Comparison of Visual, Eddy Current, Ultrasonic and Magnetic ...

BOILER

T his paper details actual inspection data and extracts from reportstaken over a nine-month period of inspections of water tube boilers.The inspections were carried out by qualified technicians andinspectors using a combination of internal visual testing (VT) using

remote cameras, eddy current testing, ultrasonic testing (UT) and magneticparticle testing (MT).

Background and Inspection PhasesIn May 2008, the failure of a wall tube on a water tube boiler triggered aninspection to ascertain the failure mechanism and to investigate for furtherindications that could lead to rupture. The initial inspections at the failure site(phase 1) necessitated the removal of five tubes to a workshop so that athorough internal and external analysis could be carried out (phase 2). Thisanalysis would then give vital information to produce the most effective tech-niques for onsite, in situ inspections (phase 3) to search for further areas thatmay have early onset discontinuities or display similar properties to those atthe failure site. All work was carried out manually; no automated or roboticapplications were utilized.

Codes, Procedures and QualificationsAll the nondestructive testing (NDT) inspections were carried in accordancewith relevant codes, procedures and qualifications. This includes ASME Boilerand Pressure Vessel Codes, Section V, Nondestructive Examination, includingArticle 1: General Requirements Non-Destructive Methods of Examination;

wx ME FEATURE

M A Y 2 0 1 2 • M A T E R I A L S E V A L U A T I O N 509

Comparison of Visual, EddyCurrent, Ultrasonic andMagnetic Particle TestingTechniques for Boiler TubeInspectionsby Michael Leonard Hodgson

TUBES

Photo credit: Michael Hodgsonof Oceaneering InternationalServices Asset Integrity

From Materials Evaluation, Vol. 70, No. 5, pp: 508–519.Copyright © 2012 The American Society for Nondestructive Testing, Inc.

ME FEATURE wx boiler tube inspection

510 M A T E R I A L S E V A L U A T I O N • M A Y 2 0 1 2

Article 9: Visual Examination; Article 7: MagneticParticle Examination; Article 6: Liquid PenetrantExamination; Article 8: Eddy Current Examination ofTubular Products; Article 4: Ultrasonic ExaminationMethod for Welds; and Article 5: UltrasonicExamination Method for Materials (ASME, 2010). Theinspections were also carried out in accordance withEN 473, Non-destructive Testing – Qualification andCertification of NDT Personnel – General Principles andRecommended Practice No. SNT-TC-1A: PersonnelQualification and Certification in NondestructiveTesting (2006) (ASNT, 2006; BS, 2008). In addition,they complied with company approved operatingprocedures, as well as with the client’s acceptancecriteria.

Phase 1 (Initial Inspection of the Failed Tubeand Surrounding Area In Situ)When first called to the failure site, it is absolutelyessential to capture as much information as possibleand record the data in a logical manner. The beststrategy is to plan ahead and carefully choose theorder of carrying out techniques so that the testing ismost efficient, sensitive and timely. An example of thiswould be to choose UT last because the couplantwould interfere with any surface crack detection tech-niques adopted. Phase 1 was carried out as follows.

Pre-clean the Area to Eliminate Extraneous MatterPre-cleaning was done with rotary wire brushes to takeoff loose rust deposits, scale and insulation that hadbecome adhered to the outer pipe wall. The rotary wirebrushes strip the surface of the steel rather than chipor impact the surface. Impact tools should not be usedto remove the scale or deposits because such toolswill cause the steel to peen over small discontinuities.Care was taken not to remove any evidence along thefracture face. This must be preserved so that the clientcan send the fracture faces to a metallurgical lab forfurther failure analysis, and this would be lost ifgrinding or excessive cleaning was applied to thefracture edge.

Photograph the Failure Site and Adjacent Area

It is vital that photographs are taken before the appli-cation of any dyes, contrast paints or couplants so thata clean record of the failed area can be stored forfuture reference. Photography can be done withstandard cameras, borescopes, flexible pipelinecameras or video recordings. This can often prove tobe a very important process as human memory canfail or different perspectives can offer varying verbalreports of what was actually seen. The photographsmust then be catalogued, dated and accessible to theclient.

