File: File:PVE-4245, Last Updated: Aug 20 2014, By: LRB
To register fittings, design validation based on code calculations, finite element analysis or proof testing is required. When a design is based on code listed materials, the code of construction provides allowable operating stress levels. If the design of the pressure containing item is simple, the regular code rules can be used and will supply a pass/fail judgement. If no code rules exist for a complex or unusual shape, Finite Element Analysis (FEA) can provide the stresses which can be compared with the listed allowables for a pass/fail judgement.
An alternate to FEA is to proof test the item at stress levels far above operating. The items actual and guaranteed minimum tensile strengths are required for the proof test. The formula used is from VIII-1 UG-101(m):
Where B is the burst test pressure and P the allowed operating pressure. The burst test has to be at least 4 times the operating pressure. E is the welding efficiency if the item is welded – typically between 0.7 and 1.0. Two more pieces of information are required – Su – the specified minimum tensile strength of the material and Suavg – the tensile test results from the item under test. Typical proof test pressures are 5-6x operating pressure, a requirement in many cases more conservative than regular code calculations or FEA.
For code listed materials, all of the required information is available for either calculations/FEA or for burst testing.
Codes B31.1 and B31.3 are useful for registering fittings because they allow unlisted materials to be adopted and because they provide fewer restrictive design rules. However, if your design looks like a small vessel you might have to use VIII-1.
An unlisted material made to a specification can be adopted if the material’s guaranteed minimum yield and tensile strength are available at the operating temperature. The code adopted strength is based on a formula using these two inputs resulting in allowable design strength. Or the minimum tensile strength can be used in the proof test. Using this process, almost any IID listed material can be adopted for use in B31.1 or B31.3.
This is a typical formula for adopting unlisted material in B31.3. Sy and St are the materials guaranteed minimum strength. More complex methods are used at higher temperatures where the materials creep properties need to be taken into account. Lack of availability of elevated temperature material properties can severely limit the adoptability of unlisted materials. Caution: see Unlisted Material Registration Problems below.
Unlisted Materials With No Specified Strength
Many fittings materials are not code listed and have no guaranteed minimum tensile or yield strength information. Two common examples: SAE1010 carbon steel and B85 A380 die cast aluminium. Both are made to chemical only specifications and both are used in fittings.
To use either of these materials in Canadian registered fittings, the purchaser has to agree with the mill/foundry what minimum tensile and yield strength level is acceptable. A specification referenced or written into each material batch purchase order is required. Chosen strength levels are obviously important. Set too high and excessive batches will be rejected upon physical testing. Set to low and the parts will not pass code calculations. Also note that the ratio between the actual and minimum tensile strength impacts the required burst test pressure. The lower the minimum specified strength, the higher the required proof test. A sample purchase order or a copy of the specification would be required with the CRN application. Caution: see Unlisted Material Registration Problems below. Setting appropriate guaranteed minimum stress levels commonly causes confusion, an example follows.
Example: A manufacturer who is investigating a new unlisted material gets some pull test results. 4 tensile test results at ambient [ksi] 47, 46, 44, 48. 4 yield stress results [ksi] 25, 26, 23, 28. The results are at ambient only, and the product will only be used at ambient so elevated temperature testing is not required. What should the guaranteed minimum yield and tensile be? Each material batch will be tested, so setting the specified minimum too high will result in batches being rejected. For example, a specified minimum tensile of 45 ksi would cause the 3rd specimen to be rejected. Some number around 40 ksi tensile and 20 ksi yield might be reasonable as is shown in this graph.
What happens if the guaranteed minimum is set too low? If the product is to be burst test, per Eqn-1, the required burst test is increased by the ratio of Su/Sur, where Su is the specified minimum burst test, and Sur the test results from the item under test. If samples 1, 2 and 3 are taken from the test object, Sur = average(47,46,44) = 45.6. If the specified minimum is 40 ksi, then the burst test ratio is 4 x 45.6/40 or 4.56x. However if the specified minimum was set too low, to 20 ksi, then the ratio would be 4 x 45.6/20 or 9.12x.
If the product will not be used at ambient, then elevated materials properties are required. For CRN applications, temperatures above 100ºF are considered elevated (source unknown). Additional elevated temperature material testing is required to cover the design conditions.
The manufacturer needs to document the minimum specified properties and other characteristics of the unlisted material with no specified strength per B31.3:
B31.3 2010 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. See also ASME BPV Code Section II, Part D, Appendix 5. Allowable stresses shall be determined in accordance with the applicable allowable stress basis of this Code or a more conservative basis.
Alberta requires that this document be published on the manufacturers web site available for unrestricted access.
If all of the above fails, many Canadian reviewers will allow a fitting to be registered with “unknown” materials if it can be proof tested to 10x operating pressure (no tensile strength testing required, no guaranteed minimum specification provided). This category includes many plastics that are not covered by the piping codes, glass, ceramics and steels that cannot be adopted by the above methods.
However, testing to 10x operating pressure is a severe test not possible with many products.