Steam pipe sizing
Steam is sized on velocity rather than on pressure drop, because the density changes by orders of magnitude with pressure: a line that is fine at 10 bar is sonic at 1 bar for the same mass flow. The specific volume therefore drives everything, and it comes from the saturated steam table rather than from the ideal gas law.
| Step | Value | Note |
|---|---|---|
| Mass flow | 2,000 kg/h | as entered |
| Pressure | 10.0 bar abs | saturation at 180 °C |
| Specific volume | 0.1944 m³/kg | saturated vapour |
| Target velocity | 25 m/s | 15–25 m/s is the usual band |
| Volume flow | 389 m³/h | mass × specific volume |
| Required bore | 74.2 mm | from A = ṁv/V |
| Nominal size | DN 80 | next standard size up |
| Actual velocity | 21.5 m/s | acceptable |
| Flash at the trap | 15.3% | dropping to 1.0 bar |
| Flash per tonne | 153 kg | recoverable |
At 10.0 bar the steam is 180 °C with a specific volume of 0.194 m³/kg, so 2,000 kg/h needs a 80 mm line to stay near 25 m/s.
A = ṁ v / V · Mass flow × specific volume ÷ velocity.
d = √(4A/π) · Then the next nominal size.
v_g(P) · From the saturated table.
(h_f1 − h_f2) / h_fg2 · Recovered at the trap.
A clear path from inputs to results.
Steam is sized on velocity rather than on pressure drop, because the density changes by orders of magnitude with pressure: a line that is fine at 10 bar is sonic at 1 bar for the same mass flow. The specific volume therefore drives everything, and it comes from the saturated steam table.
How to use
- Enter the steam flow in kg/h.
- Set the line pressure and the target velocity.
- Read the required bore and the nominal size.
- Check the actual velocity in the selected size.
The formula
Reference: IAPWS-IF97 saturated steam properties; velocity sizing practice from Spirax Sarco and Crane TP-410.
Worked example
2,000 kg/h at 10 bar
- 2,000 kg/h of saturated steam.
- 10 bar absolute, target velocity 25 m/s.
- 1
At 10 bar the saturation temperature is 179.9 °C and the specific volume is 0.1944 m³/kg.
- 2
The volume flow is 2,000 × 0.1944 = 389 m³/h.
- 3
At 25 m/s that needs a bore of 74.2 mm.
- 4
The next nominal size is DN 80, which runs at 21.5 m/s.
- 5
At 1 bar the same mass flow would need a bore more than twice as large.
74.2 mm required · DN 80 · 21.5 m/s actual.
Interpreting the result
The specific volume is the whole calculation. Because it varies as roughly 1/p, halving the pressure roughly doubles the pipe area for the same mass flow — which is why low-pressure steam mains are so much larger than the high-pressure lines that feed them.
Assumptions & limitations
- Saturated steam, dry.
- Velocity sizing, not pressure-drop sizing.
- A single nominal size with no allowance for fouling.
- Standard wall thickness and no superheat.
Frequently asked questions
What velocity should I use?
15–25 m/s for saturated steam in distribution. Above 40 m/s the line starts to erode and the noise becomes the constraint.
Does superheated steam need a bigger pipe?
Yes for the same mass flow, because its specific volume is larger. It also tolerates a higher velocity, which partly offsets it.
Why does the actual velocity differ from the target?
Because nominal sizes are discrete. The selected pipe is always at least as large as the requirement, so the actual velocity is at or below the target.
Why not size on pressure drop?
You can, and for a long main you should. Velocity sizing is the quick check that keeps erosion and noise under control.
What about wet steam?
Wetness increases the volume and carries water slugs that erode the pipe. A separator at the boiler outlet and traps at low points are the fix, not a bigger pipe.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.