Hot Tip #14 – Pipe Sizing Factors

When determining pipe size for steam systems, the following factors should be 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. Pressure drops for the total system should generally be less than 20% of the boiler operating 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.

Steam velocity: the speed of the steam flowing through the lines, in feet per minute. 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 with a maximum of 15,000 fpm.

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

Remember: When steam lines are too large, purchase, installation, and fitting costs increase substantially over the more appropriate smaller lines. Additionally, because their surface is greater, these larger lines waste more heat through radiation

(From our friends at Armstrong Traps)