What is flash steam and how is it produced?

Flash steam: When the condensate in the equipment and pipelines is discharged to the atmosphere or low-pressure area through the steam trap, the pressure suddenly drops and its boiling point also drops. Therefore, part of the condensate evaporates again due to the pressure reduction effect. The steam generated in this way is called secondary steam.

How is flash steam generated? Flash steam is normally produced through two channels: from the condensate collection tank (including the flash tank) for recovery, and from the back of the steam trap for venting to the air. As long as there’s no water accumulation in the upstream equipment and pipes, the upstream temperature of the condensate passing through the trap is usually high enough, definitely higher than the temperature of the downstream low-pressure water, thus generating flash steam. However, without any external heat during this entire process, how does this condensate become steam?

Condensate at 5 barg pressure has a saturation temperature of 159°C and an energy content of 671 kJ/kg. After passing through the steam trap, the pressure is 0 barg. According to the first law of thermodynamics, the energy content of the condensate on the low-pressure side must be the same as that on the high-pressure side, obeying the law of conservation of energy. Therefore, the energy content of the condensate on the low-pressure side is also 671 kJ/kg. However, water at 0 barg can contain a maximum of 419 kJ/kg of heat, resulting in an energy imbalance between the low-pressure side and the high-pressure side: 671 – 419 = 252 kJ/kg. This creates superheating for the water. This excess heat causes some of the condensate to boil, forming flash steam. This boiling process is called flash evaporation. Therefore, the energy originally in the high-pressure condensate is now in the water-steam mixture on the low-pressure side.

Another example:

Condensate at 5 barg pressure, but at a temperature of only 90°C, is unsaturated water and has an energy of only 377 kJ/kg, which is less than the energy of saturated water at atmospheric pressure. This means there is no excess energy to generate flash steam. After passing through the steam trap, the pressure is merely reduced, but no flash steam is generated. For example, there is no flash steam after passing through a thermostatic steam trap.

The steam pressure corresponding to 90°C is 0.7 bar. If the pressure on the low-pressure side falls below this absolute pressure (vacuum), flash steam will still be generated. In other words, when hot condensate is discharged into a low-pressure environment, its enthalpy (total heat) remains unchanged, but its saturation temperature (the temperature at which both gaseous and liquid phases of water coexist) decreases. To maintain the total heat, some of the water absorbs heat and evaporates into steam.