Rigorous Calculation of Critical Flow Conditions for Pressure Safety Devices

Sizing and verification of pressure relieving systems is an important topic in the design of process plants in order to assure equipments and people protection against malfunctions and hazards. The calculation of the critical flow (choke) condition is analysed with respect to existing standard calculations procedures (API and Omega methods) that implement approximate procedures and may not be extended to temperature/pressure regions near the thermodynamic critical point. These procedures may be replaced by a more rigorous calculation based on the evaluation of the local sonic velocity with equations of state. The method applies to systems composed of pure chemical components as well as to multi-component mixtures existing in the single phase and multi-phase regions. As a consequence of an exact calculation of the critical flow conditions, more accurate values of the discharged flowrate may be obtained. Comparisons with calculations performed using the standard API RP 520 procedure and Omega method are presented.