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Worn pump shaft
7

Worn pump shaft

Worn pump shaft

(OP)
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/getfile.aspx?folder=2...

Regards,

RE: Worn pump shaft

I might speculate that the bushes are entrapping sand particles from the crude oil, and lapping the hell out of the shaft.

Is there some way to feed the bushes with less dirty oil?

Does the pump manufacturer offer softer, e.g., plastic, bushes?



Mike Halloran
Pembroke Pines, FL, USA

RE: Worn pump shaft

I agree, it looks like trapped solids.
Perhaps some relief so that there is more flow through them.
What is the clearance on new ones?

This is why we rarely ran bushings directly on shafts.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Worn pump shaft

(OP)

Thank you very much for the feedbacks,

MikeHalloran

Entrapped sand particles, that is what i am thinking as well. Anyway how is it possible that the particle only polished the harder shaft and not the soft bush? Is it because the shaft is rotating and the bush is stationary that made this possible? When I looked at the bearing inner diameter surface under scanning electron microscope, I could not find any embedded particle on the surface.

It is almost impossible to lubricate the bush with other means but we can find out and probably filter the particle.

Yes we can try to use softer bush but how this will help to solve the issue? from what I understand the plastic bush will swell under H2S, hydrocarbon and heat the may cause the shaft to wear out, but we can try.

EdStainless
The clearance on the new one is 0.008 inch for 1.5 in shaft diameter. When we increased the clearance we found that the pump life improved, to date the pump had been operated for almost two months without failure. We will monitor the progress of the pump. The issue is can we increase the clearance beyond the maximum recommended value?

This is why we rarely ran bushings directly on shafts.
EdStainless ..do you have another method of running the bush not directly on the shaft?

RE: Worn pump shaft

Can you change to a fluted, cutless rubber bearing in a compound compatible with the pumped product, this could be your best option.

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.)

RE: Worn pump shaft

(OP)
Artisi

I am not familiar with the rubber bush as you stated but we had tried vesconite but not at the same well and found out that the bush broke in brittle manner (may be due to reaction with the crude oil) but with less wear on the shaft. However this is not really representative as the crude oil properties at that particular well probably had different properties ie less particles, etc.

Thank you for the suggestion and will look into that option. Anyway I really appreciate if you can provide more information, links etc regarding fluted, cutless rubber bearing as suggested.

For a time being we are monitoring at the pump with increased clearance. Recommend clearance is 0.004 to 0.006 inches and we used 0.008 inches which already beyond the recommended value.

Thank you

RE: Worn pump shaft

The traditional method of lapping is to embed/crush ("charge") an abrasive into a softish material like brass, and rub the now abrasive face of the brass on whatever hardened steel you are trying to polish. I'm thinking that maybe a filled plastic, e.g. Rulon, would allow the particles that you can't avoid to embed themselves a litle deeper than does the bronze, and maybe eventually even bury themselves so they stop wearing the shaft.

The 'cutless' bearing that Artisi mentioned is often found running against long propeller shafts, totally lubricated by whatever water the boat is in. They typically have a polygonal bore and a round OD, so as to maintain a pretty good flow of water to lubricate the steel/rubber interface. That might be a problem for an interstage bearing, that presumably runs with a pretty decent pressure difference across it; it would certainly reduce the pump flow capacity somewhat, with all the reverse flow.

If you have radial access to the bushes, maybe you could filter the pump output and pump cleanish high pressure fluid radially into each bush, so they'd be flushed with cleanish fluid for at least part of their length. You might have to change the filter once in a while...

Mike Halloran
Pembroke Pines, FL, USA

RE: Worn pump shaft

replica : try www.exalto.co.uk for idea's and tech. data

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.)

RE: Worn pump shaft

(OP)
MikeHalloran

I like the idea of allowing the particles to embed themselves a little deeper by using the softer bearing materials, at least softer than bronze. However as I mentioned earlier I could not find embedded particles such as sand or oxide debris on the surface of the bronze bush in contact with the shaft. So the assumption that I made is based on the presence of a lot of sand in the pump during maintenance and the nature of smooth and mirror finish worn surface. I will try to find the embedded particles on the other bushes which may throw some light to the issue. It is also possible that the particle has been washed away during cleaning. I will look at the wet and unwashed bush for the evidence. I am desperate to find the evidence that failure is due to the trapped hard particles. Is it possible that corrosion attack or erosion corrosion play rules? Just a wild guess.


