FeatureBy Ankit Vajpayee

Tube failures continue to be theleading cause of forced outages inboilers. To get your boiler back on lineand reduce or eliminate future forcedoutages due to tube failure, it isextremely important to determine andcorrect the root cause. Detecting flawsbefore they cause failures is of criticalimportance in boiler maintenance.Localized wall thinning due tocorrosion in boiler water-wall tubing isa significant inspection concern forboiler operators.

The typical methods used forinspecting boiler water walls includespot check ultrasonic testing (UT), A-scan UT, electromagnetic acoustictransducer (EMAT) UT, and scanningthermograpy. Spot check UT only givesthickness readings and gets veryminimal coverage of the total surfacearea of the furnace water walls; thechances of finding ID flaw mechanismsusing spot check UT are minimal at best.If boiler water walls have beensandblasted, A-scan UT may be used toinspect larger areas of the furnace walls;in these cases, a steady flow of water ismost often used as the couplant. TheEMAT technique requires sandblastingof any boiler water wall surfaces, anddoes not inherently get good surface areacoverage unless the inspection teamperforms multiple passes using the

EMAT probe. Scanning thermography isthe most recent development for theinspection of boiler water walls;however, it is not yet commerciallyavailable in enough capacity.

This article explains thedeployment of a robotic wall crawlerusing an electromagnetic technique toinspect boiler water walls from outsideof the tube as well as the theoreticalbackground of the technique, whichexplains the quantitative nature of theinspection.

Further, a case study is presentedfor the technique that allows theextraction of thickness informationfrom the inspection data.

Theoretical Background ofRemote Field Technology

In the 1950s, the Shell DevelopmentCo. pioneered an electromagneticnondestructive examination techniqueknown as remote field testing (RFT). Itwas first used to inspect well casings forcorrosion and wall thinning, and for anumber of years was used primarily inthe petroleum and pipeline industries. Inthe mid-1980s, this technology became asubject of sophisticated research, and thecombination of basic research andindustrial innovation has resulted inelegant theoretical models that

eventually developed into stronganalytical methods that enable a greatervariety of anomalies to be detected andquantified. Remote field testing is now awell-established inspection method forcondition assessment of ferromagnetictubes .

Principles of RFT

Remote field testing is based on athrough-transmission principle. Thefield passes from the exciter coilthrough the tube wall, along the outsideof the tube, and back in through thetube wall at the location of the detectorcoil — Fig. 1. Metal loss causes thefield to arrive at the detector coil withless travel time and less attenuation,resulting in a change in signal phaseand amplitude. The signal values ofphase (time of flight) and log-amplitude (signal strength) are directlyrelated to wall thickness in the area ofthe detector coil(s).

The RFT technique can be used forall conventional carbon steel materials,diameters, and wall thicknesses. It is,therefore, used in many different types

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Using a Robotic System to Inspect Boiler Tubes

A robotic wall crawling system utilizing electromagnetic inspection techniques inspects boiler water walls

from the outside of the tube

Fig. 1 — A typical RFT probe. In comparison with conventional eddy current,

RFT coils are widely spaced in order to measure the through-transmission field.

Fig. 2 — An inspector using UT to dothickness measurements on boiler tubewater wall tubes.

of heat exchangers, including fossilfuel boilers (especially in water walland generator bank tubes), black liquorrecovery boilers, shell and tubeexchangers, and air fin coolers. Remotefield testing operates at relatively lowfrequencies and is a noncontacttechnique, so the probes have minimalfriction with the pipe wall and requireno couplant.

The accuracy for remote fieldtesting in the straight part of the tubesis about 10% of wall thickness forgeneral wall loss. Accuracy is generallyless (20% of wall) for highly localizeddiscontinuities and near external

conducting objects because of thechanges in magnetic properties of thetube in that area and because ofshielding effects of external objects.Remote field testing is also equallysensitive to inside and outside surfacediscontinuities but usually cannotdiscriminate between them without thehelp of near-field coils. It is relativelyinsensitive to scale and magneticdebris.

Deployment of a Robotic WallCrawler Using RFT

It is very difficult to obtain accessto the inside of boiler tubes so that aninspection tool can be inserted;therefore, all inspection must beperformed from the outside of thetubes, inside the boiler. In this case, itis desirable to have an external toolthat can detect corrosion or wallthinning without exhaustive cleaning ofthe surface, or removal of coatings.

