Pump Field Performance Test: BB3 Multistage Pump

[PumpGuy101]

Would appreciate your thoughts. I am reviewing return to service results for a BB3 Multistage Pump. Flow meter located on discharge, suction and pressure gauge located more or less at respective flanges. No elevation correction required. This is a high head pump (>1300m, ~10-13 L/s), no need to consider velocity head, and the friction correction is also unnecessary (no minor losses between gauge and flange and short piping length). I reviewed all flow paths and because there is a TEE upstream of the main flow element, I added the flow from two separate flow elements to get the total flow.

After taking flow and pressure readings at operating point, I found the TDH for OEM curve comparison. Here is where I would appreciate some input.

Case A. If I assume the field flow reading is correct, and I find the TDH on the OEM pump curve at that flow rate, I find the curve TDH at measured flowrate to be within 4% deviation of the field TDH. This is within 7% therefore acceptable for a safety related pump (no evidence of degradation).

Case B. However, out of curiosity, I also did the following: if I assume the TDH field measured is correct, and I find the flow on the OEM pump curve at that TDH, I find the curve flowrate at measured TDH to be ~35% deviation from the field flowrate.

I believe Case a is the standard practice for field performance review. However I am confused as to why case b curve deviation is so much larger. To that point:

Case C: I measured BHP and estimated TDH and Flow from the BHP and Pump Curves. TDH predicted by this method was acceptable, and though estimated flow was closer to that estimated using TDH from field, it was still More then 20% deviation from measured.

Questions:
a. Any explanation for the mismatch? Why is assuming Flowrate or BHP as measured to find expected TDH on OEM curve for deviation review much closer to predicted then assuming TDH/BHP to find expected Flowrate on OEM curve for deviation review where the result indicates a large deviation from field to curve?
b. Do you concur that correct practice is performed per Case A?

 

[Snickster]

Attached is pump curves for BB3 multistage water injection pumps on a project I worked on. Notice the curves are fairly flat especially towards the lower flows. I think perhaps the flowrate measured with a meter is more accurate than the pressure measured with a pressure gage or BHP measurements. So if you are very accurate with your field flow measurements then if you go up from flow to the curve and to the left to differential head then you will be more accurate than if you take your measured differential pressure in the field and go from the head on the right to the curve and then go down to flow. This is because since the curve is flat, if you are just a little off on your field measurements of differential pressure you will be off a lot on your flow. I would go with the flowrate measurements in the field rather than the differential pressure measurements. Just my opinion as I never did such field testing.

 

[pierreick]

Hi,
Make sure your Gauges & meter are calibrated prior to test. Flat pump curves are not going to help you in your analysis.
A variation on RPM will affect the analysis.
Good luck.
Pierre

 

[Artisi]

Following Pierre's comment, discharge pressure changes at the square of the speed change and flow at the ratio of change, this can make a difference between OEM curve and operating curve, plus OEM test measurements are probably a lot more accurate than your site measurements.