1、 Standard Practice Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During a Shutdown of Refinery Equipment This NACE International standard represents a consensus of those individual members who have reviewed this document, its sc
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11、t, 1440 South Creek Dr., Houston, Texas 77084-4906 (telephone +1 281-228-6200). Revised 2012-06-23 Revised 2004-03-27 Reaffirmed 1997-Mar-10 Revised October 1993 Revised December 1984 Approved October 1970 NACE International 1440 South Creek Drive Houston, Texas 77084-4906 +1 281- 228-6200 ISBN 1-57
12、590-039-4 2012, NACE International NACE SP0170-2012 (formerly RP0170) Item 21002 SP0170-2012 NACE International i _ Foreword This standard practice provides mitigation methods to protect austenitic stainless steels and other austenitic alloys from polythionic acid (PTA) stress corrosion cracking (SC
13、C) that can occur during a shutdown of refinery equipment. A shutdown includes the actual downtime period and the contiguous periods required to shut down and start up the equipment. This standard is directed toward preventing stress corrosion cracking (SCC) caused by polythionic acid (PTA) that can
14、 be formed by reaction of oxygen and water with sulfide corrosion products (i.e., metal sulfides) or with other oxidizable sulfur species (e.g., sulfur, hydrogen sulfide H2S). Primary mitigation methods to prevent PTA SCC include appropriate selection of materials and fabrication practices, nitrogen
15、 purging to exclude oxygen, alkaline washing of equipment surfaces, and use of dry air to prevent liquid water formation. Special considerations for protection of reactors are also discussed. This standard is intended primarily for petroleum refinery materials and corrosion engineers as well as insp
16、ection, operations, and maintenance personnel. While the main focus of this standard is on equipment in refinery process units such as desulfurizing, hydrocracking, and hydrotreating, in which the incidence of PTA SCC has been comparatively high, it may be applied to equipment in other refinery proc
17、ess units that use austenitic stainless steels and other austenitic alloys, such as crude distillation units, lube distillation units, coking units, and fluid catalytic cracking units (FCCUs), when the user may have a concern for PTA SCC. This standard was originally prepared in 1970 by NACE Task Gr
18、oup (TG) T-8-19, revised in 1984 and 1993, and reaffirmed in 1997 by Group Committee T-8. It was revised in 2004 and 2012 by TG 173, “Refinery Equipment, Polythionic Acid SCC Prevention: Review of NACE Standard RP0170.” TG 173 is administered by Specific Technology Group (STG) 34, “Petroleum Refinin
19、g and Gas Processing.” TG 173 is sponsored by STG 39, “Process IndustriesMaterials Applications and Experiences,” and STG 60, “Corrosion Mechanisms.” This standard is issued by NACE International under the auspices of STG 34. In NACE standards, the terms shall, must, should, and may are used in acco
20、rdance with the definitions of these terms in the NACE Publications Style Manual. The terms shall and must are used to state a requirement, and are considered mandatory. The term should is used to state something good and is recommended, but is not considered mandatory. The term may is used to state
21、 something considered optional. _ SP0170-2012 ii NACE International _ NACE International Standard Practice Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During a Shutdown of Refinery Equipment Contents 1. General . 1 2. Selectio
22、n of Materials and Fabrication Practices 3 3. Dry Nitrogen Purging to Exclude Oxygen . 6 4. Alkaline Washing of Equipment Surfaces . 7 5. Dry Air to Prevent Liquid Water Formation . 8 6. Special Considerations for Protection of Reactors . 9 References 10 Bibliography 11 Appendix A (Nonmandatory): Ba
23、ckground Information About Polythionic Acid Stress Corrosion Cracking . 11 FIGURES: Figure A1: Dye Penetrant Inspection Showing Extensive PTA SCC in the Vicinity of Welds 13 Figure A2: PTA SCC of Austenitic Stainless Steel (200X) . 14 Table 1: Reported Sensitization Temperature Range for Some Austen
24、itic Materials 3 _ SP0170-2012 NACE International 1 _ Section 1: General 1.1 This standard practice provides mitigation methods to protect austenitic stainless steels and other austenitic alloys from PTA SCC that can occur during a shutdown of refinery equipment. 1.1.1 For the purposes of this stand
25、ard, a shutdown includes the actual downtime period and the contiguous periods required to shut down and start up the equipment. 1.1.2 For the purposes of this standard, the term austenitic materials includes austenitic stainless steels and other austenitic alloys. 1.1.3 For the purposes of this sta
26、ndard, the term other austenitic alloys refers to those alloys of nickel, iron, and chromium that may be susceptible to PTA SCC. 1.1.4 For the purposes of this standard, the term purging is defined as a flow of inert gas (in this case dry nitrogen) to free the system of impurities (oxygen). 1.2 This
27、 standard is directed toward preventing SCC caused by PTA that can be formed by reaction of oxygen and water with sulfide corrosion products (i.e., metal sulfides) or with other oxidizable sulfur species (e.g., sulfur, H2S). Appendix A (nonmandatory) provides background information about PTA SCC, in
28、cluding factors that contribute to PTA SCC, and where and under what conditions PTA SCC has been experienced. 1.3 The critical levels of sensitization and tensile stress required to initiate PTA SCC are not well understood. Therefore, austenitic stainless steel and other austenitic alloy process equ
29、ipment that may be exposed to PTA should be protected using one or more of the PTA SCC mitigation methods presented in this standard, except in those cases when the equipment operates below the sensitizing temperature range and the austenitic material has not been sensitized by any prior fabrication
