Valve Sizing for Two-Phase Flow: Wet Steam and Flashing Liquids

Two-phase flow conditions represent some of the most challenging valve sizing scenarios because the mixture of liquid and vapor phases creates flow behavior that cannot be predicted accurately using single-phase liquid or gas equations alone. Two common two-phase scenarios encountered in industrial valve applications are wet steam (a mixture of saturated steam vapor and entrained water droplets) and flashing liquid (a liquid that partially vaporizes as it passes through the valve due to pressure reduction below the vapor pressure). Both conditions require specialized approaches to valve sizing that account for the mixture properties and the phase transition behavior within the valve.

Wet Steam Valve Sizing

Wet steam is a saturated steam-water mixture characterized by its quality (the mass fraction of vapor in the mixture, expressed as a decimal from 0 to 1). Steam at quality 0.85 is 85 percent vapor and 15 percent liquid by mass. Valve sizing for wet steam must account for the fact that the mixture density and specific volume differ from dry steam at the same pressure. At quality 0.85, the mixture specific volume is approximately 85 percent of the dry steam specific volume at the same pressure, and the valve must pass this denser mixture at the same volumetric flow rate. Standard steam sizing equations that assume dry steam will undersize the valve for wet steam service. The Napier-based steam flow equations or the ISA compressible flow equations with appropriate mixture density corrections are used for wet steam sizing.

• Steam quality: mass fraction of vapor, Q=1 is dry steam, Q less than 1 is wet steam mixture

• Mixture specific volume: quality-weighted average of vapor and liquid specific volumes

• Erosion risk: liquid droplets in wet steam cause erosive damage to trim at high velocities

• Superheat margin: maintaining superheat prevents condensation and wet steam in steam valves

• Separator upstream: removing liquid before control valve simplifies sizing and reduces erosion

Flashing Liquid Service

Flashing occurs when liquid enters the valve at a pressure above its vapor pressure but the pressure drop through the valve reduces the outlet pressure below the vapor pressure, causing a portion of the liquid to vaporize within the valve. The fraction of liquid that vaporizes depends on the inlet conditions, the outlet pressure, and the thermodynamic properties of the fluid. Unlike cavitation, which involves temporary vapor bubble formation that collapses in higher-pressure regions, flashing produces a permanent two-phase mixture at the valve outlet. The mixed-phase outlet flow has much higher specific volume than the pure liquid inlet, and the valve must be sized to pass this expanded mixture. ISA S75.01 provides a sizing equation for flashing service that accounts for the increased specific volume of the flash mixture.

Valve Design for Flashing Service

Valves in flashing service require design features that accommodate the phase transition and the resulting two-phase flow. Outlet area of the valve and the downstream pipe must be sized for the high-volume two-phase mixture rather than the inlet liquid flow rate to avoid choking and excessive outlet velocity. The conversion of liquid to vapor within the trim generates very high fluid velocities and creates the potential for severe erosion of the trim and downstream body surfaces by the high-velocity vapor-liquid mixture. Hardened trim materials (Stellite or tungsten carbide) and erosion-resistant downstream body designs with smooth flow passages reduce damage. Locating the phase transition downstream of the trim by designing the valve for flashing exit conditions reduces erosion of the precision-machined seating surfaces.