Artisi

Thank you very much for the link, but after looking at the link and as mention by Mike Halloran I am afraid that the pump might loose some flow capacity due to reverse flow. But anyway at this stage anything worth trying.

RE: Worn pump shaft

Corrosion? Erosion? Sure, both are possible; you're pumping crude, right?

Mike Halloran
Pembroke Pines, FL, USA

RE: Worn pump shaft

replica, the loss through the interstage's might well be insignificant, it will depend of the pump/ impeller design which at the moment to us is an unknown.

Have you contacted and discussed the problem 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.)

RE: Worn pump shaft

(OP)
MikeHalloran,

Yes we are pumping sour crude oil, so corrosion and erosion corrosion is possible. But as stated in NACE MR0715/ISO 15156, Table A3, nitronic 50/UNS S20910 used as a pump shaft is suitable for any combination of temperature, H2S partial pressure, chloride concentration and in situ pH occurring in production environment is acceptable. However as for solid sulfur there is no data available. However it is very difficult for me to find the evidence of corrosion attack on the the polished shaft surface and trace of corrosion attack if any ( unless I know the type of inhibitor as well as drag reduction chemical used, which is not in my knowledge). Should the trace be on the surface which is very difficult to analyze by EDS, in that case we must used XPS for surface analysis which is very tedious. It is also possible that the corrosion create the passive layer of sulphite which then removed by embedded particles and further corrosion will continue. Anyway this is very difficult to prove.

What we are doing now is trying to study the topography on the surface of worn shaft and bush by Scanning Electron Microscope and try to do some EDS analysis on the embedded particle if any and establish a possible cause of failure. What bother me is that some other area where the shaft worn out ( at other bowl bush ) did not show smooth and shining surface (rather roughly abraded surface) and as in the first and second stage area. Or is it also possible that some region has more embedded particle and polish more than the other. However my believe is more on particle entrapment rather than corrosion.

Artisi

No I did not contact the pump manufacturer yet because I would like to establish the cause of the failure first.

RE: Worn pump shaft

Bear in mind, as any abrasive particles are pumped and eroding "whatever" they are also becoming less abrasive as the sharp edges are being abraded off so it is possible to see reduced wear in some areas,also any particle trapped within the "bearing" the is circulating round and round, something which doesn't usually happen with fluted cutless type bearings as they are usually passed thru'.

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.)

RE: Worn pump shaft

I work in an oil refinery that runs extremely sour, heavy Canadian crude. Our crude primary pumps are typically two or three stage vertical turbine pumps. I checked on the shaft and bushing materials that we use. About half of our primary pumps use Graphalloy bushings. These would be much softer than the bronze that you are using. The other half of our pumps use Bronze bushings called out as "660 BRZ" or "SAE 64 BRZ" on the bills of material. All of our pumps use 416 SS shafts. We do not use any outside flush to lubricate our bushings. We have not seen the sort of damage that you are describing. Our pumps typically run for several years between overhauls. I did not research the exact configuration of our bushings for features such as the axial groove shown in the picture. If we were to use rubber bushings, the only compound that would likely hold up in our service would be something like Kalrez.

Johnny Pellin

RE: Worn pump shaft

(OP)
Artisi

Thank you for the info on cutlass type bearing.

JJPellin

Thank you very much for sharing your valuable experience. It seems that I have many options for bearing materials. Considering your experience on sour crude using 416SS shaft, I may exclude the corrosion issue here since I am using Nitronic 50 (I think suppose to have better corrosion resistance under H2S environment). My main concern is the entrapped sand particles. What is your bearing clearance? Does your crude contain sand? It will be very nice to have pump that can last years between overhaul...lol

RE: Worn pump shaft

The pumps I looked at also have 1.5" diameter shafts. We work on these pumps so rarely, that I was unable to find the clearances for the bushings in our repair records. Our standard table of clearances for bushings in a pump of this configuration would call for 0.006" to 0.009" under a bronze bushing and 0.005" to 0.007" under a Graphalloy bushing. We probably make these bushings in our machine shop and probably cut them to these clearances. They are probably straight, smooth bore bushings without any axial grooves. Our primary pumps all run at 1800 rpm.

