Transcript of "Part 8 Assessment Of Weld Misalignment And Shell Distortions"
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PART 8 ASSESSMENT OF WELDMISALIGNMENT AND SHELL DISTORTIONS
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Scope:Fitness-For-Service (FFS) assessment procedures for pressurized components with weldmisalignment and shell distortion, including out-of-roundness and bulges, are covered inthis part.Applicability:The procedures in this part can be used to assess weld misalignments and shell distortionsin components made up of flat plates; cylindrical, conical, and spherical shells; and formedheads.
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Weld Misalignment – Categories covered include centerline offset, angular misalignment (peaking), and a combination of centerline offset and angular misalignment of butt weld joints in flat plates, cylindrical shells and spherical shellsThe image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again.
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Shell Distortion – Categories of shell distortion are defined as follows:a) General Shell Distortion – A deviation of a shell from an ideal geometrythat occurs in the longitudinal and/or circumferential direction and exceeds thedeviation permitted by the applicable code or standard.This type of distortion exhibits significant shape variation of the shell (i.e. multiplelocal curvatures), and typically requires a Level 3 assessment based on a numericalanalysis method.Flat spots on a shell are classified as general shell distortion.
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b) Out-of-roundness – A deviation of the cross-section of a cylindrical shell orpipe bend from an ideally circular geometry. The out-of-roundness of a cylinder isassumed to be constant in the longitudinal direction (see Figure 8.7 and paragraph8.2.5.2.d, and either global (oval shape) or arbitrarily shaped in the circumferentialdirection. The out-of-roundness of a pipe elbow is assumed to be global (oval shape) inthe mid-elbow region when the ovality at the end equals 50% of the mid-elbow value.
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c) Bulge – An outward deviation of a cross-section of a shell member from an idealgeometry that can be characterized by local radii and angular extent. The local bulgegeometry may be either spherical or cylindrical.Flat spots (infinite radius of curvature) are not considered to be bulges; they areclassified as general shell distortions.If the bulge occurs at a blister, the analysis procedures in Part 7 should be utilized for theassessment. Angular extent Local Radii
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Calculation methods are providedto rerate the component if theacceptance criteria in this part arenot satisfied.For pressurized components, thecalculation methods determine areduced maximum allowableworking pressure (MAWP) and/orcoincident temperature.For tank components (shellcourses), the calculation methodsdetermine a reduced maximum fillheight (MFH).
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The assessment proceduresonly apply to componentsthat are not operating in thecreep range; the designtemperature is less than orequal to the value in Part 4,Table 4.1.The Materials Engineershould be consultedregarding the creep rangetemperature limit formaterials not listed in thistable. Assessment procedures forcomponents operating inthe creep range areprovided in Part 10.
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Level 1 Assessment procedures are based Level I assessmenton the criteria in the original constructioncode.If these criteria are not completelydefined by the original construction codeand are not in the original owner-userdesign specification, a Level 2 or Level 3assessment may be performed. Level II assessmentLevel 1 Assessment procedures shouldnot be used if the component is in cyclicservice.A screening procedure to determine if acomponent is in cyclic service is providedin Annex B1, paragraph B1.5.2. Screening Level III assessmentCriteria For Fatigue
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The Level 2 assessment procedures inthis part apply only if all of the followingconditions are satisfied: Level I assessmenta) The original design criteria were in accordance with a recognized code or standard (see Part 1, paragraphs 1.2.2 or 1.2.3).b) The component geometry is one of thefollowing: Level II assessment 1) Flat plate 2) Pressure vessel cylindrical or conicalshell section 3) Spherical pressure vessel 4) Straight section of a piping system 5) Elbow or pipe bend that does not Level III assessmenthave structural attachments 6) Shell course of an atmosphericstorage tank
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c) The applied loads are limited topressure and/or supplemental loads (seeAnnex A) that result in a membrane state Level I assessmentof stress in the component, excluding theeffects of the weld misalignment and shelldistortion (i.e. through-wall bendingstresses in the component are a result ofthe weld misalignment or shell distortion).The assessment procedures can be usedto evaluate stresses resulting from both Level II assessmentinternal and external pressure. Level 2stability assessment procedures areprovided only for cylindrical and conicalshells subject to external pressure. TheLevel 2 stability assessment rules do notapply to cylinders subject to externalpressure in combination with Level III assessmentsupplemental loads that result insignificant longitudinal compressivestresses..
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Level I assessmentSupplemental loads include, but are notlimited to: the weight of the component,contained fluid, insulation or refractory;loads resulting from the constraint of freethermal expansion, thermal gradients ordifferences in thermal expansioncharacteristics; occasional loads due to Level II assessmentwind, earthquake, snow, and ice; loadsdue both to environmental and operatingconditions; reaction forces from fluiddischarges; loads resulting from supportdisplacements; and loads due to processupset conditions. Level III assessment
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Level I assessmentd) If the component under evaluation is acylinder with out-of-roundness, the out-of-roundness is constant along the axis ofthe cylinder. If local deviations of thecylindrical shell occur in the longitudinal Level II assessmentdirection, the Level 2 assessmentprocedure can produce non-conservativeresults, and the shell distortion should beclassified as general shell distortion. Level III assessment
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A Level 3 assessment may be performedwhere Level 1 and 2 methods do not apply,such as for the following conditions:a) Type A, B, or C Components (see Part 4, Level I assessmentparagraph 4.2.5) subject to internal pressure,external pressure, supplemental loads, andany combination thereof.b) Components with a design based on prooftesting (e.g. piping tee or reducer produced inaccordance with ASME B16.9 where thedesign may be based on proof testing). Level II assessmentc) The shell distortion is classified as generalshell distortion (see paragraph 8.2.1.2).d) The loading conditions result in significantstress gradients at the location of the weldmisalignment or shell distortion.e) The component is subject to a loadingcondition that results in compressive stresses Level III assessmentwhere structural stability is a concern.Guidelines for performing a structural stabilityassessment are provided in Annex B1,paragraph B1.4.
