Flow Phenomena

Cavitation Index (σ, Kc, xFz)

Several dimensionless numbers predict where a valve sits on the cavitation scale; the three most met in practice:

• Sigma (σ) = (P₂ − P_v) / (P₁ − P₂). Higher σ means more margin. Each valve/trim has characteristic threshold values (incipient, constant, incipient damage, choking) determined by test; comparing installed σ with the trim's σ values tells you which regime you are in.
• Kc (ISA-75.02 style) is the ratio at the incipient-damage condition — the value of (P₂−P_v)/(P₁−P₂) below which material loss begins.
• xFz (IEC 60534-8-x style) is the pressure-drop ratio of the choked condition, used with the full standard's cavitation and noise procedures.

All three encode the same physics: the margin between the downstream pressure and the vapour pressure, scaled by the pressure drop. The practical use is the same — compute your σ, fetch the trim's certified thresholds, and keep the operating point in the non-damaging region, or select multi-stage trim that raises the thresholds. Our liquid Cv calculator flags the choked-flow condition as a first screen.

Related glossary terms

• Cavitation
• Choked Flow
• Flashing