section 9 - assessment of locally thinned areas
TRANSCRIPT
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Copyright 2005, TWI Ltd World Centre for Materials Joining TechnologyWorld Centre for Materials Joining Technology
Assessment of LocallyThinned Areas
Assessment of LocallyThinned Areas
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Locally Thinned AreasLocally Thinned Areas
Many procedures:Many procedures:
API 579 Sections 4 and 5API 579 Sections 4 and 5
BS 7910 Appendix GBS 7910 Appendix G
ASME B31GASME B31G Numerous papersNumerous papers
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Locally Thinned AreasLocally Thinned Areas
Apart from API 579 Section 4 (thicknessApart from API 579 Section 4 (thickness
averaging), the theory behind theaveraging), the theory behind the
different methodology is usually based ondifferent methodology is usually based onan effective area method.an effective area method.
Where metal loss is treated as a partWhere metal loss is treated as a part
through flaw and evaluated using athrough flaw and evaluated using aFoliasFolias bulging stress magnification bulging stress magnification
factorfactor
Methodologies are validated against aMethodologies are validated against alarge burst test databaselarge burst test database
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Locally Thinned AreasLocally Thinned Areas
Most procedures require theMost procedures require thedetermination of critical thickness profilesdetermination of critical thickness profiles
(CTP)(CTP) CTP based on river bottom of theCTP based on river bottom of theminimum thickness readings in bothminimum thickness readings in both
circumferential and longitudinalcircumferential and longitudinaldirectionsdirections
Average and minimum thickness can beAverage and minimum thickness can bedetermineddetermined
Recommendations in API RP 579 toRecommendations in API RP 579 toderive inspection grid spacingderive inspection grid spacing
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Locally Thinned AreasLocally Thinned Areas
Line M critical thickness profile from inspectionLine M critical thickness profile from inspection
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Locally Thinned Areas section 4Locally Thinned Areas section 4
Section 4 basic procedureSection 4 basic procedure
Compute remainingCompute remainingthickness ratio (thickness ratio (RRtt))
Determines the lengthDetermines the lengthof thickness averagingof thickness averaging
(L) from equation given(L) from equation givenin standardin standard
Determine s and c , theDetermine s and c , thedimensions of metaldimensions of metal
loss in the longitudinalloss in the longitudinaland circumferentialand circumferentialdirectionsdirections
ttmmmm = minimum measured thickness= minimum measured thickness
D = Inside diameter of the cylinder or otherD = Inside diameter of the cylinder or othersection (see API 579)section (see API 579)
Q = Factor from Table 4.4 based on anQ = Factor from Table 4.4 based on an RSFRSFaaand the remaining thickness ratio,and the remaining thickness ratio, RRtt
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Locally Thinned Areas section 4Locally Thinned Areas section 4
S
C
S
C
Example of s and c determinationExample of s and c determination
tmin
Bold line (x) circumferential CTPBold line (x) circumferential CTP
Dashed line (o) longitudinal CTPDashed line (o) longitudinal CTP
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Locally Thinned Areas section 4Locally Thinned Areas section 4
If s < L then metal loss is acceptableIf s < L then metal loss is acceptable
If s > L then:If s > L then: Assume average thickness equal toAssume average thickness equal to
measured minimum thicknessmeasured minimum thickness
Direct averaging of the thickness readingsDirect averaging of the thickness readingsthat reside within length L.that reside within length L.
Area averaging method of the thicknessArea averaging method of the thicknessreadings that reside within length L.readings that reside within length L.
If minimum required thickness (If minimum required thickness (ttminmin) to) to
code calculation is less than or equal tocode calculation is less than or equal toaverage thickness (taverage thickness (tamam) minus the FCA) minus the FCAthen the metal loss is acceptablethen the metal loss is acceptable
Refinem
ent Si
mplicity
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Locally Thinned Areas section 4Locally Thinned Areas section 4
Critical thickness profile from inspectionCritical thickness profile from inspection
Assuming the minimum measuredAssuming the minimum measuredthicknessthickness
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Locally Thinned Areas section 4Locally Thinned Areas section 4
Average thickness in a set lengthAverage thickness in a set length
ttminmin== thickness points within Lthickness points within L
Critical thickness profile from inspectionCritical thickness profile from inspection
Set length fromSet length from
procedure (L)procedure (L)
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Locally Thinned Areas section 4Locally Thinned Areas section 4
A1
A2
Area averaging method in LArea averaging method in L
ttminmin= (A= (A11 + A+ A22) / L) / L
Critical thickness profile from inspectionCritical thickness profile from inspection
Set length fromSet length from
procedure (L)procedure (L)
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Section 4 metal loss - exampleSection 4 metal loss - example
Corrosion on a cylindrical section of pressureCorrosion on a cylindrical section of pressure
vessel has been found during an inspection.vessel has been found during an inspection.
