No more perplexing question presents itself to the engineer and steam
user than the one to be inferred from the above heading. Enormous
losses of money, danger to life and property and the loss of position
and the reputation of the engineer are involved in it. How to avoid
these actual evils is of the first importance in steam economy. The
subject at first sight seems to the average student a difficult
one to master, but like all other matters pertaining to mechanics,
investigation that is backed with reason, will show that much that
appears obscure is really very plain indeed; this is because nature,
even down to the sediment remaining in a boiler after the conversion of
water into steam, operates in its formation with infinite exactness and
along well known lines.
Question.—What is corrosion?
Answer.—_Corrosion is simply rusting_ or the wasting away of the
surfaces of metals, for particulars of which see page 126.
Question.—What is incrustation?
Answer.—_Incrustation means_ simply _a coating over_.
Water, on becoming steam, is separated from the impurities which it
may have contained, and these form sediment and incrustation.
Boilers corrode _on the outside as well as within_, and to a great
extent unless carefully cleaned and painted; but it is the damage
caused by “hard” and acidulated water within the boiler that is to be
principally guarded against.
An extreme example of incrustation has been described in that of
a locomotive type of a stationary boiler. Its dimensions were:
seventy-two inches in diameter, twenty-two feet long, with 153
three-inch tubes; shell, three-eighths; head, three-eighths, and made
of iron. The scale against the back head was nearly two inches thick
and completely filled the space between the tubes, so that circulation
was impossible, the only wonder being that the boiler did not give out
sooner than it finally did. The scale was even with the top row of
tubes, the only part of the boiler generating steam being the fire box
and the upper row of tubes, the others acting simply as smoke conduits.
There was certainly a great loss of fuel, quite fifty per cent. Had
it been a horizontal boiler it would have burned out before the scale
became so heavy.
In the above instance, the loss in fuel is estimated at one-half.
Careful experiment has proved an average loss of fuel as follows:
1/16 inch of scale causes a loss of 13 per cent. of fuel.
1/4 inch of scale causes a loss of 38 per cent. of fuel.
1/2 inch of scale causes a loss of 60 per cent. of fuel.
It must be remembered that dry steam, as it is used through the engine
or for other purposes, _carries away none of the impurities_ which pass
with the water into the boiler; hence, in a battery of boilers burning,
say, 20 tons of coal per day and evaporating 10 lbs. of water to a
pound of coal, there is a body of water going through them every day of
200 tons. Multiply this by 300 days for a year = 60,000 tons, and it
will be seen how very great is the problem of keeping the interior of
the boilers free from scale and deposit.
Chemically pure water is that which has no impurities, and may be
described as colorless, tasteless, without smell, transparent, and in a
very slight degree compressible, and, were a quantity evaporated from a
perfectly clean vessel, there would be no solid matter remaining.
But, strangely, investigation has proved that water of this purity
rapidly corrodes iron, and attacks even pure iron and steel more
readily than “hard” water does, and sometimes gives a great deal of
trouble where the metal is not homogeneous. Marine boilers would be
rapidly ruined by pure distilled water if not previously “scaled” about
1/32 of an inch.
Water is formed by the union of two gases—oxygen and hydrogen. These
two are _simple bodies_, formed by the Creator in the beginning, which
are found _in combination_ in thousands of different forms. Both when
alone are invisible. Take one volume of oxygen and mix it with two
volumes of hydrogen and they will chemically unite and form water. This
is by measure. _By weight_ water is composed of 88.9 of oxygen to 11.1
of hydrogen = 100 parts. See pages 229, 230 for further information.
It is an important point to remember that when water is expanded about
1,700 times into steam, it is simply expanded water, as ice is hardened
water, _i.e._, in expanding into steam the two constituent gases do not
separate. Hence, in dealing with the impurities inside the boiler,
it is to be observed that in no sense do they change the essential
nature of water itself. The impurities are simply _foreign bodies_,
which have no legitimate place in the boiler, and are to be expelled
as dangerous foes. As a general principle, it may be stated that it is
more profitable to soften and filter the water used in boilers than to
trust to blowing out or dissolving the sediment and scale that will
be otherwise formed, for observations show that “anti-incrustators”
containing organic matter help rather than hinder incrustations, and
are therefore to be avoided. For the remedy of foul water there are
numerous contrivances to prevent it from entering the boiler, which
is far better than trying to extract the sediment after it is there,
though there are many ingenious methods for doing that also, some of
which will be detailed hereafter.