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DRAFT - Section 3: Assessment Of Equipment For Brittle Fracture

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DRAFT - API Recommended Practice For Fitness-For-Service

SECTION 3 - Assessment Of Existing Equipment For Brittle Fracture(Draft - Revision 14 - MS Word 7.0)

3.1 General

3.1.1 This section provides guidelines for evaluating the resistance to brittle fracture of existing carbon andlow alloy steel pressure vessels, piping, and storage tanks. Assessment of other materials that could besusceptible to brittle fracture such as ferritic, martensitic and duplex stainless steels are not addressedexplicitly. However, the same principles in this section can be used to evaluate these materials. Thepurpose of this assessment is to avoid a catastrophic brittle failure consistent with ASME Code SectionVIII design philosophy. It is intended to prevent the initiation of brittle fracture; however, it does notensure against cracks resulting in leakage, or ensure arrest of a running brittle fracture. Unlike othersections, where a flaw is first found and then evaluated, this Section is used to screen for the propensityfor brittle fracture. Once crack like flaws are found, Section 9 can be used for the assessment.

3.1.2 This section includes recommendations for; data collection requirements, criteria for assessingequipment based on its type, design details and condition, and allowances to account for actual stress,heat treatment condition, and steel manufacturing practices. Guidance is also provided for usingCharpy V-notch data at one temperature for the qualification of equipment at another temperature, andfor welding on equipment which does not meet recognized toughness standards.

3.1.3 The Critical Exposure Temperature (CET) is used in this section and is defined as the lowest processor atmospheric temperature at which the equipment metal will be exposed to a given stress undereither normal or upset conditions. The CET is derived from the operating conditions the component issubject to. The CET may be a single temperature at an operating pressure or an envelope oftemperatures and pressures, e.g. vapor pressure curve for LPG streams. The CET is determined fordifferent types of equipment as follows.

3.1.3.1 Pressure Vessels - the CET is defined as the minimum metal temperature at which a component will besubjected to a general primary membrane tensile stress greater than 8 ksi. The CET may also bedefined as the minimum metal temperature at which the vessel will be subject to a pressure greaterthan 40% of the Maximum Allowable Working Pressure (MAWP) for vessels designed to the ASMECode Section VIII Division 1. For pressure vessels designed to a higher allowable stress than thatpermitted in this code, the CET may be taken as the minimum metal temperature at which the vesselwill be subject to a pressure greater than 25% of the MAWP. The CET for pressure vessels isdetermined from the anticipated process and atmospheric conditions, as listed in paragraph 3.3.4.1.

3.1.3.2 Piping - The CET is defined as the minimum metal temperature at which a component will besubjected to a general primary membrane tensile stress greater than 8 ksi. The CET may also bedefined as the minimum metal temperature at which the piping system will be subject to a pressuregreater than 40% of the design pressure. The CET for piping is determined from the anticipatedprocess and atmospheric conditions, as listed in paragraph 3.3.4.1.

3.1.3.3 Atmospheric Storage Tanks - The CET is defined as the lower of either the lowest one-day meanatmospheric temperature plus 15°F (8°C), or the hydrostatic test temperature (see paragraph 3.3.4.2).

3.1.4 The Minimum Design Metal Temperature (MDMT) is the term chosen for the brittle fracture evaluationof existing equipment. It may be a single temperature, or an envelope of acceptable operatingtemperatures as a function of pressure. With this definition, the MDMT permits consideration of lowerthan design pressure conditions. The MDMT is derived from mechanical and materials design data.


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3.2 Applicability and Limitations of the Procedure(s)

3.2.1 This section provides guidelines to assess the risk of brittle fracture of components in the followingequipment:

3.2.1.1 Pressure vessels constructed in accordance with any of the several editions of the former API/ASMECode for Unfired Pressure Vessels for Petroleum Liquids and Gases, the several editions of ASMEBoiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2, or to other recognized pressurevessel codes, or as nonstandard vessels, and for other vessels not built to a code or approved asjurisdictional special,

3.2.1.2 Piping systems constructed in accordance with the ASME Code of Pressure Piping (B31.3); but thesame guidelines may be used in assessing piping design to other codes,

3.2.1.3 Aboveground storage tankage that is either welded or riveted, is non-refrigerated, operates atatmospheric or low pressure, and was constructed in accordance with any of the several editions of theformer or current API Codes for Design and Construction of Storage Tanks; it is important to note thatthese guidelines are based primarily on experience and data associated with welded tanks.

3.2.2 The Level 1 and 2 Assessment procedures in this section can be applied to components which do nothave extensive damage resulting from locally thin areas, grooves and crack-like flaws. If this type ofdamage is determined by the Engineer to be significant, a Level 3 Assessment should be performed.

3.3 Data Requirements

3.3.1 In order to carry out a brittle fracture assessment, available data related to equipment types, the currentand foreseeable future operating conditions, mechanical design, materials of construction, andoperations and repair history should be gathered. These data, which should be available in theequipment files, are required for each pressure containing component of the equipment in order toidentify the component that governs its brittle fracture limitations. A summary of the data that isrequired for an analysis is shown in Table 3.1.

3.3.2 In addition to identification of equipment type (e.g. pressure vessels, tankage, piping) design andoperating data will be required to perform a brittle fracture assessment. These data should includedesign pressure and temperature as well as the current wall thickness and the specified material type.Previous operating pressures and temperatures should be included as well as environmental exposureconditions. These data are used to establish the most severe operating and exposure conditionsencountered during the life of the equipment. Information related to environmental exposure will alsobe needed to determine whether there is a risk of environmental cracking. Specific materials propertiestest data, such as Charpy V-notch and tensile data, if available, will be used for higher levels ofassessment. The specified material type will either establish minimum properties or provide a basiswhereby minimum properties can be determined.

3.3.3 Additional data related to maintenance and operational history are required. Key maintenanceinformation should include a history of any weld repairs (with or without PWHT) and a complete historyof any hydrotests including records documenting the test pressure and metal temperature at the time ofthe test.

3.3.4 The CET pressure-temperature envelope should be determined after complete consideration of allpotential conditions using review procedures encompassing hazard analysis or other comparableassessment criteria. Of special concern with existing equipment is any change in the operation thathas occurred after the equipment was originally placed into service which could cause a lower CETthan it was originally designed for. In determining the CET, the current process design and safetyphilosophies should be employed.

3.3.4.1 The CET pressure-temperature envelope as defined in Section 3.1.3 shall consider the followingprocess conditions and ambient factors:


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a. The lowest one-day mean atmospheric temperature, unless a higher temperature is specified(e.g., specifying a minimum required startup temperature and coincident pressure). In the lattercase, it must be confirmed that the system capabilities to carry this out are available, can bemonitored, are documented, are understood and followed by operations personnel.

b. The lowest temperature under normal operating conditions.

c. Startup, shutdown, upset conditions, standby, pressure tightness testing. The following itemsshall be considered:

1. Failure of warning and/or shut-down systems (e.g. a pump stops, control valve shuts, etc.),

2. A colder than expected warming stream,

3. Reboiler failure or stall (e.g., flow loss of reboiling medium, failure of a control valve, etc.),and

4. The possibility of future field hydrotest.

d. Potential for autorefrigeration due to depressurization, either during operations or due toequipment failure (e.g., a safety relief valve sticks open). In some services whereautorefrigeration can occur, equipment can be chilled to temperatures below the CET at anapplied pressure less than that defined in paragraph 3.1.3.1. When this occurs the possibility ofany repressurization of equipment before the material has had sufficient time to warm up to theCET must be considered. The effect of autorefrigeration on the equipment depends upon thestate of the process fluid, for example whether the vessel contents are all liquid, all gas, or amixture and how the vessel may be vented. Autorefrigeration, caused by depressurization, mayalso occur in a flowing system with a flashing liquid. As the pressure decreases, thetemperature will follow the vapor pressure curve. For a pure gas, the effect of pressure ontemperature is small and governed by Joule-Thompson cooling. However, when a vessel isdepressurized through a long line, the gas flowing through the line may be cold because it wasautorefrigerated in the vessel.

e. Shock chilling (see Appendix I); the CET should not be higher than the temperature of the liquidcausing the shock chilling.

3.3.4.2 The CET for storage tankage is the lower of the coldest one-day mean atmospheric temperature plus15°F (8°C), or the anticipated hydrostatic test temperature.

3.4 Assessment Techniques and Acceptance Criteria

3.4.1 Overview

3.4.1.1 The Level 1 assessment procedures are intended to be used for equipment that meets recognizedtoughness standards. This can be determined from impact test results, the use of industry acceptedimpact test exemption curves, or comparison of the equipment to the original design code or standardtoughness requirements.

3.4.1.2 The Level 2 Assessment procedures for pressure vessels are divided into three methods. In the firstmethod (Method A), equipment may be exempt from further assessment if it can be shown that theoperating pressure/temperature is within a safe envelope with respect to component design stress andminimum acceptable temperature. In the second method (Method B) equipment may be qualified forcontinued service based on a hydrotest, possibly in combination with acoustic emission testing. In thethird method (Method C) equipment may be qualified for continued service based on materials ofconstruction, operating conditions, service environment and past operating experience. Separate


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evaluation procedures for piping and tankage are provided which are based on a combination of thesethree methods.

3.4.1.3 A Level 3 Assessment may be used for equipment which does not meet the acceptance criteria forLevels 1 and 2. This equipment must be evaluated on an individual basis with the help of process,materials, mechanical, inspection, safety, and other specialists as appropriate. A Level 3 Assessmentwill normally involve a more detailed evaluation, using a fracture mechanics methodology (see Section9.0), the factors that control the susceptibility to brittle fracture: stress, flaw size and materialtoughness are systematically evaluated.

