Help about centrifugal multistage recurrent failure 1

[jacilore]

Hi all:

Well, I opened a controversial thread, several weeks ago, about 2-pole vs 4-pole centrifugal pumps, regarding a purchase decision among 3 equipments, because the existing one had broken. In case you find it useful, here is the link:

http://www.eng-tips.com/viewthread.cfm?qid=300244&page=2

The existing one was precisely one of the candidates (a 1.500 rpm multistage 11 RITZ pump 49.200-11 of 400 KW, 550 HP), and the failure remains a mystery.(I didn’t mention this matter in that thread as it was not relevant then)

The required flow was 300 m3/h and head 340 mts.

Now, as result of that controversy the owners are now considering to repair the broken one, so I would appreciate your help for finding the possible cause-s of this failure.

The problem happened before they contacted me, and because of some other circumstances I haven’t been able to get too deep into the problem so I can’t give you very precise data.

Anyway, here's a list of rather objective facts:

• The pump seems to have been elected properly for the desired flow and head, but the little time it could have been tested it never matched the requirements.
• The first time at 2 hours working it showed not to work properly. (Vibration and noisy).
• They reinstalled it again and the second day it broke at the end of the day.
• Even RITZ technicians came from Germany to repair but it broke again the following day they left. (Their explanation was the equipment hadn’t been well installed before, but they couldn’t explain the failure after their repair)
• The damages have been breaking bearings and couplings, but I haven’t been able to determine what was first and when.
• Axis was changed although it never broke, but neither solved the situation.
• They seem to have tried with more than one type of couplings also

Sorry to present this case in such an unformal way, (it looks like a riddle, I know), but it’s how things have become.

Anyway, from the situation I am now, it could be easier for me to collect more precise data, but I needed your support before that.

Here I enclose the only photo I was able to collect with the broken couplings.

So sorry for this long and confusing story, and...could you throw some light?

Regards

 

[GPRnD]

Honestly it could be lots of things.

Assuming you have verified that the pump is correctly sized for flow, head and NPSHa, then I would start by verifying the installation specifically:

Pipe arrangement and pipe strain
Baseplate flatness
Pump to driver alignment (initial and when warm)

These pumps are less rigid than say a BB3 and if the installation is poor it can lead to rapid failures.

I think we'll need a lot more details to help. Another area that can be fun on these pumps is balance disc construction with the wrong type of coupling. That could also lead to rapid failures.

Are you able to post a X section of the pump, a picture of the installation, details of the coupling and a summary of the duty ?

 

[JJPellin]

You really don't have enough information to make a good determination of the cause of failures in this pump. But, I can make some general comments about the basic situation and relate it to examples that I have experienced.

The general case I would use is for a new piece of equipment that has serious vibration problems a short time after placing it into service. I would start to suspect a few possible causes based on some problems machines which we have purchased over the years.

Acoustics – We purchased a large multi-stage centrifugal pump that had almost immediate high vibration. I had witnessed the test runs of the pump on the manufacturer’s test stand. In test, it ran quite well with very low vibration. In our service, it seemed to run well at first. But as soon as the process temperature increased, we experienced extremely high vibration at 5 times run speed. Each impeller in the pump had five vanes. It took over two years of work to prove to the manufacturer that we were generating a standing wave acoustic vibration in the long cross-over channel excited by the vane-pass frequency of the last impeller before the cross-over. We had to change to a six vane impeller to resolve the problem.

Resonance – We purchased a large blower that ran for many years with low vibration, but after a shutdown suddenly had extremely high vibration at run speed. All of the data suggested that we had high shaft run-out (a bent shaft). But each time we shut down, we found that the shaft was straight. It took many years of work to prove that our bearing pedestals had experienced a reduction in stiffness which allowed the rotor to be excited at first critical mode. We are in the process of redesigning the bearings and pedestals to restore the stiffness and provide a greater margin to avoid rotor resonance.

