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Inspection of Power Plant Headers Utilizing AcousticEmission Monitoring

Bryan C. Morgan∗∗∗∗ and Richard Tilley∗∗∗∗∗∗∗∗

∗ Duke Engineering & Services, 5000 Executive Parkway, San Ramon CA 94583, USA∗∗ Electric Power Research Institute, 1300 WT Harris Boulevard, Charlotte, NC 28262, USA

Fossil power plant high energy, high temperature steam headers have been found to be susceptible tothermal fatigue assisted creep degradation. These mechanisms initiate and grow cracks in chromemolybdenum headers, from the bore hole edges and stub tube-to-header welds. Linking up of multiplecracks can lead to explosive expulsion of tubes and severe shorting of the header life. In order to extendthe header life and operate safely, a better understanding of crack growth that may occur during specificplant operating conditions is needed. With that understanding, harsh operating conditions that may becausing excessive crack propagation and header damage can be curtailed. Acoustic emission monitoringof headers was performed to assist in identifying operating conditions that lead to header damage. ThisElectric Power Research Institute (EPRI) sponsored program found acoustic emission activity levelscorrelated to identified crack growth and analytically calculated stresses. Utilizing these results, draftEPRI guidelines have been developed to aid electric utilities in performing acoustic emission monitoringon superheater headers.

Keywords: acoustic emission, cracks, creep, finite element method, stress

Introduction

High energy, high temperature chrome-molybdenum(2-1/4 Cr - 1 Mo and 1-1/4 Cr - ½ Mo) superheateroutlet headers in fossil power plants have been foundto be susceptible to creep degradation, ascompounded by thermal fatigue [1][2]. This hasexhibited itself in two ways; ligament crackingbetween the stub tube bore holes, and stub tube-to-header weld cracking, as shown by Figure 1.

Tube-to-header weld cracks typically initiate either atthe weld root or at the weld toe on the outsidesurface. They grow radial and circumferential untilcompletely breaking the tube from the header.Ligament cracks initiate from thermal cracks thatradiate from tube bore holes. Link up of adjacenttube radial cracks creates the ligament crack.Linking up of multiple ligament cracks caneventually lead to explosive expulsion of tubes andsevere shorting of the header life. In order to extendthe header life and operate safely, a betterunderstanding of crack growth that occurs duringspecific plant operating conditions is needed. Withthat understanding, harsh operating conditions thatmay be causing excessive crack propagation andheader damage can be curtailed.

Figure 1 Ligament and Stub Tube-to-Header WeldCracking

A program was put together to develop AcousticEmission (AE) monitoring techniques for detectionof cracking in fossil plant high energy systemssusceptible to creep damage. This cooperativeresearch was jointly supported and funded by theElectric Power Research Institute (EPRI) and PacificGas & Electric (PG&E) company, under EPRI’sresearch program RP1893-20. The programinvestigated use of AE monitoring for detection ofcracks in seam-welded Hot Reheat Lines [1], anddetection of header ligament and stub tube-to-header

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weld cracks. This paper summarizes results for theheader program. The elements of the program werefield monitoring of a header during typical plantoperation and characterization of crack signalfeatures in the laboratory. Correlation betweenacoustic emission activity levels, identified crackgrowth, and analytically calculated stresses werefound to exist. Utilizing these results, draft EPRIguidelines have been published to aid electric utilitiesin performing acoustic emission monitoring onsuperheater headers [3].

Crack Locations

Stub tube-to-header welds contain inclusions, such aslack of penetration and lack of fusion. Cracks caninitiate at the weld root, where the inclusionstypically exist. They create stress concentrations,which are strained by material creep. Thestress/strain concentration is compounded by thepresence of additional stress mechanisms, such asresidual stresses from tube fit-up to the header andthermal fatigue. The resulting condition creates andgrows cracks radially and circumferentially until acomplete break of the tube from the header occurs.This is both a safety issue and a cause of plant downtime. Detection of the cracking can be difficult, as itmay initiate at the weld root, where the socket weldgeometry complicates inspection by ultrasonics orradiography. Typically, there is a fit-up gap betweenthe tube and header's machined socket. The gapending at the weld root. An ultrasonic signal hasdifficulty discriminating between the gap's end, andits extension by either lack of penetration or cracking.

