Recently, we have experienced a failure on our LPG pump (Multistage centrifugal 10 stages). The vibration on NDE was observed 30mm/s, with 1 X running speed, showing a severe unbalance.
Overhaul of pump was recommended. It was observed that the third stage casing wear ring locking pin was broken and the casing wear ring was seized with the impeller wear ring. Therefroe the casing wear ring was rotating with the shaft.
See the photo attached.
Now we are busy with replacement of all the wear rings and throat bushes.
Suggestions are required for preventing this sort of failure on other similar pumps.

Mr. Mohammad,

Before suggesting any possible solution, I would like to know why the locking pin broke in the first place. Were you able to examine the fracture and identify the reason(s) for the failure?

Was it, e.g., wrong material, overload, wrongly fitted, overheated (heat marks?), brittle fracture (is temperature below -20 deg. C?) etc. Without some understanding of the cause, it is difficult to suggest possible solutions.

V.Narayan.

Dear Vee

Thanks for your response.
The fractured surface cannot be examined, because of rubbing of the casing wear ring with the fractured surface of lock pin (as it was rotating with the shaft. The material of all the 18 lock pins is the same as installed by OEM. None of the other pins are damaged. This is first time the pump is overhauled due to this problem. This pump was installed in year 2000 and commissioned in August 2001. There are two pumps for this service, one is running and other standby. The product operating temperature is normally 48 deg C, so there is no chance of brittle fracture. There are no signs of overheating within the pump. The designed clearance between the wear rings is 0.017 to 0.020 inch (as per API – 610).

These are LPG product transfer pumps installed in the Tank farm area, and are not run on continuous basis. They are run when the product transfer is required for export shipment.
The Total running hrs of both the pumps are 9585 Hrs (failed pump) and 9039 Hrs (presently operating) respectively. The number of starts for the failed pumps are 214, whereas for the standby pump no. of starts are 204.

Other Spec. data of pumps is as follows:
Pump capacity: 27.7 M^3/ hr (normal), Suction Pr. : 12 Barg (Rated), Disch. Pr : 55.4 Barg, Product density @ pumping Temp. 0.524 / 0.576, Rated Speed 2980 rpm.

See another photo of failed rotor (at bottom of this page) showing the rubbing marks on casing wear ring.

From the above description what do you suspect? What might cause the pin to fail?
Is the no of starts of pumps (214) during the past 4 years, might cause Overload?

What is your Opinion / suggestion to prevent this sort of failure in future?

Awaiting for your reply.

Regards

Mohammad

This type of failure, with the wear ring, can occur for several reasons:

1. material caught between the surfaces while running or before a startup;
2. Out of round impellor or wear ring;
3. Severe unbalance of the associated impellor;
4. Thermal growth/shrinkage

You can tell by an inspection of the wear rings. If you can post a picture of the impellor and casing wear rings, I can probably identify the cause.

Howard

Assalaamu'alikum Dear Mohammad,
Last record level(30 mm/sec)was too high, according ISO 10816-3, it has been out of limit for all of various machine types.
For machanical problem, it shouldn't be occurred suddenly. It should be step by step. Except, the problem come from false-operated, un-ussually operating proccess etc.
How is the vibration level ussually? Is it normally 30 mm/sec? When did it increase?
How's about the spectrum trending?
Are you sure 1xRPM was highest since normally condition, and no another predominant amplitude (incoluding the harmonics)?
Ok, you should trace the problem first.

Wassalaam,
Rahardi
rahardi@softhome.net

Howard, Mohammad,

Howard, accoding to Mohammad there were no heat marks. With all four of your scenarios, I suspect we should see heat marks on the wear ring surfaces.

Mohammad, thank you for the extensive additional data, but I seek some more information. For some reason, I could not open your pictures, so please appreciate that I am commenting without seeing the photos (can you e-mail them to eml@effective-maintenance.com) ? Am I right in assuming that the pump has vertically split stages and is of a barrel type construction? If so, are the case wear rings a sliding fit and locked in place in two places by radial grub screws? Or are the case wear rings press fitted and then locked by axial grub screws? Are these tack welded after tightening?

The running hours and number of starts will not contribute to this type of failure. Lack of heat marks indicates that rubbing has not taken place. That I think excludes interference by thermal growth, or wrong clearance or loose grub screw. Since these pins are supplied by the OEM, material quality is not an issue.

In some wear ring designs, there is a central groove in which the grub screw (locking pin) is fitted. If it is small enough in diameter and length, it is possible that it can work loose and stay in the groove. If there are two screws on a ring, it is unlikely that both would have worked loose at the same time. But were both fitted in the first place? Sometimes the hole alignments in the wear ring and body may not match, so the mechanic may have fitted only one screw. Is it possible to verify this scenario?

All this is guesswork, we need some more data. Can you confirm there are no blue (heat) marks and that the locking pin thread were not flattened? Is there a picture of this pin? Sorry, there are more questions than answers! Regards.

V.Narayan.

Vee:

Brass impellers do not turn blue with heat, only steel ones.

The only way to determine any of the four scenarios is based upon the rub marks on both the impeller and wear ring.

