James A. Sowell
President, Capital Company
Consultant, ASME Code and ANSI-related issues
Winter 1997
Category: Design/Fabrication
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (3 printed pages)
Introduction
There are a vast number of ASME Code requirements that relate to dimensional
requirements. In a quantitative number of paragraphs, or pages, there is more
reference to dimensional requirements than other subjects such as welding,
nondestructive examination (NDE) and testing. Yet many inspectors concentrate
their activities on welding, NDE and testing, spending little time assuring
that dimensional requirements are met.
Dimensional requirements are translated from code and design calculations to
fabrication drawings. Therefore the drawing requirements become code
requirements. The ability of the completed component to safely operate during
its required design condition is directly dependent upon conformance with the
dimensional requirements and tolerances given in the construction code and
those based upon the design calculation.
When talking with a group of inspectors, diverse comments were made on whether
or not the inspectors follow the dimensional requirements of the drawings. Many
admitted it was not a high priority, although they were aware of the reference.
The inspectors believed dimensional requirements were the responsibility of the
manufacturer, the customer?s source inspector, or quality control?s
responsibility.
These and other statements indicate there is confusion and lack of uniformity
among inspectors on the subject. None of the respondents considered inspection
of dimensional requirements as shown on the fabrication drawings to be among
the most important inspections required of the inspector. The purpose of this
article is to help remove some of the confusion by citing references which make
clear the inspector?s responsibility for inspecting to the dimensional
requirements of the drawings.
Explicit Inspector Duties
Each of the construction codes explicitly addresses the inspector?s duties
regarding dimensional requirements. For example, Section VIII, Division 1,
Paragraph UG-90(c)(1)(K) states the inspector is responsible for ?making a
visual inspection of the vessel to confirm there are no material or dimensional
defects.? UG-93(e) further reinforces the requirement for the dimensional
inspections of material by stating the inspector is responsible for assuring
?that the thickness and other dimensions of material comply with the
requirements of this division.? UG-96(c) also states that the inspector is
responsible for certain dimensional checks of component parts to assure they
are of the prescribed shape and thickness. Section III of the code is even more
explicit regarding the inspector?s responsibility to inspect to the
requirements of the drawings. NCA-5220(h) states that the duties of the
inspector shall include ?reviewing drawings and inspecting in accordance with
them.? NCA-5262(a) further strengthens the requirement by stating, ?The
inspector shall satisfy himself that the item is being constructed within the
tolerances required by the design specification, design drawings, construction
specification, and this section.?
A strong case can also be made that there is an implicit requirement in each
construction code that the inspector make sufficient inspections to assure
dimensional requirements have been met. The construction codes clearly state
the manufacturer?s responsibility for code compliance. But the certificate of
shop inspection contains certification by the inspector that, to the best of
his belief and knowledge, the manufacturer has met code requirements. Since
many of the requirements contained in the code are dimensional requirements, it
is implicit that the inspector is certifying that the manufacturer has met the
dimensional requirements as given on the drawings. It can be argued that the
inspector is not responsible for inspection of certain dimensional
requirements. The classic example is nozzle location or orientation. One
inspector stated, "I don't care if it's 180 degrees out provided it meets all
code requirements." Since there are no code requirements other than correct
design and fabrication, it is true that a vessel with an incorrectly located
nozzle may meet code requirements. But the vessel buyer, and consequently
manufacturer, will be very disappointed. However, the inspector must be careful
because location of many other items is very design dependent. For example, the
location of stiffening rings is critical because location and spacing affect
the design calculations.
Design Requirement Translation
Each of the construction codes requires the manufacturer to include control of
design requirements in the quality program. The design control chapter of the
quality program will contain controls which assure that code design
requirements are correctly translated into drawings, calculations and
specifications.
Consider how this would apply to a specific example regarding material
dimensional requirements. The manufacturer makes the required calculations for
the size of a reinforcement pad for a nozzle as required by ASME Code, Section
VIII, Paragraph UG-37. The dimensional requirements will include pad thickness,
shape, internal and external diameter, and size of attaching welds. Those
dimensional requirements are then included on the fabrication drawings. Unless
those drawing-requirement dimensions are held during the fabrication process,
the code requirements have not been met. Furthermore, the vessel may be unsafe
for the intended service.
Also consider an example of dimensional requirements for weld sizes. UW-15(b)
states that strength calculations are not required if the manufacturer uses
certain attachment details. However, each of those attachment details require
the manufacturer to make a simple size (not strength) calculation. Even the
simplest attachment detail requires the manufacturer to calculate the size of
the attaching fillet-weld based upon material thickness. After calculation, the
required dimension of the fillet weld is stated on the fabrication drawing.
There are many other possible examples which could illustrate how design
requirements are translated in dimensional requirements which are then included
on the drawings. It is very clear that if these drawing requirements are not
met during the fabrication process, the design and code requirements have also
not been met.
Summary
Dimensional requirements are a result of the design calculation and specific
code requirements. The dimensional requirements are then included on the
fabrication drawing. Without compliance with the drawing requirements, the
vessel will not meet the design or the code requirements. In order to monitor
the manufacturer?s compliance with code requirements, the inspector must
inspect to the drawings to assure dimensional requirements are met.
Editor's note: Some ASME Boiler and Pressure Vessel 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 ASME Boiler and Pressure Vessel Code for current requirements.