The traditional method ofinspecting boiler water wall tubes forloss of wall thickness is by takingmany thousands of ultrasonic thicknessreadings spaced several feet apart inelevation — Fig. 2. In order to do this,the boiler must be scaffolded and thetubes must be cleaned to bare metalwhere the readings are to be taken.

Scaffolding and cleaning costsoften exceed $100,000, and theultrasonic inspection can cost the sameamount again.

If the boiler will be scaffoldedanyway, the tubes can be inspectedrapidly with a hand-held scanning toolthat delivers the equivalent of up to 2000thickness readings per foot, at a scanningspeed of up to 10 ft/min — Fig. 3.

For boilers that are not scaffolded,a magnetic “wall crawler” can be usedto carry the external pipeline integritytool (E-PIT™) RFT probe up the waterwall. The crawler can handle waterwalls up to 200 ft in height and tubesizes from 1.5 to 3.5 in. in diameter.Inspection speed is 10 ft/min so anentire wall that is 100 ft high and 100tubes wide can be inspected in lessthan three 12-h shifts. The E-PIT probeinspects the flame side of the tube towithin 3⁄8 in. of each web, using 12detection coils for high precision —Fig. 4. Pits, as small as 1⁄8 in. diameter,can be detected.

Case Study

Unit #1 of the generation station atABC Power in Canada usesOrimulsion™ as fuel. Orimulsion is abitumen-in-water emulsion producedfrom the vast reserves of the Orinocobelt in Venezuela. The emulsion contains70% natural bitumen and 30% water.This liquid fuel, resembling a black latexpaint, has relatively high energy contenton a weight basis (i.e., about 110% ofcoal and 70% of heavy fuel oil). Thescale deposition on the tubes and web insuch boilers is worse than coal-firedboilers. Figure 5 shows an example ofscale. Note that the crown is often scalefree but the spaces between the tubesalways have heavy scale.

A Vertiscan™ system from RussellNDE Systems, Inc., Edmonton, Alb.,Canada, was used to inspect the waterwall tubes in boiler #1 in the fall of2006 — Fig. 6. The system consistedof a TubeCAT™ magnetic crawler withodometer; E-PIT™ tool capable of

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Fig. 3 — For scaffolded boilers,inspectors can use a hand-held tool toperform up to 2000 thickness readings perfoot at a scanning speed of up to 10 ft/min.

Fig. 4 — An example of a roboticcrawler being used to inspect boilertubes.

Fig. 5 — Heavy scaling.

Fig. 6 — The Vertiscan system in place.

inspecting five tubes simultaneously;E-PIT™ hand-held scanner for theinspection of individual tubes;Ferroscope™ 308, 16-channel RFTinstrument; remote vision system; 200-ft umbilical; and an industrial laptopcomputer.

Calibration. The equipment wascalibrated on-site by taking ultrasonicthickness readings on at least twoseparate elevations of the same tubehaving both nominal thickness and

known wall loss. In this instance, thethinned area of tubes on one of thewalls at the burner elevation was usedto produce the calibration curvesshown in Figs. 7 and 8.

Procedure.A datum line markedon the wall was where allmeasurements were taken. In thisinstance, the datum line was at anelevation of approximately 99.5 ft. Thetool detectors were aligned with thisdatum for each of the scans performed.

From the datum line, the RFTsystem descended the wall at a speedof approximately 10 ft/min whilegathering data from the crowns of fivetubes simultaneously. Depending onthe frequency and sample rate, this canequate to (up to) 2000 thicknessreadings per foot.

Once reaching target height, thecrawler was stopped and the directionreversed. Data were also gathered onthe way up and were used to confirm

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Fig. 7 — Calibration graph for area F1. Fig. 8 — Calibration graph for area F2.

Fig. 9 — The water wall of boiler #1 is shown zoomed out. It shows the area from elevation 70 ft (below burners) to elevation 99.5ft (just below the superheater tubes). The dark area below the line showing the top of the burners was confirmed by ultrasonicreadings to be 0.098 to 0.150 in. (black to yellow colors, respectively). The dark area to the right (south corner) is due to heavyscale that lifted the scanner away from the wall. Individual tube numbers are shown at the top, and the distance scale and elevationare shown to the left.

