Both under and over sizing of steam mains has adverse impact on distribution. Long saturated steam distribution lines should be sized on pressure drop method.
The design of steam mains and the steam distribution network requires critical examination. The safety margins included should be based on engineering inputs rather than speculation.
Over sizing steam main increases the material cost and erection cost (supports, insulation, etc). Importantly oversized steam main provides a larger heat transfer surface thereby increasing radiation losses.
Under sizing steam main leads to pressure drops and could lead to steam starvation in equipments. This impacts the process quality.
For long distribution lines limiting the pressure drop is critical and hence pressure drop method is suited for sizing these lines.