section 8 - assessment of metal loss - general

8/13/2019 Section 8 - Assessment of Metal Loss - General

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Copyright 2005, TWI Ltd World Centre for Materials Joining TechnologyWorld Centre for Materials Joining Technology

Assessment of Metal LossAssessment of Metal Loss

GeneralGeneral


IntroductionIntroduction

Some important conceptsSome important concepts

General (uniform) metal lossGeneral (uniform) metal loss

Locally thinned areas (LTAs)Locally thinned areas (LTAs)

Pitting metal lossPitting metal loss

Remediation & remaining lifeRemediation & remaining life


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Remaining strength factorRemaining strength factor

The RSF (remaining strength factor) is animportant concept in metal loss assessments.

RSF =

Plastic collapse load of adamage component

Plastic collapse load of aundamaged component

The RSF is not the same as a factor of safety


Allowable RSFAllowable RSF

The RSFa (allowable remaining strength factor)is a function of the allowable stress and designcriteria of the construction code

Lower allowable stresses and conservativedesign rules result in a lower RSFa.

Recommended RSFa is 0.9 (for ASME B&PVpressure equipment in API RP 579)

A RSFa 0.9 implies that a component containinga flaw can be no less that 90% of the strength inthe as-built condition


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Future Corrosion AllowanceFuture Corrosion Allowance

FCA = future corrosion allowanceFCA = future corrosion allowance

As the name states this variable is theAs the name states this variable is theallowance for further metal loss. This isallowance for further metal loss. This is

usually set by examining inspectionusually set by examining inspection

records and determining the metal lossrecords and determining the metal loss

expected over proposed future lifeexpected over proposed future life

expectancyexpectancy

Care should be taken as not all corrosionCare should be taken as not all corrosionrates are linearrates are linear


Thickness informationThickness information

Generally two different thicknessGenerally two different thickness

measurement results:measurement results:

Point readings, manual ultrasonics with aPoint readings, manual ultrasonics with a

matrix of pointsmatrix of points Detailed manual or auto/scannedDetailed manual or auto/scanned

ultrasonicsultrasonics


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Metal loss assessmentMetal loss assessment

Most assessment proceduresMost assessment procedures

assume smooth metal lossassume smooth metal loss



Most assessment procedures are vagueMost assessment procedures are vague

or do not cover metal loss at weldsor do not cover metal loss at welds

Lack of side wall fusion

May have been safe as anembedded flaw.

If metal loss exposes it tothe surface:

Now a surface breakingflaw Stress at crack now higherdue to reduction in area


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Thus, level 1 and 2 procedures have theThus, level 1 and 2 procedures have the

following applicability:following applicability:

Equipment is designed and constructedEquipment is designed and constructed

to a recognised standardto a recognised standard

No creep, fatigue or crackNo creep, fatigue or crack--like flawslike flaws

Metal loss has smooth contoursMetal loss has smooth contours

ClosedClosed--form design equation whichform design equation which

relates pressure to thicknessrelates pressure to thickness


General (Uniform) Metal LossGeneral (Uniform) Metal Loss


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General metal lossGeneral metal loss

The general metal loss procedure is in APIThe general metal loss procedure is in API

579 Section 4579 Section 4

The procedure is based on a thicknessThe procedure is based on a thickness

averaging approach with code rules toaveraging approach with code rules to

determine acceptabilitydetermine acceptability

The code rules are for ASME and APIThe code rules are for ASME and API

equipmentequipment



For the thickness readings the coefficientFor the thickness readings the coefficient

of variation is calculated (COV)of variation is calculated (COV)

Averagethickness

Averagethickness

Standard deviationStandard deviation

COV =COV =Standard deviationStandard deviation

Average thicknessAverage thickness

FrequencyFrequency

Depth

ofreading

Depth

ofreading


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If the COV is less than 10% then it is classed asIf the COV is less than 10% then it is classed as

uniform metal loss, else its nonuniform metal loss, else its non--uniform loss anduniform loss and

the thickness averaging approach (section 4) orthe thickness averaging approach (section 4) or

section 5 local metal loss or a level 3 analysissection 5 local metal loss or a level 3 analysisshould be used.should be used.

level 1 assessment:level 1 assessment:

If minimum required thickness (tIf minimum required thickness (tminmin) to code) to code

calculation is less than or equal to averagecalculation is less than or equal to average

thickness (tthickness (tamam) minus the FCA then the metal loss) minus the FCA then the metal loss

is acceptableis acceptable



Level 2 assessment:Level 2 assessment:

Similar to level 1 except tSimilar to level 1 except tminmin is adjustedis adjusted

by using theby using the RSFRSFaa (0.9)(0.9)

Limits of applicability:Limits of applicability:

0.5t0.5tminmin or 0.1 inches (2.5mm)or 0.1 inches (2.5mm)

Not close to a structural discontinuityNot close to a structural discontinuity


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Level 3 assessment:Level 3 assessment:

Finite element analysis either elastic orFinite element analysis either elastic orelasticelastic--plasticplastic

No limitations e.g. can include fatigueNo limitations e.g. can include fatigue


General metal loss - exampleGeneral 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 to

the ASME B&P VIII division I. Evaluate if thethe ASME B&P VIII division I. Evaluate if the

vessel is fitvessel 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

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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General metal loss - exampleGeneral metal loss - example

Units in inches standard deviation is 0.0144

O'clock

0 10 20 30 40 50 60 70

0 0.559 0.555 0.576 0.551 0.552 0.547 0.552 0.578

1.5 0.589 0.584 0.560 0.581 0.549 0.545 0.551 0.574

3 0.568 0.547 0.568 0.573 0.582 0.564 0.555 0.547

4.5 0.570 0.586 0.583 0.580 0.560 0.587 0.547 0.548

6 0.586 0.584 0.561 0.572 0.562 0.567 0.590 0.569

7.5 0.591 0.558 0.554 0.579 0.577 0.570 0.546 0.587

9 0.547 0.590 0.555 0.583 0.566 0.570 0.555 0.565

10.5 0.570 0.560 0.555 0.562 0.570 0.568 0.547 0.587

Distance along shell in inches

section 8 - assessment of metal loss - general

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