What Cv Measures and What It Does Not

The flow coefficient Cv is the number of US gallons per minute of 60 °F water a valve passes at a 1 psi pressure drop. It is a capacity rating rather than a dimension, and because flow scales with the square root of pressure drop, a Cv of 100 means 100 GPM at 1 psi and about 316 GPM at 10 psi. For liquids the sizing equation is Cv = Q√(SG/ΔP) — denser fluids require more capacity for the same volumetric flow, which is why specific gravity sits under the root. The metric equivalent Kv, in m³/h at 1 bar, is about 0.865 times Cv.

Choked Flow: The Limit That Undersizes Valves

Once the pressure at the vena contracta falls to the fluid's vapour pressure, additional pressure drop produces no additional flow. The valve is hydraulically choked, and the sizing calculation must cap ΔP at ΔP_choked = F_L²(P₁ − F_f·P_v) rather than using the full available drop. Skipping this step is a common and consequential error: it produces a Cv that looks adequate on paper and a valve that cannot pass the required flow in service. This calculator applies the cap automatically for liquid service and reports when it takes effect.

Why Valve Style Changes the Answer

The pressure recovery factor F_L describes how much pressure a valve recovers downstream of its vena contracta, and it varies substantially by design. High-recovery valves — butterfly and segmented ball, with F_L around 0.6 to 0.7 — choke and cavitate at relatively modest pressure drops. Low-recovery globe valves at F_L near 0.9 tolerate far higher differentials before choking. For high-differential service this makes valve style a first-order decision rather than a detail, and it explains why an apparently oversized butterfly valve can fail where a smaller globe valve succeeds.

Cavitation, Flashing, and Sizing Margin

Cavitation and flashing begin the same way, with vapour bubbles forming at the vena contracta. If downstream pressure recovers above vapour pressure, the bubbles collapse violently — that is cavitation, and it produces the characteristic gravel noise while pitting trim and body. If downstream pressure stays below vapour pressure the bubbles persist and the fluid leaves as a two-phase mixture, which is flashing and calls for hardened trim plus enlarged downstream piping. The cavitation index σ = (P₁ − P_v)/ΔP flags the risk, with values below about 1.5 warranting anti-cavitation trim. Finally, size for 20 to 80% of rated Cv at normal flow. Oversizing is both the more common error and the more damaging one, since a grossly oversized valve spends its life near the seat where control is poor and wear accelerates.