The safe internal pressure on cylindrical shells is found according to
the following rule, which has been adopted by the United States Board
of Supervising Inspectors, and any boiler shell not found in the tables
can be determined by this rule.
RULE.—Multiply one-sixth of the lowest tensile strength found stamped
on any plate in the cylindrical shell by the thickness—expressed
in inches or parts of an inch—of the thinnest plate in the same
cylindrical shell, and divide by the radius or half diameter—also
expressed in inches—and the result will be the pressure allowable per
square inch of surface for single riveting, to which add twenty per
centum for double riveting.
The hydrostatic pressure applied, under this table and rule, must be in
the proportion of one hundred and fifty pounds to the square inch, to
one hundred pounds to the square inch of the working pressure allowed.