To prevent cavitation, you can use specialized anti-cavitation trim. This trim works by splitting the single flow pathway into multiple micro-channels, or utilizing multi-stage pressure reduction to ensure pressure never drops below the fluid's vapor pressure ($P_v$).

For flashing, which is a state-dependent phase change (since downstream $P_2 < P_v$), the erosion is handled by using hardened trim materials (such as Stellite, Tungsten Carbide) and sweep angle valves to prevent impingement on the valve body.

Valve Authority ($A$) is the ratio of the valve's pressure drop ($\Delta P_v$ at full open) to the total pressure drop of the dynamic system ($\Delta P_{system}$):

$$A = \frac{\Delta P_v}{\Delta P_{valve} + \Delta P_{piping}}$$

If authority is too low ($A < 0.25$), the valve cannot control flow effectively over its travel because dynamic pipe friction dominates. For stable PID control, engineers target $A$ between $0.25$ and $0.50$.

In gas systems, when the pressure drop ratio $x = \Delta P / P_1$ reaches the choked limit $F_\gamma x_T$, the gas velocity at the orifice throat reaches the local speed of sound (Mach 1).

Because pressure signals cannot travel faster than the speed of sound, lowering downstream pressure ($P_2$) further will have no effect on velocity or mass flow. The flow rate becomes limited by upstream density and pressure alone.

Both parameters quantify a valve's capacity, but use different unit systems:

$C_v$ (Imperial): Flow of water in US gallons per minute (GPM) at $60^\circ\text{F}$ with a $1\text{ psi}$ drop across the valve.

$K_v$ (Metric): Flow of water in cubic meters per hour ($\text{m}^3/\text{h}$) at $20^\circ\text{C}$ with a $1\text{ bar}$ drop across the valve.

Convert using: $C_v = 1.156 \times K_v$ and $K_v = 0.865 \times C_v$.

Operating temperature changes critical fluid properties:

• Density ($\rho$): Density decreases as temperature rises, which alters the mass-to-volume relationships for calculations.
• Vapor Pressure ($P_v$): Elevating temperatures increases $P_v$ exponentially. This rises the cavitation index ($\sigma$), significantly increasing the risk of cavitation and choked flow conditions.
• Viscosity ($\mu$): Affects the Reynolds number correction factor for extremely viscous laminarly flowing oils.

The parameter $x_T$ represents the pressure drop ratio limit of a control valve body type in gas calculations. It defines when the flow reaches the choked sonic velocity.

• Globe Valves: Typically $x_T \approx 0.70$ (highly tortuous path, less prone to quick choking).
• Butterfly Valves: Typically $x_T \approx 0.45$ (more straight-through profile).
• Segmented Ball Valves: Typically $x_T \approx 0.30$ (low restriction profile, chokes at very low delta-P ratios).

The liquid pressure recovery factor $F_L$ models how much pressure the fluid recovers downstream after dropping to its minimum pressure point inside the valve's throat (vena contracta):

$$F_L = \sqrt{\frac{P_1 - P_2}{P_1 - P_{vc}}}$$

Globe valves have high recovery resistance ($F_L \approx 0.9$), meaning they restrict pressure recovery, protecting against cavitation. Ball and butterfly valves are high-recovery designs ($F_L \approx 0.6$), letting pressure rebound significantly, which increases cavitation risks.

Specific gravity (S.G.) is the ratio of fluid density to standard water density. Because kinetic energy loss through the restriction depends on mass density, the flow coefficient varies directly with $\sqrt{S.G.}$.

Heavier fluids (higher S.G.) generate higher pressure drops for a given volumetric flow rate, requiring a larger flow coefficient ($C_v$) to achieve the same capacity.

The trim characteristic choice depends on how the piping system pressure drop shifts under operating loads:

• Equal Percentage: Recommended when most system pressure drops occur in the piping rather than across the valve (long pipe networks). As flow grows, the valve opens wider and expands its area exponentially, compensating for piping friction to keep control loop gain constant.
• Linear: Used when the valve pressure drop stays constant under all loads (e.g. pressure regulation bypass or short piping circuits).

The vena contracta is the point of minimum cross-sectional area and maximum velocity in a fluid stream passing through the valve throat orifice.

According to Bernoulli's equation, as velocity maximizes, pressure reaches its absolute minimum. If this minimum pressure falls below the vapor pressure of a liquid, vapor bubbles instantly form, triggering cavitation or flashing.