To find the necessary thickness in inches for the shell.Multiply the pressure of steam in pounds per square inch by the diameter of the boiler in inches, and multiply this product by 0.0002 for a copper boiler with single-riveted shell; by 0.0001563 for a copper boiler with double-riveted shell; by 0.0001316 for a wrought-iron boiler with single-riveted shell; by 0.0001111 for a wrought-iron boiler with double-riveted shell; by 0.0001 for a steel boiler with single-riveted shell; and by 0.00008333 for a steel boiler with double-riveted shell.

In illustration of the rule, suppose that it is required to find the necessary thickness for the shell of a copper boiler 60 inches in diameter, double riveted, for a pressure of 40 lbs. per square inch.

First take the product of 40 and 60, which is 2400, and multiply this by 0.0001563, which gives 0.375, or 3/8 of an inch, as the necessary thickness.