Compressor High Vibration 1

[GAL02]

Dear all, Good morning!
Regarding the subject matter we r facing sever problem of Compressor casing vibration while start up.
The short coverage is as follows. :
We have a 10 Compressor's (OEM :GE) the one which is frequently gives serious vibration (0.8 to 3.9 mm/s)is of Natural Gas compressor having 4 casings and final Recycle stage.
When compressor is started after every O/H then until it reaches minimum governing speed of 9200 to 9500 rpm keeping open recirculation valve (4th Stg.)and balancing valve (4th & Recycle stg.)and Recycle Stg. (final Stg.)shut off (I/L & O/L) valve closed it gives enormous vibration that operator can't stay side by of compressor, but when at rpm goes above 9500 to 13000 with all Shutoff valves were open (Suction & Discharge) the M/c functions normal (vibration: 0.8 to 1.2 mm/s)
We have had an RCA as well as specialist work over it but couldn't conclude what's the root cause is. Our presumption is that bundle inside the casing is not in center.
Can anybody put light on it.

With regards

 

[rotw]

"Natural Gas compressor having 4 casings and final Recycle stage" you meant the "train" that is giving troubles ?
What is 4 casings and final recycle stage ? does this mean 5 casings in total that is a very long train line!!

"We have had an RCA as well as specialist work over it but couldn't conclude what's the root cause is. Our presumption is that bundle inside the casing is not in center."
Keep in mind that bundle is rotor + stator (diaphragms) just to clarify the terminology.

The problem is poorly described, I think.
Casing vibration is different than rotor vibration. Normally centrifugal compressor (beam type) casing is not equipped with accelerometer probes as it is considered stiff construction in contrast to a gearbox or an integrally geared machined. Especially a 13000 rpm operation suggest you have low flow coefficient impellers usually this is combined with high pressure operation (high design pressure frame). If the vibration have transferred to the frame I would really worry what the hell is going on and how come the vibration probes (proximitors) did not cause trip already.

You may want to check if the 1st critical speed was originally close to the minimum operating speed, for this you may check the lateral analysis report of GE and see what are the amplification factors and separation margins to the 1st CS.

I suspect sort of surge or rotating stall phenomenon as you start up close to the surge line with a recycle valve with insufficient Cv. This is only speculation.

 

[GAL02]

Dear Mr. rotw, Good afternoon !
Here I want to clear you that Stator + rotor = bundle and bundle in Casing & coupled with other stages.
As I said there are 4 STAGES and rotor final Recycle. 1st & 2nd stg. in casing 1, 3rd Stg. in separate casing 2 and 4th Stg. & recycle is in 3rd casing.
This 3rd casing having high vibration & it has probes sensing high vibration & trip the compressor at particular set poit only, our point of discussion is what's that which cause high vibration which gets transfer to casing.

With regards

 

[GAL02]

Pl. check the P&ID for info of above.

With regards

 

[LittleInch]

Compressor surge?

This is not something that can be solved over the internet. That looks like a fearsome machine and it will take a long time to understand the gas flow and what it is doing.

The OEM needs to be involved as it could be you just hit some sort of shaft natural frequency at those RPM.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rotw]

LittleInch,

What do you think about surge cycles eventuality at low speed ?

I've seen a similar setup for synthesis gas compressors (fertilizers/urea plants); the 2nd (and last casing) has a long rotor with back to back configuration of which the 2nd stage has normally one very small wheel very low flow coefficient. This 2nd casing has particularly challenging rotor dynamics. All in all stages 1 to 3 are for make up gas and the 4th is recycle.

From the sketch I only see 3 casings so it differs from what I described above in that it has an extra stage (casing) between the two back to backs. That may relax the length of the last rotor or not, cannot guess.
But I guess the rotor dynamics of this last casing could be challenging. The sketch is unclear but still guessing the casing tag ends with a B (unless I am wrong) and that is for high pressure rating (200 - 250 bar) so getting such a heavy frame vibrating to the point the operator could not stand in front of it without the probes initiating a trip already seems very strange.

It is also underestimating the issue that based on such limited info given we are asked to "give an opinion"..

 

[GAL02]

Dear, Good morning!
It's not underestimation of issue but try to identify the possible source of high vibration.
As I told earlier when RPM reaches minimum governing speed 9500+ & shut off valves are opened it gives vibration to is normal range.0.8 to 1.8 mm/s.
Process Specialist, Reliability, Inspection forms a team to discuss and OEM is asked to assist on this issue.

Thanks for your feedback

With regards

 

[Strong]

It is a bit difficult to help solve a vibration problem by looking solely at a process diagram! Apparently your vibrations are good at full speed and with normal flow conditions. Apparently your vibrations are high during startup. If the vibration frequency is predominately at 1X shaft speed, then the cause may be rotor critical speeds (balance residence) that are excited by rotor static unbalance (mid span). If other vibration frequencies are present that indicate flow surge or stall, then process valve settings may be a cause. If the machine trips on high vibration during startup, then possible issues include excessive vibrations (as mentioned), incorrect vibration set-points, or incorrect time delay for vibration warning/trip. Shaft misalignment in alignment changes due to thermal growth they also contribute to high vibrations.
Walt

 

[rotw]

can OP provide some further info such us:

-Is the vibration monitoring system using trip multipliers during start up?
-What is minimum operating speed, the 1st and 2nd critical speeds, the log dec 1st forward as well as the amplification factors and separation margins for the subject rotor?
-What type of driver is installed ? guess it is a steam turbine? if so do you know if there is a preprogrammed start up/acceleration sequence for the turbine ?

Among many possible root causes, it can be that the machine has been designed with little margin to the 1st critical speed (border line case due to reduced degree of freedom for the operation vs.critical speeds considering the presence of many casings in the shaft line) and due to wear and tear, fouling, degradation of the seals and increase of internal leakages, etc. there is degradation of performance and/or unbalance - leading to change in rotor dynamic system behavior.

Where is the casing located in order in the shaft line ? is it drive thru casing or at the end of the shaft line ?

 

[GAL02]

Good morning Dear!
I will come back to you with more details as required.

With regards

 

[bkarim55]

Hi dear GAL02;
Did you find out what was leading your abnormal vibrations?

Regards
Karim

 

[Hesam267]

Hi
i heard we can say shaft is unbalance when there is 1x in range of 60-120 degree phase.
is it possible it happens in another shape with tilting pad journal bearing?

 

[georgeverghese]

The drawing you've posted doesnt show all recycle routes and recycle valves, especially around stage 4 and recycle stage. Also, you say there is a "balancing valve" between stage 4 and recycle stage - what is the function of this valve - could there be a capacity bottleneck on this valve?
You havent said anything about what the GE people have said or done to resolve this dangerously high vibration issue on this machine.

 

[rotw]

How come you are measuring vibrations with velocity sensors?
I would have expected centrifugal compressor (of between bearings design) to have higher stiffness casing frame ... so those are normally equipped with proximity probes only (monitoring relative displacement / shaft vibration).

Are you taking measurements with some sort of special probes (hand held or magnetic) or is it the case that the machine was originally equipped with seismic sensors (by design) ? as far as shaft vibration are concerned what info is the proximity sensor giving (axial and radial)?

Do you have time waveforms available for your type(s) of vibration measurement ?