SP0296-2010

Standard PracticeDetection, Repair, and Mitigation of Cracking in RefineryEquipment in Wet H 2S EnvironmentsThis NACE International standard represents a consensus of those individual members who havereviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone, whether he or she has adopted the standard or not, from manufacturing, marketing,purchasing, or using products, processes, or procedures not in conformance with this standard.Nothing contained in this NACE International standard is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, orproduct covered by Letters Patent, or as indemnifying or protecting anyone against liability forinfringement of Letters Patent. This standard represents minimum requirements and should in noway be interpreted as a restriction on the use of better procedures or materials. Neither is thisstandard intended to apply in all cases relating to the subject. Unpredictable circumstances maynegate the usefulness of this standard in specific instances. NACE International assumes noresponsibility for the interpretation or use of this standard by other parties and accepts responsibilityfor only those official NACE International interpretations issued by NACE International inaccordance with its governing procedures and policies which preclude the issuance ofinterpretations by individual volunteers.Users of this NACE International standard are responsible for reviewing appropriate health, safety,environmental, and regulatory documents and for determining their applicability in relation to thisstandard prior to its use. This NACE International standard may not necessarily address allpotential health and safety problems or environmental hazards associated with the use of materials,equipment, and/or operations detailed or referred to within this standard. Users of this NACEInternational standard are also responsible for establishing appropriate health, safety, andenvironmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any existing applicable regulatory requirements prior to theuse of this standard.CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may berevised or withdrawn at any time in accordance with NACE technical committee procedures. NACEInternational requires that action be taken to reaffirm, revise, or withdraw this standard no later thanfive years from the date of initial publication and subsequently from the date of each reaffirmation orrevision. The user is cautioned to obtain the latest edition. Purchasers of NACE Internationalstandards may receive current information on all standards and other NACE Internationalpublications by contacting the NACE International First Service Department, 1440 South Creek Dr.,Houston, Texas 77084-4906 (telephone +1 281-228-6200).Revised 2010-03-13Revised 2004-02-12Reaffirmed 2000-09-13Approved 1996-03-30NACE International1440 South Creek DriveHouston, Texas 77084-4906+1 281-228-6200ISBN 1-57590-013-0 © 2010, NACE InternationalNACE SP0296-2010(formerly RP0296-2004)Item No. 21078SP0296-2010________________________________________________________________________ForewordNACE International Task Group T-8-16, “Cracking in Wet H2S Environments,” was formed in 1988to conduct an organized study on the incidence and mechanisms of cracking in pressure vesselsoperating in refinery wet hydrogen sulfide (H2S) environments. Specific objectives were to (a)define the nature and extent of the problem by means of an industry survey; (b) define mechanismsfor the type of cracking found, to be accomplished primarily through a literature survey; (c) establishinspection guidelines for existing vessels; and (d) develop repair and mitigation guidelines forcracked vessels. Four work groups were formed to address these tasks. In 1990, a fifth work groupwas formed with a fifth objective, (e) to investigate material specifications and fabrication practicesfor new pressure vessels.This standard practice summarizes objectives (a), (c), and (d) listed above. A technical committeereport (NACE Publication 8X294)1 was issued to address objective (b). Finally, objective (e) washandled by another technical committee report (NACE Publication 8X194).2This standard is intended for use primarily by refinery corrosion and materials engineers and inspection, operations, and maintenance personnel. Information and guidance presented in thisstandard reflect the work of many individuals representing numerous companies worldwide.The titles and source information of the codes, specifications, and standards referred to ordiscussed in this standard are provided in Appendix A (nonmandatory) rather