This work discusses the effect of the degree of fineness of the flow volume discretization and that of the turbulence model on the accuracy of reproduction of air mass flow rates in two safety valves using the CFD software ANSYS Flo ® . Calculations show that the degree of fineness of the discretization is the decisive factor affecting the exactness of the calculations and that the best reproduction is achieved with grids where at least two cells are built on the smallest edge. The selection of the turbulence model has by far in comparison a lower impact; however, the best accuracy is obtained using the standard k-x model and the SST modification of Menter.

This work studies the influence of the physical properties of a liquid on the void fraction at the inlet of a corner valve resembling a full-lift safety valve. The test media are mixtures of air and aqueous solutions of glycerin. Our own measurements evince a reduction in the void fraction when the relative weight of glycerin in the solution is increased. If the effects of density, viscosity, and surface tension on the void fraction are accounted for by increasing the relative weight of glycerin in the solution, it results that the observed reduction of the void fraction is primarily a consequence of the enhancement of viscosity. On the other hand, the increment of the liquid density is responsible for a modest increase in the void fraction and the effect of the reduction in the surface tension is almost negligible. The enhancement of either the density or the viscosity of the liquid phase increases the relative velocity of the gas in the two-phase mixture and, therefore, the slip. The impact of the liquid properties on the void fraction in co-current vertical pipe flows is similar to that at the inlet of the corner valve. Among the most common correlations for pipe flows, the formulation of Rouhani and the homogeneous void fraction accurately reproduce the void fraction at the inlet of the valve only for two-phase flows with liquids of low viscosity. A new void fraction correlation is proposed here, which reproduces all measurements very well and correctly predicts the impact of the liquid phase properties. Despite numerical coefficients, which can be fitted to additional sets of measurements, the structure of the new correlation is also applicable outside the range of two-phase flows for which it has been explicitly validated. Introduction and PurposeFull-lift safety valves -here the definition of full-lift safety valve is in agreement with ISO 4126-1 -are installed on top of pressurized vessels to avoid human injuries and financial losses following the collapse of the units due to external fire or runaway reactions. Both accidents cause an abnormal heat inflow, leading to a partial evaporation of the liquid and a disengagement of the dissolved gaseous phases. The released gases and vapors increase the pressure in the vessel until the set pressure is reached and the safety valve opens [1]. If the liquid phase is very viscous, some gas or vapor cannot disengage and it remains entrapped in the liquid. The entrapped gas or vapor enhances the volume of the liquid that much to eventually make it swell to the top of the vessel. In that case, a two-phase discharge would pass through the safety valve. The amount of discharged mass and the duration of the two-phase release are strongly affected by the viscosity of the liquid [2].The gas content in the two-phase mixture at the inlet of the safety valve is expressed in terms of mass and volume, respectively, with the quality, _ x 1) , and the void fraction, a. On the base of the research on the two-phase discharge from pressurized vessels, the amount of gas in the mixtur...

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At the moment, very little knowledge is available about the flow of shear-thinning media in safety valves. In this paper, the flow of aqueous solutions of polyvinylpyrrolidone with a zero-shear viscosity between 0.3 and 3 Pa s in a LESER Type 441 DN 25/40 safety valve with a lift of 2.2 mm for relieving pressures up to 6.5 bar is computed using ANSYS FLUENT 12. According to these calculations, the lowest values of the dynamic viscosity are reached in the slit between the seat and the disk due to the large local velocity gradients. A more moderate increment in the fluidity of the solutions is observed also in the recirculation zone outside the disk when the stream from the rear of the valve meets the one from the front. A preliminary study suggests that the experimental flows may be turbulent in the slit and then evolve into a laminar profile in the discharge pipe. For this reason, the calculations are carried out assuming laminar, turbulent and transitional flows. The predicted mass flow rates are close to each other and to the measured values, except at relieving pressures close to ambient. The major cause of deviation seems to be the entrainment of air microbubbles, whose elongation may be responsible for additional shearing in the solutions with large polymer weight.

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The throat-to-stagnation critical pressure ratio for a frictionless and adiabatic nozzle flow of a homogeneous, nonflashing two-phase mixture can only be expressed as the numerical solution of a transcendental equation. A simple, physically plausible approximation is herein proposed, which fits well over the whole range of mass flow qualities.

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