Control Valve Sizing Calculator (Liquid)
Size a control valve for liquid service at minimum, normal and maximum flow. The workbook calculates Kv and Cv, checks for choked flow and shows how far open the selected valve will be.
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- 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 | Flow Q (m³/h) | Inlet P1 | Outlet P2 | ΔP (bar) | Choked ΔP (bar) | Sizing ΔP (bar) | Required Kv | Required Cv | Cv ratio (req ÷ rated) | Flow regime |
|---|---|---|---|---|---|---|---|---|---|---|
| Minimum | 20.0 | 9.00 | 6.50 | 2.50 | 7.27 | 2.50 | 12.6 | 14.6 | 0.14 | OK |
| Normal | 60.0 | 8.00 | 6.20 | 1.80 | 6.46 | 1.80 | 44.7 | 51.7 | 0.48 | OK |
| Maximum | 85.0 | 7.50 | 6.00 | 1.50 | 6.05 | 1.50 | 69.4 | 80.2 | 0.74 | OK |
About this Calculator
Sizing a control valve from one flow point is the most common cause of valves that hunt at low flow or run wide open at high flow. This workbook sizes the valve at three operating cases at once, applies the IEC 60534 choked flow limit and compares the result with the valve you plan to buy.
Basis: IEC 60534-2-1 liquid sizing equations for turbulent, non choked and choked flow (pipe geometry factor FP taken as 1).
How to use
- Enter the fluid specific gravity, vapour pressure and critical pressure (water: Pc = 220.64 bar a).
- Enter FL from the valve manufacturer (globe valves about 0.9, ball and butterfly lower).
- Fill in flow and inlet and outlet pressure (absolute) for the minimum, normal and maximum cases.
- Read the required Kv and Cv. If a case shows CHOKED, the sizing ΔP is limited to the choked value and the valve may cavitate.
- Enter the rated Cv of the valve you are considering and check the Cv ratios at maximum and minimum flow.
Formulas used
- ΔP = P1 − P2; choked ΔP = FL² × (P1 − FF × Pv).
- Sizing ΔP = smaller of ΔP and choked ΔP.
- Kv = Q × √(SG ÷ ΔPsizing), Cv = 1.156 × Kv.
- Cv ratio = required Cv ÷ rated Cv; aim for about 0.1 to 0.85 across the range.
Related tools
Frequently Asked Questions
How do I calculate Cv for a control valve?
For liquids, Kv = Q (m³/h) × √(SG ÷ ΔP (bar)) and Cv = 1.156 × Kv, provided the flow is not choked.
What is choked flow in a control valve?
The point where increasing ΔP no longer increases flow because the liquid is vaporising at the vena contracta. The limit is FL² × (P1 − FF × Pv).
How open should a control valve be at maximum flow?
Typically 70 to 85 % open, so there is margin for control and for errors in the process data.
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