Draining Steam Mains: Drip Legs, Traps and Warm-up Load
Every steam main makes condensate: from heat loss in service, and in a flood when it is warmed from cold. That water has to be drained at the right points, through pockets big enough to catch it, or the steam picks it up and throws it at the next bend.
You'll learn
- Explain where condensate in a main comes from
- Place drain points and size drip legs
- Calculate warm-up and running condensate loads
- Understand why warm-up load decides drainage
Before this: Sizing steam pipes, Latent heat
Where the water comes from
A steam main is hotter than the air around it, so it loses heat all the time, even when insulated. The steam inside gives up its latent heat to replace that loss, and condenses. On a cold start, far more condenses: the whole mass of steel has to be heated from ambient to steam temperature, and every kilojoule of that comes from condensing steam.
Water lying in a main is dangerous. Steam travelling at 30 m/s can pick up a slug of water and drive it into a bend or a valve like a hammer blow. The fix is to collect the water at low points and drain it away through steam traps before it can accumulate.
Drain points and drip legs
- Slope mains down in the direction of flow, so condensate runs with the steam to the drain points.
- Drain at every low point, before risers, before control valves and at the end of every main.
- On long straight runs, drain at regular intervals — commonly every 30–50 m.
- Use a drip leg (pocket) of generous diameter: a small hole in the bottom of the pipe lets most of the water rush past.
- Take the trap connection from the side of the pocket, a little above its bottom, so dirt settles below it.
Pocket size follows the main. GyanTosh’s pipe code follows EN 45510-1: On mains from DN125 to DN200 the pocket should be DN100 and at least 150 mm deep.
Warm-up load decides the drains
- Warmed from 30 °C to steam temperature (183 °C) in 20 minutes; insulated with 50 mm in service.
- Steel to heat: about 1,863 kg including fittings. Warm-up condensate: 210 kg/h during the 20 minutes.
- Running condensate once warm, insulated: 22.0 kg/h. Bare, it would be about 240 kg/h.
Drain traps sized on the running load — the figure on the heat balance — cannot clear the start-up flood. Water collects, the steam picks it up, and the main hammers every morning; the traps get blamed. Size main drains for the warm-up load at the warm-up time the plant actually uses, or warm the main more slowly with the drains open.
Key takeaways
- Mains make condensate from heat loss in service and, far more, during warm-up.
- Slope with the flow, drain every low point and riser and every 30–50 m, with full-size drip legs.
- Warm-up load on 150 m of DN80 is about 10 times the running load.
- Size main drains for warm-up, not running load.