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Flash Steam Percentage

Also known as flash steam loss

%F=hf1−hf2hfg2×100\%F = \frac{h_{f1} - h_{f2}}{h_{fg2}} \times 100

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Worked example: 100 psig condensate to atmosphere → 13.27% flash. Press Try an example to run it live, then adjust anything.

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Flash Steam Mass Rate
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See also

Steam Quality from Enthalpy: Both use liquid enthalpy at high pressure and latent heat at low pressure.Flash Steam Mass Rate: Both use flash steam percentage.Jakob Number: Both use latent heat at low pressure.Rankine Cycle Thermal Efficiency: Both use liquid enthalpy at high pressure.Steam Coil Condensate Load: Both use latent heat at low pressure.Wet Steam Enthalpy from h_f and h_g: Both use liquid enthalpy at high pressure.
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Flash Steam Percentage explained

hf1%Fhfg2hf2

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 hf1−hf2h_{f1} - h_{f2} divided by the latent heat available at the lower pressure. With hf1=309h_{f1} = 309, hf2=180.2h_{f2} = 180.2 and hfg2=970.3h_{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.

Flash Steam Percentage formula

%F=hf1−hf2hfg2×100\%F = \frac{h_{f1} - h_{f2}}{h_{fg2}} \times 100
Where
  • %F\%F= Flash steam percentage (%)
  • hf1h_{f1}= Liquid enthalpy at high pressure (J/kg)
  • hf2h_{f2}= Liquid enthalpy at low pressure (J/kg)
  • hfg2h_{fg2}= Latent heat at low pressure (J/kg)
Where this comes from

Worked from first principles: a definition, a balance or unit arithmetic, so it needs no outside source.

In this toolkit

The boiler water balance

8 formulasBoiler cycles, blowdown percent and rate, condensate return, makeup, flash steam and boiler horsepower — the steam side's water balance.

Revised 16 October 2026
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formula.expert. (2026, October 16). Flash Steam Percentage Formula & Calculator. https://formula.expert/solvers/flash-steam-percentage

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