NAC455C.242. Working pressure for nonstandard boiler.

     1. Except as otherwise provided in this section, the maximum allowable working pressure of a nonstandard boiler is determined by the following formula:

TStE
——	=	maximum allowable working pressure, in PSIG
RFS
where:
TS               =		ultimate tensile strength of shell plate, in PSIG. If the tensile strength is not known, it shall be deemed to be 55,000 PSIG for steel and 45,000 PSIG for wrought iron.
t                  =		minimum thickness of shell plate of weakest course, in inches.
E                 =		efficiency of longitudinal joint:
		For tube ligaments, E is determined by the appropriate provisions of section I of the ASME Boiler and Pressure Vessel Code, as adopted by reference in NAC 455C.108.
		For riveted construction, E is determined by the appropriate provisions of the National Board Inspection Code.
		For seamless construction, E must be 100 percent.
R                 =		inside radius of weakest course of shell, in inches.
FS               =		factor of safety permitted by this chapter.

     2. The resistance of mild steel to crushing shall be deemed to be 95,000 PSIG.

     3. If computing the ultimate strength of rivets in shear, the following values in pounds per square inch of the cross-sectioned area of the shank of the rivet must be used:

	PSIG
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

     4. If the diameter of the rivet holes in the longitudinal joints of a boiler is not known, the diameter and cross-sectioned area of rivets, after driving, may be selected from the following table or ascertained by cutting out one rivet in the body of the joint.

Sizes of Rivets Based on Plate Thickness
Thickness of plate, in inches	1/4	9/32	5/16	11/32	3/8	13/32
Diameter of rivet after driving, in inches	11/16	11/16	3/4	3/4	13/16	13/16
Thickness of plate, in inches	7/16	15/32	1/2	9/16	5/8
Diameter of rivet after driving, in inches	15/16	15/16	15/16	1-1/16	1-1/16

     5. A nonstandard boiler with welded seams may not be operated at pressures exceeding 15 PSIG for steam and 30 PSIG for water.

     6. The maximum allowable working pressure may be decreased by the inspector or special inspector if the condition and safety of the boiler warrant it.

     7. Except as otherwise provided in this subsection, the lowest factor of safety permissible on existing installations is 4.5 or as set forth in the edition of the code that was applicable at the time of construction. The lowest factor of safety permissible on existing installations is 8 for horizontal-return tubular boilers having continuous longitudinal lap seams more than 12 feet in length. If such a horizontal-return tubular boiler is removed from its existing setting, it must not be reinstalled for pressures in excess of 15 PSIG.

     8. Reinstalled or secondhand boilers must have a minimum factor of safety of 6 if the longitudinal seams are of lap-riveted construction, and a minimum factor of safety of 5 if the longitudinal seams are of butt- and double-strap construction.

     (Added to NAC by Div. of Industrial Relations by R101-02, eff. 12-15-2004)