Chap 6 - Corrosion Monitoring

7/21/2019 Chap 6 - Corrosion Monitoring

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Corrosion Monitoring

The UltraPIPE corrosion monitoring application helps you determine the condition ofcorrosion prone equipment such as piping, vessels, boilers, and tankage. Periodic wall thicknessreadings (usually obtained from an ultrasonic test instrument) along with the calculatedretirement thickness (T-MIN) at predetermined locations are stored and analyzed by this

application. In addition, special analytical settings can be applied to zero corrosion allowanceequipment that is monitored for regulatory reasons.

HELPFUL HINT

Zero corrosion allowance equipment is built to required thicknesswith no anticipated corrosion problems. Analytical Settings areavailable for this type of equipment that must be monitored forthickness and scheduled for future inspections. This logicautomatically excludes the meaningless life criteria recommendedfor equipment with a zero design corrosion allowance.

The analytical results include corrosion rates, recommended inspection dates and predicted

retirement dates for both individual TMLs and the Eq/Circ ID. UltraPIPE is versatile in that itprovides you with a wide range of available analysis options to conform to governmentregulations and/or your own in-house engineering standards.

HELPFUL HINT

The thickness inspection frequencies of API 510 - Pressure VesselInspection Code, API 570 - Piping Inspection Codeand API 653 -Tank Inspection can be met exactly using the flexible analysiscapabilities of the UltraPIPE Corrosion Monitoring Module.

The objective of the corrosion monitoring application is to incorporate user selected methods todetermine the condition of both individual TMLs as well as the complete Eq/Circ ID.

This chapter explains the analytical logic behind the corrosion monitoring application. Inaddition, it guides you through the following corrosion monitoring functions:

1. Adding/Modifying an Eq/Circ ID and TMLs

2. Adding/Modifying a Survey

Chapter


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Figure 6 - 1 TML A nalysis Calculations Dialog Box

As you are using the Corrosion Monitoring application, you will frequently encounter the dialogbox shown in Figure 6 - 1. The most likely places you will see this box are as you leave theAdd/ Edit/ Delete Eq/ Circ ID and TMLswindow, as you leave theAdd/ Edit/ DeleteSurveyswindow, and before you create a report. This box indicates that UltraPIPE isperforming the analysis calculations discussed below. It is important to keep the information in

your database current but it is not always necessary to perform the corrosion monitoringcalculations as soon as you change the data for a piece of equipment. If you do not want toperform the calculations now, you may select theCancelbutton. The calculations can beperformed at a later time. You dont even have to remember whether or not you canceled thecalculations -- UltraPIPE knows whether the information in the database is up to date and willautomatically analyze required data before a report is displayed or printed.

HELPFUL HINT

Only use the Cancel button on the TML Analysis Calculation dialogbox when you dont have time to wait for the calculations tocomplete.

All of the database fields that are calculated in the Corrosion Monitoring application short termrate, long term rate, predicted retirement date, next inspection date, etc. can be viewed on anumber of standard corrosion monitoring reports: Eq/Circ ID Analysis Report, Eq/Circ IDSummary Report, among others. Refer toChapter 18on UltraPIPE Reports for moreinformation on available reports and their contents.

Please keep in mind as you read the following sections that you have control over whichcalculations to perform, and in many cases, how the calculations are performed. There are manyuser-defined limits that you can set in each formula. This is accomplished through theAnalyti cal Settings Standard,Analytical Settings Advanced,Analytical Settings 0C.A., andAnalyti cal Sett ings Structural T-M inwindows on theSession Settingspull-

down located on theSetup UltraPIPEmenu (Refer toChapter 4on Adjusting Your SessionSettings).

HELPFUL HINT

You can control the corrosion monitoring calculations through theAnalytical Settings Standard, Analytical Settings Advanced,Analytical Settings 0 C.A. and Analytical Settings Structural T-Minscreens.


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As mentioned above, the corrosion monitoring application performs a number of calculations todetermine corrosion rates, predicted retirement dates and recommended inspection due dates forboth individual TMLs and the complete Eq/Circ ID as well as the Group Name. Thecalculations are performed in a logical order and each calculation uses information from theprevious one. For example, the T-Mins for each TML are calculated first. The remaining life foreach TML, which is calculated later, uses the data from the TML T-Min calculations. Each ofthese calculations is discussed below in the order in which they are performed.

