Flow Phenomena
Cavitation
Cavitation is the violent two-stage phenomenon that occurs when liquid flowing through a valve first drops below its vapour pressure (forming vapour bubbles) and then recovers to a pressure above it (imploding those bubbles). The collapsing bubbles release microscopic jets and shock waves with local pressures in the GPa range — enough to tear metal particles out of the hardest trim materials.
The signature damage is rough, pitted, "sponge-like" erosion just downstream of the throttling element, plus a sound often compared to gravel flowing through the line. Cavitation also causes noise, vibration, and in the worst case blocked flow conditions.
It is distinguished from flashing by the pressure recovery: if P2 stays below the vapour pressure, the bubbles never collapse — that is flashing (pure erosion, quieter). Whether and how strongly cavitation occurs is judged with the cavitation indices of IEC 60534-8-x / ISA-75.02, using the valve's recovery coefficient FL.
Countermeasures: increase the back pressure, reduce the drop (or split it between valves), use anti-cavitation cages (multi-stage, multi-path), or select cavitation-resistant trim materials.