Pipe sizing
Steam Pipe Sizing Calculator
Sizes a steam main on velocity, then checks whether the pressure still gets to the far end. It also gives the two condensate loads the line will produce — the warm-up load on startup and the running load in service — because those are what decide where the drain points go and how big the traps need to be.
Your line
The peak flow the line must carry, not the average.
Gauge pressure in the main. Higher pressure means denser steam and a smaller pipe.
Measured along the pipe.
Bends, valves and tees add resistance. As a rough guide an elbow is worth about 30 pipe diameters and a full-bore valve about 8. Leave at 0 to ignore them.
Leave on automatic, or pick a size to check one you already have.
Condensate load inputs
0 for a bare pipe.
Recommended size
DN80
28.4 m/s · 0.7 kg/cm² lost over 180 m · 6.3 kg/cm²(g) at the far end
Velocity
- Actual velocity
- 28.4 m/s
- Bore
- 77.9 mm
- One size down
- DN65
- One size up
- DN100
design range 15–40 m/s
DN80 Schedule 40
would run at 43.9 m/s
would run at 16.5 m/s
Pressure drop
- Pressure lost
- 0.7 kg/cm²
- Pressure at the far end
- 6.3 kg/cm²(g)
- Reynolds number
- 6.1e+5
- Friction factor
- 0.0180
8.7% of absolute
from 7
fully turbulent
Colebrook–White
Condensate this line will make
- Warm-up load
- 200 kg/h
- Running load
- 21 kg/h
- Warm-up is
- 9.5×
- Pipe metal to heat
- 1,863 kg
over 20 min from cold
80 W/m lost
the running load
including fittings allowance
Size the drain traps on the warm-up load, not the running load. This line needs about 3 drain points at 50 m spacing, each clearing roughly 67 kg/h on startup. A trap sized on the running load waterlogs every morning — and gets blamed for it.
With 50 mm of lagging the line loses 9.7% of what it would bare — 822 W/m becomes 80 W/m.
Drain pockets
For a DN80 main: pocket 80 mm diameter, at least 100 mm deep. On mains up to DN100 the pocket should be full bore — the same diameter as the main — and at least 100 mm deep.
Dimensions per EN 45510-1, section 10.1.14. A pocket that is too shallow lets condensate skim past at steam velocity rather than dropping in — which is why a correctly sized trap can still leave a main wet.
What a leak on this line costs
| Hole | Steam lost | Per year, continuous | As a share of this line |
|---|---|---|---|
| 1 mm | 3.8 kg/h | 33 tonnes | 0.2% |
| 3 mm | 34 kg/h | 298 tonnes | 1.7% |
| 5 mm | 94.4 kg/h | 827 tonnes | 4.7% |
| 10 mm | 377.4 kg/h | 3,306 tonnes | 18.9% |
Napier’s approximation for choked flow through an orifice. Flow scales with the area of the hole, so doubling the diameter quadruples the loss.
Laying out a new distribution system?
We fabricate and install IBR and non-IBR steam pipework, including headers, drain stations and lagging. Work out what the condensate is worth before you decide where it goes.
Talk to an engineerHow the calculation works
Two things size a steam main and they pull in opposite directions. Sizing on one alone is how plants end up with either an eroded, noisy line or a starved process 300 metres away.
- 1Velocity sets the sizeSteam volume divided by pipe area gives velocity. Above roughly 40 m/s, entrained water droplets erode bends and valve seats and the line gets noisy. Below roughly 15 m/s the pipe is needlessly large, costs more, loses more heat and holds more condensate. Distribution mains are usually designed around 30 m/s.
- 2Pressure drop checks itComputed from Darcy–Weisbach with a Colebrook–White friction factor, over the straight length plus an equivalent length for bends, valves and fittings. A pipe that is fine on velocity can still lose too much pressure over a long run — the process at the end needs a working pressure, not just steam.
- 3Warm-up loadOn startup, steam condenses to heat the pipe metal itself. This load is many times the running load and it is what a startup has to clear. Drain points sized on the running load waterlog every morning, and the trap gets blamed for it.
- 4Running loadIn service, condensate forms continuously from heat lost through the pipe wall. Computed from natural convection and radiation for a bare pipe, and from conduction through the lagging where the line is insulated — good 50 mm insulation cuts it to about a tenth.
velocity = ṁ · v_g / area · ΔP = f · (L/D) · ρv²/2
Steam capacity by pipe size
How much saturated steam each standard size carries at 30 m/s, the usual design velocity for a distribution main. Higher pressure means denser steam, so the same pipe carries more of it. Computed from IAPWS-IF97 with Schedule 40 bores.