TML T-Min and MAOP Calculations

The corrosion monitoring calculations begin by calculating the thickness minimum (T-Min) andmaximum allowable operating pressure (MAOP) for each TML. The T-Min is based on thegreater of internal pressure formulas from various design codes and the user-specified structuralrequirements. User keyed-in T-Min values, which are designated by a (U) above the T-Min valueon theAdd/ Edit/ Delete Eq/ Circ ID and TM Lswindow, are ignored in both the T-Min andMAOP calculation. The MAOP calculation is the reverse of the T-Min calculation (i.e. it solvesfor pressure for a known thickness). The specific calculations that are performed vary depending

on the design code selected. Refer to the Component Code and Material code sections of themanual for more information on the detailed calculations that are performed for each specificdesign code.

Representative TML Corrosion Rate

The next step in the corrosion monitoring calculations is to determine the corrosion rate at eachindividual TML. Up to three calculations may be performed on each TML.

HELPFUL HINT

Remember that the Representative TML Corrosion Rates areaffected by several user controlled Analytical Settings including useof nominal thickness, minimum time between dates for a calculatedcorrosion rate, growth logic and temperature correction.

1. Long Term Corrosion Rate

If theUse Long Term Ratebox is checked on theAnalyti cal Settings - Standardscreen,the Long Term Corrosion Rate will be calculated for each TML. The Long Term CorrosionRate is based on the difference between the first and last thickness measurements taken.

FirstDateLastDate

LastThickFirstThick

meLongTermTi

ssLongTermLoerrosionRatLongTermCo

==

Two or more thickness readings must be available to calculate the Long Term CorrosionRate. If theUse Nominal Thickness for Corrosion Calculationsis checked on theAnalyti cal Settings Standardwindow, the Long Term Corrosion Rate will use thenominal thickness with the first valid field thickness survey to calculate the Long TermCorrosion Rate. A fter three valid filed thickness surveys are available, the Long Term Corrosion Rate will


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always ignore the Nominal Thickness independent of the status of the Use Nominal Thickness forCorrosion Rate option.

It is important to note that before two readings will be used for a long term corrosion rate,theM inimum t ime for a calculated corrosion ratesetting as specified on theAnalyticalSett ings Standardwindow must be satisfied.

The Long Term Corrosion Rate can also be affected by the use of either the TemperatureCorrection and/or Growth Logic. These are explained in more detail in later sections.

2. Short Term Corrosion Rate

If theUse Short T erm Ratebox is checked on theAnalyti cal Settings - Standardscreen,the Short Term Corrosion Rate will be calculated for each TML. The Short TermCorrosion Rate is based on the difference between the next-to-last (previous) and lastthickness measurements taken.

eviousDateLastDate

LastThickkeviousThicimeShortTermTossShortTermLteorrosionRaShortTermC

PrPr

==

Three or more valid thickness readings must be available at a TML to calculate the ShortTerm Corrosion Rate. If theUse Nominal Thickness for Corrosion Calculationsischecked on theAnalytical Setti ngs Standardwindow, the Short Term Corrosion Ratewill be calculated after the second field survey thickness. (For this case, the nominalthickness will be used as the First Thickness in the Long Term Corrosion Rate).

It is important to note that before three readings will be used for a Long and Short TermCorrosion Rate, theM inimum t ime for a calculated corrosion ratesetting as specified ontheAnalytical Setti ngs Standardwindow must be satisfied.

The Long Term Corrosion Rate can also be affected by the use of either the TemperatureCorrection and/or Growth Logic. These are explained in more detail in later sections.

3. Best Fit Corrosion Rate

If theUse Best Fi t Ratebox is checked on theAnalyti cal Settings - Advancedwindow,the Archive Corrosion Rate will be calculated for each TML. The Archive Corrosion Rateuses all available thickness readings and utilizes the Least Squares line fit method. Thismethod will fit a straight line through all thickness data by calculating the minimum possiblesum of the squares of the vertical distance between the data points and the straight line. The

slope of this line is used to determine the Best Fit Corrosion Rate.

BestFitRate = Slope of Least Squarebest fit straight line through all valid

thickness data

If theUse Nominal Thickness for Corrosion Calculationsis checked on theAnalyticalSetti ngs - Standardwindow, the Best Fit Corrosion Rate will use the nominal.


