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Steam Theory Vapor and Flash Steam


									Steam Theory: Vapor and Flash Steam
Difference between vapor and flash steam
Vapor: The diffused reflection of light off the small water droplets of steam in the air
appears as a white cloud. In other words, vapor is water in a liquid state floating in the

Steam: A colorless transparent gas formed when water in a liquid state is converted into
gas through the transfer of the latent heat of steam.

Thinking that vapor = steam is not a problem in normal daily life, but for industries
involving the handling of steam, it is very important to make the distinction between the

Vapor released from traps
Why is vapor released from traps? If condensate has a higher heat-content than does
saturated water at atmospheric pressure, then some of the condensate released from the
trap will turn back into steam the moment it hits the atmosphere.

Immediately this condensate-turned-steam (flash steam) condenses in the air, forming
tiny water droplets that appear to the eye to be vapor being released from the trap. So
though the amount will differ depending upon the discharge temperature, traps
discharging hot condensate will always generate some vapor.

OK, but why sometimes lots of vapor?
When large amounts of vapor are seen at the trap outlet, it is sometimes mistakenly
thought to be due to steam trap failure or steam leakage. How much flash steam
appearing as vapor is actually being released?

An example: If a trap releases 10 kg/h of condensate at 1.0 MPaG to atmosphere, the
amount of flash steam generated is roughly 1.6 kg/h. This has a volume of roughly 2.7 m³
(greater than the capacity of 2 large bathtubs). As the flash steam diffuses, it becomes
vapor and can understandably lead to the impression that there is a very large quantity of
vapor. We must remember that this is from flash steam and not from steam leakage.

Discharge from free float traps on steam mains
What looks like steam leakage even from traps that are operating normally is actually
flash steam. The state of condensate and flash steam discharge varies as condensate rates

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