Centrifugal Pump Shut-Off

[Ste77]

Hello,
I simulated a centrifugal pump shut-off in two different conditions: with recycle valve open and with recycle valve closed.
In the first case recylce valve starts to open when flow reachs a set-point value from an higher rated flow and remains open for the entire shut-off period; in the second case valves remains closed.
In your opinion, the shut-off time is longer with valve closed or not? Take in consideration that at pump discharge there's a check valve provided in order to maintain a constant downstream pressure. This closes almost instantly at time of shutdown.

Thanks
Best regerds

 

[roker]

Hi,

The recycle valve is usually installed to protect the pump to maintain minimum flow to prevent overheating of the pump, if you are shutting off the pump meaning the flow is decreased till zero flow with maximum head(shut-off), the recycle valve has no meaning.
Please clarify the shut-off time.
Check valves are one way direction flow devices to prevent backflow not a pressure control device.

regards,
roker

 

[SeanB]

Pump shut off happens when there is no flow from the pump - the pump is dead headed. The recycle valve is there to maintain the minimum required flow to the pump and to prevent the pump from becoming dead headed.

 

[Ste77]

I'm sorry, I probably don't explained properly the problem.
"Recycle valve" is a minimum flow valve which tries to maintain a set point on flow through the pump. So, when pump shut off occurr, flow decrease quickly and the controller, if enabled, opens "minimum flow valve" in order to maintain set point.
In field was measured the time it takes the impeller to get 0 rpm due to his inertia with "minimum flow valve" opened.
I wonder if, with "minimum flow valve" closed during the whole time event, the impeller will stop in a longer time or not.
I am enclosing a simplified diagram of the system (fluid is water)
Thanks

 

[pumpking]

I cant really understand what you are asking or what the problem is ?

It sounds as thought the controller you refer to stops the pump (like an ITT Hydrovar system) in which case Im not understanding the purpose of the recirculation line in the first place.

Is the time the Impeller takes to physically stop an issue ? If so, it would be difficult to put a physical time on things, you can get rotation of Impellers caused by syphoning effect as systems 'balance' themselves out once powered down, but generally i would say the impeller would stop quicker if both valves were closed.

Not sure, but hope it helps ?????

Ash Fenn

http://www.cdrpumps.co.uk

 

[Artisi]

Likewise, I fail to see what you are asking or trying to achieve. Can you rephrase or add some additional info'.

 

[pmover]

Yes, the question makes no sense.

Centrifugal pump shut-off has only one condition - zero flow through the pump @ maximum head. This is regardless of recycle valve position (if properly installed). Typically, with recycle valve open, presumably there is still flow through the pump.

Good Luck!
-pmover

 

[Ste77]

The time to stop the impeller is important in term of lubrication which is provided only for a limited time.
In a first time I thought that the impeller will stop quickly with the "minimum flow valve" closed, instead the simulation result is exactly the opposite. This could be due to the fact that with valve opened the impeller gives part of its kinetic energy to the a flow; with valve closed, flow is null so kinetics energy is consumed by friction.
I just wanted to know if any of you have had this experience in the field.
Thank you very much

 

[Artisi]

If the pump casing is full at the time of shut-down, lubrication takes care of itself - so run-down time is not important for this requirement.

 

[Ste77]

I described the problem in the way in which I was placed. I have not seen the pump in operation. However I resolved

Thank you the same