(a) The factor of safety for boilers of nonstandard construction installed prior to January 1, 1918, the longitudinal joints of which are of butt-and double-strap construction, shall be not less than 5.5 for boilers over 30 years old.

1. (1) At the beginning of each five-year period thereafter, the factor of safety shall be increased by not less than five tenths except that after a thorough internal and external inspection and a hydrostatic pressure test of one and one-half 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.
2. (2) In no case shall the maximum allowable 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 construction.