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After the selected calculations are complete for a TML, the largest of the calculated short term,long term, and best fit rate for that TML is selected. Next, an optional straight line SafetyFactor is applied to the largest rate if theCalculated Corrosion Rates * (User I nput)field ischecked on theAnalyti cal Setti ngs Standardwindow. The size of the safety factor is alsoentered on this line. The Safety Factor Method allows you to artificially increase the calculatedcorrosion rates and be more conservative.

A straight line default corrosion rate is specified in theDefault Corrosion Rate = (User Input)MPYfield on theAnalyti cal Settings - Standardwindow. The TML corrosion rate justcalculated is compared to the default corrosion rate, and the greater of the two becomes theRepresentative TML Corrosion Rate for that TML.

The Best Fit Corrosion Rate can be affected by the use of either the Temperature Correctionand/or Growth Logic. These are explained in more detail in the following sections.

The formula for the TML Corrosion Rate (with all Corrosion Rates used) is as follows:

]),,,([Re DefCRBFLTSTSFMaxionRatepTMLCorros =

Temperature Correction

Because the velocity of sound decreases with increased temperature, you may wish to apply acorrection factor to any readings that were taken at high temperaturesbecause the ultrasonicinstrument was calibrated at ambient temperature. You can select/deselect this option byclicking on theTemperature Compensate Readingsbox on theAnalysis Settings -Advancedwindow. Temperature correction is activated when a check mark is shown in thebox. All logic is completed during analysis, and therefore reversible.

You are able to enter the exact correction factor in theCompensate readings over (UserInput)> F by (User I nput)% per (User I nput) Fline on theAnalyti cal Settings -Advancedwindow. A common correction for the temperature effects of thicknessmeasurements with a surface temperature greater than 150 F above calibration temperature is athickness reduction of 1% per 100 F.

Please note that the actual thickness reading measurement will be shown on UltraPIPE windows,but the values shown on the Eq/Circ ID Analysis reports will show the temperature correctedthickness used in TML calculations.

HELPFUL HINT

All logic for Temperature Correction is executed during analysisand is reversible. That is the feature can be turned on or off anddata analyzed accordingly.

Temperature correction logic occurs prior to the following Growth Logic option.


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Growths

It is common in inspection programs to find a new survey reading that is thicker than theprevious reading. This is known as a growth. Growths are counterintuitive to corrosiveenvironments where it is logical to assume that there should be a downward trend in thicknessreadings. However, because of variances in the amount of couplant used, the pressure placed on

a probe and other factors, inspection errors that occur are usually because readings are too thickrather than too thin.

You can actively manage growth through theI n Cases of Growthsection of theAnalyticalSettings Advancedwindow. If theUse the new readingoption is selected, UltraPIPEcalculations will be based on the actual growth reading (i.e. you are stating that the new, higherreading is valid).

If you feel the new, higher reading is in error, select theRevert to previous readingoption andUltraPIPE calculations will be based on the most recent previous reading at the same location.This feature forces the trend for all TMLs downward, providing several significant advantages:

A more conservative analysis -- when an actual growth reading is kept in the analysis,the analysis is inaccurate due to an excessive corrosion allowance.

Elimination of the potential for an artificially high short-term corrosion rate after thenext survey.

The net effect of this feature is a smoothing of analytical results after subsequent surveys. Alllogic is completed during analysis, and therefore reversible.

Two important logic rules associated with the Growth feature are:

1. Functions on the previous two surveys only. That is, after a third growth, the

actual measurement will be used in the analysis.

2. An initial survey thickness ignores a Nominal Thickness independently of theanalytical option to analyze nominal.

HELPFUL HINT

All logic for the Growth feature is executed during analysis and isreversible. That is the feature can be turned on or off and dataanalyzed accordingly. Growth logic will only function on the twoprevious surveys.

Representative Eq/Circ ID and Group Corrosion Rates (Advanced Option)Once the corrosion rate at each TML has been calculated, the next steps are to calculate thecorrosion rate trend for the complete Eq/Circ ID. Up to four calculations may be performedfor each Eq/Circ ID. The greatest of the selected calculations and the default corrosion rate isequal to the Eq/Circ ID Representative Corrosion Rate (RCR).

