Page 297 - Boiler plant and distribution system optimization manual

282 Boiler Plant and Distribution System Optimization Manual

remove dissolved gases from the boiler feed wa- widely. A low pressure firetube boiler can usually
ter. They are effective and oxygen can be reduced tolerate higher feedwater hardness, with proper
to trace levels; about .005 ppm. While deareators chemical treatment, while virtually all impurities
are efficient, traces of oxygen can cause a signifi- must be removed from the water used with most
cant amount of corrosion, so chemical treatment modern high pressure watertube boilers.
is also used.
While the deareator removes carbon dioxide
from the feed water, bicarbonate and carbonate WATER CARRYOVER IN STEAM
alkalinity in the boiler will produce addition-
al carbon dioxide. This will require some water The water evaporated to produce steam
treatment in the boiler. should not contain any contaminating materials,
A deareator usually consists of a heating and however there will be water droplets carried into
deareating section. The storage section of these steam due to several processes.
units are often designed to hold about 10 minutes
of rated capacity of boiler feed water. MIST CARRYOVER
The water enters the deareator and is broken A fine mist is developed as water boils. This
into a spray or mist and scrubbed with steam to process is illustrated in Figure 15.2. A bubble of
force out the dissolved gasses. water vapor (steam) reaches the water surface
Steam and noncondensibles into the vent and bursts, leaving a dent in the water. Liquid
condensing section where the steam is condensed. collapses in on the dent, with the center rising at
The released gases are discharged to atmosphere a faster rate than the edges. This results in a small
through the vent outlet. droplets being tossed free of the boiler water sur-
face. These droplets form a fine mist. This mist is
Continuous Blowdown Heat Recovery removed to a great extent in the dry portion of the
The continuous blowdown, sometimes boiler. However, any mist that remains entrained
called the surface or skimmer blowdown, is most in the steam will have the same level of contami-
effective in controlling the concentration in boiler nation as the boiler water.
water. Where continuous blowdown systems are
used, the bottom blowdown is used for removal Foaming Carryover
of precipitated impurities, especially those which The alkalinity, TDS and suspended solids can
tend to settle in the lower parts of the boiler. interact to create a foam in the boiler. A light foam
Heat exchangers can be used with the contin-
uous blowdown to recover energy from the boiler
water which is being expelled from the boiler.

How pure must feedwater be?
Feedwater purity is a matter of both quan-
tity of impurities and the nature of the impuri-
ties. Some impurities such as hardness, iron and
silica for example are more concern than sodium
salts. The purity requirements for any feedwater
depend on how much feedwater is used as well
as what the particular boiler design is. Pressure,
heat transfer rate and operating equipment on
the system such as turbines have a lot to do with
Figure 15.2—Mist formation in boiler water with
feedwater purity.
high impurity levels. The volume above the water is
Feedwater purity requirements can vary
steam, steam bubble is shown for illustration.