An explosion occurred when a process vessel was overpressurised during a process upset because a manual valve in the pressure relief vent had been left closed. Originally, it was intended to be normally left closed as a maintenance isolation valve, but a process change required it to be operated on a regular basis. Our investigation revealed that the original design basis was correct and that there had been no proper assessment of the change of use. There was no safeguard against human error because the procedure for locking the valve and checking its position had fallen into disuse.
Two people were killed while draining a pressure vessel containing an apparently non-flammable lubrication oil. The official investigation into the cause concluded that traces of oil had decomposed on the surface of an electric immersion heater in the vessel when it became exposed, and that this produced sufficient flammable vapours, which the heater ignited. Our investigation showed that crucial evidence had been ignored. It showed that the oil was continuously decomposed by other heaters in the system, such that the oil was always saturated with flammable vapour. This gave rise to a permanent flammable atmosphere inside the pressure vessel and this hazard had never been identified.