Sometimes a product test that invalidates a suspected cause of failure can be just as valuable as a test that confirms a failure mode. Such was the case for a hydrogen reciprocating compressor test intended to discover a failure called “wiped” wrist-pin bearings, a condition where the bearing surface shows excessive, premature wear from the wiping action of the journal. The new compressor appeared to behave normally during short, startup test runs and gave no obvious outward sign that the bearings were in the process of being damaged. The maintenance personnel found the wiped wrist-pin condition when the machine was disassembled after the test runs. If the condition was not discovered at that time, the machine could have suffered more severe damage such as a broken connecting rod.

Watching a Prime Suspect
Engineers decided to instrument the compressor to collect data that would help them identify the root cause of the wrist-pin bearing failures. They used strain gages and pressure sensors. The compressor manufacturer was looking for crankcase twisting or other possible deflection problems. Concurrently, Ken Singleton, manager of KCS Consulting, Bristol, Va., was hired to measure compressor vibration and analyze operating deflection. He connected an IOtech ZonicBook/618E and three expansion modules with 32 channels to measure data from accelerometers located around the compressor and a once-per-revolution phase reference signal from the crankshaft.

The number of data channels eventually totaled about 100, which included the OEM’s and plant site analyst’s sensors. Singleton used IOtech’s eZ-Analyst for data acquisition and ME’scopeVES software to calculate the operating deflection shapes (ODS). The software includes a drawing module that lets him develop 3D models in which the data, either modal or ODS, can be mapped and animated. First, data were collected during no-load and then full-load conditions. After the compressor was thermally stable, data were measured to develop the ODS model. This procedure let Singleton inspect the vibration modes of the crankcase and cylinders at different frequencies between 0.50 Hz and 1000 Hz with about 0.25 Hz resolution. Other forms of data were also calculated which included coherence and phase.