The lines of a certain great R. R. traverse a country where the
water is very hard and they are compelled to resort to some method
of precipitating the lime that is held in solution. After many tests
and experiments they have made a compound and use it as follows: in
a barrel of water of a capacity of fifty gallons they put 21 lbs. of
carbonate of soda, or best white soda ash of commerce, and 35 lbs. of
white caustic soda. The cost, per gallon, is about 2-1/2 cents.
The compound is carried in this concentrated form, in calomine cans
on the tender of each locomotive. A certain amount, according to the
necessities of the case, is poured into the tender at the water tank
at each filling. This amount is determined by analysis, and varies all
the way from two to fifteen pints to two thousand gallons of water.
The precipitating power of this compound may be taken roughly at 2/3
of a pound of the carbonate of lime, or equivalent amount of other
material, per pint of the compound. On their western lines where they
are dealing with alkali waters and those containing sulphates, the
company use merely 60 pounds of soda ash to a barrel of water. When the
water is pumped into the boiler the heat completes the precipitation
and aggregation of the particles, and this does away with all trouble
of the tenders or injector tubes clogging up.
The case is an interesting one to stationary engineers, because where
the water is pumped into the boiler from tanks the same compound
can be used, provided the water contains the proper constituents to
be precipitated by it; and where the water is taken from city water
mains, it would be a simple matter to devise an apparatus to admit the
compound to the feed pipes.
“POINTS” RELATING TO THE SCALING OF STEAM BOILERS.
The peculiarity about the sulphate of lime is that _the colder the
water the more of it will be held in solution_. Water of ordinary
temperature may hold as high as 7 per cent. of lime sulphate in
solution, but when the temperature of the water is raised to the
boiling point a portion of it is precipitated, leaving about .5 of one
per cent. still in solution. Then as the temperature of the water is
raised, still more of the substance is precipitated and this continues
until a gauge pressure of 41 pounds has been reached which gives a
temperature of about 200 degrees; at this point all the sulphate of
lime has been precipitated. Many other scale forming substances act in
a similar manner. This shows quite plainly that any temperature that
can be produced by the use of exhaust steam would not be sufficient to
cause the precipitation of all the substances which might be contained
in the water.
That boiler incrustations are the immediate causes of the majority of
steam boiler explosions is no longer a doubtable question.
Nearly all foreign matter held in solution in water, on first becoming
separated by boiling, _rises to the top in the form of what is commonly
called scum_, in which condition much of it may be removed by the
surface blow-off. If not removed, however, the heavier particles will
be attracted to each other until they have become sufficiently dense to
fall to the bottom, where they will be deposited in the form of scale,
covering the whole internal surface of the boiler below the water line,
with a more or less perfect non-conductor of heat.
It is recorded that the engineer of the French ocean steamer _St.
Laurent_ omitted to remove a bar of zinc when repairing and cleaning
out his boilers. On opening the boilers at the end of the voyage to his
great surprise he found that the zinc had disappeared, but his boilers
were entirely free from scale and the boiler plates not injured in the
least.
It has been recently determined by some German experimenters that
sugar effects a strong action upon boilers. It has an acid reaction
upon the iron which dissolves it with a disengagement of hydrogen. The
amount of damage done increases with the amount of sugar in the water.
These results are worthy of note in sugar refineries and places where
sugar sometimes finds its way into the boilers by means of the water
supplied. The experimenters in question also find that zinc is strongly
attacked by sugar; copper, tin, lead and aluminium are not attacked.
Two reasons, relating to incrustations, for not blowing out a boiler
while under steam pressure may be given as follows: One is, that the
foreign matter floating on top of the water will be deposited on the
shell of the boiler as the water gradually subsides, and, second,
the heated walls of the furnace will communicate a sufficiently high
temperature to the boiler to dry and flake the sediment that would
otherwise remain in the boiler in the shape of mud, which could easily
be washed out were it not for the baking process.
Bark, such as is used by tanners, has an excellent effect on boiler
incrustations. It may be used as follows: Throw into the tank or
reservoir from which the boilers are fed a quantity of bark in the
piece, in sufficient quantity to turn the water to a light brown
color. Repeat this operation every month at least, using only half
the quantity after the first month. Add a very small quantity of the
muriate of ammonia, about one pound for every 2,000 gallons of water
used. This will have the effect of softening as well as disintegrating
_the carbonate of lime_ and other impurities deposited by the action of
evaporation.
NOTE.—Care must be exercised in keeping the bark, as it becomes broken
up, from the pump valves and blow-off valves. This may be accomplished
by _throwing it into the reservoir confined in a sack_.
Among the best samples of boiler compounds ever sent to the laboratory
for analysis was found to be composed of:
Pounds
Sal soda 40
Catechu 5
Sal ammoniac 5
This solution was formerly sold at a good round figure, but since its
nature became more generally known, it is not found in market, but is
largely used, consumers putting it up in lots sufficient to last a year
or so at a time.
The above is strongly recommended by those who have used it, _one pound
of the mixture being added to each barrel of water used_ but after the
scale is once thoroughly removed from the boiler, the use of sal soda
alone is all that is necessary. By the use of ten pounds per week a
boiler 26 feet long and 40 inches in diameter in one of the iron mills
of New Albany, Ind., has been kept clean of scale equal to a new boiler.
There are other evils sometimes inherent in hard waters over and above
the mere production of a crust. Some waters contain a great deal of
soluble magnesia salts, together with common salt. When this is the
case there is a great chance of corrosion, for the former is acted on
by steam at high pressure in such a way that muriatic acid fumes are
produced, which seriously corrodes the boiler, and, what is far worse,
passes with the steam into the engine, and produces corrosion in the
cylinders and other delicate fittings into contact with which the
steam passes. All this can, however, be obviated by the removal of the
magnesia from the water.
There has not been, and never can be, made a mechanical device which
will precipitate all the ingredients contained in a water taken from
a natural source of supply, and if it were possible to do so it would
be the most ruinous thing one could do for the boilers, as water is
the greatest _solvent_ known to chemistry, and its nature is to hold
in solution and be impregnated with the different elements it comes in
contact with, to a certain per cent., and if its lime, magnesia, and
the mineral salts are taken away, and the pure water is pumped into
the boilers, it will take up the iron, causing pitting and grooving of
the boilers. It is better to let nature take its course, to a certain
extent, and neutralize what little mineral deposit forms in the boilers
with as small an amount of vegetable matter as possible.
It is well to note that different waters require different treatment;
what will be of benefit in one instance will be of no value whatever in
a different water, many of the “compounds” sold to prevent and remove
scale will certainly destroy a boiler if they are used persistently,
because they are composed of the exact opposite chemicals which should
be used; as an example it is stated that at one establishment one
thousand dollars were expended annually for a mixture which it is said
resulted in the reduction of the life and usefulness of the boilers of
50 per cent.