MR0103与MR0175两个规范的对比

AN OVERVIEW OF NACE INTERNATIONAL STANDARD MR0103 AND COMPARISON WITH MR0175 Don Bush Emerson Process Management / Fisher Controls Intl. LLC PO Box 190 Marshalltown, Iowa 50158 don.bush@emersonprocess.com Jeff Brown Motiva Enterprises PO Box 37 Rt 44 & 70 Convent, LA 70723 Keith Lewis Shell Global Solutions Intl., BV Badhuisweg 3 Amsterdam, 1031-CM Netherlands ABSTRACT NACE MR0103 "Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments"1 was developed by Task Group 231 to provide a standard set of requirements for materials used in sour petroleum refinery equipment. In the past, NACE MR01752, "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment", was frequently referenced for this equipment, even though refinery applications were outside the scope of MR0175. The process used to develop MR0103 is described, followed by a review of the requirements in the standard accompanied by highlights of the differences between MR0103 and the previous and current versions of MR0175. INTRODUCTION AND DOCUMENT HISTORY In 1975, NACE issued standard MR0175, "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment", to cover requirements for materials resistant to sulfide stress cracking (SSC) in sour oilfield environments. Although the scope of MR0175 includes only oilfield equipment and associated facilities (including gas production and treatment), the lack of similar standards for other industries has compelled many users in those industries to reference MR0175 for materials destined for “sour” applications. Although the process conditions that constitute the non-oilfield “sour” environments are often quite different from those defined in MR0175, the material and material condition requirements have proven to be fundamentally on target. Copyright 2004 NACE International. All rights reserved. Paper Number 04649 reproduced with permission from CORROSION/2004 Annual Conference and Exhibition, New Orleans, Louisiana. www.nace.org In the late 1990’s, the NACE T-1F-1 task group, now called Task Group (TG) 081, began working on a complete rewrite of MR0175 that included a number of fundamental changes. One of the most significant proposed changes was the expansion of the scope of the document to include chloride stress corrosion cracking (SCC), based upon the fact that most oil and gas production streams contain chlorides in sufficient levels to cause SCC in susceptible alloys. As such, the proposed rewrite included maximum temperature limits for all materials that are susceptible to chloride SCC. For example, the rewrite proposed that the temperature limit for S31600 (type 316 stainless steel) be set at 60°C (140°F) maximum. The proposed changes would mean that MR0175 would be less suitable for use in many applications, including those in petroleum refineries, where chloride ion concentrations tend to be low enough that chloride SCC isn’t a common concern. Initial discussion regarding the proposed changes to MR0175 and the potential development of a refinery-specific standard covering materials for sour environments occurred during the 1997 Fall Committee Week T-8 Information Exchange session. Further discussions, including review of drafts of proposed document sections, were held at subsequent T-8 Information Exchange sessions and at several Task Group (TG)T-8-25 ("Environmental Cracking") meetings. At Corrosion/2000, it was decided that a T-8-25 Work Group (T-8-25a) would be formed to develop a sulfide stress cracking document. This Work Group was eventually formed in June 2000 as TG (Task Group) 231 under the current NACE technical committee structure. TG 231 is administered by STG (Specific Technology Group) 34 "Petroleum Refining and Gas Processing" and sponsored by STG 60 "Corrosion Mechanisms". The task group's writing approach was to borrow pertinent concepts and requirements from the current and proposed versions of MR0175, and modify them as needed to create a new standard that would meet the needs of the oil refining industry. For example, the resulting document utilized the alloy grouping philosophy that is used in what is now MR0175-20033, but did not implement environmental limits such as H2S partial pressures, temperature limits, pH restrictions, etc. Materials and material condition requirements are based upon a mix of MR0175-2002 and MR0175-2003 requirements and refinery-specific experience. Because of this approach, there are paragraphs in MR0103 that are identical to corresponding paragraphs in one or both versions of MR0175, whereas in other instances, the requirements in MR0103 have been modified to better suit the needs of the oil refining industry. The final result is a document that differs from previous and current versions of MR0175 in the following ways: • The refinery standard guidelines for