D.C. Mun. Regs. tit. 12, r. 12-E1022
Insert new Section 1022 in the Mechanical Code to read as follows:
Maximum allowable working pressure. The maximum allowable working pressure on the shell or drum of a power boiler shall be determined by the strength of the weakest section of the structure, computed from the following information.
Computation. The maximum allowable working pressure shall be determined in accordance with the following equation:
(TS x t x E) ÷ (R x FS) = Pm
where:
Pm | = Maximum allowable working pressure (psi) (kPa) |
TS | = Ultimate tensile strength of shell plates (psi) (kPa) |
t | = Minimum thickness of shell plate in weakest course (inch) (mm) |
E | = Efficiency of longitudinal joint, per Section VIII, Division 1, Part UW, paragraph UW-12 of the ASME Code |
R | = Inside radius of the weakest course of the shell or drum (inch) (mm) |
FS | = Factor of safety required by Chapter 10 of the Mechanical Code |
Nonstandard boilers factor of safety. The factor of safety for nonstandard boilers with longitudinal joints of butt or double strap construction shall be not less than the following:
At the beginning of each subsequent 5-year period, the factor of safety shall be increased by not less than 0.5.
Allowable working pressure limitation. In no case shall the maximum allowable working pressure on old boilers be increased unless they are being operated at a lesser pressure than would be allowable for similar new boilers, in which case the changed pressure shall not exceed that allowable for new boilers of the same construction.
Standard boilers factor of safety. The factor of safety for standard boilers with longitudinal joints of butt or double strap construction shall be five for boilers not more than 25 years old. At the beginning of each subsequent 5-year period, the factor of safety shall be increased by not less than 0.5.
Exception: When a thorough internal and external inspection of a boiler more than 25 years old is conducted, and a hydrostatic pressure test is performed at 1 1/2 times the allowed working pressure of the boiler, during which no leakage or signs of distress develop, the allowed working pressure shall be allowed to continue to be calculated with a factor of safety of five.
Water-tube boilers factor of safety. The factor of safety for nonstandard boilers of the water-tube type with longitudinal joints of lap riveted construction shall be not less than the following:
At the beginning of each subsequent 5-year period, the factor of safety shall be increased by not less than 0.5.
Factor of safety for other nonstandard boilers. The factor of safety for nonstandard fire tube, flue and cylinder boilers, the shells of which are exposed to the products of combustion and which have continuous longitudinal joints of lap-riveted construction exceeding 12 feet (3658 mm) in length, shall be not less than the following:
At the beginning of each subsequent 5-year period, the factor of safety for boilers specified in this section shall be increased by not less than 0.5.
Reinstallation. When a boiler regulated by Section 1022.5 is removed from an existing setting, it shall not be reinstalled for an allowable working pressure in excess of 15 pounds per square inch (psi) (103 kPa).
Boilers with cast-iron headers and mud drums. The maximum allowable working pressure on water-tube boilers, the tubes of which are secured to cast-iron or malleable-iron headers or which have cast-iron mud drums, shall not exceed 160 pounds per square inch (psi) (1103 kPa).
Assumed tensile strengths. When the tensile strength of steel or wrought iron shell plates is not known, it shall be taken as 55,000 pounds per square inch (psi) (379 212 kPa) for steel and 45,000 pounds per square inch (psi) (310 264 kPa) for wrought iron.
Crushing strength of mild steel. The resistance to crushing of mild steel shall be taken at 95,000 pounds per square inch (psi) (655 000 kPa).
Rivets. In computing the ultimate strength of rivets in shear, the cross-sectional area of the rivet shank shall be used to determine the value of the shear strength of the rivet, based upon the provisions in Section I of the ASME Code.
Size of rivets. When the diameter of the rivet holes in the longitudinal joints of a boiler is not known, the diameter of rivets, after driving, shall be selected from Table 1022.9.1, or ascertained by cutting out one rivet in the body of the joint.
TABLE 1022.9.1
MINIMUM SIZES OF RIVETS BASED ON PLATE THICKNESS
Thickness of plate (inch) | Diameter of rivet after driving (inch) |
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 |
For SI: 1 inch = 25 mm
Inspection of inaccessible parts. When the heads of water tube boiler mud drums or headers are not accessible for inspection, the brick work shall be removed after the boiler has been in service for 10 years to facilitate inspection and at not more than 5-year intervals thereafter. Seams and parts of fire-tube boilers that are not accessible for inspection shall be exposed whenever the code official or insurance company inspector deems that the general condition of the boiler warrants further examination.
Safety valves. Each power boiler shall be equipped with one or more safety valves of the spring-pop type with a lifting device, placed as close to the boiler as possible. No valve of any description shall be placed between the safety valve and the boiler, nor on the escape pipe between the safety valve and the atmosphere. When an elbow is placed on a safety valve escape pipe, it shall be located close to the safety valve outlet or the escape pipe shall be securely anchored and supported. When an escape pipe is used, it shall be full size and fitted with an indirect drain to prevent water from lodging in the upper part of the safety valve or escape pipe. Safety valves having either the seat or disk of cast iron shall not be used. Dead weight and lever weight safety valves shall be prohibited.
Safety valve capacity. The capacity of the safety valve or valves installed on each boiler shall be such that the safety valve or valves will discharge all the steam that can be generated by the boiler without allowing the pressure to rise to more than 6 percent above the maximum allowable working pressure, nor to more than 6 percent above the highest pressure to which any safety valve is set.
Safety valves setting. One or more safety valves on every boiler shall be set at or below the maximum allowable working pressure. The remaining valves may be set within a range of 3 percent above the maximum allowable working pressure, but the range of setting of all the safety valves on a boiler shall not exceed 10 percent of the highest pressure to which any safety valve is set.
D.C. Mun. Regs. tit. 12, r. 12-E1022