3.4.2 Level 1 Assessment

3.4.2.1 Pressure Vessels

a. A simplified flow chart of an overall brittle fracture assessment of pressure vessels is shown inFigure 3.1. The detailed procedures shown in the flow chart and described in the followingparagraphs apply to carbon and low-alloy steels. Assessment Level 1 is appropriate forequipment that meets recognized toughness standards. This can be determined from impacttest results, or from the use of industry accepted impact test exemption curves. A Level 1assessment typically requires only a review of existing equipment records.

b. Pressure vessels which have a MDMT equal to or colder than the CET, as demonstrated byconformance to recognized toughness standards described below, are exempt from furtherbrittle fracture assessment provided conditions are not changed in the future. If a change in theoperating conditions is made which effects the CET, a reassessment is recommended. Thesevessels require no special treatment other than to continue their inclusion in a normal plantinspection and maintenance program encompassing generally accepted engineering practicessuch as contained in API-510 or other recognized inspection code. Also, if the vessel is in aservice where environmental cracking is possible, an appropriate inspection program should beimplemented.

c. Vessels that meet all of the following requirements of UG-20(f) of the ASME Code Section VIIIDivision 1 satisfy the requirements for a Level 1 assessment:

1. The material is limited to P-No. 1, Gr. No. 1 or 2 as defined in ASME Code Section IX, andthe thickness, as defined in paragraph (e) below, does not exceed that given in (a) or (b)below:

a) 1/2 inches (12 mm) for materials listed in Curve A of Figure 3.2, and

b) 1 inch (25.4 mm) for materials listed in Curve B, C, or D of Figure 3.2.

2. The completed vessel has been hydrostatically tested per the ASME Code, Section VIII,Division 1 or other recognized pressure vessel construction code, provided the testpressure is at least 1.5 times the design pressure.

3. Design temperature is no warmer than 650°F (343°C) nor colder than -20°F (-29°C).Occasional operating temperatures colder than -20°F (-29°C) are acceptable when due tolower seasonal atmospheric temperature.

4. Thermal or mechanical shock loadings are not a controlling design requirement.

5. Cyclic loading is not a controlling design requirement.

d. The MDMT is the lowest value determined by one of the following methods:


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1. Exemption curves in Figure UCS-66 of the ASME Code Section VIII Division 1 (shown asFigure 3.2),

2. Impact test temperature meeting the requirements in paragraph UG-84 of ASME CodeSection VIII Division 1,

3. Exemption curves in Figure AM-218.1 of the ASME Code, Section VIII, Division 2, or

4. Exemption curves from other recognized codes and standards (see paragraph 2.2.2.c ofSection 2).

e. When determining the MDMT, components, such as shells, head, nozzles, manways,reinforcing pads, flanges, tubesheets, flat cover plates, and attachments which are essential tothe structural integrity of the vessel when welded to pressure containing components shall betreated as separate components. Each component shall be evaluated for impact testrequirements based on its individual material classification (see Figures 3.2, and Table 3.3 andTable 3.4), thickness, and specified MDMT. The rules for establishing the governing thickness used to determine the MDMT are shown below.

1. Excluding castings, the governing thickness (TG) of a welded part is as follows:

a. for butt joints except those in flat heads and tubesheets, the nominal thickness ofthe thickest welded joint (see Figure 3.3(A)),

b. for corner, fillet, or lap welded joints, including attachments as defined above, thethinner of the two parts joined (see Figure 3.3(B),(C)),

c. for flat heads or tubesheets, the thinner of two parts joined or the flat componentthickness divided by 4 (see Figure 3.3(D),(E),(F)), and

d. for welded assemblies comprised of more than two components (e.g., nozzle-to-shell joint with reinforcing pad), the governing thickness and permissible MDMT ofeach of the individual welded joints of the assembly shall be determined, and thewarmest of the MDMT values so calculated shall be used as the permissibleminimum design metal temperature of the welded assembly.

2. The governing thickness of a casting shall be its largest nominal thickness.

3. The governing thickness of non-welded parts, such as bolted flanges, tubesheets, and flatheads, is the component thickness divided by 4 (see Figure 3.3(D)).

f. When using the exemption curves in Figure 3.2 the MDMT for P1 Group 1 and 2 materials inthe ASME Code can be lowered by 30°F (17°C) if the equipment was subject to PWHT and thereference thickness is less than or equal to 1.5 inches (38 mm).

3.4.2.2 Piping Systems

a. Piping systems shall meet the toughness requirements contained in ASME B31.3 (or anequivalent piping design code if that code contains material toughness requirements), orpossess a CET equal to or warmer than -20°F (-29°C).

b. Low alloy steel, such as 2� Cr - 1 Mo, and other steels may lose ambient temperature ductility ifexposed to high temperatures (above 750°F (400°C)) for long periods of time due to variousthermal aging degradation mechanisms. These piping systems may require special precautionsif a hydrotest of other low temperature pressurization is required.


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c. All weld repairs and alterations shall meet the requirements contained in API 570.

3.4.2.3 Atmospheric and Low Pressure Storage Tanks

a. Atmospheric and low pressure storage tanks shall meet the Level 1 Assessment criteriacontained in Figure 3.7, as applicable, and the accompanying notes. The Level 1 Assessmentcriteria requires that these tanks meet the toughness requirements contained in API 650, API620 or an equivalent construction code.

b. All repairs and alterations shall meet the requirements contained in API 653.


3.4.3.1 Pressure Vessels - Method A

a. Pressure vessels may be exempt from further assessment at this level if it can be demonstratedthat the operating pressure/temperature is within a safe envelope with respect to componentdesign stress and the MDMT. It is assumed that the equipment will continue to be included in anormal plant inspection and maintenance activity encompassing API-510 requirements or otherrecognized inspection codes as appropriate. Also, if the vessel is in a service whereenvironmental cracking is possible, an appropriate inspection program should be implemented.

b. The MDMT may be further adjusted from that determined in the Level 1 assessment byconsidering additional temperature reduction allowances which may apply for pressure vesselswhose actual operating stresses at the low temperature pressurization condition are below theallowable value from the original construction code at ambient temperature. For vesselsdesigned to the ASME Code Section VIII Division 1, use Figure 3.4 (ASME Code Section VIII,Div.1, Figure UCS 66.1) and the procedure outlined in Table 3.4. For vessels designed to othercodes, use Figure 3.4 and Table 3.4, but limit the temperature reduction to design stress valuesbelow 17.5 ksi (121 MPa). The MDMT shall be no colder than -150 oF (-101 oC) afteradjustments using this procedure.

3.4.3.2 Pressure Vessels - Method B

a. A vessel may be qualified for continued service based on a hydrotest, possibly in combinationwith acoustic emission testing (AET). Figure 3.5 is used to determine a minimum acceptabletemperature for operating pressures below the hydrotest pressure, using a philosophy similar tothat used to develop the temperature reduction curve in the ASME Code Section VIII, Division 1(Figure UCS 66.1 and shown as Figure 3.4 herein).

1. Test pressure should be 150% of the design pressure, corrected for the difference inallowable stresses between the design and hydrotest temperatures, but should not resultin a general primary membrane stress higher than 90% of the specified minimum yieldstrength for the steel used in the construction of the vessel.

2. The metal temperature during hydrotest, rather than water temperature, is the relevantparameter in a brittle fracture assessment. Therefore, it is preferable to measure and usethis value directly. Records of the measured metal temperature used in the assessmentshould be kept.

3. It may be advisable to perform acoustic emission testing during the hydrotest in caseswhere the quality of the welding or inspection is suspect. This may be the case for large,field fabricated pressure vessels.

4. If the hydrotest is performed at a temperature lower than the MDMT as determined by aLevel 1 assessment, it should be noted that there may be a significant risk of brittlefracture during the hydrotest.


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5. The MDMT shall be no colder than -150 oF (-101 oC) after adjustments using thisprocedure.

b. The MDMT curve is established using Figure 3.5 and plotting pressure versus permissibletemperature.

3.4.3.3 Pressure Vessels - Method C

a. Equipment assessment at this level is only applicable when no material impact test or toughnessdata are available. Service experience has been excellent with pressure vessels which havebeen built to the ASME Code Section VIII, Division 1 and other recognized standards. For thisreason, pressure vessels with a governing thickness less than or equal to 0.5 in. (13 mm.), orwhich meet all of the following criteria, may be considered to be acceptable for continued servicewithout further assessment.

1. As defined in ASME Code Section IX, P-1 and P-3 steels, where the design temperature isno higher than 650°F (343 °C). P-4 and P-5 steels may also be evaluated at this level,provided the proper precautions (e.g. preheating prior to pressurization) are taken to avoidbrittle fracture due to in-service embrittlement.

2. The equipment currently satisfies all requirements of a recognized code or standard (seeparagraph 2.2.2.c of Section 2.0) at the time of fabrication.

3. The nominal operating conditions have been essentially the same and consistent with thespecified design conditions for a significant period of time, and more severe conditions(i.e., lower temperature and/or higher stress) are not expected in the future.

4. The CET is no colder than -20°F (-29°C).

5. The nominal uncorroded governing thickness is not greater than 2 in. (50 mm).

6. The equipment is not in cyclic service (See Appendix I).

7. The equipment is not in an active environmental cracking service (see Appendix G).

8. The equipment is not subject to shock chilling (See Appendix I).

b. An assessment for brittle fracture is not required for the following product forms and thickness.

1. ASME/ANSI B16.5 ferritic steel flanges used at a metal temperature of -20°F (-29°C) andwarmer.

2. Carbon steel with a thickness less than 0.098 inches used at a metal temperature of-50°F (-46°C), or warmer.

c. Pressure vessels that are assessed using Method C of the Level 2 Assessment procedure arequalified for continued operation based on their successful performance demonstrated duringpast operation. However, if a repair is required, the guidelines in Paragraph 3.6 should befollowed to ensure that the risk of brittle fracture does not increase in the future.

3.4.3.4 Piping Systems

a. Piping systems shall meet the assessment criteria contained in Figure 3.6 and the accompanyingnotes.

b. The approach discussed in Paragraph 3.4.3.1 that provides an allowance for operating stressreduction below the allowable stress for pressure vessels, can also be applied to piping using the


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ASME Code Section VIII Figure UCS-66.1 (Figure 3.4). If this approach is used, the stress levelsused in the assessment should include system stresses (i.e. from a piping flexibility analysis) inaddition to membrane stresses from pressure loads.

c. All repairs and alterations shall be made in accordance with API 570.

3.4.3.5 Atmospheric and Low Storage Tanks

a. Atmospheric and low pressure storage tanks shall meet the Level 2 Assessment criteriacontained in Figure 3.7, as applicable, and the accompanying notes.

b. All repairs and alterations shall be in accordance with API 653.


