Valve Flow Coefficient (Cv/Kv)
Cv (US) and Kv (metric) quantify valve flow capacity—the flow rate of water at 1 psi or 1 bar pressure drop. Cv = Q/√(ΔP/SG) for liquids. Essential for valve sizing and process control.
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Cv = US GPM at 1 psi drop; Kv = m³/h at 1 bar. Choked flow: pressure ratio limits flow increase. Gas and steam need expansion factor Y. Valve characteristic (linear, equal %) affects control.
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Why: Valve sizing requires Cv/Kv to match system flow and pressure. Undersized valves restrict flow; oversized valves cause control instability. ISA and Crane standards govern sizing.
How: Cv = Q/√(ΔP/SG) for liquids. Gas and steam use density corrections. Choked flow occurs when ΔP > 0.5×P1. Kv = 0.865×Cv.
Run the calculator when you are ready.
⚽ Ball Valve - Water Flow
2-inch ball valve for water flow control in industrial process
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🚪 Gate Valve - Oil Pipeline
4-inch gate valve for oil pipeline on/off service
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🌐 Globe Valve - Steam Control
1.5-inch globe valve for steam throttling in process heating
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🦋 Butterfly Valve - Chilled Water
6-inch butterfly valve for HVAC chilled water system
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🎛️ Control Valve - Natural Gas
Control valve for natural gas pressure regulation
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📊 Cv from Test Data
Calculate Cv from actual test measurements
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🔄 Cv-Kv Conversion
Convert between US (Cv) and metric (Kv) flow coefficients
Click to use this example
For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Cv = flow (GPM) of water at 60°F with 1 psi drop.
— ISA
Kv = 0.865 × Cv for US to metric conversion.
— Crane
Choked flow when ΔP/P1 > 0.5 for gases.
— Engineering Toolbox
Liquid: Q = Cv × √(ΔP/SG); SG = specific gravity.
— ISA
What is Valve Flow Coefficient (Cv/Kv)?
The valve flow coefficient (Cv in US units, Kv in metric units) is a standardized measure of a valve's flow capacity. It represents the flow rate of water that will pass through a valve with a pressure drop of 1 unit (1 psi for Cv, 1 bar for Kv) at standard conditions.
Cv Definition (US Units)
Cv = Flow rate in GPM (US gallons per minute) of water at 60°F that will pass through a valve with a pressure drop of 1 psi.
Cv = Q (GPM) / √(ΔP (psi) / SG)
Kv Definition (Metric Units)
Kv = Flow rate in m³/h (cubic meters per hour) of water at 15-20°C that will pass through a valve with a pressure drop of 1 bar.
Kv = Q (m³/h) / √(ΔP (bar) / SG)
Cv-Kv Conversion
The conversion factor between Cv and Kv is: Kv = 0.865 × Cv
This factor accounts for the difference between US gallons and cubic meters, and between psi and bar pressure units.
How Does Valve Flow Coefficient Calculation Work?
Valve flow coefficient calculations use standardized equations that relate flow rate, pressure drop, fluid properties, and valve characteristics. The equations differ for liquids, gases, and steam due to their different flow behaviors.
🔬 Calculation Methods
For Liquids
Q = Cv × √(ΔP/SG)
Where Q is flow rate (GPM), ΔP is pressure drop (psi), and SG is specific gravity relative to water.
For Gases
Q = 963 × Cv × P1 × √(ΔP/(P1 × SG × T))
Where P1 is inlet pressure (psi), T is temperature (°R), and SG is specific gravity relative to air.
When to Use Valve Flow Coefficient Calculator
Valve flow coefficient calculations are essential for valve selection, system design, flow control optimization, and troubleshooting flow problems in piping systems.
Valve Selection
Select the correct valve size and type for your application based on required flow rates and pressure conditions.
System Design
Design piping systems with proper valve sizing to ensure adequate flow capacity and control.
Flow Analysis
Analyze existing systems to determine if valves are properly sized and identify flow restrictions.
How To Use This Calculator
- Select calculation mode: Cv from flow, flow from Cv, valve sizing, Kv conversion, or Cv from test
- Enter flow rate, pressures, and fluid properties (specific gravity)
- Select valve type and characteristic curve
- Results display Cv, Kv, and flow rates in multiple units
❓ Frequently Asked Questions
What is the difference between Cv and Kv?
Cv uses US units (GPM, psi); Kv uses metric (m³/h, bar). Conversion: Kv = 0.865 × Cv.
When does choked flow occur?
Choked flow occurs when pressure ratio exceeds critical value; flow no longer increases with greater pressure drop.
Valve Flow Coefficient Formulas
Cv Definition
ext{Cv} = Q ( ext{GPM}) / √(\text{Delta} P ( ext{psi}) / ext{SG})
Cv is the flow coefficient in US units: gallons per minute of water at 60°F with 1 psi pressure drop
Kv Definition
ext{Kv} = Q (m^{3}/h) / √(\text{Delta} P ( ext{bar}) / ext{SG})
Kv is the flow coefficient in metric units: cubic meters per hour of water at 15-20°C with 1 bar pressure drop
Cv-Kv Conversion
ext{Kv} = 0.865 imes ext{Cv}
Conversion factor between US (Cv) and metric (Kv) flow coefficients
Liquid Flow Equation
Q = ext{Cv} imes √(\text{Delta} P/ ext{SG})
Flow rate calculation for liquids using Cv, pressure drop, and specific gravity
Gas Flow Equation
Q = 963 imes ext{Cv} imes P1 imes √(\text{Delta} P/(P1 imes ext{SG} imes T))
Flow rate calculation for gases using Cv, inlet pressure, pressure drop, specific gravity, and temperature
Steam Flow Equation
Q = 2.1 imes ext{Cv} imes P1 imes √(\text{Delta} P/(P1 imes T))
Flow rate calculation for steam using Cv, inlet pressure, pressure drop, and temperature
📚 Official Data Sources
⚠️ Disclaimer: Cv/Kv values are theoretical. Actual valve performance varies with manufacturer, trim, and installation. Verify with valve supplier for critical applications.
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