REAGENTS REQUIRED IN LIME AND SODA PROCESS
FOR TREATING 1000 U. S. GALLONS OF WATER
PER GRAIN PER GALLON OF CONTAINED IMPURITIES[16]
+-----------------------+-----------+-----------+
| | Lime[17] | Soda[18] |
| | Pounds | Pounds |
+-----------------------+-----------+-----------+
| Calcium Carbonate | 0.098 | ... |
| Calcium Sulphate | ... | 0.124 |
| Calcium Chloride | ... | 0.151 |
| Calcium Nitrate | ... | 0.104 |
| Magnesium Carbonate | 0.234 | ... |
| Magnesium Sulphate | 0.079 | 0.141 |
| Magnesium Chloride | 0.103 | 0.177 |
| Magnesium Nitrate | 0.067 | 0.115 |
| Ferrous Carbonate | 0.169 | ... |
| Ferrous Sulphate | 0.070 | 0.110 |
| Ferric Sulphate | 0.074 | 0.126 |
| Aluminum Sulphate | 0.087 | 0.147 |
| Free Sulphuric Acid | 0.100 | 0.171 |
| Sodium Carbonate | 0.093 | ... |
| Free Carbon Dioxide | 0.223 | ... |
| Hydrogen Sulphite | 0.288 | ... |
+-----------------------+-----------+-----------+
The chemicals used in the ordinary lime and soda process of feed water
treatment are common lime and soda. The efficiency of such apparatus
will depend wholly upon the amount and character of the impurities in
the water to be treated. Table 16 gives the amount of lime and soda
required per 1000 gallons for each grain per gallon of the various
impurities found in the water. This table is based on lime containing 90
per cent calcium oxide and soda containing 58 per cent sodium oxide,
which correspond to the commercial quality ordinarily purchasable. From
this table and the cost of the lime and soda, the cost of treating any
water per 1000 gallons may be readily computed.
Less Usual Reagents--Barium hydrate is sometimes used to reduce
permanent hardness or the calcium sulphate component. Until recently,
the high cost of barium hydrate has rendered its use prohibitive but at
the present it is obtained as a by-product in cement manufacture and it
may be purchased at a more reasonable figure than heretofore. It acts
directly on the soluble sulphates to form barium sulphate which is
insoluble and may be precipitated. Where this reagent is used, it is
desirable that the reaction be allowed to take place outside of the
boiler, though there are certain cases where its internal use is
permissible.
Barium carbonate is sometimes used in removing calcium sulphate, the
products of the reaction being barium sulphate and calcium carbonate,
both of which are insoluble and may be precipitated. As barium carbonate
in itself is insoluble, it cannot be added to water as a solution and
its use should, therefore, be confined to treatment outside of the
boiler.
Silicate of soda will precipitate calcium carbonate with the formation
of a gelatinous silicate of lime and carbonate of soda. If calcium
sulphate is also present, carbonate of soda is formed in the above
reaction, which in turn will break down the sulphate.
Oxalate of soda is an expensive but efficient reagent which forms a
precipitate of calcium oxalate of a particularly insoluble nature.
Alum and iron alum will act as efficient coagulents where organic matter
is present in the water. Iron alum has not only this property but also
that of reducing oil discharged from surface condensers to a condition
in which it may be readily removed by filtration.
Corrosion--Where there is a corrosive action because of the presence of
acid in the water or of oil containing fatty acids which will decompose
and cause pitting wherever the sludge can find a resting place, it may
be overcome by the neutralization of the water by carbonate of soda.
Such neutralization should be carried to the point where the water will
just turn red litmus paper blue. As a preventative of such action
arising from the presence of the oil, only the highest grades of
hydrocarbon oils should be used.
Acidity will occur where sea water is present in a boiler. There is the
possibility of such an occurrence in marine practice and in stationary
plants using sea water for condensing, due to leaky condenser tubes,
priming in the evaporators, etc. Such acidity is caused through the
dissociation of magnesium chloride into hydrochloride acid and magnesia
under high temperatures. The acid in contact with the metal forms an
iron salt which immediately upon its formation is neutralized by the
free magnesia in the water, thereby precipitating iron oxide and
reforming magnesium chloride. The preventive for corrosion arising from
such acidity is the keeping tight of the condenser. Where it is
unavoidable that some sea water should find its way into a boiler, the
acidity resulting should be neutralized by soda ash. This will convert
the magnesium chloride into magnesium carbonate and sodium chloride,
neither of which is corrosive but both of which are scale-forming.