Conduct Close Visual Testing for Telltale SignsUsing magnifying aids and good lighting, this part ofthe testing gives a more intimate approach and helpsguide the inspection towards a more tailored result.This is because smaller detail is noted that can oftengo unnoticed if only cursory VT is carried out. Duringthis part of the inspection, internal indications wereobserved through the combination of low magnifica-tion and small bright torches, which cast shadowsfrom the edge of the internal discontinuity. This thencalled for further NDT down the bore of the failed tubeto verify this observation.

Conduct Internal Liquid Penetrant TestingOnce there was visual evidence of internal discontinu-ities, the application of PT down the bore of the failedtube was thought to be the most advantageousapproach since there was no way of applying MT tothe bore of the failed tube due to the small diameter.The PT system utilized was water washable red dye,coarse droplet water spray, air drying using plant airline and hard white contrast paint, which has a longerdevelopment time but leaves sharp indications once ithas developed through. The great benefit of thistechnique is that, after taking the photograph andreporting the findings, the indications remain visiblelong enough for the site engineer to witness thembefore they blot out. All relevant indications wererecorded and sized.

Conduct External Magnetic Particle Testing Once the data had been collated from the bore PT, theexternal area of the failed tube could be examined.Conventional MT was chosen, using an alternatingcurrent 110 V yoke because of its speed and sensi-tivity. The same contrast paint was utilized, savingtime and cost. It was important to ensure good yokespacing and field strength at the areas of interest,which were the weld toes and heat affected zones atthe seal welds that are fused to adjacent tubes. Allrelevant indications were recorded and sized.

it is absolutely essentialto capture as muchinformation as possible

Conduct Internal and External Eddy Current Testing

The data collected from the internal and externalsurface crack detection techniques showed that thediscontinuities were longer in the bore than they wereshowing externally. In order to investigate this further,an eddy current weld probe with a 90° head andradius shoe was selected to scan the bore.Sensitivities were set using an electrical dischargemachining (EDM) notched reference of 0.5 mm (0.02 in.) depth and then increasing the gain until a signal height was attained at full screen from thecenter. The probe body gave approximately 100 mm(3.9 in.) of bore scanning, which was enough to notethat cracking was indeed longer internally than it wasfound externally. To confirm this, conventional eddycurrent weld probes were used to scan the externalfillet weld and heat-affected zones after adjustmentswere made to the sensitivity to compensate for thechange in geometry from moving from internal tubebore to the external tube to fillet weld. All indicationswere recorded.

Phase 1 Results

Eddy current testing found two external tears (cracks)at the north side of the rupture area, plus a furtherfour cracks internally adjacent to where the externalweld toe was sited. This is shown in Figure 1a as aschematic view of discontinuities, in Figure 1b as aphotograph of the failure site and in Figure 1c as anend view showing orientation of internal cracks. Figure 2 also shows photographs of the reportedcracks T, U, W, X, Y and Z. MT found the two tears, T and U, on the external side, but no other reportableindications. PT of the tube bore highlighted thecracking (discontinuities W, X, Y and Z) that had beenfound with eddy current testing.

Phase 1 ConclusionAll of the data collected gave clear evidence that thebore of the tube would be the area of interest forfuture inspections. Both PT and eddy current testingfound that the internal cracking was longer than theexternal cracking, and this information was the key tounlocking the future developments of the project.


Figure 1. The failure site and surrounding area: (a) location of discontinuities; (b) rupture window; (c) end view showingthe orientation of internal cracks. Cracks W, X, Y and Z were located opposite the external weld toe, as indicated inFigure 1c.

Tube 1 Tube 3

W, X, Y and Z

ZX

U

T

W Y

Tube 3 Tube 2 Tube 1

Failure sitewindow rupture

(a)

(b)

(c)

MEFEATURE wx boiler tube inspection


From this first inspection, the decision was takento provide a technique that would screen the tubesand identify which had the potential of having internaldiscontinuities. In order to do this, a request wasmade to extract some full tubes and carry outextensive testing within a workshop environment,utilizing test equipment and parameters that would beadopted for onsite inspections.

