Steam Mains should be engineered to facilitate flow of condensate by gravity to the steam trap. This ensures adequate removal of condensate.
As steam looses heat it turns back into water. Steam inevitably begins to do this as soon as it leaves the boiler due to heat loss via radiation.
If condensate is allowed to accumulate, the overall effective cross sectional area of the pipe also reduces. This causes steam velocity to increase above the recommended limits. The mixture of steam and condensate at high velocities is damaging as it leads to erosion and water hammer. Thus the steam distribution mains should be engineered to have a slope of 1m in every 100m. This ensures that the condensate generated flows by gravity itself to the steam traps, thus avoiding condensate accumulation.
Condensate removal should be via drain pockets. In drain pockets steam trap lines should be taken from a point above the bottom of the pocket. This allows space for any dirt and scale to settle, preventing it from choking the trap.
In normal operation, steam carries condensate while flowing along the mains with speeds up to 25m/s (i.e. 90km/hr). Slugs of condensate get carried along the pipe length and collide with pipe walls at bends or with pipe accessories eroding them.