Shop Compressor Shaft and Casing Fatigue Study and Modification

A new design of motor-driven shop compressor in France was needed to run reliably and quietly. MSI analyzed the casing and shaft/rotor assembly (both laterally and torsionally) for fatigue under expected worst-case variable loading. A fatigue problem in the casing discharge end corner fillet was identified under circumstances of rapid load cycling. The manufacturer at first did not agree that the analysis was realistic, but agreed to perform a shop test under cyclic conditions which were worst-case, but possible in many services. After accelerated cycling that represented several months of operation, a fatigue crack occurred, and a through-crack punctured the compressor casing, causing an explosion hazard. The design was revised per the original recommendations, and performed without structural hazards.