3 Condensate pumps not pumping equally
3 Condensate pumps not pumping equally
(OP)
We have 3 condensate pumps that pump directly out of a condensor into the deareator tank. I have tested the 3 motors and pumps recently and all are working within 10 to 15% of each other. They have all been rebuilt within the last year. They are sitting next to each other at a distance of 6 ft centerline to centerline. All three discharge into the same discharge pipe. If we run any 2 combinations of pumps they appear to be pumping the same. I am looking at the amps that the motors are drawing and they are the same. When we turn on all three, the end pump ( call it the east pump) does very little work (amps drop to 25% of rated work amps. The middle pump drops amps slightly and the west pump ends up exceeding the motor rated amps. To run all three pumps the operators throttle the discharge valve on the west pump until the amps fall into the normal range. I have looked to see if maybe the condensor was configured in a way that might cause one pump to work harder, but I can find nothing that makes any sense. Has anyone dealt with this kind of problem before? Any help would be greatly appreciated.





RE: 3 Condensate pumps not pumping equally
RE: 3 Condensate pumps not pumping equally
RE: 3 Condensate pumps not pumping equally
Offhand, this doesn't sound like a hydraulically balanced configuration maybe aggrevated by pumps with relatively flat curves.
BigInch
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RE: 3 Condensate pumps not pumping equally
I can imagine only two causes. Either the east pump is running backwards, or the combined delivery from the east and middle pumps is somehow causing a venturi effect as it flows past the branch from the west pump delivery.
Do you have pressure gauges on the pump deliveries and on the combined discharge manifold? Watching these gauges as you run the pumps individually, then in pairs and then all three together might give you some clues. Do the pumps have NRV's in the delivery piping?
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RE: 3 Condensate pumps not pumping equally
Two guesses are:
-Check the suction of the two low amp pumps for cavitation. If NPSHa is borderline, flow surge when all pumps are switched on could cause mid and East pumps to lose suction and not recover. This would leave West pump to take up the slack.
-Are the pump curves continuously rising, or do they fall as flow approaches zero.
RE: 3 Condensate pumps not pumping equally
Can you describe your piping system arrangement (both suction and discharge) more completely? It seems most likely that the piping configuration is a major controlling factor. Seemingly minor issues can be very important. I would want to pay very close attention to the arrangement of manifold branches, elbows, pipe sizes, size transitions, tees, etc.
As mentioned above, paralleling pumps with relatively flat head vs. flow curves is inviting lots of trouble. Unless you are dealing with unusually carefully matched pumps, operation within about 10% - 15% of each other is not unusual. How are you measuring the balancing of the flows from the pumps? If you are relying on motor amps, you would do well to find a more accurate method. (Site and cost constraints will probably dictate the choices available.)
Motor amp draw is a tolerable indicator, but it is not nearly as good a basis as actual true power measurement. It may be helpful to monitor the condensate temperature leaving each pump with very sensitive sensors. You may find significant heating from churning at the unloaded pump.
From what you have described, it seems likely that there may be suction flow problems at the unloaded pump.
It would be helpful to know more about your pumps, too.
Since the problems appear to be associated with higher flow rates, it seems likely that there may be an effect from variations in deaerator pressure and condenser temperature as the flow (or system load) varies. This couldf shift the operation of the pumps to a less favorable region of their head vs. flow curve while making NPSHa vs. NPSHr less favorable.
RE: 3 Condensate pumps not pumping equally
It sure sounds like the pumps do not have equal performance. Perhaps a different number of stages, different types or trim of impellers, or severe wear ring wear on the weak pump.
Possibly even the weak pump is flowing backwards when in parallel with the strongest pump.
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RE: 3 Condensate pumps not pumping equally
As far as the layout, all three pumps get their suction out of the center of the condenser water box ( the center pump is in line with the centerline of the waterbox and the other pumps are 6 ft on center to it). They all pump into a header pipe with the closed end next to the east pump. 8 ft after the west pump the header makes a 90 degree turn up and starts it's ascent back to the boiler. I had the mechanics inspect all of the inlet strainers, expansion joints, discharge valves, etc. when the unit was off last time. They found nothing of real significance.
