Valve Sizing for Two-Phase Flow and Flash Service

Two-phase flow and flashing service present unique valve sizing challenges because the fluid changes state as it passes through the valve. When liquid flashes to vapor downstream of the valve, the actual volumetric flow is much larger than predicted by single-phase liquid calculations. Incorrect valve sizing for flash service results in oversized valves that hunt and oscillate, or undersized valves that cannot achieve the required flow, along with accelerated erosion from high-velocity two-phase flow.

Flashing and Cavitation Mechanisms

Flashing occurs when the fluid pressure drops below the vapor pressure and a portion vaporizes, remaining in the vapor phase at valve outlet. Cavitation occurs when the fluid pressure drops below vapor pressure in the vena contracta (minimum pressure point) but recovers above vapor pressure downstream, causing vapor bubbles to collapse violently. Flashing causes erosion of downstream trim and body components by high-velocity vapor-liquid mixture. Cavitation causes severe vibration, noise, and pitting of metal surfaces within the valve.

• Flashing: pressure stays below vapor pressure - sustained vapor formation

• Cavitation: pressure drops then recovers - bubble collapse causes vibration and pitting

• Vena contracta: minimum pressure point in valve where cavitation initiates

• Choked flow: maximum flow condition limited by vapor formation

• Flash fraction: mass fraction of liquid that vaporizes through the valve

IEC 60534 Sizing Methods for Two-Phase Flow

IEC 60534-2-1 provides methods for sizing control valves for liquid, gas, and two-phase flow. For flashing service, the sizing method accounts for the reduction in effective pressure drop available for flow (due to vapor pressure limitation) using the liquid pressure recovery factor FL and the combined liquid pressure recovery factor FLP. The effective pressure drop is limited to the choked flow condition, preventing the valve from being sized on a differential pressure that exceeds the choking pressure drop. Manufacturer-specific correction factors may be required for highly accurate sizing in flash service.

Valve Material Selection for Flash Service

Flash service requires hard-faced or hardened trim materials to resist erosion from high-velocity two-phase flow. Common solutions include Stellite-faced seats and plugs, tungsten carbide-coated trim, and hardened stainless steel (17-4 PH or 440C). Anti-cavitation trim designs with multi-stage pressure reduction can reduce fluid velocity within the trim, lowering erosion rates and extending trim life. Downstream body components, including the outlet bore and downstream piping, should also be protected with hard-facing or erosion-resistant liners where flow impingement occurs.