Pipe Sizing Factors

When determining pipe size, the following factors are considered.

Initial steam pressure: the output pressure at the boiler (or the main for branch piping).

Allowable pressure drop: the total pressure drop allowed from the source to the end of the line. This includes all drops from line loss, elbows, valves, etc. Pressure drops are usually due to friction between the steam and piping. Pressure drops are usually measured in psi/100 ft (mm WC/m). Pressure drops for the total system should generally be less than 20% of the maximum boiler pressure.

Flow rate: the amount of steam that must be supplied to the heat exchangers connected to the steam lines. Flow rate is measured in lbs/hr (kg/h).

Steam velocity: the speed of the steam flowing through the lines, in feet per minute (m/sec). Erosion and system noise increase with velocity. In general, steam velocities in process steam systems should be maintained between 6,000 and 12,000 fpm (30 m/sec and 60 m/sec) with a maximum of 15,000 fpm (75 m/sec).

Future expansion: lines should be sized with the foreseeable future in mind. When in doubt, remember oversized lines usually present fewer problems than undersized lines.