Sinosidal Vibration On Compressor

We are facing problem in ammonia compressor having mechanical seal installed at both ends to seal ammonia. The lube oil is provided for lubrication of bearings and mechanical seal faces. During changeover of lube oil pumps the pressure changes from 31 kg/cm2 to 23kg/cm2 and then back to 33kg/cm2. After this fluctuation the compressor is running with sinusoidal vibrations on all 04 vibration probes of machine. The sinusoidal vibrations are +x and -x from the base values i.e. the increase and decrease of vibration is same from base value of vibration. The phase angle fluctuates in a same way as that of vibrations on all 04 vibration probes of compressor.

Note that the cyclic variations are of following type

VT-401A/B: compressor outboard 25 ~ 32 micron within 3 to 5 sec

VT-402A/B: compressor inboard 14 ~ 23 micron within 3 to 5 sec

The ADRE data signatures show

· The main component is 1x.

· No sub synchronous component present

· The polar plot is perfect circle equal to change in phase angle.

· Orbit is same as before the problem occur.

Following actions were taken to subside this change.

· Oil supply temperature changed from 45deg C to 50 deg C in steps but no change

· 230 RPM reduced and increased but no change (11200 to 10970 rpm and back to 11100 rpm)

· Seal oil flow increased from 40 GPM to 42 GPM but no change

· The vibration probes health being ensured

· The coaxial cable health being ensured

· All this problem arises during changeover of lube oil pump, so the lube oil pump changed over again but vibrations do not subside

The suggestions are required to subside the problem

P.S. No change in bearing temperatures, seal oil pressure, differential pressure, reference gas pressure, sour oil flows or compressor axial vibration was noted.