Control Valve Cᵛ / Kᵛ (ISA S75.01)

What this is: Cv (Imperial) and Kv (metric) are the universal flow-coefficient definitions used by every control-valve manufacturer. They tell you how much flow the valve passes per unit of pressure drop. Pick a valve whose published full-open Cv comfortably exceeds your required Cv at maximum design flow. Sizing rule of thumb:

• Required Cᵛ should fall between 30% and 80% of the valve's max Cᵛ at min and max flow.
• Below 30% — valve runs near seat, poor controllability, accelerated wear.
• Above 80% — no margin to handle upset conditions or future flow increase.

Choked flow: when downstream pressure drops low enough that vapour bubbles form in the vena contracta, additional ΔP does not increase flow. The formula returns the maximum useful ΔP; if your operating ΔP exceeds it you are likely cavitating, which destroys trim. Fᴸ depends on valve style — globes have high recovery (less prone to cavitation), butterflies and ball valves are lower-recovery. Brownfield tip: when a process change pushes flow up 30% on an existing valve, do not just throttle further open. Re-run this calc with the new flow, the actual pressure drop available, and the actual Pᵛ / temperature. Many "noisy" or "shaking" valves on revamped units are simply choking. The fix is a different trim (or the next size down so you regain authority), not more positioner tuning.