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NACE MR0175

materials_q_and_a_graphicQ: I've heard there are major changes to the 2003 revision of NACE MR0175. Why was the standard changed so dramatically and how will the changes affect valves?

A: NACE MR0175 has been revised many times since it was initially released in 1975. In fact, since 1990, the document has been revised on an annual basis. The 2003 revision involved some major changes in the scope of the document, and included many changes in material requirements and environmental application limits. For the first time ever, materials were actually deleted from the document. In addition, MR0175-2003 is in the process of being converted to an ISO standard (ISO 15156 "Petroleum and Natural Gas Industries-Materials for Use in H2S-Containing Environments in Oil and Gas Production"), which will undoubtedly bring more changes.

A brief review of the document history will help to explain the motivation for some of these changes. When MR0175 was initially released, its scope was limited to valves, as indicated by the title: "Materials for Valves for Resistance to Sulfide Stress Cracking in Production and Pipeline Service." After several sulfide stress-cracking failures occurred in Texas in the mid-1970s, the Texas Railroad Commission adopted MR0175 as a standard requirement for all production equipment in sour service. The scope of the document was expanded in the 1978 revision to cover general use, and the title was changed to "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment."

Since that time, many letter ballots have been processed to modify the document. Most of the ballot items have pertained to the addition of new materials and/or new material processing options.

As the document evolved, so did voting practices. Some voters began to recognize that many of the newer corrosion-resistant alloys (CRAs) being balloted were only used in hot, chloride-containing environments, and they began requesting submission of high-temperature H2S/chloride stress corrosion cracking test data in those instances. These test data were then used to establish environmental limits (i.e., limits on maximum temperature, chloride content, H2S partial pressure, pH, elemental sulfur, etc.) for the material.

Because of this shift in voting and negative vote resolution practices, newer materials were sometimes subject to application restrictions even though similar materials that had been balloted in previous years were not, creating an uneven playing field for the various manufacturers of CRAs.

The 2003 revision attempts to alleviate some of these discrepancies, among many other changes. The scope of MR0175-2003 was expanded to cover chloride stress-corrosion cracking, as indicated in the new title: "Metals for Sulfide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments."

Changes in requirements for carbon and alloy steel base materials were minor. However, the CRA section, which essentially covers everything from 400-series stainless steels through the non-ferrous materials, changed quite dramatically as a result of the expanded scope and to bring the document into alignment with some of the policies adopted by the major petroleum production companies. Following are a few of the changes that will likely affect the valve industry:

  • The welding requirements for P-No. 1 carbon steels in pre-2003 versions have always been somewhat ambiguous and were often misinterpreted. Although the intent has not changed, the requirements in the 2003 version of MR0175 have been re-worded to be much clearer, especially with regard to requirements for welding without post-weld heat treatment. Some companies may need to review their welding practices to ensure they comply with the clarified wording.
  • MR0175-2003 defines acceptable austenitic stainless-steel grades using a chemical composition range rather than listing each individual alloy. This allows the use of many international stainless steel-grades that previously were technically unacceptable. However, austenitic stainless steels are restricted to 140°F (65°C) maximum to prevent chloride stress cracking. There are H2S limits and elemental sulfur restrictions as well. This means that 300-series stainless-steel valves and/or trim are no longer acceptable per MR0175 for a large number of applications where they have historically been used successfully.
  • Wrought S17400 (17-4) was removed completely from the general section. Wrought S17400 and S15500 (15-5), and cast CB7Cu-1 and CB7Cu-2 (17-4 and 15-5) are allowed for internal, non-pressure retaining components in valves,pressure regulators, and level controllers. In general, this will mean that these materials may no longer be used for valve stems and shafts, since they are considered pressure retaining. Wrought S17400 is allowed for pressure-retaining components in wellhead and Christmas tree valves, but there are severe environmental restrictions.
  • N05500 (alloy K500) and N07750 (alloy X750) are no longer listed in the general section. Both are allowed for internal, non-pressure-retaining components in valves, pressure regulators, and level controllers, and N07750 is still allowed for use in springs. However, neither may be used for pressure-retaining bolting that is considered "exposed."
  • N04400 and M35-1 (wrought and cast versions of Monel®) are no longer listed anywhere in the document. These alloys can no longer be used for any MR0175 application.

There are "loopholes" in the document allowing the use of unlisted materials based upon successful documented field experience and/or laboratory data. However, these are of limited usefulness to valve companies, who generally do not have the historical information or resources to pursue these options.

Again, this is only a brief summary of some of the major changes. For more information, a copy of MR0175-2003 can be obtained from NACE International. In addition, a free summary of inquiries and responses pertaining to MR0175-2003 is available online at www.nace.org/nace/content/technical/MR0175InquiriesandResponses.pdf.

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