To find the number of braces, necessary for the flat surface above the
tubes.
1. _Find the area of the segment of the boiler head, which is to be
braced, in square inches._
2. _Multiply the area, thus found, by the steam pressure in pounds per
square inch._
3. _Multiply the cross sectional area of one brace by the number of
pounds, which it is allowed to carry, per square inch of section._
4. _Divide product 2 by product 3, and the result is the number of
braces, required for the head_.
Table No. 1 gives the total area in square inches. No. 2, areas to
be braced. No. 3, number of braces of one inch round iron required,
allowing seven thousand five hundred pounds per square inch of section
at one hundred pounds steam pressure.
Table No. 3 will be found of more practical use than Table 2, for it
gives directly the number of braces required in any given boiler,
instead of the area to be braced. It was calculated from Table 2. The
iron used in braces will safely stand a continuous pull of 7,500
pounds to the square inch, which is the figure used in computing the
foregoing table. A round brace an inch in diameter has a sectional area
of .7854 of an inch, and the strain that it will safely withstand is
found by multiplying .7854 by 7,500, which gives 5,890 pounds as the
safe working strain on a brace of one-inch round iron.
In a 60-inch boiler, whose upper tubes are 28 inches below the shell,
the area to be braced is, according to table 2, 930 square inches. If
the pressure at which it is to be run is 100 pounds to the square inch,
the entire pressure on the area to be braced will be 93,000 pounds, and
this is the strain that must be withstood by the braces. As one brace
of inch-round iron will safely stand 5,890 pounds, the boiler will need
as many braces as 5,890 is contained in 93,000, which is 15.8. That is,
16 braces will be required. The table is made out on the basis of 100
lbs. pressure to the square inch, because that is a very convenient
number.