SOLUBILITY OF MINERAL SALTS IN WATER (SPARKS)
IN GRAINS PER U. S. GALLON (58,381 GRAINS), EXCEPT AS NOTED
+------------------------------+------------+-------------+
|Temperature Degrees Fahrenheit| 60 Degrees | 212 Degrees |
+------------------------------+------------+-------------+
|Calcium Carbonate | 2.5 | 1.5 |
|Calcium Sulphate | 140.0 | 125.0 |
|Magnesium Carbonate | 1.0 | 1.8 |
|Magnesium Sulphate | 3.0 pounds | 12.0 pounds |
|Sodium Chloride | 3.5 pounds | 4.0 pounds |
|Sodium Sulphate | 1.1 pounds | 5.0 pounds |
+------------------------------+------------+-------------+
CALCIUM SULPHATE AT TEMPERATURE ABOVE
212 DEGREES (CHRISTIE)
+------------------------------+----+----+-------+----+---+
|Temperature degrees Fahrenheit|284 |329 |347-365| 464|482|
|Corresponding gauge pressure | 38 | 87 |115-149| 469|561|
|Grains per gallon |45.5|32.7| 15.7 |10.5|9.3|
+------------------------------+----+----+-------+----+---+
Table 15 gives the solubility of these mineral salts in water at various
temperatures in grains per U. S. gallon (58,381 grains). It will be seen
from this table that the carbonates of lime and magnesium are not
soluble above 212 degrees, and calcium sulphate while somewhat insoluble
above 212 degrees becomes more greatly so as the temperature increases.
Scale is also formed by the settling of mud and sediment carried in
suspension in water. This may bake or be cemented to a hard scale when
mixed with other scale-forming ingredients.
Corrosion--Corrosion, or a chemical action leading to the actual
destruction of the boiler metal, is due to the solvent or oxidizing
properties of the feed water. It results from the presence of acid,
either free or developed[15] in the feed, the admixture of air with the
feed water, or as a result of a galvanic action. In boilers it takes
several forms:
1st. Pitting, which consists of isolated spots of active corrosion which
does not attack the boiler as a whole.
2nd. General corrosion, produced by naturally acid waters and where the
amount is so even and continuous that no accurate estimate of the metal
eaten away may be made.
3rd. Grooving, which, while largely a mechanical action which may occur
in neutral waters, is intensified by acidity.
Foaming--This phenomenon, which ordinarily occurs with waters
contaminated with sewage or organic growths, is due to the fact that the
suspended particles collect on the surface of the water in the boiler
and render difficult the liberation of steam bubbles arising to that
surface. It sometimes occurs with water containing carbonates in
solution in which a light flocculent precipitate will be formed on the
surface of the water. Again, it is the result of an excess of sodium
carbonate used in treatment for some other difficulty where animal or
vegetable oil finds its way into the boiler.
Priming--Priming, or the passing off of steam from a boiler in belches,
is caused by the concentration of sodium carbonate, sodium sulphate or
sodium chloride in solution. Sodium sulphate is found in many southern
waters and also where calcium or magnesium sulphate is precipitated with
soda ash.
Treatment of Feed Water--For scale formation. The treatment of feed
water, carrying scale-forming ingredients, is along two main lines: 1st,
by chemical means by which such impurities as are carried by the water
are caused to precipitate; and 2nd, by the means of heat, which results
in the reduction of the power of water to hold certain salts in
solution. The latter method alone is sufficient in the case of certain
temporarily hard waters, but the heat treatment, in general, is used in
connection with a chemical treatment to assist the latter.
Before going further into detail as to the treatment of water, it may be
well to define certain terms used.
_Hardness_, which is the most widely known evidence of the presence in
water of scale-forming matter, is that quality, the variation of which
makes it more difficult to obtain a lather or suds from soap in one
water than in another. This action is made use of in the soap test for
hardness described later. Hardness is ordinarily classed as either
temporary or permanent. Temporarily hard waters are those containing
carbonates of lime and magnesium, which may be precipitated by boiling
at 212 degrees and which, if they contain no other scale-forming
ingredients, become "soft" under such treatment. Permanently hard waters
are those containing mainly calcium sulphate, which is only precipitated
at the high temperatures found in the boiler itself, 300 degrees
Fahrenheit or more. The scale of hardness is an arbitrary one, based on
the number of grains of solids per gallon and waters may be classed on
such a basis as follows: 1-10 grain per gallon, soft water; 10-20 grain
per gallon, moderately hard water; above 25 grains per gallon, very hard
water.
_Alkalinity_ is a general term used for waters containing compounds with
the power of neutralizing acids.