The presence of air in the feed water which is sucked in by the feed
pump is a well recognized cause of corrosion. Air bubbles form below the
water line and attack the metal of the boiler, the oxygen of the air
causing oxidization of the boiler metal and the formation of rust. The
particle of rust thus formed is swept away by the circulation or is
dislodged by expansion and the minute pit thus left forms an ideal
resting place for other air bubbles and the continuation of the
oxidization process. The prevention is, of course, the removing of the
air from the feed water. In marine practice, where there has been
experienced the most difficulty from this source, it has been found to
be advantageous to pump the water from the hot well to a filter tank
placed above the feed pump suction valves. In this way the air is
liberated from the surface of the tank and a head is assured for the
suction end of the pump. In this same class of work, the corrosive
action of air is reduced by introducing the feed through a spray nozzle
into the steam space above the water line.
Galvanic action, resulting in the eating away of the boiler metal
through electrolysis was formerly considered practically the sole cause
of corrosion. But little is known of such action aside from the fact
that it does take place in certain instances. The means adopted as a
remedy is usually the installation of zinc plates within the boiler,
which must have positive metallic contact with the boiler metal. In this
way, local electrolytic effects are overcome by a still greater
electrolytic action at the expense of the more positive zinc. The
positive contact necessary is difficult to maintain and it is
questionable just what efficacy such plates have except for a short
period after their installation when the contact is known to be
positive. Aside from protection from such electrolytic action, however,
the zinc plates have a distinct use where there is the liability of air
in the feed, as they offer a substance much more readily oxidized by
such air than the metal of the boiler.
Foaming--Where foaming is caused by organic matter in suspension, it may
be largely overcome by filtration or by the use of a coagulent in
connection with filtration, the latter combination having come recently
into considerable favor. Alum, or potash alum, and iron alum, which in
reality contains no alumina and should rather be called potassia-ferric,
are the coagulents generally used in connection with filtration. Such
matter as is not removed by filtration may, under certain conditions, be
handled by surface blowing. In some instances, settling tanks are used
for the removal of matter in suspension, but where large quantities of
water are required, filtration is ordinarily substituted on account of
the time element and the large area necessary in settling tanks.
Where foaming occurs as the result of overtreatment of the feed water,
the obvious remedy is a change in such treatment.
Priming--Where priming is caused by excessive concentration of salts
within a boiler, it may be overcome largely by frequent blowing down.
The degree of concentration allowable before priming will take place
varies widely with conditions of operation and may be definitely
determined only by experience with each individual set of conditions. It
is the presence of the salts that cause priming that may result in the
absolute unfitness of water for boiler feed purposes. Where these salts
exist in such quantities that the amount of blowing down necessary to
keep the degree of concentration below the priming point results in
excessive losses, the only remedy is the securing of another supply of
feed, and the results will warrant the change almost regardless of the
expense. In some few instances, the impurities may be taken care of by
some method of water treatment but such water should be submitted to an
authority on the subject before any treatment apparatus is installed.
[Illustration: 3000 Horse-power Installation of Cross Drum Babcock &
Wilcox Boilers and Superheaters Equipped with Babcock & Wilcox Chain
Grate Stokers at the Washington Terminal Co., Washington, D. C.]
Boiler Compounds--The method of treatment of feed water by far the most
generally used is by the use of some of the so-called boiler compounds.
There are many reliable concerns handling such compounds who
unquestionably secure the promised results, but there is a great
tendency toward looking on the compound as a "cure all" for any water
difficulties and care should be taken to deal only with reputable
concerns.
The composition of these compounds is almost invariably based on soda
with certain tannic substances and in some instances a gelatinous
substance which is presumed to encircle scale particles and prevent
their adhering to the boiler surfaces. The action of these compounds is
ordinarily to reduce the calcium sulphate in the water by means of
carbonate of soda and to precipitate it as a muddy form of calcium
carbonate which may be blown off. The tannic compounds are used in
connection with the soda with the idea of introducing organic matter
into any scale already formed. When it has penetrated to the boiler
metal, decomposition of the scale sets in, causing a disruptive effect
which breaks the scale from the metal sometimes in large slabs. It is
this effect of boiler compounds that is to be most carefully guarded
against or inevitable trouble will result from the presence of loose
scale with the consequent danger of tube losses through burning.
When proper care is taken to suit the compound to the water in use, the
results secured are fairly effective. In general, however, the use of
compounds may only be recommended for the prevention of scale rather
than with the view to removing scale which has already formed, that is,
the compounds should be introduced with the feed water only when the
boiler has been thoroughly cleaned.