Page 294 - Boiler plant and distribution system optimization manual

Chapter 15

Boiler Water Treatment

IMPURITIES IN BOILER FEEDWATER the low points of the boiler. Table 15.1shows the
CONCENTRATE IN THE BOILER effect of improper or inadequate water condi-
tioning.
All boiler water contains dissolved solids.
When the feed water is heated, it evaporates and
goes off as distilled steam leaving impurities be- CYCLES OF CONCENTRATION
hind. As more and more water is distilled in the
boiler, more feed water is added to replace it. As All of the impurities dissolved in water are
a result the amount of these solids dissolved in usually termed Total Dissolved Solids, referred
the boiler water gradually increases. After a while to as TDS. Modern methods utilize electronic in-
there is so much of these highly soluble solids in struments to measure the conductance, the op-
solution in the boiler water that it does not boil posite of resistance, of boiler water. These read-
like ordinary water, it boils more like a syrup. ings are called “mhos” or “micromhos” and can
The bubbles of steam which rise to the surface be mathematically converted to parts per million
of the water do not readily break free from the sur- with respect to sodium ions by simply using a
face. Instead big bubbles form. When they break, multiplier.
they carry with them into the steam space some One part per million (ppm) is one pound in
of the film that formed the bubble material. This a million pounds of water. Since water weighs
condition is commonly referred to as carry over. approximately 8.33 pounds per gallon, one ppm
In addition some highly soluble materials is one pound in 120 thousand gallons of water.
are changed by the high boiler temperature to If a given water had a total dissolved solids
materials of low solubility such as calcium car- of 500 ppm and we concentrated this water two
bonate which are then precipitated. Much of this times or two cycles, the TDS level would be 1,000
precipitation takes place where the boiler water ppm. At three cycles the TDS would be 1,500 ppm
is the hottest, where water is in contact with high and at four cycles 2,000 ppm and so on.
heat transfer zones. The precipitate is deposited In the case of a boiler as small as 100 horse-
on the heating surface and forms a scale build up. power, it can evaporate more than 10,000gallons
This scale is a good insulator, reducing heat of water in 24 hours. If this water had a hardness
transfer. As scale builds up, the steam and water of 340 ppm, 28 pounds of residue would be left
is unable to keep the tube metal surfaces cool and behind in the boiler every day.
it begins to overheat. This overheating destroys Energy loss through blowdown is mini-
the strength of the tubing causing tube failure. mized by maintaining the boiler water cycles of
This can occur as blistering which ruptures or concentration as close to the recommended limit
general melting, depending on the circumstanc- as possible. This can be best accomplished by au-
es. One of the purpose of a water treatment pro- tomating the continuous blowdown. F1gure 15.1
gram is to keep certain scale forming solids in shows how blowdown losses can be reduced by
solution. Other scale forming solids are turned increasing cycles of concentration of boiler wa-
into a soft fluffy precipitate and carried down to ter.

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