Toughness Testing and ASME B31.3 Process Piping Code
The other day in a conversation with a customer a question arose regarding the Toughness Test requirements of the B31.3 Process Piping Code as they apply to cryogenic piping. As Department of Energy National Laboratories respond to the requirements of 10 CFR 851 Appendix A Part 4, we at Meyer Tool are seeing the ASME B31.3 Process Piping Code being specified as the code governing the design, fabrication, and inspection of pressure piping being used at National Laboratories. When the pressure piping involves the transport of cryogenic fluids such as nitrogen and helium, consideration regarding the Code’s requirements for Low Temperature Toughness Tests must be considered.
Cryogenic temperatures are those defined as -150C (-238F). Cryogenic liquids are defined as those liquids that have boiling points at atmospheric pressure above this temperature limit. Common cryogenic liquids are argon (-186F), helium (-452F), hydrogen (-423F), nitrogen (-321F), oxygen (-297F), and methane (-256F).
The most common materials used in cryogenic pressure piping are austenitic stainless steel (304/304L and 316/316L) and aluminum. For the most part the application of the Toughness Test requirements of B31.3 to these metals is straightforward. Only when helium or other liquids subcooled below –425F are involved does the application of the Code rules become unclear.
Chapter III “Materials” of B31.3 addresses Toughness Test requirements of metals at lower temperature limits in paragraph 323.2.2 and Table 323.2.2. We will review how these requirements apply to austenitic stainless steels and aluminum piping at cryogenic temperatures.
When deciding the Toughness Test requirements of a material we first follow the instructions of paragraph 323.2.2 (a) which directs us to review Table A-1 “Basic Allowable Stresses in Tension for Metals” for the minimum temperature allowed for use without toughness testing. In the case of aluminum this temperature is –452F. The paragraph then directs us to Table 323.2.2 and for aluminum to cell A-6 of that table. This cell states that there are no additional Toughness Test requirements for aluminum. Thus for the aluminum materials listed in Table A-1 used at cryogenic temperatures no Toughness Tests are required for temperatures at or above –452F.
Austenitic Stainless Steel:
Repeating the above procedure, Table A-1 tells us for austenitic stainless steel (limiting ourselves to the common grades of 304/304L and 316/316L) that the minimum temperature allowed for use without toughness testing is –425F. Not bad; that covers all the common cryogenic liquids except helium. However Table 323.2.2 has further restrictions to avoid impact tests. These restrictions are (1) carbon content less than 0.1% and (2) the material must be solution annealed. Neither of these restrictions are big issues, the first limits us to the low carbon 304L and 316L grades, while solution annealing is required manufacturing practice for these materials when ordered to the ASTM specifications listed in Table A-1.
Table 323.2.2 A-4(b) has a further restriction for austenitic stainless steels. This restriction, however, is modified by paragraph 322.2.2 (f). The final determination of these combined restrictions is that Toughness Tests are required on a production weld specimen per Table 323.3.1 at design temperatures below –150F. There is one final caveat for manufacturers, such as Meyer Tool, who have weld procedures qualified per the ASME Boiler and Pressure Vessel Code Section IX. Per Note 2 of Table 322.2.2, if the manufacturer has weld procedure qualifications in place that meet the requirements of Table 323.2.1 (i.e. procedures qualified at or below the design temperature) the requirement for a production weld specimen is waived.
To summarize, so far for austenitic stainless steels at minimum design temperatures at or above –425F.
Toughness Tests are not required if:
What about below –425F?
According to paragraph 323.2.2 (c) we may use austenitic stainless steels below –425F if we follow the requirements of Table 323.2.2 Column B cell B-4. Unfortunately the requirement of cell B-4 (after the reading of several notes and other paragraphs) requires us to impact test at the minimum design temperature. For liquid helium piping this would mean toughness tests performed at –452F, essentially impossible.
However, the Code does give us some guidance on how to proceed. In Chapter I “Scope and Definitions” there are several paragraphs giving us direction. These paragraphs are included in Section 300 (b) “Responsibilities” and 300 (c) “Intent of the Code”. A reading of these paragraphs, in our opinion, gives the owner, designer and manufacturer some latitude in this situation, where the Code doesn’t prohibit the use of these materials but the testing requirements are not physically possible to perform. In these situations, we have recommended to the owner that good engineering practice would be to apply the requirements of a more restrictive Code, namely the ASME Boiler and Pressure Vessel Code Section VIII Division I to this situation. Part UHA “Requirements for Pressure Vessels Constructed of High Alloy Steel”, specifically paragraph UHA-51, addresses Impact Tests (e.g. Toughness Tests). The rules for austenitic stainless steels in this situation are summarized below.
Thus Meyer Tool’s recommendation to the owner in this situation is to: