For a major food processing company, we performed a failure analysis of a horizontally mounted low pressure process vessel. The sequential failure of several of the nuts holding the closure head caused it to blow off, seriously injuring a worker.
A review of the original design showed that the vessel had not been built with the number of nuts and swing bolts shown on the drawings or on the Manufacturer’s Data Report. Metallurgical analysis of the nuts which had failed showed that they were fabricated of weaker material than originally specified. Information provided by the operating personnel indicated that several of the nuts were generally not used because of inconvenient access.
A finite element analysis of the closure head accounted for the mal-distributed nuts. These errors taken together exactly used up the safety-factor of five required for pressure vessel bolting design.