Technical Data Handbook

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Inspection and Remaining Life

Evaluation of Process Plant Equipment


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Proceedings of the Process & Power Plant Reliability Conference, November 2002. Copyright 2002 by ClarionTechnical Conferences and the authors. All rights reserved. This document may not be reproduced in any mannerwithout permission of the copyright owners.

Inspection and Remaining Life Evaluationof Process Plant Equipment

By Carl E. Jaske and Brian E. Shannon

ABSTRACT

This paper reviews the types of inspection that are used to assess the condition of process plantequipment. The types of damage and/or defects identified and characterized by each inspection techniqueare reviewed. For each type of damage/defect, the advantages and disadvantages of each inspectiontechnique are discussed. The technique or technique combinations preferred for remaining life evaluationis highlighted. Methods for evaluating remaining life are reviewed with emphasis on data that are

provided by in-service inspection. Methods of integrating the inspection data directly with remaining lifeevaluation are reviewed. This integrated approach provides results that the plant operator can utilize tomake timely decisions regarding the fitness-for-service of equipment. Run, repair, or replace decisionsthen can be made to minimize downtime and impact on production.

Examples are presented to illustrate the integrated approach to inspection and remaining life prediction.These examples are a pressure vessel with corrosion damage and a furnace tube with creep damage. Witheach example, the benefits of automating the data processing and analysis by means of integratedcomputer software are pointed out.

INTRODUCTION

Managing equipment integrity is essential to the safe, reliable operation of process plant equipment. Thisequipment includes pressure vessels, piping, storage tanks, valves, pumps, compressors, boilers, firedheaters, and turbines. Figure 1 shows an example of the framework for an integrity management

program, which is adapted from API Standard 1160.1

The first step in establishing such a program is togather, review, and integrate all of the relevant data on the plant equipment. This is usually done using acomputerized database. Using the initial data, an initial risk assessment of the plant is performed. Theresults of the risk assessment are used to prioritize equipment inspection and assessment and develop a

baseline assessment plan. Once the baseline plan is completed, the remaining four items in Figure 1 areperformed continually throughout the life of the plant. These involve inspection and mitigation, datareview and integration, updating the risk assessment, and revision of plan. During this process, theeffectiveness of the program is evaluated to make sure that it is achieving the desired goals.

The current condition of equipment and remaining life must be evaluated to perform the risk assessment.Results of the inspection are used to assess the current condition, whereas stress analysis and materialsdegradation models are used to assess remaining life. This paper reviews the inspection methods and

remaining life assessment procedures that can be applied to process plant equipment.

Equipment condition depends of the type of material damage, such as corrosion, fatigue, or creep.Inspection is used to quantify this damage. The type of inspection employed must be tailored to the typeof material damage that is expected in service. Thus, selection of the inspection technique(s) is based onanalysis of the equipment operating conditions and past experience. Also, the remaining life evaluationdepends on the material damage mechanism expected in future service. For example, inspection must


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4 Process & Power Plant Reliability Conference, November 13-14, 2002

Gather, review, andintegrate initial data

Perform initial riskassessment of plant

Develop baselineassessment plan

Perform inspectionand/or mitigation

Review, update, andintegrate data

Revise inspection andmitigation plan

Update and revise riskassessment

Evaluateprogram

Figure 1. Example Framework for an Integrity Management Program.1

estimate fatigue crack growth when fatigue cracking under cyclic loading is the expected damage

mechanism.

Both inspection and remaining life evaluation can be improved if they are employed using an integratedapproach. The inspection technique and type of data collected are then optimized for the remaining lifeassessment that is to be performed. Also, the models used for remaining life assessment are based uponrealistic values that can be measured during inspection. Thus, the inspection and remaining life

prediction plans should be developed concurrently to obtain the best results in a cost-effective manner.


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Inspection and Remaining Life Evaluation of Process Plant Equipment 5

INSPECTION METHODS

Defect location, orientation, size, and shape need to be characterized to assess current equipmentcondition, fitness for service, and remaining life.

The suitability of a particular technology or technique must fully be understood for the specific damage

mechanism. This requires application specific techniques that provide technical merit through acombination of appropriate technology, procedure selection, and personnel qualification.

The nondestructive examination data input for engineering analysis requires determination of an absenceof flaws, with a known level of probability of detection (POD), a presence of flaws with a known POD,and the estimated size of flaws with a known degree of accuracy.

When considering NDE techniques for specific damage attributes, a selection can be made from one ofthree general groupings:

1. Surface TechniquesOffering only general detection and length/width sizing

2. Volumetric TechniquesOffering varying degrees of quantitative information on flaw size, location, orientation, etc.

3. Screen or Global TechniquesOffering a cursory general condition status, which will require follow up examinations withmore definitive techniques

Surface Techniques

These include the following methods:

Visual Inspection Liquid Penetrant Examination Magnetic Particle Examination Eddy Current Alternating Current Field Measurement (ACFM)

Visual Inspection. Visual inspection, more than any other technique can determine the initial overallcondition of a structure or component. The addition of dimensional testing tools can greatly improve theeffectiveness of the technique as can the use of video cameras, borescopes, or remote operational tools.The techniques are applied at first pass method where suspect areas are further examined using othermore quantitative methods.

Liquid Penetrant Examination. Liquid penetrant is normally applied to non-magnetic materials todetermine the presence of surface breaking flaws. The technique employs a series of dyes and contrastdevelopers to denote the presence of flaws. For the purpose of determining the full extent of flawshowever, it is a poor technique as it only offers the surface dimension (length/width).

