UFC 3-240-13FN
25 May 2005
NOTE: This calculation does not incorporate any phosphate reaction and precipitation
with hardness (see paragraph 3-2.4.3), so the practical amount of HMP needed would
be slightly higher.
3-2.4.3
Preventing Sludge Deposits. In either low-pressure or high-pressure
boilers, the feedwater hardness (dissolved minerals) can precipitate in the boiler. To
prevent the formation of adherent sludge deposits, natural or synthetic (or both) water-
soluble organic chemicals are added to the boiler water. Organic chemicals help to
create sludge by distorting the crystal structure of scale-forming compounds and
preventing the formation of scale. Properly formed and conditioned sludge is fluid
enough to be removed through bottom blowdown of the boiler mud drum. A typical
natural organic compound, quebracho tannin, has been used traditionally with
satisfactory results in military boilers, but its use is diminishing in favor of the new
synthetic, water-soluble polymers. Tannin is effective for the control and minimization of
sludge and contributes to corrosion control since it absorbs a small amount of dissolved
oxygen and helps to form a protective film on mild steel surfaces. To be effective, tannin
levels should be maintained at several hundred ppm (of active component) in the boiler
water. Polymer materials are typically maintained between 5 to 20 ppm (of active
component). The most recently developed synthetic water-soluble polymers have been
shown to be more cost-effective than tannins. For example, polyacrylates or copolymers
of methacrylate and sulfonated styrene are dispersants, sludge and scale inhibitors
commonly used for minimizing the formation of deposits in boilers. Polymer treatments
are commonly found in water treatment formulations available from vendors.
3-2.5
Corrosion in the Boiler. Corrosion within the boiler results from an
improper pH level (below 10.3), a situation that contributes to general (overall)
corrosion, and from oxygen not being removed from the feedwater, a situation that
causes pitting of the mild steel tubes and drums.
3-2.5.1
General Corrosion. "General corrosion" is a term that refers to an overall
uniform corrosion of metal surfaces. Adequate prevention of general corrosion is
achieved by maintaining a proper pH, thus allowing the formation of a protective iron
oxide coating known as magnetite. Magnetite is a self-limiting form of corrosion that
forms in the outermost surface layers of, and that adheres to, the mild steel tube
surfaces. Magnetite is composed of an iron oxide complex of Fe3O4 + FeO + Fe2O3.
3-2.5.1.1
Proper pH for Boilers up to 6205 Kilopascals. The proper pH range for
boilers up to 6205 kilopascals (900 pounds per square inch gauge) is between 10.3 and
12.0. Adjustment of the boiler water to within this pH range is achieved through cycling
up the natural alkalinity contained in the feedwater and, if necessary, adding a chemical
alkalinity agent such as sodium hydroxide (caustic soda), sodium carbonate (soda ash),
or an alkaline phosphate treatment.
3-2.5.1.2
Proper pH for Boilers Over 6205 Kilopascals. Proper pH adjustment for
boilers over 6205 kilopascals (900 pounds per square inch gauge) is achieved by using
water treatment chemicals, which are added because the demineralized feedwater is
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