than listed infootnotes throughout the standard. Confining this information to one appendix should help readerswho have any interest in further research. This standard was originally prepared in 1996 by formerTask Group (TG) T-8-16, “Cracking in Wet H2S Environments.” It was reaffirmed in 2000 by GroupCommittee T-8, and revised in 2004 and 2010 by TG 268, “Wet H2S Cracking in PetroleumRefinery Pressure Vessels.”TG 268 revised this standard in 2010 to address a number of items raised by Specific TechnologyGroup (STG) 34 members as well as to respond to revisions in other applicable NACE standardssuch as SP0472.3 The original emphasis of this standard was on pressure vessels, and thisemphasis remains. However, with this revision, some limited information on piping has beenincluded at the request of TG 268 members and other members of STG 34.TG 268 is administered by STG 34, “Petroleum Refining and Gas Processing.” This standard isissued by NACE International under the auspices of STG 34.In NACE standards, the terms shall, must, should, and may are used in accordance with the definitionsof 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 something consideredoptional.________________________________________________________________________SP0296-2010________________________________________________________________________NACE InternationalStandard PracticeDetection, Repair, and Mitigation of Cracking in RefineryEquipment in Wet H2S EnvironmentsContents1. General (1)2. Mechanisms of Cracking (2)3. Inspection for Cracking (4)4. Repair of Cracked or Blistered Equipment (11)5. Mitigation Considerations for Operation (14)References (15)Bibliography (16)Appendix A: Cited Codes, Specifications, and Standards (17)Appendix B: Nature and Extent of Problem—Results from 1990 T-8-16a Survey (19)Appendix C: Typical Cracks Found in Wet H2S Environments (25)FIGURESFigure C1: SSC in HAZ of head-to-shell weld of FCCU absorber tower. The crack is on theASTM A 516-70 shell side. The numbers in the photograph are Knoop hardness values.(nital etch) (25)Figure C2: Hydrogen blister in ASTM A 516-70 amine contactor/water wash tower. (26)Figure C3(a): Hydrogen blisters on ID surface of amine contactor/water wash tower. (27)Figure C3(b): Cross-section of plate shown in upper photo illustrating HIC (“stepwise”cracking). (27)Figure C4: SOHIC in soft base metal extending from the tip of SSC in a hard HAZ of arepair weld in the shell of a primary absorber (deethanizer) column in an FCCU gasplant. The ASTM A 212-B steel shell was given PWHT at original fabrication, but therepair weld was not. (nital etch) (28)Figure C5: ASCC (carbonate cracking) of non-PWHT ASTM A 285-C steel shell of FCCUmain fractionator overhead accumulator. Cracking was found near welds in the lowerportion of vessel. (29)TABLESTable B1: Overall Summary (19)Table B2: Cracking Reported by Company (20)Table B3: Cracking by Process Unit (20)Table B4: Cracking vs. Operating Temperature (21)Table B5: Cracking vs. H2S Concentration (21)Table B6: Cracking vs. Steel Specification (22)Table B7: Cracking vs. Steel Grade (22)Table B8: Cracking vs. PWHT (22)Table B9: Cracking vs. Blistering History (23)Table B10: Cracking vs. Weld Repairs (23)Table B11: Depth of Cracking (23)Table B12: Crack Penetration (24)Table B13: Disposition of Cracked Pressure Vessels (24)________________________________________________________________________SP0296-2010 ________________________________________________________________________Section 1: General1.1 This standard is intended to be a primary source of information on cracking in wet H2S petroleum refinery environments and provides guidelines on the detection, repair, and mitigation of cracking of existing carbon steel refinery equipment in wet H2S environments.1.1.1 For the purposes of this standard, the term equipment refers to pressure vessels and piping made ofcarbon steel plate material. Refinery pressure vessels include items such as, but not limited to, columns or towers, heat exchangers, drums, reboilers, and separators.1.1.2 Limited cracking has been noted in seamless piping; therefore, the information in this standardconcentrates on longitudinally seam-welded pipe fabricated from plate.1.1.3 Information on fabrication and inspection practices for new pressure vessels (never in service) is inNACE Publication 8X194.1.2 For the purposes of this standard, the term wet H2S environments includes, but is not limited to, refinery process environments known to cause wet H2S cracking resulting from hydrogen entry into the steel, as defined in NACE Standard MR0103.4 Some