Understanding Cavitation & Flashing Regimes
In a control valve, fluid velocity increases as it passes through the narrow seat orifice (the **Vena Contracta**), causing local pressure to drop. If local pressure drops below the fluid's vapor pressure ($P_v$), vapor bubbles instantly form. What happens next determines the regime:
1. Cavitation
If the outlet pressure ($P_2$) recovers and rises back **above the fluid vapor pressure**, the vapor bubbles collapse violently. This bubble collapse generates micro-jets with shockwaves exceeding **100,000 psi**, chipping metal off seats and plugs, causing a distinct "gravel-like" noise and intense vibration.
2. Flashing
If the downstream outlet pressure remains **below the fluid vapor pressure**, the bubbles do not collapse. The fluid exits the valve as a high-velocity liquid-vapor mixture, causing severe smooth erosion along the body outlet.
The Sizing Ratios:
- xF < xFz: Safe. No cavitation bubbles form.
- xFz ≤ xF < FL²: Cavitation regime. Bubbles form at the seat and collapse downstream. Multi-stage severe-service trims are needed to step down pressure gradually.
- xF ≥ FL²: Choked Cavitation. Maximum mass flow capacity is reached. Downstream expansion or flashing is imminent.
- P2 ≤ Pv: Flashing. Downstream vapor remains expanded. Chrome-moly alloy valve bodies are required to resist wear.