Technical paper
Is Your Relief Valve Sizing Method Truly Rigorous?
Pressure relief valves are devices that protect equipment from excessive overpressure. In case of emergency situations, they should ensure a sufficient discharge of mass to reduce the pressure below the recommended pressure limits. The purpose of relief sizing is to determine the required area that can hold the required mass discharge from the valve under different overpressure scenarios. The discharged mass can be vapor, liquid, supercritical fluids or two-phase fluids. This article focuses on relief valve sizing methods for vapor phase and supercritical fluids at choked flow. The American Petroleum Institute (API Standard 5201,2) recommends basing vapor-phase sizing methods on an ideal gas flow assumption.1 This assumption has been addressed by several groups in the industry and may lead to high levels of deviation in cases of near critical and supercritical fluids 3,4. In the most recent version of API 520, a rigorous approach for calculating mass flux through the valves is introduced in addition to the existing, ideal gas based models.2 Furthermore, API Standard 520 suggests using a real gas isentropic coefficient calculation method as an alternative to the ideal gas specific heat ratio for sizing relief valves. This paper will compare different vapor and supercritical vapor vent sizing approaches and compare their performance against a rigorous model. The rigorous model performs many isentropic flashes using appropriate thermodynamic packages offered in ProMax7. The performance of each method is assessed through both pure component and mixture examples. In ProMax, a rigorous sizing method can be conveniently applied along with other alternative methods for API 520 sizing calculations7.
Pressure relief valves are devices that protect equipment from excessive overpressure. In case of emergency situations, they should ensure a sufficient discharge of mass to reduce the pressure below the recommended pressure limits. The purpose of relief sizing is to determine the required area that can hold the required mass discharge from the valve under different overpressure scenarios. The discharged mass can be vapor, liquid, supercritical fluids or two-phase fluids. This article focuses on relief valve sizing methods for vapor phase and supercritical fluids at choked flow. The American Petroleum Institute (API Standard 5201,2) recommends basing vapor-phase sizing methods on an ideal gas flow assumption.1 This assumption has been addressed by several groups in the industry and may lead to high levels of deviation in cases of near critical and supercritical fluids 3,4. In the most recent version of API 520, a rigorous approach for calculating mass flux through the valves is introduced in addition to the existing, ideal gas based models.2 Furthermore, API Standard 520 suggests using a real gas isentropic coefficient calculation method as an alternative to the ideal gas specific heat ratio for sizing relief valves. This paper will compare different vapor and supercritical vapor vent sizing approaches and compare their performance against a rigorous model. The rigorous model performs many isentropic flashes using appropriate thermodynamic packages offered in ProMax7. The performance of each method is assessed through both pure component and mixture examples. In ProMax, a rigorous sizing method can be conveniently applied along with other alternative methods for API 520 sizing calculations7.