Straight talk on relief-valve sizing. Engineering Practice: Sizing relief valves for two-phase flow. A number of factors must be kept in mind when using the different methods to size these critical equipment components. This article profiles several different valve-sizing methods, and cautions the user on how to best use each one. Pressure-relief valves are widely used to protect plants against overpressure. Over the years, many methods have been proposed for sizing such valves for two-phase (gas-liquid) flow. The resulting valve sizes can vary by a factor of two or three, depending on how specific flow conditions and fluid properties are handled. This article discusses the key issues to consider, and offers recommendations for cutting through the confusion.