Sizing Steam Pipes: Velocity First, Then Pressure Drop
A steam pipe is sized for two things: velocity, so the steam does not erode and hammer, and pressure drop, so enough pressure reaches the far end. Get it wrong one way and the process is starved and noisy; the other way and money is spent on steel and lost through extra surface.
You'll learn
- Size a steam main by velocity from mass flow and specific volume
- Check the pressure drop along the main
- Know typical design velocities
- Understand the costs of undersized and oversized pipes
Before this: Reading steam tables, Gauge vs absolute pressure
Why pipe size matters
Too small a pipe forces steam through at high speed. Fast steam whistles, erodes bends and valve seats with the water droplets it carries, and loses so much pressure on the way that the user at the end of the line gets less than it needs. Too large a pipe costs more to buy, install and insulate, holds more condensate, and loses more heat from its larger surface.
Because steam takes far less space at high pressure, distributing it at high pressure lets the pipes be smaller; pressure is reduced again close to the user.
Sizing by velocity
| Symbol | Meaning | Unit |
|---|---|---|
| Steam velocity | m/s | |
| Steam flow | kg/h | |
| Specific volume at line pressure | m³/kg | |
| Internal cross-section of the pipe | m² |
| Service | Target, m/s | Range, m/s |
|---|---|---|
| Distribution main | 30 | 15–40 |
| Short branch line | 40 | 20–50 |
| Superheated steam | 45 | 25–60 |
- Specific volume 0.1803 m³/kg; volume flow 361 m³/h. Target 30 m/s for a distribution main.
- Try standard Schedule 40 sizes from small to large; the first at or under 30 m/s is DN80 (bore 77.9 mm), at 21.0 m/s.
- Check pressure drop over 100 m: 0.29 kg/cm², about 2.6% of line pressure.
Pressure drop, and why both checks are needed
| Symbol | Meaning | Unit |
|---|---|---|
| Friction factor, from Colebrook–White for the pipe roughness | — | |
| , | Length (including fittings) and bore | m |
| Steam density, | kg/m³ |
Pressure drop rises roughly with the square of velocity, so it punishes undersizing hard. Forcing the same 2,000 kg/h through DN50 gives 46.3 m/s and a drop of 2.23 kg/cm² over 100 m — 20% of the line pressure lost before the steam reaches its user.
Velocity sizing alone can still starve a user at the end of a long main, where a comfortable velocity accumulates too much drop over hundreds of meters. Pressure-drop sizing alone can produce a pipe that runs fast and noisy on a short run. Check both, and include fittings as equivalent length.
Key takeaways
- Velocity = mass flow × specific volume ÷ area; size mains for about 25–40 m/s.
- 2,000 kg/h at 10 kg/cm²(g) needs DN80, at 21 m/s.
- Check pressure drop too: it grows with velocity squared and with length.
- Distribute at high pressure, reduce near the user; allow for growth.