James A. Sowell
President, Capital Company
Consultant, ASME Code and ANSI-related issues
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (3 printed pages)
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.
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 of the ASME Boiler and Pressure Vessel Code for current requirements.