Vibration Case Study - Screw Compressor Problems

Case 1: Thrust Bearing Problems

Application: Refrigeration/ NH3 High Stage compressor Compressor Model: Mycom model 200LUD-MX

Thrust bearing problems are the most common problem found in screw compressors. In this case study the compressor is presumed to have run for more than 70,000 hors(Hour meter broken) . The thrust bearing used was a double row angular contact spherical ball thrust bearings located on each rotor. Damage occurred on the inside bearing of the male rotor. This was a typical failure pattern for compressors with long operating hours.

Vibration analysis determined the problem existed 18 months before the major overhaul was required. There was no back up compressor to run through a heavy production time.

The outer race, on the left, shows spallings and abrasion were seen on the raceway.

The inner race and balls shows the same damage as the outer race which was caused by fatigue.

This shows the velocity vibration spectrum before the compressor was pulled out for overhaul. It shows a typical pattern of the bearing problems. Vibration peaks on the spectrum are defect frequencies of the outer race, inner race, and ball spin.

This, enveloped acceleration spectrum, was taken at the same location on the compressor as the velocity data. Note, it shows increased noise floor due to the abrasion on the raceways.

This time waveform data was taken at the same location. It shows a 20G-peak which is relatively low considering the damage to the bearings. The rough edges were already rounding off due to abrasion.

Note the random waveform pattern at smaller peaks.

Case 2: Oil injection problems

Application :NH3 Refrigeration/Booster compressor

Most flooded type screw compressors use cooled oil injected into the rotor cavities. This process is called "oil injection" and its purpose is to cool the compressed gas and to seal the space between the rotors and the casing. Too much or too little oil injection will cause inefficient compressor operation.

Because of an oil injection problem this NH3 booster compressor emitted a loud screaming noise for a long period of time. A factory service tech told the customer that the compressor was assumed to be OK because the vibration level(velocity) was low.

Is that so? Let's see the vibration data.

This velocity spectrum was taken at bearing head and showed several harmonics with overall vibration at 0.13 ips.

This acceleration spectrum was also taken at the same bearing head. It shows an abnormal vibration pattern.

This time waveform was taken at the same bearing head. Note that it shows very high peaks (+/- 30G) at regular interval, which is equal to rotor meshing frequency.

After performing the vibration analysis and doing some field experiments, it was determined that too much oil was being injected into the compressor. A factory service technician was called in to properly adjust the oil flow for injection.

Another vibration analysis was done for comparison.

This velocity spectrum shows the O/A now at 0.06 ips.

This acceleration trend data now shows a reduction in the O/A after the adjustment was done.

This time waveform was taken after the adjustment was done. After the oil flow adjustment, the vibration level is back to normal.; however, a low rumbling noise problem still existed as shown on this time waveform.