Section 408.4212. Maximum allowable working pressure.

(1) The maximum allowable working pressure  on  the    shell   or drum of a 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 of the ligaments between tube holes in the shell or drum (whichever is  the least), the inside diameter of the outside course and the  factor of  safety allowed by these rules.

TStE/RFS = Maximum allowable working pressure, psi where   TS = ultimate tensile strength of shell plates, psi.

t  = minimum thickness of shell plate, in weakest course, inches E  = efficiency of longitudinal joint:

For riveted construction, E shall be determined by the rules based on the ratio which the strength of the joint bears to the strength of the solid plate.

For fusion-welded construction, E shall be determined by the rules applying to the class of welding used.

For tube ligaments, E shall be determined by the rules based on the ratio of the cross-sectional area of the ligaments

to the entire section of the plate.

For seamless construction, E shall be considered 100%.

R  = one half the insude diameter of the weakest course of shell or drum, inches

FS = factor of safety permitted by these rules.

(2)   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)   In computing the efficiency of riveted  joints,   the    resistance    to crushing of mild steel shall be taken as  95,000   psi  of  cross-sectional area.

(4)   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:

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 When the diameter of the rivet holes in the longitudinal joints of a boiler is not known, the diameter and cross- sectional area of rivet shanks may be selected from table 1, or they may be ascertained by cutting out  1 rivet  in the body of the joint.

(5)  In no case shall a boiler be designed for a pressure   less   than   30 psi. TABLE 1

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 ....

History: 1979 AC.