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]

Sorry - very incomplete original post.
Two vertical turbine can pumps pulling water out of a ground level tank.
Common suction line out of tank to manifold.
Only one pump operates at a time - duty/standby
Each pump during their respective tests during startup, on the same gauge (moved from one discharge to another), produce identical head (139').
I'm really baffled as to why at the same discharge head on these pumps that performed materially identical to each other at the factory would now produce different flows (2150 gpm vs 2350 gpm).
It's likely that there are design errors in the piping and valving on both the suction and discharge sides of the pump cans, but the cans and pumps are designed identical to each other from the suction flange on the cans to the discharge pressure gauges...identical discharge head should produce identical discharge flow, correct? Could there be an obstruction in the suction that would decrease the discharge flow without effecting the discharge head?

Again, thank you!

 

[bimr]

You have provided little information.

Have you measured the operating amps on the pumps?

It's likely that there are design errors in the piping and valving on both the suction and discharge sides of the pump cans,

It should be obvious that the differences between the pump installation is what is causing the discrepancy. What are the design errors? How about posting some schematics, pictures, pump curves, etc.

Based on what you have posted, one would expect that the suction head is more restricted in the pump with the lower flow. Possibly the suction pipe has too small of a diameter. Do you have the fluid velocities and pipe length in the suction header for each pump?

 

[UtahWater]

Thanks, Bimr

Available test data below and layout attached:
5/2/2018 3Ph/480V
North Pump
sequence Q (GPM) P (ft) I (amps)
2 1357 162 112
3 2146 139 122
4 2178 139 122
1 2300 145 123

5/2/2018 3Ph/480V
South Pump
sequence Q (GPM) P (ft) I (amps)
4 1332 162 113
3 2345 139 ?
1 2345 143 125
2 2350 139 ?
5 2355 143 125

 

[UtahWater]

Flow is from right to left in the plan sheet above. 14" suction pipe on the right side from a ground level tank (about 40' of pipe; NPSHa calculates to about 1.6xNPSHr at low water level), 14" tee and elbow, 20" diameter cans with 14" suction flanges, 8' vertically from suction flange to 12" discharge flange, 180 degrees from each other, 12" discharge through building out to a 12x14 reducer outside the building on the left side of the plan sheet.

There are likely many things that are not optimal in this design. Am I oversimplifying to ignore those flaws and say that the degree of inefficiency should be about the same in the two pumps, and wonder what could cause a different flow at the same head? I appreciate all of the experienced designers in this forum. I'm pretty inexperienced in this arena and appreciate any input you might provide. Contact with the pump manufacturer, distributor and regional rep have all been made with no return contact over the past 5 days. Thank you!

 

[bimr]

Is the suction the light gray piping to the right? No pipe sizes are shown.

Are there isolation valves on the suction side?

Where is the flow being measured?

Are the discharge isolation valves closed when the opposite pump is operating?

 

[UtahWater]

bimr, yes the gray pipe on the right is the suction piping. It is 14" diameter. The discharge piping on the left is 12"
There is a mag meter (tube) in a vault on the suction side, out of view to the lower right.
There is an isolation valve on each side of the meter, but no isolation valves on the suction lines for each can separately.
The discharge isolation valves (keynote 7 in the plan view) are left open at all times and are only there for maintenance. There are check valves (keynote 6)in the discharge lines to prevent reverse flow.

 

[SandCounter]

I plotted your points on an x-y plane as flow and head coordinates and they all fall pretty well on a 2nd order polynomial with the exception of your north pump sequence numbers 3 and 4. You may want to re-run those scenarios. There appears to be a branch on the north discharge line. Make sure you are not getting any leak through.

Also, not sure what pumps you are using, but I have experience with smaller turbine pumps with multiple impellers where the lower impeller shaft(s) would break off or the impellers unscrew and sit static in the bottom of the can. The effect was reduced flow.

I used to count sand. Now I don't count at all.

 

[UtahWater]

Thanks, SandCounter.

These are Flowserve 12EQH (100 hp, AC drivers at 1770 RPM) with 9.25" impellers. Application is for potable water service. The branch on the north line is a pressure relief valve. No leak through.