Magnetic Particle Examination. Magnetic particle examination is utilized for the detection of surfaceand near surface flaws in ferromagnetic materials. The component or surface being examined ismagnetized, and discontinuities that lie in a direction transverse to the magnetic field, will cause a leakage


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in that field. The presence of this leakage field, caused by the discontinuity distorting the magnetic field,permits detection of the flaw when fine ferromagnetic particles are applied over the component surface.

The technique is extremely sensitive to flaw detection and surface discontinuities, but it does not indicatethe depth and true extent of the flaw or defect.

Eddy Current. Eddy current inspection is based on the principles of electromagnetic induction and isused to identify or differentiate between a wide variety of physical, structural and metallurgical conditionsin electrically conductive ferromagnetic and non-ferromagnetic metals and metal parts. Eddy currentinspection can be used as follows:

1. To measure or identify such conditions and properties as electrical conductivity, magneticpermeability, heat treatment condition, hardness, and physical dimensions.

2. To detect seams, laps, cracks, voids, and inclusions.3. To sort dissimilar metals and detect differences in their composition, microstructure, and other

properties.4. To measure the thickness of a nonconductive coating on a conductive metal, or the thickness of a

nonmagnetic metal coating on a magnetic metal.

Because eddy current inspection is an electromagnetic induction technique, it does not require directelectrical contact with the part being inspected. The eddy current method is adaptable to high speedinspection, and because it is nondestructive, it can be used to inspect an entire production output ifdesired. The method is based on indirect measurement, and the correlation between the instrumentreadings and the structural characteristics and serviceability of the parts being inspected must be carefullyand repeatedly established.

Eddy current inspection is extremely versatile, which is both an advantage and disadvantage. Theadvantage is that the method can be applied to many inspection problems provided that the physicalrequirements of the material are compatible with the inspection method. In many applications, however,the sensitivity of the method to the many properties and characteristics inherent within a material can be a

disadvantage; some variables in a material that are not important in terms of material or part serviceabilitymay cause instrument signals that mask critical variables or are mistakenly interpreted to be caused bycritical variables.

Alternating Current Field Measurement (ACFM). An alternative inspection method that has beenapplied to the detection of surface breaking cracks is the Alternating Current Field Measurement (ACFM)technique. This is a non-contacting electromagnetic technique that uses an induced electric current to

produce a magnetic field in the weld area to be inspected. Two small sensors located orthogonally arethen used to detect changes in the magnetic field components indicating the presence of cracks. Theinduction coil and the sensors are all combined in a single probe that can be applied directly to weldinspection. See Figure 2.

Because the technique is non-contacting, it can be used to inspect through non-conducting coatings. Withnormal protective paint coatings, no adjustment to the system gain would be required, but as the coatingincreases, the gain on the system would have to be increased to take into account the separation betweenthe surface and the probe face.

Rough non-coated surfaces can be examined with standard instrument settings except in extremesituations. There is a range of probes available for use with the ACFM system at normal temperaturesand elevated temperatures up to 270 degrees Fahrenheit and special probes can be used up to 1000Fahrenheit. New probes have been developed which have greater sensitivity to small surface breaking


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defects, less than 40 mils deep, and are not affected by edge effects. New improved software has alsobecome available which aids the operator in selecting the speed of scanning, setting up the instrument andsizing defects.

The ACFM technique offers an alternative method of inspection for the detection of surface breakinghydrogen assisted and stress related cracking, with little prior surface preparation and no refurbishment

after inspection required. Table 1 compares ACFM with magnetic particle inspection.

Table 1. Comparison of ACFM with Magnetic Particle Inspection.

Criteria ACFMMagnetic Particle

Inspection

Crack Detection Yes Yes

Crack Length Yes Yes

Crack Depth Yes (Max 1.5) No

No Need to Clean Yes No

Automatic Recording Yes No

High Temperature Applications Yes No

Non-Magnetic Materials Yes No

Limited Access No Yes

Automated Interpretation Yes No

Calibration Required No No

Operator Skill Required Medium-High Medium

Figure 2. Typical HIC Damage that Can Be Detected and Sized by ACFM.

Having found a defect indication using ACFM, a crack size estimate is made as follows:

The length is found by moving the probe along the weld until the Bz peak (see Figure 3) associated withone end of the defect is maximized. The probe position is then marked on the specimen, using a magneticarrow for example. The Bz trough associated with the other end is similarly marked and the distance

between them is measured with a flexible tape. This length is close to the true defect length and is used asan input to the sizing routine.


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Two measurements are taken from the Bx plot (see Figure 3). These are the background level either sideof the defect, and the minimum value in the middle of the defect trace. These two signal levels and theinitial crack length estimate are then entered into a software routine which searches through theoretically,

based look tables, to produce the final estimates of the length and depth of the defect.

Figure 3. Qualitative Explanation of the Nature of Bx and Bz Above a Notch.

Volumetric Techniques

Volumetric techniques include radiography and ultrasonic inspection.

Radiography. Three elements are required for radiography as follows: a radiation source (usually x-rayor gamma ray), a test piece, and a recording medium, usually film.

Radiation from the source is absorbed by the piece under test; the flaws absorb different amounts ofradiation. Radiation impinges on the film in differing degrees from that of the surrounding good material.This produces a two dimensional latent image when the film is developed.

The issue with regard to providing information necessary for engineering assessment is that again thetechnique provides only certain information; i.e. extent of corrosion, both width and depth, are easilyobtained. But in the case of planar flaws (lack of fusion or cracking), only the width and length areavailable.

In a recent European study, the probability of detection using radiography for new welds was in somecases as low as 53%.


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Ultrasonic Inspection. Ultrasonic inspection is a nondestructive method utilizing beams of highfrequency sound waves that are introduced into the material being inspected, to detect surface andsubsurface flaws. The sound waves travel through the material with some attendant loss of energy(attenuation) and are reflected at interfaces. The reflected beam is detected and analyzed to define the

presence and location of flaws.

