from forming, or has enough alkalinity to keep the silica
soluble. no great difficulty is encountered. The amount
of soluble silica can be limited by continuous or routine
boiler blowdown to prevent buildup of excessive
Corrosion control occurs with the problem of scale
control. Boilers, feedwater heaters, and associated
piping must be protected against corrosion. Corrosion
results from water that is acidic (contains dissolved
oxygen and carbon dioxide). Corrosion is prevented by
removing these dissolved gases by deaeration of
feedwater, by neutralizing traces of dissolved gases in
effluent of the deaerating heater by use of suitable
chemicals. and by neutralizing acidity in water with an
METHODS OF TREATMENT
The specific method of chemical treatment used
varies with the type of boiler and the specific properties
of the water from which the boiler feed is derived. In
general, however, the chemical treatment of feedwater
and boiler water is divided into two broad types or
methods-external treatment and internal treatment of
makeup water for alkalinity control and for removal of
scale-forming materials and dissolved gases (oxygen and
carbon dioxide) before the water enters the boiler.
"Internal treatment" means that chemicals are put
directly into the boiler feedwater or the boiler water
inside the boiler. Frequently, both external and internal
chemical treatments are used.
External treatment. frequently followed by some
internal treatment, often provides better boiler water
conditions than internal treatment alone. However,
external treatment requires the use of considerable
equipment, such as chemical tanks, softening tanks, filters,
or beds of minerals, and the installation costs are high.
Such treatment is therefore used only when the makeup
water is so hard or so high in dissolved minerals or when
internal treatment by itself does not maintain the desired
boiler water conditions. What is the dividing line between
the hardness and the concentration of dissolved matter in
water? What factors other than the dividing line determine
the need for external treatment? These factors are the
physical makeup of the plant. the type and design of the
boilers used, the percentage of makeup water being used,
the amount of sludge the boiler can handle, the space
available, and the adaptability of the operators. Many
methods of INTERNAL TREATMENT are in use. Most
of these treatments use carefully controlled boiler water
alkalinity, an alkaline phosphate, and organic material.
One of the organic materials used is tannin. Tannin is a
boiler water sludge dispersant; that is, it makes
precipitates more fluid and prevents their jelling into
masses that are difficult to remove by blowdown.
Because of treatment costs and simplicity of chemical
concentration control, the alkaline phosphate-tannin
method of internal treatment is perhaps the most widely
used. When properly applied and controlled, this
treatment prevents formation of scale on internal boiler
surfaces and prevents corrosion of the boiler tubes and
BOILER WATER TESTING
As we have just seen. boiler water must be treated
with chemicals to prevent the formation of scale on the
internal surface of the boiler and to prevent deterioration
of the boiler metal by corrosion. Boiler water must be
tested to determine the sufficiency of chemical residuals
to maintain clean boiler surfaces. As a Utilitiesman, you
should be able to make various boiler water tests (fig.
1-28). The procedures for a few types of tests that you
may have to make is given heretests for hardness.
phosphate. tannin. caustic alkalinity (with and without
tannin), sodium sulfite, and pH. A test kit is provided for
the different tests. Each test kit contains the equipment
and materials for the specified test. If a kit is not
available, you have to use the laboratory equipment (figs.
1-29 and 1-30) provided in the boiler or water treatment
Figure 1-28.Testing boiler water is an important job.