“The reverse applies to the thickness of the plate. For instance, if we take two cylindrical boiler shells, each 5 feet in diameter, the first one made of plate 1⁄2 inch thick, and the second twice that, or 1 inch thick, and if the first is equal to a safe working pressure of 90 lbs. per square inch, then the second is equal to a safe working pressure of twice as much, or 180 lbs. per square inch, providing, of course, that the riveted seams are of equal strength in each case, and that both boilers are allowed the same margin for safety; hence we have:
| Diameter of boiler. | Thickness of shell. | Safe working pressure. | ||||||
| 5 | feet. | 1⁄2 | inch. | 90 | lbs. | per | square | inch. |
| 5 | „ | 1 | „ | 180 | „ | „ | „ | „ |
“These principles (namely, that the strength of a boiler is, all other things or elements being equal, inversely proportional to its diameter, and directly proportional to its thickness) afford us a groundwork upon which we may lay down rules for determining by calculation the strength of the solid part[50] of any boiler shell, and the bases of these calculations are as follows:
[50] In the case of the riveted joints or seams other considerations come in, as will be shown [hereafter].
Fig. 3237.
Fig. 3238.