Piping failure in a superheated steam service – A case study

Paper deals with the failure analysis of a PSV (pressure safety relief valve) piping, attached to a superheated-steam header, at a petrochemical plant in the Middle East. In this case, the pipe reducer and associated-weld suffered from the severe leakage, within a short period of installation (4 years) in the service against a design-life of 20 years. As a consequence, pre-matured replacement of the piping assembly was carried out by the operating company. In order to explore the root-cause of failure, damaged components were subjected to the detailed investigations, using advanced- analytical and characterization tools and techniques (e.g., metallurgical, microscopic, spectroscopic, finite element, etc.). As a result, the rupture of reducer was diagnosed to be a consequence of “thermal shock” as well as “thermal-expansion difference between the two materials” (of joined components), dominantly. Some probable contribution of the thermal fatigue is also attributed to result in the current failure. These anomalies could have been caused by (i) the use of an unspecified material of construction and; (ii) incompatibility of PSV piping material with that of the service pipe; and (iii) poor welding/post-weld heat treatment. To minimize or eliminate the re-occurrence of such failures, the use of appropriate material (as per design specification) is recommended, on mandatory basis. Further, it is advisable that all components of plant-piping should be identified and verified (by testing and inspection) for their material and integrity, before putting them into real service. Welding activities should be carried out with strict compliance to the weld specification, including post-weld heat treatment and subsequent NDT inspections. Moreover, upgrade of existing material of construction of PSV piping to a superior one (having better thermo-mechanical properties), is recommended