determining whether an environment is "sour" are quite different from the sour environment definitions provided in previous and current versions of MR0175. • The refinery standard does not include environmental restrictions on materials. • Materials and/or material conditions are included in the refinery standard that are not listed in previous and/or current versions of MR0175. • Materials and/or material conditions are included in previous and/or current versions of MR0175 that are not listed in the refinery standard. • Because welding is prevalent in refinery piping and equipment, extra emphasis is placed upon welding controls in several material groups, most notably the carbon steels. The document was developed using the approved NACE work process. Various sections were drafted, reviewed at Corrosion and Fall Committee Week meetings, and then finalized based upon the feedback that was received. The "final" draft was sent out for formal letter ballot in mid-July 2002. This initial ballot resulted in 4 negative votes and 17 affirmative votes with comments. The document was modified to address the negative votes and other comments, and was sent out for reballot in January 2003. The reballot passed with a 97% affirmative vote after negative vote resolution. The MR0103 standard "Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments" was published in mid-April 2003. Following is an overview of the document, including discussion of pertinent differences among MR0175-2002, MR0175-2003, and MR0103. NACE CORROSION/2004 Paper 04649 Page 2 www.nace.org APPLICABILITY OF MR0175 AND MR0103 Both MR0175 and MR0103 include sections that describe the applicability of each of the Standards. Within each of these sections there are sub-sections that describe the material and environmental factors that affect susceptibility of materials to SSC and also provide guidelines to the user on how the Standard should be applied. It is extremely important to note that in both MR0175 and MR0103 the user is responsible for determining and judging whether the environmental conditions are such that the material requirements of the Standard should be applied. One of the key differences between the MR0175 and MR0103 Standards lies in the guidelines addressing the environmental conditions under which SSC is likely to occur. This difference between the upstream (oil and gas production) and downstream (refining and gas processing) environments was one of the principal reasons why NACE STG 34/TG 231 decided to write the MR0103 Standard. MR0103 is more focused on a broader range of sour environments conditions experienced in downstream process units. The MR0175 definition of sour service environments in upstream processes is very well known and understood, having remained essentially unchanged for almost 30 years. In the 2003 version of MR1075 the environmental conditions likely to cause SSC are described in Paragraphs 1.4.1 and .4.2 with sample calculations in Appendix A. Simply summarized, these conditions consist of a partial pressure of H2S in the wet gas phase of a gas, gas condensate or crude oil equal to or exceeding 0.0003MPa abs (0.05 psia). For gas systems there is a low-pressure cut-off (i.e., total system pressure below which SSC is not expected to occur) of 0.45 MPa abs (65 psia) and for multiphase phase systems the low-pressure cut-off is 1.83 MPa abs (265 psia), (with other conditions applying). The MR0175 definition of sour service has also been widely and successfully applied by users in many downstream facilities either directly in company specifications and practices or indirectly via the application of API equipment specifications such as API RP 6104, 6175 and 6186. However, for downstream applications many users, engineering contractors and suppliers have over the years developed their own practices on how and when MR0175 material requirements should be applied. These practices have ranged between: • No application at all, irrespective of H2S level since some downstream users have considered MR0175 strictly applicable to upstream applications, • Application of MR0175 material requirements to any process containing H2S, including trace levels in services with no free water present. In the new MR0103 Standard an attempt has been made to develop consensus guidelines on what constitutes sour service in downstream units based on: • User’s plant experience and practices; • Existing NACE and industry recommended practices and reports (i.e. NACE RP02967, 8X1948, 8X2949, API Publication 58110); • A fundamental understanding of atomic hydrogen generation in the sour service corrosion reaction and the subsequent rate of hydrogen flux into the process-contacted steel i.e., combined effects of pH, H2S and HCN. A significant difference between upstream and