Worn pump shaft 7

[replica]

Hi all,
I am new to this forum and would like to ask whether anyone had experience on worn nitronic 50 pump shaft used in VS6 pump. The length of the shaft is 4.35m and pump is 19 stage pump used to transfer crude oil offshore. It was observed that all 19 locations where the bowl bush bearings were placed on the shaft shown the severe wear on the shaft surface and not on the bronze bush. The length and locations of the worn shaft region has similar length as the length of the bush. The clearance between bush and the shaft was self lubricated with the transferred crude oil. The worn shaft at the first and second stage shows severe wear with smooth and shiny surface (mirror finish) where else the worn shaft in contact with bowl bearing at other stages have rough surface. The shaft which is in contact with the line shaft bearing also showed a rough surface and some are not concentric (uneven wear). The diameter of the shaft is 1.5 inches. The bush bearings are made of C836 bronze and the shaft is made of Nitronic 50. The hardness of the shaft is 44HRC and the hardness of the bush is 90HRB. All the bush bearings observed have either no sign of wear or minimum wear and tear. This problem happened about a year ago and kept on reoccurring even though the shaft and bush bearing were replaced during maintenance using the same new spare parts. The longest the pump in operation after maintenance was for two months and the shortest was two weeks before the same failure happened. The shaft and bush bearings can normally last for a year. The main issue is why the shaft worn out and not the bush bearing. Normally from what I understand, the bronze bush should wear out and not the shaft. Any suggestions for possible causes of the worn shaft are highly appreciated. Is it possible for softer bush to abrade the harder shaft and under what condition?
Attached herewith are the photographs of a portion of 4.35m pump shaft showing worn shaft region and the unaffected bush bearing.

http://files.engineering.com/getfil...edd-b0e2-753650ac6f94&file=wornshaftforum.pdf

Regards,

 

[replica]

romke

Agree with you but in my case i do not think that the lapping of the shaft is due to insufficient lubrication because if it happened than the bearing will probably seized or galled on shaft.

JJPellin

Thank you very much for bringing up another good point. So less groove or without groove is better as it prevents from fresh abrasive to enter providing it does not jeopardize the shaft stability. However my thinking is that do these abrasive particle will also be carried out by the crude oil since the groove is the open end type (am i got wrong here?).
I may consider the options on the bush materials and design in the future but at this time I will rather try to monitor the performance of the pump with the increased clearance of 0.008 inch.

 

[1gibson]

Not sure that bronze will ever gall to Nitronics. Also the grooves look too shallow to really allow any flow if there is no differential across them (especially if they're filled with particles.) I'd guess problems aren't as bad in the case bearings, that do have differential pressure across them.

I vote Graphalloy.

 

[EdStainless]

Our 3600 rpm subs ran with smooth bushings, no grooves.
When we had to pump a lot of sand we either ran rubber bushings if temperature allowed, or we went to ceramic ones. A lot of tricks were required to get the ceramics to work, but in the end they were great. We always ran with sleeves on the shaft, shafts are expensive.
You might consider one of the softer bronzes, and as a test try no grooves

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P.E. Metallurgy, Plymouth Tube

 

[replica]

1gibson

Agree...probably bronze will not gall to Nitronic 50 due to vast hardness difference between the two (specify the minimum difference of 50 BHN as in API 610) . However I think, the surface of the shaft would not be polished as it was if it were due to overheating-lack of lubrication. I did not see any sign of overheating either on shaft or on bush.

EdStainless

Thank you very much for the suggestion. Will try all options whatever possible.

 

[DubMac]

One option would be to consider an enclosed lineshaft; probably should have been designed this way from the start. Without a doubt you should contact the manufacturer and get their input. That is what they are there for.

By the way, an excellent post all the way around. Very good and detailed info from replica, and usual expertise from the usual suspects.

 

[replica]

I had already concluded that the caused of shaft failure was due to entrapped sand in the axial groove of the bush. However one question that puzzles me is that how can the sand abraded harder shaft more than the softer bush. Is there any possible explanation on this? Is the embedded abrasive on softer brass bush polished the shaft as mentioned by Mike Halloran applied? Or is it possible also that the trapped particle will rolled around the between shaft and bush and caused the failure as mentioned by Artisi? When we have problem with the entrapped solid particle in the clearance, which I considered the common problem in the pump, what is the best explanation on the mechanism for this when dealing with hard shaft and soft bush? Which one is a common failure, worn out shaft or bush?