Stub tube bore hole radial cracks are commonoccurrences and, by themselves, are not a majorconcern. They are generally self-limiting, but theirpotential link-up into ligament cracks createsconcerns. As ligament cracks, they may grow into athrough-wall crack with potential for major structuraldamage to the header. Importantly, ligament cracksin creep damaged thick-walled chrome-molybdenumheaders have not be demonstrated to be repairable.The ultimate solution is replacement of the headerwith substantial replacement cost and plant downtime. Another possible solution is curtailment ofplant operation to minimize the rate of temperaturechange during plant load cycling and plant heat-up/cool-downs.

As thermal fatigue is the driving mechanism, theidentification of acceptable temperature ramp rates isan important issue. Calculation of header stresses byfinite element analysis and fracture mechanics crack

growth rates is one method. Results from suchcalculations would define limits on plant operation.

Real-time feedback on the actual crack growth ratesthat occur during plant operation can be of greatvalue for comparison to assumed crack growth rates.Some evidence suggests that these ligament crackscan become self-arresting or experience reducedgrowth under many of the plant operating conditions[4]. The difficulty is to identify changes in crackgrowth rate. Sizing individual crack depth and lengthextension can be difficult due to the numerous radialcracks growing from the bore holes. Figure 2 showsa wet fluorescent particle exam of a header insidediameter (ID) at a tube row.

Figure 2 Radial Cracks from Tube Bore Holes

A possible method of verifying that crack growth hasself-arrested or is limited to specific plant operatingmodes is by AE monitoring. The basic approach is toattach, with waveguides, AE sensors to the headernear selected known crack locations, and to monitorAE events during a typical range of plant operatingconditions. The intent is to identify which, if any, ofthe operating conditions are producing crack growth.

Field Monitoring of High Energy Header

The primary superheater outlet header at PG&E’sPittsburg Unit 6 power plant in Pittsburg Californiawas monitored for crack growth from September1991 through March 1992. The header had

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substantial ligament cracks with evidence of creepand creep-fatigue damage. The largest crack being2.9 cm in depth and connecting two adjacent tubebore holes. At the start of monitoring, the header wasestimated to have a residual life of less than 5 years,based on this initial crack size and continuedoperation at design temperature limits.

The decision was made to restrict temperature ramprates, while monitoring header and tube temperatures.Frequent ultrasonic inspections at critical cracklocations were conducted to monitor crack growth.The intent was to replace the header in the nearfuture, but in the interim provide augmentedinspection and monitoring. This presented anexcellent opportunity to determine the degree ofcorrelation between AE activity levels, change inultrasonic size, and expected crack growth.

Acoustic emission sensors were located near thedeepest crack locations. Figure 3 shows themonitoring equipment setup. Waveguides werewelded to the header outside surface surrounding thecrack locations. The resonant sensor frequency wasselected to high-pass the steam flow noise.Thermocouples were mounted on both the tubes andon the header surface for recording the temperaturetime histories.

Figure 3 AE and Thermocouple Monitoring Setup atPittsburg Power Plant Unit 6

Results showed that the AE generation was related toheader/tube temperature changes. This is shown byFigure 4 for a two-day period when the plant load(Net Megawatts) was cycled. The figure shows thehighest AE rate correlated to the greatest temperaturechanges. The temperatures were measured at twooutside diameter (OD) skin locations - on the headerand at Tube No. 40B.