3.4.4.1 Pressure vessels, piping and tankage which do not meet the criteria for Levels 1 and 2 assessmentsmust be evaluated on an individual basis with the help of process, materials, mechanical, inspection,safety, and other specialists as appropriate. Level 3 assessments will normally involve moresophisticated determinations of one or more of the three factors that control the susceptibility to brittlefracture: stress, flaw size and material toughness.

3.4.4.2 Section 9 shall be used as a basis for a Level 3 Assessment. This approach may involve theassignment of partial safety factors. A risk analysis considering both the likelihood and potentialconsequences of a brittle fracture in the specific service should also be considered in a Level 3Assessment.

3.4.4.3 At this assessment level, the engineering judgment of the specialists involved may be used to applysome of the principles of Levels 1 and 2 without the specific restrictions used at those levels. Examplesof some other approaches which may be considered are:

a. Perform a heat transfer analysis to provide a less conservative estimate of the lowest metaltemperature which the vessel will be exposed to in service.

b. If loadings are always quasi-static, consider additional credits due to the temperature shiftbetween dynamic (e.g., Charpy V-notch) and quasi-static toughness.

c. Inspect all seam welds and attachment welds to the pressure shell for surface cracks using dyepenetrant or magnetic particle techniques at the next scheduled turnaround and provideguidance on acceptable flaw sizes based on a flaw assessment (see Section 9.0). The extent ofsubsequent inspections should be based upon the severity of the service considering theconditions given in paragraph 3.3.4.1. Ultrasonic examination from the outside for cracks on theinternal surface is permissible if a vessel will not be opened or has an internal concrete/refractorylining.

3.4.4.4 Weld repairs should be avoided, if possible. In many cases, weld repairs can be avoided by utilizing theFFS assessment procedures in this document to evaluate a damaged component. However whennecessary, all proposals for weld repairs or alterations to the primary pressure shell should be reviewedby appropriate specialists.

3.4.4.5 It may be necessary to evaluate stresses using advanced techniques such as finite element analysis.Consideration should be given to localized or other loads (e.g. at nozzles), thermal transient effects,and residual stress. These additional considerations may result in different criteria for different locationswithin a piece of equipment. Assumed locations and orientations of crack-like flaws should bedetermined to guide the stress analyst.

3.4.4.6 A Level 3 assessment will normally rely on a determination of maximum expected flaw sizes atlocations of high stresses. In general, these postulated flaws should be assumed to be surface


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breaking, and to be oriented transverse to the maximum stress. For welded structures, this oftenimplies that the flaw is located within the residual stress field of a longitudinal weld. The maximumexpected flaw size should be sufficiently large to assure that there is a reasonable probability ofdetection using practical NDE techniques. The detectable flaw size will depend on factors such assurface condition, location, accessibility, operator competence, and NDE technique. Section 9.0 shouldbe used to derive limiting depths for crack-like flaws. In addition, the aspect ratio of the assumed flawshould be large enough to ensure that the calculations are not highly sensitive to small variations in flawdepth in the through thickness direction. To reduce this sensitivity, a minimum crack-like flaw aspectratio of 6:1 is recommended. However, it may be appropriate to verify the sensitivity of the result byplotting the critical length versus depth.

3.4.4.7 The use of material toughness data from appropriate testing is the preferred basis for advancedassessments. Where this is not practical, appropriate and sufficiently conservative estimates must bedetermined. Methods for obtaining or estimating fracture toughness are described in Appendix F.

3.5 Remaining Life Assessment - Acceptability for Continued Service

3.5.1 Remaining life is not normally an issue associated with an equipment’s resistance to brittle fracture.Therefore, equipment evaluated using a Level 1 or 2 assessment procedure should be acceptable forfuture operations as long as operating conditions do not become more severe and there is no activematerial degradation mechanism that can result in propagation of crack-like flaws. If this is not thecase, a Level 3 assessment should be performed, and a remaining life associated with the time a flawgrows to critical size can be calculated.

3.5.2 Pressure vessels constructed of materials which satisfy the requirements of a Level 1 or Level 2assessment are considered acceptable for continued service. Pressure vessels can be fullypressurized within the limits of their design parameters at any metal temperature above the MDMT.

3.5.3 Piping systems constructed of materials which satisfy the requirements of a Level 1 or Level 2assessment are considered acceptable for continued service. Piping systems can be fully pressurizedwithin the limits of their design parameters at metal temperature above the MDMT. The acceptabilityof piping systems for continued service can be determined by using similar methods as those toevaluate pressure vessels. There are two facts which distinguish piping from pressure vessels andmake piping less likely to experience brittle fracture. Firstly, because the metal thickness is generallyless than ¾ inch, even for high pressure systems, the MDMT for a particular thickness as illustrated inFigure 3.2 is more easily attainable. Secondly, there is less likelihood to have to crack-like flaws inpiping systems because there are fewer longitudinal weld seams (i.e. seamless pipe).

3.5.4 Storage tanks constructed of materials which satisfy the requirements of a Level 1 or Level 2assessment are considered acceptable for continued service. For materials represented by Curve A orfor unknown materials, the minimum shell metal temperature is 60°F or as shown in Figure 3.6.Storage tanks can utilize maximum liquid levels at a metal temperature warmer than the MDMT.

3.6 Remediation

3.6.1 Methods are available which can be used, either alone or in combination, to reduce the risk of acatastrophic brittle fracture of a pressure vessel, piping system, or storage tank.

3.6.2 The remediation methods described in this paragraph are not the only acceptable methods but areexamples of the type of methods that have been successfully employed in the past. New methods andmodifications of these techniques are always being developed and refined.

3.6.2.1 Limiting Operation - The limitation of operating conditions to within the acceptable operating pressure-temperature envelope is the simplest type of remediation effort. This method, however, may beimpractical in many cases because of the requirements for stable process operation. The mostsuccessful, and effective, technique for limiting operation has been the controlled start-up procedure.


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This is due to the fact that many petroleum and chemical processes that undergo this type ofassessment for brittle fracture were originally designed for substantially warmer temperatures, abovethe temperature range where the risk of brittle fracture must be addressed.

3.6.2.2 Controlled Start-Up Procedure - Using a controlled start-up procedure to control the pressurization ofequipment within the limitations of an acceptable pressure-temperature operating envelope is a viableand often used method to overcome the limitations of low material toughness properties and risks ofcatastrophic brittle fracture.

3.6.3 Pressure vessels that are assessed using Method C of the Level 2 Assessment procedure are qualifiedfor continued operation based on their successful performance demonstrated during past operation.However, whenever repair welding, PWHT and hydrotesting are conducted, the following guidelinesshould be followed to ensure that the risk of brittle fracture does not increase in the future. Theseguidelines are also recommended for piping systems. Recommendations for repairs to atmosphericstorage tanks are included in API 653.

3.6.3.1 Welding Guidelines - the following are recommended for repairs:

a. All weld repairs or alterations shell should be in accordance with API-510 or API 570, asapplicable.

b. When possible, cutting though or along existing pressure retaining weld seams, or within adistance of the greater of 1 inch (25 mm) or twice the plate thickness from the edge of the weld,should be avoided. When this cannot be avoided, procedures should be established to removethe old weld and heat affected zone material.

c. Base plate should be inspected prior to making any cuts into the shell to first assess the materialcondition. The size of the repair, or location of a new nozzle or alteration, should be adjusted asrequired based on the findings of the base metal inspection.

d. All weld bevels prepared by oxyacetylene cutting should be ground to eliminate the potential forany heat affected material remaining on the weld bevel surface.

e. All weld bevels, and a band of adjacent base material that is at least the larger of 2 in. (50 mm)or twice the nominal plate thickness wide, should be inspected by liquid penetrant (PT) ormagnetic particle (MT) techniques prior to any welding to identify surface breaking flaws. Allflaws not meeting the acceptance criteria in the ASME Code Section VIII Appendices 6 and 8shall be removed.

f. All Category A, B and D welds (as defined in ASME Code Section VIII Division 1) should be fullpenetration and full fusion. Back gouging, followed by PT or MT of the back side of the rootpass, is also required when the weld is made from both sides.

g. For carbon steel materials, preheat to a minimum of 200°F (93°C) whenever the plate thicknessexceeds 1 in. (25 mm). For thinner plates, use a 60°F (16°C) minimum preheat.

h. All final welds should be examined on both the inside and outside surfaces where accessible,using either PT or MT techniques. This inspection should be performed after any requiredPWHT. All detected flaws should be removed.

I. All design and/or repair details with any required calculations, specified inspection, and testingshould be documented and included in the vessel’s permanent file.

3.6.3.2 PWHT Guidelines - welds should be subject to PWHT for any of the following cases:

a. The plate material and the site of any new welds are such that PWHT is required by the Codeused in the design, fabrication, modification and repair of the vessel.


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b. PWHT is required based on the vessel service.

3.6.3.3 Hydrotest Guidelines - hydrotesting in accordance with the original construction code is recommendedfor the following:

a. Addition of nozzles over 2 in. (50 mm) nominal pipe size into shells or heads over 0.5 in. (13mm) in thickness.

b. Removal, replacement, or addition of any shell or head plate, where the longest weld dimensionof the replacement plate exceeds 12 in. (300 mm).

c. Complete or partial removal and replacement of more than the following lengths of pressureretaining weld (partial removal of a weld is defined as removal of more than one-half of theoriginal weld thickness):

• 24 in. (600 mm) of circumferential weld in cylindrical shell sections

• 12 in. (300 mm) of all other Category A, B or D welds as defined in ASME code SectionVIII, Division 1.

3.7 In-Service Monitoring

3.7.1 There is little that can be accomplished by in-service monitoring of equipment to alleviate the risk ofbrittle fracture because the factors that contribute to this phenomena, stress level, material toughness,and flaw size are difficult to monitor.

3.7.2 Monitoring for Degradation of Low Alloy Steel Notch Toughness - Certain materials, such as thechromium-molybdenum low alloy steels suffer from a loss of notch toughness due to exposure at hightemperatures. This degradation is often monitored over the service life by means of sentinel materialincluded within a pressure vessel. Periodically, a portion of this material is removed and tested tomonitor for the degradation of material toughness as a result of the exposure at temperature. Thedegradation of properties is evaluated against minimum acceptable criteria for resistance to brittlefracture which have previously been established. The evaluation of fitness-for-service for pressurevessels which no longer meet the original criteria generally requires that a Level 3 assessment beconducted.

