Flash Steam: What It Is and How Much You Get
Condensate leaves a steam trap at the saturation temperature of the higher pressure. On the low-pressure side that water is too hot to stay liquid, so part of it boils instantly. That is flash steam: free, low-pressure steam that is either recovered or wasted.
You'll learn
- Explain why flash steam forms
- Calculate the flash fraction from steam tables
- Avoid the common error in the flash formula
- See why flash steam, not water, sizes condensate return lines
Before this: Latent heat, Saturation pressure and temperature
Why hot condensate boils
Steam condensing in a heat exchanger at 10 kg/cm²(g) leaves as water at 183 °C. That is fine while it is still at 10 kg/cm²(g). But when it passes through the trap into a return line near atmospheric pressure, water can only exist as liquid up to 100 °C. The extra heat has to go somewhere, and it goes into boiling part of the water. The steam that appears is flash steam.
The plume you see at a trap discharging to atmosphere is mostly flash steam, not a leak. A failed trap blowing live steam looks different: a sharp, clear jet close to the outlet rather than a lazy white cloud. Telling the two apart is a basic trap-survey skill.
How much flashes
| Symbol | Meaning | Unit |
|---|---|---|
| Enthalpy of condensate at the higher pressure | kJ/kg | |
| Enthalpy of saturated water at the lower pressure | kJ/kg | |
| Latent heat at the lower pressure | kJ/kg |
The numerator is the surplus heat; the denominator is what it costs to turn a kilogram of water into steam at the lower pressure.
| Condensate pressure, kg/cm²(g) | Flash, % of condensate |
|---|---|
| 2 | 6.3 |
| 3 | 8.2 |
| 5 | 11.0 |
| 7 | 13.3 |
| 10 | 15.9 |
| 14 | 18.7 |
| 17.5 | 20.7 |
Computed from IAPWS-IF97, discharge to 0 kg/cm²(g).
- at 10 kg/cm²(g) = 778.0; at atmosphere = 419.0 and = 2,256.5 kJ/kg.
- Flash fraction:
- Flash steam: 1,000 × 0.1591 = 159 kg/h.
The common error, and what flash does to return lines
Flash is small by mass and enormous by volume. In the example, 159 kg/h of flash steam at atmospheric pressure occupies about 266 m³/h, while the 841 kg/h of water left behind occupies about 0.88 m³/h. The flash is over 300 times the volume of the water. A return line sized for water alone is undersized by that much, which is why return lines downstream of traps run as two-phase flow and why they hammer and back up when sized as if they carried water.
Recovering it is often worth more than any other trap-side improvement. A flash vessel held at 1 kg/cm²(g) separates 12.4% of 10 kg/cm²(g) condensate as usable low-pressure steam for a heater, a deaerator or a tank, and sends the remaining water on to the feed tank. Module 08 covers flash vessel design and the economics.
Key takeaways
- Condensate above the saturation temperature of a lower pressure partly flashes to steam.
- Flash fraction = — latent heat in the denominator, not .
- About 16% of 10 kg/cm²(g) condensate flashes at atmospheric pressure.
- Flash is small by mass but hundreds of times the water’s volume: it sizes return lines.