Check Valve Cv Calculation Guide: Formulas & Sizing
What Is Check Valve Cv Calculation?
Cv (Flow Coefficient) is the universally accepted numerical index quantifying a valve’s flow capacity. It is defined as the number of US gallons per minute (GPM) of water at 60°F (15.6°C) that flows through a fully open valve with a pressure drop of exactly 1 psi. Accurate check valve Cv calculation is the foundation of reliable valve selection, preventing excessive pressure drop, disc flutter, and water hammer in Indian industrial piping systems.
Check Valve Cv Calculation Formulas
There are two primary formulas used in check valve engineering. The forward formula calculates the minimum Cv required when you know the flow rate and allowable pressure drop. The inverse formula calculates the actual pressure drop when you know the flow rate and the selected valve Cv.
Check Valve Cv Calculation: Forward Formula
Check Valve Cv Calculation: Inverse Formula
Variable Definitions for Cv Calculation
| Variable | Symbol | Units | Description |
|---|---|---|---|
| Flow Coefficient | Cv | dimensionless | Valve capacity index (GPM per psi drop) |
| Flow Rate | Q | GPM | Volumetric flow rate of fluid through the valve |
| Specific Gravity | SG | dimensionless | Fluid density relative to water (water = 1.0) |
| Pressure Drop | ΔP | psi | Pressure loss across the fully open valve |
Kv to Cv Conversion for Check Valve Calculation
Indian engineering projects predominantly use metric units: flow rate in cubic metres per hour (m³/hr) and pressure in bar. The metric flow coefficient is Kv. The relationship between Cv and Kv is fixed by unit conversion. For practical engineering work, Cv is approximately 16% higher than Kv for the same valve (Cv = 1.156 × Kv).
Check Valve Cv Values by DN Size
The following table presents typical Cv values for single plate wafer check valves. These values represent the flow coefficient when the valve disc is fully open and flow is turbulent, positioned in the higher range due to KELOR’s optimised disc profile.
| DN Size | NPS (inch) | Cv (Typical KELOR) | Kv (Typical) |
|---|---|---|---|
| DN50 | 2″ | 22 | 19.0 |
| DN80 | 3″ | 52 | 45.0 |
| DN100 | 4″ | 78 | 67.5 |
| DN150 | 6″ | 165 | 142.7 |
| DN200 | 8″ | 290 | 250.9 |
| DN300 | 12″ | 650 | 562.3 |
Step-by-Step Check Valve Cv Calculation Examples
Check Valve Cv Calculation Example 1: Water Pump
Given: Water pump delivering 150 GPM, maximum allowable ΔP = 1.5 psi. Water at 25°C, SG = 1.0.
- Step 1: Apply forward formula: Cv = 150 / √1.5 = 150 / 1.225 = 122.4
- Step 2: Apply 30% sizing margin: Required Cv = 122.4 × 1.30 = 159.1
- Step 3: Select DN size: DN150 (KELOR Cv = 165) exceeds 159.1.
- Result: DN150 single plate wafer check valve — actual ΔP = (150 / 165)² = 0.83 psi (within limit).
Check Valve Cv Calculation Example 2: Metric Data
Given: Chilled water flow = 25 m³/hr, maximum allowable ΔP = 0.2 bar. SG = 1.0.
- Step 1: Convert to imperial: Q = 25 × 4.403 = 110.1 GPM, ΔP = 0.2 × 14.504 = 2.90 psi
- Step 2: Calculate required Cv: Cv = 110.1 / √2.90 = 110.1 / 1.703 = 64.6
- Step 3: Apply 30% margin: Required Cv = 64.6 × 1.30 = 84.0
- Result: Select DN125 single plate wafer (Cv 115) or DN100 dual plate wafer (Cv 88) to satisfy the margin.
Factors Affecting Check Valve Cv Calculation
Effect of Disc Design on Cv Calculation
The internal disc design is the largest factor affecting Cv. For the same DN size, a swing check valve has the highest Cv (100% baseline). A dual plate wafer is 80–90%, a single plate wafer is 70–85%, and a lift check valve is 50–70% of the swing valve Cv due to varying levels of flow obstruction.
Viscosity and Temperature Corrections in Cv Calculation
Standard Cv assumes water (turbulent flow). For viscous fluids, a viscosity correction factor must be applied based on the Reynolds number. If the Reynolds number is below 4,000, the actual pressure drop can be 2 to 5 times higher than the water-based prediction, requiring the valve to be oversized by 1–2 DN sizes.
Flow Velocity and Cv Calculation Relationship
The recommended velocity range for water service is 1.0 to 3.0 m/s, which corresponds to operating the valve at approximately 40% to 80% of its rated Cv capacity. Velocities above 3.0 m/s indicate the valve is undersized, risking disc flutter and water hammer.
5-Step Check Valve Cv Calculation Procedure
Identify Design Flow
Determine Q in GPM or m³/hr from the pump curve. Use maximum continuous flow.
Set Allowable ΔP
Determine max pressure drop. Pump discharge: 0.5–2.0 psi. General service: 1.0–5.0 psi.
Calculate Minimum Cv
Apply Cv = Q × √(SG / ΔP). Apply viscosity correction if the fluid is not water.
Select DN with Margin
Choose a valve with published Cv at least 20–30% above the calculated minimum.
📧 Need Cv Data Sheets for Your Sizing Calculation?
Tell us your flow rate, fluid, and allowable ΔP — KELOR sends the correct check valve size with published Cv within 2 hours.
Get Cv Sizing Help Email Flow Data