Power plant safety and relief value vent stacks have generally been sized according to the methods and design criteria published by Max W. Benjamin in the 1940s. Although the method might have been satisfactory at that time utilizing basic flow dynamics, it not only requires laborious calculations, but also contains some inadequate approaches which may contradict the theory of compressible fluid mechanics available today. This paper presents a complete analysis of steam conditions and its properties from power plant safety/relief valve inlet through its vent stack to the atmosphere. The analysis is based on the theory of compressible fluid flow under irreversible adiabatic process to establish a method and criterion for vent stack design. The method developed herein is applicable to both choking and nonchoking conditions at the valve exit and/or vent stack outlet, and greatly reduces complex trial-and-error solution. Furthermore, an accurate determination of fluid conditions will facilitate the calculation of reaction forces on safety/relief valve and its stack. This paper also presents the results of sensitivity analyses to verify the validity of applying perfect gas equations to steam.