_Causticity_, as used in water treatment, is a term coined by A. McGill,
indicating the presence of an excess of lime added during treatment.
Though such presence would also indicate alkalinity, the term is
arbitrarily used to apply to those hydrates whose presence is indicated
by phenolphthalein.
Of the chemical methods of water treatment, there are three general
processes:
1st. Lime Process. The lime process is used for waters containing
bicarbonates of lime and magnesia. Slacked lime in solution, as lime
water, is the reagent used. This combines with the carbonic acid which
is present, either free or as carbonates, to form an insoluble
monocarbonate of lime. The soluble bicarbonates of lime and magnesia,
losing their carbonic acid, thereby become insoluble and precipitate.
2nd. Soda Process. The soda process is used for waters containing
sulphates of lime and magnesia. Carbonate of soda and hydrate of soda
(caustic soda) are used either alone or together as the reagents.
Carbonate of soda, added to water containing little or no carbonic acid
or bicarbonates, decomposes the sulphates to form insoluble carbonate of
lime or magnesia which precipitate, the neutral soda remaining in
solution. If free carbonic acid or bicarbonates are present, bicarbonate
of lime is formed and remains in solution, though under the action of
heat, the carbon dioxide will be driven off and insoluble monocarbonates
will be formed. Caustic soda used in this process causes a more
energetic action, it being presumed that the caustic soda absorbs the
carbonic acid, becomes carbonate of soda and acts as above.
3rd. Lime and Soda Process. This process, which is the combination of
the first two, is by far the most generally used in water purification.
Such a method is used where sulphates of lime and magnesia are contained
in the water, together with such quantity of carbonic acid or
bicarbonates as to impair the action of the soda. Sufficient soda is
used to break down the sulphates of lime and magnesia and as much lime
added as is required to absorb the carbonic acid not taken up in the
soda reaction.
All of the apparatus for effecting such treatment of feed waters is
approximately the same in its chemical action, the numerous systems
differing in the methods of introduction and handling of the reagents.
The methods of testing water treated by an apparatus of this description
follow.
When properly treated, alkalinity, hardness and causticity should be in
the approximate relation of 6, 5 and 4. When too much lime is used in
the treatment, the causticity in the purified water, as indicated by the
acid test, will be nearly equal to the alkalinity. If too little lime is
used, the causticity will fall to approximately half the alkalinity. The
hardness should not be in excess of two points less than the alkalinity.
Where too great a quantity of soda is used, the hardness is lowered and
the alkalinity raised. If too little soda, the hardness is raised and
the alkalinity lowered.
Alkalinity and causticity are tested with a standard solution of
sulphuric acid. A standard soap solution is used for testing for
hardness and a silver nitrate solution may also be used for determining
whether an excess of lime has been used in the treatment.
Alkalinity: To 50 cubic centimeters of treated water, to which there has
been added sufficient methylorange to color it, add the acid solution,
drop by drop, until the mixture is on the point of turning red. As the
acid solution is first added, the red color, which shows quickly,
disappears on shaking the mixture, and this color disappears more slowly
as the critical point is approached. One-tenth cubic centimeter of the
standard acid solution corresponds to one degree of alkalinity.
[Illustration: 2640 Horse-power Installation of Babcock & Wilcox Boilers
at the Botany Worsted Mills, Passaic, N. J.]
Causticity: To 50 cubic centimeters of treated water, to which there has
been added one drop of phenolphthalein dissolved in alcohol to give the
water a pinkish color, add the acid solution, drop by drop, shaking
after each addition, until the color entirely disappears. One-tenth
cubic centimeter of acid solution corresponds to one degree of
causticity.
The alkalinity may be determined from the same sample tested for
causticity by the coloring with methylorange and adding the acid until
the sample is on the point of turning red. The total acid added in
determining both causticity and alkalinity in this case is the measure
of the alkalinity.
Hardness: 100 cubic centimeters of the treated water is used for this
test, one cubic centimeter of the soap solution corresponding to one
degree of hardness. The soap solution is added a very little at a time
and the whole violently shaken. Enough of the solution must be added to
make a permanent lather or foam, that is, the soap bubbles must not
disappear after the shaking is stopped.
Excess of lime as determined by nitrate of silver: If there is an excess
of lime used in the treatment, a sample will become a dark brown by the
addition of a small quantity of silver nitrate, otherwise a milky white
solution will be formed.
Combined Heat and Chemical Treatment: Heat is used in many systems of
feed treatment apparatus as an adjunct to the chemical process. Heat
alone will remove temporary hardness by the precipitation of carbonates
of lime and magnesia and, when used in connection with the chemical
process, leaves only the permanent hardness or the sulphates of lime to
be taken care of by chemical treatment.