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 — sized for the vessel's 1,200 kg/h datasheet rating (the calculated duty is 760 kg/h; lesson A10 explains why the datasheet legitimately says more, and lines are sized for the stated maximum). It arrives at 10.5 kg/cm²g and 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.
FAQ
Is there one right velocity for sizing steam pipes?
No — there are bands, chosen by service. Distribution mains sit at 15–25 m/s because pressure drop and erosion accumulate over long runs; short branches can afford 25–35; very wet or exhaust steam stays at 15–20 because droplets at speed shot-blast elbows. Pick the band first, then let the arithmetic pick the DN.
Why must the pipe grow after a pressure-reducing valve?
Because the same kilograms occupy far more room at lower pressure. At 10.5 kg/cm²g a kilogram of steam fills about 0.17 m³; at 3.5 kg/cm²g, 0.42 m³ — two and a half times more. Run the velocity method on both sides and the downstream pipe typically comes out two sizes bigger. A reducing station piped straight through at one size is the most commonly built mistake in Indian steam pipework.
How do I check whether an existing line is undersized?
Run the method backwards: estimate the flow, look up vg at the line pressure, divide the volumetric flow by the pipe's actual bore area, and read the velocity. Above 30 m/s on a main, expect wetness, noise and eroded elbows — put it on the audit list. The line-sizing tool does the arithmetic for you.
Is an oversized steam pipe a problem too?
Not an emergency, but a quiet tax. Below about 10 m/s the line is carrying more metal surface than the duty needs, and every square metre of that surface loses heat all shift — which is C7's business. You also paid for the extra pipe, flanges and supports once at purchase.
Why do branch connections come off the top of the main?
Because real steam is never perfectly dry: droplets condense and run along the bottom of the pipe. A branch taken from the side or bottom scoops that water straight into the equipment; a top take-off hands over the driest steam the main can offer. The droplet story is told in A4.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.