Phase 2 (Follow-up Inspection and TechniqueVerification)Once the client granted approval, phase 2 wasinitiated. To prove suitability of the techniques, it wasfirst necessary to select the highest risk locations withthe greatest stresses and then screen these tubes to

identify all those with internal visually suspect areas.This necessitated follow-up with a technique thatwould give clear data for acceptance or rejection as adiscontinuity. The tubes selected for phase 2 areshown in Figure 3.

The first pass was carried out with the tubes intact,and various internal bore cameras were tried out tofind the most convenient one that would also find allindications. All the inspections were carried out inworkshop conditions and with good lighting of >500 lux (46.45 fc).

Remote Visual TestingFour camera systems (Camera A, Camera B, Camera Cand Camera D) were trialed to determine which gave

Figure 2. Reported cracks: (a) 30 mm (1.2 in.) external crack T; (b) 20 mm (0.8 in.) external crack U; (c) 80 mm (3.2 in.)internal crack X and 90 mm (3.5 in.) internal crack Z; (d) 65 mm (2.6 in.) internal crack W and 95 mm (3.7 in.) internalcrack Y.

WY

Z

X

T

U

(a) (b)

(c) (d)


the best combination contrast, brightness, clarity,manipulation and ease of use.

The bores of the extracted tubes were inspectedwithout any further cleaning down the bore. Thisensured that the condition that they would have hadin situ would be maintained. Each camera was passedthrough the tube from end to end, and adjustmentswere made to light levels, focus and speed of travel toattain optimum parameters for each camera. Noteswere made regarding the ease of use, time taken andclarity of image until all camera systems had beenutilized. Observations were also made of any internalindications that would require follow-up NDT.

Results of Remote Visual TestingCamera A provided good image quality and easilyadjusted light, but was a forward-facing camera thatcould not view the intrados of bends and thereforemight miss discontinuities. This unit was easy to useand set up, and was very good at locating the discon-tinuities on the extrados of the bend. The pushrodsystem on which it was designed allowed for easyprogression along the tube length. The unit might becumbersome at the manway entrance or restrictive inconfined locations. This is not intrinsically safe, so asite fire permit would have to be issued, and thecamera would need electrical checks from the siteelectrician prior to use. Discontinuities were clearlyseen on the bend inner wall surface and the video-outport allowed easy recording via the portable digitalvideo recording (DVR) system.

Camera B provided poor image quality for itsintended use. It offered easily adjusted light, but wasalso a forward-facing camera that could not view theintrados of bends and, therefore, might also missdiscontinuities. This unit was easy to use and set upand would be suitable at locating blockages or foreignmatter that may have entered the tubing. The pushrodsystem allowed for easy progression along the tubelength. The unit is compact and suitable for use atmanways or at confined areas. It is also not intrinsi-cally safe, so a site fire permit would have to beissued, and the camera would need electrical checksfrom the site electrician prior to use. Discontinuitieswere observed seen on the bend inner wall surface;however, they were difficult to assess due to themonochrome image. The video-out port allowed easyrecording via the portable DVR system.

Camera C provided good image quality and easilyadjusted light, but it was another forward-facingcamera that might not view the intrados of bends. Thisunit was easy to use and set up, and was very good atlocating the discontinuities on the extrados of thebend. The pushrod system again allowed for easy

progression along the tube length. The unit might becumbersome at the manway entrance or restrictive inconfined locations. This system was intrinsically safeand could be utilized with the plant online followingconfirmation from the site electrician. Discontinuitieswere clearly seen on the bend inner wall surface, andthe video-out port allowed easy recording via theportable DVR system.

Camera D provided exceptional image quality,easily adjusted light levels and a fully articulatedhead, which made inspecting the bend intradospossible. It was a very portable and space-saving unit,which would be unobtrusive at the actual onsitelocation. However, it was not intrinsically safe, so asite fire permit would have to be issued, and thecamera would need electrical checks from the siteelectrician prior to use. Discontinuities were clearlyseen on the bend inner wall surface, and the video-out port allowed easy recording via the portable DVRsystem.

Remote Visual Testing ConclusionEvaluation of the results was based upon the client’srequirements and ASME Boiler and Pressure VesselCodes, Section V, Article 9: Visual Examination (ASME,2010). It was clear from the results that all futureremote VT should be carried out using Camera D dueto its image quality, ease of use, articulation of thehead and the relatively small unit size. The next stepwas to section the tubes in half to allow for internalNDT to prove the remote VT findings. The section wascold cut so as to not disturb the internal surface.

