Maximum allowable working pressure for nonstandard boilers

1. The maximum allowable working pressure on the shell of a nonstandard boiler must be determined by the strength of the weakest section of the structure, computed from the thickness of the plate, the tensile strength of the plate, the efficiency of the longitudinal joint or tube ligaments, the inside diameter of the weakest course and the factor of safety allowed by this article. TStE RFS = Maximum allowable working pressure, per square inch gauge where: TS = Ultimate tensile strength of shell plates per square inch t = Minimum thickness of shell plate, in weakest course, inches E = Efficiency of longitudinal joint: For tube ligaments and riveted construction, E shall be determined by the rules given in section I, part PR, of the American Society of Mechanical Engineers Code for power boilers. For seamless construction, E shall be considered one hundred percent. R = Inside radius of the weakest course of the shell, in inches FS = Factor of safety permitted 2. When the tensile strength of steel or wrought iron shell plate is not known, it must be taken as fifty-five thousand pounds per square inch [386.11 megapascals] for steel and forty-five thousand pounds per square inch [310.26 megapascals] for wrought iron. 3. The resistance to crushing of mild steel must be taken at ninety-five thousand pounds per square inch [655 megapascals] of the cross-sectional area. 4. When computing the ultimate strength of rivets in shear, the following values, in pounds per square inch [megapascals] of the cross-sectional area of the rivet shank must be used: POUNDS PER SQUARE INCH MEGAPASCALS Iron rivets in single shear 38,000 262.00 Iron rivets in double shear 76,000 524.00 Steel rivets in single shear 44,000 303.37 Steel rivets in double shear 88,000 606.69 When the diameter of the rivet holes in the longitudinal joints of a boiler is not known, the diameter and cross-sectional area of rivets, after driving, may be selected from the following table, or as ascertained by cutting out one rivet in the body of the joint. SIZES OF RIVETS BASED ON PLATE THICKNESS Thickness of plate, inches 1/4 9/32 5/16 11/32 3/8 13/32 Diameter of rivet after driving, inches 11/16 11/16 3/4 3/4 13/16 13/16 Thickness of plate, inches 7/16 15/32 1/2 9/16 5/8 Diameter of rivet after driving, inches 15/16 15/16 15/16 1-1/16 1-1/16 5. The following factors of safety must be increased by the inspector if the condition and safety of the boiler demand it: a. The lowest factor of safety permissible on existing installations is four, except for horizontal-return-tubular boilers having continuous longitudinal lap seams more than twelve feet [3.66 meters] in length, when the factor of safety is eight; when this latter type boiler is removed from its existing setting, it may not be reinstalled for pressures in excess of fifteen pounds per square inch gauge [103 kilopascals]. b. Reinstalled or secondhand boilers must have a minimum factor of safety of six when the longitudinal seams are of lap-riveted construction, and a minimum factor of safety of five when the longitudinal seams are of butt-and-double-strap construction. Steam traction engines must be considered as secondhand boilers for purposes of determining their factors of safety.