The principal advantages of ultrasonic inspection as compared to other methods for nondestructiveinspection of metal parts are listed below:

Superior penetrating power, which allows the detection of flaws deep in the part. Ultrasonicinspection is done routinely to depths of several feet on many types of parts, and to depths ofabout 20 feet in the axial inspection of parts, such as long steel shafts, or rotor forgings.

High sensitivity, permitting the detection of extremely small flaws.

Greater accuracy than other nondestructive methods in determining position of internal flaws,estimating their size and characterizing their orientation, shape, and nature.

Only one surface need be accessible.

Operation is electronic, which provides almost instantaneous indications of flaws. This makesthe method suitable for immediate interpretation, automation, rapid scanning, in-line production

monitoring, and process control. With most systems, a permanent record of inspection results canbe made for future reference.

Volumetric scanning ability, enabling inspection of a volume of metal extending from frontsurface to back surface of a part.

Is not hazardous to operations or to nearby personnel, and has no effect on equipment andmaterials in the vicinity.

Portability.

Disadvantagesof ultrasonic inspection include the following:

Manual operation requires careful attention by experienced technicians.

Extensive technical knowledge is required for the development of inspection procedures.

Parts that are rough, irregular in shape, very small or thin, or not homogeneous, are difficult toinspect.

Discontinuities that are present in a shallow layer immediately beneath the surface may not bedetectable and require specialized procedures for satisfactory results.

Couplants are needed to provide effective transfer of ultrasonic wave energy between transducersand parts being inspected.

Reference standards are needed, both for calibrating the equipment and for characterizing flaws.

In addition to traditional manual ultrasonic examination, there are several advanced ultrasonic techniquesthat offer significant improvements in terms of coverage and accuracy of inspection data. These includeAutomated Ultrasonic Imaging Techniques (AUT) and Time-of-Flight Diffraction (TOFD).

Automated ultrasonic imaging techniques (AUT) have the capability of providing detailed dimensions forflaw assessment and damage progression monitoring. The work can be carried out on-line up to 750F.Information on both corrosion and flaw detection can be provided.

The typical AUT presentations include C-scan (plan view), B-scan (cross-sectional) and the recorded A-scans or digital RF waveforms. Additional presentations include volumetric projection views (overlaiddata), 3D histograms along with data post processing.


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The B-scan and projection views provide cut-away slices. These are excellent and easy-to-read visualmeans of determining if blisters are propagating and linking because of stepwise cracking.

Ch:2 0 Degree longitudinal data Standard A, B, andC-scan views

Figure 4. AUT Examination of HIC/Blistering Sample.

Time-of-Flight Diffraction TOFD differs from conventional pulse echo ultrasonics in that it depends ondiffracted energy rather than reflected energy, monitored in the form of signal height or amplitude. Theconventional energy response is compared to an equivalent defect, normally in the form of a side-drilled hole or notch of a specific size for a specific material thickness. TOFD, however, relies on time-

based, low energy, diffracted signals from the tips of flaws, allowing an exact or absolute position andsize for the flaw to be measured and imaged.

The TOFD technique is implemented using two probes, a transmitter and a receiver; normally placedeither side of the weld joint or area to be examined. Because the technique is not amplitude dependent, asin conventional ultrasonics, it does not suffer from the constraints of probe location to flaw orientation,

poor coupling, and uneven or changing material surfaces. TOFD also uses compression energy only,allowing mode converted signals to be screened out, simplifying the characterization and positioning offlaws.

Data from the scan is collected in raw form and digitized in RF format (A Scan). The signals are thenstacked electronically on top of each other to provide a cross sectional image in the form of a D Scan,representing both the length of the flaw and the through-wall extension or depth (see Figure 5). This is aunique fact in that the corresponding image and position of the flaw are absolute and can be readilyapplied by plant engineers in a critical assessment of equipment integrity using a fracture mechanics.

TOFD technique sees everything between the pulser (or outside surface, the inside surface, and thereceiver - outside surface, again), and displays all flaws and data in a proportional sense. Therefore,TOFD is a very effective detection tool, providing excellent information on the nature and location of theinherent flaws. This provides plant management with a superior monitoring tool for reliability decisions.

As detection and sizing are carried out in the same scan, this has the added benefit of allowing instantdetermination of the acceptability of flaws. Sizing using TOFD is carried out using direct measurements,

not equivalent responses. Accuracy has been reliably demonstrated at 1 mm (0.040 inch) or better, bothin sizing of length and through-wall height.


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Figure 5. Example of TOFD Measurements.

Because of the tandem setup of the TOFD probes, and by nature of the broad divergent beam employed,TOFD can examine the full volume of weld metal, heat affected zones and the adjacent base material in asingle scan. This has been demonstrated to be much faster than radiography or manual ultrasonics.Scanning rates of two to six inches per second have been achieved in various field applications. As thematerial under examination gets thicker, there is no corresponding increase in the relative scan time.Another consideration is that the effect of mismatched or complex geometries has no real effect on eitherthe performance of the technique or the scan time required.

Other benefits of TOFD include the following:

Can be applied to new construction welds, in lieu of radiography (ASME Code Case 2235),providing an excellent record of vessel or pipe condition at birth.

Can also be used for in-service defects, such as cracking, corrosion, erosion, etc.

Has a very high temperature limitation (700F), with only a slight loss in sensitivity. See

Figures 6 and 7. Detects flaws regardless of their orientation within the weld.

Can be applied without the need to shut down other activities in the area (which radiographydemands).

Can be applied either during welding or immediately after completion of the weld, allowinginstant determination of weld quality.

Allows flaw growth monitoring for run/repair decisions.


