Abstract

When designing control valves for multiphase applications using conventional valve sizing methodologies, it is assumed that the flow properties such as mixture density and volume fraction of phases are uniform throughout the valve trim. In reality, however, the properties may be different due to the difference in the gas and liquid densities as well as velocities within the valve which cause phase stratification. Furthermore, complex geometric effects can also cause phase non-uniformity within the valve. Current valve sizing methodologies are based on global parameters such as overall valve coefficient (Cv) which do not consider local phase non-uniformities within the trim during the design phase. Among these methodologies, it has previously been shown that the harmonised Cvs method is the most accurate method of sizing control valves under multiphase flow conditions across a wide range of flow regimes. The current study provides additional equations to be used with the harmonised Cvs method to ensure that the local flow quality compliance is achieved, which seems to be the major weakness of the existing method. In order to obtain local gas and liquid flow rates, pressure drops and phase fractions, well-validated computational fluid dynamics (CFD) simulations were carried out. Two valve opening positions of 60% and 100% were considered each with 5, 10, and 15% inlet air volume fractions to simulate real life conditions. The results show that there is severe non-uniformity in the local gas and liquid distributions within the valve trim. To quantify the phase non-uniformities observed, a distribution parameter () based on the drift-flux model was used. The harmonised method was used to calculate the local Cvs and the local equivalent area factor (). A new equation was derived that considers the local variation of the factor within the trim and which incorporates the distribution parameter. Based on the foregoing analyses, additional flow distribution equations have been proposed that when used with the harmonised Cvs valve design method for multiphase flow applications will ensure that the deviation in performance of the valve is minimal from the design conditions.