We process heavy, sour Canadian crude in addition to diluted tar-sands bitumen and synthetic crude cracked from tar sands bitumen. I could make the argument that our crude is among the nastiest on the planet. I have never seen it myself, but I am told that when we open up our crude tanks for maintenance, we find sand bars as much as 8 feet deep. I believe we do pass some sand through our primary crude pumps. But, I am not sure of the concentration or particle characteristics.

Johnny Pellin

RE: Worn pump shaft

Johnny brought up another good point. The axial vrooves feed fresh abrasive with every revolution. Did the bushes always have them?

Mike Halloran
Pembroke Pines, FL, USA

RE: Worn pump shaft

(OP)
JJPellin

Thank you very much for sharing and bringing a good point regarding bush design, for your information, our pump runs at 3650rpm. I know that Graphalloy bearing has graphite in it which is very good lubricator and graphite is always use in self lubricating bearing (without lubrication fluid) It seems that your bronze bearings do not have grooves on them . I am not sure about the benefit of having groove other than for lubrication purpose. Which is better with groove or without groove? I think it probably depends on the type and design of the pump. I have to find out weather it is permissible to have bearing without groove or even with more grooves.

MikeHalloran

All our bearings have 3 axial grooves on them. Is it possible by having more grooves will provide better flow and prevent the entrapment of solid particles? or the opposite? Any suggestions? Johnny experience had shown better performance without groove.

RE: Worn pump shaft

one point of interest that struck me in this discussion was the fact that using a larger clearance resulted in longer shaft/bush life. can it be that lack of lubrication plays a part in all this, in the sense that insufficient "crude oil lubricant" enters into the bushing? "crude oil" of course being a "lubricant" with a lot of shortcomings, since it usually contains dirt, can be highly corrosive and most likely has a viscosity that will not help in getting sufficient fluid into a bush that needs to be lubricated.

RE: Worn pump shaft

If those bushings have three grooves, they are probably there for the purpose of shaft stability. A vertical shaft will tend to be unstable in a smooth bore cylindrical bearing. It would tend to be prone to whirl/whip. A tri-lobe design would be a classic solution to this problem. We had a set of liquid sulfur pumps that were converted from 1800 to 3600 rpm and had vibration problems. We solved these with a tri-lobe bearing (bushing) configuration. Knowing that your pumps are running at 3600 rpm, I would recommend that you be very cautious about removing those grooves.

Is it possible that the grooves are contributing to the problem by feeding a fresh stream of sand into the bushings? It is certainly possible. I would consider shortening those grooves so that they don’t break through both ends of the bushings. This would keep the stabilizing influence of the tri-lobe design, but block the entry path for more solids.


Johnny Pellin

RE: Worn pump shaft

(OP)
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.

RE: Worn pump shaft

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.

RE: Worn pump shaft

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

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Worn pump shaft

(OP)
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.

RE: Worn pump shaft

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.

RE: Worn pump shaft

(OP)
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?

RE: Worn pump shaft

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.)

RE: Worn pump shaft

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

RE: Worn pump shaft

(OP)

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

RE: Worn pump shaft

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

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Worn pump shaft

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.)

RE: Worn pump shaft

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.)

RE: Worn pump shaft

(OP)
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?

RE: Worn pump shaft

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.)

RE: Worn pump shaft

(OP)
Artisi

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

RE: Worn pump shaft

(OP)
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

RE: Worn pump shaft

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.

RE: Worn pump shaft

(OP)
1gibson

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

RE: Worn pump shaft

limestone formation reservoir?

RE: Worn pump shaft

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Worn pump shaft

To answer your question of 4 May, limestone is softer than silica, but it's still an abrasive.
... and as DubMac points out, limestone may be coming up with the crude oil.
... and as EdStainless points out, it could be coming up dissolved in whatever water is in the crude, and precipitating.


Mike Halloran
Pembroke Pines, FL, USA

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