Flash Steam

Flash steam is a name given to the steam formed from hot condensate when the pressure is reduced.

Flash steam is no different from normal steam, it is just a convenient name used to explain how the steam is formed. Normal or “live” steam is produced at a boiler, steam generator, or waste heat recovery generator – whereas flash steam occurs when high pressure / high temperature condensate is exposed to a large pressure drop such as when exiting a steam trap.

High temperature condensate contains an excess of energy which prevents it from remaining in liquid form at a lower pressure. The result is that the excess energy causes a percentage of the condensate to flash [into steam].

Condensate discharged out of the orifice of a trap partially evaporates (flash evaporation) due to the pressure difference.

Flash steam occurs because the saturation point of water varies according to pressure. For example, the saturation point of water is 100 °C (212 °F) at atmospheric pressure, but is 184 °C (323 °F) at 1.0 MPaG (145 psig).

So what happens when condensate kept under pressure at 184 °C (363 °F) is released to atmosphere? The condensate contains too much energy (enthalpy) to remain entirely liquid, and a portion of it evaporates, causing the temperature of the remaining condensate to drop to the saturation temperature (i.e., 100 °C or 212 °F if discharging to atmosphere). This phenomenon is known as flash evaporation.

In other words, when hot condensate is discharged into a lower pressure environment, its enthalpy (total energy) remains the same, but its saturation point drops (the temperature at which condensate can exist in both the liquid and gaseous state). To compensate for the excess amount of energy, part of the water molecules absorb the excess energy as latent heat and evaporate to form steam.
from http://www.tlv.com/global/US/steam-theory/flash-steam.html

See Also

The Doom of Steam
Page last modified on Monday 12 of March, 2018 04:34:56 MDT

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