environmental conditions known to cause wet H2S cracking are those containing an aqueous phase and:(a) > 50 ppmw total sulfide content in the aqueous phase; or(b) ≥ 1 ppmw total sulfide content in the aqueous phase and pH < 4; or(c) ≥ 1 ppmw total sulfide content and ≥ 20 ppmw free cyanide in the aqueous phase and pH > 7.6; or(d) > 0.3 kPa absolute (0.05 psia) partial pressure H2S in the gas phase associated with the aqueous phase of aprocess.However, the threshold total sulfide content in the aqueous phase required for cracking to occur has not been clearly established. Therefore, selective application of this standard may be appropriate when experience has indicated the presence of cracking or blistering in comparable service, regardless of total sulfide content.Alkaline environments such as alkanolamine solutions that contain sulfides and carbonate-containing sour waters also are included in the term wet H2S environments and thus are within the scope of this standard. Two forms of alkaline stress corrosion cracking (ASCC) are commonly found in these alkaline wet H2S environments. Amine stress corrosion cracking (commonly referred to as amine cracking) can occur in amine service under certain conditions, which are discussed in API(1) RP 945.5 Alkaline carbonate stress corrosion cracking (commonly referred to as carbonate cracking) can occur in alkaline carbonate-containing sour waters under certain conditions. NACE Publication 341086 describes where carbonate cracking has occurred in process equipment in petroleum refining service, the refining community’s current theory(ies) on the conditions and mitigation techniques that may have an impact on this type of cracking, and analytical and inspection techniques that have been used to address the issue.1.3 Increased industry attention to the potential for cracking of carbon steel pressure vessels began in 1984 with the rupture of a monoethanolamine (MEA) absorber tower at a Lemont, Illinois refinery. The ensuing explosion and fire resulted in fatalities and extensive damage to the facility.7 In response to this incident, NACE Task Group T-8-14, “Stress Corrosion Cracking of Carbon Steel in Amine Solutions,” was formed in the fall of 1984. An industry survey to determine the nature and extent of the cracking problem was conducted by T-8-14. The results of the T-8-14 effort have been reported separately.8(1)American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070.SP0296-20101.4 In 1988, some new results on vessel inspections and the cracking found were reported to the industry.9 Among the significant findings was the observation that cracking problems were occurring in other wet H2S environments, not just in MEA. It was further reported that inspection techniques commonly used at the time (visual, liquid penetrant, and dry magnetic particle testing) were not sensitive enough to find these cracks. In response to this new information, NACE Task Group T-8-16, “Cracking in Wet H2S Environments,” was formed in the spring of 1988. Work Group T-8-16a conducted a survey of cracking experiences in wet H2S environments to better identify the extent of the problem. Appendix B (nonmandatory) summarizes the 1990 T-8-16a survey findings.________________________________________________________________________Section 2: Mechanisms of Cracking2.1 The objective of this section is to define the terms used to describe cracks that occur because of exposure to wet H2S environments and describe the mechanisms of cracking. Photographs of typical cracks found in wet H2S environments are shown in Appendix C (nonmandatory).2.2 Definitions2.2.1 Sulfide Stress Cracking (SSC): Cracking of a metal under the combined action of tensile stress andcorrosion in the presence of water and H2S. SSC is a form of hydrogen stress cracking resulting from absorption of atomic hydrogen that is produced by the sulfide corrosion process on the metal surface. SSC usually occurs more readily in high-strength steels or in hard weld zones of steels. (See Figure C1.)2.2.2 Hydrogen Blistering: The formation of subsurface planar cavities, called hydrogen blisters, in a metalresulting from excessive internal hydrogen pressure. Growth of near-surface blisters in low-strength metals usually results in surface bulges. Hydrogen blistering in steel involves the absorption and diffusion of atomic hydrogen produced on the metal surface by the sulfide corrosion process. The development of hydrogen blisters in steels is caused by the accumulation of hydrogen that recombines to form molecular hydrogen at internal sites in the metal. In its