Assembly Errors – We installed two large barrel pumps which experienced high vibration at first stage vane-pass frequency after start-up. The vibration was indicative of a horizontal bearing housing resonance on the inboard end. We found that the contractor that installed the pump had not installed it correctly to accommodate thermal growth. The alignment pin blocks on the base were not welded down. And the outboard hold-down bolts that were supposed to be left at a very low torque were torqued down fully. When the pump heated up, the case was distorting, resulting in an uneven gap at the internal clearances. The uneven gap resulted in higher vane-pass frequency pressure pulses. The vane-pass frequency pressure pulses excited a bearing housings resonance at about that same frequency. We corrected the pin-blocks and hold down bolts and the problem was resolved.

Pipe Strain – A new set of pumps experienced high vibration indicative of misalignment. We are not allowed to perform a hot alignment at our plant. But, when we checked the cold alignment, we found that we had the proper offsets as called out by the manufacturer. However, when we took readings on the housings as the pump was heated up, we found extreme movement well beyond the predicted thermal growth. After some long investigation, we found that the piping support design was not correct. When the piping heated up, it was imposing high pipe strain on the pump which was distorting the pump and resulting in extreme coupling misalignment.

Thermal Effects – In a similar example, we had high vibration indicative of misalignment on a new installation. We found that the insulator had installed the insulation blankets incorrectly. The support pedestals were designed to be cold supports; outside of the insulation. Instead, the insulator found it easier to box in the entire pump which placed the support pedestals inside the insulation; serving as hot supports. In another example, the pump was protected from freezing with electric tracing. The tracing included a temperature probe that was supposed to be installed under the insulation so that it could automatically control to a relatively low temperature (70 °F). Instead, the electrician had installed the temperature probe outside the insulation. On the first cold day, the tracing system had overheated the pump to well over 200 °F trying to achieve set-point at the temperature probe. This distorted the pump and caused high misalignment.

Johnny Pellin

 

[DubMac]

Jacilore,

Wish you were back with better news.

Sounds like either misalignment or a critical speed problem.

Misalignment can either be internal to the pump (ring cases not exactly centered around the shaft, bearing housing misaligned on case, etc.), or external (pump not perfectly flat and aligned to motor).

I would guess the Ritz mechanics would have caught pump/motor misalignment. Possibly a "soft foot" on pump that only shows up at running speed??

Resonant frequency issues sound probable as well; have a good vibration guy come out and do an analysis on the system if you can get it running again. Their charge is well worth it.

I hate to say it, but once a ring section pump becomes a dog, it seems like it goes from one problem to another until it is replaced. Hope I'm wrong, but that's been my experience.

 

[jacilore]

Hello all:

First, thanks for your dedication in your replies; I’m truly glad to have met this forum.

And second, sorry for the delay, but I’ve been very busy, and for me it’s quite difficult to “process” your help. I am Spanish, new in this field, and I lack a lot of vocabulary. I have to google translate a lot every time I write or read here, so I will go replying as soon as I can.


Bradshsi, About the proper installation, I think the guy that first installed the machine (employed of the owners’ company), is very skilled and it’s really no surprise technicians from Germany didn’t get things any better. It seems to be a subtle issue what’s happening here, so I'll try to collect more data regarding your approach.

JJPellin: Thanks for your extensive and professional reply; talking about subtle issues it seems the solution should be in your list, although I’ll really need time to get into those possibilities. But I will do and tell you.

Dubmac: Thanks again for your kind reply. The news are rather bad than "new" because the problem already existed when I contacted you, so it’s not really that things have gone worse; only that it’s now that they are focusing on the repair, because they couldn’t decide between 1.500 or 2.900 rpm. new equipments.

Anyway, before anylising things very deep, I have read that the simple fact of the pump not working on the ideal conditions (duty was never achieved from the start), could itself lead to failures.
So that would be a cause that would require another explanation, but perhaps simpler.
Do you think that problems could come from working in a non-desired point?

