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SS316L in thermowell applications and NACE MR0175

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Question:

I have a technical query related MR0175/ISO 15156 and the use of 316 stainless steel for sour service application. This standard imposes restrictions on the use of 316 SS in environments operating above 60°C. My question is can 316 SS be used above 60°C for non-stressed vessel internals or for items such as thermowells located into sour lines or vessels? I ask this because I note that the standard need not be applied to parts loaded in compression (Table 1). The implication may be that parts have to be stressed for SCC to be an issue. As a similar situation to vessel internals and thermowells, please could you advise on the use of 316 stainless steel for valve internals in a sour application, operating above 60°C. Of particular interest is the use of solid 316 SS balls for ball valves.

Answer:

1.0 The scope of NACE MR0175/ISO 15156 Part 3, Paragraph 1, Sentence 1 defines the applicability of the standard. The standard need not be applied for equipment not covered by this sentence. In addition, in Table 1, parts loaded in compression are included among those considered to be “permitted exclusions.” SCC requires a tensile stress (applied and/or residual) to occur. There is no provision for any of the alloys in the standard for a threshold tensile stress below which failure cannot occur.
2.0 The Maintenance Panel cannot analyze the design of equipment. It is up to the manufacturer and equipment user to agree whether or not the scope or any of the listed exclusions in Table 1 apply for a given design.

This question is in relation to NACE MR0175/ISO 15156-3 Table A.2

Reference: ISO 15156 Maintenance Panel Inquiry #2005-03

 

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