Motor over amping when discharge valves are closed
Motor over amping when discharge valves are closed
(OP)
I am working on a problem where the end-user is runing single stage, vertical pumps. They are running with the discharge valves 75% closed. What continues to happen is that they are experiencing over amping and even some shafts breaking. These are 400hp 1200rpm motors. Right now, I do not know the pump models but they are Goulds.
They are thinking they need to up the HP of the motors. I once worked for a manufacture of small (5hp and below) centrifugal pumps. Although it has been years, I thought that if you start running back towards shut off, out of the pumps normal working range, that HP requirements begin to go up.
Anyone have any thoughts on this and am I totally wrong about the HP requirements going up?
By the way, first post. Seems like a nice forum that could prove to be very helpful as I get back into fluid systems.
They are thinking they need to up the HP of the motors. I once worked for a manufacture of small (5hp and below) centrifugal pumps. Although it has been years, I thought that if you start running back towards shut off, out of the pumps normal working range, that HP requirements begin to go up.
Anyone have any thoughts on this and am I totally wrong about the HP requirements going up?
By the way, first post. Seems like a nice forum that could prove to be very helpful as I get back into fluid systems.





RE: Motor over amping when discharge valves are closed
Axial flow pumps have highest BHP at low flow.
Radial flow pumps have highest BHP at low flow.
Mixed flow are somewhwere in between.
I think single-stage pumps are usually pure axial or pure radial (in your case pure axial would give the behavior you described)
http://www.gouldspumps.com/cpf_0009.html
I would think you would trip the motor long before you break the shaft from overload. I would suspect something else going on to cause a shaft break. First step to analyse might be a vibration analysis.
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RE: Motor over amping when discharge valves are closed
As for the shafts, I agree, something else is happening. We are going to do VA signatures to see what is up there.
Thanks again!
RE: Motor over amping when discharge valves are closed
RE: Motor over amping when discharge valves are closed
* Misalignment between the line bushings causing a bending moment in the shaft. The fracture would show signs of fatigue in bending.
* Design error. If the shaft was under-designed for the loads or the design was changed without accounting for shaft size, it could be failing from overload. The fracture would show signs of torsional failure.
* Torsional resonance. This can be very difficult to diagnose. The radial vibration may show very little sign of a problem. The fracture would show signs of fatigue in torsion.
The last one seems to match up to the fact that you run with a pinched discharge. If the pump is pinched back too far, you could be seeing vane pass frequency vibration from suction recirculation. This could be exciting the torsional resonance. I am no expert on rotor dynamics, but there are simple methods available to calculate the torsional criticals that might indicate if this is an issue.
Save a sample of the fracture surface of a broken shaft and have a metallurgist look at it. They can tell you why it failed. We use a lab called Materials Evaluation and Engineering, Inc. in Plymouth, Minnesota that gives us excellent results. We have a standing contract with them and send them samples regularly.
RE: Motor over amping when discharge valves are closed
Axial flow pumps have highest BHP at low flow.
Radial flow pumps have highest BHP at HIGH flow.
Mixed flow are somewhere in between.
Mixed flow pumps sometimes have a humped BHP curve that drops at low flow and at high flow.
RE: Motor over amping when discharge valves are closed
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RE: Motor over amping when discharge valves are closed
rmw
RE: Motor over amping when discharge valves are closed
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RE: Motor over amping when discharge valves are closed
Thanks! Great forums!
RE: Motor over amping when discharge valves are closed
Regards checman
RE: Motor over amping when discharge valves are closed
If this unit is in fact an axial flow pump (vertical propeller pump) then a rise in BHP is normal with a reduction in capacity from BEP as stated already. Though often this rise will exceed the driver rating, breaking shafts makes me suspect other issues.
If this is a mixed flow (typical vertical turbine), there are definitley models within several manufacturers product line with annomolous BHP curves.
One important variable to check in both cases is the impeller/ bowl lateral setting. This is the axial adjustment via the coupling or headshaft nut either above or below the motor.
This adjustment ussually requires calculation of the shaft elongation for a given head (hydraulic thrust load + static)plus column elongation to arive at the correct operating clearance for the impeller.
I have seen several pumps destroyed when this setting is made based on published bowl lateral settings and shaft strech is ignored. All vertical pump manuals cover this very well.
This setting procedure is typical for most vertical lineshaft pumps where the thrust loads are carried by the driver regardless of the type of impeller/propeller.
Check the factory specs for this unit and then check the actual. It could well be a case of "we keep changing the shafts, doing everything the same each time, and it keeps breaking..."
I hope everyone enjoyed the last day of summer.
RE: Motor over amping when discharge valves are closed
Maybe I should have said it the other way: multi-stage pumps are almost always mixed flow. If you're going to find pure radial or pure axial, it will be on a single-stage pump. Right?
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Thank electricpete
for this valuable post!"
In days gone by - I sold many multi-stage axial flow pumps ranging from 6" to 24" diameter, as deep-well high-flow units (6"- 8" - 10") and as drainage and flood irrigation units in the bigger sizes.
The original posting sounds like 2 separate problems, and I would investigate each problem individually,
1. to see why the pump goes out on overload, is it a mixed-flow or an axial flow unit etc etc.
2. get the shaft failure analysed to establish the mode of failure - I think it very unlikely the shaft has failed from overload.
Re-look at what JJPellin (Mechanical) has outlined. Go one step at a time - eliminate the know factors first.
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Thailand
RE: Motor over amping when discharge valves are closed
Could be a factor in both trip and breakage.
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