downstream sour environments is that in many refinery sour water environments dissolved ammonia is present which increases the pH thereby NACE CORROSION/2004 Paper 04649 Page 3 www.nace.org increasing the solubility of H2S, which in turn increases the bisulfide ion concentration and corrosivity. Ammonium bisulfide corrosion in these high pH environments generates a relatively high rate of hydrogen flux. Furthermore, the presence of cyanides at an elevated pH further aggravates the degree of atomic charging and hydrogen flux into the steel by poisoning the surface reaction that results in a stable and protective iron sulfide scale from forming. The outcome of the consensus approach, embodied in MR0103, has resulted in the following guidelines (with additional explanation in parenthesis) on what constitutes a sour enough service in downstream units to justify the application of the Standard’s material requirements (Note: the presence of a free water phase is a prerequisite for aqueous corrosion and SSC): • >50 ppmw dissolved H2S in the free water (recognition that significant levels of dissolved H2S can result in SSC even in low pressure systems), or • A free water pH < 4 and some dissolved H2S present (recognition that in low pH environments significant charging of materials with atomic hydrogen can take place irrespective of H2S level), or • A free water pH > 7.6 and > 20 ppmw hydrogen cyanide ion (HCN) and some H2S dissolved in the free water (recognition that at high pH the HCN ion is stable and results in significant charging of ferritic materials by poisoning the formation of a protective iron sulfide scale), or • >0.0003 MPa abs (0.05 psia) partial pressure H2S in a process with a gas phase (based on historical MR0175 definition of sour service, without low-pressure cut-offs). Another key difference between the MR0175 and MR0103 Standards is the way the user is expected to use the guidelines on environmental conditions. In MR0175 the user is obligated apply the material requirements of the Standard when it is judged that the environmental conditions prescribed in the Standard have been exceeded; however, there is relatively little judgment required since the environmental conditions for SSC are tightly defined with sample calculations provided in Appendix A. In MR0103 the user is also obligated to determine whether the equipment falls within the scope of the standard; however, more judgment of the environmental conditions is permitted, and the user may supplement the environmental guidelines in the Standard with actual plant experience and risk based analysis to make a determination on applicability (API Publication 581 provides a methodology for such an analysis). However, when making this judgment the MR0103 user is expected to consider all plant operating scenarios including operational upsets, start-up/shutdown conditions etc. MATERIALS OF CONSTRUCTION Carbon Steels Carbon steels are the workhorse materials in refineries, and as such they have received a great deal of attention in previous NACE activities. For the most part, refineries use carbon steels classified as P-No. 1 Group 1 or 2 in Section IX of the ASME Boiler and Pressure Vessel Code11 (grades such as ASTM A10512 forgings, ASTM A21613 WCC and A35214 LCC castings, ASTM A51615 Grade 70 plate, ASTM A106 Grade B pipe) for piping and vessels. Unlike MR0175-2002 and MR0175-2003, MR0103 imposes no base metal hardness requirements on these materials due to the fact that these grades have maximum tensile strength requirements that effectively limit their bulk hardness. Other carbon steels are required to meet a 22 HRC maximum requirement. NACE CORROSION/2004 Paper 04649 Page 4 www.nace.org MR0103 shares the following requirements with MR0175-2002 and MR0175-2003: • Carbon steels must be in one of the following heat treatment conditions: (a) hot-rolled (b) annealed (c) normalized (d) normalized and tempered (e) normalized, austenitized, quenched, and tempered (f) austenitized, quenched, and tempered • Carbon steel materials that are cold worked to produce outer fiber deformation greater than 5%, must be stress relieved to ensure that the material is below 22 HRC. Welding of Carbon Steels Welding introduces the potential for creation of hard regions in carbon steels. As such, controls must be imposed to ensure that weldments will be soft enough to resist sulfide stress cracking in service. MR0103 requires that welds in P-No. 1 carbon steel materials be performed per the methods outlined in NACE Standard RP0472 “Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments”16. RP0472 is a recommended practice document that was issued by the T-8 Unit Committee on Refinery Corrosion in 1972. Note that this document actually pre-dates MR0175, although the scope and requirements have