Flow Phenomena
Flashing
Flashing occurs when the liquid passing a valve partially vaporizes because the pressure somewhere in the trim falls below the vapour pressure — and stays below it at the valve outlet. Part of the flow leaves the valve as vapour bubbles mixed with liquid.
Consequences: the mixture expands enormously (a flashing service can need a huge outlet pipe), the sizing calculation must use the standard's two-phase (or the vendor's proprietary) method, and the vapour bubbles travelling downstream collapse later — in the pipe or the next component — causing erosion, vibration and noise there. Trim damage from flashing is smooth, polished erosion (no pitting), unlike the spongy damage of cavitation.
Diagnosis at the valve: if P2 < Pv, you are flashing. Mitigation: hard trim materials (tungsten carbide), larger outlet bodies and tapered diffusers to slow the mixture, angled-flow or eccentric bodies to protect the casing, and process changes that raise P2 or reduce the drop. Flashing plus cavitation often coexist in high-ΔP liquid service.