Possible Axial Vibrations in an Integrally Geared Compressor

[msr57965]

Dear All

We are facing problem of repeated Dry Gas Seal failure on an Integrally Geared Centrifugal Compressor. The Compressor consists of Bull Gear Driving 03 Pinions with each Pinion carrying 02 Impellers i.e. 01 Impeller at each end.

Each Pinion is axially located by 02 Thrust Collars that are running against Bull Gear Face. Bull Gear is Axially located by Thrust Bearing. Axial Probes are installed only on Bull Gear Shaft.

The Dry Gas Seal manufacturer has attributed the failure to Axial Vibrations. I am trying to understand the source of Axial Vibrations.

Is it possible that Axial Runout on Pinion Thrust Collar Face is causing To & For Axial Motion of Pinion in each revolution e.g. if Axial Runout on Pinion Thrust Collar Face is 0.012 mm TIR (API limit) and Pinion RPM are 21945, resulting axial movement will be 0.012 mm/revolution which implies Axial Vibration at rate of 4.389 mm/sec

Is it possible similar effect will be created by Face Runout of Bull Gear resulting either from machining tolerance or Bull Gear Shaft position within its Bearings (Say extreme right on Non Drive End Bearing and extreme left in Drive End Bearing) e.g. for 0.25 mm Bearing Clearance, Gear Dia to Bearing Span ratio of 1.5 and Gear RPM 1490 resulting movement will be 9.3 mm/sec

Please refer to the attached pdf for schematic of what I am thinking of

 

[Strong]

Unless you have some data (vibrations or shaft runout) you are just guessing or asked for others to guess. The technical equivalent for guess is Hypothesis.

Do the seal issue occur on one or all stages (pinions)?
Do you have radial proximity vibration probes on each pinion shaft?
Do you have axial/thrust probes on any shaft?
Do you have any historical vibration data from shaft probes or casing measurements?

A basic troubleshooting approach:
If there is an inspection port on bull gear, then do visual inspection of thrust faces and gear teeth.
If there is an inspection port on bull gear, then measure axial and radial (if possible) runout.
Measure high frequency tooth mesh vibrations in radial and axial directions at several locations on casing. Look for modulation sidebands corresponding to the speed of each shaft (3-pinions and bull gear)to possible verify Case-1 or Case-2 on your attached sketches.

Walt

 

[msr57965]

Hi Walt
Thanks for the reply. Below us answer to some of your questions

[ul]
[li]Seal Issue is much more frequent only on one stage. In fact the Seal on opposite end of same pinion is performing much better. We are already looking on the other possibilities but there has been much emphasis on possible Axial Vibrations so I was trying to understand what can be the potential Sources[/li]
[li]Radial Vibration Probes are installed on each pinion and values are well within alarm level. In fact on the pinion where seal is failing the shaft vibration alarm is at 45 microns and values are within alarm (Though I will reconfirm latest values)[/li]
[li]Axial Probes are installed only on Bull Gear Shaft[/li]
[li]Visual Inspection of Gear Teeth and Pinion Thrust Collar has revealed no abnormality.[/li]
[li]Radial Run outs have been checked at Pinion Shaft and found well within limits. Bull Gear Face Axial Runout couldn't be checked due to difficulties in installing 02 Dials at 180 degrees[/li]
[li]I will try to obtain and share the historic trends Shaft Vibration Probe Readings and Casing Accelerometer Readings[/li]
[li][/li]
[/ul]

 

[msr57965]

Dear Walt
In the attached zip folder have put screenshots of historic trends for casing vibration from Accelerometer mounted on Gear Casing as well as Historic Trend of Shaft Vibrations of Pinion where we are facing Seal Failure Problem. Trends form last Startup are also there

 

[kryz0]

Hi msr57965,

Are you still having issues? I repair shipboard gas compressors for a living; maybe I can be of some assistance. Please let me know if this is indeed shipboard.

It's certainly possible for the axial runout to be the cause of this issue. Often times this is because the bearings are starting to go bad. Are they tilt-pad bearings? If so, you can do "lift-checks" and check the indicated reading matches what's in the manual. This will tell you if the clearance is good or bad. If the clearance is bad, it is often caused by lack of oil getting to the bearings. If the dry-gas seal keeps being destroyed, it's likely the bearing clearances are too high, causing your seal to act as the bearing instead. The ID of the seal should be slightly greater than the ID of the bearings.

The next thing to check is the gear tooth contact of each pinion using prussian blue. You should get consistent contact across all pinions, on all teeth. This will tell you if any pinion is cocked. Runout with indicators would not tell you this.

The next thing to check is the alignment of the gearbox to motor, which will require removal of the bulkhead seal. I would caution you to either hire the OEM to do this or another gas compressor specialist. This is a tricky process.

Do you mind sharing who the OEM/model of this gearbox? Perhaps I may have some info on it.

Hope I could be of some insight