Page 270 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 255

working. The saturation pressure has to be above atmo- point of energy waste and water treatment. Read the
spheric or there’s no pressure to push the air and gases portion on water in the section on consumables for
out. I’ve found at least three plants that were operating further information on this subject. That section was
in the 180° to 190°F range and thought there system was concerned with wasting water, now we’re going to talk
working just fine. So did their sulfite salesman! about wasting some of it to maintain boiler water qual-
Deaeration, getting the oxygen out of the water ity. Blowdown, and I do mean continuous blowdown or
before it gets to the boiler is principally done to reduce so-called surface blowdown on steam systems and low
the cost of chemically treating the water to remove the point blowdown on hydronic systems, is used to reduce
oxygen. If the oxygen isn’t removed it will create pits in the concentration of solids dissolved in the water.
the boiler metal, something that looks almost as if it was Even if we have demineralizers or are using dis-
done with an electric drill. Oxygen pitting can destroy tilled water for makeup we will still get a growing con-
a boiler in short order so the sulfite is always added to centration of solids in the water in the system. Some will
remove the little bit of oxygen that slips past a deaerator come from corrosion of piping and other parts of the
even when it’s working fine. In order for the sulfite to be system by our condensate. Even in tight hydronic sys-
effective and remove the oxygen that gets past the de- tems we’ll get increasing solids from gradual dissolving
aerator and before it gets to the boiler the sulfite should of materials left in the system during construction and
be added to the deaerator storage section. minor vapor leaks that aren’t always apparent. In steam
Sulfite generates sulfate ions when it reacts with boilers all the solids remain in the boiler water, concen-
the oxygen in the water and, since sulfate salts form the trating there as the water leaves the boiler as steam. If
hardest scale, you don’t want to put in any more than some of the solids carry over with water droplets in the
absolutely necessary so maintaining proper operation of steam they’re returned in the high pressure condensate
the deaerator is important. so the boiler is where all the solids end up.
If your plant happens to be one of those where the The amount of solids and some liquids dissolved
sulfite is added before the deaerator you should change in water has an effect on the surface tension of the water.
that so it gives the deaerator something to do. The sulfite There are two sticky properties of fluids, cohesion and
salesman won’t be happy but your boss will be. I prefer adhesion. Cohesion is a measure of how the material
to see the sulfite fed right below where the water drops sticks to itself. Adhesion is a measure of how much the
from the deaerator (but below the low water line) so it material sticks to something else. Water is high in both.
can start doing its job immediately. You’ll notice that water actually climbs the sides of a
Since the first edition I’ve encountered a problem glass because it adheres to the glass. High cohesion is
with corrosion of a deaerator associated with the injec- evident at the surface of water where it sticks to itself.
tion of sulfite. Apparently sodium sulfite solution is When separated from a large body of water a small
corrosive when concentrated and heated and it will pro- droplet becomes perfectly round because of the high
duce a pattern of corrosion on the bottom of the deaera- cohesion at its surface, what we call surface tension.
tor storage section that looked like it was sandblasted The combination of adhesion and cohesion con-
unevenly if the sulfite isn’t distributed properly. Having tributes to the capillary action of water. It will literally
stainless steel sulfite distribution piping in the storage pull itself up into narrow spaces after adhering to the
tank with a long row of perforations to distribute the surrounding walls then reach out again. It’s what makes
chemical just below the low level should prevent that water flow up those three hundred feet high redwood
kind of damage. trees in California.
Hot water boiler plants don’t normally have the As the quantity of dissolved solids increases the
experience of constant makeup. Many of them are physical characteristics of the water change, increasing
treated with sodium nitrite. The nitrite ion converts to the surface tension of the water until eventually the wa-
nitrate, absorbing oxygen in the process. It’s only usable ter starts to foam and carry over into the steam piping.
at the low pressure hot water heating temperatures. While this is one way to get the solids out of the boiler it
doesn’t do the steam piping a lot of good. Increasing sol-
ids can also result in saturation of the water with solids
BLOWDOWN in the risers so some of the dissolved materials drop out
as scale.
I do hope you know the difference between blow- We need a way to limit the concentration of solids
down and blowoff. It’s rather important from the stand- in the boiler water to a value just below that point of