When is a pressure vessel not really a pressure vessel? A question that comes up often. And the answer may surprise you. The NBIC and ASME agree on this topic…
With a volume of less than 5 cubic feet and a MAWP of less than 250 psig, it is considered a “fat pipe” (not my term, but how I was taught in the 1990’s).
Other exemptions include 3 cubic feet at 350 psi, 1.5 cubic feet at 600 psi, or any vessel with an inside diameter not exceeding 6 inches.
ASME Section VIII Exemption Criteria
Vessels are generally exempt from the “U” stamp requirement if they meet the following “volume-pressure” thresholds:
Volume and Pressure:
The vessel does not exceed
- a design pressure of 250 psig with a volume less than 5 ft3, or
- a design pressure of 350 psig with a volume less than 3 ft3, or
- a design pressure of 600 psig with a volume less than 1.5 ft3
The “Fat Pipe” Rule refers to the 6-inch Diameter Rule. According to U-1(c)(2)(i), vessels having an inside diameter, width, height, or cross-section diagonal not exceeding 6″\ 152 mm are exempt from the scope of Section VIII, regardless of length or pressure.
These vessels, often along with smaller volume/higher pressure combinations, are frequently categorized under the UM (Unfired Miniature) Stamp rather than the standard U stamp.
U-1 SCOPE
...
(j) Pressure vessels exclusive of those covered in (c), (g), (h), and (i) that are not required by the rules of this Division to be fully radiographed, that are not provided with quick-actuating or quick-opening closures (see UG-35.2 and UG-35.3, respectively), and that do not exceed the following volume and pressure limits may be exempted from inspection by Inspectors, as defined in UG-91, provided that they comply in all other respects with the requirements of this Division:
(1) 5 ft3 (0.14 m3) in volume and 250 psi (1.7 MPa) design pressure; or
(2) 3 ft3 (0.08 m3) in volume and 350 psi (2.4 MPa) design pressure;
(3) 11/2 ft3 (0.04 m3) in volume and 600 psi (4.1 MPa) design pressure.
In an assembly of vessels, the limitations in (1) through (3) above apply to each vessel and not the assembly as a whole. Straight line interpolation for intermediate volumes and design pressures is permitted. Vessels fabricated in accordance with this rule shall be marked with the “UM” Symbol in Figure UG-116, sketch (b) and with the data required in UG-116. Certificates of Compliance shall satisfy the requirements of UG-120(a).