Question

To avoid cavitation the valve should be designed such that

• A. high pressure recovery characteristics with high $F_L$ and $K_c$
• B. high pressure recovery characteristics with low $F_L$ and $K_c$
• C. low pressure recovery characteristics with high $F_L$ and $K_c$
• D. low pressure recovery characteristics with low $F_L$ and $K_c$

Hint:

High pressure recovery valves (like ball or butterfly valves) are highly prone to cavitation because their pressure drops sharply at the narrowest point and recovers rapidly.
Globe valves (low recovery) are much better suited for high pressure-drop applications prone to cavitation.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the optimal control valve design parameters (pressure recovery behavior, liquid pressure recovery factor $F_L$, and cavitation index $K_c$) required to prevent or minimize the risk of cavitation.

Step 2: Detailed Explanation:

• Cavitation occurs in control valves when the local fluid pressure drops below the vapor pressure of the liquid at the vena contracta, causing vapor bubbles to form.

• If the downstream pressure subsequently recovers above the vapor pressure, these bubbles violently collapse, causing physical damage to the valve trim and body.

• To avoid cavitation, we want the pressure drop inside the valve to be gradual, minimizing the pressure drop at the vena contracta.

• Low Pressure Recovery: Low recovery valves (like globe valves) do not allow the downstream pressure to recover sharply, which means the pressure at the vena contracta does not have to drop as low, preventing it from dipping below the vapor pressure.

• High $F_L$ (Liquid Pressure Recovery Factor): A higher $F_L$ indicates that the valve is less prone to pressure recovery and can handle larger pressure drops before cavitation begins.

• High $K_c$ (Cavitation Index): A high cavitation index coefficient ($K_c$) indicates that the valve design is highly resistant to the onset of cavitation.

• Thus, to avoid cavitation, the valve should feature low pressure recovery characteristics combined with high values of $F_L$ and $K_c$.

Step 3: Final Answer:
To avoid cavitation, the valve must have low pressure recovery characteristics with high $F_L$ and $K_c$.