And another possibility I have been reading issues regarding axial thrust, so I found in the pump manual diagram the number 600 component, named “Axial balance pipe”.

Could any of you tell me what that’s for?

Thanks again and regards. I will feedback with more information anyway.

 

[JJPellin]

The axial balance pipe would normally be called a balance line. It diverts the fluid that leaks across the balance drum back to the pump suction. There is an FAQ regarding the function of the balance drum. If this balance line were plugged, obstructed or blocked in, an excessive amount of thrust could cause a nearly immediate failure of the thrust bearing. In a pump of this arrangement, a thrust failure can be among the most destructive. Once the thrust bearing fails, the impellers can rub axially into the diffusers, potentially destroying both. I could check that this axial balance pipe is properly routed and unobstructed. A closed valve in this line could cause almost immediate destruction. But the manufacturer’s technicians would have certainly noticed this.

Johnny Pellin

 

[JJPellin]

It was not clear to me if you feel that the pump may have failed because it was running at low flow or that the pump was running at low flow because it failed. Running a pump below minimum flow can result in problems that may be acute or chronic. Now that you have provided a cross-section of the pump, I can make more specific comments.

If this pump is operated below minimum flow a number of possible problems could be created. Running below minimum flow can result in a form of cavitation known as suction recirculation. This can occur even if there is excessive Net Positive Suction Head available. Cavitation produces broad band vibration that can excite resonances in the pump, rotor or structure. So, if you have a poorly damped critical anywhere in the system, the cavitation could drive it to respond violently.

This pump is represented as having five stages in the drawing. But, it looks like a generic drawing. How many stages does this pump have? We have one pump of this configuration that has case sections for 13 stages but only has 11 impellers. Once these pumps get very long, accommodations have to be made for sag in the rotor. It becomes very difficult to maintain concentricity. Our long stacked diffuser pump has two wedge shaped diffusers that are supposed to be installed in a particular orientation to force the diffuser stack to assume a bow shape similar to the expected rotor bow.

If your pump has a large number of stages, it will be sensitive to start-up conditions. It needs to be well vented and cannot be operated against a blocked valve for start-up. Even a few seconds blocked in can generate an incredible amount of energy.

This pump uses a stacked rotor design that has serious problems. All of the impellers, sleeves and bearings stack up with a single nut on the end to lock it all together. If there is any run-out on the faces of these parts, the rotor will be bowed when the nut is tightened. This is a poor design. The impellers should be located individually with steps, snap rings or split rings to avoid this problem. Since the final assembly of the pump has to be stacked impeller, diffuser, impeller diffuser, it is impossible to check during final assembly if the stacked arrangement is still straight. If that lock nut on the thrust end is too tight, this makes the problem worse. When the rotor is stacked up without the diffusers for balancing and run-out checks, it is critical that it is stacked exactly the same way it will be in service including how that nut is torqued.

A similar problem exists with the tie-rods that clamp the diffusers together. They must be tightened in a proper pattern to the correct stress. If they are tightened unevenly or over-tightened, the diffuser stack will distort and not be straight.

Either a bowed rotor or bowed diffuser stack can result in hard rubs and almost immediate failure.

In some pumps of this configuration, the thrust bearing cover has a gap at the face. When the cover is tightened it must have a certain torque applied to the bolts or the thrust bearing will be crushed and loose all internal clearance. This is unusual in API pumps, so our mechanics have a hard time with this strange requirement. I cannot tell by the drawing if your pump has this feature.

Johnny Pellin

 

[GPRnD]

jacilore, sorry I should have asked for this in my previous post. Generally the way I start looking at these types of failures (after you've checked the obvious stuff), is to review the operating history prior to failure (flows, NPSHa, duration) together with vibration data.

The vibration data in particular is going to give you some good info on pump problems. Was any vibration measurement performed ? If so what was the resulting spectrum obtained ?

