Failure analysis of packing plate in a hyper compressor at a petrochemical plant

Cracks in one of the packing plates of a two stage multi-cylinder 250 MPa polymer hyper compressor at a petrochemical plant provoked a gas leak and a plant stop. Fractographic and micrographic analyses and mechanical modelling revealed that fatigue cracks developed in both inner and outer discs of the fretted packing plate. First cracks initiated in the inner disc from pre-existing micro-cracks in the wall of a 3 mm sealing oil channel, fatigue crack growth time was 70 days. Several small cracks were found on the surface of the oil channel, intergranular with few ramifications. These micro-cracks initiated by low cycle fatigue, after 1000 stop and run cycles. Microcracking in other packing plates in this and other similar compressors was found to be likely. The influence of excessive wear and possible non-destructive assessment techniques or replacement strategies is discussed.