A blind flange was being removed to replace corroded bolts. The valve was connected to a system which had been previously depressurised.
There was a sudden, uncontrolled release of pressure when the last retaining bolt was removed. The technician sustained injuries.
Depressured systems can contain pockets of pressure. This should be covered during the Toolbox Talk to ensure all the team are aware. Control of Work processes and breaking of containment procedures must be followed.