D.C. Mun. Regs. tit. 12, r. 12-E1026
Insert a new Section 1026 in the Mechanical Code to read as follows:
Maximum allowable internal working pressures. The maximum allowable working pressure for a pressure vessel shall be determined in accordance with Sections 1026.1.1 or 1026.1.2.
Standard Pressure Vessels. The maximum allowable working pressure for standard pressure vessels shall be determined in accordance with the applicable provisions of the ASME Code or the API-ASME Code under which they were constructed but shall not exceed the working pressure shown on the manufacturer's nameplate stamping and data report.
(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 = Lowest thickness of shell plate in weakest course (inch) (mm)
E a = Efficiency of longitudinal joint depending upon construction
Use values as follows:
For riveted joints = calculated riveted efficiency
For fusion welded joints:
- Single "V" weld = 50%
- Double "V" weld = 70%
- Single lap weld = 40%
- Double lap weld = 50%
- Forge weld = 80%
- Brazed steel = 80%
- Brazed copper = 90%
E a = Efficiency for tube ligaments between openings as calculated in Section I, Part PG, paragraphs PG-52 and PG-53 of the ASME Code
R = Inside radius of the weakest course of the shell (inch) (mm). If the thickness of the shell exceeds 10 percent of the inside radius, the outer radius shall be used
FS = Factor of safety required by Chapter 10 of the Mechanical Code
a. Where there are both riveted joints and tube ligaments to consider, the lowest calculated efficiency, E, shall be used.
Maximum allowable external working pressure. The maximum allowable working pressure for cylindrical vessels subjected to external or collapsing pressure shall be determined by methods in Section I, Part PG, paragraph PG-28 of the ASME Code, except that the factor of safety used to calculate the working pressure shall be in accordance with the requirements of Section 1026.3.
Factor of safety. The maximum permissible exterior working pressure for existing pressure vessels of other than lap-seam construction shall be calculated using a factor of safety of not less than 4.5.
Pressure vessels of lap-seam construction. The maximum permissible exterior working pressure for existing pressure vessels with longitudinal lap joints shall be calculated based on the age of the vessel, using the factors of safety in Table 1026.3.1.
TABLE 1026.3.1 MINIMUM SAFETY FACTORS FOR EXISTING PRESSURE VESSELS WITH LONGITUDINAL LAP SEAMS
Age of the Vessel | Safety Factor |
0 to 10 years | 4 |
10 to 20 years | 4.5 |
20 to 25 years | 5 |
25 to 30 years | 5.5 |
Age limit. The age limit of a pressure vessel having a longitudinal lap joint and a working pressure over 50 pounds per square inch (psi) (345 kPa) shall be 30 years.
Stress limits. In checking the tensile stresses in the walls of existing vessels, the effect of static head shall be considered in order to verify that such tensile stresses do not exceed the ultimate tensile strength of the material, divided by the applicable factor of safety required by the Mechanical Code.
Inspection of inaccessible parts. Where, in the opinion of the code official, as the result of conditions disclosed at the time of an inspection, it is deemed necessary to remove interior or exterior lining, covering or brick work to expose certain parts of the vessel not visible at the time of regular inspection, the code official is authorized to require the removal of such material to permit proper inspection and to ascertain hidden conditions and remaining thicknesses.
Lap-seam cracks. The shell or drum of a pressure vessel in which a lap seam crack is discovered along a longitudinal riveted joint shall be immediately discontinued from use. If the vessel is not more than 15 years of age, and when approved by the code official, the owner or user is authorized to make repairs consisting of the installation of a complete new course of the original shell thickness. Patching shall be prohibited. For the purpose of this section, a "lap-seam crack" is the typical crack frequently found in lap seams, extending parallel to the longitudinal joints and located either between or adjacent to rivet holes.
Tensile strength. When the ultimate tensile strength of steel shell plates is not known, it shall be taken as 55,000 pounds per square inch (psi) (379 000 kPa) for equipment operating at temperatures not exceeding 700°F (371°C).
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 values of the material shear strength contained in Table 1026.9, to be applied to the cross-sectional area of the rivet shank, shall be used.
TABLE 1026.9 ULTIMATE STRENGTH OF RIVETS IN SHEAR
Pounds per square inch (psi) | |
Steel rivets in single shear | 44,000 |
Steel rivets in double shear | 88,000 |
Cross-Sectional Area. The cross-sectional area used in the computations shall be that of the rivet shank after driving.
Diameter. When the diameter of the rivet holes in the longitudinal joints of a pressure vessel is not known, the diameter of the rivet after driving shall be ascertained from Table 1022.9.1 or by cutting out one rivet in the body of the joint, and the cross-sectional area of the rivet shall be calculated from the obtained diameter.
Safety appliances. Each unfired pressure vessel shall be protected by such safety and relief valves and indicating and controlling devices as will ensure its safe operation. These valves and devices shall be so constructed, located and installed that they cannot readily be rendered inoperative. The relieving capacity of safety valves shall be such as to prevent a rise in pressure in the vessel to more than 10 percent above the maximum allowable working pressure, taking into account the effect of static head. Safety valve discharges shall be carried to a safe place of disposal.
D.C. Mun. Regs. tit. 12, r. 12-E1026