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In this Part, the following component definitions are used indetermining the permissible assessment level for a component.a) Type A Components – A component that has a design equationthat specifically relates pressure (or liquid fill height for tanks)and/or other loads, as applicable, to a required wall thickness.Examples of Type A components are shown below.1) Pressure vessel cylindrical and conical shell sections2) Spherical pressure vessels and storage spheres3) Spherical, elliptical and tori spherical formed heads4) Straight sections of piping systems5) Elbows or pipe bends that do not have structuralattachments6) Cylindrical atmospheric storage tank shell courses
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In this Part, the following component definitions are used in determining thepermissible assessment level for a component.b) Type B Components – A component that does not have a design equationthat specifically relates pressure (or liquid fill height for tanks) and/or otherloads, as applicable, to a required wall thickness.These components have a code design procedure to determine an acceptableconfiguration, examples are shown below. Type B components typically exist ata major structural discontinuity and involve the satisfaction of a localreinforcement requirement (e.g. nozzle reinforcement area), or necessitate thecomputation of a stress level based upon a given load condition, geometry, andthickness configuration (e.g. flange design). These rules typically result in onecomponent with a thickness that is dependentupon that of another component. Design rules of this type have thicknessinterdependency, and the definition of a minimum thickness for a component isambiguous. Examples of Type B components are shown below.1) Pressure vessel nozzles, tank nozzles and piping branch connections2) The reinforcement zone of conical transitions3) Cylinder to flat head junctions4) Integral tube sheet connections5) Flanges6) Piping systems (see paragraph 4.4.3.3.b.3)
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In this Part, the following component definitions are used indetermining the permissible assessment level for a component.c) Type C Components – A component that does not have a designequation which specifically relates pressure (or liquid fill height fortanks) and/or other loads, as applicable, to a required wallthickness. In addition, these components do not have a codedesign procedure to determine local stresses.Examples of Type C components are shown below.1) Pressure vessel head to shell junctions2) Stiffening rings attached to a shell3) Skirt and lug-type supports on pressure vessels lug-4) Tank shell bottom course to tank bottom junction
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Data Requirements(1) Original Equipment Design DataAn overview of the original equipment data required for an assessment is provideda) Data for pressure vessels and boiler components may include some or all of the following: 1) An ASME Manufacturers Data Report or, if the vessel or system is not Code stamped, other equivalent documentation or specifications. 2) Fabrication drawings showing sufficient details to permit calculation of the MAWP of the component containing the flaw. If a rerate to a different condition of pressure and/or temperature is desired (i.e. increase or decrease in conditions), this information should be available for all affected components. Detailed sketches with data necessary to perform MAWP calculations may be used if the original fabrication drawings are not available. 3) The original or updated design calculations for the load cases in Table A.1 of Annex A, as applicable, and anchor bolt calculations. 4) The inspection records for the component at the time of fabrication. 5) User Design Specification if the vessel is designed to the ASME Code, Section VIII, Division 2. 6) Material test reports. 7) Pressure-relieving device information including pressure relief valve and/or rupture disk setting and capacity information. 8) A record of the original hydro test including the test pressure and metal temperature at the time of the test or, if the metal temperature is unavailable, the water or ambient temperature.
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Data Requirements(1) Original Equipment Design DataAn overview of the original equipment data required for an assessment is provided in Part 2,paragraph 2.3.1. 2.3.1.b) Data for piping components may include some or all of the following: 1) Piping Line Lists or other documentation showing process design conditions, and a description of the piping class including material specification, pipe wall thickness and pressure-temperature rating. 2) Piping isometric drawings to the extent necessary to perform a FFS assessment. The piping isometric drawings should include sufficient details to permit a piping flexibility calculation if such an analysis is deemed necessary by the Engineer in order to determine the MAWP (maximum safe or maximum allowable operating pressure) of all piping components. Detailed sketches with data necessary to perform MAWP calculations may be used if the original piping isometric drawings are not available. 3) The original or updated design calculations for the load cases in Table A.1 of Annex A, as applicable. 4) The inspection records for the component at the time of fabrication. 5) Material test reports. 6) A record of the original hydrotest including the test pressure and metal temperature at the time of the test, or if the metal temperature is unavailable, the water or ambient temperature.
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Data Requirements(1) Original Equipment Design DataAn overview of the original equipment data required for an assessment is provided in Part 2,paragraph 2.3.1. 2.3.1.c) Data for tanks may include some or all of the following: 1) The original API data sheet. 2) Fabrication drawings showing sufficient details to permit calculation of the maximum fill height (MFH) for atmospheric storage tanks and the MAWP for low-pressure storage tanks. Detailed data with sketches where necessary may be used if the original fabrication drawings are not available. 3) The original or updated design calculations for the load cases in Table A.1 of Annex A, as applicable, and anchor bolt calculations. 4) The inspection records for the component at the time of fabrication. 5) Material test reports. 6) A record of the last hydro test performed including the test pressure and metal temperature at the time of the test or, if the metal temperature is unavailable, the water or ambient temperature.
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Data Requirements(2) Maintenance and Operational HistoryAn overview of the maintenance and operational history required for an assessment isprovided in Part 2, paragraph 2.3.2. 2.3.2.a) The actual operating envelope consisting of pressure and temperature, including upsetconditions should be obtained. If the actual operating conditions envelope is not available,an approximation of one should be developed based upon available operational data andconsultation with operating personnel. An operating histogram consisting of pressure andtemperature data recorded simultaneously may be required for some types of FFSassessments (e.g., Part 10 for components operating in the creep regime).b) Documentation of any significant changes in service conditions including pressure,temperature, fluid content and corrosion rate. Both past and future service conditions shouldbe reviewed and documented.c) The date of installation and a summary of all alterations and repairs including requiredcalculations, material changes, drawings and repair procedures, including PWHT proceduresif applicable. The calculations should include the required wall thickness and MAWP (MFHfor atmospheric storage tanks) with definition and allowances for supplemental loads such asstatic liquid head, wind, and earthquake loads.
