Many gas-pbase reactions result in a pressure buildup that is very fast, but slower than that for an explosion (that is, a deflagration). Sizing a relief vent by the conventional method, which assumes a steady state discharge mass flow rate that is sufficient to remove energy from the vessel at a rate equal to the maximum rate at which it is input into or released within in the vessel, can result in an unacceptably large vent size. A metbod previously proposed by DuPont, which is based on an experimental rate constant for the deflagration reaction (the "violence coefficient") and an empirical expression for the vent mass flux gives the vent size that limits the maximum transient pressure to less than the maximum allowable vessel pressure results in more realistic vent sizes. This paper compares the results of these two methods, as well as a modified metbod using a more general relation for the valve flow capacity, with the data for an ethylene decomposition reaction reported by DuPont. The modified metbod can be used for any gaseous relief with a known reaction rate constant and gas density as a function of pressure. 9 2006 American Institute of Chemical Engineers Process.