WE have a 3 stage, 4 cylinder horizontal opposing type arrangement recip compressor fitted with rod drop monitoring system. Non lubricated cylinders with PTFE piston & rider rings.
Rod drop monitoring system has two 11mm prox probes mounted 180deg apart (top & bottm) for redundancy near to gas packing rings flange in distance piece.
This is employed in compressing a hydrocarbon gas( not very dry) with condensation moisture extracted interstages automatically. The cylinders are water jacketed.
The last maintenance was carried out in month of Aug 2009 when all stage piston,rider rings, gas packings, scraper rings and valves were replaced by new ones. Also 2ns stage cyl liner was replaced since we observed few scoring marks due to complete wear out of rider rings.
AFTER STARTUP ( with rod drop monitoring system reset )the 2nd stage rod drop trend has fallen to alarm levels steadily over a period of 9 months by 900 micorns. ( alarm: 880 microns and danger:1100 microns)we recommended to plan for an inspection and replacement of rider/piston rings if calls for.
But suddenly after reaching alarm the rod drop values trend is fluctuating like a sinusoidal waveform (from +100 microns rise to -1000 microns drop)for both probes. The instrumentation is verified for gap voltage measurement and it tallies with amount of change Vs scale factor(100 mv/mil).
This has totally confused me!!! what could cause this fluctuation.( I guess one possibility of rings rotating inside !!! but last time when it was opened we found rider rings wear out only at bottm side). These rings are of scarf joint type single piece, Piston dia 500 mm.
Anyone requiring more data or clarification i would be happy to provide. Kindly revert with your inputs.