We spend a little extra time on machine faults that seem to occur again and again...we call them "adverse trends". We are researching one of these adverse trends on a gear type positive displacement oil pump. It's driven by a 10HP, 6-pole motor. It has a mechnical shaft seal that has failed at a rate of about once per year over the past few years.

They just found and corrected a coupling misalignment condition, along with replacing a failed seal. Some of my coworkers believe misalignment to be the cause of the seal failures. I don't see it. Pump casing vibration has been very low...less than 0.04 ips-p. In the absence of high vibration, I don't see how coupling misalignment will contribute to mechanical seal failure? I'm not saying it can't, but I just don't see the relationship.

The reason this is significant around here, is that we like to identify the causes of these adverse trends and correct them...not just do something unrelated and try to get lucky. I'm looking for advice as to whether misalignment, in the absence of vibration, can cause mechanical seal failure.

Thanks in advance,
George

What kind of clearances and floats are we dealing with? Total stack of all axial floats, etc... I'm not totally sure what the unit is or looks like or how it is setup. So, under specific conditions misalignment can cause seal failure without the expected magnitude of vibration one would think necessary for damage. I had one in a pump in 2000 like this and switched to a different but the situation was different I'm sure.

You probably have already looked at this but asking for further information.

What is the pump RPM? What is the base bolt distance front to back and side to side? My thought is if you have borderline softfoot problem could the seal be operating in an elliptical path causing excessive wear?

Also what is it pumping?

Don

Thank you Sam and Don.

I have some questions to try to answer:
1. Clearances...it's a face rubbing mechanical seal, so I have to say that there is zero clearance...except for possibly a few microns as the product finds its way between the seal faces.
2. Pump arrangement...It has two shafts, one driver and one idle. Each has an eleven lobe herringbone gear. These gears mate to one another to drive the product...I think typical for a gear type positive displacement pump.
3. Axial float...I tried to figure this out from the vendor docs. It appears that the idle shaft is totally enclosed, and positioned axially with a thrust plate in each direction. The herringbone gear locks the axial position of the driver shaft to that of the idle shaft. The bearings for each shaft are straight roller bearings, offering no resistance to axial float. So...the axial float seems to be that allowed between the idle shaft and the thrust plates. I would have to ask our craft to perform a thrust check to determine how much this is.
4. Pump RPM...Six pole motor minus slip, or about 1180 rpm.
5. What is it pumping? GST 32 turbine oil.
6. Bolt distance front to back...4.5 inches. Side to side...9.75 inches.

Don...I have thought of the possibility that pump softfoot may cause the seal to spin eliptically and shorten its service life. I suspected, however, that it would be accompanied by increased vibration?

George,
I have to agree with you about the softfoot issue and the unit having roller bearings.
What directions are you measuring vibration? How is it axially?

With clean turbine oil, you would think the seal would last for years. Could seal installation or bad seals from the factory be an issue? Not putting blame on anyone but that is a possibility.

Could bearing to bearing misalignment be checked? This too could cause minor elliptical movement but this sounds like a stretch to me.