Control Valve Cv Sizing Calculator

Choked flow means the pressure drop has reached (or exceeded) the valve's critical limit — xT × P1 for liquid, or Fk × xT × P1 for gas — beyond which further reducing downstream pressure no longer increases flow through the valve. In liquid service this is a proxy for cavitation risk, and in either case operating right at or past that point causes noise, vibration, and accelerated trim wear, so the tool caps the effective ΔP used in the Cv equation at that limit.

Liquid is essentially incompressible, so its Cv equation only needs specific gravity and the (capped) pressure drop: Cv = Q/(Fp·√(ΔP/Gf)). Gas expands as it drops in pressure, so its sizing equation instead needs the expansion factor Y, absolute temperature, and molecular-weight-driven specific gravity to account for that compressibility — that's why gas service also asks for temperature while liquid service doesn't use it.

The calculator compares the required Cv against a table of common manufactured valve sizes and picks the smallest one that still meets or exceeds it, since manufacturers only build valves in these discrete sizes rather than to an arbitrary Cv. The rangeability output (selected Cv ÷ required Cv) tells you how much margin that selection leaves — keeping it roughly between 1 and 3–4 avoids picking a valve so oversized it spends its life barely cracked open.

xT is the critical pressure-drop ratio at which the valve chokes, and it directly sets both the effective pressure drop used in the Cv formula and the FL/expansion-factor terms — a lower xT means the valve chokes at a smaller pressure drop. It varies by valve trim style (globe ≈0.7, ball ≈0.55, butterfly ≈0.5 by default here), but the real value should always come from the specific valve manufacturer's data sheet rather than these generic defaults.