1. (a) The maximum allowable working pressure on the shell of an unfired pressure vessel is determined by the following:

   1. (1) The strength of the weakest course completed from the thickness of the plate.
   2. (2) The tensile strength of the plate.
   3. (3) The efficiency of the longitudinal joint.
   4. (4) The inside diameter of the course.
   5. (5) The safety factor allowed by the ASME Code.

2. (b) The equation for computing the maximum allowable working pressure is:

   1. (1) TS equals the ultimate strength of the shell plates in psi. If the tensile strength is not known, 55,000 psi must be used for temperatures not exceeding 700° F.
   2. (2) T equals the maximum thickness of shell plates of weakest course in inches.

   3. (3) E equals the efficiency of longitudinal joint depending upon construction.

      1. (i) ANSI/NB 23, Appendix C, sections A-1 to A-9 must be used to calculate efficiency for a riveted joint.

      2. (ii) Fusion welded joints must have the flowing E values:

         1. (A) Single lap weld is 40%.
         2. (B) Double lap weld is 60%.
         3. (C) Single butt weld is 60%.
         4. (D) Double butt weld is 75%.
         5. (E) Forge weld is 70%.
         6. (F) Brazed steel and brazed copper is 80%.
   4. (4) R equals the inside radius of the weakest course of the shell in inches if the thickness of the shell does not exceed 10% of the radius. The outer radius is used in the equation if the thickness is over 10% of the radius.

   5. (5) FS equals the minimum safety factor allowed by this section. The minimum allowable safety factors are as follows:

      1. (i) For unfired pressures vessels, except those of lap seam construction, the minimum safety factor is five.
      2. (ii) For unfired pressure vessels with longitudinal lap joints the minimum safety factor is 5 1/2.
      3. (iii) For unfired pressure vessels with reinstalled or secondhand lap seamed construction the minimum safety factor is six.
      4. (iv) For unfired pressure vessels with reinstalled or secondhand butt strap or welded construction the minimum safety factor is 5 1/2.

   TS x t x E = Maximum allowable working pressure in psi where:

   R x FS

3. (c) The ASME Code, section VIII, Division 1 is incorporated as the maximum allowable working pressure for cylindrical unfired pressure vessels subjected to external or collapsing pressure.
4. (d) The formulas in ASME Code, section VIII, divisions 1 and 2 or ASME, section X are incorporated and must be used to calculate the maximum allowable pressure for the head of an existing unfired pressure vessel that was not constructed in accordance with this chapter.
5. (e) The effect of static head must be considered in checking an existing vessel’s maximum allowable working pressure.

Cross References

This section cited in 34 Pa. Code § 3a.3 (relating to scope).