1. Average TML Corrosion Rate


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If theAverage Corrosion Rates * (User Input)box is checked on theAnalyticalSettings - Advancedwindow, the Average TML Corrosion Rate will be calculated for eachEq/Circ ID. The Average TML Corrosion Rate is based on the arithmetic mean of theRepresentative TML Corrosion Rates (as calculated above). The average TML rate is thenmultiplied by an optional statistical safety factor, and is known as the Safety Factor Rate. Thesafety factor allows you to artificially increase the calculated corrosion rates and be moreconservative.

2. Maximum Average TML Corrosion Rate

If theMax (User Input)% of TML Rates, Min (User Input) TM Lsbox is checked ontheAnalyti cal Settings - Advancedwindow, the Max Average TML Corrosion Rate willbe calculated for each Eq/Circ ID. The Max Average TML Corrosion Rate is based on thearithmetic mean of the highest X% Representative TML Corrosion Rates (as calculatedabove). This calculation helps assure that the final statistical corrosion rate reflects an area ofthe Eq/Circ ID with greater than average corrosion rates.

3. Formula Circuit Corrosion Rate

If theFormula Circui t Rate, Sigma = (User Input)box is checked on theAnalyticalSetti ngs - Standardwindow, the Formula Circuit Corrosion Rate will be calculated for eachEq/Circ ID. The Formula Circuit Corrosion Rate is based on a certain probability that thereis no higher corrosion rate in the Eq/Circ ID. The Sigma value you enter specifies a certainprobability confidence level. For example, a Sigma of 1.28 provides a 90% confidence leveland a 1.60 provides a 95% confidence level.

FCCR AvgTMLCorrosionRate SigmaAvgTMLCorrosionRate

NumTMLs= + *

This formula assumes that the standard deviation of the data is equal to the arithmeticaverage of all corrosion rates. This assumption has been approximately correlated in studiesof actual refinery corrosion data and is inherently more conservative as high corrosion rateEq/Circ IDs will always have a high assumed standard deviation.

4. Default Corrosion Rate for Statistical Analysis

A statistical default corrosion rate in mils per year is specified in theDefault Corrosion Rate= (User Input)MPYfield on theAnalyti cal Settings - Advancedwindow.

After the selected calculations are complete for an Eq/Circ ID, the largest of the active 1) Safety

Factor Average, 2) Max. Ave., 3) Formula Circuit Rate and 4) Default Corrosion Rate for thatEq/Circ ID is selected and becomes our Representative Eq/Circ ID Corrosion Rate for thatpiece of equipment.

The calculations described in this step will always be performed for the Eq/Circ ID and canalways be accessed in reports, but will only be used in the TML Retirement Date calculation (seenext step) if theUse Statistical Method(s)box is checked on theAnalyti cal Settings -Advancedwindow.


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Group Corrosion Rate -The Group Corrosion Rate is defined as the maximum Eq/Circ IDRepresentative Corrosion Rate of all Eq/Circ IDs linked to the Group Name. A Group canhave several linked Eq/Circ IDs so this value represents the most corrosive section of a Group.

TML Retirement Date

The TML Retirement Date is simply the remaining life of the TML, in years, added to the mostrecent TML inspection date. Remaining life is calculated as follows:

RemainingLife(Yrs)MostRecentReading TMin

ActiveCorrosionRate=

TheActiveCorrosionRatewill be one of the following, depending on selection of straight lineanalysis, statistical analysis or both as checked on the standard and advanced analysis dialogs:

1. IfonlytheUse Straight L ine Method(s)box is checked on theAnalyticalSetti ngs - Standardwindow, the ActiveCorrosionRate will be equal to the

Representative TML Corrosion Rate. This method is recommended if:

You are confident TML readings are accurate.

The TML sampling is very high.

The internal corrosive environment in the Eq/Circ ID is not approximatelyuniform.

Insufficient calculated TML corrosion rates are available for a valid statisticalRepresentative Eq/Circ ID Corrosion Rate.

2. IfonlytheUse Statistical Method(s)box is checked on theAnalyti cal Sett ings -

Advancedwindow, the ActiveCorrosionRate will be equal to the RepresentativeEq/Circ ID Corrosion Rate. This method is recommended if the selected Eq/CircID has the same approximate corrosive internal environment throughout and ifreading accuracy and/or location are not reliable. Both of these conditions aretypical for many inspection programs.