Figure 4 AE Activity Related to Plant Load Change

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Monitoring regions of a header section, as shown byFigure 5 separated the located AE events.Comparing AE waveform arrival times at the sixwaveguide/sensor locations defined these regions(AE monitoring zones). Zones were created using azone calibration system built into the AE monitoringequipment (B&W model AET-5500). AE signalswere generated by lead breaks and pulsers on aheader section – on the OD, ID and inside tubes. Byapplying many signals throughout a region, a look-uptable of wave arrival times (detected at least at threesensors) was generated. The range of arrival timesfor each sensor, and the order in which they arrivedprovides the boundary of acceptable AE events thatwill locate within the zone. The lookup tablesprovide a method of filtering AE hits. If the arrivaltimes and order of arrival fall within a boundary theAE hit is defined as a located event within the relatedzone. If not, then the AE hit is either noise ororiginates outside all zones. Typically, if only asingle sensor detected an AE hit, it was assumed tobe noise and rejected.

Two small regions (1 & 2) were specifically designedto identify events occurring on the header ID near orat the bore holes. Ligament cracks existed directlybelow the six AE waveguide attachment points (TubeRows No. 39, 40, & 41). The other regions (3, 4 &5) provided coverage mainly for the header OD.Figure 6 shows AE activity by monitoring region -the majority of events were located in Regions 3, 4,and 5. This corresponds to the outside diameter neartube-to-header welds. Very little AE appeared to beoriginating from the header ID near the existingligament cracks.

Figure 5 Schematic of the monitoring regions of a headersection.

Figure 6 AE Activity by Header Region

Inspections were performed before and aftermonitoring. Prior to monitoring, ultrasonic sizing ofthe ligament cracks showed crack depths rangingfrom 2.0 to 2.9 cm. After monitoring in March 1992,the cracks were again ultrasonically sized. They didnot show any measurable increase in ligament crackdepths during the monitoring period.

Exams of the stub tube-to-header weld areasindicated significant creep damage. Magneticparticle exams in 1990, identified weld cracks ontubes 39B and 40B. Tube 40C had completely failedand was ejected. Replications of the tube 39C basemetal, heat affected zone (HAZ), and weld metalshowed aligned creep voids (3 of 5 on damage scale).The damage scale being: 1– isolated cavities, 2-orientated cavities, 3– cavity linkage, 4-microcracking, 5- macrocracks. Tube 40B showedisolated creep voids (2 on damage scale) in theheader base metal and weld to micro creep cracks (4on damage scale) in the HAZ. The 39B, 40B and40C welds were repaired prior to the AE monitoring.Again in 1992 after the header was replaced, spotreplicas were performed. Tube 40C, which did notshow any damage in 1990, showed isolated creepvoids in 1992.

From these correlations and other evidence, itappears that the AE events were associated with stubtube-to-header welds and not ligament cracks.Overall, these results are not surprising. Thecurtailed operation likely reduced or stoppedligament crack growth but failed to stop stub tube-to-header weld damage.

Acoustic Emission Laboratory Testing

The superheater outlet header was removed andreplaced in March 1992. Three header sections werecut out and brought into the laboratory for testing.

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The section with the largest ligament cracks washeated to grow, and AE monitor, cracks in acontrolled environment. Figure 7 shows the headersection in the laboratory, with mounted waveguidesand AE sensors. Heat induction coils were attachedto a pipe and positioned inside the header for heating.Using controlled temperature ramp rates, the coilsheated the header section to 480 °C at atmosphericpressure. Thermocouples were attached on theoutside and inside header surfaces, as well as in thetube bore holes. This allowed for inside-to-outsidediameter and adjacent tube temperature differentialsto be measured.

The first heating cycle used a relatively slowtemperature ramp rate that peaked at 83°C/hr on boththe heat-up and cool-down. It produced maximumtemperature differentials (inside to outside surface) of+28 °C for heating and -11 °C for cooling. Despitethe fact that tube holes were blocked and no airflowed through them, the maximum temperaturedifferential between adjacent tube surfaces was foundto be 28 °C. These ramp rates and temperaturedifferentials greatly exceeded those monitored in thefield testing. Using the higher rates insured thatligament cracks would be grown during the labtesting.

