Steam Main Capacity from Size and Velocity

The inverse of the steam-main sizer: how much steam an existing Sch 40 main carries within an allowable velocity - capacity = velocity x 60 x internal area / specific volume, the area from the Sch 40 ID. A 2 in main (ID 2.067) at a 6,000 ft/min ceiling and 13.7 ft3/lb carries 612 lb/hr; a 4 in at the same velocity carries 2,323, so a load the sizer put in 4 in fits with margin. The velocity band (supply mains ~6,000-12,000 ft/min) is a recommendation, not a code limit; the engineer of record governs.

area_ft2 = (pi/4)(ID_in/12)^2 from the Sch 40 ID; capacity_lbhr = vel_ceiling_fpm x 60 x area_ft2 / spec_vol_ft3lb.

First-principles continuity, the inverse of the steam-main sizer; the recommended velocity band (supply mains ~6,000 to 12,000 ft/min) and the saturated-steam specific volumes are from ASHRAE Fundamentals / Systems, by name. Sch 40 IDs are ASME B36.10M nominal mill dimensions.

Continuity (volumetric flow / specific volume = mass flow) is public; the steam specific volume comes from the published saturated-steam table at the line pressure.

Estimate. AHJ and licensed professional govern.

Field names used by the API: nps, spec_vol_ft3lb, vel_ceiling_fpm, capacity_lbhr, area_in2

• Continuity capacity = velocity x internal area / specific volume (the inverse of the main sizer)first-principles continuity
• Specific volume saturated-steam specific volume from the steam table at the line pressure (user-supplied for an off-table pressure)ASHRAE Fundamentals
• Pipe schedule Sch 40 nominal inside diameters per ASME B36.10M mill dimensionsASME B36.10M