Pump surging on a VFD

[BoomerSooner7]

Okay BigInch, I need your help here. Or anyone who has a clue for that matter. I recently replaced a DOL starter on a centrifugal booster pump with a Yaskawa VFD. We use this pump to boost pressure to another test pump in a closed loop test with flow control valves, hitting several points along the performance curve. When the DOL starter was in place, we ran the booster pump at 60 hz and it put out about 35-45 PSI average depending on the flow rate, no problems. Now we are trying to maintain 25-30 PSI by adjusting the speed on the new VFD. Everything was okay until we started increasing the flow rate (the VFD speed was around 55hz at that point) and the pump started surging and eventually lost pressure altogether at the test pump suction. The only thing I can figure is the volts/hertz pattern is not set correctly and we didn't have enough torque (or volts) at that speed to keep up with the flow rate. Please help with any advice you may have as to why this might be happening.

Best regards,

 

[Artisi]

Possibly the speed is too low to maintainthe head/flow and the pump is running out on its curve and cavitating.

Remember - head is a function of speed^2 while flow is a function of speed, at 50hz the 35-45psi will become something like 24-31psi.

 

[SNORGY]

Is the downstream test pump single speed (60 Hz only)? How flat is the characteristic on the booster? I am inclined to try running the booster to speed first before starting the downstream test pump, then backing the speed off on the booster pump gradually (the flatter the characteristic, the more gradual the speed reduction) until you hit the operating point you want. Yes, you will be briefly dead-headed (or close to it) on the booster, but it might help the downstream pump avoid trying to run away on you.

I'm not a pump guy - I'd need to play with the curves and think about it slowly. I think I am probably thinking something similar to what Artisi might be; once the booster runs out and starves, it can't re-prime fast enough and the whole system goes down.

Regards,

SNORGY.

 

[BigInch]

I think its probably like artisi says, although maybe not so much a function of the pump speed itself, but perhaps more dependent on the system curve shape, or what the control valves are doing at lower flows.

If the loop's system curve is basically flat (paralleling the Q flow axis on a H head vs Flow curve for the pipe system) in the test flow range, a large change in flowrate will not produce a significant change in head, or v/v, any given approximate discharge head will be able to produce a wide range of flows with little variation in pressure. If the system is being controlled via a pressure signal, it could be that control function is essentially lost when trying to use such a small variation in pressure that doesn't correspond well to one specific flowrate.

Its hard to think about these things without knowing the exact pump and system curves, so I'm really not sure enough to tell you what your system is doing, but you might check to see if it is by partially closing a valve in the pipe loop and thus raising your pipe system curve into a range where a change in Hz produces a larger change in pressure head at the lower flowrates and consequently a better feedback signal. You mention flow control valves, so perhaps those valves might be doing that automatically and by the time you get to the pressures and flows at 60 Hz, they are almost or fully open and the system curve has dropped lower on the pump curve where a large change in flow can be had for a lesser variation in Hz.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[Artisi]

As BigInch has stated - a lot more detail is required for a meaningful study to be undertaken.
My initial comment was just off the top of my head to get some discussion going with the OP - but we have nothing back so far.

 

[BigInch]

Right artisi. I really only offered more at this time, because I felt somewhat "invoked" to do so.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[BoomerSooner7]

Thanks for the replies fellas. Sorry I have a bad habit of not checking posts (and e-mails). I think we may have the surge problem under control. Like I said, when the booster pump was on a DOL starter, this problem was a rare occurance, caused by entrapped air. I messed with some settings on the new VFD to turn off any "intelligent" features trying to take control of the motor under load and put in a linear V/F pattern.