Figure 7 Laboratory AE Monitoring of Header Section

After the first heating cycle, a wet fluorescent particleexam was conducted on the inner surface. Holes 36A& 36B had new radial cracks initiated and existingones grown. Crack extensions were also observed fortube row 39. Two large radial cracks on each side oftube hole 39B had widened and lengthened. The

ligament crack between holes 39A & 39B hadwidened, becoming more visible to this surface exam.

AE was monitored during heating and cooling,recording waveforms and signal features. Figure 8top plot shows AE events generated during the firstheating. These AE events originated at the headerregion between tubes 39A & 39B. This regionincludes the inside surface and part way up the tubebore holes. It specifically excludes the headeroutside surface. The bottom plot shows inside surfacetemperature and temperature differential betweeninside and outside surfaces. It shows a highertemperature inside surface on the heat-up. Aftertemperature peaked and cooling begun, the insidesurface is placed in tension. If the tensile stress wassufficiently high, this is the likely point where cracksshould open and extend. The top plot shows thecorresponding AE event rate. Most AE wasgenerated on cooling, stopping completely at about650 minutes, where the temperature differentialreaches its maximum negative value (i.e., whereinside surface is coolest compared to outsidesurface).

These are results for just one location. Otherlocations with known crack growth wereinvestigated. Their evaluation shows that mostlocatable AE events were generated from headerinside diameter locations that had visible evidence ofcrack extension. In addition, little acoustical noisewas generated from other possible sources, such asinside surface oxide fracture. As such, it is believedthat the generated AE shown in Figure 8 wasassociated with crack movement and extension.

Figure 8 AE Activity from Laboratory Heating for Tubes39A & 39B

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Stress Analysis and Comparison to AEActivity

The thermocouple measurements made in the field-testing were used as input in a finite element modelto calculate principal stress time-histories. The stresstime-histories were used for comparison to the AEactivity time periods - determining if periods of highstress matched periods of increased AE activity.

Figure 9 shows the finite element model. It uses theCOSMOS finite element software to determine tri-axial stress time histories[5]. Figure 10 shows anexample of high stresses at a particular time point.They were generated on a thick wall structure in theligament areas. The figure shows tensile stresses onthe OD and compressive on the ID for stress in thepipe axial direction. Typically stress is tensile on theID and compressive OD, but when a slug of coolersteam enters the header it lowers ID temperatures tonear or below OD temperatures. This is themaximum stress condition.

Figure 9 Three-Dimensional Finite Element Model of aheader section with a row of three tubes. As only thetemperature difference between tubes and header are ofinterest, the bottom half of the header is not included in themodel. The mesh is designed specifically for calculatingstresses from temperatures in the ligament and tube-to-header weld areas.

Figure 11 plots the stress time history to showcorrelation between AE activity during a 12 hourperiod of the field monitoring. The top curveindicates the temperature difference between tubeand header. Essentially the tube temperature is thesame as the steam temperature, as the tubes have thinwalls. Therefore the temperature difference is thedifference across the header thickness. When thecooler steam slug occurs, the difference is negative.The bottom curve provides the pipe axial stress for a

finite element node in the ligament area. Note thehigh stress points occur at the negative temperaturedifferences. The middle curve is the AE activity forevents located by the zones. It shows that activityoccurs for the negative temperature difference. Italso shows a lack of activity at other times.

Figure 10 Thermal stresses generated by thermocoupletemperature input for the Finite Element Model. This figureshows the change in stress from tensile on the OD tocompressive on the ID.

