Maximum Allowable Working Pressure for Nonstandard Boilers

Maximum Allowable Working Pressure for Nonstandard Boilers 1. The maximum allowable working pressure on the shell of a nonstandard boiler shall 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 these rules and regulations. TStE ÷ RSF = maximum allowable working pressure, psig where: TS = ultimate tensile strength ofshell plates, psi 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 ASME Code for Power Boilers. For seamless construction, E shall be considered 100 percent. R = inside radius of the weakest course of the shell, in inches FS = factor of safety permitted 2. Tensile strength: When the tensile strength of steel or wrought iron shell plates is not known, it shall be taken as 55,000 psi for steel and 45,000 psi for wrought iron. 3. Crushing strength of mild steel: The resistance to crushing of mild steel shall be taken at 95,000 psi cross-sectional area. 4. Strength of rivets in shear a. When computing the ultimate strength of rivets in shear, the following values, in pounds per square inch, of the cross-sectional area of the rivet shank shall be used PSI (Table 1) Iron rivets in single shear 38,000 Iron rivets in double shear 76,000 Steel rivets in single shear 44,000 Steel rivets in double shear 88,000 b. 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 Table 1, or as ascertained by cutting out one rivet in the body of the joint. Table 2: 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 or rivet after driving, inches 15/16 15/16 15/16 1-1/16 1-1/16 5. Factors of safety: The following factors of safety shall 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 shall be 4.5, except for horizontal-return-tubular boilers having continuous longitudinal lap seams more than 12 feet in length, when the factor of safety shall be eight. When this latter type of boiler is removed from its existing setting, it shall not be reinstalled for pressure in excess of 15 psig. b. Reinstalled or secondhand boilers shall 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.