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Data Requirements(2) Maintenance and Operational HistoryAn overview of the maintenance and operational history required for an assessment isprovided in Part 2, paragraph 2.3.2. 2.3.2.d) Records of all hydro tests performed as part of any repairs including the test pressure andmetal temperature at the time of the tests or, if the metal temperature is unavailable, thewater or ambient temperature at the time of the test if known.e) Results of prior in-service examinations including wall thickness measurements and otherNDE results that may assist in determining the structural integrity of the component and inestablishing a corrosion rate.f) Records of all internal repairs, weld build-up and overlay, and modifications of internals.g) Records of "out-of-plumb" readings for vertical vessels or tank shells.h) Foundation settlement records if corrosion is being evaluated in the bottom plate or shellcourses of the tank.If some of these data are not available, physical measurements should be made to providethe information necessary to perform the assessment.
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Data Requirements(3) RequiredData/Measurements for aFFS Assessment* The information typicallyused for a Level 1 and Level 2Assessment is summarized inTable 8.1.** The information requiredto perform a Level 3Assessment.The description of the weldmisalignment or shelldistortion should include fieldmeasurements thatadequately characterize thedeformed shape.
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Data Requirements(4) Recommendations for Inspection Technique and Sizing RequirementsMeasurement of the radial (offset) and angular (peaking) misalignment at the weld joint isrequired to use the assessment procedures for weld misalignment.a) For flat plates, these two quantities can be established by knowing the plate plates thicknesses, disposition of the surfaces of the plates in the weld joint (e.g. internal surfaces are flush), the maximum offset between the plate centerlines at the weld joint, and for angular misalignment, the effective height and length used to characterize the deviation.b) For cylindrical or spherical shells, the radial misalignment can be established by shells knowing the plate thicknesses and the disposition of the surfaces of the plates in the weld joint (e.g. internal surfaces are flush). The angular misalignment at the joint can be established by using a template as shown in Figure 8.8. The arc length of the template should extend beyond the locally deformed region resulting from the angular misalignment (or the contact point, see Figure 8.8), and should be established using the inside or outside radius of the cylinder, as applicable.
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Data Requirements(4) Recommendations for Inspection Techniqueand Sizing Requirementb) For cylindrical or spherical shells shells,Using this technique, themaximum deviation can be calculated usingeither of the following equations.For a center template,
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Data Requirements(4) Recommendations for Inspection Technique and Sizing RequirementsMeasurement of the radius and associated deviation from the mean radius at positionsaround the circumference are required to use the assessment procedures forcircumferential out-of-roundness of cylindrical shells.a) For the case of global out-of-roundness, the maximum, minimum and mean diameters are required. If these quantities are difficult to measure in the field, the measurement procedure for arbitrary out-of roundness presented in item (b) below is recommended.b) An accurate measurement of the cylinder radius at various stations should be made in order to apply the assessment procedures for an arbitrary circumferential out-of- roundness.
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Data Requirements(4) Recommendations for Inspection Technique and Sizing RequirementsMeasurement of the radius and associated deviation from the mean radius at positionsaround the circumference are required to use the assessment procedures forcircumferential out-of-roundness of cylindrical shells.b) An accurate measurement of the cylinder radius at various stations should be made in order to apply the assessment procedures for an arbitrary circumferential out-of- roundness. 1) Radii at an even number of equally spaced intervals around the circumference of the cylinder sufficient to define the profile of the cross section under evaluation should be measured (see Figure 8.9). The recommended minimum number of measurement locations is 24. If access to the inside of the vessel is not possible, an alternative means for measuring the cross section profile will be required. For a vessel that has stiffening rings, the shape deviation of the shell located between the stiffening rings can be measured by placing a level on the outside diameter of the rings and measuring the radial offset to the deformed surface. This method can produce accurate results if the stiffening rings are not significantly out-of-round. For a vessel without stiffening rings, a vertical level or plumb line placed alongside the vessel shell can be used to measure the radial offset.
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Data Requirements(4) Recommendations for Inspection Technique and Sizing RequirementsAn estimate of the local radius, L R ,Is required to use the assessmentprocedures for local imperfections incylindrical shells subject to externalpressure. The local radius, L R , can beestimated using the guidelines shownin Figure 8.10
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Data Requirements(4) Recommendations for Inspection Technique and Sizing RequirementsEstimates of the local bulge radii and the bulge angular extent are required to use theassessment procedures for bulges.In addition, if the bulge is caused by local heating, hardness values and other insitu testingshould be considered to evaluate the condition of the material.
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Evaluation Techniques and Acceptance CriteriaOverviewAn overview of the assessment levels is provided in Figure 8.1. a) The Level 1 assessment is based on the fabrication tolerances of the original construction code. If the current geometry of the component conforms to the original fabrication tolerances, the Level 1 assessment criteria are satisfied. Additional analysis is not required unless the component is in cyclic service as defined in Annex B1, paragraph B1.5.2, or if a fatigue analysis was conducted as part of the original design; in these cases, a Level 2 or Level 3 assessment is required. b) Level 2 assessments provide a means to estimate the structural integrity of a component with weld misalignment or shell distortion characterized as out-of- out-of- roundness. Level 2 assessments can consider pressure and supplemental loads as well as complicated geometries (e.g. pipes with different wall thickness and locations of welds). c) Level 3 assessments are intended for the evaluation of components with general shell distortions, distortions complex component geometries and/or loadings. Level 3 assessments involve detailed stress analysis techniques including fracture, fatigue, and numerical stress analysis. Level 3 assessments typically require significant field measurements to characterize the weld misalignment or shell distortion.
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Level 1 Assessment** The Level 1 assessment procedures are based on the fabrication tolerancesprovided in the original construction code. Tables 8.3 through 8.7 provide anoverview of these tolerances for the following construction codes. For equipmentor components designed to other recognized construction codes, standards, orspecifications, fabrication tolerances in those documents may also be followed(see paragraph 8.2.5.1). a) ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 and Division 2 – see Table 8.3 b) ASME B31.3 Piping Code – see Table 8. 4 c) API 620 Standard – see Table 8.5 d) API 650 Standard – see Table 8.6 e) API 653 Standard (reconstructed tanks) – see Table 8.7If the component does not meet the Level 1 Assessment requirements, then aLevel 2 or Level 3 Assessment can be conducted.
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Level 2 Assessment** The Level 2 assessment procedures are computational procedures forassessment of a weld misalignment or shell distortion in a component subject topressure and supplemental loads.Calculation methods are provided to rerate the component if the acceptancecriteria in this part are not satisfied.