Figure 3. Tubes selected for phase 2 were removed from the boiler and taken to aworkshop. These included half of tube 2, all of tubes 3 and 4, and half of tube 5.

12

34

5

Window pane area(rupture area)top bend tube 2

Eddy Current Testing

The eddy current testing unit chosen for this surveywas a complex plane analysis unit, small andportable, with good discontinuity finding capabilities.All probes were calibrated using an EDM notchedreference block with a 1 mm (0.04 in.) notch. Oncecalibrated, the settings were noted and scanningcould commence along the bores of the tubes to verifythe findings of the remote VT survey. All positive indi-cations were recorded and compared against otherprobe findings. The probes utilized are as follows:Probe A was a 100 kHz self-comparison differentialbridge type probe; Probe B was a 200 kHz self-comparison differential bridge type probe; and Probe C was a 200 kHz absolute probe.

Eddy Current Testing ResultsProbes A and B both offered excellent discontinuitydetection and were very easy to manipulate. Eachdetected a total of eight indications, some of whichthe remote VT could not detect. Both probes werecapable of depth estimation using EDM notches ofvarying depths as comparators up to a limit of 3 mm(0.12 in.) deep.

Probe C offered excellent discontinuity detection,though the small probe tip had a tendency to causesome problems with scanning at any comfortablespeed. Additionally, the coil was too sensitive tochanges in material properties and influenced by thegrain structure changes from the far side seal welds. It is better suited for small area scans at slow scanspeeds for discontinuity detection.

Eddy Current Testing ConclusionEvaluation of the results was based upon the client’srequirements and ASME Section V, Article 8: EddyCurrent Examination of Tubular Products (ASME,2010). Excellent results were noted with both of thebridge probes; these probes have a curved probe tipand are a good size to handle. All future inspections ofthe boiler tubes are to be carried out using 100 to 200 kHz self-comparison bridge type probes.

Magnetic Particle Testing Standard MT was carried out using a 110 V Y6 yoke,type 2 field flux strips, white contrast aid paint andblack magnetic particle suspension (ready to use).These products are the most sensitive and ready-to-use products for this type of inspection and are regarded as some of the industry standards.All indications were recorded.

Magnetic Particle Testing ResultsEight internal discontinuities were noted and sized forlength. These were comparable with the findings fromthe eddy current testing.

Ultrasonic Testing The wall thickness and external radius of the tubesnarrowed the probes selection down to a 45° shearwave with nominal frequency of 2.25 to 4 MHz. Theprobe had a shoe shape to suit the curvature of thetube and could be seated inside a metal holderpurposely built to fit the external radius and providestable scanning (see Figure 4).

Reference samples were made using tube material identical to those of the tubes to be tested.



Figure 4. The ultrasonic probe curved shoe.

Once calibrated, the settings were notedand scanning could commence along thebores of the tubes to verify the findings


The reference tube was cut in half, and then EDMnotches were made in the internal surface. Thenotches were selected to give variation in depth andlength, as shown in Figure 5.

Once the probe, holder and reference samples weremade, the test parameters were set to give a workableprocedure that could be repeated out in the field.

Contact surfaces were presented for inspection,clean of weld spatter, dirt, rust, grease and anyroughness that would interfere with the freemovement of the search unit or prevent the transmis-sion of ultrasonic vibrations; a minimum standard of120 µm was recommended.

The distance of surface preparation was a 40 mm (1.6 in.) wide band adjacent to the weld toe.The instrument calibration checks were performedprior to first use and weekly thereafter. The signatureof the operator or technician on the report docu-mented satisfactory calibration checks. The checksincluded screen height linearity and amplitudecontrol linearity. Scanning speed could not exceed150 mm/s (6 in./s).

A verification of calibration was performed whenany part of the test system was changed, at thecompletion of each test (or series of similar tests)performed, when testing personnel were changed, andany time malfunction was suspected. The verificationcheck was performed on at least one of the reflectorsin the appropriate calibration block or on a knownreference reflector.