Figure 11 Correlation of AE Activity and Header ThermalStress during a 12 hour Period

Figures 12 and 13 focus on the time periods in Figure11 with AE activity for 3 hour periods. They showAE activity matching very closely in time with thestress peaks. Some variation is expected in thematch, as the finite element time step is relativelycourse. Thus the actual stress peak may vary by 10minutes from calculated stress peak. These stress toAE activity correlations suggest that a thresholdeffect is occurring. Below certain stress levels, littleAE activity is evident. This was very evident duringthe field monitoring. While constant AE activity

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occured through out the 6 month monitoring, therewere time periods when no zone located AEoccurred. Thus supporting the expectation that theunlocated AE hits were related to noise and not crackextension or initiation. This stress to AE correlationalso suggests an on-line monitoring method ofindicating when material damage is occurring. Theappearance of locatable AE events suggests thatdamage is occurring, and the plant operatingconditions should be abated. Typically increasing thetime to change from one plant power level to anotherprovides the abatement by reducing the temperatureramp rate and correspondingly stresses. It is a trade-off between preventing damage and reducing theplant's ability to quickly change power levels to meetdemand.

Figure 12 First Period of AE Activity (detail from Figure 11)

Figure 13 Second Period of AE Activity (detail from Figure11)

Monitoring Guidelines Development

Using the field and lab tests, draft guidelines havebeen developed to use AE monitoring for crackgrowth detection in high energy headers [3].Guidance is provided for two uses: detection ofcracking over the entire header length, and

monitoring known cracks in a localized header areafor growth during plant operation. Both of these useswould entail short term monitoring (1 to 2 weeks).The intent is to provide periodic condition assessmentto determine if a particular plant operating mode iscausing material damage. The monitoring interval isdetermined by crack growth rates, header residuallife, and plant operating schedule.

In general, the guidelines follow the testing methoddescribed in this paper for monitoring known cracks.For example, a header section with the most criticalcracking is chosen for AE monitoring. Thewaveguide/sensor array and location algorithm areselected to differentiate AE events near the knowncracks from others away from the cracks. As well as,filtering out AE hits not locatable and likelyassociated with flow or other types of backgroundnoise. The plant operating history is reviewed todetermine enveloping operating conditions. Dividingthem into plant start-up, shut-down, on-line steadystate, and on-line load cycling to providerepresentative stressing of each type.Thermocouples are added as needed to header tubesand the header OD, to identify temperaturedifferentials and correlation to the operating modes.Temperatures, pressures and AE are monitoredtogether during periods representative of the differentoperating modes. Extremes in temperaturedifferentials and ramp rates are then correlated to theAE generated for identification of operating modes todetermine levels above which substantial AE activityis generated. These levels can then be utilized toprovide warnings of possible damage.

Acknowledgments

This work was supported by the Electric PowerResearch Institute. The authors would like toacknowledge the assistance of Dr. ManuchehrShirmohamadi of Material Integrity Solutions fordevelopment of the finite element model and stressanalysis. We are indebted to Chuck Foster of PacificGas and Electric Company for his support anddirection.

References

1. Morgan, B.C., Foster, C.L. 'Acoustic EmissionMonitoring of High-Energy Steam PipingVolume 1: Acoustic Emission MonitoringGuidelines for Hot Reheat Piping', ElectricPower Research Institute Report, TR-105265-V1, 1995.

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2. Coulter, J.E., Stevens, D.M., Gehl, S.,Scheibel, J.R., 'Acoustic Emission Monitoringof Fossil-Fuel Power Plants', MaterialsEvaluation, 46, 1988, pp 230-237.

3. Morgan, B.C., Rodgers, J.M., 'AcousticEmission Monitoring of High-Energy HeadersVolume 1: Acoustic Emission MonitoringGuidelines for Superheater Outlet Headers',Electric Power Research Institute Report, TR-107839-V1, 1997.

4. Grunloh, H.J., et al, 'Life Assessment of BoilerPressure Parts, Volume 3: Heavy Section CrackInitiation and Propagation', Electric PowerResearch Institute Report, TR-103377-V3, 1993,Sections 2 & 7.

5. Shirmohamadi, M., 'Developing Stress-TimeHistories at Selected Locations of a BoilerHeader Field and Lab Testing Events for Supportof Acoustic Emission Research', MaterialIntegrity Solutions Inc. Report, Project EPRI-96-1, 1996.