Taking on ASME Section VIII, Div. 1, Pressure Vessel Efficiency

This article by staff member Robert D. Schueler Jr. was originally published in the summer 2006 National Board BULLETIN. It has been edited for space. Some code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the National Board Inspection Code and ASME Boiler and Pressure Vessel Code for current requirements.

Presented here is a list of questions and corresponding answers addressing common inquiries about the 2004 Edition with 2005 Addenda of ASME Code Section VIII, Div. 1. While the answers are meant to be helpful, they are merely the author’s explanation of the more complex rules found in the code book itself.

Q. Where do the requirements for pressure part efficiency begin?

A. Look at the formulas given for each pressure part where the term “E” denotes efficiency. The nomenclature will refer to the rules in UW-12 for joint efficiency. Paragraph UW-12 includes subparagraphs (a) through (f), which refer to UW-11(a) and UW-11(a)(5). For the condition applicable to no radiographic examination, the path from the formula to UW-12 and then to UW-12(c) is correct. Unfortunately for the other plans, this does not direct the user to the true starting point, which can be found in UG-116(e). Paragraph UG-116(e)(1) through (4) provides the definitions of each of the radiographic plans and sends the user along the proper path.

Q. What is the difference between an RT-1 and an RT-2 vessel?

A. The definitions for the RT-1 and RT-2 are provided in paragraph UG-116(e) and, by reference, UW-11(a). Paragraph UW-11(a) defines both plans as full radiography. The RT-1 plan requires all butt-welded joints be fully radiographed over their entire length using the criteria in paragraph UW-51. The RT-2 plan requires all category A and D butt-welded joints be radiographed over their entire length using the criteria in paragraph UW-51. All category B and C butt-welded joints must be spot radiographed per UW-11(a)(5)(b) using the criteria in paragraph UW-52. Depending on the welded joint type employed for welded components, the efficiency will normally be established by a category A or D butt-welded joint (UG-27 footnote 15). A vessel complying with either plan will be 100 percent efficient for both components having type 1 welded joints (Table UW-12 column [a]) and seamless head or shell sections (UW-12[d]).

Q. Can RT-2 be used to satisfy the radiographic requirements of special service lethal construction or must an RT-1 plan be used?

A. RT-1 must be applied. This is a function of the rule provided in paragraph UW-2(a), which requires compliance with paragraph UW-11(a)(4). Paragraph UW-11(a)(4) ties in the rules in UW-11(a)(1) and UW-11(a)(3) which sets the condition RT-1 as defined in paragraph UG-116(e)(1). Paragraph UW-11(a)(5) was not part of this set of requirements and is therefore not applicable to special service lethal constructions.

Q. The vessel has a number of longitudinal and circumferential welded joints along with a category D butt-welded joint, all affecting a single cylinder shell section of the vessel. With each of these joints having its own welded joint efficiency, how do you determine what value of “E” is to be used in the formula in UG-27?

A. The definition of the term “E” in UG-27(b) refers to UW-12 for welded joint efficiency. Based on the requirements for each joint, making contact with the cylindrical shell being considered, a list of all such welded joints and their corresponding joint efficiencies must be compiled. The joint efficiency must be expressed in terms of equivalent longitudinal efficiency (see UG-27 footnote 15) for each joint to permit the selection of the controlling item (most severe case).

Example:

The vessel is to be stamped RT-4. The cylinder has a type 1, fully radiographed longitudinal joint in accordance with UW-51. A nozzle conforming to Figure UW-16.1 sketch (f-4) is installed in the cylinder using a type 1 joint which is spot examined per UW-11(b). Seamless 2:1 ellipsoidal heads are attached at both ends and are type 1 butt-welded joints, spot examined per UW-11(a)(5)(b) (also see UW-52[b][4] for limitations). There are no ligament conditions on the cylinder.

Expressed in terms of equivalent longitudinal efficiency:
Ligament efficiency
– UG-53 not applicable to this example
Longitudinal cylinder joint
– Table UW-12 column (a) = 1.0
Circumferential joints
– Table UW-12 column (c) = 0.70 x 2 = 1.4
Nozzle joint
– Table UW-12 column (b) = 0.85

Based on this, the lowest value of “E” used in the equation will be 0.85 resulting from the nozzle joint.

Q. Given a seamless head or shell section, other than a hemispherical head (see UG-32), what is the design efficiency of the seamless section?

A. Paragraph UW-12(d) answers this question with a question, as follows: Was the weld(s) joining the seamless head or seamless shell spot examined per the rules given in UW-11(a)(5)(b)? If yes, the seamless head or shell efficiency is set at 100 percent. If no, the seamless head or shell efficiency will be set at 85 percent.

Q. When following an RT-3 plan per UG-116(e)(3), can seamless head or shell sections have an efficiency of 100 percent?

A. No, RT-3 complies with the rule in UW-11(b). The requirement that would permit a higher efficiency is found in paragraph UW-11(a) and is not applicable to a UW-11(b) spot radiographic plan. Therefore, the rule in UW-12(d) will set the efficiency at 85 percent. Note: UW-11(a)(5)(b) cannot be applied with RT-3 (see UW-52[b][4]).

Q. If the answer to the previous question is no, what would be required to permit a higher efficiency for seamless head and shell sections?

A. It will be necessary to select an RT-1, RT-2, or RT-4 plan in which the requirements of UW-11(a)(5)(b) will be satisfied.

Q. Can a nonradiographed vessel have aligned vessel longitudinal joints between courses?

A. No, with a nonradiographed construction, the rule in UW-9(d) takes on a different meaning and must be read as mandating the joints be staggered a distance greater than five times the plate thickness.

Q. How can one determine the applicable RT number from the data listed on the Manufacturer’s Data Report?

A. Based on the information provided, with the exceptions of an RT-4 and nonradiographed vessel, the RT level cannot be determined from the data report. Only a limited amount of weld joint efficiency and degree of radiographic examination information is required on the report. The actual RT number only appears on the vessel stamping (see UG-116[e]).