Steam Main Size from Flow and Velocity

Required area = (lb/hr x specific volume) / (velocity x 60), then the smallest Sch 40 nominal whose ID clears it and the actual velocity in that size. The volume a pound of steam occupies collapses as pressure rises, so the same flow needs very different pipe at 5 vs 100 psig. The velocity band (supply mains ~6,000-12,000 ft/min) is a recommendation, not a code limit; the engineer of record governs.

req_area_ft2 = (steam_flow_lbhr x spec_vol_ft3lb) / (vel_ceiling_fpm x 60); req_dia_in = sqrt(4 x req_area_ft2 / pi) x 12; then the smallest Sch 40 nominal whose ID >= req_dia_in, and actual_fpm = (flow x spec_vol) / (chosen_area_ft2 x 60).

First-principles continuity; 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 (mass flow x specific volume = volumetric 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: steam_flow_lbhr, spec_vol_ft3lb, vel_ceiling_fpm, req_area_in2, chosen_nps, actual_fpm

• Specific volume saturated-steam specific volume from the steam table at the line pressure (user-supplied for an off-table pressure)ASHRAE Fundamentals
• Velocity band supply mains ~6,000 to 12,000 ft/min is a recommendation, not a code limitASHRAE Systems
• Pipe schedule Sch 40 nominal inside diameters per ASME B36.10M mill dimensionsASME B36.10M