Valve sizing differs for liquids and gases. Gases expand as pressure drops, and once the outlet falls below about half the inlet pressure the flow chokes – it no longer increases as the outlet pressure drops further.
How to use the Valve Flow Coefficient Calculator
- Fluid
- Find
- Specific gravity (water = 1, air = 1 for gases)
- Inlet pressure P₁ (absolute)
- Outlet pressure P₂ (absolute)
- Gas temperature
- Liquid flow rate
- Gas flow rate (scfh)
- Flow coefficient C_v
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the fluid type and what to calculate, then enter the values. Pressures are absolute.
Inputs explained
| Field | What to enter |
|---|---|
| Fluid | Choose one: Liquid, Gas |
| Find | Choose one: Flow coefficient C_v, Flow rate |
| Specific gravity (water = 1, air = 1 for gases) | A number |
| Inlet pressure P₁ (absolute) | Units: psi, bar, kPa |
| Outlet pressure P₂ (absolute) | Units: psi, bar, kPa |
| Gas temperature | Units: K, °C, °F |
| Liquid flow rate | Units: US gpm, m³/h, L/min |
| Gas flow rate (scfh) | A number |
| Flow coefficient C_v | A number |
Example values
How the main result changes when Specific gravity changes and everything else stays at the example values above:
| Specific gravity (water = 1, air = 1 for gases) | Required C_v |
|---|---|
| 0.5 | 15.8114 |
| 0.75 | 19.3649 |
| 1 | 22.3607 |
| 1.25 | 25 |
| 1.5 | 27.3861 |
| 2 | 31.6228 |
Formula
Liquid: Q = C_v·√(ΔP/SG) (gpm, psi) · Gas, subcritical (P₂ > P₁/2): Q = 1360·C_v·√(ΔP·P₂/(SG·T))
Gas, critical: Q = 660·C_v·P₁/√(SG·T) (Q in scfh, P absolute psia, T in °R)
Worked example
Inputs — Fluid: Liquid; Find: Flow coefficient C_v; Specific gravity (water = 1, air = 1 for gases): 1; Inlet pressure P₁ (absolute): 100 psi; Outlet pressure P₂ (absolute): 80 psi; Liquid flow rate: 100 US gpm.
Results — Required C_v = 22.3607; Equivalent K_v = 19.342.
Key terms
- Choked flow
- Flow limited by sonic velocity.
Frequently asked questions
What is choked flow?
When gas reaches sonic speed in the valve, flow stops increasing as P₂ falls.
Why absolute pressure?
Gas density depends on absolute pressure.
What is scfh?
Standard cubic feet per hour at 60 °F and 14.7 psia.
How much margin should a control valve have?
Size so the valve is 60–80% open at normal flow.