Page 259 - Boiler_Operators_Handbook,_Second_Edition

244 Boiler Operator’s Handbook

you add chemicals to water and increase its pH from 7 piping with water that you want properly treated. An
to 8 it will take ten times as much to increase it from 8 to initial fill of a hot water boiler system can be calculated
9 and one hundred times as much to raise the pH from 9 by estimating the total length of pipe, multiplying the
to 10. pounds of water per foot from the table in Appendix D
The value of pH provides a measure of the acid- then adding that result to the number of pounds to fill
ity or alkalinity of water. When the pH is less than 7 it the boiler. To establish the initial charge of sodium ni-
is called acidic and when the pH is greater than 7 it is trite in the water (to achieve a content of 60 ppm) divide
called alkaline. Acidic water is very corrosive. Highly al- the weight of water by one million then multiply by 60.
kaline water is also very reactive, highly alkaline water The result is the number of pounds of nitrite to put in
will react violently with aluminum and generate some the chemical feed pot, sodium nitrite is 63% nitrite so
very toxic gases. Normal values of pH in a boiler plant multiply by 1.58 to determine how much of the actual
are 7 to 8 for make-up and feed water, 10 to 11 for boiler chemical to add then divide that result by the purity of
water, and 5 to 8 for condensate. Water supply plants in what your chemical supplier provides.
the United States are required by law to maintain pH in We treat water for two principal reasons, to pre-
the range of 7.6 to 8.5. vent corrosion and to prevent scale formation. The most
We measure all the other things that dissolve in common form of corrosion is destruction of metal by
water using a scale that is a lot simpler than pH. The hydrogen ions but other chemicals dissolved in water
standard units of measure are parts per million (ppm) can also attack the metal in our systems. Another form
which is a ratio, the number of pounds of material that of corrosion is oxidation, where the oxygen in the air or
would be dissolved in a million pounds of water. Some water combines with the metal to form rust. A severe
operators find it easier to think in terms of pounds per form of oxygen corrosion is oxygen pitting.
million pounds of water. Of course we don’t have to Scale formation coats the heat exchange surfaces of
have a million pounds of water to determine the ratio. the boiler to act like a heat insulator. The scale being on
Some water treatment departments will measure the inner surface of the boiler separates the water and
the concentrations of ions in solution in terms of micro- metal so the water can’t cool the metal. When enough
grams per deciliter. That value is very close to ppm so scale builds up the metal overheats and fails. The vari-
use it as such unless you’re trying to do some critical ous water treatment processes serve to prevent corro-
evaluation of your water treatment facilities. sion and scale formation by pretreatment which changes
Occasionally you will see an analysis described as the corrosive and scale forming properties of the water
“ppm as CaCO ” to describe a condition of water that before it gets to the boiler and chemical treatment which
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includes a combination of materials dissolved. Since changes the properties of the feedwater and boiler wa-
the materials have different weights, they are corrected ter.
so the analysis can be expressed as an equivalent to
calcium carbonate (CaCO ). If you should ever need to
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know the precise concentration of a substance dissolved WATER TESTING
in water there are tables of equivalents that give you a
multiplier. You shouldn’t have to be doing this though, Testing of water is required to learn what’s in the
it’s best left to the water treatment specialists. water, what other people and other systems have done,
Most of the time we don’t need to know how much and to check on the actions you have taken to maintain
of a chemical is in the water, only its proportion com- quality water for the system be it boiler, chiller, or cool-
pared to the amount of water. Therefore parts per mil- ing water. Most operators do water testing and I’ve seen
lion is an easy way to measure the chemicals dissolved variations in that activity ranging from something equal
in the water. In those rare instances when we need to to hospital grade testing to something I can describe
know how much is in the water it’s a simple calculation. only as early cave man. Before you decide to skip this
Find out how much water is in the boiler; (or whatever it part ask yourself if you’re absolutely certain you can’t
is you’re working with) if the value is gallons then mul- learn anything new about testing water.
tiply by 8.33 to convert to pounds; dividing the number The first requirement of water testing is to draw
of pounds of water by one million then multiplying by what us engineering types call a “representative sam-
ppm tells you how many pounds of chemical is in the ple.” That means the sample of water you take to the test
water. Normally this only comes up when you’re charg- bench should be the same as the water in the system you
ing a system, filling the boiler and in some cases the took the sample from. If the sample is drawn from blow-