Now to illustrate
what I mean, we will go a little deeper and then still deeper, and you
will begin to appreciate the simple way of putting the things which you
as a plain engineer are interested in.
For example on page 114 we talked about the safe working pressure of
different sized boilers. It was most likely natural for you to say "How
do I find the safe working pressure?" Well, to find the safe working
pressure of a boiler it is first necessary to find the total pressure
necessary to burst the boiler. It requires about twice as much pressure
to tear the ends out of a boiler as it does to burst the shell, and as
the weakest point is the basis for determining the safe pressure, we
will make use of the shell only.
We will take for example a steel boiler 32 inches in diameter and 6 ft.
long, 3/8 in. thick, tensile strength 60,000 lbs. The total pressure
required to burst this shell would be the area exposed times the
pressure. The thickness multiplied by the length then by 2 (as there
are two sides) then by the tensile strength equals the bursting
pressure: 3/8 x 72 X 2 x 60,000 = 3,240,000 the total bursting pressure
and the pressure per square inch required to burst the shell is found by
dividing the total bursting pressure 3,240,000 pounds by the diameter
times the length 3,240,000 / (32 x 72) = 1406 lbs.
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