What is the difference between flashing and cavitation?
Flashing and cavitation are two distinct phenomena that can occur in control valves, although they are related and share some common characteristics. The key differences between them are:
- Pressure Recovery:
- Flashing: In flashing, the pressure does not recover to the vapor pressure of the fluid. The vapor bubbles formed at low pressure remain in the vapor phase downstream of the valve.
- Cavitation: In cavitation, the pressure recovers to the vapor pressure of the fluid, causing the vapor bubbles to collapse. This collapse results in intense shock waves and damage to the valve and surrounding equipment.
- Bubble Collapse:
- Flashing: In flashing, the vapor bubbles do not collapse. Instead, they remain in the vapor phase, reducing the flow capacity and causing erosion of the valve components.
- Cavitation: In cavitation, the vapor bubbles collapse, releasing energy that causes damage to the valve and piping through shock waves and high-velocity liquid impacts.
- Sound and Damage:
- Flashing: Flashing typically produces a hissing sound due to the continuous flow of vapor and liquid.
- Cavitation: Cavitation produces a popping or implosive sound due to the sudden collapse of vapor bubbles.
- Effects on Valve Performance:
- Flashing: Flashing reduces the flow capacity of the valve by increasing the specific volume of the fluid, leading to choked flow.
- Cavitation: Cavitation causes wear and tear on the valve, noise, and vibration that can damage surrounding equipment.
Flashing occurs when the pressure does not recover to the vapor pressure, and the vapor bubbles remain in the vapor phase, while cavitation occurs when the pressure recovers, causing the vapor bubbles to collapse and release energy.