Section 10 - Assessment of Pitting Metal Loss

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

Pitting Metal LossPitting Metal Loss


Primary published procedure:Primary published procedure:

API Section 6:API Section 6:

The theory is based on pit coupling andThe theory is based on pit coupling and

limit load analysislimit load analysis

Main challenge is the quality of theMain challenge is the quality of the

inspection datainspection data

Pitting Metal Loss


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d d d

w w w

P



Four pitting cases can be assessed:Four pitting cases can be assessed:

Widespread pittingWidespread pitting

Localised pittingLocalised pitting

Locally thinned area in widespreadLocally thinned area in widespread

pittingpitting

Pitting within a locally thinned areaPitting within a locally thinned area


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Widespread pitting:Widespread pitting:

Assessments based on representativeAssessments based on representativesamples of pittingsamples of pitting

For example:For example:

Uniform 10 to 20 sample pit couplesUniform 10 to 20 sample pit couples

NonNon--uniform greater than 50uniform greater than 50

samplessamples



Localised pitting:Localised pitting:

Assessments based on equivalent locallyAssessments based on equivalent locally

thinned area (then use Section 5)thinned area (then use Section 5)

Pitting within a LTA & LTA withinPitting within a LTA & LTA within

widespread pittingwidespread pitting

Assessment based on combined RSF forAssessment based on combined RSF for

locally thinned area and pittinglocally thinned area and pitting


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Level 1 assessment:Level 1 assessment:

RSF calculated on pitRSF calculated on pit--couplecouple


RSF calculated on pitRSF calculated on pit--couple withcouple with

respect to stress orientationrespect to stress orientation




Finite element analysis of individualFinite element analysis of individual

pitspits

Finite element analysis of effectiveFinite element analysis of effective

material properties e.g. elasticmaterial properties e.g. elastic

modulusmodulus


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Possible futurePossible future

assessmentassessment

procedure to useprocedure to usecomparator chartscomparator charts

of pit densityof pit density


Metal lossMetal loss

Remediation

& Remaining life

Remediation

& Remaining life


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Remediation & Remaining lifeRemediation & Remaining life

Remediation actions:Remediation actions:

Weld repairWeld repair

Coatings and lining systemsCoatings and lining systems

Change operating environmentChange operating environment

ReRe--rate equipmentrate equipment

Inhibitors, etcInhibitors, etc


Remediation & Remaining lifeRemediation & Remaining life

Remaining life:Remaining life:

Thickness approach based on metalThickness approach based on metal

loss rate with timeloss rate with time Maximum allowable workingMaximum allowable workingpressure (MAWP) approachpressure (MAWP) approach

Use depends on design equationUse depends on design equationrelates pressure to wall thicknessrelates pressure to wall thickness

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Section 10 - Assessment of Pitting Metal Loss