Abstract

The low density polyethylene (LDPE) tubular reactor tube is fabricated from a material with extremely high fatigue strength. During severe runaway decomposition reaction, the decomposing gas rapidly heats up the reactor tube bore surface to a temperature exceeding the austenitizing temperature of the steel (AISI 4333). Subsequent quenching by the incoming colder gas from the opening of the hydraulic safety blow-off valve hardens the bore surface. The fatigue life of the reactor tube can be negatively affected by the presence of the hardened layer. A maximum limit of 2 mm for the depth of hardening is specified in the operating manual for safe operation. A hardness test is conducted on the internal surface cross section of the lens ring (gasket connecting the two high pressure tubes) to establish whether the hardening has exceeded the 2 mm depth as a result of decomposition. Due to differences in chemical composition between the reactor tube and lens ring, a correlation between the depth of hardening of the lens ring and reactor tube is investigated. The remaining life of a reactor tube with a 3.4 mm depth of hardening was predicted through experimental fatigue cyclic test approach. Results of the prediction suggested that the depth of hardening of the tube was close representative of the hardened lens ring, while the experimental cyclic fatigue test suggested that the 3.4 mm hardening depth of tube could endure a significant numbers of pressure cycles before the tube fails (i.e. a through-wall crack that leaks).

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