I'd agree in general with JJPellin's comments on the criticality of straightness, runout etc on these stacked rotor pumps.m The X section you posted is of a rather old design where the rotor stack is carried all the way through the pump. I don't like this because of the possibility to overtighten and bow the shaft. More modern designs not use sleeves or shaft nuts in this way.

 

[jacilore]

Hi JJPellin:

Thanks again for your input.

About the axial balance line, I just didn't see that was the connection with the balance chamber. I thought it could be some kind of regulator, but I didn't even understand.

And about my doubt, I figured out that a possible cause of the final failure could be working below design, but in the hypothesis of that, I should search by another failure. (Tension supply, higher hydraulic losses than expected, equipment not reaching goals).

As you have confirmed me, that can be a real possibility, but then it opens some other questions.

The rest of the things you said I'll need time to check, but it's really appreciated all that information, so the least I can do is to try to immerse as much as I can on it, and try to reply.

I really wish my findings can be useful in return for this forum.

Regards

 

[JJPellin]

I should make one more comment about the stacked rotor design. Even if the mechanics find that they can reduce the run-out on the assembled rotor by controlling how tight they tighten that nut, you can still have problems. We found this on our pump once. They got it to pass the run-out check by tightening that nut very little. But, when the pump is placed in service, the hydraulic thrust from the impellers will pull the stack-up much tighter than the nut and the rotor will bow. So, you need to torque that nut to full torque when you stack up the bare rotor to see what is likely to happen when it runs.

Johnny Pellin

 

[jacilore]

Bradshsi, no vibrations measurements were made; only that when German technics repaired it didn't seem to exist abnormal vibrations but after the machine broke again.

I will try to collect all informations I can, but first I needed to be more familiar with this.

I don't know if I have said but the installation is in another island (Canary Islands) from mine, so I'd need to have as good picture as I can of the possible causes to request data to the current owners of the installation, because besides, they are not the same existing when problems arised.

I am planning to travel next week to collect more information from there (spoken and documents).

 

[PumpSmart]

JACILORE,
-PARA EVITAR LOS PROBLEMAS QUE INDICA JJPELLIN, TU PUEDES HACER UNA LECTURA EN TECNICA DE VIBRACIONES FFT, STAR-UP - COAST DOWN O HACER UN BUMP TEST PARA VER SI HAY RESONANCIA EN EL SISTEMA DE TUBERIAS, Y ESTA LECTURA QUE SEA PARTE DE UNA STAR UP COMMISIONING CHECK LIST. LAS PERSONAS QUE TE DAN APOYO DEBERIAN HACER UNA LISTADO DE COSAS ESTAN HACIENDO.
LA NORMA API RP 686, DA UN LISTADO MUY AMPLIO DE COMO ARRANCAR UN EQUIPO DE BOMBEO, BOMBA CENTRIFUGA.
QUE VAN DESDE ALINEAMIENTO, PRUEBA HIDRAULICA, TOMA DE VIBRACIONES, ETC.
-EL EQUIPO REPARADO RITZ, TIENE ALGUNA PRUEBA EN TALLER? TIENES ALGUN REPORTE DE REPARACION, ESO AYUDA A SABER POR LO MENOS QUE ENCONTRARON EN EL EQUIPO.

 

[jacilore]

Hello all:

After some time of being involved in other things, finally managed to speak with a fellow who doesn't work now at the company, but was at charge, when the pump was first installed and failed.

He told me that the vibrations of the machine were rather normal, but after the first working hours, the thrust ball bearing started to heat anormally, apart from the pump never achieving the required flow. Only 80% of it. (It seems vibrations were not so relevant, as the owners had told me, and by the way he told me he seems more reliable).

More or less the same happened after the german technicians came to repair the pump, but no report about that was found.

I am travelling there next week for other reasons, but I will try to collect more information, and I will tell.

Regards and sorry for this messy case.

P.S: PUMPSMART, thanks for the input and for the Spanish. As I said before, it seems vibrations were not so relevant, and no report was given.

