A high pressure heat exchanger may have a gas at 100 to 400 bar inside the tubes and cooling water at 20 bar outside the tubes. If a tube ruptures the high pressure gas is released into the water and the pressure quickly rises within the shell. The shells of these heat exchangers may only be capable of withstanding relatively low pressures and a level of safety is achieved by providing bursting disks and pressure relief systems. However, such pressure relief systems do not act instantaneously. The problem is to determine if the rapid pressure rise that follows a tube rupture can rupture a shell before the pressure relief system can act to vent the pressure.