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LEVEL II assessment for weld- weld-misalignment
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Identify the component and misalignment typeSTEP 1STEP 2STEP 3STEP 4STEP 5STEP 6
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Determine the wall thickness to be used in theSTEP 1 assessment using either of the following equations:STEP 2STEP 3STEP 4STEP 5STEP 6
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Determine the membrane stress from pressure, σmSTEP 1 For cylindrical shells, should be used for misalignmentSTEP 2 of longitudinal joints. and should be used forSTEP 3 misalignment of circumferential joints.STEP 4 For centerline offset in flat plates and circumferential joints inSTEP 5 cylindrical shells, determine the resulting longitudinal membrane stress using the following equation if supplemental loads exist.STEP 6
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Determine the membrane stress from pressure, σmSTEP 1 (see Annex A). For cylindrical shells, should be used for misalignmentSTEP 2 of longitudinal joints. and should be used forSTEP 3 misalignment of circumferential joints.STEP 4 For centerline offset in flat plates and circumferential joints inSTEP 5 cylindrical shells, determine the resulting longitudinal membrane stress using the following equation if supplemental loads exist.STEP 6
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Calculate the ratio of the induced bending stress to the appliedSTEP 1 membrane stress : Using the equations in Tables 8.8, 8.9, 8.10, and 8.11 based on the type of component and weld misalignment, and the misalignment,STEP 2 thickness determined in STEP 2. This ratio equals zero if no centerline offset or angularSTEP 3 misalignment exists. * The quantity is the ratio of the induced bending stress to the applied membrane stress resulting from pressure. pressure.STEP 4 * * While is the ratio of the induced bending stress to the membrane stress resulting from supplemental loads. loads.STEP 5STEP 6
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Calculate the ratio of the induced bending stress to the appliedSTEP 1 membrane stress : Using the equations in Tables 8.8, 8.9, 8.10, and 8.11 based on the type of component and weld misalignment, and the misalignment,STEP 2 thickness determined in STEP 2. For Flat Plates (note that a pressureSTEP 3 induced bending stress shall be categorized as and is applicable) For CylindersSTEP 4 Circumferential Joints Longitudinal Joints (Longitudinal Stress) (Circumferential Stress)STEP 5STEP 6
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Calculate the ratio of the induced bending stress to the appliedSTEP 1 membrane stress : Using the equations in Tables 8.8, 8.9, 8.10, and 8.11 based on the type of component and weld misalignment, and the misalignment,STEP 2 thickness determined in STEP 2. For Spheres Circumferential Joints (Circumferential Stress)STEP 3STEP 4STEP 5STEP 6
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Determine the remaining strength factor using Equation (8.21)STEP 1STEP 2 WithSTEP 3 (the expression abs[x] means the absolute value of x).STEP 4 AndSTEP 5STEP 6
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Evaluate the results.STEP 1 If a RSF ≥ RSFa , then the weld misalignment is acceptable per Level 2; If this criterion is not satisfied, then the component may beSTEP 2 rerated using the equations in Part 2, paragraph 2.4.2.2. It should be noted that in the cases where the Rb factor is a function of pressure, an iterative analysis is required to determine the final MAWP .STEP 3 RSF : Estimated remaining strength factorSTEP 4 RSFa: Allowable remaining strength factorSTEP 5STEP 6
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Part 2, paragraph 2.4.2.2STEP 1 Remaining Strength Factor Structural evaluation procedures using linear elastic stress analysis with stress classification and allowable stress acceptance criteria provide only a rough approximation of theSTEP 2 loads that a component can withstand without failure. A better estimate of the safe load carrying capacity of a component can be provided by using nonlinear stress analysis to: develop limitSTEP 3 and plastic collapse loads, evaluate the deformation characteristics of the component (e.g. deformation or strain limits associated with component operability), and assess fatigue and/or creep damage including ratcheting.STEP 4STEP 5STEP 6
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Part 2, paragraph 2.4.2.2STEP 1 Remaining Strength Factor a) In this Standard, the concept of a remaining strength factor is utilized to define the acceptability of a component for continued service. The Remaining Strength Factor (RSF) isSTEP 2 defined as: b) With this definition of the RSF, acceptance criteria can be established using traditional code formulas, elastic stressSTEP 3 analysis, limit load theory, or elastic-plastic analysis. For example, to evaluate local thin areas (see Part 5), the FFS assessment procedures provide a means to compute a RSF. IfSTEP 4 the calculated RSF is greater than the allowable RSF (see below) the damaged component may be placed back into service. If the calculated RSF is less than the allowable value, the component shall be repaired, rerated or some form of remediation can beSTEP 5 applied to reduce the severity of the operating environment. The rerated pressure can be calculated from the RSF as follows:STEP 6
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LEVEL II assessment forOut-Of-Out-Of-Roundness – Cylindrical Shells And Pipe Elbows
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Determine the following variables based on the type of out-of- out-of-STEP 1 roundness. roundness. 1) For Global Out-Of-Roundness (see Figure 8.7), the following parameters are required:STEP 2 2) For General (Arbitrary Shape) Out-Of- Roundness (see Figure 8.9), the parameter θ and the cross sectional profile of the cylinder at variousSTEP 3 angles around the vessel circumference when the cylinder is not pressurized are required. The cross sectional profile data can be representedSTEP 4 by the Fourier Series in Equation (8.2). The coefficients to this Fourier Series may be determined using the procedure in Table 8.2.STEP 5STEP 6
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Determine the following variables based on the type of out-of- out-of-STEP 1 roundness. roundness.STEP 2STEP 3STEP 4STEP 5STEP 6
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Determine the wall thickness to be used in the assessment, c t ,STEP 1 using Equation (8.10) or Equation (8.11), as applicable.STEP 2STEP 3 orSTEP 4STEP 5STEP 6
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Determine the circumferential membrane stressSTEP 1 using the thickness from STEP 2 (see Annex A). For cylindrical shells, should be used for misalignmentSTEP 2 of longitudinal joints. and should be used forSTEP 3 misalignment of circumferential joints.STEP 4 For centerline offset in flat plates and circumferential joints inSTEP 5 cylindrical shells, determine the resulting longitudinal membrane stress using the following equation if supplemental loads exist.STEP 6