Inspection Trends / April 20114

any indications of wall loss detected.Results. The software

semiautomatically generated a detailedreport (spreadsheet) for each wall. Inadditon, field notes and a collage of the“color map” of the full-scan data fromeach wall was made — Fig. 9.

Confirmations. The inspectionresults were backed up using ultrasonics— Figs. 10, 11.

Summary

• The VertiScan system proved effectivein identifying general and localthinning near the welds at elevation90.5 ft. Thinning was confirmed byultrasonic thickness readings.

• The scaffold gap must be a minimumof 10 in. from the crown of the tubesto allow the system to pass by.

• For future inspections, it would helpto have a fourth person doing on-sitedata analysis only. The operation ofthe RFT system when scaffolds arepresent is a three-person job (twopeople if no scaffolds are present).

• The system provides the best valuewhen there is no scaffold in the boiler.

• Generally, one full water wall can bescanned per shift if just one system isin use.Capabilities. The technique is

sensitive to all types of wall thinning,including the following:• Hydrogen damage, • Underscale pitting and graphitization, • Flame and soot blower erosion, • Blister and local overheating, • Creep damage (thermal fatigue), • Elephant skin and rhino hide,

• Dents and gouges, and • Internal pitting.

Works Consulted

1. MacLean, W. R. 1951. Apparatusfor magnetically measuring thickness offerrous pipe. U.S. Patent 2573799.

2. Schmidt, T. R. 1989. History ofthe remote-field eddy current inspectiontechnique. Materials Evaluation 47(1):pp. 14, 17, 18, 20–22. Columbus, Ohio:American Society for NondestructiveTesting.

3. Atherton, D. L., and Czura, W. M.1994. Finite element calculations for eddycurrent interactions with collinear slots.Materials Evaluation 52(1): 96–100.Columbus, Ohio: American Society forNondestructive Testing.

4. Hoshikawa, H., Koyama, K.,Koidoand, J., and Ishibashi, Y. 1989.Characteristics of remote-field eddycurrent technique. Materials Evaluation47(1): 93–97. Columbus, Ohio:American Society for NondestructiveTesting.

5. Schmidt, T. R. 1984. The remotefield eddy current inspection technique.Materials Evaluation 42(2): 225–230.Columbus, Ohio: American Society forNondestructive Testing.

6. Lord, W., Sun, Y.-S., Udpa, S. S.,and Nath, S. 1988. A finite elementstudy of the remote-field eddy currentphenomenon. IEEE Transactions onMagnetics Vol. 24, pp. 435–438. NewYork, N.Y.: Institute of Electrical andElectronics Engineers.

7. Mackintosh, D. D., Atherton, D.L., and Puhach, P. A. 1993. Through-

transmission equations for remote-fieldeddy current inspection of small-boreferromagnetic tubes. MaterialsEvaluation 51(6): 744–748. Columbus,Ohio: American Society forNondestructive Testing.

8. Sun, Y.-S., Udpa, L., Udpa, S.,Lord, W., Nath, S., Lua, S. K., and Ng,K. H. 1998. A novel remote-field eddycurrent technique for inspection of thickwalled aluminum plates. MaterialsEvaluation 56(1): 94–97. Columbus,Ohio: American Society forNondestructive Testing.

9. Kilgore, R. J., andRamachandran, S. 1989. Remote fieldeddy current testing of small-diametercarbon steel tubes. Materials Evaluation47(1): 32–36. Columbus, Ohio:American Society for NondestructiveTesting.

10. Atherton, D. L., Macintosh, D.D., Sullivan, S. P., Dubois, J. M. S., andSchmidt, T. R. 1993. Remote field eddycurrent signal representation. MaterialsEvaluation 51(7): 782–789. Columbus,Ohio: American Society forNondestructive Testing.

11. ASTM E 2096-00, StandardPractice for In Situ Examination ofFerromagnetic Heat-Exchanger TubesUsing Remote Field Testing. 2000. WestConshohocken, Pa.: ASTMInternational.

Fig. 11 — Ultrasonic test results of the same tubes evaluatedin Fig. 10 (wall loss detected).

ANKIT VAJPAYEE

(avajpayee@russelltech.com) is

with Russell NDE Systems, Inc.,

Edmonton, Alb., Canada.

Fig. 10 — Butt joint weld with possible loss below (east wall

62–66).