Influence of the Liquid Phase Physical Properties on the Void Fraction at the Inlet of a Full‐Lift Safety Valve

Abstract

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...

Critical air/water blow-down in safety valves at low qualities

“…Additional hints about the behavior of the mixtures in the safety valve may be extracted from the analysis of the flow patterns in the outlet pipe. Since no photographic or statistical recording is available to estimate the flow regimes, the generalized flow pattern map of Tai- Flow regime in the discharge outlet piping midway between the safety valve and the back pressure valve for flows of air and water (empty points) and air and glycerine (full points) from Moncalvo and Friedel [16] using the generalized map of Taitel and Dukler [18]. tel and Dukler [18] is applied here following the recommendation of Hewitt [14].…”

“…From the database of Moncalvo and Friedel [16] air/glycerine and additional air/water flows in the same valve for relieving pressures between 5 and 7 bar are introduced in the flow pattern map of Fig. 5 for comparison.…”

“…2 and has been already described by Moncalvo and Friedel [16], has a seat diameter of 23 mm and the lift is fixed at 5.5 mm.…”

“…Test safety valve with the disk at the lift of 5.5 mm, see Moncalvo and Friedel[16]. Special thanks to Mr. B. Pallaks for his precious cooperation to realize this CAD drawing.…”