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Figure 6. TOFD Scan Performed at 780F.

Figure 7. TOFD Scan Performed at 780F.

Lateral WaveI.D. Notch

Back Wall

Notch

Lateral Wave

Back Wall


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Pulsed Eddy Current

This technique is designed to detect corrosion in insulated vessels (up to 4 inches thick) and pipingsystems. It allows screening of pressure systems with a high temperature profile (up to 1000F).

The system averages thickness values over the eddy current field, and displays a volumetric assessment of

pipe or vessel wall. The information is again qualitative and is usually verified by a more finitetechnique. Figure 8 illustrates a pulsed eddy current system.

Figure 8. Schematic of a Pulsed Eddy Current System.

Long Range Ultrasonics

A recent development for the inspection of piping systems over extended distances is the guided waveconcept. The overall idea is to induce various low frequency ultrasonic waves (longitudinal, torsional,flexural) onto an area of exposed piping See Figure 9. The waves propagate within the pipe wall over a

predetermined distance along the pipe. Deviations from known values are reflected back to thetransducers where special software displays the information.

The technique is sensitive to wall loss of approximately 10% of wall volume. Areas of concern arenormally examined by a more quantitative method, usually automated ultrasonics, to determine specificflaw dimensions. Tens of meters of pipe are examined from one location. Difficult to inspect areas, suchas roadways and insulated pipe, can be screened for defects.

Acoustic Emission Inspection

Acoustic emission is defined as the high frequency stress waves generated by the rapid release of strainenergy that occurs within a material during crack growth, plastic deformation or phase transformation. Itcan be used in the detection of sub-critical growth of flaws, such as fatigue crack growth, stress corrosioncracking and hydrogen embrittlement.

Significant applications include continuous surveillance of pressure vessels and nuclear primary pressureboundaries for the detection and location of active flaws, and determination of the onset of stresscorrosion cracking and hydrogen damage in susceptible structures.


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Figure 9. Guided Wave Ultrasonic Inspection of Piping.

Figure 10. Testing of Large Diameter (6 to 24 inch) Pipes Using an Inflatable Ring.

Acoustic emission inspection provides the following advantages over traditional nondestructiveinspection methods such as ultrasonics and radiography:

It provides a complete integrity analysis of a structure.

It can detect and evaluate the structural significance of flaws that may be inaccessible totraditional nondestructive inspection methods.

It requires only limited access and downtime for the requalification of in-service structures.


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Assessment of the integrity of pressure vessels by acoustic emission analysis is convenient because theloading mechanism (pressure) is quiet and does not interfere with the acoustic emission inspection

procedure. The technique was incorporated into a hydrostatic acceptance test of large pressure vesselsand was found capable of detecting and locating acoustic emission sites (flaws) as small as 0.1 inches inmaximum dimension.

Acoustic emission inspection of thick-wall pressure vessels during hydrostatic testing has been effectivein locating discontinuities and other flaws in the wall that can propagate under stress and result infracture.

The technique also has been applied to in-service monitoring of pressure vessels in nuclear reactor plantsto detect and locate growing sub-critical flaws. Continuous, on-line monitoring of central station nuclearreactor plants and monitoring of rocket chamber components have been performed.

Utilizing a Combination of Techniques to Quantify Creep Damage in Cast Materials

Combining NDE techniques allows optimized inspection for a specific material damage mechanism. Castheat-resistant alloys utilized in reformer furnaces offer a large challenge for inspection technology.

Because of the varying degrees and types of damage, it is necessary to employ a combination oftechniques to quantify the true extent of damage.

Specifically, creep damage as shown below is readily found by volumetric ultrasonic inspection;however, non-radial or outside surface connected damage is more readily detected using eddy currentinspection. Combining the results of both offer a realistic determination on the through wall extent ofcracking. By also measuring the wall thickness and outside diameter (creep strain) one can obtain a true

picture of tube condition.

Figure 11. Creep Damage in HP 45 Material.


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Table 2 summarizes the applicability and effectiveness of the various common inspection techniques.This information provides guidance for selection of an inspection technique for a specific piece ofequipment in a process plant.

REMAINING LIFE EVALUATION

Computing estimates of remaining life requires a model of the material damage mechanism. Moreaccurate damage models and knowledge of operating conditions will yield more accurate calculations ofremaining life. For an in-service component, the current condition of a component must be determined

before remaining life can be estimated. The current condition can be determined by computing thecurrent material condition using damage accumulation models and past operating conditions or by meansof nondestructive examination (NDE). The former procedure is typically less accurate than the latter

because good data on past operating conditions is generally difficult to obtain. The latter procedure isthus preferred if there are reliable NDE techniques for characterizing the current material condition.

Once the current material condition is defined, estimated remaining life is calculated using damageaccumulation models and forecasts of future operating conditions. When the damage models are very

accurate and future operations are well defined, accurate estimates of remaining life can be calculated. Asuncertainties about the material damage model and future operating conditions increase, conservatismsare employed to ensure that the calculations produce estimates of minimum remaining life. The estimatesof remaining life are used to make repair/replace decisions and to set re-inspection intervals. In caseswhere remaining life cannot be estimated because there is no quantifiable material damage mechanism orthe future operating conditions cannot be forecast, equipment should be carefully monitored during futureoperation.

Integrated Approach

Predictions of remaining life are best made using an integrated approach where NDE is used tocharacterize the current material condition and damage models and future operations are used to compute

damage accumulation and remaining life. When NDE and life prediction are coupled both activities canbe optimized and completed in a cost-effective manner. This approach avoids getting too little, too much,or the wrong type of inspection data and avoids trying to use damage models that are not compatible withthe NDE data. An integrated approach also makes sure that the appropriate damage mechanism isaddressed. For example, performing elegant creep damage calculations is not appropriate if fatigue crackgrowth is the most likely in-service damage mechanism and inspection reveals crack-like indications.