RE: 3 Condensate pumps not pumping equally
BigInch
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http://virtualpipeline.spaces.msn.com
RE: 3 Condensate pumps not pumping equally
terminator4 you said the end pump was doing the least work and the first pump was doing the most.
The first pump has plenty of liquid to pump, the most in fact, then the second pump has less but more than the last pump, which is sand bagging it.
The inlet manifold has to be symmetrical to get even flow.
RE: 3 Condensate pumps not pumping equally
I meant to say "The first pump has plenty of liquid to pump, the most in fact, then the second pump has less but more than the last pump, which is sand bagging it because it has no liquid to pump.
RE: 3 Condensate pumps not pumping equally
A flat curve pump system will get a very tiny change in head for a large change in flow, thus can experience large deviations from an initial upset of one variable without having much of an effect on the other. The other pumps also get large deviations before they can help redistribute flows and equalize heads too, which only excerbates any initial upsetting condition, so all pumps tend to diverge and each one will run-away along the curves as far as they can go. Upset conditions come and go in some form or another all the time, but a header that is not hydraulically balanced assures that upsetting conditions are continuously present.
The operating point for pump & pipe system is the intersection of both the pump H-Q curve (or the combined curve of all pumps) and the pipe H-Q curve. The intersection point is hard to find both for our eyes and Mother Nature's eyes when those curves are only flat parallel lines. Steeper pump and pipe system curves add more hydraulic stability to the system.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: 3 Condensate pumps not pumping equally
That was why I said earlier that the only way I can see the flow on the west pump increasing is if its head has decreased and I suggested that maybe the flow from the other two pumps was causing a bit of a venturi effect where the west pump discharge joined the main header. Terminator4 has confirmed that the configuration is such that the flow from the east and mid pumps does flow past the west pump's tie-in point, so it is at least theoretically possible. If the pump curves are flat, the pressure at the west pump discharge only has to be reduced slightly to cause it to go into an over-capacity situation. Do you think this is feasible? The operators' remedy of throttling back on the discharge valve to increase the head the pump "sees" seems to validate this theory.
Terminator4 - is there any way you can let us see a photograph of the discharge header? And perhaps give an indication of the pipe sizes in the discharge header and branches. Do you have pressure gauges on the pump discharges and on the header?
regards
Harvey
RE: 3 Condensate pumps not pumping equally
The first time we had a problem getting balanced flow from the pumps was right after an overhaul. We ended up finding a leak between the pump and the can. Since it is under vacuum, it was leaking air into the pump suction at a rate sufficient to keep that pump from pumping.
The second time we had a problem was shortly after a turnaround on the condenser. We ended up finding a piece of gasket that the pipe fitters had dropped into the condenser lodged in the pump suction line.
The main difference between our pumps and yours is the size. Ours are only rated for about 200 gpm each at much lower head. We only pump the water to a DA and have another set of pumps that pump it back to the boiler. Your pumps must pump all the way to the boiler.
Rather than focusing on the piping configuration, I would suggest that you first rule out all of the usual suspects. Verify that all pumps are identical in terms of number of stages, impeller diameters and shaft rotation. If your system is under vacuum, check for air leaks. Depending on the configuration of your pumps, even a leaky mechanical seal can be a source of air. If you can, calculate the total flow based on steam load and verify that the flow is adequate to satisfy the minimum flow requirements of three pumps running.
I believe that minimum flow is your problem. If two pumps in any combination pump well, then most of the things I just suggested can’t be the problem. But if the total flow is too low for three pumps to run above minimum flow, then it matches your description. With three pumps in parallel and no measurement of flow, all three could be running below minimum. The two weaker pumps are driven back to shut-off where they gas pump and loose flow completely. The single pump that is still pumping takes the full load and runs out to high amps.
Why do you need three pumps running? If the head required to get into the DA is too high for two pumps, can you try dropping the DA pressure? If your pumps are vertical turbine configuration, you may need to add stages to get the extra head with only two pumps running.
Johnny Pellin
RE: 3 Condensate pumps not pumping equally
BigInch
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http://virtualpipeline.spaces.msn.com
RE: 3 Condensate pumps not pumping equally
This really does sound like a header flow dynamics issue.
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RE: 3 Condensate pumps not pumping equally
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com