To register a fitting or equipment for CRN, the material of construction has to conform to the applicable codes and standards. Allowable materials are listed within the codes and standards in which the design is registered to. The design could be based on either code calculation, Finite element Analysis (FEA) or proof testing. For each, the material properties need to be taken from the provided tables of material. These tables contain allowable stress levels in design temperatures as well as maximum temperature limit.
Codes B31.1 and B31.3 allow for unlisted materials to be used for construction and provide less restrictive rules. Note that even though the Codes provide methods for adopting unlisted materials, some provinces implement more restrictive rules which prevents some designs to be registered.
To make the discussion more clear, let’s categorize the unlisted materials to two category of material:
Based on our experience each category can be dealt in separate way which is acceptable by most of Canadian jurisdictions. Here we continue our discussion based on B31.3 Code provisions for this matter. For B31.1, pretty much same approach is employed, but, we still recommend to contact to the regulatory bodies in each design case or simply contact deconeq Engineering Group.
B31.3 requirements for Unlisted Materials
Note that for each category of unlisted materials, the subject material needs to conform to a “published specification covering chemistry, physical and mechanical properties, …”. The Code allows for determination of allowable stresses “in accordance with the applicable allowable stresses basis of Code or more conservative basis.”
323.1.2 Unlisted Materials. Unlisted materials may be used provided they conform to a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control, and otherwise meet the requirements of this Code. Allowable stresses shall be determined in accordance with the applicable allowable stress bases of this Code or a more conservative basis.
The other point a designer needs to consider while using either of categories of unlisted materials, is the validity of the material properties within the specified design temperature range. Para. 323.2.3 requires an unlisted material to be qualified for service at all design temperatures, from design minimum temperature to design maximum temperature. For low temperature toughness test Table 323.2.2 requires an unlisted material to conform to a published specification and furthermore meet the requirement of a comparable listed material, in term of composition, heat treatment and product form.
More details are given in para. 323.2.4 for verification of serviceability. It is, in fact, the designer’s responsibility to demonstrate the validity of the allowable stresses, and the approach taken for establishment of this values, as well as other limits in design. It also requires the data for the development of design limits to be obtained from a sound scientific program carried out in accordance with recognized technology. Meaning that a designer needs to consider applicability and reliability of the data, resistance of the material to deleterious effects of the fluid service, and determine the allowable stresses in accordance with para. 302.3.
Category 1 unlisted materials
Material properties are available at design temperature for this category of materials. For the allowable stresses, para. 302.3.2 provides guidelines for both bolting and other materials. Basic allowable stress values at temperature for materials other than bolting materials, cast iron, and malleable iron shall not exceed the following:
Note: see para. 302.3.2 for more details.
Category 2 unlisted materials
Category 2 are those unlisted material in which conform to a published specification but their material properties are not available in design temperature. It is assumed that the material properties are available at room temperature. In this case many jurisdictions accept to adopt a material out of listed materials in B31.3 Code. The idea is to use a “more conservative basis” of determining the allowable stresses, as stated in para. 323.1.2.
The adopted material from B31.3 code, Table A-1, is the closest material to the unlisted material in terms of chemical characteristics and composition, physical properties, method and process of manufacture, heat treatment, and quality control.
Here an unlisted sample material forged Aluminum “X” is considered. Material properties are listed in the following table. Allowable stress is evaluated using the Code basis (para. 302.3.2(d)).
|Temp. (F)||Tensile Strength (psi)||Yield Strength (psi)||1/3 of Tensile (psi)||2/3 of Yield (psi)||Allowable Stress (psi)|
Material Aluminum Alloy B247 – 6061 – T6 is adopted from B31.3 Code. The variation of allowable stress versus temperature is evaluated from Table A-1 (Curve A). It is considered that unlisted material “X” follows the same variation of stress vs temperature as the adopted material (Curve B). To have more conservative basis (as per para. 323.1.2), the values are discounted %5 (Curve C).