Fifteen months before the incident occurred it had been noticed that the flare line isolation valve V17 was passing. It was decided however to wait for a scheduled shutdown of the catalytic cracker unit and No 1 flare before commencing work on the valve. Gases from the remaining operating units were re-routed to No 2 and No 3 flares. This flare arrangement would allow the pipelines at V17 to be isolated.

When senior refinery staff prepared a plan for the isolation of the flare system, they concentrated on the operational and safety requirements of the flare system, making sure that no operational areas of the plant were inadvertently isolated. The details of the removal of V17 were not considered and left to those who would be responsible for the work.

Four workers were involved with the removal of the valve. When the majority of the bolts were undone the joint opened slightly and liquid dripped from a small gap between the flanges. The workers sought advice. The valve was checked and it was concluded that it was safe to carry on. Non ferrous hammers were provided before continuing with the removal. All the bolts were removed and the crane took the weight of a spacer and started to remove it, at which point gallons of liquid poured from the valve. A flammable vapour cloud formed from the rapidly spreading pool. The cloud reached the nearby air compressor, ignited and flashed back around the working area.

Two workers managed to escape the fire but a fitter and a rigger were engulfed by the flames and killed. The fire was allowed to burn in a controlled manner for almost two days while the rest of the refinery was shut down and the flare system purged with nitrogen

KEY ISSUES:
• DESIGN CODES – PIPEWORK
• MAINTENANCE PROCEDURES
• ISOLATION

Report: https://www.icheme.org/media/13700/the-fires-and-explosion-at-bp-oil-grangemouth-refinery-ltd.pdf

Image Credit: HSE