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Guru
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Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

05/31/2012 8:59 AM

In a four stage integrally geared centrifugal compressor a large bull gear shaft is coupled to 5 MW electric motor (1500 rpm). Bull gear is meshed with two pinions, 10000 and 12000 rpm (180 deg apart). Each pinion is fitted with two overhung impellers on either side.

While starting the compressor, just at the time when it is reaching to rated speed in 20 seconds, its bull gear shaft got sheared at the shaft labyrinth groove towards coupling side. Sheared surface is straight perpendicular to the shaft surface.

What could be the reason for this kind of shaft failure?

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Guru

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#1

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

05/31/2012 9:14 AM

We need a little more info.

1st time running? or running for months?, years?

any shock loads?

Has anyone verified the calculations used to size the shaft?

Potential causes could be

1. too sharp of a radius leading to a stress concentration point.

2. fatigue

3. too weak of material

4. bad calculations during sizing of shaft

any chance that you could attach a picture or 2?

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Guru
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#3
In reply to #1

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/01/2012 5:25 AM

Thanks ddk for your response, GA.

1. It's running since over 20 years.

2. Bearing vibration, shaft position and bearing temp were all normal until failure.

3. Shaft has a machined groove for oil labyrinth where it sheared (min. dia. point).

4. About 5 year back there was exactly the same failure in another machine. It was sheared exactly at same place and sheared surface look exactly the same. That time bull gear rotor and both pinion rotors were replaced with new ones as OEM advised to replace all the 3 rotors. Both removed pinion rotors were in good condition. This time also OEM may advise the same and we may be stock piling removed pinion rotors. We are worried about another 2 machines out of 4 installed, if failure in waiting.

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Guru

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#4
In reply to #3

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/01/2012 7:28 AM

pritam,

It is looking like fatigue IMHO. But this machine is over 20 years old, so it is not unexpected.

Some questions your company should be asking.

1. How long do we expect these machines to last before they wear out?

2. How long does the OEM think they will last before they wear out?

3. Is it more cost effective to rebuild or replace?

good luck.

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Guru
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#5
In reply to #4

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/02/2012 2:15 AM

ddk,

What is IMHO?

We have many integrally geared centri copressors but this failure occured for two of the same set of machines in 5 years. Such machines are expected to give quite a long life. Even many places running since over 50 years without such failures.

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Guru

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#11
In reply to #5

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/06/2012 7:33 AM
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#12
In reply to #3

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/08/2012 11:57 AM

20 years assuming 24x7 service is about 15 x 109 cycles. Hot topi for fatigue failure as someone has pointed out and from your description it seems quite likely.

The labyrinth groove failure also points to the same since the groove is unlikely to have the required corner radius to avoid the notch effect and another thing (which I have earlier seen in some failed shafts) is the finish at the groove (chattering) due to plunge cut that excerbates the situation.

Why don't you (if feasible) carry out an NDT in rest of the shafts to find the initiation of the fatigue and take action before it fails?

In the replacements isn't it possible to avoid the groove?

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Guru
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#13
In reply to #12

Re: Reasons for gear shaft failure in integrally geared centrifugal compressor

06/09/2012 12:29 PM

You are right about no of cycles, but as two out of four compressors are on continuous service, it has approximately completed 7.5 x 109 cycles.

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Guru

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#2

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

05/31/2012 3:15 PM

Poor Quality Control by the manufacturer would be my guess given this little information.

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#6

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/02/2012 6:48 AM

This one of strange failure i have red! There should be a huge force to shear out shaft, and Motor should trip on high amps before that. First check all protections of your machine. this kind of failure should not considered as sudden failure, you have to review and investigate history of the machine, is it running at high vibration, what was the speed when it sheared off, is it at critical speed or not. what is the amps of the Motor at that time of failure, what is the discharge pressure of both stages. Shaft material to be analyzed, is there any equipment connected at other end (at thrust side).

Fahad

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Guru
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#7
In reply to #6

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/02/2012 1:04 PM

Thanks Fahad for your comment. 5 years back when first machine failed exactly the same way, at the same place and in the same circumstances, it was strange failure for me too. Now it is repeated for one more machine, it's no more strange for me. It's an air compressor to deliver 44000 Nm3/hr at 8 bar to supply in a plant network.

All protections were in place. As I stated earlier, all shaft vibrations were well within the norm in previous run. Full speed of sychronous motor and bull gear rotor is 1500 rpm. Sychronisation time is about 16 seconds from start to attain full speed. As per the regular operation practice, machine is slowly loaded by opening its inlet guide vanes (IGV) only after attaining full speed. While it was being loaded (after about 3 minutes from start) it was noticed that flow rate is reducing from 16000 Nm3/hr and became zero. All shaft vibrations also became zero. While IGV was being checked whether it is really opening at site, metal spark was observed from bull gear shaft. Then motor was stopped. I shall come back on motor Amps at the time of failure, as it is already sychronised, maximum phase was over. No any other equipment connected in downstream.

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Guru

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#8
In reply to #7

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/04/2012 2:57 PM

My first reading I envisaged the I-R Centac unit or an Elliott PAP unit but after rereading your description I realized that there is no mention of manufacturer.