I was a field service and shop pump mechanic (rewinder, machinist, etc.) as a motor repair journeyman for many years, while working on my Ph.D. With that experience, I would like to note that the failure is not necessarily 'instantaneous.' But, if there is slight rub, for any reason, then it can cause the pins to wear, with the number of starts having a direct effect.

In this type of pump, the wear ring is a snug fit with pins to ensure that the wear ring does not spin. It is manufactured this way because the casing is cast and the wear ring is brass (or similar alloy) with different expansion rates.

Howard

Naturally one fitted with a long shaft span between bearings is fragile and sensitive to any inaccuracies especially if near a resonant frequency. Setup is everything as the OEM design is probably good.

A good installation with no soft feet anywhere along the machine train is a must. Good 'hot' alignment is must in my opinion. And there must not be any induced stresses.

How were the operational parameters? Were there operating upsets and/or cavitation? Anything that could set the shaft in motion causing a sinuodial pattern that could rthymatically excite the shaft causing it to loose clearance and rub?

Case distoration can come from piping stresses and affect performance reducing clearances.

On ititial setup you may want to consider shooting-in the feet to ensure they are all in one plane prior to setting the pump on its base. This will ensure a known starting point and eliminate soft foot.

How well was the commissioning documented? And was resonance testing performed?

Mohammad et al,

It will be easier to work from the evidence than, as I have been doing, guessing waht it might be. Perhaps Mohammad, you can clarify the points raised by Howard, Sam and myself so we can try and assist.

Howard, I respect your knowledge of pumps, so I was not trying to question that; Mohammad had not mentioned brass wear rings, so I did not assume they would be made of brass. In my xeperience with LPG pumps, they have always been made of low alloy steel. However, it will be easier to diagnose the fault when we have the correct data. Let us wait to see those.

Regards.

V.Narayan.

Thanks to all (MotorDoc,Sam,Vee,Rahardi) for your comments.
Sorry for the late reply. Actually our Holy month has been started last week.
During this holy month of fasting (Ramadhan),
our official timings have been reduced. So there is little time for us during this month. Also there is extra prayers during the night time. Therefore overall, during this holy month, we have less time for technical matters.

Hope I will send the answers to your querries on the posting board today evening.

Thanks for your interest in solving this problem.

Reagrds

Mohammad

Dear All...Sorry for the late reply...

The material for the wear rings (case and impeller) is A48 - CL30.

The case wear rings are locked (actually provided with anti rotation locking pins) at one location in the radially split barrel assembly. These pins are not threaded, just press fitted.

It has been observed, surprisingly, that the 2nd stage impeller wear ring is missing (It is suspected that it was not installed by the OEM, mistakenly) See photo attached below. Note that the 3rd stage casing wear ring pin was fractured, just nearby to the missing impeller wear ring.
Will this might cause unbalance of associated impeller. What is your opinion?

The disassembly of pump has not been started yet. I will take the photos of the impeller / case wear ring contact surfaces when disassembled and post on the forum.

Best Regards

Mohammad

Howard, Vee and others
At Last, I have taken the photos of Impeller / Casing wear ring after dissassembly (See photos below).
Rubbing marks can be seen at the contact surface between the Impeller & Case wear ring. Also there are traces of material caught & deposition between the rings, possibily before startup. At the location of deposition on case wear ring, the impeller wear ring location got grooved. It appears that the initial rubbing and then consequent seizure of rings caused the pin to broke. Eventually there was severe unbalance resulting into high vibration at 1 X running speed.

In my previous post, I told about a missing impeller wear ring of 2nd stage. Actually, it appears that the impeller wear ring of 2nd stage got broken into pieces due to severe vibration and rubbing of near by rings.

Howard & Vee ..... your comments will be appreciated to prevent this hidden failure.

Thanks in advance,

Regards,

Mohammad

Was unable to attach Impeller Wear Ring photo in previous post. See it below......

Mohammad,

I worked on a LNG pump (two Sulzer pumps side by side on top tank) that had frequent failures. I found that the pump rotor was sensitive to unbalance because the motor structure had a natural frequency close to 1xSS. I designed and tested a modification to detune the structure, and the premature failures stopped.

In your case, you need to determine if the vibrations was the Result of a failure or a Cause of the failure. If you can resolve this issue, then the failure solution will become more evident.

Walt
w_f_strong at msn dot com

There are cases of feedpumps where wear of seal, wear rings etc. change the tuning of the crtical speed of the rotor to get close to running speed and cause final failure or shaft splitting in 2 parts by fatigue crack. So if there are parts that was not mounted that change length btw. bearings or bearing stiffness it may make a difference on machine life length. So if you verify opertion on the rebuilt unit by running som coast down testing or similar it may verify that you now have a good unit, if not, then you know what problem you are looking at. On the other hand you can by careful balancing, monitoring and frequent rebuilding keep hi pressure pumps operating very close or on critical speed but to a hi cost. Olov

All of the suggestions sound very logical. I do not have any real expierence on this style of pump. I would have a look at the spare as soon as possible with a boroscope (video style)to see if there is anything that would suggest fatigue with the pins etc.

Just a thought!

Good luck

Hooch