(a) The lowest factor of safety for boilers of the water-tube type of nonstandard construction installed prior to January 1, 1918, the longitudinal joints of which are of lap riveted construction shall be not less than the following:

1. (1) 5 for boilers not over 20 years old;
2. (2) 5.5 for boilers over 20 and not over 25 years old;
3. (3) 6 for boilers over 25 and not over 30 years old; and
4. (4) at the beginning of each five-year period thereafter, the factor of safety shall be increased by not less than five tenths.

(b) The lowest factor of safety for all fire-tube, flue and cylinder boilers of nonstandard construction installed prior to January 1, 1918, the longitudinal joints of which are of lap riveted construction, except as provided for in subdivision (c) of this section, shall be not less than the following:

1. (1) 5 for boilers not over 10 years old;
2. (2) 5.5 for boilers over 10 and not over 15 years old;
3. (3) 5.75 for boilers over 15 and not over 20 years old;
4. (4) 6 for boilers over 20 years old; and

5. (5) at the beginning of each five-year period thereafter, the factor of safety shall be increased by not less than five tenths.

   (c)

6. (1) The lowest factor of safety for all fire-tube, flue and cylinder boilers of non-standard construction installed prior to January 1, 1918, the shells of which are exposed to the products of combustion and which have continuous longitudinal joints of lap riveted construction exceeding 12 feet in length, shall be not less than the following:

   1. (i) 6 for boilers not over 10 years old;
   2. (ii) 6.5 for boilers over 10 and not over 15 years old;
   3. (iii) 7 for boilers for 15 and not over 20 years old; and
   4. (iv) at the beginning of each five-year period thereafter, the factor of safety shall be increased by not less than five tenths.
7. (2) This type of boiler when removed from an existing setting shall not be reinstalled for a pressure in excess of 15 pounds.