Motor over amping when discharge valves are closed 6

[landonab]

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.

 

[electricpete]

Here is a link that shows the shape of the BHP vs flow curve.


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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[landonab]

Thanks for the link. I am hoping it is axial flow as that would explain the overamping. I will no more tomorrow regarding what the pumps are.

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!

 

[rzrbk]

Another thought is that if the discharge valves are gate type, the restriction of a 25% open valve may not be significantly more than if it was wide open. If pumps are radial type there is still a good chance they could be running on the right side of the curve despite the pinched valves.

 

[JJPellin]

In a low speed vertical pump, potential causes of repeated shaft fracture could be:
* 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.

 

[JJPellin]

I assume that electricpete meant to say:

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.

 

[electricpete]

Yup, that's what I meant. Thx

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[rmw]

Single stage pumps are quite often mixed flow type.

rmw

 

[electricpete]

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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[landonab]

Thanks to all that responded to this. Like I said, unfortunately this was all engineered by someone else and now we are getting drug into it. Our initial responsibility is to PM the motors but with that, we are finding ourselves assisting with troubleshooting the failures. As everyone knows, it can be rather difficult to read someone elses mind when this system was designed and thus far, even more difficult with the lack of information I have been able to gather. Hopefully, I can piece the details together over some meetings and will also take all of your ideas and look into them.

Thanks! Great forums!

 

[checman]

electricpete, Every axial flow impeller I have seen has been in a single stage pump moving mass amounts of water at a very low pressure. They were moving water from an elevated reservoir into a gravity flow distribution pipeline. As far as for radial vane impellers I have worked with many multistage process pumps that have radial vane impellers many with diffusers. They are very common in high pressure applications. I agree that their impeller is most likely a mixed flow impeller. A valve 75% closed says very little as to where the pump is on the curve but assuming that the pump is running back on the curve impeller/bowl end clearance is critical. I have seen vertical turbines shafts stressed to the point of contact between the impeller and bowls because of high axial thrust loads. The extra drag required extra horse power and would kick the motor out. Normally the pump was assembled without the correct clearances for temperature and or pressure. The pumps turned free when assembled but needed additional clearance to run under the actual operating condition.

Regards checman

 

[coastpumpguy]

Just my two cents.

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.

 

[Artisi]

"electricpete (Electrical) 20 Sep 05 8:14
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.

Naresuan University
Phitsanulok
Thailand

 

[EdStainless]

The shaft setting could be a big issue. If these are running at reduced flow for long periods they could be getting a lot hotter than was intended. This could through off the setings, both verical and axial.
Could be a factor in both trip and breakage.

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