In general, rupture discs are designed to give a virtually instantaneous pressure relief opening to a relief system at a predetermined pressure. For safe design of pressure relief systems, consideration must be given to the sizing and the specification of the rupture disc but also to the reaction forces that result when a gas is discharged. These reaction forces can be excessive in the case of venting at high pressures.

When a disc ruptures, a shock wave propagates through the gas in the piping behind the rupture disc. This exerts a transient reaction force on the pressure relief system. In the case of steady supersonic flow choking often occurs at one or more points in the pressure relief line and a shock is formed in the pipe. The pressure in the relief line may increase considerably during blowdown as a result of choking at the end of the pipe. Depress-surization experiments were conducted with nitrogen at high pressures.

The results of the pressure measurements in the piping during blowdown are compared with exploratory calculations. The reaction forces exerted on a pressure relief system can be assessed from the pressures and the mass flow rate of the gas as a function of time. For safe design of pressure relief systems, both steady-state flow reaction forces and the initial transient forces should be considered.