How is this possible? Q re: "identical" booster pumps 1

[UtahWater]

Please help me understand flows in a newly installed pump station. Two booster pumps that came with NW factory tests have materially identical performance curves, yet in practice, they produce identical discharge head, but with 7% difference in flow. How theoretically or practically is that possible? The 7% makes quite a difference in this application. Thank you!

 

[UtahWater]

We pulled the pump today. No obstructions, nothing apparently out of place.
The suction piping is a bit different between the two pumps, but my most recent theory is that the butterfly valves on the discharge lines are mated directly to the silent check valves. The butterfly valves are oriented differently from each other, so while neither one can fully open, the south one can open more than the north one. Those valves are being separated today and tests will be run next week. What I'm opining is that the discharge pressures during operation were not recorded precisely, and flow from both pumps will improve with the BFV/NRV reposition, with the north pump improving to the greatest degree, bring them both into roughly the same production.

IRstuff, Thanks for pointing out the similarities in the data.

LittleInch, the curves are attached, so is the section view (Pump 2 = north)

Inlet water level was not the exact same for each test. It was 1'-2' different
flow was varied during the pump tests by partially closing the BFV on the unique discharge line for each pump when operating.
Operation is effected by flow. This pump station transfers water from a well into the distribution system after disinfection. At the higher production rate of the south pump, the well is able to pump at its capacity (the pump station can keep up). At the lower flow of the north pump, the pump station is out-produced by the well feeding it, and well flow has to be restricted.

 

[Artisi]

Just a quick one to check - do the 2 pumps operate at the same speed, not the catalogue or name plate speed - but actual measured speed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Artisi]

you might be better having this moved to Pump Engineering - forum407.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[UtahWater]

The butterfly valves were not able to open completely to varying extents between the two pumps. The rearranged valves on the discharge allowed them both to open fully and the pumps now operate nearly identically.

Lessons learned: make sure that valves can operate completely, design the suction and the discharge identically when the pumps are intended to operate identically, fully design what needs to be fully designed rather than letting or expecting a contractor on a low-bid-wins budget to do it. Probably a lot more lessons out there. thank you all for your generous contributions of time and thought!