3.7.3 Monitoring for Criticality of Growing Flaws - Flaws which develop or propagate during the service life ofequipment can have a detrimental affect on the risk of brittle fracture. The assessment of each type offlaw is prescribed in other sections of this Recommended Practice, see Section 2 for an overview.

3.7.4 Assessment of Non-Growing Flaws Detected In-Service - During the course of in-service inspections orother forms of examination for other purposes, flaws may be detected. These could include originalmaterial or fabrication flaws. Furthermore, these defects may or may not be in excess of therequirements of the original design and construction code. In addition, while these flaws may havebeen innocuous, their presence may affect current or altered design and operating parameters.Alternatively, flaws may have developed or resulted from service exposure, excessive operatingconditions, or maintenance-related activities. The influence of such flaws on the increasedsusceptibility for brittle fracture should be assessed. This assessment will generally require either aLevel 2 or 3 analysis.

3.8 Documentation

3.8.1 The documentation for each level of brittle fracture assessment should include the information cited inparagraph 2.8 of Section 2.0 and the following specific requirements:

3.8.1.1 Level 1 Assessment - A report covering the assessment evaluation conducted, specific data used, andthe criteria which have been met by the results obtained from that evaluation.


3-12

3.8.1.2 Level 2 Assessment - Documentation shall consist of a report written by a qualified technical specialistand included in the equipment history record file. This report shall address the reason for theassessment, the assessment level used, the engineering principles employed, the source of all materialdata used, identification of any potential material property degradation mechanisms and the associatedinfluence on the propagation of flaws, and the criteria applied to the assessment procedure.

3.8.1.3 Level 3 Assessment - The documentation shall cover the reason(s) for performing a Level 3assessment, all fitness-for-service flaw assessment issues, and shall be written by a qualified technicalspecialist. This report shall also address the engineering principles employed including stress analysismethods and flaw sizing, the source of all material data used, identification of any potential materialproperty degradation mechanisms and the associated influence on the propagation of flaws, and thecriteria applied to the assessment procedure.

3.8.2 All documents pertaining to the assessment for brittle fracture shall be retained for the life of theequipment in the equipment history file. This includes all supporting documentation, data, test reports,and references to methods and criteria used for such assessments and evaluations. For vesselsexposed to identical conditions, a single document with appropriate references is adequate.

3.9 Referenced Publications, Tables and Figures

3.9.1 API, "Pressure Vessel Inspection Code: Maintenance Inspection, Rerating, Repair and Alteration,"ANSI/API Std. 510, American Petroleum Institute, Washington D.C., 1992.

3.9.2 API, "Inspection, Repair, Alteration, and Rerating Of In-Service Piping Systems," ANSI/API 570,American Petroleum Institute, Washington D.C., 1993.

3.9.3 API, "Design and Construction of Large, Welded, Low-Pressure Storage Tanks," ANSI/API Std. 620,American Petroleum Institute, Washington D.C., 1992.

3.9.4 API, "Welded Steel Tanks for Oil Storage," ANSI/API Std. 650, American Petroleum Institute,Washington D.C., 1992.

3.9.5 API, "Tank Inspection, Repair, Alteration, and Reconstruction," ANSI/API Std. 653, American PetroleumInstitute, Washington D.C., 1992.

3.9.6 ASME, ”Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels Division 1,” American Societyof Mechanical Engineers, NY, 1992.

3.9.7 ASME, ”Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels Division 2 - AlternativeRules,” American Society of Mechanical Engineers, NY, 1992.

3.9.8 ASME, “Process Piping” ASME Code For Pressure Piping, B31, ASME B31.3, American Society ofMechanical Engineers, NY 1992.

3.9.9 ASME, ”Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels Division 1,” American Societyof Mechanical Engineers, NY, 1992.

3.9.10 McLaughlin, J.E., Sims, J.R., "Assessment of Older Equipment for Risk of Brittle Fracture," ASME PVP-Vol. 261, American Society of Mechanical Engineers, New York, 1993, pp. 257-264.


3-13

Table 3.1Overview Of Data For The Assessment Of Brittle Fracture

A summary of the data that should be obtained from a field inspection is provided on this form.

Equipment Identification: Equipment Type: _____ Pressure Vessel _____ Storage Tank _____ Piping ComponentComponent Type & Location: Year Of Fabrication:

Data Required for Level 1 Assessment (V - indicates data needed for pressure vessels, T - indicates dataneeded for tankage, and P - indicates data needed for piping)Design Temperature {V,T}: Original Hydrotest Pressure {V,T]: Temperature During Original Hydrotest Pressure {V,T}: Nominal Wall Thickness of all components {V,T,P}: Critical Exposure Temperature (CET) {V,T,P}: Minimum Design Metal Temperature (MDMT) {V}: PWHT done at initial construction? {V}: PWHT after all repairs? {V}:

Additional Data Required for Level 2 Assessment (In Addition to the Level 1 Data):Weld Joint Efficiency (level 2) {V,T,P} : Corrosion Allowance {V,P}: Maximum Operating Pressure {T,P}: Charpy Impact Data, if available {V,T,P}:


3-14

Table 3.2Assignment of Materials to Curves in Figure 3.2

Curve Material

A All carbon and all low alloy steel plates, structural shapes and bars not listed in Curves B, C, and D below. Thefollowing specifications for obsolete materials are also included in Curve A: A7, A10, A30, A70, A113, A149,A150, S1, S2, S25, S26, S27, A201, A212.

B 1. SA-285 Grades A and BSA-414 Grade ASA-442 Grade 55>1 in. if not to fine grain practice and normalizedSA-442 Grade 60 if not to fine grain practice and normalizedSA-515 Grades 55 and 60SA-516 Grades 65 and 70 if not normalizedSA-612 if not normalizedSA-662 Grade B if not normalized

2. All materials of Curve A if produced to fine grain practice and normalized which are not listed for CurveC and D below;

3. Plates, structural shapes, and bars, all other product forms (such as pipe, fittings, forgings, castings,and tubing) not listed for Curves C and D below;

4. Parts permitted under UG-11 shall be included in Curve B even when fabricated from plate thatotherwise would be assigned to a different curve.

C 1. SA-182 Grades 21 and 22 if normalized and tempered.SA-302 Grades C and DSA-336 Grades F21 and F22 if normalized and temperedSA-387 Grades 21 and 22 if normalized and temperedSA-442 Grades 55 < 1 in. if not to fine grain practice and normalizedSA-516 Grades 55 and 60 if not normalizedSA-533 Grades B and CSA-662 Grade A

2. All material of Curve B if produced to fine grain practice and normalized and not listed for Curve Dbelow

D SA-203SA-442 if to fine grain practice and normalizedSA-508 Class 1SA-516 if normalizedSA-524 Classes 1 and 2SA-537 Classes 1 and 2SA-612 if normalizedSA-662 if normalized

Notes:1. When no class or grade is shown, all classes or grades are included.2. The following shall apply to all material assignment notes.

a. Cooling rates faster than those obtained in air, followed by tempering, as permitted by the materialspecification, are considered to be equivalent to normalizing and tempering heat treatments.

b. Fine grain practice is defined as the procedures necessary to obtain a fine austenitic grain size asdescribed in SA-20.


3-15

Table 3.3Impact Test Exemption Temperature for Bolting Materials

Specification Grade Impact Test Exemption Temperature (°F)

SA-193 B5 -20

SA-193 B7 -40

SA-193 B7M -50

SA-193 B16 -20

SA-307 B -20

SA-320 L7, L7A, L7M, L43 Impact Tested per Specification

SA-325 1, 2 -20

SA-354 BC 0

SA-354 BD +20

SA-449 --- -20

SA-540 B23/24 +10

Note: Bolting materials are exempt from assessment due to loading conditions.


3-16

Table 3.4Procedure for Determining MDMT

Step Required Actions

1 Establish nominal thicknesses [General Note (1)] of welded parts, non-welded parts, and attachments underconsideration both before and after corrosion allowance is deducted (tn and tn - FCA respectively), and other pertinentdata applicable to the nominal thicknesses such as:o All applicable loadings and coincident minimum design metal temperature (MDMT)

(General Note 2)o Materials of constructiono Weld joint efficiency, E (General Note 3)o Nominal uncorroded thickness TN (in) (General Note 1)o Required thickness in corroded condition for all applicable loadings, tr (in), base on the applicable joint

efficiency (General Notes 2 and 3)o Applicable material toughness curve(s) of Figure 3.2.o Corrosion allowance, FCA (in)

2 Select MDMT from Figure 3.2 (General Note 4) for each nominal uncorroded governing thickness (General Note 5)

3Determine the ratio

t E

t FCAr

n

*

− or

S E

SE

* *

(General notes 3,6 and 10)

4 Using the ratio from Step 3 to enter ordinate of Figure 3.7, determine the reduction to be applied tothe MDMT found in Step2 (General Notes 7 and 8)

5 Determine the adjusted MDMT for governing thickness under consideration.

6 Repeat for all governing thicknesses (General Note 5) and take warmest value as the lowest allowable MDMT to bemarked on nameplate for the zone under consideration (General Note 9), see UG-116. See UG-99 (h) for coldestrecommended metal temperature during hydrostatic test (General Note 6). See UG-100(c) for coldest metaltemperature permitted during pneumatic test General Note 6).

General Notes: (paragraph references UG and UW refer to the ASME Code, Section VIII, Division 1)1. For welded pipe where a mill undertolerance is allowed by the material specification, the thickness after mill

undertolerance has been deducted shall be taken as the uncorroded nominal thickness tn for determinationof the MDMT to be stamped on the nameplate. Likewise, for formed heads, the minimum specifiedthickness after forming shall be used as tn .

2. Loadings, including those listed in UG-22 (see Table 4.1), which result in general primary membranetensile stress at the coincident MDMT.

3. E is the joint efficiency (Table UW-12) used in the calculation of tr; E* has a value equal to E except that E*

shall not be less than 0.80. For castings, use quality factor or joint efficiency E whichever governs design.4. The construction of Figure 3.2 is such that the MDMT so selected is considered to occur coincidentally with

an applied general primary membrane tensile stress at the maximum allowable stress value in tension fromTable 1A of Section II Part D.

