Sizing relief valves for real gas or vapour: An appraisal of relief valve discharge area sizing methods for gas or vapour

摘要

At pressures and temperatures close to the critical regions,most gases or vapours generally do not behave like an ideal gas.The compressibility factor,Z,is reportedly a typical indicator of deviation from the ideal gas behaviour.Relief valve discharge area sizing methods derived from ideal gas assumptions could result in significant under-sizing of relief valves in gas or vapour services close to the critical regions.API 520 Part I states that the gas or vapour may be in these regions if the associated Z is less than approximately 0.8 or more than approximately 1.1,and three relief valve discharge area sizing methods for gas or vapour in these regions are described-isentropic expansion coefficient,Omega Leung method,and direct integration.These methods require a set of thermodynamic property data and may not lead to identical discharge areas.The required property data can be conveniently generated from commercial process simulation programs,but the generated data could vary depending on the selected property package available in the program.Moreover,the three sizing methods stated by API may not result in the same relief discharge areas for a given relief case.This article discusses three examples of relief valve discharge area sizing cases using the ideal and real gas isentropic expansion coefficients and compares the results with those calculated by the direct integration methods.Additionally,the variation in the areas calculated from using different property packages is also presented.