Page 274 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 259

do the job. For protection from corrosion due to oxygen you have to be very careful of leaks in the boiler because
in the water we normally maintain a residual of 30 ppm evaporating water leaves a concentrated solution where
of sulfite. To stop hardness, a residual of 60 ppm of the pH is way too high and severe damage to the boiler
phosphate (less with chelant and polymer) is common. near the leak can result. The damage is said to be the
There’s one problem with sulfite use. When it’s result of caustic embrittlement.
done the job the sulfite ion is a sulfate ion and sulfate The other cause of corrosion in a boiler is dissolved
ions can combine with calcium and magnesium to form oxygen. We all know that oxygen in the air will combine
the hardest, toughest scale there is. Low pressure hot with iron to form rust but the conditions in a boiler are
water boiler systems and chilled water systems oc- different. Oxygen in a boiler will produce what we call
casionally use sodium nitrite for oxygen removal. The “pitting.” It looks as if some strange worm tried to eat a
mode of oxygen removal is the same as sulfite. Neither hole straight out through the metal or someone used a
the nitrite nor the sulfite produce desirable elements in poor drill on it. Oxygen pitting is usually easy to iden-
waste water so science is still looking for a better solu- tify because it happens where water is heated to free the
tion. oxygen from solution and the oxygen comes in contact
Chemicals can’t reduce the solids content of the with the metal.
boiler water; they actually increase it as we add them. Heating of boiler feed tanks and deaerators re-
Most of our water treatment chemicals are sodium move most of the oxygen but we need some chemical
based, consisting of sodium and other molecules that treatment to get the little bit that leaks through. If we
dissolve in the water. The sodium ions tend to remain don’t have a heated feed tank or deaerator we’ll need
dissolved so they are not a problem. The other ions a lot of chemical to make certain the oxygen doesn’t
from the material are what we use to treat the problems eat away our boilers. The standard chemical for steam
of corrosion and scale formation. You don’t test for so- plants and lots of hot water plants is sodium sulfite
dium, you test for the ions that do something and TDS (NaSO ) which dissolves to free sulfite ions to remove
3
which is a measure of all the ions in the water. oxygen. It takes two sulfite ions to remove a molecule of
-
-
oxygen gas (2SO + O => SO ) so it takes a while for
3 2 4
two of them to get around to ganging up on that oxygen
PREVENTING CORROSION to remove it from the water. That’s one reason we feed
the sulfite back at the boiler feed tank or deaerator, so
There are two basic ways corrosion occurs in a the sulfite has time to work.
boiler and an additional one for condensate systems Other reasons for feeding the sulfite there include
and piping. As the number of hydrogen ions in water protecting the feed system, storage tank, pumps, and
increases the pH gets lower and the free hydrogen ions piping along with any economizer we have on the
attack the metal in the boiler, changing places with the boiler. I know I’m probably going to take some heat for
iron molecules in the steel. Preventing this kind of cor- this next one, but… Many chemical salesmen try to sell
rosion is solved by adding hydroxyl ions to the water catalyzed sodium sulfite. It’s supposed to have some
to combine with the hydrogen ions, making water mol- special ingredient in it that makes it operate faster. I’m
ecules, so there are very few hydrogen ions and they sorry, but I don’t know of any chemical that will make
can’t attack the iron. The chemical normally added to ions move around in water any faster. The ions move
boiler water to raise the pH (which means fewer hydro- around and the sulfite ions will contact the oxygen in
gen ions) is sodium hydroxide (NaOH). proportion to the temperature of the water (molecules
It’s easy to envision that chemical dissolving into and ions move around faster as they’re heated) and mix-
-
+
sodium (Na ) and hydroxyl (OH ) ions in the water. ing of the water, not some additional chemical. Like the
Enough is added to keep the pH of the boiler water in guy on TV says, “don’t waste your money;” catalyzed
the range of 10 to 12. Adding too much caustic soda will sulfite will not necessarily do any better than regular
raise the pH so high that other problems, caustic em- sulfite and if you have the recommended installation of
brittlement and caustic cracking, will occur. the feeder the regular stuff has lots of time to find and
In some localities the water is already caustic so interact with those oxygen molecules.
additions of caustic soda are not required. Some of those What about that business with the condensate? I’m
actually require additional blowdown to prevent the pH sure you’ve seen many a condensate line eaten up, usu-
going too high, usually allowing it to go as high as 12. ally by a groove at the bottom of the pipe, by carbonic
When you have a problem with caustic water or high pH acid. The question has to be how can the condensate