Code of Maryland Regulations
Chapter 09.12.01 - Board of Boiler Rules - Power Boiler - Existing Installation: Maximum Allowable Working Pressure

Universal Citation: MD Code Reg Power Boiler - Existing Installation: Maximum Allowable Working Pressure

A. The maximum allowable working pressure on a standard boiler shall be determined in accordance with the ASME Code under which it was constructed and stamped.

B. 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 by multiplying TS by t and E and dividing the product by R multiplied by FS:

(TS x t x E) / (R x FS) = maximum allowable working pressure, PSIG, where:

(1) TS = ultimate tensile strength of the shell plate, PSI, which, if unknown, shall be:

(a) 55,000 PSI for steel; and

(b) 45,000 PSI for wrought iron;

(2) t = minimum thickness of the shell plate of the weakest course, in inches;

(3) E = efficiency of the longitudinal joint, which is, for:

(a) Tube ligaments and riveted construction, established by Section I of the ASME Code for power boilers; and

(b) Seamless construction, 100 percent;

(4) R = inside radius of the weakest course of the shell, in inches;

(5) FS = factor of safety required by this regulation;

(6) The resistance to crushing of mild steel is assumed to be 95,000 PSI of cross-sectional area;

(7) 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:

(a) Iron rivets in single shear 38,000 PSI;

(b) Iron rivets in double shear 76,000 PSI;

(c) Steel rivets in single shear 44,000 PSI; and

(d) Steel rivets in double shear 88,000 PSI;

(8) When the diameter of a rivet hole in a longitudinal joint of a boiler is not known, the diameter and cross-sectional area of the rivet, after driving, shall be:

(a) Ascertained by cutting out one rivet in the body of the joint; or

(b) As indicated in the following table:

Plate Thickness in Inches Rivet Diameter
1/4 11/16
9/32 11/16
5/16 3/4
11/32 3/4
3/8 13/16
13/32 13/16
7/16 15/16
15/32 15/16
1/2 15/16
9/16 1-1/16
5/8 1-1/16

C. The maximum allowable working pressure on a:

(1) Water-tube boiler having tubes secured to cast iron or malleable-iron headers, or having cast-iron mud drums, shall be 160 PSIG;

(2) Cast-iron boiler, except a hot-water boiler, shall be 15 PSIG;

(3) Boiler having lap-riveted longitudinal joints, operated at a pressure greater than 50 PSI, when removed from an existing setting and reinstalled, shall be 15 PSIG; and

(4) Reinstalled horizontal-return tubular boiler having a continuous longitudinal lap seam more than 12 feet long, shall be 15 PSIG.

D. Except as otherwise provided by this regulation, the lowest factor of safety shall be, for:

(1) An existing installation: 4.5;

(2) A horizontal-return tubular boiler having continuous longitudinal lap seams more than 12 feet long: 8;

(3) A reinstalled or secondhand boiler having longitudinal seams of lap-riveted construction: 6; and

(4) A reinstalled or secondhand boiler with longitudinal seams of butt and double-strap construction: 5.

(Regulations .22 adopted effective June 1, 1989 (16:10 Md. R. 1106)
Regulation .22B amended effective July 3, 1995 (22:13 Md. R. 966))
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