changed somewhat since its initial release. RP0472 requires that the weld deposit meet a hardness limit of 200 HBW maximum. It allows control of heat-affected zone (HAZ) hardness by several different methods. Those methods include: Post-weld heat treatment (PWHT): PWHT serves two purposes. As a tempering process, it reduces the hardness of the weld deposit and the heat affected zone (HAZ). As a stress relieving process, it reduces residual stresses in the weldment through stress relaxation. Both of these effects tend to reduce the probability of failure due to SSC. Although some of the ASME codes allow the option of using lower temperatures for longer times, this option is not recommended. Using lower temperatures for longer times may provide reduction in residual stresses, the primary concern of the ASME codes, but is less likely to reduce HAZ hardness, which is the primary factor in reducing susceptibility to SSC. Base metal chemistry controls: This technique involves controlling of carbon content and/or carbon equivalent and levels of micro-alloying elements in base metals to such low levels that low hardness is virtually guaranteed in the weld deposit and HAZ regardless of welding process parameters. The carbon equivalent of a particular heat of material is calculated from the heat chemistry using the following equation: 5 %V)%Mo(%Cr 15 %Cu)(%Ni 6 %Mn %CCE ++ + + ++= NACE Committee Report 8X194 states that a maximum carbon equivalent of 0.43 is commonly specified for base materials when this technique is employed. Deliberate additions of micro-alloying elements (greater than 0.01% each of Cb, V, and Ti, or greater than 0.0005% B) are usually prohibited to ensure that hardenability will remain low. NACE CORROSION/2004 Paper 04649 Page 5 www.nace.org HAZ hardness testing during welding procedure qualification: When this method is used, a procedure qualification record (PQR) specimen is created using either actual production material or a coupon of representative material with an actual carbon equivalent corresponding to the maximum carbon equivalent value that is to be applied to production base material. Welding variables (such as filler metal, preheat, current, voltage, travel speed, interpass temperature, etc.) are controlled and documented during the creation of the PQR specimen. The PQR tests include a hardness traverse performed using the 5-kgf or 10-kgf Vickers scale or the Rockwell 15N scale to demonstrate that the weldment hardness does not exceed 248 HV or 70.5 HR15N in the weld metal, HAZ and base metal. The resulting welding procedure specification (WPS) is written to contain restrictions to ensure that the PQR specimen is actually representative of production weldments. Those restrictions include the following: • The procedure may only be used to weld a base metal of the same specification, grade, and class as that of the PQR specimen. In other words, a procedure qualified on ASTM A516 Grade 70 plate material could not be used to weld ASTM A516 Grade 60 plate material, ASTM A105 forgings, or ASTM A216 Grade WCC castings, even though all are within the same ASME Section IX P-No. 1 category. • The maximum CE and micro-alloying element contents of production material must be controlled to values less than or equal to those of the PQR specimen. • The heat input used during production welding must not deviate from the heat input used during creation of the PQR specimen by more than 10% lower or 25% higher. For the shielded metal arc welding (SMAW) process, the maximum bead size and the minimum length of weld bead per unit length of electrode used in creation of the PQR specimen can be imposed as an alternate requirement in the WPS. • Preheat and interpass temperatures must be at least as high as those utilized in production of the PQR specimen. • If preheat was not utilized for the PQR specimen, the maximum base metal thickness of production weldments must not be allowed to exceed the thickness of the PQR specimen. Other restrictions apply to fillet welds, submerged-arc welding (SAW), gas metal arc welding (GMAW), flux-cored arc welding (FCAW) processes, welding procedures involving bead-tempering techniques and other techniques that are sensitive to weld-bead sequence, and materials containing intentional additions of microalloying elements such as Nb (Cb), V, Ti, and B. This method may not be suitable for certain applications such as repair welding of castings. It is generally utilized for establishment of an acceptable welding procedure for a particular heat of material for a large job. An example would be the fabrication of a large vessel from a single heat of plate material which doesn't have chemistry restrictions that are adequate to guarantee low weldment hardness. The wording regarding welding of carbon steels in MR01