Thanks for the idea that maybe something happened to an impeller on installation or subsequently. It's been a pretty controlled environment through construction, so debris in the suction line seems unlikely, but I think there's enough suspicion to pull it and verify.

 

[ashtree]

Is most of the total head , static head or friction losses?


Just some comments on the factory testing. It concerns me that you say the two supplied curves are identical.
Many companies produce pumps to standard specs and tolerances and do little or no factory testing beyond that it works and falls with the specification plus or minus a certain amount. A non specific pump curve is supplied.
If you buy one of these pumps they will normally achieve the company supplied data and if it doesn't the company will normally replace it. The cost of these pumps are reduced and avoids the cost and hassle of very detailed testing and certification. At the same time very few pumps fail to make the grade.
However if the client requires testing(which you might based on the consequences of not achieving the standard required) the factory does a higher standard of testing, with sophisticated testing facilities, but charges accordingly.

So what level of "factory" testing has been done?
If the pump curve is specific it should quote the pump serial number at least , and other details such as test conditions , testing officer etc. A generic pump curve typically won't have these.



Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[UtahWater]

Thanks, ashtree.

The pumps were ordered with a non-witnessed test.

The "Performance test results" included serial number and tester name as well as tabular data along with the resulting curve. Each pump has its own unique test result...two differeent S/N.

 

[ashtree]

Thanks for the clarification.

I am guessing that most of the total head is static lift. Is that correct?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

One of the check valves may be leaking. Suggest that you run a flow test with the isolation valve closed on the out of service pump.

 

[Pinwards]

If identical, swap the two pumps and redo your tests. See if the problem follows the pump, or reveals an issue with the setting/piping at one location.

#

 

[bimr]

If identical, swap the two pumps and redo your tests. See if the problem follows the pump, or reveals an issue with the setting/piping at one location.

Swapping 100 HP pumps is not an inexpensive task.

 

[IRstuff]

Seems to me that the data is really only different specifically at 139, both 162 and 143/145 are pretty well matched. This might suggest that the slight asymmetry of the layout causes aberrant behavior at 139, like some sort of cavitation.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[UtahWater]

Thank you all for your input! We are pulling and inspecting the pump with low flow later this week. Will post whatever determination we come to.

 

[fel3]

I found this pump curve at

https://www.wcc.nrcs.usda.gov/ftpref/wntsc/Pump Curves/FlowServe/1800/12EQH.pdf.

I couldn't find it on Flowserve's website, but that was probably operator error. Please note the note about this pump being renamed in 2001.

This pump's family of performance curves has a dip/flat spot in the 1500-2000 gpm range (depending on impeller size), but your data is showing dips at higher flows (north pump) and even higher flows (south pump). If these dips are real as measured, it would mean that the standard factory curve isn't very good and that the two pumps are actually a bit different. Regardless, I avoid pumps with flat spots and dips.

One other point: the two pump have slightly different piping arrangements. The north pump has a very slightly longer suction run and slightly different minor losses compared to the south pump. These difference are not big enough to create the problem, but I would account for them anyway.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[LittleInch]

A few things:

1) can you post the two pump curves?. If indeed they look like those posted by bimr, I'm not surprised you're getting differences as any v small imbalance will be seen.
2) Leaking NRV might be the cause, but doesn't leap out
3) was the inlet water level in the tank the exact same for both tests?
4) How did you vary the flow in your tests?
5) If these are can pumps why is the inlet flange 8 feet lower than discharge? Can you add a sectional view?
6) regardless of tests, does it make a difference in operation?


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

 

[djs]

A totally different issue, I once was involved with controlling two identical new RAS pumps in a WWTF. The functional description called for only one pump to run at time, and to alternate every 24 hours. One pump worked fine, however the other would run and when shut down would lock the impeller and shaft to a point where it needed to be broken loose with a 36" pipe wrench! Once broken loose, the shaft rotated freely. Everyone on the job knew this, but the GC came after me stating that I was not controlling the pump carefully. When I stated that the issue was with the impeller locking, up he went after the electrical contractor because he had not set up the VFDs correctly. Finally the GC got the pump people out to take a look at both pumps, and the one that locked up was assembled incorrectly.

Moral of the story is that two identical pumps may not be identical! And of course the pump supplier will not admit to it.