Misalignment of piston rod and packer rings cause another leak path. Rod-to-ring surface contact area is reduced. In addition, edges of rings at the bore become worn.
This permits gas flow from one radial cut in the ring to another. In API 618, the maximum allowable rod runout at operating temperature is 0.00015 inch per inch of stroke. Alignment of cylinder, distance piece and crosshead guides should be adjusted in order to meet this guideline. Runout should always be checked following installation of piston rods, crossheads, wrist pins, etc. Noncontacting probes can be used very effectively to measure and monitor rod runout.