Steam Basics - What is Flash Steam?
Flash steam is vapor or secondary steam formed from hot condensate discharged
into a lower pressure area. It is caused by excessive boiling of the
condensate which contains more heat than it can hold at the lower pressure.
Flash steam occupies many times the volume of water from which it forms. For
example, flash steam created by hot condensate flowing from 15 PSIG to an
atmospheric pressure will have nearly 1,600 times the volume of the high pressure
hot water.
If the pressure is increased both the boiling point and the heat content
at boiling go up. Conversely, if the pressure of the boiling water is
reduced, the water must reduce its temperature and heat content to those corresponding
to the lower pressure. This means that a certain amount of heat must
be released, and this excess heat will be absorbed in the form of latent heat,
causing part of the water to flash into steam.
Flash steam can be valuable in some industrial processes, and some equipment
can be operated by flash steam. It may also be piped to the plants low
pressure heating system or run through a heat exchanger to provide hot water
or it may be piped into a lower pressure process. The only requirement
is that when flash steam is used in any one of these ways it must always be
at a pressure lower than the original process, for example, condensate formed
from an operation using 100 PSIG may be discharged or flashed into a 50 PSIG
operation, then the 50 PSIG condensate flashed into a 25 PSIG system, etc.
This is commonly called the “cascading of pressures.”
When condensate is discharged to atmosphere it is usually quite easy
to tell whether the steam formed is flashed steam or live steam. If a
strong jet or blast issues from the discharge line and is colorless at
first, some live steam is present, a sign of a leaking trap, but if it is all
white with no clear jet it is flash steam mixed with condensate, a normal phenomena.