Flow Measurement · Free Calculator
Gas Velocity and Mach Number Calculator
Find the actual velocity of a gas in a pipe from its mass or standard flow at line pressure and temperature, with the speed of sound, Mach number and momentum (ρv²) for line sizing and noise checks.
Actual velocityMach numberSpeed of soundρv²
Nm³/h or kg/h
°C
g/mol
k
Z
Velocity above about 20 m/s: check noise, pressure drop and company line sizing limits.
Actual velocity32.77 m/s
Mach number0.0939
Speed of sound349.1 m/s
ρv² (momentum)7,403 Pa (kg/m·s²)
Actual volume flow562.27 m³/h
Gas density at line conditions6.894 kg/m³
How this result was calculated
- ρ = P M ÷ (Z R T) = 6.894 kg/m³
- Qactual = ṁ ÷ ρ; v = Qactual ÷ A = 32.77 m/s
- c = √(k Z R T ÷ M) = 349.1 m/s; Mach = v ÷ c
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Gas Velocity Formulas
ρ = P × M ÷ (Z × R × T)
v = ṁ ÷ (ρ × A)
c = √(k × Z × R × T ÷ M), Mach = v ÷ c
v = ṁ ÷ (ρ × A)
c = √(k × Z × R × T ÷ M), Mach = v ÷ c
P is absolute line pressure, M the molecular weight, Z the compressibility, R = 8314 J/kmol·K, T absolute temperature, ṁ the mass flow (from normal flow × normal density) and A the pipe area. ρv² is used as a momentum limit for erosion, vibration and inlet devices.
Worked example
3000 Nm³/h of air at 6 bar a and 30 °C in a 3" Sch 40 line (77.9 mm) has an actual flow of 562 m³/h and a velocity of about 32.8 m/s, Mach ≈ 0.09.
Key insight: standard flow is constant along a gas line but actual velocity rises as pressure falls, so check velocity at the lowest pressure point, often the end of the line or downstream of a control valve.
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Frequently Asked Questions
Why calculate actual velocity?
Line sizing, noise and erosion depend on actual velocity at line conditions, not on standard flow.
What is a typical gas velocity limit?
Company standards often limit general gas lines to about 15 to 30 m/s, lower for noise sensitive areas, and check ρv².
What is the speed of sound used for?
To find the Mach number. Near Mach 1, flow chokes; above about 0.3, compressibility and noise become significant.
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