Orifice Flow Cv/Kv & Choked Flow

Restriction-orifice sizing, Cv and Kv flow coefficients, choked sonic gas flow and Reynolds-number viscosity correction.

Restriction-orifice sizing

Orifice Flow Cv/Kv & Choked Flow calculations for restriction-orifice sizing are computed deterministically from published formulas, so the same inputs always produce the same result and the same OAS-XXXX verification hash.

Cv and Kv flow coefficients

Orifice Flow Cv/Kv & Choked Flow calculations for cv and kv flow coefficients are computed deterministically from published formulas, so the same inputs always produce the same result and the same OAS-XXXX verification hash.

Choked sonic gas flow

Orifice Flow Cv/Kv & Choked Flow calculations for choked sonic gas flow are computed deterministically from published formulas, so the same inputs always produce the same result and the same OAS-XXXX verification hash.

Engineering summary

Governing parameters for the Orifice Flow Cv/Kv & Choked Flow silo.

Governing parameters and computed results for this calculation
Parameter Value
Media covered 8 fluids and gases
Orifice diameters 0.031–1.5 in
Sharp-edge discharge coefficient ≈ 0.62
Critical pressure ratio (air) ≈ 0.528
Cv to Kv conversion Kv = 0.865 · Cv

Frequently asked questions

When does gas flow through an orifice become choked?
Gas flow chokes when the downstream-to-upstream absolute pressure ratio falls to or below the critical ratio (2/(k+1))^(k/(k−1)) — about 0.528 for air. Below that ratio the flow is sonic and further downstream pressure reduction adds no flow.
How do I convert between Cv and Kv?
Kv = 0.865 · Cv. Cv is the US gallons-per-minute of water at 60 °F that flows through the restriction at a 1 psi drop; Kv is the metric equivalent in cubic metres per hour at a 1 bar drop.
What discharge coefficient should a sharp-edged orifice use?
A sharp-edged, square-shouldered orifice uses a discharge coefficient near 0.62 for a fully turbulent liquid. The coefficient falls in the laminar regime, which is why a Reynolds-number viscosity correction matters at low flow.