UFC 3-240-13FN
25 May 2005
3-2.4.1.2.1 Precipitating Water Treatment Programs. A precipitating program often
uses phosphate, which will react with calcium to form a calcium phosphate sludge called
"hydroxyapatite" (3Ca3[PO4]2 Ca[OH]2), and act as both as a sludge conditioner or
dispersant. Magnesium reacts with hydroxide and silica to form a magnesium silicate
sludge called "serpentine" (2MgSiO3 Mg[OH]2 H20). Sufficient hydroxyl alkalinity
(causticity) is required to allow the formation of the proper types of sludge. Sludge that
is properly formed and conditioned is fluid enough to be removed through bottom
blowdown of the boiler. Sodium hydroxide may be required to maintain the appropriate
hydroxyl alkalinity (causticity). Phosphate precipitating agents are prepared as either
orthophosphate ("ortho" means single phosphate) or as one of several forms of
polyphosphate ("poly" means multiple phosphate molecules bonded together). Sodium
hexametaphosphate, sodium tripolyphosphate, and sodium pyrophosphate are
examples of polyphosphates. In the boiler, polyphosphate breaks down to the
orthophosphate form due to the higher temperature. Polyphosphates can be added
either to the feedwater or directly to the steam drum. Orthophosphate should only be
fed into the steam drum, not to the feedwater, since it can react with the hardness
(mineral ions) to form scale in the feedwater lines.
3-2.4.1.2.2 Solubilizing Water Treatment Programs. A solubilizing program uses
one or more of the following chemicals to keep scale-forming materials in solution:
phosphonates (also known as organic phosphate), chelants, or polymers. Solubilizing
agents, such as phosphonates (not to be confused with phosphate), chelants, and
polymers are common chemicals used in boiler water treatment formulations. Chelants,
such as ethylene diamine tetra-acetic acid (EDTA) or nitrilo-triacetic acid (NTA), bind
(chelate) calcium, magnesium, and iron. Using phosphonates and chelants requires
special injection systems and excellent feed control. Excess (free) phosphonate or
chelant can attack metal surfaces, causing corrosion, although phosphonates are less
aggressive than chelants. Water-treating polymers are long-chain, water-soluble
compounds containing electrochemically active side branches that perform various
functions such as solubilization, dispersion, agglomeration, and crystal distortion to
prevent boiler deposits. Polymers do not attack metal surfaces and can be fed into the
feedwater line or directly into the boiler steam drum with proper injection equipment.
3-2.4.1.2.3 Boiler-Specific Treatment Programs. High-pressure boilers over 6205
kilopascals (900 pounds per square inch gauge) use high-quality feedwater that is
produced by extensive external treatment; thus, scale-forming materials are not
normally present in the water and cannot form scale in the boiler. These boilers often
use water treatment programs designed specifically for that particular boiler. These
programs require excellent control of water chemistry and involve applying coordinated
phosphate, congruent phosphate, equilibrium phosphate, and all volatile treatment.
Boiler pressures of this level are usually found in operations used to drive turbines for
the generation of electrical power.
3-2.4.2
Determining Chemical Treatment Feed Rates. Development of a
chemical treatment program involves selecting the type of chemical to be used,
selecting appropriate concentration levels for each chemical (when used in combination
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