STRESS CORROSION AND SUBSEQUENT FATIGUE CRACKING IN A HIGH PRESSURE POLYETHYLENE REACTOR TUBE

摘要

At the end of May 1992 a serious explosion and fire occurred in the reactor bay of a high pressure, low density polyethylene (LDPE) reactor in Tarragona, Spain. This paper details an investigation which has been carried out to determine the cause of the leakage in a high pressure reactor tube which necessitated the shut down of the reactor, indirectly culminating in an ethylene gas release from another point in the reactor which caused the fire and explosion. On the basis of the investigation it was concluded that the high pressure reactor tube was caused to leak through the initiation and growth of a through-wall fatigue crack lying in the axial-radial plane of the tube. Most unusually, the fatigue crack was initiated at the mid-wall of the high pressure tube through the presence of a part through pre-existing transverse stress corrosion crack. This stress corrosion crack was confined to the circumferential-radial plane and had been initiated at the outside surface of the tube in contact with the cooling water. Cracking of the high pressure tube was directly attributable to the use of a steel with too high a tensile strength to resist aqueous stress corrosion cracking in other than perfect cooling water conditions. The reactor was satisfactorily reinstated after all suspect straight and bend tubes had been subjected to crack detection at the outside surface and a heat treatment cycle sufficient to stress relieve the tubes and reduce the tensile strength of the steel to a level more appropriate for good stress corrosion resistance