Gas Flow Coefficient Calculator

Calculator Inputs

Calculation Mode

Pressure Unit

Temperature Unit

Flow Unit

Upstream Pressure P1 (absolute)

Downstream Pressure P2 (absolute)

Gas Temperature

Specific Gravity, Gg (air = 1)

Compressibility Factor, Z

Critical Pressure Ratio Factor, xT

Known or Target Flow

Used when sizing Cv or Kv from a required flow.

Known Coefficient Value

Used when predicting flow from a known Cv or Kv.

Example Data Table

Scenario	P1	P2	Temp	Gg	Z	xT	Known Flow	Approx. Cv	Approx. Kv
Natural gas sizing	8 bar(a)	5 bar(a)	25 °C	0.60	0.98	0.72	1500 Nm³/h	12.455	10.773
Air service sizing	6 bar(a)	4.8 bar(a)	20 °C	1.00	1.00	0.70	850 Nm³/h	11.270	9.748
Flow prediction	10 bar(a)	6 bar(a)	35 °C	0.55	0.97	0.68	Cv = 15.000	15.000	12.975

These entries are sample engineering cases for orientation. Actual valve sizing should still be checked against valve trim, noise, piping geometry, and manufacturer guidance.

Formula Used

This calculator uses a practical compressible-gas sizing relationship for valve-style flow coefficient work under standard volumetric flow conditions.

Q = 1360 × Cv × P1 × Y × √(x / (Gg × T × Z))

x = (P1 - P2) / P1

x_effective = min(x, xT)

Y = max(2/3, 1 - x_effective / (3 × xT))

Kv = 0.865 × Cv

Cv = Kv / 0.865

Where:

Q = standard gas flow rate in SCFH
Cv = U.S. style flow coefficient
Kv = metric style flow coefficient
P1 = upstream absolute pressure in psia
P2 = downstream absolute pressure in psia
Gg = specific gravity relative to air
T = absolute gas temperature in °R
Z = compressibility factor
xT = valve critical pressure ratio factor
Y = gas expansion factor

This method is excellent for quick design checks and screening studies. Final valve selection should use full manufacturer data and service-specific standards.

How to Use This Calculator

Select whether you want to size Cv, size Kv, or predict flow from a known coefficient.
Choose consistent pressure, temperature, and flow units from the dropdown lists.
Enter absolute upstream and downstream pressures.
Provide gas temperature, specific gravity, compressibility factor, and xT.
Enter either the required flow or the known coefficient, depending on your chosen mode.
Press the calculation button. The result will appear above the form, directly below the page header.
Review Cv, Kv, flow regime, expansion factor, and the Plotly performance curve.
Use the CSV or PDF buttons to save the result summary for reports or design records.

FAQs

1) What does the gas flow coefficient represent?

It represents a valve or restriction’s capacity to pass gas under a defined pressure drop and set of fluid conditions. Larger coefficients indicate higher flow capability.

2) Why must the pressures be absolute?

Compressible-gas equations depend on true thermodynamic pressure. Gauge pressure omits atmospheric pressure, which changes the pressure ratio and can distort the calculated coefficient.

3) What is choked flow in this calculator?

Choked flow occurs when the pressure drop ratio reaches the valve’s critical factor xT. Beyond that point, more downstream pressure reduction does not increase flow proportionally.

4) When should I use Cv instead of Kv?

Use Cv when your sizing references U.S. customary practice. Use Kv when your project documentation or vendor data is based on metric valve coefficients.

5) What does the compressibility factor Z do?

Z corrects ideal-gas behavior. Real gases deviate from ideal predictions at many practical pressures and temperatures, so Z helps keep the result more realistic.

6) Are SCFH and Nm³/h the same thing?

No. Both are standard volumetric flow units, but they reference different standard temperatures. This calculator converts between them for convenience.

7) Can I use this for final control valve procurement?

Use it for strong preliminary engineering, checking, and comparison. Final procurement should still verify trim style, noise, cavitation risk, piping geometry, and manufacturer sizing methods.

8) What does the graph show?

The graph plots predicted standard flow against downstream pressure while holding the gas, temperature, upstream pressure, and coefficient fixed. It helps visualize sensitivity and choking behavior.