We are facing problem of repeated shimpack coupling failure on recycle gas compressor train.
The equipment was originally commissioned in 2009. In 2011 there was damage to LP compressor rotor (a piece at discharge of 1st stage 1st impeller broke) while in operation and rotor had to be replaced. In 2014 compressor was overhauled again on preventive basis and at that time subject coupling (LP to HP) was found damaged “two of the flexible-disc screws being found sheared off and the disc (shim pack) had a fracture”. In 2014 rotor was replaced with repaired rotor (same that got damaged in operation in 2011) due to fouling on 1st and 2nd stage impeller
May 2017 Failure
In April 2017 compressor train was shut down due to some other problem and On May 4th 2017, compressor was started. Following sequence of events observed during startup
Within 8 to 10 seconds of reaching the full speed on 13,300 rpm, the LP compressor tripped on High Vibration .
Subsequently, the compressor train was started four times and operated each time for about 40 to 70 minutes, during 4 and 5 May 2017.
On 5 May the it was observed that the System1 was not receiving any vibration data for HP Compressor after the first start up and trip. It was also revealed that HPC was not developing pressure during the last four startups.
Upon investigation, it was observed that the HP Compressor was not rotating during the last four startups since key-phasor was not showing any speed indication and the bearings were not indicating any vibration amplitudes.
Further it was noticed that during the first start up, HPC compressor reached only about 12,600 rpm and then coasted down to zero speed, meanwhile, LPC reached the rated speed of 13,300 rpm and with 8 to 10 seconds tripped on high vibration.
Based on this coupling between LP and HP Compressor was inspected and found badly damaged. Coupling transmission unit was replaced and compressor was started. Alignment readings could not be taken
Oct 2017 Failure
In June 2017, nearly one month after coupling replacement vibration values increased on LP compressor. In October last week compressor train was shut down again due to some other problem and subject coupling was inspected. Coupling was again found damaged. 5 out of 6 bolts on LP compressor side shim pack found broken. Also LP compressor shims were found damaged.As found alignment was checked and found as follows
Parallel: HP compressor 0.1mm down
Angular: 0.1mm/100mm close at bottom
Parallel: 0.2 mm
Coupling Manufacturer Feed Back
The couplings damaged in May 2017 and Oct 2017 were sent to coupling manufacturer and as per their feed back torsional vibrations cause loss of pre tension in drive bolt which eventually results in coupling damage. In Oct 2017 damage the HP compressor side drive bolts and shim pack were in tact and as per finding from coupling manufacturer tightening torque on these bolts was found to be 4 NM against design/set value of 10 NM.
Vibration analysis report prepared by our Condition Monitoring section also indicates presence of low low amplitude spikes at gear box at frequency matching with 1st torsional critical
what can be possible source/cause of torsional vibrations ?
Please note that the stripper bolts no 4 in coupling drawing which attach transmission unit to hubs are also torqued to 10 NM. Why these bolts are not loosened due to torsional vibrations ?