1. The Vena Contracta Effect

As fluid flows through the restrictive orifice of a control valve, its velocity increases to conserve mass. According to Bernoulli's principle, as velocity increases, pressure decreases. The pressure reaches its lowest point slightly downstream of the physical restriction; this point is called the Vena Contracta.

Crucially, the pressure at the Vena Contracta ($P_{vc}$) can drop significantly lower than the outlet pressure ($P_2$). This "pressure dip" is determined by the valve geometry, quantified by the **Liquid Pressure Recovery Factor ($F_L$)**.

2. Flashing vs. Cavitation: Know the Difference

Both phenomena start when $P_{vc}$ drops below the liquid's Vapor Pressure ($P_v$), causing bubbles to form.

• Flashing: The pressure never recovers above $P_v$. The bubbles persist downstream. The liquid turns into a liquid-vapor mixture. The damage mechanism is erosion (sandblasting effect) due to high velocity. It cannot be prevented by valve design, only accommodated (e.g., using hardened trims).
• Cavitation: The pressure recovers above $P_v$ downstream. The vapor bubbles formed at the Vena Contracta suddenly collapse (implode) as the pressure rises. These implosions generate microscopic shockwaves (>100,000 psi) and micro-jets that pit and eat away metal. This is the "Gravel Sound". It destroys valves rapidly.

3. The Cavitation Index ($\sigma$)

The Sigma index is the ratio of "forces resisting cavitation" ($P_1 - P_v$) to "forces promoting cavitation" ($\Delta P$).

• $\sigma > 2.0$: No Cavitation.
• $1.7 < \sigma < 2.0$: Incipient Cavitation (Light noise).
• $1.0 < \sigma < 1.5$: Constant Cavitation (Damage likely without special trim).
• $\sigma < 1.0$: Choked Flow / Flashing (Maximum flow limit reached).

4. The Role of $F_L$

$F_L$ measures how much pressure recovery occurs after the Vena Contracta.
High Recovery ($F_L \approx 0.5-0.6$): Rotary valves (Ball/Butterfly). They have a streamlined flow path, so pressure recovers efficiently. However, this means the pressure dipped VERY low to get there. They cavitate easily.
Low Recovery ($F_L \approx 0.9$): Globe valves. The tortuous path prevents pressure recovery. The pressure dip is shallow, making them resistant to cavitation.

5. Choked Flow

When cavitation becomes severe, the vapor bubbles occupy so much volume that they choke the flow passage. Increasing the pressure drop ($\Delta P$) no longer increases the flow rate. The flow is saturated. Calculating the Choked Pressure Drop ($\Delta P_{choked}$) is vital for sizing to ensure the valve can actually pass the required flow.