IfboththeUse Straight L ine Method(s)and theUse Statisti cal M ethod(s)boxes arechecked, the ActiveCorrosionRate will be equal to the greater of the Representative TMLCorrosion Rate and the Representative Eq/Circ ID Corrosion Rate for each TML. Thismethod is most conservative because the inherent strengths of the Straight Line and Statisticalanalyses have a canceling effect on each others inherent weaknesses. This method isrecommended for most applications.

Define TMLs as Zero Corrosion Allowance or Non-Zero Corrosion

Allowance

If theUse C.A. Logicbox is checked on theAnalyt ical Settings 0 C.A. window, anyTML with a database entry of Nominal Thickness = T-Min is defined as a zero corrosion


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allowance TML. No TMLs in the Corrosion M onitoring database wil l be defined as zerocorrosion allowance unless thi s logic is activated and all Eq/ Circ IDs are analyzed.

HELPFUL HINT

By definition, a zero corrosion allowance TML has a Nominal

Thickness equal to Retirement Thickness (T-Min).

TML Inspection Due Date for Non-Zero Corrosion Allowance TMLs

UltraPIPE allows you to enter inspection scheduling criteria information for a thickness(ultrasonic) inspection activity based on a calculated inspection interval and a maximum interval.The inspection scheduling information is specified in theU/ Tfields on theAnalyti cal Settings- Standardwindow. Note:The information you enter in these fields will be used for both theTML Inspection Due Date and the Eq/Circ ID Inspection Due Date calculations.

HELPFUL HINT

The thickness inspection frequencies of API 510 - Pressure VesselInspection Code, API 570 - Piping Inspection Codeand API 653 -Tank Inspection can be met exactly using the flexible analysiscapabilities of the UltraPIPE Corrosion Monitoring Module.

1. Calculated Interval for non 0 C.A. TMLs (IE: Nominal not equal to T-Min)

The calculated interval is based on the Remaining TML Life prediction:

InspectionIntervalRemainingLife Yrs

N=

( )

The constant N is specified in theRemaining Life / (User I nput)field in theU/ Tsectionof theAnalyti cal Settings - Standardwindow. Check the box for this field to make sure itis included in the inspection due date calculation.

2. Maximum Interval

The maximum interval is a fixed time-span that allows an inspection interval to be setindependent of the calculated remaining life of the TML. The maximum interval is specifiedin the (User I nput) Year Maximumfield in theU/ Tsection of theAnalyti cal Sett ings -Standardscreen. Check the box for this field to make sure it is included in the inspectiondue date calculation.

The maximum interval (without the calculated interval) is useful for low corrosion proneequipment with minimal design corrosion allowance. Many alloy vessels are designed withzero corrosion allowance and fall into this category. However, the zero corrosionallowance logic discussed below is a better way to handle this case. In addition, this methodprovides a fail safe mechanism to schedule inspections when bad data artificially causeslong life during the Eq/Circ ID analysis.


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Depending on which methods are selected, the calculated U/T Inspection Interval will be theCalculated Interval, the Maximum Interval, or the shorter of the two. Most applications will useboth methods.

TML Inspection Due Date for Zero Corrosion Allowance TMLs

UltraPIPE allows you to enter inspection scheduling criteria information for thickness(ultrasonic) inspection activity based on fixed intervals for zero corrosion allowance TMLs. Forthis logic to function, the zero corrosion allowance logic must be active in theAnalyticalSett ings 0 C.A. window. Note: The information that you enter in theAnalytical Settings 0 C.A. window will be used for both zero corrosion allowance TML inspection Due Date andthe zero corrosion allowance Eq/Circ ID Inspection Due Date calculations.

HELPFUL HINT

The life based thickness inspection frequencies of API 510 Pressure Vessel Inspection Code, API 570 Piping Inspection Codeand API 653 Tank Inspectionare not reasonable for zero corrosionallowance TMLs and Eq/Circ IDs as the remaining life of zerocorrosion allowance is zero years by design.

Fixed Interval -The fixed interval is the fixed time-span that allows an inspection interval to beset independent of the calculated remaining life of the TML. This is important as remaining lifeis zero by design. The fixed interval is specified in theFixed Interval for 0 C.A. TMLs and 0C.A. Eq/ Circ Ids (User Input) Yearsfield of theAnalyt ical Settings 0 C.A. window. Thecheckbox toUse 0C.A. logicfor UT / RT Inspection Due DatesMUST be selected for zerocorrosion allowance logic to function.