molecular state, hydrogen is too large to diffuse through the steel. Typical sites for the formation of hydrogen blisters are large nonmetallic inclusions, laminations, or other discontinuities in the steel. This differs from the voids, blisters, and cracking associated with high-temperature hydrogen attack.Hydrogen blistering is much more common in plate materials used for pressure vessels or longitudinally seam-welded pipe than in seamless pipe materials or forgings. (See Figure C2.)2.2.3 Hydrogen-Induced Cracking (HIC): Stepwise internal cracks that connect adjacent hydrogen blisters ondifferent planes in the metal, or to the metal surface (also known as stepwise cracking). No externally applied stress is needed for the formation of HIC. In steels, internal cracks that may develop (sometimes referred to as blister cracks) tend to link with other cracks by a transgranular plastic shear mechanism. This occurs because of internal pressure resulting from the accumulation of hydrogen. The link-up of these cracks on different planes in steels has been referred to as stepwise cracking to characterize the nature of the crack appearance. HIC is commonly found in steels with (a) high impurity levels that have a high density of large planar inclusions, and/or(b) regions of anomalous microstructure produced by segregation of impurities and alloying elements in thesteel. Because HIC is caused by the same fundamental mechanism that causes hydrogen blistering, it also is much more common in plate materials used for pressure vessels or longitudinally seam-welded pipe than in seamless pipe materials or forgings. (See Figure C3.)2.2.4 Stress-Oriented Hydrogen-Induced Cracking (SOHIC): Arrays of cracks, aligned nearly perpendicular tothe stress, that are formed by the link-up of small HIC cracks in steel. Tensile stress (residual or applied) is required to produce SOHIC. SOHIC is commonly observed in the base metal adjacent to the heat-affected zone (HAZ) of a weld, oriented in the through-thickness direction. SOHIC may also be produced in susceptible steels at other high stress points such as from the tip of mechanical cracks and defects, or from the interaction of hydrogen blisters on different planes in the steel. (See Figure C4.)2.2.5 Alkaline Stress Corrosion Cracking (ASCC): Cracking of a metal produced by the combined action ofcorrosion in an aqueous alkaline environment containing H2S, CO2, and tensile stress (residual or applied). TheSP0296-2010cracking is branched and intergranular in nature, and typically occurs in non-stress-relieved carbon steels. This form of cracking has often been referred to as carbonate cracking when associated with alkaline carbonate-containing sour waters, and as amine cracking when associated with alkanolamine treating solutions. NACE Publication 34108 discusses carbonate cracking and API RP 945 discusses amine cracking. ASCC may occur in both vessels and piping. (See Figure C5.)2.3 Environmental Parameters Affecting Cracking2.3.1 Several cracking mechanisms in wet H2S environments, including SSC, hydrogen blistering, HIC, andSOHIC, are related to the absorption and permeation of hydrogen in steels. The key variables involved in hydrogen permeation in steels are pH and the composition of the service environment. Typically, the hydrogen permeation flux in steels has been found to be minimal in neutral solutions (pH 7), with increasing flux at both lower and higher pH values. Corrosion at low-pH values is caused by H2S, whereas corrosion at high-pH values is caused by increasing concentrations of ammonium bisulfide (higher ammonia levels in H2S-dominated environments).2.3.2 Hydrogen permeation has also been found to increase with increasing H2S partial pressure and with thepresence of cyanide at alkaline pH levels.2.3.3 SSC susceptibility increases with increasing H2S partial pressure. Based on investigations in oil and gasproduction environments, 0.3 kPa absolute (0.05 psia) and greater partial pressure of H2S in the presence of free water may produce SSC in susceptible steels.2.3.4 ASCC can occur over a wide range of temperatures, but susceptibility appears to increase withincreasing temperature. ASCC generally occurs in alkaline solutions with a pH in the 8 to 11 range, but its occurrence is highly dependent on the solution composition. This form of cracking has occurred in refinery services such as sour water streams and alkanolamine solutions containing H2S and CO2. ASCC is promoted by carbonates in the presence of weak sulfiding agents such as thiosulfate and thiocyanate. The mode of cracking involves local