Lowest speed I can run a centrifugal pump on VFD? 1

[Easterly_Tiger]

Hello,

I am using a centrifugal pump for a process that requires it to run two different fluids. One is sucrose and the other is water. I sized a pump for the application but wanted to know what is the lowest speed I can run my pump at on Variable Frequency Drive?

 

[don1980]

That's a detail that is specific to the pump you are using. For this specific pump, you need info from the pump supplier that identifies the head vs flow at a range of different speeds. That is, you need multiple pump curves, each showing the head/flow relationship at different speeds. From that, and from the system head requirement, you can determine the lowest pump speed which generates the necessary head & flow demanded by the system.

 

[RVAmeche]

As described by don, it varies depending on the pump and process information.

For the pump itself, there's something called the minimum continuous stable flow (MCSF) which is the lowest point you can run the pump if you're going to run it for any period of time. Don't mix this up with the minimumm continuous thermal flow (lower than stable flow). I've had vendors quote the thermal flow as "minimum flow" and the result was the pump was inadequate and damage ensued.

The specifics of your piping system will dictate what point your VFD can run and still actually pump fluid. Just because the you could slow the pump down by a significant amount doesn't mean you'll actually be able to pump through your system (still need to develop enough head).

 

[LittleInch]

Can't add much more than Don 1980.

Remember that the affinity laws work here and head is proportional to square of speed / rpm.

So lower rpm by 50% and head falls by 75%.

A number of VFDs I've seen go from 40 to 70 htz so also depends on your VFD panel.

40 htz for a single pole motor is 2400 rpm.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rotw]

To add to previous comments, I would like to say that if the VFD speed control range is not the limiting factor for going lower in speed, the rotordynamics shall be verified for the minimum operating speed in that, sufficient separation margins to the closest critical speed shall be provided and amplification factors shall also be acceptable. I suppose that when this aspect becomes of relevance, pump speed range validation by the OEM is in order.

https://www.ksb.com/centrifugal-pump-lexicon/critical-speed/192234/

 

[EdStainless]

If you need similar amounts of each then this might work (water has lower pumping losses, lower head required, lower speed) but if the flow needs are greatly different then I doubt that it will work.
You also have to be very particular about the motor. VFD will run the motor hotter so special rated motors are used.
If you will pump one of these a lot of the time and the other only a little bit then look into it. But if the times that you will be pumping each are evenly divided just use 2 pumps each sized for the application. It would be more reliable and more efficient.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Also beware that the cooling fan on the motor might restrict your minimum rpm as that is also a function of motor speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

This topic was discussed some months ago and my recollection was that you could get down to as low as 10% of rated speed on a centrifugal pump - it is related to the torque required at low speed. Min permissible speed is higher with pd pumps since rotational torque remains constant at low speed. While torque decays with speed on centrifugals since developed head is much lower at low speed. See if you can find other discussions on this topic here on E-Tips. Else repost this on the electric motor forum

 

[rotw]

Interesting article about pump's critical speeds, Lomakin effect, VFD's limitations vs. motors number of poles...

http://www.pumped101.com/critical speed.pdf

so, do you have 2 or 4-pole (or more poles) motor?

 

[nackra]

if there are check valves in the discharge piping, then the turn down may be limited by the valve

 

[LittleInch]

I think we're talking to ourselves. The OP hasn't logged in since Monday.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.