isolating the pressure control valve from the heat exchanger. The operator was aware that the shell-side was full of pressurised ammonia but was unaware that the isolation valve remained closed form the operations from the day before. The operator assumed that any over-pressuring that might occur would be able to vent through the valve to the pressure relief valve (USCSHIB, 2011U.S. Chemical Safety and Hazard Investigation Board, 2011).