Flash Steam Percentage

Condensate at 100 psig sits at 338 °F, and it can only do that under pressure. Open a trap to an atmospheric receiver and the water is suddenly 126 °F hotter than it is allowed to be, so it borrows its own excess sensible heat as latent heat and part of it boils instantly. The bookkeeping is pure energy balance: the surplus liquid enthalpy h_f1 − h_f2 divided by the latent heat available at the lower pressure. With h_f1 = 309, h_f2 = 180.2 and h_fg2 = 970.3 BTU/lb, 128.8/970.3 = 13.3% of the condensate flashes.

Thirteen percent by mass is more than a thousand percent by volume, which is why a receiver vent that "blows steam" is usually normal flash and not a failed trap — the classic misdiagnosis on a steam survey. Tell them apart by watching the plume: flash pulses with each trap discharge, live steam blows continuously. Better still, do not vent it. A flash tank recovering that steam into a low-pressure header, or a heat exchanger putting it into the makeup, pays back in months, and the same arithmetic applied to boiler blowdown is where blowdown heat recovery gets its numbers.