Corrosion General and Local

Two approaches are used to predict remaining corrosion life. The first method is to utilize a futurecorrosion allowance (FCA) and then check to make sure that the component thickness (t) is adequate aftertaking the FCA into account. With this approach, the remaining life (RL) is implicit in the FCA. The

second approach is to compute RL directly from the corrosion rate, measured wall thickness (tm), andminimum required wall thickness (tmin). This approach gives RL explicitly. Values of tminare computedfrom standard design formulas for the component.

For the first approach,

tmtmin+ FCA (1)


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Table 2. Comparison of Common Inspection Techniques.

Damage Attribute

Visual

Exam

Penetrant

Testing

Magnetic

Particle

Radio-graphy

Manual

Ultrasonic

Automated

Ultrasonic ACFM

Acoustic

Emission

Electro-

Magnetic

Erosion (General) B N/A N/A B C A N/A N/A C

OD Corrosion C N/A N/A B C C N/A N/A C

ID Corrosion D N/A N/A B B A N/A N/A D

Weld Flaws Non-Planar B* B* B* B C B B N/A C*

Cracking / LOF C* B* B* C C A A* 1B B*

Surface Flaws B* B* B* C C C A* 1B B*

Hydrogen Surface Cracking - Connected N/A N/A B D C A A N/A N/A

Hydrogen Blistering Embedded N/A N/A N/A D C A N/A N/A N/A

Creep Damage Embedded N/A N/A N/A D C A N/A C N/A

High Temperature Hydrogen Attack N/A N/A N/A N/A A A N/A N/A N/A

Legend

A Highly Effective (Recommended)

B Usually Effective

C Fairly Effective

D Poorly Effective

E Ineffective (Not Recommended)

* Inspected Side Only

** Remote Side Only

N/A Not Applicable

1 Only If The Defect Is Emitting An Acoustic Energy Signal (GrowingDefect)


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For the second approach,

RL = (tmtmin)/CR (2)

Equations (1) and (2) apply to both general and local corrosion. For general corrosion, FCA and CRrelate to the overall corrosion of the component in service. For local corrosion, FCA and CR relate to the

corrosion process at the local area of the component that is being evaluated. Also, for localized corrosion,the allowed local value of wall thinning may be less than the value of t mincomputed from standard designformulas when the surrounding material provides reinforcement, depending on the extent and depth oflocal corrosion, stress level, and material properties. Methods for assessing to allowed degree of localizedcorrosion have been developed for application to pressure vessels, piping, and storage tanks.

2

Stress-Corrosion Cracking

Computing remaining stress-corrosion-cracking (SCC) life is often more complicated than computingcorrosion life. SCC behavior is sometimes difficult to quantify, and in those cases equipment monitoringis recommended. When SCC can be quantified, the rate of crack growth (da/dt) is related to fracturemechanics parameters, such as the linear stress intensity factor (K) or the J integral:

da/dt = f(K) or f(J) (3)

SCC RL is then computed by integrating Equation (3) from the initial crack size (ao) to final the finalcrack size (af). In some cases, da/dt is observed to be independent of K or J over a wide range of SCC,and RL can be simply computed from a constant SCC rate within that range. For example, Jaske, et al.

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found that SCC of pipeline steels under rising loads and in a near-neutral pH groundwater environmentcould be characterized using the following power law function:

da/dt = G Jg (4)

where G and g are material-environment dependent constants. However, for the range of loading

typically experienced by underground gas pipelines, they found that the average and maximum SCC rateswere approximately 1 x 10

-8mm/s and 2 x 10

-8mm/s, respectively.

Fatigue and Corrosion-Fatigue

Fatigue and corrosion-fatigue remaining life are computed using two different approaches. The firstapproach is based on stress amplitude (Sa) versus number of cycles to failure (N f) curves developed fromthe results of fatigue testing. Figure 12 shows an example design fatigue curve for carbon and low-alloysteels from the ASME Code.

6 Such curves are usually based on total fracture of unflawed specimens

tested under laboratory conditions. Appropriate safety factors are applied to the test results in developingdesign curves, such as the one in Figure 12.

For constant amplitude cycling, remaining fatigue life (Nr) can be easily computed from Nf and thenumber of prior cycles (Np):

Nr= Nf- Np (5)

However, in most practical applications, the load amplitude varies during service, so the cycles in theloading history must be counted and a cumulative fatigue damage rule must be employed to evaluateremaining fatigue life.

7 Rainflow cycle counting

8,9 is widely used to quantify the number of discrete

cycles of loading in a variable amplitude history. Figure 13 shows an example of rainflow cycle counting


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ASME Design Fatigue Curve for Carbon and Low-Alloy

Steels at Temperatures Not Greater Than 371 C

10

100

1,000

10,000

10 100 1,000 10,000 100,000 1,000,000

Number of Cycles

Sa,

MPa

UTS < 552 MPa

Figure. 12. Design Fatigue Curve of Carbon and Low-Alloy Steels from Section VIII, Division 2 of theASME Boiler & Pressure Vessel Code.

6

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

0 1 0 2 0 3 0 4 0 5 0 6 0

Pressure,

kPa

T ime, h r

a

b

c

df h

eg

Pressure

Cyc le Range, kPa Frequency, 1/hr

a- f 4137 0.0512

g-h 3896 0.0419

d-e 3448 0.0646

b - c 2413 0 .0952

Figure 13. Example of Rainflow Cycle Counting for a Variable Pressure History.


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for a variable pressure history on an underground pipeline.10

The pressure range and frequency areindicated for each discrete cycle.

