The Elk County advocate. [volume] (Ridgway, Pa.) 1868-1883, 1875-02-11
ABOUT STEAM BOILERS.
A Practical Engineer Tells us why they are
Weakened and Explode.

!
Weakened and Explode.
Safety and the production of large
quantity of dry steam are the two essen
tials of good boiler, said James Whit
ney before the New York Society of
Practical Engineers, but these two re-
qliirements are antagonistic to each
other, and the best boilers are the result
of compromise. If we could have q
perfect hollow sphere of metal, we would
have theoretically the strongest form of
steam generator, and in the infancy of
steam engineering some spherical
boilers were made in England, and con-
tinued in use nearly to our own time.
But it is possible to bring the fire in
contact with only small portion of the
surface, and hence we have little steam
for good deal of fuel. The form,
| therefore, is changed to a cylinder, with
the fire underneath and playing upward
against the sides. This affords a greater
heating surface, but the ends do not
bear the same relation to the pressure as
the sides, and must be stayed by braces
that shift a portion of the strain on the
heads to other portions of the shell. If,
now, our fire is started under the boiler,
the lower part heats faster than the upper
part, and consequently expands more.
This bends the lower part outward on
the arc of circle, which, although i'm
perceptible to the eye, is the result of
an irresistible force. This expansion of
the lower part of the boiler brings o
strain upon the upper part; when the
boiler is allowed to cool it resumes its
original form. At each change of tenn
perature, therefore, there is a to and fro
movement of the Tapped edges of the
plates where they are riveted together.
The final effect is that the plate
cracks along the line of its least resist
ance. In some boilers the fracture may
be along line of rivets, and be entirely
hidden from any possible view. But if
the riveting be extremely good the min
ute bending to and fro is more apt to
occur in one plate close to the over
tapping edge of the plate to which it is
riveted, and when the crevice opens to
the interior of the boiler, any acid in the
water will act chemically to increase it.
This is termed "grooving " by the Eng
lish engineers, and is not confined to any
particular type. For if, instead of
placing our fires beneath the cylinder,
we extend large cylindrical fue through
the cylinder or shell of the boiler, we
simply change the locality in which the
effects of this alternate expansion and
contraction are exerted. The hues are
fixed in the ends of the shell, and are
hotter than the shell, and of course ex-
pand more. Being confined against the
ends, the f1ues bend upward. If we put
on a steam drum in which the otherwise
waste heat from the escaping products
of combustion may dry the steam from
its contained vesicles of vapor, we ac-
complish an increased eoonotny of fuel;
but by adding vertical cylinder inter
secting a horizontal one, we have an
additional joint between portions subject
to quite different temperatures, and at
which growing" is liable to occur. If
the joint is made with angle iron, in
which the grain runs parallel with the
sharp corner in the angle of the iron,
the danger is much increased, and this
has led American engineers to discard
the English plan of using angle iron in
steam boilers. The new and better sys-
tenn is to make the joint on curve hav-
ing q radius Of about four inches. The
ends of the shells or cylinders of boilers
are often fitted in their places with angle
iron, but the same objection holds as in
the filling of the domes. AS rule,
sharp and rigid corners should be avoid
ell between plates liable to unequal ex-
pansion from heat. This gradual de-
terioration of boilers from the cause in
dicated is one of the most insidious of
all the causes of explosions, and it is
one that is inherent in all notifications
of the cylinder type. Skill and good
Judgment may reduce the danger but
cannot wholly obviate it.