Page 275 - Boiler_Operators_Handbook,_Second_Edition

260 Boiler Operator’s Handbook

have acid in it if it’s distilled water? The problem is asso- PREVENTING SCALE FORMATION
ciated with carbon dioxide gas coming from bicarbonate
ions in the water. We mentioned in testing for alkalinity Now that we’ve taken care of the corrosion prob-
that the methyl orange or methyl purple test showed lems all that’s left is preventing scale formation. Scale is
either phosphate or carbonate alkalinity and it’s the re- the result of all the rocks that water dissolved as it trav-
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sult of those ions. Bicarbonate ions (HCO ) in the water eled from the rain cloud to the makeup water piping in
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break down when the water is heated in the boiler to your plant. Once the water leaves the boiler as steam it
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form hydroxyl ions and carbon dioxide gas (HCO => leaves all those dissolved rocks behind. Frequently the
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OH + CO ) the gas leaves the boiler and travels with water has so much dissolved in it that it isn’t a matter of
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the steam. converting it to steam, all you have to do is heat it up to
Decarbonators and degassifiers mentioned earlier get scale formation. I recall one application where well
help remove the bicarbonate but, like other pretreatment water at 57°F formed scale in a heat exchanger that only
processes, they don’t always get it all. raised the temperature 6°F. Water with that kind of scale
When the condensate forms the carbon dioxide is forming property is going to plug up service water heat-
dissolved in the condensate to return to bicarbonate, ers with scale, let alone a boiler.
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leaving a hydrogen ion in the process (CO + H O => H Softeners and other forms of pretreatment can re-
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+ HCO ) It’s those hydrogen ions that do the corroding duce the amount of scale forming ions in the water by,
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of condensate lines after the carbon dioxide is dissolved with the exception of demineralizers, swapping them
again. The only effective treatment is to put something with ions that normally don’t form scale (sodium). But
in the water to raise the pH (just like we did in the boil- that doesn’t eliminate the potential for scale and some
er) but it’s not a simple matter of adding caustic soda. If of the scale forming ions always manage to sneak past
we were to add caustic soda we would have to put it in all that pretreatment. Chemicals are added to the water
at every little condensate trap in the system and then try to either convert the scale forming salts to sludge or “se-
to come up with a way of controlling it. We can’t put it in quester” (the word means to surround and isolate) them
the steam because it would be a dry chemical and plug to prevent them forming a scale.
up the steam lines. Both methods work fine as long as some water
Special chemicals called “amines” and cyclohexyl- remains to hold the sludge or sequestered ions in solu-
amine in particular will flow with the steam as a vapor tion. If all the water is boiled away to steam then the
then dissolve in the condensate along with the carbon dissolved solids that remain will appear as scale no mat-
dioxide and act to raise the pH of the condensate to pre- ter what we do. After all, when salt water is evaporated
vent the acidic corrosion. I can remember using “filming there will be crystalline salt left and it will be called scale
amines” which were supposed to coat the piping to pro- if it’s on the boiler tubes.
tect it from corrosion at a lower cost than the “neutral- There are several chemicals that will combine
izing amines” which raised the pH but most of those with the magnesium and calcium ions that tend to
chemicals were discontinued because they cause cancer. form scale and convert them to a sludge. The idea is the
Even cyclohexylamine is questionable for cancer so you sludge isn’t going to stick to the heating surfaces of the
should limit its use to what’s necessary. boiler but will settle out in the mud drum (where it can
When I was sailing we were using another wa- be removed by bottom blowoff) to eliminate the scale
ter treatment product called Hydrazine (N H ) which forming salts from the water. Sources of treatment that
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combined with oxygen to produce water and gaseous accomplished this ranged from potato peels (a source of
nitrogen. It also formed ammonia which flowed with tannin which is the actual chemical) to the many blends
the steam to dissolve in the condensate and raise the of phosphates that are in use today.
pH of the condensate. While it still may be used in some An advantage of the sludge forming treatments
plants the concern over it’s caustic properties, potential is they combine with the salts to form a solid thereby
as a carcinogen and generation of poisonous ammonia reducing the TDS of the boiler water, they don’t con-
require special handling and operations so its use is not tribute to the dissolved solids content. Disadvantages of
general. sludge forming treatments include problems handling
So, preventing corrosion is simply a matter of the sludge and problems in certain boilers where there
maintaining the pH and removing oxygen. I should add isn’t enough room in the mud drum to reduce water
that it’s also keeping oxygen out but that’s addressed in velocity to the point where the sludge can settle out. If
many other places in this book. the sludge doesn’t settle out it can be swept around by