Ultrasonic Testing System Calibration With the signal maximized from the 2 mm (0.08 in.)deep, 20 mm (0.79 in.) long reflector, the emissionpoint will be at the mark indicating a true 45° angle(see Figure 6). The mark is 12 mm (0.47 in.) from the tangential radius of the reflector, indicating at 1.5 skips. A maximum deviation from 45° is +2° or –3°, giving a maximum deviation from the 45°mark of +0.9 mm (0.04 in.) and –1.2 mm (0.05 in.).

The calibration was performed corresponding tothe surface of the component from which the testingwas performed.

The sweep range was calibrated to cover the areaof interest required for the test. The search unit for themaximum first indication from the internal bore cornerof the calibration reference sample block was posi-tioned. The beam path was set to 5.7 mm (0.2 in.).Next, the search unit for the maximum indication fromexternal bore corner of the reference sample block thebeam path was positioned at 11.4 mm (0.4 in.). Thesurface distance was 4 mm (0.2 in.) for a half skip and8 mm (0.3 in.) for a full skip from the corners to theemission point. Figure 6. Angle verification.

2 mm

12 mm

0.9 mm

–1.2 mm

4 mm

Figure 5. Reference electrical discharge machining notches in side and end view:(a) variable depth and constant length; (b) constant depth and variable length.

100 mm

20 mm20 mm20 mm20 mm20 mm

3 mmdeep

2 mmdeep

1 mmdeep

10 mm

1 mmdeep

1 mmdeep

1 mmdeep

15 mm5 mm Notch depth

Notch depths1 mm2 mm3 mm

4 mm

4 mm

1 mm

100 mm

(a)

(b)


The signal from the 1 mm (0.04 in.) deep, 20 mm(0.79 in.) long slot was maximized, and the gain wasadjusted to give a response of 80% full screen height.For scanning, 6 dB were added. This was validated asa scanning sensitivity against known cracks in the cali-bration reference sample.

The extra 6 dB were removed for testing. All indica-tions greater than 6 mm (0.24 in.) in length had thelength tested using the 6 dB drop; indications with alength of less than 6 mm (0.24 in.) were not recorded.

Probability of DetectionWith strict adherence to the procedure, there was ahigh degree of probability of detection (POD) in cracksgreater than 6 mm in length (0.24 in.) and greaterthan 1 mm (0.04 in.) in depth, parallel to the weldaxis within the suspect area. Any deviation was

recorded on the report and had a detrimental effect onPOD.

POD increased by the use of a complementary NDTtechnique. Remote VT outlined within this paper fulfillsthese criteria and may be used as a screeningtechnique to focus the UT and as a verification aid forindications found by UT.

No discontinuities were detected that did not havea major axis parallel to the ligature weld. Only indica-tions within 6 mm (0.24 in.) of the primary beam axisincidence to the area designated as the suspect areawere detected.

Ultrasonic Testing ResultsDuring the UT, a number of discontinuities wereobserved directly opposite the tube-to-tube ligatureweld zone. These were sized and recorded. The


Figure 7. Discontinuity locations in tubes 2 through 5: (a) end view; (b) side view.

G

G

A

A

C

C

D

D

H

H

B

B

E

E

F

F

Tube 5Tube 5Tube 4

Tube 4Tube 3

Tube 2Tube 3Tube 2

(a) (b)


results from UT were then compared alongside theresults from the other techniques used and itshowed good confirmation that the technique hasthe ability to find inner wall discontinuities from theexternal surface.

Ultrasonic Testing ConclusionEvaluation of the results was based upon the client’srequirements and ASME Section V, Article 4 and ASMESection V, Article 5 (ASME, 2010).

As long as all the criteria for surface conditionsand sensitivity parameters were set correctly, and aslong as suitably qualified technicians were used, itcan be concluded that UT is an excellent tool for thedetection of this kind of boiler tube failure, evenbefore it has propagated through wall. The limita-tions of this technique are that the surface isrequired to be smooth, bare metal, free of scale, andthat the probe must have access to the test area.Any attachments spanning or touching the weld willcause a restriction.

