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Pump performance unstable.

Pump performance unstable.

Pump performance unstable.

This is a follow on from a previous question...see attached sketch
I have a large condenser water system. Cooling tower , pumps etc. Back to 3 chillers

When ever all the pumps atop the water continues to fill into the towers from the FLOW IN side and it overflows the towers. Is this due to siphoning? Also whenever pumps are started or stopped quickly through their VFDs the discharge gauge starts vibrating and shaking and there is a fierce racket as if air has hot into them. I cannot u understand if there is a fundamental hydraulic problem in the overall piping et up
Your thoughts

RE: Pump performance unstable.

in my opinion, overflowing due to siphoning.is the NRV functioning??

RE: Pump performance unstable.

Well like I said on your previous post I think your issue is to do with balancing levels and flows.

If the water keeps coming into the towers then it has to be coming from somewhere or air is getting in somewhere.

So the questions are:
Does this happen for all the towers or only some?
Where is your instrumentation (Pressure, flow, level)
Can you mark up your diagram with their location
What is this instrumentation telling you before you turn it off?
How do you balance flows and levels?
Your previous diagram shows a balance line - how big is this?
What volume are these towers from normal liquid level to overflow
what overflows? the whole tower or the basins?
If the basins what volume is required to overflow?
What does the VFD control on? Flow?, level?,
Why is your pump inlet line cross connected? Can you isolate one set of two CTs from the other?
Is the air vent manual or automatic / vacuum release
You show 111 l/sec aka 400 m3/hr per pump - What is the "normal" or Max flows in the whole system?
Any pictures or diagrams or links to the tower design?

As said before for the start/stop thing I think you're getting preferential flow into different parts of the system and hence drawing one tower down before the others and then getting air or vortexing, but this is just one possibility.

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