5. See Paragraph 3.4.1.1.d for definitions of governing thickness.6. If the basis for calculated test pressure is greater than the design pressure [UG-99(c) test], a Ratio based

on the tr determined from the basis for calculated test pressure and associated appropriate value of tn -FCA shall be used to determine the recommended coldest metal temperature during hydrostatic test andthe coldest metal temperature permitted during the pneumatic test. See UG-99(h) and UG-100(c).

7. Use of Figure 3.7 to reduce the MDMT is limited to stationary vessels. See UCS-66(b).8. For reductions in MDMT up to and including 40°F, the reduction can be determined by: reduction in

MDMT = (1.0 - Ratio)100 °F.9. A colder MDMT may be obtained by selective use of impact tested materials as appropriate to the need

(see UG-84). See also UCS-68(c).10. Alternatively, a Ratio of S*E* divided by the product of the maximum allowable stress value in tension from

Table 1A of Section II Part D times E may be used, where S is the applied general primary membranetensile stress and E and E* are as defined in General Note (3).


3-17

Figure 3.1Overall Brittle Assessment Procedure for Pressure Vessels

ObtainEquipment

Data.

Determinethe CET.

EvaluationMethod:

Level 1

MDMT <=CET?

Level 2 -Method A

Operationwithin the MDMT

Envelope?

Level 2 -Method B


Envelope?

Level 2 -Method C


Envelope?

Level 3


Envelope?

Vessel Is NotSuitable For

Operation!

Vessel Is SuitableFor Service.

MaintainInspection

Per API 510.

Change InService?

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3-18

Figure 3.2Minimum Allowable Metal Temperature For Pressurization of Equipment without Impact Testing

Nominal Thickness, inches

0 1 2 3 4 5 6

Min

imum

Des

ign

Met

al T

empe

ratu

re, °

F

-80

-60

-40

-20

0

20

40

60

80

100

120

140

0.394

A B

C

D

Impact Testing Required

-50

Notes:1. Curves A through D define material specification classes in accordance with Tables 3.2 and 3.3.2. Equipment whose CET is above the appropriate material curve is exempt from further brittle fracture

assessment.3. This figure is identical to Figure UCS-66 of ASME Code Section VIII, Division 1.


3-19

Figure 3.3Some Typical Vessel Details Showing the Governing Thicknesses

X

X

tg1=tatg2=ta (seamless) or tb (welded)

(A) Butt Welded Components

ta

C

1 B A

C2

B A

C3

B A

tg1=min (ta, tb) tg2=min (tb, tc) tg3=min (ta, tb)

2

tc

tb ta

tc

tb ta

tc

tb ta

Note: Using tg1, tg2, and tg3, determine the warmest MDMT and as the permissible MDMT for the weld assembly.

(B) Welded Connection with Reinforcement Plate Added

A

1

B

tb

Section X-X


3-20

Figure 3.3 (continued)Some Typical Vessel Details Showing the Governing Thicknesses

Groove

1

taA

B

2

C

tc

Groove

ta

1

A

2

B

tb

(C) Bolted Flat Head orTubesheet and Flange

(D) Integral Flat Head or Tubesheet

Groove

taA

1

2

B

tb

(E) Flat Head or Tubesheet with a Corner Joint

tg1= (for welded or nonwelded)

ta4

A tg1= (for welded or nonwelded)

ta4

A

tg2=tctg2=tb

tg1= (for welded or nonwelded)

ta4

A

tg2=min (ta, tb)

The governing thickness ofis max (tg1, tg2)A

The governing thickness ofis max (tg1, tg2)A


3-21

Figure 3.3 (continued)Some Typical Vessel Details Showing the Governing Thicknesses

Notes: tg = governing thickness of the welded joint as defined in UCS-66.

(F) Welded Attachments as Defined in UCS-66(a)

1B

A

Pressure Part Pressure Part

tg1=min(ta, tb)

1 B 1

A

ta

tb

tbta


3-22

Figure 3.4Reduction in Minimum Design Metal Temperature Based on Available Excess Thickness

PART UCS - Carbon and Low Alloy Steel Vessels

°F

0 20 40 60 80 100 120

Rat

io:

t rE*/

(tn-

FC

A)

or

Ars

0.0

0.2

0.4

0.6

0.8

1.0

Notes (references are to General Notes of Table 3.4):1. Nomenclature:

tr = required thickness of the component under consideration in corroded condition for allapplicable loadings [General Note (2)], based on the applicable joint efficiency E[General Note (3)] in.

tn = nominal thickness of the component under consideration before corrosion allowance isdeducted, in.

FCA = Future corrosion allowance, in.E* = as defined in General Note (3)

Ars = Alternate ratio based on stress; A S E SErs = * * where S* is the applied general

primary membrane tensile stress, S is the maximum allowable stress value in tensionfrom Table UCS-23 times E and E and E* are as defined in General Note (3).

2. The value of Ratio (y-axis) is equal to 0.4 for all temperatures exceeding 105 oF.


3-23

Figure 3.5Minimum Allowable Metal Temperature Reduction

Based on Hydrostatic Proof Testing

Permissible Temperature ReductionBelow Hydrotest Temperature, °F

0 10 20 30 40 50 60 70 80 90 100 110 120

Max

imum

Exp

ecte

d O

pera

ting

Pre

ssur

e/H

ydro

test

Pre

ssur

e

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Corresponds toDesign Pressure

Notes:1. The pressure ratio for all temperatures less than or equal to 35 oF is 0.67.2. The pressure ratio for all temperatures greater than or equal to 105 oF is 0.25.


3-24

Figure 3.6Level 2 Brittle Fracture Assessment of Carbon Steel Piping

Piping Does NotMeet the Level 1

Assessment Criteria.

Obtain InformationFor A Level 2Assessment.

Is MetalCET <= -150 F

(1)?

Is ShockChilling Possible

(2)?

Is Vibration OrLow Temperature Impact

Possible (3)?

Is NominalPipe Wall Thickness

> 0.5 in.?

DoesComponent Meet

Operating ExperienceCriteria (4)?

Is Cir. Stress> 7 ksi Or Long. Stress

> 9 ksi (5)?

Is Metal CET<= -50 F (1)?

Piping Meets Level2 Assessment

Criteria!

High Level Of Concernfor Brittle Fracture -Level 3 Assessment

Required!

No

Yes

No

Yes

No

Yes

Yes

No

Yes

No

No

Yes

No Yes


3-25

Notes for Figure 3.6

1. Experience suggests that brittle fracture of piping is usually associated with unanticipated low tem-perature excursions.

2. Shock chilling is a rapid decrease in metal temperature caused by a sudden flow of liquid, which iscolder than -20oF (-29oC), and which is 100oF (55oC) or more, below the metal temperature of theequipment before cooling. In addition to a liquid, a two phase fluid may need to be considered whenevaluating potential shock chilling. One example of this is a flare header that receives sub-cooled orflashing liquid from a safety valve discharge.

3. Visible vibrations of a portion of the piping system could initiate cracks which are cause for a high levelof concern of brittle fracture. If the system could be subject to an impact load while it is below -20oF (-29oC), a high level of concern exists. Impact loads include hammering from flow or aggressiverepeated strikes from tools or mobile equipment. etc. Minor impacts from hand tools other thandeliberate hammering, should not be a concern.

4. Acceptance based on successful operating experience is based on the following:

a. The nominal operating conditions have been essentially the same and consistent for a significantperiod of time and more severe conditions (i.e., lower temperature and/or higher pressure) arenot expected in the future. In addition, the maximum design temperature and pressure have notbeen exceeded for a significant period of time. (Note safety valve discharge lines usually do notmeet this criteria.) (See Note 1)

b. The piping is not in a stress corrosion cracking environment such as non-PWHT piping in DEA,MEA, NaOH, or KOH. This restriction does not apply to seamless pipe in wet H2S service orseamless and welded pipe in anhydrous ammonia service unless there are clear indications ofcracking in the piping.

c. Is the piping system in good condition as determined by inspection?

d. Does the piping system have adequate flexibility by virtue of freedom/condition of supports asdetermined by visual inspection?

5. Guidelines for stress calculations are as follows:

a. The circumferential stress should be calculated from nominal wall thickness less corrosionallowance, less manufacturing tolerance, or the actual measured thickness.

b. The total longitudinal stress should consider pressure, weight, thermal expansion/contractionand imposed displacements, but exclude stress intensification factors. The thermal stress doesnot have to consider a full design range, such as would result from a system with a high designtemperature. It should best reflect the actual stress imposed at low temperature.


3-26

Figure 3.7Brittle Fracture Assessment for Storage Tanks

Obtain Informationfor a Brittle Fracture

Assessment.

Level 1Assessment.

Tank MeetsToughness Requirements

In Current ConstructionCode (1)(2)?

Level 2Assessment.

PriorHydrotest Demonstrates

Fitness-For-Service(3)?

TankThickness <= 0.5

inches (4)?

OperatingTemperature Above

60 F (5)?

MembraneStress <= 7 ksi

(6)?

TankExempt from

Impact Testing(7)?

Tank Fullat Lowest One DayTemperature (8)?

Perform Level 3Assessment?

Level 3AssessmentSatisfied?

Hydrotestto Demonstrate

Fitness-For-Service(3)?

Rerate TankBased on Prior Operating

History (9)?

Tank ContinuesTo Operate in the Same

Service (10)?

Is theFuture ServiceMore Severe

(11)?

Tank Is Suitablefor Continued

Operation!

RetireTank!

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3-27

Notes for Figure 3.7

The assessment procedure as illustrated in Figure 3.7 shall be used for Level 2 assessment of aboveground atmo sphericstorage tanks in petroleum and chemical services. Each of the key steps on the decision tree are numbered corre spondingto the explanation provided as follows:

1. Atmospheric storage tanks constructed in accordance with API Standard 650 (seventh edition or later) includerequirements to minimize the risk of failure due to brittle fracture. Tanks constructed to earlier version of thisStandard may also be shown to meet these the API 650 (seventh edition or later) toughness requirements byimpact testing coupon samples from a representative number of shell plates.