Eq/Circ ID and Group Retirement Dates

The Eq/Circ ID Retirement Date is calculated using theUse average of (User-Input) % ofthe lowest life TM Ls (M in of (User-Input)) TMLs for an Eq/ Circ ID Ret. Datefield ontheAnalyti cal Settings - Standardwindow. Based on the values you enter in these fields, theearliest X% retirement dates from all the TMLs on the Eq/Circ ID (or the specified minimum)will be selected when calculating the Eq/Circ ID Remaining Life. For example, entering a 25in this field specifies that the earliest 25 percent of TML retirement date calculations (or thespecified minimum) will be averaged to represent the Area-Eq/Circ ID Retirement Date. AllTMLs used for the Eq/Circ ID life prediction are highlighted on several Fixed Format TextReports. In addition, these TMLs can be color-coded using the Report Configuration.

Based on the values you enter in these fields, several of the earliest X retirement dates from all

the TMLs on the Eq/Circ ID will be selected and averaged to calculate the Eq/Circ IDRetirement Date. The most recent survey date for any of the TMLs will then be subtractedfrom this retirement date, yielding the remaining life in years for the Eq/Circ ID. This value isused to calculate the thickness inspection interval used below to calculate the Eq/Circ IDInspection Due Date

The most conservative setting is zero percent, minimum one. That is, the Eq/Circ IDretirement date is equal to the minimum TML Retirement date. A less conservative setting of


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25% minimum one has been demonstrated to extend inspection frequencies with minimumadditional risk. (The UltraPIPE Eq/Circ ID Inspection Due Date can never exceed theminimum TML Retirement Date by analysis logic as described below.)

Group Retirement Date -The Group Retirement Date is always the minimum Eq/Circ IDRetirement Date of all Eq/Circ IDs linked to the Group Name. A Group can have severallinked Eq/Circ IDs so this value represents the earliest Retirement Date within a Group.

Define Eq/Circ IDs as Zero Corrosion Allowance or Non-Zero Corrosion

Allowance Eq/Circ IDs

No Eq/Circ IDs in the Corrosion Monitoring database will be defined as zero corrosionallowance unless the logic is activated and all Eq/Circ IDs are analyzed. By definition, a zerocorrosion allowance Eq/Circ ID meets one of the conditions established by the user as shownbelow. The logic is based on zero corrosion allowance TMLs defined as a Nominal Thicknessequal to Retirement Thickness (T-Min).

If theUse 0 C.A. Logiccheckbox is selected on theAnalyt ical Settings 0 C.A. window,any TML with a database entry of Nominal Thickness = T-Min is defined as a zero corrosionallowance TML. In addition, there are three ways to identify a zero corrosion allowance Eq/CircID on theAnalytical Settings 0 C.A. window.

1. All TMLs on the Eq/Circ ID are zero corrosion allowance.

2. There are less than or equal to a minimum percent (with a maximum value) of non-zero corrosion allowance TMLs on the Eq/Circ ID.

3. One or more TMLs on the Eq/Circ ID are zero corrosion allowance.

Eq/Circ ID and Group Inspection Due Dates for Non-Zero Corrosion

Allowance Eq/Circ IDs

UltraPIPE allows you to enter inspection scheduling criteria information for an internal(ultrasonic) inspection activity based on a calculated inspection interval and a maximum interval.The inspection scheduling information is specified in theU/ Tfields on theAnalyti cal Settings- Standardwindow. Note: the information you enter in these fields will be used for both theTML Inspection Due Date and the Eq/Circ ID Inspection Due Date calculations.

1. Calculated Interval

The calculated interval is based on the Eq/Circ ID Remaining Life prediction:

InspectionIntervalRemainingLife Yrs

N=

( )

The constant N is specified in theRemaining Life / (User I nput)field in theU/ Tsectionof theAnalyti cal Settings - Standardwindow. Check the box for this field to make sure itis included in the Inspection Due Date calculation.