Once cycles are counted, the well-known Miners Rule is used to calculate cumulative fatigue damage(Df), as follows:

Df= (n/Nf) (6)

where n and Nfare the number of cycles and corresponding number of cycles to failure at each stressamplitude. When the value of Df = 1, fatigue failure is predicted. Thus, remaining fatigue life isdetermined from the remaining damage fraction (Dfr):

Dfr= 1 - Dfp (7)

where Dfpis the prior damage fraction. The anticipated future loading history is then used to compute thetime associated with Dfr.

The second approach for computing remaining fatigue or corrosion-fatigue life is based crack growth andengineering fracture mechanics. A crack-like flaw of some initial size (ao) is identified by nondestructiveexamination (NDE) or assumed to exist, and the number of cycles required for it to grow to a critical finalsize (af) is computed. The value of afis either the crack size that will produce a leak or the crack size thatis predicted to cause a sudden failure. From the results of laboratory testing, the cyclic crack-growth rate

(da/dN) is characterized as a function of the range of the stress intensity factor (K), ratio of minimum tomaximum cyclic load (R), and frequency (f):

da/dN = f(K, R, f) (8)

Fatigue cracking above the threshold region of very low growth rates, is typically characterized using thewell-known Paris Law:

da/dN = C (K)n (9)

Figure 14 shows a plot of typical Paris-Law curves for carbon steel and pipeline steel welds. The curvefor carbon steel is an upper bound relationship from API RP 579

2, it also provides a reasonable upper

bound for the data on welds.

Equation (8) or (9) is integrated from ao to af to compute remaining fatigue crack-growth life. NDE isused to measure aoor the value of ao is assumed to be the largest flaw that may have been missed by

NDE. Fatigue analysis can be used to predict the areas of largest expected fatigue damage in acomponent; NDE can then focus on these areas. Also, the accuracy of the NDE directly influences the

predicted remaining fatigue crack-growth life. Therefore, integration of fatigue analysis and NDE helpsoptimize the assessment of a component.

Creep

High-temperature equipment is subject to creep damage during long-term service. As with fatigue,remaining creep life can be evaluated in terms of failure life or in terms of crack-growth life. The formerapproach is most often employed because creep damage is typically associated with the formation ofmultiple voids and microcracks, rather than a single dominant crack that can be analyzed using fracturemechanics.


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1 0-7

1 0-6

1 0-5

1 0-4

1 0-3

1 0-2

1 00

1 01

1 02

C a r b o n S t e e l ( R e f e r e n c e 2 )

A P I X 5 2 W e l d 1

A P I X 5 2 W e l d 2

A P I X 5 2 W e l d 3

da/dN

(mm/cycle)

K ( MP a - m0. 5

)

Figure 14. Fatigue Crack Growth Curves for Pipeline Steels in Air at Room Temperature.

Creep-rupture life (tr) is a function of stress () and absolute temperature (Ta) and is typically measured

by testing tensile specimens in the laboratory. Values of trdecrease as T and increase. This behavior ischaracterized using parametric relationships, such as the well-known Larson-Miller parameter (LMP):

LMP = Ta(CLM+tr) (10)

where CLM is the Larson-Miller constant. Values of CLMare determined by curve fitting the test results

and are typically in the range of 15 to 25 for engineering alloys. Figure 15 shows a typical plot of

versus LMP. It is for the HK-40 alloy is from API STD 530.

11

Both average and minimum rupturestrength curves for Ta in degrees K are shown in Figure 15. If a component has been in service atconstant temperature and stress, then the remaining creep-rupture life is the total life computed usingEquation (10) minus the time of prior service.

The stress and temperature are usually not constant during the operating life of high-temperatureequipment. For this reason, the well-known Robinson Rule is employed to cumulative compute creepdamage (Dc), as follows:


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1

10

1 0 0

18 20 22 24 26

Average Rupture St rength

Min imum Rupture S t rength

Stress,

MPa

Larson-Mi l ler Parameter (C = 15)

HK-40 f rom F igure 3Q of API RP 530

Figure 15. Larson-Miller Parameter Curves for the HK-40 Alloy.11

Dc=(t/tr) (11)

where t and tr are the time interval and corresponding time to failure at each stress level. Failure ispredicted to occur when Dc = 1. Remaining creep damage (Dcr) is computed by subtracting the creepdamage from prior service (Dcp):

Dcr= 1 - Dcp (12)

Then, the anticipated operating history and Dcrare used to compute remaining creep life. The predictionof remaining creep life can be greatly improved when Dcpis measured by NDE because there is usually ahigh degree of uncertainty in defining the past operating conditions needed to compute Dcp. Theuncertainty in forecasting future operations is the same for either approach. Therefore, the integrated

NDE and analytical approach is best for predicting remaining creep life.

Remaining creep life can be based crack growth and engineering fracture mechanics when the creepdamage is localized at the region of cracking, which is typically a notch or stress concentration. Again,an initial crack-like flaw is identified by nondestructive examination (NDE) or assumed to exist. Thetime required for the flaw to grow to a critical final size that results in a leak or rupture is then computed.From the results of laboratory testing, the creep crack-growth rate (da/dt) is characterized as a function ofthe C* integral, as follows:


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da/dt = D (C*)d (13)

where D and d are material constants that may be a function of temperature. However, for manymaterials, these values are independent of temperature.

Equation (13) is integrated from aoto afto compute remaining creep crack-growth life. As with fatigue,NDE is used to measure aoor the value of aois assumed to be the largest flaw that may have been missedby NDE. Stress analysis can be used to predict the areas of largest expected creep damage in acomponent; NDE can then focus on these areas. The accuracy of the NDE directly influences the

predicted remaining creep crack-growth life. Therefore, integration of the creep analysis and NDE helpsoptimize the assessment of a component.

