Flash Steam Mass Rate

The companion arithmetic to the flash percentage, and the step that turns a percentage into something you can size a vent or a flash vessel on. Condensate at 100 psig leaving a trap into an atmospheric receiver flashes about 13.3% of its own mass, so a trap passing 2,000 lb/h of condensate releases 266 lb/h of flash steam and drops 1,734 lb/h of hot water into the receiver. In metric, 1,000 kg/h of condensate at the same conditions gives 133 kg/h of flash and 867 kg/h of water.

The mass split looks modest and the volume split does not, which is why this calculation matters more than it first appears. At atmospheric pressure that flash steam occupies 1.673 m³/kg while the condensate beside it occupies 0.00104, so the 13.3% by mass is more than 1,600 times the volume of the water it came out of. A return line sized on the condensate alone will be choked by its own flash, back-pressure the traps and stall the equipment upstream, and this is the classic reason a heat exchanger will not hold temperature on a cold morning. Two habits. Use it to argue for recovery rather than venting, since a flash vessel feeding a low-pressure header, or a heat exchanger putting that energy into makeup water, usually pays back inside a year. And use it as a diagnostic: a receiver vent plume that pulses with each trap discharge is honest flash, while one that blows steadily is a failed trap passing live steam, and running this calculation tells you which plume the numbers say you should be seeing.