 

[jacilore]

Hello, all again:

I have just opened a new thread regarding another issue with pumps, so I’ll take the opportunity to update about this one I myself opened about a RITZ mod. 49.200 centrifugal 11-stages pump, also thanking the interest you showed about it.

Actually I haven’t got as deep as all your inputs, but the fact is that some pumps supplier known by the owners was at the installation and offered to check repair the pump at low cost so the owners accepted.

He found some misalignments in the former installation and also thrust bearings were the mounted wrong way.

So he repaired it and the pump has been working so far without problem, but there’s one controversial question I want to expose to you:

The maintainance technician of the company is convinced that the bearings temperature is playing a role in the former problem, as he says it shouldn’t reach more than 60ºC. (140ºF) and from the beggining, Despite manual instructions say bearings may reach 110ºC-230ºF without showing problem.

When the pump was reinstalled last September, at more or less 30ºC-86ºF ambiental, bearings temperature reached about 85ºC-185ºF

So, given the bad experience, doesn’t dare to leave the machine working without cooling with fresh water outside the bearings, not letting the temperature surpass 50ºC-122ºF

So, what do you think about this?. Can it be really a sign and/or a cause of problem that bearings get that temperature (85ºC-185ºF)?

Thanks and regards.

 

[DubMac]

Hello Jacilore,

Even though your roller bearings are manufactured to operate at temperatures up to 110C/230F; the question to ask is "what is causing the bearings to operate at these temperatures if the ambient temp is about 30C/86F?".

If the cause of the rapid heat buildup is not ambient or fluid temperature, then it must be generated by high friction in the bearings. Your mechanic is right when he says the bearing temperature should not go above about 60C/140F; that may be even a little too high.

At your low ambient temp, you should be able to put your hand on the bearing housing and it should feel somewhat warm, but it should not burn you.

Adding cooling water to the bearings is only covering up your root problem.

Three possible causes I am thinking of:

- Balance line not functioning properly. This line provides hydraulic assistance to the thrust bearing. If it is not functioning, the thrust bearing is carrying too much thrust load in the axial direction and will heat up or fail. This could very well be the problem, and you should ask the Ritz people to give you all of the specifics on the balance line operation (required balance line flows/pressures for different operating points).

- Critical/resonant speed problem. Only a good vibration consultant can determine this for you; they are not expensive and well worth it. But of course the pump must be running to perform this check.

- Bearing/shaft misalignment of some sort. The Ritz mechanics probably have checked the external alignments, shaft/coupling, baseplate, etc. and lets assume they are fine.
If you will notice, your bearing housings are bolted onto the pump case. If they are not exactly aligned and centered with the bore of the pump case, you would expect the problems you are seeing. This is very common with this type of pump (ring section).

If you will note from the cross-section drawing, the thrust bearing housing is bolted onto the seal housing (stuffing box), which is then bolted to the pump case. This gives two chances for misalignment. Also, once the pump does heat up only on one end, this can cause uneven thermal growth along the pump axis, which causes more misalignment.

You would need a somewhat sophisticated machine shop or a very good pump mechanic to assemble the pump and measure the concentricity of the bearing housings to the pump case.

Those would be the first steps I would take, but I am sorry to say that in my experience, this style of pump is famous for misaligment problems.

I wish you well.

 

[jacilore]

Thank you very much, Dubmac; What you say sounds very professional. I'll just need some time to figure it all out. I will surely come with more questions.

Regards.

 

[GPRnD]

I'd personally be uncomfortable with newly serviced pumps operating with 185 deg F bearing temperatures. That is a level at which I've typically seen the failure rate rapidly increase.

In addition to what Dubmac listed, I'll add that balance drum sizing is very critical on these pumps especially at high stage numbers (>10). It may be that the axial thrust is simply higher than the expected design value which in turn will cause higher bearing temperatures.

The only way to confirm this is with thrust load testing which is beyond the means of most people. However it may be worth asking Ritz what their design thrust load is and seeing the response.