anodic dissolution of iron at breaks in the normally protective corrosion product film on the metal surface. Laboratory tests have shown that cracking occurs in a relatively narrow range of electrochemical potential that corresponds to a destabilized condition of the protective film. This film destabilization occurs at very low ratios of the sulfide concentration to the carbonate/bicarbonate concentration in the solution, and is possibly affected by a number of contaminants in the solution.10,11 This form of cracking is not directly associated with the above-mentioned forms of hydrogen-related damage. However, in sour waters and alkanolamine services containing H2S, cracking as a result of HIC, SOHIC, and SSC is possible, in addition to ASCC.2.4 Material Parameters Affecting Cracking of Carbon Steels in Wet H2S Environments2.4.1 Sulfide Stress Cracking2.4.1.1 SSC has not generally been a concern in the carbon steel base metals typically used for pressurevessels and piping in refinery wet H2S environments because these steels generally have a tensile strength less than 620 MPa (90 ksi).2.4.1.2 Carbon steel weld metal is generally considered resistant to SSC if its hardness is limited to 200HBW maximum in corrosive petroleum refining environments in accordance with NACE SP0472.3 However, weldments (weld metal, HAZ, and adjacent base metal zones subject to residual stresses from welding) may contain localized zones of high hardness. SSC in carbon steel weldments frequently is limited to hard HAZs of the last weld pass, which are not tempered by subsequent weld passes. Data show that, depending on the severity of the service environment, small hard regions of up to 248 HV (237 HBW) can be tolerated without the occurrence of SSC. The Rockwell Superficial Hardness equivalent to 248 HV is 70.5 HR15N.These values are a direct conversion from the 22 HRC maximum specified in NACE Standard MR0103 for ferritic materials to be used in petroleum refining environments.SP0296-20102.4.1.3 SSC has generally not occurred in seamless carbon steel piping that has been welded from oneside only because it does not contain hard weldments that are exposed to the process on the inside diameter (ID) of the piping. The initial weld pass is usually tempered by the subsequent weld passes, which helps to control the hardness.2.4.2 Hydrogen Blistering and Hydrogen-Induced Cracking2.4.2.1 Hydrogen blistering and HIC have been encountered in the lower-strength carbon steels platematerials typically used in refinery wet H2S environments for pressure vessels and longitudinally seam-welded piping.2.4.2.2 These cracking mechanisms are associated with the formation of hydrogen blisters caused by anaccumulation of molecular hydrogen at internal laminations, nonmetallic inclusions, or other discontinuities in the steel. Reducing the inclusion level of the steel by lowering the sulfur content increases the resistance to hydrogen blistering and HIC. In addition, control of sulfide inclusion morphology by calcium or rare earth metal additions to produce a spheroidal sulfide shape, in conjunction with use of lower-sulfur steels, has been found to increase resistance to hydrogen blistering and HIC.2.4.2.3 Base metal heat treatments, such as normalizing or quenching and tempering above 593 °C(1,100 °F), increase resistance to crack growth.2.4.3 Stress-Oriented Hydrogen-Induced Cracking2.4.3.1 Generally, the material parameters affecting hydrogen blistering and HIC are expected to apply toSOHIC.2.4.3.2 Susceptibility to SOHIC is increased by increasing local tensile stresses. Notch-like welddiscontinuities and/or local differences in microstructure present in the area of a weldment may increase the localized stresses. Postweld heat treatment (PWHT) is expected to reduce the susceptibility to SOHIC when it is influenced by residual stress. PWHT can also reduce local HAZ hardness, thereby reducing the possibility for SSC, which can initiate SOHIC.2.4.3.3 SOHIC has been found in pressure vessels constructed with conventional steels in refinery wetH2S environments. In laboratory tests, SOHIC has been produced in a variety of steels. In severe hydrogen-charging laboratory tests, SOHIC has also been produced in steels processed to optimize resistance to HIC.2.4.4 Alkaline Stress Corrosion Cracking2.4.4.1 ASCC has occurred in a variety of steels. Field experience to date has not indicated anysignificant correlation between susceptibility to ASCC and steel properties or product form.2.4.4.2 Susceptibility to ASCC increases with increasing tensile stress level. Areas of deformationresulting from cold forming or localized high residual stresses in weldments are more prone to ASCC.Surface discontinuities, especially in areas adjacent to welds, often serve as initiation sites for ASCC because they act as localized stress raisers. ASCC can be effectively controlled by PWHT and proper heat treatment after cold forming.