The vessel was designed and constructed toThe vessel was designed and constructed tothe ASME B&P VIII division I. Evaluate if thethe ASME B&P VIII division I. Evaluate if the
pipe vessel is fitpipe vessel is fit--forfor--service.service.
Design Conditions = 300 psig @ 350Design Conditions = 300 psig @ 350ooFF
Inside Diameter = 48 inchesInside Diameter = 48 inches
Nominal Thickness = 0.75 inchesNominal Thickness = 0.75 inches
Future Corrosion Allowance = 0.10 inchesFuture Corrosion Allowance = 0.10 inches
Uniform metal loss = 0 inchesUniform metal loss = 0 inches
Material = SA 516 Grade 70 (S = 17500 psi @Material = SA 516 Grade 70 (S = 17500 psi @
temperature)temperature)
Weld Joint Efficiency = 0.85Weld Joint Efficiency = 0.85
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Section 4 metal loss - exampleSection 4 metal loss - example
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Section 4 metal loss - exampleSection 4 metal loss - example
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Locally Thinned Areas section 5Locally Thinned Areas section 5
API 579 Section 5 MethodAPI 579 Section 5 Method
Level 1Level 1 Screening curves longitudinalScreening curves longitudinal
extent of a flawextent of a flaw
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Locally Thinned Areas section 5Locally Thinned Areas section 5
API 579 Section 5 MethodAPI 579 Section 5 Method
Level 1Level 1 Screening curve circumferentialScreening curve circumferential
extent of a flawextent of a flaw
c = Circumferential dimension ofc = Circumferential dimension of
the metal lossthe metal loss
D = Inside diameter of theD = Inside diameter of the
cylinder or other section (see APIcylinder or other section (see API
579)579)
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Locally Thinned Areas section 5Locally Thinned Areas section 5
Assessment includes groove like flawsAssessment includes groove like flaws
(not shown refer to API 579)(not shown refer to API 579)
Screening curves for bothScreening curves for both
circumferential and longitudinalcircumferential and longitudinal
directionsdirections
Level 2 similar to level 1 but provide aLevel 2 similar to level 1 but provide a
better estimate of the RSFbetter estimate of the RSF
Includes variation for thickness profileIncludes variation for thickness profile
to find RSF (based on the RSTRENGto find RSF (based on the RSTRENGslicing methodology)slicing methodology)
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Locally Thinned Areas section 5Locally Thinned Areas section 5
AAii = area of metal loss for increment length= area of metal loss for increment length
AAiioo = Original area for increment length (= Original area for increment length (ttminminxx SSii))
RSF calculated for each incremental lengthRSF calculated for each incremental length
Variation for thickness profile to find RSFVariation for thickness profile to find RSF
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Locally Thinned Areas section 5Locally Thinned Areas section 5
ttmin
min
SS11
R
SF
R
SF
incrementincrement
SS11
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Locally Thinned Areas section 5Locally Thinned Areas section 5
ttmin
min
SS22
R
SF
R
SF
incrementincrement
SS11
SS22
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Locally Thinned Areas section 5Locally Thinned Areas section 5
ttmin
min
SSii+n+n
R
SF
R
SF
incrementincrement
SS11
SSii+n+n
Minimum RSFMinimum RSF
for assessmentfor assessment
SS22
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Locally Thinned Areas section 5Locally Thinned Areas section 5
Section 5 Level 3Section 5 Level 3
Finite element analysis either elasticFinite element analysis either elasticor elasticor elastic--plasticplastic
No limitations e.g. can includeNo limitations e.g. can include
fatiguefatigue
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Locally Thinned Areas section 5Locally Thinned Areas section 5
A region of localized corrosion has beenA region of localized corrosion has been
found on a pressure vessel during afound on a pressure vessel during a
scheduled turnaround. The vessel wasscheduled turnaround. The vessel wasdesigned and constructed to the ASMEdesigned and constructed to the ASME
B&PV Code, Section VIII, Division 1.B&PV Code, Section VIII, Division 1.
Determine if the vessel is acceptable forDetermine if the vessel is acceptable forthe current MAWP using a Section 5the current MAWP using a Section 5
Level 1 Assessment.Level 1 Assessment.
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Locally Thinned Areas section 5Locally Thinned Areas section 5
Vessel DataVessel Data
Design Conditions = 300 psi @ 650Design Conditions = 300 psi @ 650ooFF
Inside Diameter = 96 inchesInside Diameter = 96 inches
Fabricated Thickness = 1.25 inchesFabricated Thickness = 1.25 inches
Uniform Metal Loss = 0.10 inchesUniform Metal Loss = 0.10 inches
FCA = 0.125 inchesFCA = 0.125 inches Material = SA 516 Grade 70 (S= 17500 psi)Material = SA 516 Grade 70 (S= 17500 psi)
Weld Joint Efficiency = 0.85Weld Joint Efficiency = 0.85
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Locally Thinned Areas section 5Locally Thinned Areas section 5
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Locally Thinned Areas section 5Locally Thinned Areas section 5