30、 practices (e.g., hot forming, welding, heat treatment). PTA SCC mitigation methods are listed in Paragraphs 1.3.1 through 1.3.4, and more details on each mitigation method are provided in later sections of this standard. Users may select one or more of these mitigation methods depending on their ne
31、eds and assessment of exposure risk, which includes the ability of the selected mitigation method(s) to reduce the PTA SCC risk and the possibility of creating additional exposure risks when implementing the selected mitigation method(s). 1.3.1 Selection of Materials and Fabrication Practices Select
32、ion of materials and fabrication practices are made that result in a fabricated component or process equipment resistant to sensitization, supported by an assessment of the risk of PTA SCC associated with such selections. When the risk associated with potential PTA SCC is judged to be acceptable, th
33、e user may not require the application of other mitigation methods. 1.3.2 Nitrogen Purging to Exclude Oxygen A dry nitrogen purge is used to exclude oxygen from the equipment. Use of a dry nitrogen purge may also exclude water from the equipment. 1.3.3 Alkaline Washing of Equipment Surfaces Alkaline
34、 washing of equipment surfaces is used to neutralize any PTA that may form. Field experience has demonstrated that austenitic stainless steels and other austenitic alloys are effectively protected when alkaline wash solutions are properly applied to all equipment surfaces. NOTE: The user must consid
35、er other factors such as the effect of the alkaline chemicals on catalysts, as well as the appropriate means and protective equipment required for handling these chemicals. 1.3.4 Dry Air to Prevent Liquid Water Formation The use of dry (dehumidified) air for protection from PTA SCC is acceptable if
36、the dew point temperature of the air entering the equipment is maintained a minimum of 22 C (40 F) lower than the internal surface metal temperature.1 SP0170-2012 2 NACE International 1.4 Regardless of the mitigation method(s) selected, the user must take appropriate confirmation steps to validate c
37、ompliance with the requirements of this standard to ensure that protection from PTA SCC is provided. 1.5 If process equipment remains unopened and “hot” (i.e., above the water dew point temperature of the vapor in the equipment), additional protection from PTA SCC is unnecessary. 1.6 The PTA SCC mit
38、igation methods described in this standard are not designed to remove process-related chloride deposits, such as ammonium chloride (NH4Cl) that can cause corrosion or chloride SCC, from the equipment. The alkaline washing method of PTA SCC protection entails some risk of chloride SCC. However, the a
39、lkaline washing procedures described in this standard should minimize the likelihood of chloride SCC by the alkaline wash solutions. 1.7 Heaters used in process units often have austenitic stainless steel or other austenitic alloy tubes, which are subject to coking, for preheating reactor feed or re
40、cycle gas, or both, containing H2S and other sulfur compounds. The austenitic stainless steel tubes in these services can be susceptible to internal PTA SCC. If not decoked, the heater tubes should be kept dry; effectiveness of alkaline wash is low when coke is present. If the heater tubes will be i
41、nternally exposed to air (for maintenance work) and alkaline washing is the selected PTA SCC mitigation method, decoking of the tubes should be done prior to or concurrent with the alkaline washing to allow the alkaline wash solution to reach the internal tube metal surfaces. 1.7.1 Thermal decoking
42、procedures should ensure that the heater tubes are not subject to condensation of water prior to completion of decoking, and PTA SCC protection should be provided immediately after decoking. 1.7.2 Pig decoking procedures should use alkaline wash solutions during and after decoking. 1.7.3 There have
43、been numerous cases in which alkaline wash solutions have been very difficult to fully drain from vertical heater tubes. Boiling/evaporation of the residual alkaline wash solution during startup has resulted in concentration of carbonate and chloride salts, leading to SCC or solids plugging (tube ov
44、erheating from loss of flow). Alkaline washing of vertical heater tubes should be done only if effective draining and drying procedures such as nitrogen sponge pigging are developed. Another option is to circulate hydrocarbon (turbulent flow required) to flush out alkaline wash solutions. 1.8 PTA SC
45、C protection of the external surfaces of austenitic stainless steel and other austenitic alloy heater tubes should be considered when sulfur-containing fuels have been used for heater firing. 1.8.1 In many applications, combustion conditions in the firebox result in formation of oxide scales on the
46、external surfaces of the heater tubes rather than formation of the sulfide scales that are a precursor to PTA formation. External surfaces of austenitic stainless steel and other austenitic alloy heater tubes with oxide scales do not require PTA SCC protection. 1.8.2 When combustion practices (e.g.,
47、 fuel-rich combustion) lead to reducing conditions that generate sulfide scales externally on heater tubes, moisture exposure during turnarounds should be minimized. PTA SCC protection during a shutdown should be considered based on a risk analysis. If warranted, one of the following PTA SCC mitigat
48、ion methods should be used: 1.8.2.1 Alkaline wash the external surfaces of the heater tubes. 1.8.2.2 Prevent moisture condensation on the external heater tube surfaces by maintaining tube and firebox temperatures above the water dew point. Depending on the dew point temperature, this may be accomplished either by keeping pilots burning or keeping a burner at minimum firing level when personnel entry into the firebox is not needed and safety procedures allow. External or internal electric heaters may also be used to heat the
