Prediction of the maximum full lift safety valve two-phase flow capacity

During an emergency blowdown of pressurized plant equipment, a discharge of a two-phase mixture across the safety valve, at least temporarily, can occur. Most calculation methods applicable for this flow condition do not contain geometrical data of the safety valve as parameters and are not yet, or only in small parameter ranges: validated. Therefore, experiments with commercially available full lift safety valves with flashing and non-flashing single- and two-component two-phase mixtures were carried out. The results of comparisons between the measured and the calculated mass fluxes demonstrate that merely with the methods according to the Isentropic Homogeneous Equilibrium Model, the omega-method of DIERS, the Homogeneous Frozen Flow by Nastoll (1985) and the procedure by Gosslau and Weyl (1989) the measurements are qualitatively reproducible in limited parameter ranges. However, large deviations appear, particularly in the transition range to the single-phase flow of liquid and of vapour/gas, so that with no model an adequate prediction in the whole two-phase flow range is possible. By inserting a two-phase flow discharge coefficient, in analogy to the approach used in single-phase flow, the predictive accuracy of the Homogeneous Frozen Flow Model for non-flashing two-component flow is improved. In the case of flashing single-component flow, because of the boiling delay, the disequilibrium model of Henry and Fauske (1970) is extended by the two-phase flow discharge coefficient, and a better prediction accuracy is also obtained. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.