General
"Boiler water" is the term used to specifically refer to the water
which is heated inside the boiler. It is that water which is provided by
the boiler feedwater and withdrawn from the boiler as either heated
water or steam.
Aside from the BFW treatment, corrosion of boiler water systems is
further controlled by proper boiler operation and in some cases by
"internal" treatments with chemical additives which adjust pH and
control solids precipitation and sludge formation. The chemicals are fed
either to the boiler feedwater or to the boiler drum.
Water Treatment
General
The internal treatment may consist of a treatment with one or more sodium phosphate compounds (standard "phospate program" or "coordinated phosphate program"), and/or with complex-forming chemicals ("chelation program").
Alternative treatments include the so-called All Volatile Treatment
(AVT) which reduces or eliminates the formation of sludge, and which has
become common practice in nuclear power stations.
Specific Water Treatment Programs
An alkaline pH of the order of 9 to 10 (or even higher : 10.5 to
11.5) is usually desired. Specific ratios of mono- or disodiumphosphate
(NaH2PO4, Na2HPO4) to trisodiumphosphate (Na3PO4) are commonly employed
for this purpose ("coordinated phosphate program"), minimizing the
possibility of exposure to free caustic.
Chelating agents (e.g., sodium salts of either ethylene diamine
tetraacetic acid - EDTA - or nitrilotriacetic acid - NTA -) may also be
added to tie up specific cations (e.g. metal ions) in a harmless form
(i.e., dissolved complexes, which remain stable in solution in the
boiler water). The concentration of these complexes is controlled by
continuous boiler blowdown.
Related Information
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