessel, I’m pumping water in at 4 bar gauge into this vessel.​
Is the 4bar measured at the valve/pump or in the tank?
 
russ_watters said:
Is the 4bar measured at the valve/pump or in the tank?
At the tank
 
Fellps said:
At the tank
Then the pressure at the valve is roughly 0 absolute and opening the valve to atmosphere will cause the water level to fall. How much it falls will depend on how the tank is being pressurized. If there is compressed air at the top of the tank, the water level will drop about 10m. If there is no air in the tank and it is just solid water, it will only drop a little and the tank pressure will increase to 5 bar gauge since water is incompressible.

...of course, that begs the question: why do you need a pump to begin with? Why couldn't you just let gravity fill the tank?
 
russ_watters said:
Then the pressure at the valve is roughly 0 absolute and opening the valve to atmosphere will cause the water level to fall. How much it falls will depend on how the tank is being pressurized. If there is compressed air at the top of the tank, the water level will drop about 10m. If there is no air in the tank and it is just solid water, it will only drop a little and the tank pressure will increase to 5 bar gauge since water is incompressible.

...of course, that begs the question: why do you need a pump to begin with? Why couldn't you just let gravity fill the tank?
Thank you very much, the need for the pump comes from the need for control. The vessel must be remotley flooded at a controlled rate.

The vessel will contain compressed air. Should a dewatering process be required, further compressed air will be pumped into the vessel to 'push' the water the extra 10 m and induce a flow rate
 

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