Root Cause and FFS Analysis of a Dent in a 22mm Thick Elliptical Head

This paper describes the failure of a jacketed vessel. The product pressure is 16 barg (inner vessel; D = 2200 mm) and is heated with thermal oil in the jacket (operated at 4barg). The jacket is split up in different zones which can be opened and closed separately. After a shut down, several valves were not opened properly. This resulted in blocking-in of the jacket on the top head of the vessel during the start-up and operation of the vessel. The vessel was heated to the operating temperature (±250°C), causing a pressure increase of the blocked-in thermal oil. The jacket has a wall thickness of 10 mm, and the vessel head has a wall thickness of 22 mm. Because of the pressure increase, failure occurred at a nozzle weld on the inner pressure vessel shell. This resulted in a leakage of thermal oil into the vessel. The jacket itself deformed but did not fail. Based on a detailed FE analysis, it has been concluded that failure occurred as a result of (local) buckling of the 22mm thick elliptical head (diameter of ±2.200mm). This paper describes the failure that occurred and the assessments performed to determine and validate the root cause of the failure. A level 3 assessment according to ASME VIII div 2 Part 5 (1) was used to determine if the vessel is still safe for operation.