EXAMPLE APPLICATIONS

Two example applications are reviewed. The first deals with a pressure vessel,12

while the second dealswith an ammonia reformer furnace tube.

Corrosion of a Pressure Vessel

A region of corrosion is discovered in the wall of a pressure vessel. The corrosion is in base metal anddoes not cross any welds. The following parameters apply:

The maximum operating pressure (p) is 250 psig at 100 F.

The inside diameter (ID) is 75 in., so the inside radius (Ri) is 37.5 in.

The wall thickness is 0.541 in., where there is no corrosion loss.

The 10-year future corrosion allowance (FCA) is 0.050 in.

The material is ASME SA516 Grade 70 steel; from Section II, Part D of the ASME Boilerand Pressure Vessel Code, the allowable stress (Sa) is 17,500 psi.

The weld efficiency factor (E) is 0.85.

The minimum distance of the flaw from a major structural discontinuity (Lmsd) is measuredand found to be 32 in.

Wall thickness in the corroded region is measured at points on a 25-mm (1-in.) grid, as illustrated inFigure 16. This is done in anticipation of the need for remaining life assessment of the vessel. Themeasurements are then plotted, as illustrated in Figure 17, to determine the critical thickness profilesthrough minimum values in the longitudinal (M) and circumferential (C) directions, as follows:

Longitudinal: 0.541, 0.502, 0.403, 0.495, and 0.541 in.Circumferential: 0.541, 0.483, 0.403, 0.511, and 0.541 in.

The minimum measured thickness (tmm) is thus 0.403 in. The tasks are to determine if the vessel isacceptable for continued operation based on a API RP 579

2 Level 1 assessment and to estimate the

remaining corrosion life of the vessel.

First, use the standard code formulas to compute minimum required wall thickness values as follows:

in.0.5410.6(250)17500(1.0)

)05.0250(37.5

0.6pES

FCA)p(Rt

a

icmin =

+=

+= (14a)


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Figure 16. Illustration of Inspection Planes for Measuring Critical Thickness Profiles.

Figure 17. Illustration of Critical Thickness Profiles Through Planes of Maximum Metal Loss.


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.in267.0)250(4.0)0.1)(17500(2

)05.05.37(250

p4.0ES2

)FCAR(pt

a

iLmin =

+

+=

+

+= (14b)

Even though E = 0.85 for the vessel, E = 1.0 is used in the above calculations because the corroded regionis in base metal. The minimum wall thickness (tmin) is 0.541 in.

Compute the remaining thickness ratio (Rt), as follows:

Rt= (tmm FCA)/tmin= (0.403 0.05)/0.541 = 0.652 (15)

Obtain the factor Q from Table 4.4 of API RP 579 using Rt= 0.652 and RSFa= 0.9, or compute Q asfollows:

Q = 1.123[{(1 - Rt)/(1 - Rt/ RSFa)}2- 1]

0.5 or (16a)

Q = 1.123[{(1 - 0.652)/(1 - 0.652/0.9)}2- 1]

0.5= 0.866 (16b)

Next, determine the length for thickness averaging (L), as follows:

L = Q(ID(tmin))0.5

= 0.866(75(0.541))0.5

= 5.51 in. (17)

Now, determine the longitudinal extent of the corrosion flaw (s), as illustrated in Figure 17. In this case t= tmin= 0.541 in., so s = 4.00 in., which is the actual length of the region of corrosion. When t > tmin, swill be less than the actual length of the corroded region, and the value of s has to be determined as shownin Figure 17. Since the minimum required thickness for longitudinal stress = 0.267 < 0.403 0.05 =0.353, evaluating the circumferential extent of the flaw (c) is not required.

Because s < L (4.00 < 5.51), check the criteria of Paragraph 5.4.2.2.d of API RP 579:

Rt= 0.652 > 0.20 Okaytmm- FCA = 0.353 > 0.10 Okay

1.8(ID tmin)0.5

= 11.5 < Lmsd= 32 Okay

In addition, check the criterion of Paragraph 5.4.2.2.g of API RP 579 for c. Using Figure 5.7 of API RP579, Rt= 0.652 > 0.20 at c/D = 0.05 Okay.

Based on the results of this Level 1 assessment per API RP 579, the corrosion flaw is found to beacceptable. The remaining corrosion life is implicit in the 10-year FCA for this example. Thus, theminimum remaining corrosion life is 10 years. The calculations directly used inspection data that weretaken for the purpose of fitness for service and remaining life assessment.

Ammonia Reformer Furnace Tube

This example involves the creep life assessment of HK-40 alloy catalyst tubes used in an ammoniareformer furnace. The design parameters for these tubes are listed below:

Outside diameter (Do) = 4.10 in.

Minimum wall thickness (tmin) = 0.670 in.


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Maximum tube metal temperature (Tmax) = 1700 F

Maximum internal pressure (pmax) = 500 psig

Design life = 100,000 hours

After 52,000 hours of service, a Level 1 analysis gives a remaining tube life of 48,000 hours.

A review of operating records revealed that the maximum temperature and pressure had been 1675 F and485 psig, respectively. For the Level 2 analysis, the mean-diameter formula was used to calculate the

maximum hoop stress () for operating conditions as follows:

= pmax(Do/tmin- 1)/2 = 485 (4.10/0.670 - 1)/2 = 1240 psi (18)

For the minimum creep-rupture strength of the HK-40 alloy at 1240 psi, API STD 5306gives a Larson-

Miller parameter value of 43,340 for Tain degrees R (24,078 x 1.8 in Figure 15). The minimum LMP forthe HK-40 alloy is given by the following expression:

LMPmin= (T + 460)(15 + log Lmin) (19)

where T is the temperature in degrees F and Lmin is the minimum life in hours. Setting Equation (19)equal to 43,340 with T equal to 1675 F and solving for Ldgives a design life of approximately 199,000hours. Therefore, the remaining life estimate for Level 2 analysis was 199,000 - 52,000 = 147,000 hours.

