Valve · calculator
Valve Sizing & Flow Coefficient (Cv) Calculator
Size liquid control valves for required Cv, or calculate flow rate from known Cv and pressure drop.
Method record
What this calculator measures
The valve coefficient (Cv) isolates resistance across the valve orifice. Solving for Cv allows sizing a new valve, while rearranging for Q predicts discharge through an existing valve.
Required Cv = GPM × √(SG ÷ ΔP) | Q = Cv × √(ΔP ÷ SG) Use it correctly
Calculation steps
- Set maximum and normal operating liquid flow.
- Allocate allowable valve pressure drop (typically 25–50% of total dynamic piping friction).
- Check manufacturer Cv trim tables, rangeability, and cavitation index.
Applicability
Use the model within its limits
High pressure drops across liquid valves risk cavitation, flashing, and severe acoustic noise. Verify the cavitation index (sigma) against manufacturer trim limits.
What this result represents
- System
- Valve
- Formula
- Required Cv = GPM × √(SG ÷ ΔP) | Q = Cv × √(ΔP ÷ SG)
- Review
- Verify safety-sensitive work with qualified guidance.
Worked Field Example
Worked Example: Control Valve Flow Coefficient (Cv) Sizing
Field Scenario: Sizing a commercial globe control valve for a chilled water coil passing 60 GPM with an allocated pressure drop of 4.0 PSI.
Given Input Parameters
Step-by-Step Mathematical Solution
- Cv formula: Cv = Q × √(SG / ΔP).
- Cv = 60.0 × √(1.0 / 4.0) = 60.0 × √0.25 = 60.0 × 0.50 = 30.0.
- Commercial selection: Select a 2-inch control valve with rated full-open Cv of 35 to 40 so that normal 60 GPM operation sits at 75–80% valve lift.
Required valve Cv is 30.0.
Professional Guidance: Avoid oversizing control valves; operating near the seat (<15% stroke) causes flow instability and wire-drawing seat damage.
Engineering FAQs
Frequently Asked Questions
What does valve flow coefficient (Cv) actually measure?
Cv is defined by ISA/ANSI standards as the volume of 60°F water in US gallons per minute that will flow through a wide-open valve with a pressure drop of exactly 1.0 PSI across the valve body.
How do you choose an allowable pressure drop (ΔP) when sizing control valves?
For good control authority, the valve pressure drop should represent 25% to 50% of total dynamic friction loss in the branch loop, or a minimum of 3 to 5 PSI. If ΔP is too small, the valve is oversized and exhibits hunting/instability near the closed seat.
What is the metric equivalent of Cv?
The metric flow coefficient is Kv, defined in cubic meters per hour (m³/h) with a pressure drop of 1 bar. The conversion is: Cv = 1.156 × Kv (or Kv = 0.865 × Cv).