Condensate Return Line Size From the Flash Steam
The return line size from the FLASH steam that re-boils off the condensate at the lower return pressure, not the liquid: flash = load x flash fraction, volumetric flow = flash x specific volume / 60, required area = flow / a return-velocity ceiling, then the smallest Sch 40 nominal that carries it (800 lb/hr at 13% flash to a 0 psig return -> 1-1/2 in). A return sized for the gallons of water floods and water-hammers; pair the flash fraction with flash-steam-pct. The lower velocity ceiling (~4,000 to 5,000 ft/min) and a wet, dry, or vacuum return each change the design; the engineer of record governs the return scheme.
Formula and source
flash_lbhr = condensate_lbhr x flash_fraction; vol_cfm = flash_lbhr x spec_vol_ft3lb / 60; req_area_ft2 = vol_cfm / vel_ceiling_fpm; req_dia_in = sqrt(4 x req_area_ft2 / pi) x 12; then the smallest Sch 40 nominal whose ID >= req_dia_in.
First-principles continuity on the FLASH steam; the return-velocity ceiling (~4,000 to 5,000 ft/min, lower than a supply main) per ASHRAE / Spirax Sarco return-sizing practice, by name. Sch 40 IDs are ASME B36.10M nominal mill dimensions.
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