Is Pump Runout possible with VFD?

[jantoffman]

Am I missing the boat? The problems with pump runout as I understand it, are possible overloading the motor and possible cavitation with resulting vibratation and pump damage due to excessive flow.
I'm looking at a system that will be at low head for one step in the process, and high head at the next step in the process. I want to protect the pump against runout during the short low head step in the process.
Now I've sized my motor way above the runout BHP so I shouldn't have a problem with the amps/HP.
My question goes to the other issue with runout (cavitation/vibration). If I lower the speed of the pump during the low head phase of pumping, as I understand it I effectively drop down the curve and can find a speed that hits the low head point at the flow I want. But what If I'm limited to 40% pump speed on my VFD and that still hits the low head value at the far right end of the curve?
If I'm not really moving the maximum flow rate of the pump would I still have a problem? Would efficiencies drop off so much that I still see high amps?
And lastly, can I just control the VFD to a certain # of amps to protect against runout? Is that considered runout protection? Effectively that's what I'm trying to do.
Thanks for any input.
-JT

 

[HEC]

jantoffman,

When a centrifugal pump runs off the end of the curve it is a problem. For the low head step what is the flow requirement? Is it possible to throttle that destination such that the flow/head conditions do not fall off the end of the curve. Sizing the motor excessivly above the non overloading case is unnecessary if the flow/head conditions are controlled. Oversizing the motor may also create additional enefficiencies in that the motor runs only partly loaded.
"Runout" protection i.e. falling off teh end of the curve, can only be guarded against by controlling the pump diferential pressure.


Mark Hutton

 

[Artisi]

to fully understand what is happening you need to draw the system curves for the various duties across the pump curve - this will identify what is going on and give you the chance to makes some sound engineering decisions.

 

[ozmosis]

jantoffman
Firstly, I must state for reasons of fairness/openess that I work for the company that makes the VFD I am going to suggest in a possible answer to your thread. I trust that my view is not seen as biased in any way as I do not know of any other VFD manufacturer who can provide this feature:
"5.5.2 Dry pump
This feature is useful for detecting a condition when the pump is running but no water is in the system. A dry pump condition can cause pump damage
if not detected and corrected. Dry pump detection does not require the use of external differential pressure switches or flow meters and associated wiring.
If there is no water in the system, the pump will not produce pressure. The frequency converter will go to maximum speed to try to produce pressure.
Because there is no water, the load on the motor will be low and power consumption will be low. If the frequency converter is running at the maximum
speed and the system power consumption is low, a warning or alarm is generated to notify the operator of the condition.

5.5.3 End of curve
This feature is used to detect leakage in a pipe system or the loss of pressure in the system. End of Curve detection does not require the use of external
pressure sensors or flow meters and associated wiring.
End of curve occurs if a pump is delivering a large volume of water but cannot maintain the set static head. When there is a water leak in the pipe system,
the pump will not produce full pressure. The frequency converter speed increases to maximum speed to attempt to produce the full pressure. If the
frequency converter is running at the maximum speed and the system pressure is low, a warning or alarm is generated to notify the operator of the
condition."
Click on link below and then choose >Programming Guide(2) in the selection of documents at bottom of page:

http://www.danfoss.com/Products/Cat...Drive+FC+100&country=US&language=en&dyn_Lang=

See page 235 of the VLT HVAC FC 100 Programming Guide[US version] MG11C322 dated Mar 7 2007:

 

[TenPenny]

No matter what, without laying out the system curve(s) and pump curves, you'll have no idea how the system will perform.

There are several 'intelligent' pump control/vfd devices on the market (ITT's PumpSmart being one,

http://www.pumpsmart.com),

but unless you understand how the system will perform, there's no guessing if it will help, or what it will do.

 

[BigInch]

Why would you want to size a motor "way above" the runout? You should probably be shutting down the pump by then, or at least controlling the flow to keep it near BEP! Much better than running out-of-control. As it is, you've probably spent too much on the pump, the motor and the electrical power distribution system feeding the pump, not to mention having a very inefficient design.

If you really need that flow, you need a larger pump where BEP is nearer to your present runnout flow, or you need a more flowrate flexible multiple pump configuration.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain