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1、Item No. 24236 NACE International Publication 34108 This Technical Committee Report has been prepared by NACE International Task Group 347,* “Petroleum Refineries, Environmental Cracking: Reviews of Carbonate Stress Corrosion Cracking” Review and Survey of Alkaline Carbonate Stress Corrosion Crackin
2、g in Refinery Sour Waters This NACE International (NACE) technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or usin
3、g products, processes, or procedures not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifyi
4、ng or protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable c
5、ircumstances may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and f
6、or determining their applicability in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report
7、. Users of this NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of t
8、his report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE repo
9、rts may receive current information on all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281-228-6200). Foreword This technical committee report describes the state of the art regarding alkaline carbona
10、te stress corrosion cracking (ACSCC) experiences in certain sour waters found in petroleum refining plants. This report outlines where ACSCC has occurred in process equipment in petroleum refining service, the refining communitys current theory(ies) on the conditions and mitigation techniques that m
11、ay have an impact on this type of damage, and analytical and inspection techniques that have been used to address the issue. The main focus of this report is on fluidized catalytic cracking units (FCCUs) and light ends recovery systems and associated sour water processing equipment where the majorit
12、y of the damage has been reported. The technical committee has also chosen to include ACSCC in sour water stripper (SWS) units. This report is intended to provide the refining industry with a resource for current information that is available on ACSCC. The main sections of this report include: What
13、Is Alkaline Carbonate Stress Corrosion Cracking? Where Has ACSCC Occurred? What Conditions Are Observed When ACSCC Occurs? Literature-Proposed Regions for ACSCC Susceptibility Chemical Analysis in ACSCC Likelihood Assessment Inspection for ACSCC Mitigation of ACSCC Supplemental information is provid
14、ed in the following appendixes: Appendix A: Examples of ACSCC Appendix B: Graphs of TG 347 Survey Data Appendix C: Proposed Guidelines for ACSCC Susceptibility Appendix D: Examples of CO2/Bicarbonate/ Carbonate Equilibrium Curves _ * Chair John Wodarcyk, Conoco Phillips, Ponca City, OK. Copyright NA
15、CE International Provided by IHS under license with NACELicensee=Boeing Co/5910770001 Not for Resale, 01/31/2009 01:24:52 MSTNo reproduction or networking permitted without license from IHS -,-,- NACE International 2 This technical committee report was prepared by Task Group (TG) 347, “Petroleum Ref
16、ineries, Environmental Cracking: Review of Carbonate Stress Corrosion Cracking,” which is administered by Specific Technology Group (STG) 34, “Petroleum Refining and Gas Processing,” and is sponsored by STG 60, “Corrosion Mechanisms.” This technical committee report is issued by NACE International u
17、nder the auspices of STG 34. NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific applications of corrosion mitigation technology, whether considered successful or not. Statements used to
18、 convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to be recommendations for general application of this technology, and must not be construed as such. Wha
19、t Is Alkaline Carbonate Stress Corrosion Cracking? ACSCC, or “carbonate cracking” as it is often referred to, manifests itself as surface-breaking cracks that typically develop at or near carbon steel welds under a combination of tensile stress and corrosion in an alkaline carbonate- containing envi
20、ronment. The location of these cracks is usually in the base metal within 2 in (50 mm) of the weld. There have been individual reports of cracking occurring more than 3 in (80 mm) from the weld in the base metal of a highly cold- worked elbow and in a 10 x 6 in (250 x 150 mm) reducer that cracked in
21、 the middle of the reducer (see Appendix A, Example 4). There have been at least two reports of cracking in the weld metal itself. When cross-sections of damaged areas were analyzed, the damage has consisted of intergranular oxide-filled cracks having an appearance similar to caustic stress corrosio
22、n cracking and amine stress corrosion cracking. In the past, this type of damage has been found mainly in the pipeline industry on the outside surface of buried carbon steel pipelines in certain soil environments. Also, ACSCC has been found in carbon steel process vessels and piping that contain aqu
23、eous solutions with high concentrations of carbonate (e.g., potassium carbonate) that are used in the carbon dioxide (CO2) removal facilities associated with steam methane reformers, and in which CO2 and carbonate are the primary forms of corrosive compounds. Those environments are not addressed in
24、this report. This report includes environments where sulfides (e.g., dissolved H2S) are usually present. From information available in literature, ACSCC appears to occur when certain parameters exceed a threshold value. These parameters include the carbonate ion(1) (CO32) concentration and the pH of
25、 the aqueous solution. Also, ACSCC appears to occur when the corrosion (open- circuit) potential is in a certain range (active-passive transition). Other parameters may have an impact on the susceptibility to ACSCC. (See section titled “What Conditions Were Observed When ACSCC Occurred?” for additio
26、nal information.) Where Has ACSCC Occurred? ACSCC of carbon steel exposed to refinery process streams has been reported since the early 1980s. Most of the reports have been associated with the FCCU main fractionator overhead systems and FCCU gas plants, but some reports have been associated with oth
27、er refinery process services. The reports of ACSCC damage have been sporadic, but the number of reported incidents has appeared to increase since 2000. This increase may be attributed to the fundamental operating change to the production of low- sulfur (S) products directly from the FCCU. To further
28、 define this trend, TG 347 undertook an industry survey.1 Results of the survey are discussed in this report and summarized in graphs included in Appendix B. The responses received included 44 process units in 36 refineries. Survey responses were split into three process groupsFCCU, SWS, and cokers.
