The lowest factor of safety that will be permissible on existing installations
under any conditions is 4.5. Secondhand boilers shall have a minimum factor of
safety of 5 when the longitudinal joints are of butt- and double-strap
construction, and a minimum factor of safety of 6 when the longitudinal joints
are of lap-riveted construction.
Horizontal-return tubular boilers having continuous
lap-riveted joints of more than 12 feet in length shall have a factor of safety
of not less than 8.
factors of safety for all standard boilers having longitudinal joints other
than lap-riveted construction shall be increased at the end of its specified
age limit by not less than 5/10, except that after a thorough internal and
external inspection and a hydrostatic pressure test of 1 1/2 times the pressure
allowed and held for a period of at least 30 minutes, during which no distress
or leakage develops, the pressure may be continued at a factor of 5.5. These
factors may be increased by the inspector as the condition and safety of the
boiler may dictate, and in no case shall the factor of safety be less than
called for by the rules for new construction.
(3) In no case shall the maximum allowable
working pressure on old boilers be increased, unless they are being operated at
a lesser pressure than would be allowable for new boilers, in which case the
changed pressure shall not exceed that allowable for new boilers of the same