Choosing pipe size bigger than required for a particular flow requirement leads to energy and monetary losses.
While setting up a new plant or laying a new steam line, it is a practice to factor in a safety margin that would perhaps be sufficient to accommodate any future needs. At some places this safety margin can be 100% leading to almost twice the line size being selected! While this may cater to probable future requirements, bigger line sizes have a daily expense attached to them.
Oversizing of steam lines increases the surface area of the pipe resulting in increased radiation losses and thus higher distribution loss. Thus, especially in plants which operate in batches, the large pipe size will lead to higher start up time. It is recommended to size the steam lines based on the steam flow and velocity requirements. If a line is being consciously over sized, the benefits should be balanced against these daily losses.
The steam carrying capacities (in kg/hr) for some of the typical line sizes is presented below.
The acceptable velocity of steam flow is limited to 25m/s.