Full results of all the tests made in phase 2 areshown in Figures 7, 8 and 9. The following informationwas taken directly from all the inspections carried outin phase 2. Figure 7 shows the discontinuity locations.Figure 8 shows the internal cracks that were found,labeled A through H. Figure 9 shows an internal indi-cation in Figure 9a and a no internal indications inFigure 9b.

Discontinuities found were labeled cracks A to H.In Tube 2, crack A was 220 mm (8.7 in.) long; crack Gwas 250 mm (9.8 in.) long; and crack H was 150 mm(5.9 in.) long. In Tube 3, crack C was 620 mm (24.4 in.) long and crack D was 600 mm (23.6 in.)long. In Tube 4, crack E was 10 mm (0.4 in.) long andcrack F was 100 mm (3.4 in.) long. In Tube 5, no indi-cations with found with remote VT, UT, eddy currenttesting or MT.

Table 1 shows the detectability of each crack witheach technique utilized.

Figure 8. Internal cracks found: (a) 1 to 4 mm (0.04 to 0.2 in.) deep, 220 mm (8.7 in.) long crack A; (b) 1 to 4 mm (0.04 to 0.2 in.) deep, 100 mm (3.9 in.) longcrack B; (c) 1 to 2 mm (0.04 to 0.08 in.) deep, 590 mm (23.2 in.) long crack C; (d) <1 mm (0.04 in.) deep, 500 mm (19.7 in.) long crack D; (e) <0.2 mm (0.08 in.)deep, 10 mm (0.4 in.) long crack E; (f) <0.2 mm (0.08 in.) deep, 100 mm (3.9 in.)long crack F; (g) 1 to 4 mm (0.04 to 0.2 in.) deep, 250 mm (9.8 in.) long crack G;(h) 1 to 4 mm deep, (0.04 to 0.2 in.) 150 mm (5.9 in.) long crack H.

(a) (b)

(c) (d)

(e) (f)

(g) (h)

TABLE 1Detectability of each crack with each technique utilized

Crack Detectable with Detectable with Detectable with Detectable withremote visual testing eddy current testing magnetic particle testing ultrasonic testing

A Yes Yes Yes YesB Yes Yes Yes YesC Yes Yes Yes YesD Yes Yes Yes YesE No Yes Yes NoF No Yes Yes No

G Yes Yes Yes Yes

H Yes Yes Yes Yes


Phase 2 Conclusions

The decision to adopt the following set of techniqueswas based upon ASME Section V, NDT guidelines andalso the information gathered from the client regardingtheir concerns at being able to screen for and detectearly stages of corrosion fatigue (ASME, 2010; ASNT,2006; BS, 2008).

Based upon the entire set of trials and results, itwas concluded that future inspections of the boilertubes would be carried out as follows: tubes associ-ated with critical high stress areas where the possi-bility of corrosion fatigue is greatest would beselected; external eddy current testing would beconducted at tangent welds using bridge weld probesand sensitivity set on a 1 mm (0.04 in.) depth EDMnotch to give a signal deflection full screen heightfrom center. In addition, external MT would beconducted at tangent welds using standard 110 V Y6yokes and color contrast consumables. External UTwould be conducted at tangent weld areas using 45°shear waves with a nominal frequency between 2.25and 4 MHz, shaped to suit the tube outer radius.Finally, internal inspections would be conducted usingan advanced video borescopes with fully articulatedfiber optic cameras.

This set of inspections and the application of theprocedures developed would give the best chance atearly detection of crack initiation and propagationand, if carried out on a regular frequency, could reduceplant downtime and the possibility of further rupturesin the future.

Phase 3 (In Situ Inspection Application ofProcedures Developed in Phase 2)The critical areas selected for inspection and the appli-cation of techniques are shown in Figure 10.

The inspections were carried out as per the specifi-cations in Table 2, which were proven to be the mostcost effective and time saving, having only to cleanthe external surfaces once prior to the first externaltechnique. The first order of inspection would includeinternal VT, in which there is an initial search orscreening for potential areas of thermal fatigue


Figure 9. Internal view of: (a) tube 2 showing internalindications of cracking; (b) tube 4 showing no internalindications or evidence of visible cracks.

(a)

(b)

Figure 10. Critical areas selected for testing in phase 3.

