Centrifugal Pump Low Flow limit vs. Speed Reduction

[Dokuen]

In regards to thread407-167783.


I too have not seen anything in writing about variable speed and the minimum flow requirement for pumps; however, the "standard" practice I have seen in the field is to take the same percentage of the new BEP at the reduced speed.

For example if the pump company states 10% of nominal flow for their minimum. Look at the performance curve at lowest speed you will operate in the application and take 10% of that reduced speed curve's BEP.

As flow is linearly related to speed this is simply the percentage of speed times rate minimum flow. So if standard min. flow is 10GPM then at 80% speed you could consider the minimum flow to be 8GPM. Most variable speed pumps I have seen that come from the factory with drive installed still use the rated minimum flow of the pump. But the mfr will always lean conservative.

Please also keep in mind this is all assuming cool clean water. If this is a boiler feed application with high temp water then your minimum flow will be higher to begin with.

 

[Artisi]

The problem with becoming more mature it that the brain can't keep up with the 2 finger typing. My earlier comment re speed change was refering to a half speed change ie, flow at the same ratio as the change, head at the the square (1/4) and power at the cube (1/8) of change. However, whatever the speed change the factors influencing minimum flow reduce at a rate greater than the speed change.

TenPenny
Head varies as the square of the speed change, therefore speed X 0.5 squared = 0.25

 

[BigInch]

Well ya... not "by" 1/4 its "to" 1/4

So the question amounts to...

Do you have a curve like #1 or #2?

http://virtualpipeline.spaces.msn.com

 

[TenPenny]

"TenPenny
Head varies as the square of the speed change, therefore speed X 0.5 squared = 0.25"

I've known that for about 20 years now. But, as BigInch commented, the devil is in the details - 'reduced BY 1/4' is not the same as 'reduced TO 1/4'.

Surely to God we could expect engineers to speak properly about mathematics, if nothing else.

 

[quark]

OK. The mistake of using a wrong proposition is on my part but I didn't mean it that way.

BigInch,

That is a rudimentary consideration about selecting a variable speed pumping system and when I said 'perfectly recirculating system' I mean it is the curve 1.

Anyhow, I will open up a new thread about my system for a sanity check of what I am going to do.

 

[Artisi]

My mistake with not thinking clearly, or thinking too much therefore, please change by to read to.

 

[quark]

Damn it! There is a mistake again Read pro- as pre-

 

[BigInch]

Quark, I did assume you meant "to", but wasn't sure what you meant about "perfectly recirculating.

Ya. Post the ref to the new thread.

http://virtualpipeline.spaces.msn.com

 

[d23]

Dokuen

I believe your original question was what do VFD’s or trimming impellers do to recommended flow rates. Based solely on the pump affinity laws speed change and flows should be linear. If you have a BEP of 100 GPM and a min recommended flow 10% below BEP then at half speed the BEP should be 100 * 0.5 = 50.

The min recommended flow should be (100 GPM * (1 - 0.1)) * 0.5 = 45 GPM

The same should apply to trimming impellers however; these equations are not exactly true. As you slow down or trim impellers the flow paths through the impellers continually change up to some point where you get into re-circulation. Re-circulation is Eddie currents that occur in the eye of the impeller or at the tip (discharge) of the impeller due to low velocity. The vane openings and distance between vane tips and volute are designed assuming “X” amount of flow. As you slow down or trim impellers you change the geometry of the flow path. You could have a well-designed pump at 3500 RPM, if you operate it at 1750 RPM that pump curve will most likely become a drooping pump curve on the left. For field calculations the affinity laws are OK, but if your designing a system you may want to consult the manufacture for actual test data at various speeds or trims.

I saw an article on this forum that was named something like “Stop Falling Off The Curve” that had actual test data at several speeds and several trim impellers. I haven’t found the link since, but it did identify flow patterns through the impeller and expected deviations from the affinity laws.

D23