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We need urgent help in form of vibration analysis and diagnostics of a very production critical multistage centrifugal compressor, driven by a steam turbine with speed increasing gearbox in between. The gas handled by the compressor is carbon dioxide which is first compressed in low pressure casing and then in this high pressure casing.  The layout of the complete compressor train is attached as Annexure-I.

The overhauling of this compressor casing was performed to address the issue of high acid number of lubrication oil due to expected ingress of gas as result of high end seal losses. In this overhauling, rotor was reused along with existing stage separation seal (Material; Stainless Steel). Inter-stage (Material; Aluminum), end seal (Material; Aluminum) and bearings were replaced with new ones. Bearing clearance was set 0.11 mm (90 x 31 mm: LOP). The operating speed of the compressor is 13200-13800 with 1st critical speed as 7700-9800 rpm. Rotor total leghth:1401 mm and bearing span is 1027 mm The cross-sectional drawing of the compressor is attached as Annexure-I.

On 18th Jan, 2022 during start-up, the compressor tripped twice on high vibration of NDE radial bearing VT-205B and it touched 108 micron. Vibration on all other probes also increased but remained less than alarm (75 micron) and danger limit (100 micron).  After internal discussion, it was decided to go a 2nd attempt by lowering the trip limit to 80 micron and reducing time delay from 01 second to 03 second. The compressor tripped in the same fashion and at this point, it was decided to perform detailed internal inspection Vibration signatures of both these trippings are available in form of M/s. Baker Hughes System-1 are attached as Annexure-II.

On dismantling the compressor bundle, the following two salient observations were noted;

  • Rub between rotor (nearly 30% circumference) and stator (Inter-Stage and stage separation seal). All inter-stage seals are of aluminum and stage separation seal is of stainless steel with integral teeth and inter-locking configuration. The same is shown in pictures.
  • Liquid accumulation in sealing gas piping, vent piping as well as in compressor internals.
  • The observations of both overhauling’s are attached as Annexure-III.

Our findings for System-1 vibration signatures is;

  1. Referring Bode plot of 1st tripping, there was significance difference between ramp-up and coast down in form of direct vibration, 1X and phase angle. At the same speed of the shaft, this difference is more than 80 micron in VT-205B and it kept decreasing during the coast down.
  2. Referring the same bode plot of 1st tripping, there was a speed variation of more than 1000 rpm in less than 10 seconds and it repeated in nearly 2.5 cycles before tripping.
  3. During 2nd tripping, the ramp-up vibration again matched the ramp-up of 1st tripping by showing that rotor was exposed to temporary unbalance during the 1st The rotor responded with significant phase change (more than 90°) at VT-205B during the critical speed range of 7700 - 9800 rpm whereas no such behaviors exist in 1st trippings. After a few minutes, the vibration of compressor again increased with phase change but reverted back and finally it tripped on high vibration of VT-205B of 80 micron.
  4. In both these trippings, we were in loading phase of the compressor where recycle valve was being closed and compressor was being pressurized to meet the system requirement.

Your expert opinion is requested for the following queries;

  1. What are possible cause of both these trippings events?
  2. Is the liquid accumulation in a centrifugal compressor can cause temporary unbalance and sudden speed variation?
  • If thermal bow generated by rub is the cause of high vibration, what is the plausible explanation for source of rub?
  1. Is any symptoms of mechanical fit-up issue are present in the vibration signatures?

Attachments:

  1. Machine Layout and Drawing as Annexure-I
  2. Sysytem-1 Vibration signatures as Annexure-II
  3. Observations of 1st and 02nd Overhauling’s as Annexure-III.

Regards,

Attachments

Tags: Centrifugal Compressor, vibration, Rub, Bearing

Replies sorted oldest to newest

- Check the main block/cover surface flatness (possible overheating in the past?)

- Measure the rotor eccentricity, especially on the position of the problematic seal (rotor could get deformed before/during overhaul)

- Recheck proper seal installation, block-seal surface maching

RM

Dear ,

I have a similar case with BB5 pump that suffer from resonance at the operating speed due to rubbing issue .

It's obvious from orbit plot that you have a rubbing issue in the compressor

Also the bode plot confirm the shifting of the critical speed of the system as the rubbing increase the stiffness of the system .

Good luck

RM

The question is what caused is the rubbing as being an effect. We are searching for cause which resulted temporary unbalance as showed  in bode plot of 1st and 2nd run. Is the luquid carryover in compressor can cause such effect or not?

Secondly also there is speed variation of almost 1000 rpm just before 1st tripping. What are the possible causes as prime mover governor of steam turbine was heathy in all possible aspects and no such behaviour found in 2nd attempt.

RM

Were there any obvious material removal from the the corroded impellers ?  Were there any machining activities performed on corroded impellers ?

If not , I suggest that there is no permanent unbalance and bow in the rotor ,  However the  critical speed shifted to the 1x rpm and amplified the unbalance vector 1x rpm and changed the phase by 90 degree only ( still in critical band )  because of existing rubbing issue (higher stiffness effect ).

About speed variation , you should review the operating parameters and DCS signal such as liquid accumulation, pressure ,flow rate, Temperature , BOV , IGV and possible surge  as you may have a high loading issue and reversed torque on compressor that could reflected on speed .

I recommend to perform onsite balancing on the coupling as a temporary action to reduce the 1x component until you figure out the source of rubbing .



Note , I attached a similar case with similar bode plot , These pump suffered from resonance issue and onsite balance reduced the vibration

Attachments

Images (1)
  • P-1502-A Bode Plot: Resonance due to rubbing
RM

Dear SAB262015,

Liquid carry-over can cause un-balance of rotor due to presence of incompressible fluid in Gas. Also the passages of impellers discharge blocked the gas flow. Being very high speed rotor (13,800 rpm), impact is very severe. This unbalance at high speed will create very high radial vibrations which in turn will cause rubbing of seals with Rotor. Change of phase and shift of critical speed is clear evidence of this phenomenon due to stiffness change / rubbing. Any un-balance already present in the rotor, above API limits, can exaggerate this behavior. I believe you have cleaned the rotor (impeller & diaphragms) before re-installation due to fouling / deposition of material? 

One question. Please inform the reason of using SS inter-stage labyrinth instead of Aluminium labyrinths? Is this original design or upgraded by you due to corrosion issues by Carbonic Acid (Water + CO2)?

Regards, 

RM
@Registered Member posted:

Dear SAB262015,

Liquid carry-over can cause un-balance of rotor due to presence of incompressible fluid in Gas. Also the passages of impellers discharge blocked the gas flow. Being very high speed rotor (13,800 rpm), impact is very severe. This unbalance at high speed will create very high radial vibrations which in turn will cause rubbing of seals with Rotor. Change of phase and shift of critical speed is clear evidence of this phenomenon due to stiffness change / rubbing. Any un-balance already present in the rotor, above API limits, can exaggerate this behavior. I believe you have cleaned the rotor (impeller & diaphragms) before re-installation due to fouling / deposition of material?

One question. Please inform the reason of using SS inter-stage labyrinth instead of Aluminium labyrinths? Is this original design or upgraded by you due to corrosion issues by Carbonic Acid (Water + CO2)?

Regards,

The OP has not visited these forums since

RM

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