Page 266 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 251

is high because the filter has very tiny holes in it and use a dealkalizer but other equipment was used, and is
some of the water has to be used to constantly carry the still used today, to remove the carbon dioxide before it
dissolved stuff away (sort of like blowdown). The filter ever gets to the boiler. These are caused decarbonators
membranes, depending on their make, can be suscep- or degassifiers and consist of a tank, usually wooden or
tible to heat or certain chemicals in the water, chlorine fiberglass, with wood slats or pieces of plastic stacked
being one, so you may have to pretreat the water before inside to form what we call “fill.” Treated water is
it gets to the RO unit. Reverse osmosis performance var- dumped into the top and trickles down over the fill
ies as well, expect anything from 70% to 99% efficiency. while air is forced by a blower into the degassifier and
Note that while they eliminate ions indiscriminately up through the fill. The water has to be treated so the
they don’t get them all so boiler internal water treatment carbon dioxide gas will separate from the bicarbonate
is still needed despite what the salesman says. ion. In some plants the treatment simply consisted of
High quality RO requires wasting a considerable adding acid, usually sulfuric, to the water to lower the
amount of the water to carry off the contaminants, nomi- pH so the bicarbonate ions would break down. The
nally about 20% of the water fed to the unit. The purified other pretreatment consists of running some or all of
water is called “permeate” because it penetrated the the water through a cation unit. The hydrogen ions ex-
membranes, and is, therefore, about 80% of the makeup changed for others lowers the pH of the water. In many
water supply. Lower waste rates usually accompany demineralizers the cation and anion units are separated
lower efficiency but some can be low efficiency with by a degassifier because the bicarbonate is broken down
high waste rates. and removing it as carbon dioxide gas takes load off the
This is one piece of equipment that requires read- anion units. The carbon dioxide, now a dissolved gas, is
ing the instruction manual immediately. The mem- “stripped” from the water by the air flowing up through
branes can’t be allowed to dry out. If they sit too long the degassifier so it can’t recombine with a hydroxyl ion
without water flow there’s danger of microbiological to form a bicarbonate ion again.
(very little bugs) growth. You can’t shut it down for the A dealkalizer is an ion exchanger regerated with
summer and walk away. Feeding with a biocide (bug salt, taking on chloride ions that are exchanged for the
killer) during idle periods is required. They require bicarbonate ions.
some chemical treatment at their inlet to prevent chlo-
rine damage. Cleaning at intervals as frequent as every
month is necessary to keep the capacity up. BOILER FEED TANKS AND DEAERATORS
Finally, the membrane cartridges have to be re-
placed about every five years. Current replacement cost Boiler feed tanks with heaters and deaerators are
is about $100 for every gallon per minute capacity. other common pieces of pretreating equipment. They
Some water sources, especially those in the middle have three principal functions, removing oxygen from
of the country, have a high concentration of bicarbonate the boiler feedwater, heating, and storing boiler feedwa-
ions. The bicarbonate produces two problems for boiler ter. In the case of some deaerators the three functions are
operation. In the boiler, where the water is heated, the served by separate tanks, a deaerator and separate stor-
bicarbonate breaks down to form carbon dioxide gas age tank. Both systems remove air from the water but
and hydroxyl ions. That raises the pH and alkalinity of there are variations in equipment construction and dif-
the boiler water, frequently so much so that blowdown ferences in how much air is removed. Neither removes
is based on alkalinity, not dissolved solids. oxygen completely. A boiler feed tank can only remove
The carbon dioxide that evaporates in the boiler oxygen to small values. Deaerators, operated properly,
flows with the steam to the steam users where it is ab- will remove oxygen to minimal amounts.
sorbed in the condensate that forms. Each molecule of Removal of the oxygen is achieved by raising the
carbon dioxide dissolved into the water produces a bi- temperature of the water. As the water temperature
carbonate ion by combining with a hydroxyl ion. When approaches the boiling point the amount of oxygen the
it obtains the hydroxyl ion another molecule of water is water can hold decreases. Heating the water to 180°F
dissolved to replace the hydroxyl ion and increase the reduces the maximum oxygen absorption to less than
number of hydrogen ions in the water. The result is con- 2 ppm. Raising the temperature to boiling reduces that
densate with a very low pH and corrosion of the piping to 0.007 ppm. When the water is ready to boil every
and other parts of the condensate system. molecule of water is prepared to change to steam so the
The best approach for high bicarbonates today is to water has very little ability to hold dissolved oxygen.