Steam main air volume

A cold one-pipe steam main is full of air. End-of-main vents dump that air so steam can fill the run. Vent count is not a guess from pipe size alone — it starts from how many cubic feet of air the main actually holds, then divides by the minutes you will give it to clear.

This guide covers only the volume measurement. Capacities, ounce columns, and vent counts live in the main vent sizer and the Gorton #2 how-many guide. No CFM chart numbers are invented here.

From the boiler header out to each venting point, note every section that shares the same end-of-main tapping:

1. Nominal pipe size — the stamped or measured size (for example 2" or 2-1/2").
2. Length of that section — linear feet along the run, including offsets that still vent through the same tapping.

Split the takeoff wherever the size changes. Add only the sections that empty through the tapping you are sizing.

The calculator maps each nominal size to a Schedule 40 internal diameter (ASME B36.10M for wrought steel pipe), then computes cubic feet of air per linear foot from that ID:

Multiply by section length, then sum the sections. That sum is the air volume the vents on that tapping must clear.

Gill & Pajek publish a compact “how much air is in a pipe?” table for common mains. The Schedule 40 computation used here tracks that published table within ordinary chart rounding — use the calculator for the site numbers rather than copying a row from memory.

Main-vent sizing is volume divided by target minutes, then matched to a published vent capacity at your working ounce column:

A longer or fatter main raises cubic feet and therefore raises the required rate for the same clear time. The volume step is what makes that difference visible before you pick #2 vs #1 counts.

For the ounce columns, the ~3-minute working default, and how those feed the capacity charts, see vent ounces and minutes.