An undersized steam main collects its tax forever: pressure drop, wet steam, noise, erosion, a process that never quite reaches temperature at peak. An oversized one taxes you once at purchase and then quietly every hour through its bigger heat-losing surface. Between the two sits a thirty-second calculation this lesson teaches you to do — and to check on any existing line.
Steam is bought by mass, but flows by volume
The boiler raises kilograms; the pipe carries cubic metres. The exchange rate between them is the specific volume vg from the steam table — and it changes violently with pressure. At the practice plant's 10.5 kg/cm²g, a kilogram of steam occupies 0.17 m³; at 3.5 kg/cm²g, 0.42 m³ — two and a half times more room for the same mass. Hold that thought; it is about to size two very different pipes.
The velocity method
Divide the volumetric flow by a sensible velocity and you have the pipe area:
Everything hangs on choosing the velocity. The working bands, for saturated steam:
| Service | Velocity band | Why |
|---|---|---|
| Distribution mains | 15–25 m/s | Pressure drop and erosion stay civil over long runs |
| Short branches to equipment | 25–35 m/s | Short runs can afford the pressure drop |
| Very wet or exhaust steam | 15–20 m/s | Droplets at high speed are shot-blasting; slower is kinder |
Faster than the band and the pipe announces itself: droplet erosion of elbows and valve seats, roaring, and pressure drop stealing the temperature you paid for. On long mains (hundreds of metres), velocity alone is not enough — check the pressure drop too; that is Tool T03's job.
The boiler main — 6,000 kg/h at 10.5 kg/cm²g (vg ≈ 0.173 m³/kg):
The vessel line across its PRS — 1,200 kg/h arrives at 10.5 kg/cm²g, leaves the reducing valve at 3.5:
That downstream growth is the most commonly built mistake in Indian steam pipework: a reducing station piped the same size straight through. The low-pressure side must grow — the steam table says so.
Reading an existing line
The method runs backwards just as well. Measure or estimate the flow, look up vg at the line pressure, divide by the pipe's actual area, and you have its velocity. Above 30 m/s on a main: expect wetness and erosion trouble, and put the line on the audit list. Below 10 m/s: the line is oversized — no emergency, but its surface is losing more heat than the duty needs, which is the insulation lesson's business.
- Size on the peak sustained flow, not the average — and state the design margin once, deliberately, instead of adding "a bit" at every step of the calculation.
- Every pressure-reducing valve: check the downstream pipe grew. If it didn't, the calculation above tells you what it should be.
- Branch connections come off the TOP of the main — droplets run along the bottom (the phase-diagram lesson explains why there are always droplets).
- A singing or roaring line is telling you its velocity. Believe it before the elbows wear through.
- kg/h × vg ÷ 3600 = m³/s; divide by velocity; round up to standard DN.
- Mains 15–25 m/s; short branches up to 35; wet steam slow.
- Lower pressure = fatter pipe. After every PRS the pipe grows — usually two sizes.
- Long runs: velocity sizes the pipe, pressure drop confirms it.
Why does a "100 mm" pipe have a 102 mm bore, while a "25 mm" pipe's bore is 26.6? DN numbers are labels, not measurements — inherited from a century of wrought-iron pipe standards where the outside diameter was fixed so that threads and fittings interchanged, and the bore was whatever the wall thickness of the day left over. The walls changed; the labels stuck. Always size with the actual bore of the schedule you are buying — never the label.