Fretting Corrosion Analysis on Packings of Hypercompressors Available to Purchase

Secondary compressors for Low Density Poly-Ethylene are very critical machines. Safety aspects and reliability expectations determine a thorough evaluation of the service and the parameters involved. The solution used for the machine requires a long experience of positive results in these applications. Cylinder performance is strongly influenced by the design and also related to the combination of operation and maintenance activities. The capability to withstand the high fatigue stresses and the need to avoid any leakage of gas around the compressor area involves due considerations and design solutions to minimize such occurrences also during abnormal operations and emergencies. In addition to the cylinder valves, packings represent important components with great influence on the availability of machine and plant. The operating conditions of the packing, due to the high pressures involved, and the geometry of the components, create the condition for a relative movement of the cups mating surfaces. In general at disassembly, when maintenance and reconditioning activities are carried out wear, pitting, corrosion and local defects can be detected on the cups and sealing elements, as a function of the life of the cylinders. The condition of the cups may also evidence some damages, with evident deposit of polymers, oil and wear of the components. Fretting corrosion phenomena may be sometime the reason of failure of packing in hyper-compressors for LDPE production. In some cases the repeated occurrence of various abnormalities on the packing life and particular wear pattern on the cups bring to need to discover the reason of the problems and require an accurate investigation, by using advanced laboratory analyses. The results evidence that the origin of such phenomena can be found on the surfaces of the components and on the organic acids, as in such conditions they are able to originate and develop the corrosion reactions. There is a strong connection with process parameters and operating conditions. These situations and particular applications have to be evaluated to put in place adequate actions to improve or have more chances of a reliable and economic operation.