25 May 2005
corrosion-forming conditions and minimize the internal water treatment required to
protect the boiler system components.
External Water Treatment for High-Pressure Boilers
. The higher the
pressure of the boiler, the greater the need for high-purity feedwater. A 5-kilogram-per-
second (40,000-pound-per-hour) water tube boiler operating at 6205 kilopascals
(900 pounds per square inch gauge) requires deionized feedwater. External treatment
options include RO followed by demineralization. Guidelines for feedwater purity are
described in paragraph 3-3.4.
External Treatment for Low-Pressure Boilers
. Low-pressure boilers can
operate with simple external treatment or sometimes no external treatment at all. A
0.44-kilogram-per-second (3450-pound-per-hour) (100 horsepower) fire tube boiler
operating at less than 103 kilopascals (15 pounds per square inch gauge) may require
using only a sodium zeolite softener for water treatment. A small heating boiler that is
returning over 99% of the condensate may not require any external treatment,
particularly if the makeup water is low in hardness and the condensate is not
Internal Treatment of Boiler Water. Internal treatment of boiler water is a
process of adding chemicals to the boiler to control deposition and corrosion. Internal
water treatment, together with proper blowdown control, controls the water impurities
that have not been removed or reduced through external treatment.
Preventing Scale Formation. Internal boiler water treatment for scale
prevention can be performed using either a solubilizing chemical treatment program or a
precipitating chemical treatment program. The solubilizing treatment program uses
chemicals designed to keep scale-causing materials with hardness (mineral ions) in
solution, whereas the precipitating treatment program uses chemicals designed to react
with hardness-causing materials and precipitate them as a sludge that will not adhere to
tube surfaces. Both the solubilizing approach and the precipitating approach require
good blowdown control to keep hardness and sludge levels within chemical
. Low-pressure steam boiler systems (103
kilopascals [15 pounds per square inch gauge] and less) that use little or no makeup or
blowdown are usually not chemically treated for scale control because, due to low
makeup water demands, there is no continuous addition of scaling agents (present in
the makeup water) to the boiler. If there are high makeup requirements (over 1% per
month) or if the makeup water is over 300 ppm total hardness, a treatment program is
required to protect the boiler system.
. High-pressure boilers (103
to 6205 kilopascals
[15 to 900 pounds per square inch gauge]) must use either a precipitating-type water
treatment chemical program or a solubilizing-type chemical treatment program.