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Determine the circumferential membrane stress using the thickness from STEP 2 (see Annex A).STEP 1 For cylindrical shells, should be used for misalignmentSTEP 2 of longitudinal joints. and should be used forSTEP 3 misalignment of circumferential joints.STEP 4 For centerline offset in flat plates and circumferential joints inSTEP 5 cylindrical shells, determine the resulting longitudinal membrane stress using the following equation if supplemental loads exist.STEP 6
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Determine the ratio of the induced circumferential bendingSTEP 1 stress to the circumferential membrane stress at the circumferential position (denoted by the angle θ ) of interest. interest. 1) Global Out-Of-Roundness of a cylinder, θ is measured fromSTEP 2 the major axis of the oval:STEP 3STEP 4 2) General (Arbitrary Shape) Out-Of-Roundness of a cylinder:STEP 5STEP 6
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Determine the ratio of the induced circumferential bendingSTEP 1 stress to the circumferential membrane stress at the circumferential position (denoted by the angle θ ) of interest. interest. 3) Global Out-Of-Roundness Of Long Radius Elbow (note:STEP 2 limitations for the terms of Equation (8.26) are shown in Equations (8.28) and (8.29), see subparagraph 4 if these limitations are not satisfied).STEP 3STEP 4STEP 5STEP 6
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Determine the ratio of the induced circumferential bendingSTEP 1 stress to the circumferential membrane stress at the circumferential position (denoted by the angle θ ) of interest. interest. 4) Global Out-Of-Roundness Of An Elbow Or Pipe Bend (noSTEP 2 limitation on bend radius)STEP 3STEP 4STEP 5STEP 6
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Determine the remaining strength factor using Equation (8.21)STEP 1STEP 2 WithSTEP 3 (the expression abs[x] means the absolute value of x).STEP 4 AndSTEP 5STEP 6
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Evaluate the results.STEP 1 If a RSF ≥ RSFa , then the out-of-roundness is acceptable per Level 2; otherwise, refer to paragraph 8.4.3.7.STEP 2 8.4.3.7 Rerating Components If a RSF ≥ RSFa , the component is acceptable per Level 2. If this criterion is not satisfied, then the component may be rerated using the equations in Part 2, paragraph 2.4.2.2. It should beSTEP 3 noted that in the cases where the Rb factor is a function of pressure, an iterative analysis is required to determine the final MAWP .STEP 4 RSF : Estimated remaining strength factorSTEP 5 RSFa: Allowable remaining strength factor see part 2STEP 6
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RSFa: Allowable remaining strength factor see part 2STEP 1 2.4.2.2 Remaining Strength Factor d) The recommended value for the allowable Remaining Strength Factor, a RSF , is provided inSTEP 2 Table 2.3. The values in Table 2.3. have been shown to be conservative (see Annex H). These values may be reduced based upon the type of loading (e.g.STEP 3 normal operating loads, occasional loads, short-time upset conditions) and/or the consequence of failure. For example, a lower a RSF could be utilized for lowSTEP 4 pressure piping containing a flaw that conveys cooling water, or for a shell section containing a flaw subject to normal operating pressure and designSTEP 5 wind loads.STEP 6
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RSFa: Allowable remaining strength factor see part 2STEP 1 2.4.2.2 Remaining Strength FactorSTEP 2STEP 3STEP 4STEP 5STEP 6
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LEVEL II assessment forCombined Weld Misalignment and Out- Out- Of-Roundness In Cylindrical Shells Of- Subject To Internal Pressure
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Determine the wall thickness to be used in the assessment, tc ,STEP 1 using Equation (8.10) or Equation (8.11), as applicable. applicable.STEP 2STEP 3STEP 4STEP 5
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Determine the circumferential membrane stress using theSTEP 1 thickness from STEP 1 (see Annex A). (seeSTEP 2STEP 3STEP 4STEP 5
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Calculate the ratio of the induced bending stress to the appliedSTEP 1 membrane stress for weld misalignment using paragraph 8.4.3.2, and for circumferential out-of roundness using paragraph 8.4.3.3 (note:STEP 2 when computing or Rb per paragraph 8.4.3.3, do not take the absolute value of the result as indicated in STEP 4).STEP 3STEP 4STEP 5
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Determine the remaining strength factor using Equation (8.21)STEP 1 with the value of Rb determined in STEP 2STEP 2STEP 3 AndSTEP 4STEP 5
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Evaluate the results.STEP 1 If a RSF ≥ RSFa , then the weld misalignment and out-of- roundness is acceptable per Level 2; otherwise, refer to paragraph 8.4.3.7.STEP 2 8.4.3.7 Rerating Components If a RSF ≥ RSFa , the component is acceptable per Level 2. If this criterion is not satisfied, then the component may be reratedSTEP 3 using the equations in Part 2, paragraph 2.4.2.2. It should be noted that in the cases where the Rb factor is a function of pressure, an iterative analysis is required to determine the finalSTEP 4 MAWP . RSF : Estimated remaining strength factorSTEP 5 RSFa: Allowable remaining strength factor
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LEVEL II assessment forOut-Of-Out-Of-Roundness – Cylindrical Shells Subject To External Pressure (Buckling Assessment)
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Cylindrical shells subject to external pressureshould satisfy the stress criteria in paragraph8.4.3.3 or 8.4.3.4, as applicable, and the bucklingcriteria set out in this paragraph.
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Determine the following variables (see Figure 8.10):STEP 1STEP 2STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
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Determine the wall thickness to be used in the assessment, c t ,STEP 1 using Equation (8.10) or Equation (8.11), as applicable.STEP 2STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
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If a conical shell is being evaluated, determine the equivalentSTEP 1 length and outside diameter as defined in Annex A, paragraph A.4.8. The equivalent length and equivalent outside radiusSTEP 2 are to be used in all subsequent steps.STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
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Find the value of n (the number of waves into which theSTEP 1 cylinder will buckle in the circumferential direction to give a minimum value of ) for the perfect cylinderSTEP 2 i) Approximate method – determine the value of n using theSTEP 3 equations in Annex A, paragraph A.4.2.a.1 with ii) Exact method – determine the value of n in the following equation that results in a minimum value of .STEP 4 This calculation assumes the value of n to be a floating point number. Starting with n = 2 , increase n in incrementsSTEP 5 of 0.10 until a minimum value of is found.STEP 6STEP 7STEP 8STEP 9
83.