| Size | Bore | 3.5 kg/cm²(g) | 7 kg/cm²(g) | 10.54 kg/cm²(g) | 14 kg/cm²(g) |
|---|---|---|---|---|---|
| DN15 | 15.8 mm | 50 kg/h | 86 kg/h | 122 kg/h | 157 kg/h |
| DN20 | 21.0 mm | 89 kg/h | 153 kg/h | 216 kg/h | 278 kg/h |
| DN25 | 26.6 mm | 144 kg/h | 247 kg/h | 350 kg/h | 449 kg/h |
| DN32 | 35.1 mm | 249 kg/h | 428 kg/h | 606 kg/h | 779 kg/h |
| DN40 | 40.9 mm | 340 kg/h | 583 kg/h | 826 kg/h | 1,062 kg/h |
| DN50 | 52.5 mm | 558 kg/h | 958 kg/h | 1,357 kg/h | 1,744 kg/h |
| DN65 | 62.7 mm | 796 kg/h | 1,367 kg/h | 1,935 kg/h | 2,488 kg/h |
| DN80 | 77.9 mm | 1,230 kg/h | 2,112 kg/h | 2,991 kg/h | 3,846 kg/h |
| DN100 | 102.3 mm | 2,119 kg/h | 3,637 kg/h | 5,151 kg/h | 6,623 kg/h |
| DN125 | 128.2 mm | 3,330 kg/h | 5,717 kg/h | 8,096 kg/h | 10,410 kg/h |
| DN150 | 154.1 mm | 4,810 kg/h | 8,258 kg/h | 11,694 kg/h | 15,037 kg/h |
| DN200 | 202.7 mm | 8,328 kg/h | 14,297 kg/h | 20,246 kg/h | 26,034 kg/h |
These are velocity limits only. On runs beyond roughly 100 metres, pressure drop often forces a size up beyond what this table suggests — use the calculator above to check the run you actually have.
Where drain points go
A main sized perfectly but drained badly will still hammer. Put a drain point:
- Every 30 to 50 metres along a straight horizontal run.
- At every low point, where condensate collects by gravity.
- At the bottom of every rise, before the steam has to lift it.
- Ahead of every isolating valve, control valve and reducing station.
- At every dead end and at the end of the main.
Lay the main with a fall of about 1 in 100 in the direction of steam flow, so condensate runs towards the drain points rather than away from them.
Common questions
What steam velocity should I design for?
For saturated steam distribution mains, 25–40 m/s is the normal design range and 30 m/s is a sound default. Short branch lines can run to 40–50 m/s because there is less length for pressure drop to accumulate over, and superheated steam tolerates more still because it carries no water droplets. The upper limit exists because entrained condensate travelling at steam velocity erodes bends, valve seats and thermometer pockets.
Should I size a steam pipe on velocity or on pressure drop?
Size on velocity first, then check pressure drop — that is the order practising engineers work in, because velocity is what damages the pipe and pressure drop is what starves the process. On short runs velocity almost always governs. On long runs, above roughly 100 metres, pressure drop frequently forces a size up beyond what velocity alone would suggest. Both need checking; neither alone is sufficient.
Why does my steam main hammer on startup?
Almost always because condensate is not being cleared fast enough. On startup the pipe metal is cold and steam condenses on it at many times the running rate — a 150 metre DN80 main warmed over 20 minutes produces roughly ten to fifteen times its running condensate load. If the drain points are spaced too far apart, sized on the running load, or the pockets are too shallow to catch condensate travelling at steam velocity, that water collects and gets picked up as a slug. Opening the main slowly and warming it through gently is the operational fix; more and better drain points are the engineering one.
How far apart should drain points be on a steam main?
Every 30–50 metres on a straight horizontal run, and additionally at every low point, at the bottom of every rise, ahead of every isolating or control valve, and at every dead end. The drain pocket matters as much as the spacing: on mains up to DN100 it should be full bore and at least 100 mm deep, so condensate travelling at steam velocity drops into it rather than skimming past. EN 45510-1 section 10.1.14 sets these dimensions.
How much steam does a small leak actually waste?
Far more than people expect, because the flow is choked and scales with the area of the hole. A 3 mm hole in a 7 kg/cm²(g) main loses roughly 34 kg of steam an hour — about 300 tonnes a year on continuous running. A 6 mm hole loses four times that. This is why leak surveys pay for themselves quickly, and why a visible plume at a gland or flange is worth fixing on the day it appears rather than at the next shutdown.
Is insulation worth it on a steam main?
Overwhelmingly. A bare DN80 main at 7 kg/cm²(g) loses around 900 W per metre; the same line with 50 mm of lagging loses under 100 W per metre. Over a 150 metre run that is the difference between roughly 210 kg/h and 14 kg/h of condensate formed purely by heat loss — steam you have paid to raise and are throwing away, plus the extra condensate load on every trap. Insulation on steam mains typically pays back in months.
Is this a steam pipe size calculator or a steam line size calculator?
Both — steam line sizing and pipe sizing are the same job under two names. Give it the flow, the pressure and the run length and it returns a bore on velocity, then checks the pressure drop over that length so a line that is fast enough at the header is still adequate 300 metres away.
What steam pipe dimensions does it return?
Nominal bore to ASME B36.10M Schedule 40, with the outside diameter and wall thickness it used. Mass per metre is computed from those rather than looked up, so the bore and the mass can never disagree with each other.