2. Many tank continue to operate successfully in the same service were not constructed to the requirements of APIStandard 650 (seventh edition or later). These tanks are potentially susceptible to failure due to brittle fractureand require a Level 2 Assessment.

3. For purposes of this assessment, hydrostatic testing demonstrat es that an aboveground atmospheric storagetank in a petroleum or chemical service is fit for continued service and at minimal risk of failure due to brittlefracture, provided that all governing requirements for repair, alterations reconstruction, or change in service are inaccordance with API Standard 653 (including a need for hydro static testing after major repairs, modifica tions orreconstruction). The effectiveness of the hydrostatic test in demonstrating fitness for con tinued service is shownby industry experience.

4. If a tank shell thickness is no greater than 0.5 inch, the risk of failure due to brittle frac ture is minimal, providedthat an evaluation for suitability of service per API 653, Section 2 has been per formed. The original nominalthickness for the thickest tank shell plate shall be used for this assessment.

5. No known tank failures due to brittle fracture have occurred at shell metal temperatures of 60 oF or above. Similarassurance against brittle fracture can be gained by increasing the metal temperature by heating the tankcontents.

6. Industry experience and laboratory tests have shown that a membrane stress in tank shell plates of at least 7 ksiis required to cause failure due to brittle fracture.

7. Tanks constructed from steel listed in Figure 2-1 of API Standard 650 can be used in accordance with theirexemption curves, provided that an evaluation for suitability of ser vice per Section 2 of API Standard 653 hasbeen performed. Tanks fabricated from steels of unknown toughness thicker than ½ inch and operating at a shellmetal temperature below 60oF can be used if the tank meets the requirements of Figure 3.8. The origi nal nominalthickness for the thickest tank shell plate shall be used for the assessment. For unheated tanks, the shell metaltemperature shall be the design metal temperature as de fined in 2.2.9.3 of API Standard 650.

8. The risk of failure due to brittle fracture is minimal once a tank has demonstrated that it can operate at a specifiedmaximum liquid level at the lowest expected temperature without failing. For the purpose of this assess ment, thelowest expected temperature is defined as the lowest one day mean temperature as shown in Figure 2-2 of APIStandard 650 for the continental United States. It is necessary to check tank log recorded and meteorologicalrecords to ensure that the tank has operat ed at the specified maximum liquid level when the one-day meantemperature was as low as shown in Figure 2-2 of API Standard 650.

9. An evaluation can be performed to establish a safe operating envelope for a tank based on the past operatinghistory. This evaluation shall be based on the most severe combination of temperature and liquid levelexperienced by the tank during its life. The evaluation may show that the tank needs to be rerated or operateddifferently; several options exist:

a. Restrict the liquid level,

b. Restrict the minimum metal temperature,

c. Change the service to a stored product with a lower specific gravity, or

d. Combinations of a., b., and c., above.

10. An assessment shall be made to determine if the change in service places the tank at greater risk of failure due tobrittle fracture. The service can be considered more severe and creating a greater risk of brittle fracture if theservice temperature is reduced (for example, changing from heated oil ser vice to ambient temperature product),or the product is changed to one with a greater specific gravity and thus increasing stresses.

11. A change in service must be evaluated to determine if it increase the risk of failure due to brittle fracture. In theevent of a change to a more severe service (such as operating at a lower temperature or handling product at ahigher specific gravity) it is necessary to consider the future service conditions in the fitness-for-serviceassessment.


3-28

Figure 3.8Exemption Curve for Tanks Constructed from Carbon Steel of Unknown Toughness Thicker Than � inch

and Operating at a Shell Metal Temperature Below 60oF

Shell Thickness, in.

0.0 0.5 1.0 1.5 2.0

She

ll m

etal

tem

pera

ture

, °F

0

10

20

30

40

50

60

70

Safe for Use

Additional Assessment Required

0.875

Safe for Use

Note: The above exemption curve between 30 oF and 60 oF is based on Curve A in Figure 3.2. The otherparts of the curve were established based on successful operating experience.


3-29

3.10 Example Problems

3.10.1 A pressure vessel, 1 inch thick, fabricated from A-285 Grade C in caustic service was originally subjectto PWHT at the time of construction. Determine the MDMT.

Solution - Based on Curve A in Figure 3.2, an MDMT of 68° (20°C) was established for the vesselwithout any allowance for PWHT. Applying the allowance for PWHT reduces the MDMT by 30°F(17°C) and established a new MDMT of 38°F (3°C).

3.10.2 A horizontal drum 1.5 inches (38 mm) thick is fabricated from A-516 Grade 70 steel which was suppliedin the normalized condition. There is no toughness data on the steel. Determine the MDMT.

Solution - Since A-516 Grade 70 is manufactured to a fine grain practice and was supplied in this casein the normalized condition, Curve D of Figure 3.2 may be used. In this case, the MDMT is -15°F (-26°C).

3.10.3 A reactor vessel fabricated from A-204 Gr B (C-½ Mo) has the following material properties anddimensions. The reactors were designed to ASME Code Section VIII, Division 1. Develop a table ofMDMT as a function of pressure based on Paragraph 3.4.3.1 and the allowances given in Figure 3.4and Table 3.4.

Vessel Information

Allowable stress = 17,500 psi (121 MPa)

Design pressure = 390 psi (26.5 atm)

Inside Diameter = 234 inches (5943.6 mm)

Operating pressure = 240 psi (1633 atm)

Wall Thickness = 2.72 inches (69 mm)

Startup pressure = 150 psi (10.2 atm)

Weld Joint Efficiency = 1.0

Corrosion Allowance = 1/16 in. (1.6 mm)

MDMT at Design Pressure = 110°F (43°C) (see Curve A of Figure 3.2)

Impact test data is not available.

Solution - The membrane stress for a cylindrical pressure vessel as a function of the pressure is (seeAppendix A):

R

t

S E P P

c

c

=−

=

= − =

= +

=

234 0 0625

2116 97

2 72 0 0625 2 66

116 97

2 660 6 44 6

" . ". "

. " . " . "

.

.. .* *


3-30

Using this relationship, a table of MDMT can be established as a function of pressure based onParagraph 3.4.3.1 and the allowances given in Figure 3.4 and Table 3.4.

P

psi (atm)

S*E*

psi (MPa)A

S E

SErs =* * ∆T

°F (°C)

MDMT

°F (°C)

390 (26.5) 17,400 (12) 1.00 0 (0) 110 (43)

240 (16.3) 10,700 ( 74) 0.61 38 (21) 72 (22)

157 (10.7) 7,000 (48) 0.40 105 to 260

(58 to 144)

5 to -150°F

(-15 to -101)

The operating pressures and corresponding values of the MDMT in this table must be compared tothe actual sphere operating conditions to confirm that the metal temperature (CET) cannot be belowthe MDMT at the corresponding operating pressure.

3.10.4 A sphere fabricated from BS 1501 - 213 Grade 32A LT (ASME SA414 Grade G equivalent) has thefollowing material properties and dimensions. Develop a table of MDMT as a function of pressurebased on Paragraph 3.4.3.1 and the allowances given in Figure 3.4 and Table 3.4.

Vessel Information

Allowable stress = 30,500 psi (211 MPa)

Design pressure = 250 psig (17 atm)

Inside Diameter = 585.6 inches (14874.2 mm)

Wall Thickness = 1.26 in. (32 mm)

Weld Joint Efficiency = 1.0

Corrosion Allowance CA = 1/16 in. (1.6 mm)

MDMT at Design Pressure = 75°F (24°C), (see Curve A of Figure 3.4)

Impact test data is not available.

Solution - The membrane stress for a spherical pressure vessel as a function of pressure as (seeAppendix A):

R

t

S EP

P

c

c

=−

=

= − =

= +

=

585 6 0 06252

292 77

126 0 0625 1198

2

292 77

11980 2 122

. " . ". "

. " . " . "

.

..* *


3-31

Using this relationship, a table of MDMT can be established as a function of pressure based onParagraph 3.4.3.1 and the allowances given in Figure 3.4 and Table 3.4. Note that in developing thistable, temperature reductions for vessels fabricated to Codes other than ASME Section VIII, Division 1are not permitted for stresses above 17,500 psi.

P

psi (atm)

S*E*

psi (MPa)A

S E

SErs =* * ∆T

°F (°C)

MDMT

°F (°C)

250 (17.0) 30,500 (211) NA NA 75 (24)

143 (9.7) 17,500 (121) 1.0 0 (0) 75 (24)

100 (6.8) 12,200 ( 84) 0.7 30 (17) 45 ( 7)

57 (3.9) 7,000 ( 48) 0.4 105 to 225

(58 to 125)

-30 to -150

(-34 to -101)


3.10.5 A spherical pressure vessel has the following properties and has experienced the following hydrotestconditions. Using Paragraph 3.4.3.2 and Figure 3.5, prepare a table showing the relationship betweenoperating pressure and MDMT.

Hydrotest pressure = 300 psi (20.7 bar)

Design pressure = 200 psi (13.8 bar)

Metal temperature during hydrotest = 60°F (16°C)

Solution - The maximum measured metal temperature during hydrotest was 50°F. To be conservative,10°F is added to this and the analysis is based on a hydrotest metal temperature of 60°F.

Operating Pressure

psi (bar)

Operating Pressure/

Hydrotest Pressure

Temperature Reduction

°F (°C)

MDMT

°F (°C)

200 (13.8) 0.67 35 (19.4) 25 (-4)

180 (12.4) 0.6 43 (24) 18 (-8)

150 (10.2) 0.5 55 (31) 5 (-15)

120 (8.3) 0.4 70 (39) -10 (-23)

90 (6.2) 0.3 90 (50) -30 (-34)

75 (5.1) 0.25 105 to 210

(58 to 117)

-45 to -150

(-43 to -101)


3-32


3.10.6 A demethanizer tower in the cold end of ethylene plants typically operate colder in the top portion andwarmer at the bottom which is kept warm with a stream circulated through a reboiler. As a result, thetop portion of the tower is normally constructed from a 3½% Ni steel toughness tested down toapproximately -101°C (-150°F), while the lower portion of the tower is normally constructed from a fullykilled fine grained and normalized carbon steel which is toughness tested down to approximately -46°C(-50°F). The potential for brittle fracture exists because if the reboiler does not operate, cold liquid willflow down the tower into the carbon steel section, causing it to operate significantly lower than -46°C (-50°F), the temperature at which the steel was toughness tested. Perform a brittle fracture assessmentof ethylene plant demethanizer tower considering all aspects of operation.

