Control valve in flashing service
A liquid valve is sized on the square root of differential pressure only while the drop is below the choked value. Past that the flow stops rising: the vena contracta has reached the vapour pressure, the liquid is boiling there, and lowering the downstream pressure further does nothing. Sizing on the full ΔP therefore undersizes the valve.
| Term | Value | Note |
|---|---|---|
| Flow | 50.0 m³/h | as entered |
| Upstream pressure | 10.00 bar | absolute |
| Downstream pressure | 1.00 bar | absolute |
| Available ΔP | 9.00 bar | P₁ − P₂ |
| Vapour pressure | 1.50 bar | at the flowing temperature |
| F_F | 0.937 | 0.96 − 0.28√(P_v/P_c) |
| Choked ΔP | 6.96 bar | F_L²(P₁ − F_F P_v) |
| Choked | Yes | the drop exceeds the choke point |
| Required Cv | 18.95 | on the choked drop |
| Cv if the full ΔP were used | 16.67 | undersized |
| Cavitation index σ | 0.94 | below 1.5 is a risk |
The downstream pressure is below the vapour pressure, so the liquid flashes in the valve body — this is flashing service, not cavitation. The choked ΔP is 6.96 bar, so the 9.00 bar available is beyond the choke point and the Cv must be sized on the choked drop.
0.96 − 0.28√(P_v/P_c) · Critical pressure ratio factor.
F_L²(P₁ − F_F P_v) · IEC 60534-2-1.
Q √(G/ΔP) · Liquid, unchoked.
Q / (F_L √((P₁ − F_F P_v)/G)) · Liquid, choked.
A clear path from inputs to results.
A liquid valve is sized on the square root of differential pressure only while the drop is below the choked value. Past that the flow stops rising: the vena contracta has reached the vapour pressure and the liquid is boiling there.
How to use
- Enter the flow and both pressures.
- Set the vapour pressure and the fluid’s critical pressure.
- Choose the valve trim’s F_L.
- Read the required Cv and whether the service is choked.
The formula
Reference: IEC 60534-2-1, industrial-process control valves, sizing equations for incompressible fluid flow.
Worked example
50 m³/h from 10 bar to 1 bar
- 50 m³/h, P₁ = 10 bar, P₂ = 1 bar.
- P_v = 1.5 bar, P_c = 220 bar, F_L = 0.9, SG = 1.
- 1
The available drop is 9 bar.
- 2
F_F = 0.96 − 0.28√(1.5/220) = 0.937.
- 3
The choked drop is 0.81 × (10 − 1.405) = 6.96 bar.
- 4
Since 9 bar exceeds it, the valve is choked.
- 5
Cv is 50/(0.9 × √(8.595)) = 18.95, against 16.67 from the full ΔP — 14% undersized.
Choked · Cv 18.95 · 14% larger than the unchoked answer.
Interpreting the result
The choked condition is not a fault — it is a physical limit that every valve reaches at a large enough pressure drop. The engineering response is to size on the choked drop and, if the service flashes, to choose a trim that tolerates it rather than one that erodes.
Assumptions & limitations
- Incompressible liquid, single phase at the inlet.
- A single valve, not a split range.
- F_L from the trim, not from the installation piping.
- No flashing allowance for the downstream pipework.
Frequently asked questions
What is F_L?
The liquid pressure recovery factor — how much pressure the trim recovers downstream of the vena contracta. A globe valve is about 0.9, a butterfly about 0.7, and a high-recovery trim chokes earlier.
Why does the flow stop rising?
Because the throat reaches sonic velocity for the mixture. Lowering the downstream pressure further cannot increase the mass flow through a choked throat.
Does the same apply to gas?
Gas has an analogous choked condition with the critical pressure ratio, but the equation and the coefficients are different — the relief valve page covers the gas case.
What is the difference between flashing and cavitation?
Flashing keeps the outlet pressure below the vapour pressure, so the bubbles persist downstream. Cavitation has the outlet above it, so the bubbles collapse in the valve or just after, and that is what damages the trim.
How do I avoid cavitation?
Reduce the pressure drop across the valve (a larger valve, or a restriction downstream), use a hardened trim, or use an anti-cavitation cage with staged pressure drop.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.