 

[Artisi]

I might be harping on the use of elastomer "rubber" bearings, so why is that you might ask, well my experience with line shaft driven bore pumps using rubber lineshaft bearings with entrained sand has always been good, also for pumping fine abrasive particles using centrifugal slurry pumps the choice of materials for the wetted parts is "rubber". Now whether an elastomer to suit your product is available is not in my scope, although I did see Johnny Pollin has offered a suggestion of material.
I would certainly discuss your concerns with the pump manufacturer.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

I have seen many examples of a softer material grinding into a harder material. I believe that there can be two mechanisms involved. We often see mechanical seals fail in this manner. The softer carbon face looks pretty good, but the harder (tungsten carbide or silicon carbide) face has a deep groove. This would be a result of hard particles imbedding in the soft material. This is the reason that we are able to hand lap T-C seal faces on a lapping plate made of cast iron. The cast iron is charged with diamond paste. The imbedded hard particles grind away at the hard seal face while the softer cast iron is spared.

The other example that we commonly see would be a soft rubber lip seal cutting a groove into a steel shaft. There is a reason that API pumps no longer use rubber lip seals as bearing housing seals. Since hard particles would not normally imbed into soft rubber, there is probably another mechanism at work. The soft rubber continuously rubs off the passivating corrosion products on the surface of the steel. New metal is constantly exposed to allow more corrosion. There may be hard particles involved in this example as well. But I don’t feel that hard particles, alone provide a good explanation.

I feel that your example is better explained by imbedded hard particles. But, since I do not know the properties of Nitronic-50, I cannot rule out the corrosion cause.


Johnny Pellin

 

[replica]

Artisi

Thank you very much, I will discuss the issue with the manufacturer.

JJPellin

Thank you very much for the suggestion for the mechanism. Since I had found the embedded particles on bush....this will be a very good explanation for the mechanism.

I would like to express my deepest appreciation to everyone, MikeHalloran, EdStainles, JJPellin, Artisi, 1gibson, romke and DubMac for spending time, sharing the experience and ideas in this discussion. This discussion is very beneficial to me especially on the cause of the shaft failure and suggestions for improvement to prevent the failure from reoccurring. I will consider all the possible bush materials and design after the performance of the pump running offshore right now with the increased bush clearance is not satisfactory.

I will talk to the manufacture on this issue.

Thank You very much

 

[EdStainless]

Replica,
And we thank you for staying engaged in the discussion, asking for clarification and follow up questions, and then closing at the end.
I honestly look forward to further interaction with you. You handled this very professionally.
Regards, Ed

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P.E. Metallurgy, Plymouth Tube

 

[Artisi]

Yes, good clear questions and considered responses from replica, unlike many posts where we need to squeeze every bit of data from the OP.bit by bit as if it were classified top secret of national importantance.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Artisi]

JJPellin: fair, reasonable and correct comments regarding rubber cutting into much harder materials, this of course is usually the result of the rubber being "loaded" against the other material. In the case of cutless rubber bearings / shaft this situation is not in play as there is running clearance and hopefully lubrication between.
My thoughts are still to give serious consideration to rubber bearings for this application (provided material is compatible with the product.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[replica]

Hi all,
other than worn shaft. I has also experienced seizure of the wear ring. Is the seizure of impeller wear ring and casing wear ring always related to galling? or is there any other mechanism?

 

[Artisi]

Is it the result of excessive wear of the shaft allowing the impeller / case to touch.
Actually, you might be better to start another thread - unless of course the comment above is valid.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[replica]

Artisi

Thank you very much. Agree with you, I think I better start with a new thread

 

[replica]

Hi all,

A little bit of update. When I looked at the surface of the bush and shaft I found limestone (CaCO3)or CaO embedded in addition to SiO2. Any idea where this limestone came from and is it abrasive? I know limestone is used for toothpaste..lol

 

[1gibson]

Is the ground at your site "crusher run" ? 2" and smaller grey rock, limestone? Someone may be dragging the bottom of the pump around when lifting from horizontal to vertical, and dropping rocks in with the pump. Or they are getting in some other way.

 

[replica]

1gibson

No ..I don't think so...it must come by other means...

 

[DubMac]

limestone formation reservoir?

 

[EdStainless]

CalCarb is less soluble as the temp increases, so if the water has a lot of minerals in it, and the bushings run warm you could be getting scale formation there.

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P.E. Metallurgy, Plymouth Tube