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The Remaining Life in Years used for the life based inspection interval is calculated asdiscussed above in theEq/Circ ID and Group Retirement Datessection. However, theUse 0 C.A. logicoption on theAnalytical Settings 0 C.A. window will determinewhich TML to use for the Remaining Life in order to calculate the Inspection Due Date. If0 C.A. logic is active, only non-zero corrosion allowance TMLs are used for the RemainingLife calculation to determine the Eq/Circ ID Inspection Due Date.

2. Maximum Interval

The maximum interval is a fixed time-span that allows an inspection interval to be setindependent of the calculated remaining life of the Eq/Circ ID. The maximum interval isspecified in the(User I nput) Year M aximumfield in theU/ Tsection of theAnalyticalSetti ngs - Standardwindow. Check the box for this field to make sure it is included in theinspection due date calculation.

The maximum interval (without the calculated interval) is useful for low corrosion proneequipment with minimal design corrosion allowance. Many alloy vessels are designed with

zero corrosion allowance and fall into this category. In addition, this method provides afail safe mechanism to schedule inspection s when bad data artificially causes long lifeduring the Eq/Circ ID analysis.

Depending on which methods are selected, the initial calculated U/T Inspection Interval wouldbe the Calculated Interval, the Maximum Interval, or the shorter of the two. Most applicationswill use both methods.

Only when the calculated interval is used for the inspection due date, a final check is made for afinal calculation of the Eq/Circ ID Inspection Due Date. For these cases, the initial InspectionDue Date is compared to the earliest TML Retirement Date. The final Eq/Circ ID InspectionDue Date will be the earliest of the initial calculated Eq/Circ ID Inspection Due Date or the

earliest TML Retirement Date. This final logic check allows a decrease in conservatism for thecalculated Eq/Circ ID Retirement Date calculation and an extension of most inspection intervalswith minimum increased risk.

The minimum Retirement Date TML used for this final check will only include non-zerocorrosion allowance TMLs when 0 C.A. logic is active. TheUse 0 C.A. logicoption ontheAnalyt ical Settings 0C.A. window controls this logic.

Eq/Circ ID and Group Inspection Due Dates for Zero Corrosion Allowance

Eq/Circ IDs

UltraPIPE allows you to enter inspection scheduling criteria information for thickness(ultrasonic) inspection activity based on fixed intervals for zero corrosion allowance Eq/CircIDs. For this logic to function, the zero corrosion allowance logic must be active in theAnalytical Settings 0 C.A. window. Note: The information that you enter in theAnalyticalSett ings 0 C.A. window will be used for both zero corrosion allowance TML Inspection DueDate and the zero corrosion allowance Eq/Circ ID Inspection Due Date calculations.


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HELPFUL HINT

The life based thickness inspection frequencies of API 510 Pressure Vessel Inspection Code, API 570 Piping Inspection Codeand API 653 Tank Inspectionare not reasonable for zero corrosionallowance TMLs and Eq/Circ IDs as the remaining life of zerocorrosion allowance is zero years by design.

Fixed Interval -The fixed interval is the fixed time-span that allows an inspection interval to beset independent of the calculated remaining life of the TML. This is important as remaining lifeis zero by design. The fixed interval is specified inThe Fixed Interval for 0 C.A. TMLs and 0C.A. Eq/ Circ Ids (User Input) Yearsfield of theAnalyt ical Settings 0 C.A. window. Thecheck box toUse 0 C.A. logicfor UT / RT I nspection Due DatesMUST be selected for zerocorrosion allowance logic to function.

Group Inspection Due Date -The Group Inspection Due Date is always the minimumEq/Circ ID Inspection Due Date of all Eq/Circ IDs linked to the Group Name. A Group canhave several linked Eq/Circ IDs so this value represents the earliest Inspection Due Date within

a Group.

HELPFUL HINT

The thickness inspection frequencies of API 510 - Pressure VesselInspection Code, API 570 - Piping Inspection Codeand API 653 -Tank Inspection can be met exactly using the flexible analysiscapabilities of the UltraPIPE Corrosion Monitoring Module.Recommended Intervals for 0 C.A. Eq/Circ IDs is not mentioned bythe API documents.

Caution TML Calculation

You can specify TMLs that need special attention by using the Caution TML settings. ACaution TML is defined as follows:

RepresentativeTMLCorrosionRate XXX MPY

And Or

TMLRemainingLife XXX Yrs

>

Chap 6 - Corrosion Monitoring

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