________________________________________________________________________Section 3: Inspection for Cracking3.1 The objective of this section is to provide guidelines on inspection for cracking of existing carbon steel pressure vessels and piping made from carbon steel plate in petroleum refinery wet H2S environments. Where appropriate, guidelines are also included for piping.SP0296-2010 3.2 The scope of this section is the inspection of weldments. This includes pressure-retaining circumferential, longitudinal, and nozzle welds, and internal attachment welds to the pressure boundary.3.3 Inspection guidelines for new pressure vessels (never in service) are beyond the scope of this standard. However, initial inspection of new vessels during fabrication with methods of comparable sensitivity to anticipated in-service inspection methods is of significant value in assessing subsequent inspection results.3.4 These guidelines incorporate risk-based principles to determine the need and frequency for inspection. (Risk is defined as the likelihood [or probability] of failure times the consequence of failure.) See API RP 58012 and API RP 581.13 Also included are guidelines for inspection personnel qualifications, nondestructive examination (NDE) procedures, areas of inspection, surface preparation, inspection techniques, acceptance criteria, reporting of results, and reinspection.3.5 Application of these inspection guidelines shall be made by engineers and/or inspection personnel who are knowledgeable in the technical aspects of this section.3.6 Inspection Priorities and Intervals3.6.1 Each refinery should prioritize equipment in wet H2S environments. When prioritization is done, theranking for equipment shall consider the consequences of a leak or a failure on the surrounding area, operating conditions (temperature, pressure, and contents), criticality of the equipment, and the fabrication, inspection, and repair history. Priorities can be established by assessing the risk that cracking represents to the refinery.Evaluation of risk should use industry-approved approaches such as those in API RP 580, API RP 581, ASME(2) PCC-3,14 or similar procedures/methodologies unique to the owner/user. Regardless of the approach used, the risk assessment process shall address the likelihood of cracking and the consequence of failure.3.6.2 Some factors that should be considered when assessing the likelihood of cracking and blistering in wetH2S environments are the following. These guidelines are based on survey data, literature information, and industry experience.(a) History of cracking and blistering. Equipment with a history of blistering is more likely to be cracked.Also, equipment in service comparable to that of other equipment that has cracked is more likely to be cracked.(b) Materials, fabrication, and repair history. Equipment without PWHT or those with non-postweld-heat-treated repairs should be given higher priority when setting inspection requirements. NACE Publication 8X194 provides some background information on materials and fabrication practices typically used for vessels in wet H2S service.(c) Type of vessel. Trayed columns or drums in which an aqueous phase can condense, splash, oraccumulate are more susceptible to cracking and blistering. Vapor spaces where condensation occurs or where sections are intermittently wetted are often the most severely damaged.(d) Type of piping. Piping fabricated from plate material, such as large-diameter, longitudinally seam-welded piping, is potentially susceptible to wet H2S cracking similar to vessels. The plate material used to fabricate longitudinally seam-welded pipe is similar to that used to fabricate pressure vessels. Seamless piping, forgings, and castings are generally considered to be resistant to wet H2S cracking. Although several factors have been identified to explain this favorable experience with these product forms, a frequently cited reason is the shape and distribution of impurities in these product forms. However, for seamless piping, the fabrication history, environment, and experience should be considered because some instances of wet H2S cracking of seamless piping have been reported.(2) ASME International (ASME), Three Park Avenue, New York, NY 10016-5990.。