Solution - A brittle fracture assessment consistent with Paragraph 3.4.4 (Level 3 assessment) can beperformed on the demethanizer tower. The approach is illustrated with reference to a typical oldervintage demethanizer tower as illustrated in Figure 3.1E.


3-33

Feed 1

Feed 2

Tray 62

Tray 33

Tray 24

Tray 32Feed 3

Tray 1

29 mm

3-1/2 % Ni

Position

-100 -50 0Temperature (oC)

Detail ATypical Design

Detail BTemperature Profile Along The Length Of

The Tower

Material: ASTM A516 Grade 70 (KCS)Minimum Recorded Yield Strength at RoomTemperature: 45 M/mm2Pressure: 37.2 Bar-gToughness: 34/32J @ -46 oFPWHT: YesWeld Joint Efficiency: 1.0

Original MDMT(based on

Impact Tests)

Normal Operation

Potential Excursion

PotentialViolation

2300 mm

Figure 3.1ESchematic Of Demethanizer

However the principles should be equally applicable to a wide range of process equipment. Twonotable exceptions are fixed tubesheet exchangers and piping, since the highest applied stresses inthis equipment may be caused by thermal loadings, not internal pressure. The basis is to useestablished fracture mechanics principles as outlined in Section 9, to estimate the limiting flaw size in avessel, and to review how this changes as the temperature in the vessel drops during an excursion.The resulting graph of limiting flaw size versus temperature is referred to as a Fracture ToleranceSignature (FTS). The FTS provides an overview of the "safety factor" in terms of limiting flaw size,


3-34

both against a datum such as a new vessel meeting all code rules, or other similar pieces of equipmentwhich have demonstrated satisfactory performance. In addition, it is possible to select a lowerexcursion limit by establishing a flaw size which can be detected with sufficient confidence usingpractical non-destructive examination (NDE) techniques. The FTS can then be used to develop amodified MDMT diagram, onto which the excursion limits can be superimposed.

It is an assumption in the development of the methodology that for this particular Level 3 assessment,the equipment has been fabricated to code standards at the time of construction, as well as beingimpact tested and subject to full radiography. It is also a requirement that the vessel materialspecifications are known, and documented, as is the inspection history. This is essential to enablereasonable assumptions to be made about basic toughness properties, stress levels, and likelihood offabrication or service induced flaws. It is specifically not intended to apply this methodology to processequipment of unknown quality or uncertain history.

Assessment Approach

Since the approach is to calculate a limiting flaw size, it is clear that applied stress and materialtoughness must be known. The fracture assessment process used is a Level 3 Fracture Assessmentas outlined in Section 9, and assessment is limited to the lower carbon steel section of the tower, sincethis is the only section to experience an MDMT violation (see Figure 3.1E).

Flaw

Since no specific flaw is being considered, a reasonably conservative yet representative hypotheticalsurface breaking crack, with an aspect ratio (2c:a) of 6:1, is assumed to lie on the inside surface of thevessel, within and parallel to a longitudinal weld seam. Potential flaw could be assumed elsewhere inthe vessel and, if necessary, can be considered by adjusting the applied stresses, or the stress intensitysolution. However, as will be seen latter, the relative nature of the results as expressed by the FTS arenot significantly affected by such variations, though the minimum excursion temperature will be.

Stress

Four load sources typically describe the relevant stress applied to the vessel. These are hoop stressfrom internal pressure, residual stress in welds, local stress effects from nozzles and attachments, andthermal transient stresses during the upset. In addition, consideration must be given to occasionalloads such as wind, or deadloads from weight on a horizontal drum. The latter are ignored in thisexample. For optimum use of a Level 3 Assessment from Section 9, stresses are classified intoprimary and secondary, as well as membrane and bending.

The pressure stress is evaluated using the code formulae as a primary membrane stress (seeAppendix A). The residual stress can be estimated based on whether post weld heat treatment(PWHT) has been performed. For the PWHT condition, the residual stress used in our assessmentequals either 15% or 30% of the weld metal room temperature yield strength. Since low temperaturevessels are generally subject to PWHT, the residual stress in this example is taken as 15% Sy and isclassified as a secondary bending stress. It is not practical in a global assessment to adequatelyevaluate the effect of local stresses at nozzles and attachments. This can be partly achieved by theapplication of a stress multiplier. However, depending on the multiplier selected an on the estimatedmagnitude of these local stresses, supplementary detailed analyses at these local areas may berequired. However, alternatives can be considered such as intense NDE at all local stressconcentrations. In addition, it is not a forgone conclusion that these areas are any more vulnerable tofracture than the shell, considering proper reinforcement, the random orientation and positioning offlaws relative to maximum stress, and the beneficial effects of shakedown or warm prestressing duringhydrostatic test. In this study, a nominal stress multiplier has been applied to all primary stresses.

Thermal transient stresses are of particular interest since their magnitude and effect are not obvious.This is particularly important since one of the fundamental objections to grandfathering is the cyclicaspect of the excursion. These may be evaluated by a variety of closed form solutions or finite element


3-35

studies. In this example an excursion model consisting of a piston of cold liquid at constanttemperature defined by the specific contingency, is assumed to move down the tower, cooling thevessel from its pre-excursion steady state temperature to the cold liquid temperature. Heat transferstarts instantly and is made via a film coefficient. No heat flows from the atmosphere. Finite elementstudies provide a description of the stress versus time history in Figure 3.2E, and confirm that themaximum transient stress can be readily evaluated from the equation also shown in Figure 3.2E.

45

40

35

30

25

20

15

10

5

00 20 40 60 80 100 120 140 160 180 200

Figure 3.2EEvaluation Of Transient Thermal Stress

Transient Thermal Stress - -35 oC to -100 oC with h=1135 W/m 2-oC

Time (seconds)

Str

ess

(N/m

m2 )

Notes:

1. The equation for the transient thermal stress is:

( )σ

α

β βν

=+ −

−

−

E T∆

15325

0 516

1..

. exp

where,

E = Modulus of Elasticity, N/mm2�∆T = Temperature difference; the difference between the steady state wall

temperature before the excursion and the temperature of the fluid causingthe excursion, oC,

α = Thermal expansion coefficient, 1/oC,

β = Biot Modulus (see Note 2),

ν = Poison’s ratio,

σ = Thermal stress N/mm2.


3-36

2. The Biot Modulus is given by the following equation:

β =hL

k

where,

h = ILOP�FRHIILFLHQW��:�P��R&�k = thermal conductivity of the shell material,�:�P�R&�L = shell wall thickness, m.

The finite element study defines the controlling stress as a through thickness bending stress withtension on the inside surface. The resultant stress is considered to be a primary stress and for furtherconservatism in this example, it is broken down into equal membrane and bending components. Thetransient thermal stress has a relatively small magnitude. -100°C (-148°F) liquid flowing over a -35°C (-

31°F) shell gives rise to a maximum stress of 41 N/mm2 (6 Ksi), which, if treated as a cyclic stress, isnegligible with respect to fatigue crack growth.

The final treatment of stresses involves the addition of a "safety factor" in the form of a stress multiplierto all primary components. In this example a factor of 1.3 has been used. This provides allowance forpotential variations in thickness and minor structural discontinuities, such as tray support rings. Thebasis of 1.3 lies in the assumption that the effect of a local stress concentration factor (e.g. at a weldtoe) will rapidly decrease from a high value at the surface to a much lower value at a crack tip which islocated below the surface. Therefore, the influence of a surface residual stress on a crack tip ofreasonable depth will be greatly diminished. This principle is generally supported by the treatment ofcracks at the toe of fillet welds in a Level 3 Assessment in Section 9. Applied stresses are summarizedin Table 3-6.1E.

Table 3.1EInspection Summary Required For The Assessment Of Local Metal Loss

Source Of Stress Stress Classification

Pressure Stress = 153 M/mm2

( )P N mmm = +

⋅ =153

20

213 211 2. /

Residual Stress = 67 M/mm2

( )P N mmb =

⋅ =

20

213 13 2. /

Transient Stress = 153 M/mm2Q N mmm = 67 2/

Notes:1. A stress concentration factor of 1.3 is used in the analysis.2. The thermal transient is based on 72o C liquid on a 35o C shell.

Toughness

Since definitive toughness data on process equipment is normally only available in the form of Charpyenergy at one temperature, it is necessary to adopt a lower bound approach to describe the variation oftoughness with temperature. The most widely used data is the KIR curve from Figure F.3 in AppendixF, shown in Figure 3.3E. To use this curve it is necessary to estimate the Nil Ductility Temperature(NDT) in order to position the temperature axis on an absolute scale. For this assessment we selected40 Joules (30 Ft-lbs.) as the NDT. Charpy V energy is obtained from a longitudinal specimen. This hasbeen adopted as the basis in this example. It should be noted that Appendix F recommends the less


3-37

conservative value of 20 J (15 Ft-lbs). Where an impact temperature corresponding to 40 J is notavailable, actual values are extrapolated to give an effective 40 J test temperature using therelationship 1.5 J/°C (0.6 Ft-lbs./°F). For this assessment the lowest average Charpy value was usedfor determining the NDT, as opposed to the lowest minimum. The use of actual values is illustrated inFigure 3.3E.

-89 -67 -44 -22 0 22 44 67 89 111

-160 -120 -80 -40 0 40 80 120 160 200

220

200

180

160

140

120

100

80

60

40

20

KIR

(ks

i {nc

hes}

0.5 )

Temperature (oC)

Temperature (oF)

Shabbiis (WCAP - 1623)

Ripling and Crosley HSST, 5thAnnaula Information Meeting,1971, Paper No. 9

Unpublished Data

MRL Arrest Data 1972 HSSTInfo MIG

Figure 3.3EToughness Evaluation Using The KIR Curve

Notes:1. Actual Charpy data: 33/32 Joules at -46 oC2. Equivalent temperature at 40 Joules from: -46 oC + (40 oC - 33 oC)/1.5 = -41 oC; therefore,

NDT (0) in this figure, indexes to -41 oC.