Determine the value of the local radius, RL , of the imperfectionSTEP 1 using the procedure shown in Figure 8.10 with the measured value of the deviation from the true cylinder, ed , and the valueSTEP 2 of n determined in STEP 3.STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
84.
Substitute RL for Ro into Equation (8.33) and find a new value ofSTEP 1 n along with the associated value of the minimum elastic buckling pressure and designate this pressure as Pec.STEP 2 In calculating Pec, start with n = 2 and increase n in increments of 0.10 until a minimum value of Pec is found.STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
85.
Determine the inelastic buckling pressure using Equation (8.35).STEP 1 The parameter hc F in this equation is determined by Annex A paragraph A.4.4.a.3 with the value of he F calculated usingSTEP 2 Equation (8.36). Note that for materials other than carbon and low alloy steels,STEP 3 the correction described in Annex A paragraph A.4.1.d shall be applied. appliedSTEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
87.
Evaluate the results.STEP 1 If Pext ≥ P , then the component is suitable for continuedSTEP 2 operation; otherwise, refer to paragraph 8.4.3.7.STEP 3STEP 4STEP 5STEP 6STEP 7STEP 8STEP 9
89.
A Level 2 Assessment procedure for determining the acceptability of abulge is currently not provided; refer to paragraph 8.4.4 for Level 3Assessment options.
91.
If RSFa ≥ RSF , the component is acceptable per Level 2.If this criterion is not satisfied, then the component may be rerated using theequations in Part 2, paragraph 2.4.2.2.It should be noted that in the cases where the Rb factor is a function ofpressure, an iterative analysis is required to determine the final MAWP.2.4.2.2 Remaining Strength FactorStructural evaluation procedures using linear elastic stress analysis with stressclassification and allowable stress acceptance criteria provide only a roughapproximation of the loads that a component can withstand without failure. Abetter estimate of the safe load carrying capacity of a component can beprovided by using nonlinear stress analysis to: develop limit and plastic collapseloads, evaluate the deformation characteristics of the component (e.g.deformation or strain limits associated with component operability), and assessfatigue and/or creep damage including ratcheting.
92.
2.4.2.2 Remaining Strength Factora) In this Standard, the concept of a remaining strength factor is utilized todefine the acceptability of a component for continued service. The RemainingStrength Factor (RSF) is defined as:b) With this definition of the RSF, acceptance criteria can be established usingtraditional code formulas, elastic stress analysis, limit load theory, or elastic-plastic analysis. For example, to evaluate local thin areas (see Part 5), the FFSassessment procedures provide a means to compute a RSF.If the calculated RSF is greater than the allowable RSF (see below) the damagedcomponent may be placed back into service. If the calculated RSF is less than theallowable value, the component shall be repaired, rerated or some form ofremediation can be applied to reduce the severity of the operatingenvironment. The rerated pressure can be calculated from the RSF as follows:
93.
2.4.2.2 Remaining Strength Factorc) For tankage, the RSF acceptance criteria is:d) The recommended value for the allowable Remaining Strength Factor, a RSF ,is provided in Table 2.3. The values in Table 2.3. have been shown to beconservative (see Annex H).These values may be reduced based upon the type of loading (e.g. normaloperating loads, occasional loads, short-time upset conditions) and/or theconsequence of failure. For example, a lower a RSF could be utilized for low-pressure piping containing a flaw that conveys cooling water, or for a shellsection containing a flaw subject to normal operating pressure and design windloads.
95.
If the component is in cyclic service, or if a fatigue analysis was performed as service,part of the original design calculations, the fatigue strength including the calculations,effects of the weld misalignment or shell distortion should be checked.A screening procedure to determine if a component is in cyclic service isprovided in Annex B1, paragraph B1.5.2.The procedure for fatigue assessment that follows may be used to evaluatefatigue in components with weld misalignment, shell out-of-roundness, or a out-of-combination of weld misalignment and shell out-of roundness subject to the out-restrictions in paragraphs 8.4.3.2, 8.4.3.3, or 8.4.3.4, respectively.A Level 3 Assessment is required for a component with bulges.
96.
Determine the nature of loading, the associated membrane ,STEP 1 stress (see Annex A), and the number of operating cycles. .STEP 2STEP 3STEP 4STEP 5
97.
Determine the ratio of the induced bending stress toSTEP 1 membrane stress, Rb , resulting from weld misalignment, shell out-of- out-of-roundness or combination of weld misalignment and shell out-of roundness, as applicable, using the procedures in out- paragraphs 8.4.3.2, 8.4.3.3, or 8.4.3.4, respectively.STEP 2STEP 3STEP 4STEP 5
98.
Using the loading history and membrane stress from STEP 1 andSTEP 1 the Rb parameters from STEP 2, calculate the stress range for the fatigue analysis using Table 8.12.STEP 2STEP 3STEP 4STEP 5
99.
Compute the number of allowed cycles using the stress rangeSTEP 1 determined in STEP 3 using Annex B1, paragraph B1.5.3 or B1.5.5, as applicable. Paragraph B1.5 provides three methods for determining the permissible number of cycles:STEP 2 1) Elastic Stress Analysis and Equivalent Strength in accordance with paragraph B1.5.3 2) Elastic-Plastic Stress Analysis and Equivalent Strain inSTEP 3 accordance with paragraph B1.5.4 3) Elastic Stress Analysis and Structural Stress in accordance with paragraph B1.5.5 Since an elastic-plastic stress analysis has not been conducted,STEP 4 the permitted number of cycles will be determined using Methods 1 and 3. In both cases the stresses considered consist of those due to pressure loading, stresses from supplementarySTEP 5 loads and thermal gradients are considered negligible.
100.
Evaluate the results. If the computed number of cyclesSTEP 1 determined in STEP 4 equals or exceeds the number of operating cycles in STEP 1, then the component is acceptable per Level 2.STEP 2STEP 3STEP 4STEP 5
101.
