Dump tanks are used as an initial treatment to separate the liquid and gas phases of the discharge from the emergency relief of a runaway chemical reactor. Methods are developed for sizing such vessels for vapour pressure, 'gassy' and hybrid systems, requiring iterative solution. A simplified methodology is given, eliminating the need for iteration at the expense of a small error. Analysis allows evaluation of the maximum error and accurate prediction of the actual error involved in each specific case. Examples of sizing methods are given for vapour pressure and 'gassy' systems.