I would assume that this is a 4 stage unit since you're only going to about 115 psi but you're running at 1500 RPM input. I seem to recall that some of the earliest I-R units had some issues that the PAP units did not. This was not some sort of rebuilt I-R unit was it?

From the sound of it you indicate that the variable inlet controls are functioning properly so you're not getting any abrupt loading/unloading shock issues. Any chance that you're getting anything passing through the inlet filters that could lodge in the impeller creating an imbalance? I know that these used to be open radial twisted vane impellers but a larger chunk of rust or foreign material at the high speeds they run at could create a problem. When you say "no other equipment was connected downstream" is it running deadheaded against a closed valve or is there a valve that automatically opens when the compressor is up to full pressure?

The inlet guide vanes generally throttle the flow so as to reduce HP/KW requirements as demand changes but the discharge pressure is based on the RPM/impeller diameter. If the inter-coolers are clean and functioning the temperatures shouldn't get much higher at reduced flows.

"I shall come back on motor Amps at time of failure.... maximum phase was over". I don't get your meaning by this statement.

The fact that the shaft sheared at the same location, which is obviously a stress riser location, I would look for excessive loading/unloading operation such as too much capacity on-line resulting in constant cycling, intake issues or marginal shaft design or material. Sometimes "after market" suppliers or maintenance departments skip on using the same quality shaft material either to save costs or because they just don't know what materials were used. A steel shaft can have the correct dimensions but not have the physical characteristic required by the design. If the manufacturer uses only 17-4ph for his shafts and the replacement isn't of the same quality premature failure is often a consequence.

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Guru
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#10
In reply to #8

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/06/2012 4:29 AM

Thanks Spino & GA for detailed comment.

Here I want to amend my own post at #7. I wrote- "While it was being loaded (after about 3 minutes from start) it was noticed that flow rate is reducing from 16000 Nm3/hr and became zero." Actually these machines are being loaded after synchronisation by gradually opening IGV. But flow rate started reducing from about 20th second and became zero after 3 minutes. So from the trend recorded, shaft broke at approx. at 20th seconds from start.

Now coming to your comments, it's neither I-R nor Elliot, but a reputed one. You wrote- "I would assume that this is a 4 stage unit since you're only going to about 115 psi but you're running at 1500 RPM input." You need not assume it, I mentioned it at original post. And yes, it is run with 1500 rpm input at bull gear shaft which drives two pinions rotating at 10000 & 12000 rpm (described earlier) . 1st & 2nd stage impellers are fitted with 10000 rpm pinion shaft, 3rd & 4th with the other.

Yes, variable inlet controls are functioning properly. In the beginning it is loaded manually by operator, only after attaining required discharge pressure and connecting to supply header (by remote opening of valve) its controls are put in auto. Inlet filters are intact. No foreign materials observed at suction. There was no issue with the inter-coolers, all temperatures were well within limits. I couln't get the trend of motor current.

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Guru

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#9
In reply to #7

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/04/2012 2:59 PM

If you know someone with a Spectrometer set up for steel have them spark the broken shaft and check what alloy it is.

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#14
In reply to #7

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/15/2012 10:36 PM

We have 6 stages Integrally Geared Centrifugal Air compressor of capacity 30,000nm3/hr, KOBE Make . One bull gear and 3 pinions with impellers on Both sides.

My question is :-

1. Is anybody having similar compressor installed

2. What are the common problems associated .(My compressor is 10 yr old ).

3. One of the Problems commonly encountered by me is the shaft seal leakage from 3 and 4th stages. The manufacturer says its a common thing and designed to leak. However air being a scarcity for us , we dont accept any kind of leakage . Can anybody highlight more info on this .

REGARDS

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Guru

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#15

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/18/2012 8:10 AM

If the surface is perpendicular to the shaft surface the crack is due to a bending strain. The crack progressed from the surface to the the center till the remaining section became so small that the starting torque lead to a shear stress over the limit and the shaft broke.

Although the number of cycles is important it is NOT compulsory to get a fatigue failure if the stress level is low enough. The fact that it took such a long time shows that the level is low but the fact that you had a similar failure already years ago shows that the material is sensitive to the grove.

I would suggest:

- you check the remaining shafts for a crack at the critical place this is easy to do with fluorescent dye

- if you can you roll-burnish the grove where you do not already have an incipient crack. The compression strains in the material will reduce the stress peaks and increase the life expectancy. Attention DO NOT DO it where you notice a crack.

I would also suggest to analyse where a bending could come and try to reduce its amplitude.

The analysis could be better done if some pictures and dimensions would be available.

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Guru
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#16
In reply to #15

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/29/2012 1:21 AM

The surface is perpendicular to the shaft surface, but I don't think it is due to bending because is supported by journal bearing very near to the failed surface. Bearing condition is good and its clearance is in order.

Yes, whenever we open we check it with DPT.

Picture I tried to enclose but not successful.

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Guru

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#17
In reply to #16

Re: Reasons for Gear Shaft Failure in Integrally Geared Centrifugal Compressor

06/29/2012 3:18 AM

Torsion failure is at about 45° with respect to axis since the maximal tension stress is 45° with respect to the shear stress and in torsion there is only shear. If the failure is normal to axis this means that the crack opening stresses are parallel to axis this can occur ONLY if bending appears in a turning shaft.

As for the picture if it is in JPG (check since not always it is the codification) it is no problem to present it.

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