The furnace operator recognized that cyclic operation can significantly affect tube life and that the Level1 and 2 analyses did not directly account for the effects of cyclic operation. For this reason, a moredetailed Level 3 analysis was performed using the WinTUBE computer program. The following

parameters were used to model the cyclic operation of the furnace tubes:

Outside diameter = 4.10 in.

Wall thickness = 0.670 in.

Outside tube metal temperature = 1675 F.

Inside tube metal temperature = 1610 F.

Internal pressure = 485 psig.

A H-Scaninspection of the tube revealed that it was Grade 2 material.

A repeated operating block consisting of a cold start followed by 1500 hours of steady operation,a 4-hour hot trip, 1200 hours of steady operation, a 2-hour hot trip, 1100 hours of steadyoperation, and an 8-hour cold shutdown.

The block of cyclic operation was patterned to model a typical repetitive segment of past operations. Itwas assumed that future operations would be similar to the past ones.

Before computing the expected creep life for simulated operations, the creep life was computed for steady

operations using the computer program. In this case, the total minimum life was calculated to be 268,000hours, which is 35% greater than the value of 199,000 hours computed in the Level 2 analysis. The totalminimum life for simulated operations then was computed to be 66,400 hours, which gave a Level 3estimated minimum remaining life of 66,400 - 52,000 = 14,400 hours. In this case, the more accurateLevel 3 analysis predicted a much lower remaining life than the Level 1 and 2 analyses because operatingconditions were modeled more closely.

The operator was uncertain that the simulated operating conditions actually modeled past operations. For

this reason, the furnace tube was inspected using the H-Scantechniques discussed previously and was


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found to be Grade 2 material. Based on this material condition and the same operating conditions, theminimum remaining tube life was computed to be 50,330 hours, which is approximately 3.5 times thevalue computed by modeling both past and future operations. Since using inspection to characterize tubecondition is usually more accurate than the modeling, the same combined inspection-assessment

procedure was use to evaluate all the tubes in the furnace. In addition to the measured material grade,measured tube diameter and wall thickness was used for each calculation. The process was automated by

modifying the inspection software to produce a data file for the creep life analysis. The operator usedthese results to schedule re-tubing of the furnace in a timely fashion and thus to avoid a costlyunscheduled shutdowns for tube replacements.

CONCLUSIONS

Various NDE methods can be used to characterize the condition of material in operating equipment.They are especially useful for quantifying flaws. The information on flaw type, size, and location thencan be used in engineering fracture mechanics models to evaluate equipment integrity and remaining life.Multiple parameter inspection techniques help to accurately quantify material damage, such as creepdamage in cast heat-resistant alloys. Optimal results are obtained when inspection and remaining life

assessment are integrated and coordinated. Methods of predicting remaining corrosion, SCC, fatigue, andcreep life were reviewed. Two examples of combining inspection and life prediction were presented toillustrate the benefits of the integrated inspection-life assessment approach.

REFERENCES

1. Managing System Integrity of Hazardous Liquid Pipelines, API Standard 1160, First Edition,American Petroleum Institute, Washington, D.C., November 2001.

2. Fitness-For-Service, API Recommended Practice 579, First Edition, American Petroleum Institute,Washington, D.C., January 2000.

3. Jaske, C. E., and Beavers, J. A., Effect of Corrosion and Stress-Corrosion Cracking on Pipe Integrity

and Remaining Life, Proceedings of the Second International Symposium on the MechanicalIntegrity of Process Piping, MTI Publication No. 48, Materials Technology Institute of the ChemicalProcess Industries, Inc., St. Louis, pp. 287-297, 1996.

4. Jaske, C. E., Beavers, J. A., and Harle, B. A., Effect of Stress Corrosion Cracking on Integrity andRemaining Life of Natural Gas Pipelines, Paper No. 255, Corrosion 96, NACE International,Houston, 1996.

5. Jaske, C. E., and Beavers, J. A., Fitness-For-Service Evaluation of Pipelines in Ground-WaterEnvironments, Paper No. 12, Proceedings for the PRCI/EPRG 11th Biennial Joint Technical

Meeting on Line Pipe Research, Arlington, VA, 1997.6. ASME International, Section VIII - Rules for Construction of Pressure Vessels Division 2 -

Alternative Rules, Boiler and Pressure Vessel Code, New York, 1998.7. Suresh, S.,Fatigue of Materials, Cambridge University Press, Cambridge, UK, 1998.

8. ASTM Designation: E 1049 - 85, Standard Practices for Cycle Counting in Fatigue Analysis,American Society for Testing and Materials, West Conshohocken, PA, 1995.

9. Fatigue Design Handbook, AE-10, Second Edition, The Society of Automotive Engineers,Warrendale, PA, 1988.

10. Jaske, C. E., Vieth, P. H., and Beavers, J. A., Assessment of Crack-Like Flaws in Pipelines, PaperNo. 02089, Corrosion 2002, NACE International, Houston, 2002.

11. Calculation of Heater-Tube Thickness in Petroleum Refineries, API STD 530, American PetroleumInstitute, Washington, D.C., 1996.


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12. Jaske, C. E., Assuring the Safety of Ammonia Plant Vessels and Piping Using API RP 579, Paper1C, Safety in Ammonia Plants and Related Facilities Symposium, Vol. 43, American Institute ofChemical Engineers, New York, 2002.

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