If the component does not meet the Level 2 Assessmentrequirements, then the following, or combinationsthereof, should be considered:a) Rerate, repair, replace or retire the component.b) Adjust the FCA by applying remediation techniques(see Part 4, paragraph 4.6).c) Conduct a Level 3 Assessment.
102.
4.6 Remediation4.6.1 A FFS assessment provides an evaluation of the condition of acomponent for continued operation for a period based upon a future corrosionor degradation rate. However, in many cases, future degradation rates are verydifficult to predict, or little or no further degradation can be tolerated.Therefore, remediation methods may be applied to prevent or minimize therate of further damage.4.6.2 Remediation methods for general corrosion/erosion as well as localcorrosion/erosion and pitting are provided below.These methods may also be suitable for mitigation of crack-like flaws in someprocess environments.The methods described below are not inclusive for all situations, nor are theyintended to be a substitute for an engineering evaluation of a particularsituation.The Owner–User should consult a qualified Metallurgist/Corrosion Engineer andMechanical Engineer as to the most appropriate method to apply for therelevant damage mechanism(s).
103.
4.6 Remediation4.6.2.1 Remediation Method 1 – Performing Physical Changes to the ProcessStream; the following can be considered.a) Increasing or decreasing the process temperature, pressure, or both – If the degradation mode is temperature or pressure sensitive, a process change may minimize the progression of the damage. However, the component must be evaluated so that the design still meets the changed conditions. Note that a reduction in the pressure or temperature may result in a reduction of the minimum required wall thickness, thereby increasing the life of the component.b) Increasing or decreasing the velocity of the stream – Some damage mechanisms, such as erosion, sour water corrosion, under–deposit corrosion, and naphthenic acid corrosion are very velocity sensitive. A slight decrease or increase in stream velocity can change the rate of damage.c) Installing scrubbers, treaters, coalescers and filters to remove certain fractions and/or contaminants in a stream.
104.
4.6 Remediation4.6.2.2 Remediation Method 2 – Application of solid barrier linings or coatingsto keep the environment isolated from the base metal that has experiencedprevious damage.a) Organic coatings – The coating must be compatible with the service (temperature and stream composition) and must be resistant to all service conditions, including steaming–out. Surface preparation, particularly filling of pits, cracks, etc., is critical to achieve a solid bond. Curing conditions are also very important to assure a reliable lining. These fall into the following general classes: 1) Thin film coatings – Typically, these include epoxy, epoxy phenolic, and baked phenolic coatings applied in dry film thickness less than 0.25 mm (10 mils). 2) Thick film coatings – Typically, these include vinyl ester and glass fiber reinforced coatings that are applied in dry film thickness greater than 0.25 mm (10 mils).
105.
4.6 Remediation4.6.2.2 Remediation Method 2 – Application of solid barrier linings or coatingsto keep the environment isolated from the base metal that has experiencedprevious damage.b) Metallic linings – These fall into three general classes: 1) Metal spray linings – Various metal spray processes are available. In general, higher velocity processes such as HVOF (high velocity oxy–fuel) produce denser coatings, which are less susceptible to spalling or undermining. Coatings are often applied in multiple layers, with different compositions in each layer. The coating material in contact with the process environment should be corrosion resistant. Surface preparation is critical in achieving a solid bond. One advantage of metal spray linings is that the base material is not heated to high temperatures as in welding. 2) Strip linings – Thin strips of a corrosion resistant metal are applied to the area of concern. They are fastened to the backing metal by small welds, which help to minimize the size of the weld heat affected zone. Note that strip linings may crack at the lining attachment-to-component wall weld and may need periodic maintenance. In addition, corrosion of the underlying wall by leaking fluid at these cracks may be difficult to detect. 3) Weld overlay (see paragraph 4.6.2.4).
106.
4.6 Remediation4.6.2.2 Remediation Method 2 – Application of solid barrier linings or coatingsto keep the environment isolated from the base metal that has experiencedprevious damage.c) Refractory linings – Many materials fall into this category. Depending on thedamage mechanism, insulating refractories can be used to decrease the metaltemperature, erosion resistant refractories can be used for erosion protection,and corrosion resistant refractories can be used to protect the base material.Selection of the refractory type and anchoring system, and curing of therefractory are critical elements for this remediation method.A refractory specialist should be consulted for details.
107.
4.6 Remediation4.6.2.3 Remediation Method 3 – Injection of water and/or chemicals on acontinuous basis to modify the environment or the surface of the metal.Important variables to consider when injecting chemicals are: the particularstream contaminants, injection point location and design, rate of injection,eventual disposition and any adverse reactions, the effect of process upsets,and monitoring for effectiveness. Examples of this type are as follows: a) Water washing to dilute contaminants – This strategy is often applied in fluid catalytic cracking light end units and hydrodesulphurization reactor outlet systems. Important variables to consider when stipulating a retrofit water wash installation are location of injection, distribution of water, water rate, water quality, injection point design and disengagement, and monitoring for effectiveness.
108.
4.6 Remediation4.6.2.3 Remediation Method 3 – Injection of water and/or chemicals on acontinuous basis to modify the environment or the surface of the metal.Important variables to consider when injecting chemicals are: the particularstream contaminants, injection point location and design, rate of injection,eventual disposition and any adverse reactions, the effect of process upsets,and monitoring for effectiveness. Examples of this type are as follows: b) Injection of chemicals to change the aggressiveness of the solution Neutralizing chemicals as used in atmospheric distillation unit overheads, polysulfide, and oxygen scavengers all fall into this category. Important variables to consider include; the injection location and design, possible adverse side effects, and monitoring for effectiveness. c) Injection of filming type chemicals to coat the metal surface – Filming chemicals attach to the metal surface to form a thin barrier that protects the metal. Important variables to consider are; the injection location and design, response to upsets, and monitoring effectiveness.
109.
