The material of a boiler being satisfactory and the plates being
thoroughly and skillfully riveted there remains the important matter
of strengthening the boiler against the enormous internal pressure not
altogether provided for.
[Illustration: Fig. 42.]
To illustrate the importance of attention to this point it may be
remarked that a boiler eighteen feet in length by five feet in
diameter, with 40 four-inch tubes, under a head of 80 pounds of steam,
has a pressure of nearly 113 tons on each head, 1,625 tons on the shell
and 4,333 tons on the tubes, making a total of 6,184 tons on the whole
of the exposed surfaces.
Not only is this immense force to be withstood, but owing to the fact
that the boiler grows weak with age—_a safety factor_ of six has been
adopted by inspectors, _i.e._, the boiler must be made six times as
strong as needed in every day working practice.
[Illustration: Fig. 43.]
BRACES IN THE BOILER.—The proper bracing of flat surfaces
exposed to pressure, is a matter of the greatest importance, as the
power of resistance to bulging possessed by any considerable extent
of such a surface, made as they must be in the majority of cases of
thin plates, is so small that _practically the whole load has to be
carried by the braces_. This being the case, it is evident that as
much attention should be given to properly designing, proportioning,
distributing and constructing the brace as to any other portion of the
boiler.
All flat surfaces should be strongly supported with braces of the best
refined iron, or mild steel, having a tensile strength of not less than
58,000 lbs. to the square inch. These braces must be provided with crow
feet or heavy angle iron properly distributed throughout the boiler.
[Illustration: Fig. 44.]
Fig. 42 shows the method usually followed in staying small horizontal
tubular boilers. The cut represents a 36-inch head and there are five
braces in each head: two short ones and three long ones. The braces
should be attached to shell and head by two rivets at each end. The
rivets should be of such size that _the combined area_ of their shanks
will be at least equal to the body of the brace, and their length
should be sufficient to give a good large head on the outside to
realize strength equal to the body of the brace.
In boilers with larger diameters, 5 to 8 feet, stay ends are made of
angle or T iron; by this arrangement the stays can be placed further
apart, the angle irons very effectively staying the plate between the
stays, and thus affording more room in the body of the boiler. The size
of the stays have to be increased in proportion to the greater load
they have to sustain. See Fig. 43.
In a 66-inch boiler it is proper to have not less than 10 braces in
each head, none under three feet in length, made of the best round iron
one inch in diameter, with ends of braces made of iron 2-1/2 × 1/2
inches with three pieces of T iron riveted to head above the tubes to
which the braces are attached with suitable pins or turned bolts. See
Fig. 44.
STAYING OF FLAT SURFACES.—When boilers are formed principally of
flat plates, like low-pressure marine boilers, or the fire-boxes of
locomotive boilers, the form contributes nothing to the strength, which
must, therefore, be provided for by staying the opposite furnaces
together. Fig. 45 shows the arrangement of the stays in a locomotive
fire-box. They are usually pitched about 4 inches from centre to
centre, and are fastened into the opposite plates by screwing, as
shown, the heads being riveted over. Each stay has to bear the pressure
of steam on a square _aa_, and the sectional area of the stay must be
so chosen that the tensile strength will be sufficient to bear the
strain with the proper factor of safety.
[Illustration: Fig. 45.]
If the spaces between the stays are too great, or the plate too thin,
there is a danger of the structure yielding through the plate bulging
outwards between the points of attachment of the stays, thus allowing
the latter to draw through the screwed holes made in the plates.
In designing boilers with stayed surfaces, care should be taken that
_the opposite plates connected by any system of stays should, as far as
possible, be of equal area_, otherwise there is sure to be an unequal
distribution of load in the stays, some receiving more than their
proper share, and moreover, the least supported plate is exposed to the
danger of buckling.