(120b) Case Study of High Pressure Steam Header Failure Rcfa

This paper presents a case study on two separate failure incidents at an ethylene cracking facility in which steam leaks were discovered at the same piping "tee" connection. The failed tee is located where Very High Pressure Steam (XS) is let down to a High Pressure Steam (HS) header. Upon investigation it was determined that the leaks were due to cracks in the circumferential welds of the tee to the HS header and letdown piping. To avoid further failures, or a significant process safety event, each steam leak incident resulted in a plant wide shut down, and repairs of the steam line weld cracks were carried out.

Qatar Chemical Company Ltd. (Q-Chem) and KnightHawk Engineering, Inc. (KHE) performed a root cause failure analysis (RCFA) on the HS tee, which included metallurgical analysis, process assessment, field measurements, and numerical analyses (finite element analysis and computational fluid dynamics). This RCFA concluded that the tee failures were caused by off-design cool temperatures in the XS-HS letdown piping, which allowed for condensate to develop in the letdown line. High thermal stresses developed at the tee when this condensate met with a flow of superheated steam that is present in the HS header during normal operations. Several short term remediation measures were taken to effectively address the root cause of the failures. A long term solution plan was also developed. Finally, a plant wide survey was conducted on other letdown stations to check if similar conditions existed.