4.6 Remediation4.6.2.4 Remediation Method 4 – Application of weld overlay for repair of thebase material or for the addition of a corrosion resistant lining. If weld overlayis applied, the weldability of the base metal considering the effects of theenvironment should be evaluated (see Annex G). Note that the application of aweld overlay may necessitate a PWHT. a) Repair of Base Material – Weld overlay with the same chemistry (P Number) as the base metal of the component is added to the component to provide the necessary increase in wall thickness to compensate for corrosion/erosion. Note that this method does not eliminate/reduce the rate of degradation. The weld overlay may be added either to the inside or outside surface regardless of the surface on which the metal loss is occurring. For some applications, a repair procedure can be developed which permits deposition of the weld overlay while the component is in operation. Since this process changes the geometry of the component, an analysis considering bending stresses should be made to determine the acceptability of the proposed design. b) Application of corrosion resistant lining – A corrosion/erosion resistant material is applied to the surface of the base material.
111.
Level 3 AssessmentThe stress analysis techniques in Annex B1 can be used to assess the weldmisalignment or shell distortion discussed in this part in pressure vessels,piping, and tankage.Linear stress analysis and the stress categorization techniques discussed inAnnex B1 , paragraph B1.2.2 can be used to analyze misalignment at weldjoints. In the Level 2 Assessment, the induced bending Stress resulting frommisalignment is considered a secondary bending stress for most applications.In some cases, this stress should be taken as a primary bending stress if elasticfollow-up occurs. The limit load techniques described in Annex B1 , paragraphB1.2.3 may be utilized in the analysis to resolve issuespertaining to stress categorization.
112.
Level 3 AssessmentThe non-linear stress analysis techniques described in Annex B1 , paragraphB1.2.4 may be utilized to analyze general shell distortions. a) Typically, the localized bending stresses resulting from general shell distortion will tend to decrease due to the rounding effect of the shell when subject to internal pressure. This effect is more pronounced in thinner shells and can be directly evaluated using a non-linear analysis that includes the effects of geometric nonlinearity. The rounding effect is introduced in a Level 2 analysis of out-of-roundness through the correction factor, f C . If material nonlinearity is included in the analysis, the plastic collapse strength of the component can also be determined and used to qualify the component for continued operation.
113.
Level 3 AssessmentThe non-linear stress analysis techniques described in Annex B1 , paragraphB1.2.4 may be utilized to analyze general shell distortions. b) An accurate representation of the deformed shell profile is critical in obtaining accurate analysis results. This is especially important for a shell with significant deviations (or kinks) in the longitudinal an circumferential direction. To obtain an accurate profile of the shell geometry, a grid should be established over the deformed region and measurements taken to determine the actual profile of the shell. The data should then be curve-fit with piece-wise cubic splines to obtain an accurate representation of the deformed shape and ensure that the slope and curvature of the deformed shell profile is continuous. c) If a kink or sharp bend exists in a shell, traditional shell theory will not provide an accurate estimate of the stress state. In this case, a continuum model including the effects of plasticity is recommended for the evaluation.
114.
Level 3 Assessment8.4.4.4 If the component is subject to a compressive stress field, the nonlinearstress analysis techniques described in Annex B1, paragraph B1.2.4 may be used forthe assessment. If geometric non-linearity is included along with materialnonlinearity in the assessment, the stability of the component can be evaluated inthe same analysis utilized to determine the plastic collapse strength. Alternatively,the stress categorization and structural stability techniques discussed in Annex B1,paragraph B1.2.2 and Annex A, paragraph A.14, respectively, may be utilized in theassessment.8.4.4.5 If the component is operating in the creep range, a nonlinear analysis thatincludes both material (plasticity and creep) and geometric nonlinearity should beperformed. Stresses due to a localized weld misalignment or shell distortion may notsufficiently relax with time due to the surrounding compliance of the component. Inthis case, creep strains can accumulate and could result in significant creep damageor cracking. The assessment procedures in Part 10 should be considered.8.4.4.6 If the component contains a weld misalignment or shell distortion withhighly localized stresses, a detailed non-linear stress analysis and assessmentshould be performed. This assessment should also include an evaluation of thematerial toughness requirements. Otherwise, repair or replacement of thecomponent is recommended.
115.
8.5 Remaining Life Assessment8.5.1 A remaining life assessment of components with a weld misalignment orshell distortion generally consists of one of the following three categories:a) Metal Loss Resulting From A Corrosive/Erosive Environment – In this case,adequate protection from a corrosive/erosive environment can be establishedby setting an appropriate value for the future corrosion allowance. Theremaining life as a function of time can be established using the MAWPApproach described in Part 4, paragraph 4.5.2.b) Cyclic Loading – The Level 2 assessment procedures include a fatigueevaluation for weld misalignment and out-of-roundness (see paragraph 8.4.3.8and Annex B1, paragraph B1.5). The remaining life can be established bycombining the results from this analysis with the operational history of thecomponent.c) High Temperature Operation – If the component is operating in the creepregime, the assessment procedures in Part 10 should be utilized to determine aremaining life.8.5.2 If the component’s operation is not within one of the above categories, adetailed Level 3 analysisshould be performed to determine the remaining life of the component.
116.
8.6 Remediation8.6.1 Weld misalignment, out-of-roundness and bulges may be reinforcedusing stiffening plates and lap patches depending on the geometry,temperature and loading conditions. The reinforcement, if utilized, should bedesigned using the principles and allowable stresses of the original constructioncode.8.6.2 Cylindrical shell sections that are out-of-round can be brought to withinoriginal fabrication tolerances or to a shape that reduces the local stress towithin acceptable limits by mechanical means. Hydraulic jacks have been usedsuccessfully to alter the out-of-round shape of stiffened cylindrical shells. Thedesign of the jacking arrangement and loads should be carefully established andmonitored during the shaping process to minimize the potential for damage tothe shell and attachments.
117.
8.7 In-Service Monitoring8.7.1 The weld misalignment and shell distortion covered in this part do notnormally require in-service monitoring unless an unusually corrosiveenvironment exists and future corrosion allowance cannot adequately beestimated, or if the component is subject to a cyclic operation and the loadhistory cannot be adequately established. In these cases, the in-servicemonitoring usually entails visual inspection and field measurements of thecomponents weld misalignment or shell distortion at regular intervals. The typeof measurements made depends on the procedure utilized in the assessment.8.7.2 In-service monitoring is typically required when a Level 3 assessment isperformed to qualify a component that contains weld misalignment or shelldistortion with a groove-like or crack-like flaw for continued operation.
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