1. (a) Allowable working pressure. The maximum allowable working pressure of a non-standard 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 the rules of this Chapter. Non-standard boilers with welded seams shall not be operated at pressures exceeding 15 psig for steam, 30 psig for water.

   	EqnTStEFRS = maximum allowable working pressure, psi
   where:
   		TS	=	ultimate tensile strength of shell plate, psi
   		t	=	minimum thickness of shell plate, in weakest curse, in inches
   		E	=	efficiency of longitudinal joint:
   		For tube ligaments, E shall be determined by the rules given in Section I of the ASME Code. For riveted construction, refer to the National Board Inspection Code. For seamless construction, E shall be considered 100%.
   		R	=	inside radius of weakest course of shell, (inches) provided the thickness does not exceed 10% of the radius. If the thickness is over 10% of the radius, the outer radius shall be used.
   		FS	=	factor of safety allowed by these rules.

2. (b) Tensile Strength. When the tensile strength of steel or wrought iron shell plates is not known, it shall be taken as 55,000 psig for steel and 45,000 psi for wrought iron.
3. (c) Crushing Strength of Mile Steel. The resistance to crushing of mild steel shall be taken at 95,000 psi.

4. (d) Strength of Rivets in Shear.

   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

   1. (1) 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.
   2. (2) 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.
5. (e) Factors of Safety. The working pressure shall be decreased by the Inspector if the condition and safety of the boiler warrants it using the applicable code as a guideline. The following factors of safety represent minimum values to be used. 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, the factor of safety shall be 8. When this latter type of boiler is removed from its existing setting, it shall not be reinstalled for pressures in excess of 15 psig.

Amended at 10 Ok Reg 3075, eff 6-25-93