Mechanical Properties

Actual material properties obtained from equipment records should be used for yield strength andCharpy energy. Other properties are readily available from sources such as ASME Section 2; however,a correction function can be adopted to increase the value of yield strength at low temperature. Whilethis was used in the example its effect is primarily a higher plastic collapse limit, which is not a typicallimiting factor for low temperature brittle fracture.

Fracture Tolerance Signature (FTS)

The stress and toughness parameters described above are solved for a series of temperatures andexpressed as a plot of limiting flaw size versus temperature as illustrated in Figure 3.4E. The criticalflaw depth is in the through thickness dimension, and is expressed as a percentage of all thickness witha 6:1 aspect ratio maintained. The absolute factor of safety in the critical flaw size is undetermined, butis a function of the assumptions made with respect to lower bound toughness, stress, stress multiplier,and the NDT indexing temperature.


3-38

Figure 3.4E illustrates the influence of the transient operation on the limiting flaw size. Line segment A-B represents steady operation, and defines the limiting flaw for gradual cool down to -36°C (-33°F)where the limiting flaw is 25% of the wall thickness. The exposure to cold liquid at -72°C (-100°F),begins at B and results in an almost instantaneous drop in limiting flaw size to 21% of the wall thicknessat C. This occurs as a result of the applied thermal stress. The initial impact of the thermal transientdecreases as the shell cools, decreasing the temperature difference between the shell and the coldliquid. During this period the toughness falls, but the thermal stress reduces, with the net result that thelimiting flaw size reduces to 17% of the wall thickness at D. At this point the metal temperature reachesequilibrium with the cold liquid, and over line segment D-E a return to steady state cool down continues.The limiting flaw size is 12% of the wall thickness at E, the minimum temperature reached.

A

B

CD

E

-140 -120 -100 -80 -60 -40 -20 0 20

100

90

80

70

60

50

40

30

20

10

0

Cra

ck D

epth

Per

cent

For A Design Pressure of 37.2 Bar-g &Temperature Excursion of -36 oC to -72 oC

Temperature (oC)

Crack Depth = 16%

Figure 3.4EFracture Tolerance Signature

The shape of the FTS curve in Figure 3.4E follows that of the KIR curve, and is modified only theimpact of the transient effect. More or less conservative stress/flaw assumptions will lower or raise thecurve vertically, and assuming a lower NDT will move the curve horizontally to the left. For this reasonthe curve provides useful insight into brittle fracture resistance during an excursion. However, theflatness of the curve makes limiting temperature predictions highly sensitive to the minimum flaw size.This in turn is greatly influenced by type and extent of inspection, and factors such as probability ofdetection (POD) of flaws. While work still needs to be done to clarify POD issues, application ofdetailed NDE to a vessel should enable a minimum flaw size to be assumed with sufficient confidenceto enable the FTS to be used to specify a minimum excursion temperature. Figure 3.5E suggests aflaw depth of 4.5 mm (0.177 in) should be detectable using a suitable magnetic particle technique (MT),with a confidence level greater than 90%. For the 6:1 aspect ratio assumed in developing the FTS, thisimplies a crack of length 27 mm (1.063 in).


3-39

1

0.8

0.6

0.4

0.2

00 2 4 6 8 10 12 14

UT - Nordtest++ +

+

+

AE + UT MPI UT - NordtestUT20 +

Defect Size - Crack Depth (mm)

PO

D -

Pro

babi

lity

Of D

etec

tion

++

+

Inspection Method

Figrue 3.5EComparison Of Inspection Methods - Probability Of Detection Curves

Specific Example

Figure 3.6E summarizes the evaluation of a potential thermal excursion for the demethanizer towerillustrated in Figure 3.1E. Table 3-6.1 list the stresses and other factors assumed in conducting theevaluation. An important aspect of the required data is a realistic estimate of the critical exposuretemperature (CET). This is the measured metal temperature, or more likely the vessel temperaturepredicted by process simulation programs during an excursion. The excursion temperature in theexample illustrates that an MDMT violation will not occur in the 3.5% Ni section above tray 33. Hencethe evaluation need only consider the lower carbon steel section.


3-40

++

+++

++

20

10

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

-100

-11062 57 52 43 40 33 32 29 28 25 24 18 6 1

Excursion Limit

Normal operation

Tray Numebr

Tem

pera

ture

(o C

)

ExcursionTemperature

MDMT

-66 oC

-80 oC

-101 oC

Coldest KCSTemperature = -72oC Largest Excursion

Temperature = -49 oC

Figure 3.6EDemethanizer MDMT verse Location

The excursion temperature plotted in Figure 3.6E defines two cases to be considered. 1) The lowesttemperature in the carbon steel section is at tray 32 and a pre-excursion temperature of -35°C (-33°F)and an excursion delta of 37°C (67°F) to -72°C (-100°F). 2) The largest delta of 49°C (88°F) occursfrom a steady state temperature of -12°C (+10°F) at tray 24 to give an excursion temperature of -61°C(-78°F).

To illustrate the influence of inspection on the results, it is assumed that the tower has been 100%visually inspected internally. In addition, it is assumed that all internal weld seams are inspected by wetfluorescent magnetic particle methods, and angle probe ultrasonics, from the bitmetallic weld to acircumferential weld between trays 24 and 25. It is assumed that nay flaw indications would beremoved by light grinding. As part of such an assessment it would also be reasonable to conduct ahydrostatic test at 150% of design pressure. These assumptions allow the carbon steel section to beevaluated by two approaches. 1) The visually inspected region can be assessed using basic MDMTprinciples in accordance with the "code compliant approach." 2) The MT/UT inspected region can beassessed using the more sophisticated FTS approach.

Figure 3.7E illustrated the MDMT approach for two constant flaw sizes. One is 6.2 mm deep, (22% ofthe wall thickness), and was selected to pass through original design conditions. For clarity, the effectof the transient stress is ignored in Figure 3.7E. The 22% curve illustrates that the excursiontemperature at tray 24 of -61°C (-78°F) is within the acceptable MDMT zone and, provided thatadditional transient stresses can be accommodated within the excursion margin, the MDMT can beset at -66°C (-87°F) based on operating rather than design pressure. This check is made by evaluatingthe critical flaw size during the excursion, using an FTS for tray 24, and ensuring it is always above22%. The check is made using tray 24 temperature and excursion conditions, with operating pressureapplied rather than design. The check confirms that in this case -66°C (-87°F) is an acceptableexcursion limit below tray 24.


3-41

45

40

35

30

25

32

20

15-140 -120 -100 -80 -60 -40 -36 -20

Required Excursion Limit- Tray 32 = -72 oC

16% Defect (4.5mm)

22% Defect (6.2mm)Pre

ssur

e (b

ar-g

)

Temperature (oC)

Normal Operation

Required Excursion Limit - Tray 24 = -61oC

Potential Margin For Region

Inspected By MT

Excursion Margin -Tray 5 & Below = 5oC

MDMT asDefined by the

Impact TestTemperature

Figure 3.7EPressure Temperature Relationship for Contsant Defect Size - Killed Carbon Steel Section

The second feature apparent from the 22% curve is that a violation still exists at tray 32. Tray 32 ishowever, located in the section of the tower that was subject to MT/UT inspection. Thus it can beassessed on the basis of a smaller flaw size. The 4.5 mm (16% of the wall thickness) curve in Figure3.7E represents this criterion as proposed earlier. It is clear that the -72°C (-168°F) excursion isaccommodated, even at design pressure.

The FTS curve, (Figure 3.4E), suggests that a 4.5 mm limiting flaw is critical below -80°C (-112°F)when analyzed at full design pressure. In practice the contingency is unlikely to violate designconditions, hence there is an in-built conservatism over the more realistic operating case. An FTS forthe operating case results in -111°C (-168°F) as the limiting temperature.

To be of value to operating personnel, and to compare it with the excursion temperature, it is useful toexpress the result in the form of an excursion limit for the tower, as shown in Figure 3.6E. This allows adirect comparison of normal operation, excursion temperature, MDMT and excursion limits. Thisdistinction between the MDMT and the excursion limits is to differentiate between the "code compliant"and non code compliant aspects of the assessment. The purpose of the analysis is to establishreasonable excursion limits and to quantify the risk associated with excursions below the MDMT. It isnot meant to encourage normal operation at temperatures lower than the MDMT, as might be impliedby redefining the MDMT to below code compliant limits.

In practice, alarms and operating procedures would be put in place to minimize the likelihood ofreaching the excursion limits, and process modification such as moving cold feeds higher up the towercould be considered. However alarms and procedures themselves have a probability of failing.

Recommendations and Conclusions

For this particular type of Level 3 assessment only, equipment to be evaluated should meet thefollowing criteria:

1. Meets the design and fabrication requirements of a recognized code of construction.

2. Demonstrates, by measured values, minimum toughness of weld, HAZ and plate materials.


3-42

3. Was inspected 100% radiographic standards or volumetric equivalent.

Where a Level 3 assessment is made, its acceptability should be subjected to suitable criteria such asthe following:

1. Where no additional detailed inspection for surface breaking flaw is performed by an appropriateNDE technique, the excursion limits should be no lower than the MDMT as developed by a "codecompliant" approach.

2. Where MT or equivalent is carried out around nozzles and attachments, the MDMT may bebased on a 25% or 6 mm (0.25 in) deep flaw, whichever is the lower.

3. Where an appropriate NDE technique is used to preclude the existence of flaws with sufficientconfidence, (typically 100% MT), the excursion limit can be based on the FTS using a minimumflaw size of 4.5 mm (0.177 in).

4. The assessment is only valid if the service conditions in the vessel are essentially unchanged orless severe than those experienced in the past.

5. Poor operation in terms of control techniques leading to frequent cycling, should be discouragedby limiting the number of excursions allowed during the life of the vessel.

6. Hydrostatic testing at a temperature sufficiently high to ensure material toughness is above thelower shelf is recommended, preferably in combination with acoustic emission monitoring.

This is an example of a Level 3 Assessment. It is not intended to be a "prototype" for all Level 3assessments, since there are many different approaches which can be used successfully at this level.

chap 03

Documents

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controlling design requirement

acoustic emission testing

american petroleum institute

nominal wall thickness

impact test results

qualified technical specialist

successful performance demonstrated