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Centrifugal Pump BEP @ different pump speeds (rpm)

thread407-166705

Centrifugal Pump BEP @ different pump speeds (rpm)

Centrifugal Pump BEP @ different pump speeds (rpm)

(OP)
When a centrifugal pump is driven by a VSD (AFD), there is a different Head (H) vs. Flow Rate (Q) characteristic curve for each value of the speed (rpm).
Using Affinity Laws, it is possible to calculate points of the same efficiency over the different curves (e.g. from the values of H and Q of the 70 % efficiency point over the H-Q curve at 1800 rpm, calculate H and Q values for the 70 % efficiency point over the 900 rpm curve). That is, Affinity Laws represent iso-efficiency lines (quadratic)that pass through the origin.

QUESTION: is the BEP at the same % efficiency value for all values of speed (rpm) ??
(e.g. pump BEP = 80 % at 1800 rpm and the same 80 % for all other speeds also)
Another way to put it: H and Q values for BEP would be different for different speeds but they all would be over a common iso-efficiency line ?

One of the reason for asking is that although Affinity iso-efficiency lines are quadratic, there are publications that indicate the BEP to move along a straight line for different speed (rpm) values.
If that is true, then BEP does not move over an iso-efficiency line. Meaning that the % value of the efficiency at the BEP is not the same for all values of speed.
See for instance  http://www.mcnallyinstitute.com/13-html/13-08.htm

thanks and regards
MS

MS
www.3rps.com

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

If I am calculating the new BEP due to a speed change and lacking any other information from the manufacturer, I would apply the affinity laws rules to give an indication of performance,particularly if the speed change was not great. However, the only way to be get true and accurate information is to performance test the pump at various speeds, especially in your case - 1800 to 900 rpm.   

Naresuan University
Phitsanulok
Thailand

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

(OP)
Thanks BigInch, good article.

It does provide a reasonable explanation on how diff. conditions and systems affect the way points of same efficiency move from a pump curve at one speed to another at different speed (e.g. deviations from Affinity Law calcs.).

In fact, the ref. I mentioned from McNally does refer to a Boiler Feed Water system (e.b. high static head) and according to your ref. this causes deviations from Affinity. That's probably why MaNally indicates that BEP moves along a straight line and not over an iso-efficiency line as per Affinity.

Thanks and regards,

MS

MS
www.3rps.com

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

(OP)
Artisi,

Your are right that Affinity gives good predictions if speed changes are not too great.

A note on pump testing and performance. Many times people mistaken "pump characteristic curve" or performance curve with "pump test curve".
The former are usually generic (e.g. manufacturer's standard curves for a given pump model or type). The latter, actual performance testing of the pump, is usually costly and, at least in my experience, justified only for large, expensive pumps. But you are right that it is the way to obtain truly accurate information.
(by the way, 1800 and 900 were figures just for the sake of the example, not actual case).

Thanks and regards,

MS
www.3rps.com

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

I feel that there is some obtuse relationship I could prove, if I wanted to, as to how efficiency changes with rpm that might bare some relationship to how the specific speed changes with rpm, RPM x Q^0.5 / H^0.75, but in any case, extrapolating efficiency should not be done too far away from BEP in any direction.  In other words, the relationship between efficiency and rpm can be closely approximated by only using a straight line varying directly with Q (or rpm) and it doesn't need to be complicated further by correlating to the more complex relationship of specific speed by using the Q^0.5 and H^0.75 terms.  At least it is so, over the very tiny ranges in which efficiency extrapolations can be done with any validity at all.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Centrifugal Pump BEP @ different pump speeds (rpm)


This is what Sulzer has to say on the subject of efficiency as function of speed of rotation:

Quote:

Where the system curve derives wholly or mainly from friction losses, speed control is best, because pump efficiency then remains prctically constant...With small speed changes (up to 10%) efficiency remains virtually unchanged. With bigger speed changes the velocity in the channel alters and with it the Reynolds number. The efficiency factors must be downgraded at lower speeds and upgraded at higher ones.

Sam Yedidiah:

Quote:

A comparison between the data calculated from tests performed at 1740 RPM and the actual data from the test at 3550 RPM leads to the following conclusions:

1. As far as the QH curves are concerned, the data calculated from tests at one speed seem to be reasonably accurate for the other speed. However, at the lower speed, the QH curve seems to be able to go out to a greater Q/Qd ratio than at the higher speed, possibly owing to cavitation in the casing throat.
2. The efficiency curves show certain differences, but they rarely seem to exceed ±3%.
3. The NPSH requirements at 3550 RPM, calculated from the results at 1740 RPM, vary considerably from the NPSH requirements determined from the tests performed at 3550 RPM. In terms of suction specific speed, the pump seems to have a different suction specific speed at each of the two tested operating speeds.

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

That's pretty much how I see it, although I never made a a study of it when I did run into that kind of data.  It was something I always intended to do, but never had time.  It was much easier just to use the straight line approx and get on with it.  

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

BigInch
Great article, I haven't had time to read it all but a good read so far, seems you must have plenty of spare time at the moment to search them out.

Naresuan University
Phitsanulok
Thailand

RE: Centrifugal Pump BEP @ different pump speeds (rpm)

I thought it was a really nice one.  It's worth the read.

Just very efficient at web mining.  Actually I am doing some work right now.  Got another refined product storage system, similar to one shown on my web page... speaking of which...

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

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