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NACE MR0175, UNS S31600, S30940 and types 3a and 3b, what is the difference?

Question:

We are doing a pipe skid system that requires ASME B31.3 and NACE MR0175 as project requirements. According to A.3.2 of NACE MR0175, Table A.8 lists some material UNS numbers. We are wondering what NACE means by “type 3a” and “3b” (max temp 140 deg F). The pipe we were contemplating using was ASTM A312 Gr TP 316. According to another chart we saw that the equivalent URN (UNS) number for A312 is S30940. What therefore is the maximum temperature rating on S30940 (ASTM A316 Gr TP316). Our application is on frame oil where the design values are -18 to 160 deg C, 300 lb.

Answer:

The definition of materials type 3a and 3b is given at the bottom of Table A.8. It is based on alloy chemical composition. The temperatures given in the tables are the maximum known acceptable values with the given H2S partial pressure, pH, and chlorides limits. The alloys UNS S31600 and S30940 you are referring to are austenitic stainless steels and are covered by Table A.2 and Clause A.2. The MP does not answer questions concerning specific applications.

This question is in relation to NACE MR0175/ISO 15156-3  A.3.2, Table A.8

Reference: ISO 15156 Maintenance Panel Inquiry #2010-12

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