The calculation of the mass flow rate through throttling devices is difficult when handling two-phase flow, especially when boiling liquids flow into these fittings. Safety valves are typically oversized by a significant extent, if sizing methods like the ω-method (originally developed by J. Leung), are used in case of low-quality inlet flow. Within this method the boiling delay of the liquid and the influence of the boiling delay on the mass flow rate are not considered. In this paper the HNE-DS model is proposed, where the compressibility coefficient ω is extended by adding a boiling delay coefficient. It includes the degree of thermodynamic nonequilibrium at the start of the nucleation of small mass fractions of vapor upstream of the fitting.
In Part 1 the sizing of safety valves is described. Additionally, the derivation of the HNE-DS method is given in detail. Part 2 considers the mass flow rate through short nozzles, orifices, and control valves. The HNE-DS model can be used for all those fittings. A comparison with experimental results on safety valves with steam/water and air/water flow has emphasized the excellent accuracy of the new model. © 2004 American Institute of Chemical Engineers Process Saf Prog, 2004