29、 The FCCU and SWS responses were split for further analysis into two populations: one with no ACSCC reported (85%) and the second with ACSCC reported either in piping or in vessels (15%). Unless otherwise stated, the statistics for the TG 347 survey presented in this report are based on individual c
30、ases (segments of the system operating under similar conditions) rather than units. Though requested based on some anecdotal reports of ACSCC in cokers, only one case of ACSCC in a delayed coker light ends unit was reported in the two survey responses involving cokers. The cracking occurred in the p
31、iping between the second stage separator and downstream fractionator. _ (1) The carbonate ion discussed here is calculated from the composition of the solution and is not the total carbonate value typically reported by the laboratory. Copyright NACE International Provided by IHS under license with N
32、ACELicensee=Boeing Co/5910770001 Not for Resale, 01/31/2009 01:24:52 MSTNo reproduction or networking permitted without license from IHS -,-,- NACE International 3 Only 38% of ACSCC reported occurred in vessels including drums, exchangers, compressor knockout drums, and tanks. ACSCC was reported in
33、only two vessels that had received a postweld heat treatment (PWHT). Mitigation steps (e.g., PWHT) that typically have been taken to reduce the potential for wet H2S cracking in carbon steel vessels may have mitigated ACSCC also. This could not be confirmed by the TG 347 survey data because only 13%
34、 of the respondents provided any information on PWHT. Most reported ACSCC damage occurred in piping components (62% of ACSCC reported). Many (60%, 9 of 15) of those reporting ACSCC in piping have not reported ACSCC in vessels. Piping typically has been deemed not as vulnerable to wet H2S cracking da
35、mage and therefore did not often have PWHT as an additional mitigation measure in place. Of the 58% who provided information on PWHT in the TG 347 survey, no respondent indicated having performed PWHT on piping. The vessels and piping in which ACSCC has occurred were in service for less than one yea
36、r to well over 50 years. In at least one case, a section of piping in service for five to six months is known to have leaked as a result of through-wall ACSCC. See Appendix A, Examples 2 and 4, for details. FCCU Main Fractionator Overhead and Gas Plant The reported incidents of ACSCC include overhea
37、d lines, exchanger shells, sour water piping, reflux piping, and piping downstream from the coolers on the last stage of wet gas compression in the FCCU gas plant.1,2,3,4,5 In most refineries where ACSCC has occurred, the ACSCC typically has not been found beyond the last stage of wet gas compressio
38、n in the FCCU gas plant. One incident has been reported in the overhead system of a fractionator in a gas plant downstream from the FCCU wet gas compression section. One failure of a sour water tank that only received sour water from an FCCU was confirmed to be ACSCC by metallographic analysis (see
39、Appendix A, Example 1). The tank was repaired and coated, but did not receive PWHT. A compilation of the areas in the FCCU where ACSCC has been reported during Technology Exchange Group (TEG) 205X/Group Committee T-8 corrosion information exchanges, and information reported to committee members thro
40、ugh the TG 347 survey, is presented in Figure 1. Copyright NACE International Provided by IHS under license with NACELicensee=Boeing Co/5910770001 Not for Resale, 01/31/2009 01:24:52 MSTNo reproduction or networking permitted without license from IHS -,-,- NACE International 1 Figure 1 Simplified pr
41、ocess flow diagram of a typical FCCU showing areas where ACSCC has been reported. (These data are not an example of a specific unit in which ACSCC was found but a compilation of data from various units.) SWS Unit Piping in a SWS unit was reported to have failed as a result of ACSCC. In this particul
42、ar unit, the damage was found in the pumparound return line to the top section of the stripping tower. The ACSCC occurred near the weld in a 4 in (100 mm) section of piping (see Appendix A, Example 3). This unit is the most common type of SWS that uses one stripping tower to strip both the hydrogen
43、sulfide (H2S) and ammonia (NH3) from the sour water. During the STG 34 information exchange (TEG 205X) at Corrosion Technology Week 2007, an ACSCC failure of the U-bends in a carbon steel bundle in a SWS pumparound exchanger was reported. The U-bends of the ASME(2) SA-1796 tubes had been induction h
44、eat- treated and still failed. Others at the meeting reported success with a stress relief performed in a furnace to prevent stress corrosion cracking in similar services. The refiner chose to use a floating head bundle instead of the U-tube bundle to eliminate the cold-worked U-bends. Some SWS unit
45、s use two separate stripping towers in series to strip the H2S and NH3 from the sour water. In this particular type of SWS unit, most of the H2S and some of the NH3 are stripped from the sour water in the first stripping tower. The partially stripped sour water from the bottom of the first tower is
46、then sent to a second stripping tower, in which the majority of the NH3 and remaining H2S are stripped from the sour water. Some believe that this type of SWS unit has a higher potential for ACSCC because of the higher NH3 concentrations in the second stripping tower. However, no survey responses in
47、dicated ACSCC problems in this type of SWS unit. _ (2) ASME International (ASME), Three Park Ave., New York, NY 10016-5990. 4 Copyright NACE International Provided by IHS under license with NACELicensee=Boeing Co/5910770001 Not for Resale, 01/31/2009 01:24:52 MSTNo reproduction or networking permitt
48、ed without license from IHS -,-,- NACE International 1 What Conditions Are Observed When ACSCC Occurs? This section discusses process parameters that could cause the environment in FCCUs and SWSs to move into or out of the region of ACSCC susceptibility. This information may be useful for identifyin
49、g possible influences that an operator could use to trigger reassessment of the likelihood of ACSCC within a unit. The chemistry of the resulting environment generated in each of the units in which ACSCC occurs is influenced by the feed, the operation of the various systems in the unit (especially fractionation), and wa