Valve Flow Coefficient (Kv / Cv) Sizing Calculator for Liquids
This calculator gives the required flow coefficient for a liquid control valve and checks whether the pressure drop you entered exceeds the choked flow limit. Results are in both Kv (metric) and Cv (US).
Your numbers
Relative to water at 15 C. Water is 1.0, light crude about 0.85.
Water is 220.6 bar a. Hydrocarbons are typically 30 to 50 bar a.
From the valve datasheet. Globe valves are often 0.85 to 0.9.
Your results
Basic incompressible flow sizing without piping geometry factors. Use the valve manufacturer sizing software for final selection, including noise and cavitation checks.
Required Cv vs flow rate
Sweeps Flow rate from 5 to 200, holding your other inputs fixed.
Structure valve data in your ERP
Optional review of how to hold Cv, class and trim data on the item master.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the flow rate, the specific gravity, the inlet pressure and the pressure drop you can afford across the valve. Supply the fluid vapour pressure and critical pressure, plus the valve's FL factor from its datasheet. The tool calculates Kv = Q x sqrt(SG / dP) and converts to Cv. If your pressure drop is larger than the choked limit, flow stops rising and the calculator sizes at the limit instead and flags the result. A margin of 10 to 25 percent is typical so the valve works at a sensible opening.
Formulas
For non-choked liquid flow, Kv = Q x sqrt(SG / dP) with Q in cubic metres per hour and dP in bar. Cv is 1.156 times Kv. Choked flow occurs when dP exceeds FL squared x (P1 - FF x Pv), where FF = 0.96 - 0.28 x sqrt(Pv / Pc). This is the standard approach in IEC 60534 and ISA 75 for incompressible fluids, simplified here by neglecting pipe reducer and viscosity corrections.
- Higher FL means better pressure recovery resistance.
- Vapour pressure close to inlet pressure lowers the choked limit.
- Flashing and cavitation need a separate, specialist check.
Worked example
For 60 cubic metres per hour of light oil at SG 0.85, a 4 bar drop and a 12 bar a inlet, Kv is 60 x sqrt(0.85 / 4) = about 27.7 and Cv is about 32. With a 20 percent margin you specify roughly Cv 38. The choked limit is 0.81 x (12 - 0.95 x 0.2), so about 9.6 bar, well above 4 bar: no choking.
Common mistakes
Sizing on normal flow only leaves no room for the maximum case, and oversizing for a worst case leaves the valve nearly closed at normal flow and prone to poor control. Using gauge pressure where absolute is needed distorts the choked limit. Always compare the final Cv against the valve's own characteristic curve for its travel. In the ERP, keep the valve datasheet Cv, trim and rated pressure class on the item so the right body is bought.
Frequently Asked Questions
What is the relation between Kv and Cv?
Kv is flow in cubic metres per hour of water at 1 bar drop. Cv is flow in US gallons per minute of water at 1 psi drop. Cv equals 1.156 times Kv, or Kv equals 0.865 times Cv.
Why does choked flow cap my result?
When the pressure at the vena contracta falls to the vapour pressure, bubbles form and flow cannot increase with more pressure drop. The valve passes a maximum flow, so sizing beyond that drop gives a wrong, too small Cv.
Does this work for gas or steam?
No. Gases and steam are compressible and use a different equation with an expansion factor. This tool is for liquids only.
What margin should I add?
Many specifications ask for the normal case to sit at 60 to 80 percent open and the maximum case at 90 percent or less. A 10 to 25 percent Cv margin over the maximum flow case is a common starting point.
If valve data lives in spreadsheets and datasheets, we can help you structure it in the ERP item master so engineering and purchasing see the same Cv, class and trim.
Related Tools
Hydrostatic Test Pressure Calculator
Compute the shop hydrotest pressure from design pressure and the allowable stress ratio, then check the gauge range you need on the test rig.
Industry TemplatesPipeline Erosional Velocity API RP 14E Calculator
Compare actual line velocity with the API RP 14E erosional limit for a two-phase or liquid line, and find the smallest inside diameter that stays under it.
Industry TemplatesASME B31.3 Pipe Wall Thickness Calculator
Pressure design thickness for straight pipe under internal pressure per the B31.3 equation, with corrosion allowance and mill tolerance rolled into the wall to order.
Industry TemplatesHydrotest Water Volume and Fill Time Calculator
Estimate the water to fill a pipe section, the time at your pump rate and the extra water to pressurise to test pressure.
More Industry Templates tools
See all- Pressure Vessel Shell and Head Thickness Calculator
- Pipe Thermal Expansion and Restrained Force Calculator
- Distributor Free Freight Policy Template
- Job Shop Quote Cover Letter Template
- Monthly Operations Review Meeting Pack Template
- OEE Improvement A3 Charter Template
- Sheet Metal Job Traveler Template
- Press Brake Setup Sheet Template
- Sheet Metal Cut List and Material Takeoff Worksheet