Control Valve Sizing Calculator (Gas and Vapour)
Size a control valve for gas or vapour at three flow cases. The workbook calculates inlet density, pressure drop ratio, expansion factor Y, choked flow, Kv and Cv, and compares them with the selected valve.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 25 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Sizing cases (pressures in bar a)
| Case | Mass flow W (kg/h) | Inlet P1 | Outlet P2 | T1 (°C) | ρ1 (kg/m³) | x = ΔP ÷ P1 | Sizing x | Y | Required Kv | Required Cv | Cv ratio / regime |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Minimum | 400 | 10.00 | 8.50 | 40 | 7.05 | 0.150 | 0.150 | 0.925 | 4.9 | 5.6 | 0.12 OK |
| Normal | 1,000 | 10.00 | 8.00 | 40 | 7.05 | 0.200 | 0.200 | 0.900 | 10.8 | 12.5 | 0.27 OK |
| Maximum | 1,500 | 9.50 | 7.00 | 40 | 6.70 | 0.263 | 0.263 | 0.869 | 15.5 | 17.9 | 0.39 OK |
About this Calculator
Gas valves are sized differently from liquid valves because the gas expands through the valve and the flow chokes at a pressure drop ratio set by xT. This workbook applies the IEC 60534 compressible flow equations at three operating cases at once, the same method as the online gas valve sizing calculator.
Basis: IEC 60534-2-1 compressible flow equations with mass flow: Kv = W ÷ (27.3 × Y × √(x × P1 × ρ1)), Fγ = γ ÷ 1.4, choked at x = Fγ × xT (pipe geometry factor FP taken as 1).
How to use
- Enter gas molecular weight, specific heat ratio and compressibility at inlet conditions.
- Enter xT from the valve manufacturer (globe valves about 0.7, ball and butterfly lower).
- Fill mass flow, inlet and outlet pressure (absolute) and temperature for the three cases.
- Read Y, Kv and Cv. CHOKED means the outlet pressure no longer affects flow.
- Enter the rated Cv of your valve and check the Cv ratios.
Formulas used
- ρ1 = P1 × M ÷ (Z × R × T1).
- x = (P1 − P2) ÷ P1; sizing x = smaller of x and Fγ × xT.
- Y = 1 − x ÷ (3 Fγ xT).
- Kv = W ÷ (27.3 Y √(x P1 ρ1)); Cv = 1.156 Kv.
Related tools
Frequently Asked Questions
What is the expansion factor Y?
A correction for the change in gas density from the inlet to the vena contracta. It falls from 1 at small pressure drops to 2/3 at choked flow.
What is xT?
The pressure drop ratio factor of the valve at choked flow, given by the manufacturer; it depends on valve style and trim.
Why use mass flow?
Mass flow avoids confusion between standard and actual volume units, which is a common source of gas sizing errors.
Learn more on the blog
Control valve flow coefficient (Cv and Kv)
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article