Case Studies of Sizing Pressure Relief Valves for Two-Phase Flow

The most widely accepted methods of sizing safety valves for two-phase-flow, namely the homogeneous equilibrium model of Leung, the HNE-DS model proposed in ISO 4126-10, and the latest version of the API 520 two-point omega-model were compared to each other. The thermodynamic and mechanical nonequilibrium effect included in the ISO (HNE-DS) model dominates the more rigorous property data calculation (API) at low and moderate reduced pressures, whereas the critical flow rates become closer when approaching the thermodynamic critical point. In addition, the predictive accuracy of the ISO (HNE-DS) model was analyzed by means of roughly 3000 experimental data points from the literature. This data includes flashing steam/water, non-flashing air/water, and initially sub-cooled water flowing through safety valves from several manufacturers. The calculated flow rates are in good agreement with the measurements.