TABLE 2Test technique order and reasons

Order of testing Test technique used Reason for order

1 Internal remote visual testing Initial search/screening for potential areas of thermal fatigue cracking 2 External magnetic particle testing Applied first so that the contrast aid paint would adhere3 External eddy current testing Applied second because it could be applied on top of contrast aid paint

and was not affected by residual magnetic inks4 External ultrasonic testing Applied last as the couplant can leave greasy film residue, which would

need extra cleaning if other techniques were to follow


cracking. The second order of testing consists ofexternal MT, which is applied first so that the contrastaid paint will adhere. The third order of inspectioncomprises external eddy current testing, which is notaffected by contrast aid paint or residual magneticlinks. The final order of inspection involves externalUT, which is applied last, as the couplant can leavegreasy film residues, which would need extra cleaningif other techniques were to follow.

ResultsRemote VT noted two tubes with internal linear indica-tions at tube 1 and tube 2. No further indications wereobserved. MT found no evidence of reportable externaldiscontinuities. Eddy current testing found noevidence of reportable external discontinuities. UTconfirmed the presence of internal cracking at the twobends and no further indications were found.

Phase 3 ConclusionsTesting was carried out to the sentencing criteria givenby the client. Each visual indication was gradedaccording to appearance and location. The follow-upUT, eddy current testing and MT results clearly identi-fied two tubes as falling outside of the acceptancecriteria. The tubes were removed from the boiler andsectioned in half, which confirmed that the indicationsnoted were the start of corrosion fatigue.

This action gave the client confidence in the tech-niques and procedures that had been developed andthe written scheme of testing was rewritten to includethese extra NDT techniques at regular intervals whenthe boiler is off-line.

It is therefore conclusive that the techniquesdeveloped can target early stages of thermal corrosionfatigue before they rupture or break through to theouter surface.

SummaryThe techniques developed and utilized for this inspec-tion are individually useful in their own right, but whenthey are combined the overall effectiveness of usingremote VT alongside UT, eddy current testing and MT

was greatly improved (HSE, 2011). The screeningprocess worked remarkably well at finding linear oraligned corrosion, and the follow-up NDT techniquesquickly confirmed the findings.

However, these techniques each have limitations,and it is the limitations that reduce the overall thor-oughness of the testing. When evaluating the testing,all limitations must be noted, such as externalsteelwork adjacent to, touching, spanning or weldedacross the area of interest, which would restrict theaccess for the external MT, eddy current testing and UTtechniques and would leave the inaccessible areasuninspected. Not only are these areas uninspected,but also the introduction of the localized stresses fromthe welded attachments can add to the high stressesthat are already present in the boiler tube, whichwould make these locations critical to inspect.

Currently, the use of the internal remote VT witharticulation is used to extensively inspect theserestricted areas and to note down linear or alignedcorrosion that could be indicative of corrosion fatigue.The results can be saved in digital video and archivedfor future reference. wx

AUTHOR

Michael Leonard Hodgson: Oceaneering International,Piccadilly Offices, Wilton International, Middlesbrough,TS10 4RG, United Kingdom; 44 (0) 1642 770800; fax 44 (0)1642 466547; e-mail [email protected].

REFERENCES

ASME, ASME Boiler and Pressure Vessel Codes, Section V,Nondestructive Examination, American Society of Mechan-ical Engineers, New York, New York, 2010.

ASNT, Recommended Practice No. SNT-TC-1A: PersonnelQualification and Certification in Nondestructive Testing(2006), American Society for Nondestructive Testing,Columbus, Ohio, 2006.

BS, EN 473, Non-destructive Testing – Qualification andCertification of NDT Personnel – General Principles,European Committee for Standardization, Brussels,Belgium, 2008.

HSE, “Corrosion Fatigue Failure of Tubes in Water TubeBoilers,” Control of Major Accident Hazards (COMAH) Alerts,14 July 2011, Health and Safety Executive, London, UnitedKingdom, www.hse.gov.uk/comah/alerts/corrosion.htm.

Comparison of Visual, Eddy Current, Ultrasonic and Magnetic ...

Documents

Transcript of Comparison of Visual, Eddy Current, Ultrasonic and Magnetic ...