Inspect for MaintenanceWith NOT Methods

Based on theirexperience inmaintenance

and turnaroundsat an ammonia

plant, theseengineers

compare impor-tant diagnostic

tools fornondestructive

testing.

Manfred Dressel,Gunter Heinke, and

Udo Steinhoff, BASF AG

! n principle, all nondestructive testing| (NOT) methods are suitable for in-specting plant equipment. The follow-

! ing provides a survey of these develop-ments and demonstrates our experience inpractical applications in various parts ofthe plants.

Ultrasonic testing. We use ultrasonictechniques for wall-thickness measurementsand for crack detection at hidden surfacesof weld seams and parent metal. Because we

leaks in the tube-to-tubesheet welds of heatexchangers. In 1970, we developed a newradiographie method for testing thesewelds. Figure 1 shows a schematic of thetest device we developed. A gamma raysource (the radionuclide iridium-192) is in-troduced into the tube, and the gamma rayspenetrate the weld seam and "expose" aradiographie film that is in external con-tact with the tube-to-tubesheet weld. Theweld seam is shown on the developed ra-diation image as a circle. Because of itslight weight, the device can be used easily

• Figure 1. Gamma ray device f or NDT of tube-to-tubesheet welds.

do not use special equipment or techniques,this NDT method is not discussed.

Radiographie examination of tube-to-tubesheet welds in heat exchangers. Insome of our plants we had problems with

and quickly. With auxiliary equipment, itcan be used overhead.

More than 300,000 exposures have nowbeen made on critical heat exchangers priorto start-up. The radiographie examination

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of the tube-to-tubesheet welds de-scribed here, however, cannot beconsidered as a substitute for leaktesting because small defects likelack of fusion or cracks cannot be de-tected by hard radiation.

Eddy current testing. We do notapply ultrasonic methods to detect theextent of crack propagation in cen-trifugal casting tubes in steam re-formers (1 ). The water necessary forcoupling the ultrasonic probes to thesurface cannot be removed, and, as aresult, the reformer would becontaminated.

On the other hand, the sensitivityof the eddy current method could beincreased by developing adequatecoil systems. The influence of inter-fering outer magnetic fields could besuppressed. A prototype for manualand automatic inspections is cur-rently being tested in the laboratory.In small reformers the tubes are par-tially checked by manual eddy cur-rent testing, which, from our experi-ence, is a good, economical solution.In large reformers the tube surface isscanned from outside by a mechanicalscanner.

Figure 2 shows that the scannermoves along the vertical tubes whilean eddy current probe rotates aroundthe circumference and scans thewhole volume of the tube. Indications ofthe damaged areas can be both stored anddisplayed as images. This makes it possi-ble to verify the results after repeated in-spections and to recognize any damageprogress. This is, however, recommendedonly for special cases where the ratherhigh expenditure is justified.

We can already detect cracks with 30%wall-thickness depths quickly and reli-ably. From our experience we know thattubes with such cracks still have a resid-ual lifetime of at least one year. Duringthat period, an exchange of the tubes canbe prepared.

Figure 3 shows the trace of the eddycurrent signal over a complete circumfer-ence. Two damaged areas with less than70% wall thickness are clearly visible.Such a tube must be removed immediately.

Magnetic particle and liquid pene-trant testing. Magnetic particle testing(MPT) and liquid penetrant testing (PT)are generally used to examine surface

• Figure 2. Eddy current testing of reformer tubes.

..Eddy-current

Crack-depth in %100-

90 .>••.." JSOv: Î270Circumference of ibe: tuba :

• Figure 3. Eddy current testing of reformer tubes,principle and data representation.

cracks. At BASF, we prefer using MPTwith ferromagnetic materials because italso discovers closed cracks, which PTfails to do. This was observed especiallyin cases of cracks induced by thermalshocks. On the other hand, there aresometimes diffuse indications when MPTis used.

The aircraft industry in the UnitedStates, for instance, studied how the pow-der concentration and the strength of themagnetic field influenced the inspectionsensitivity and the occurrence of diffuseindications. The best results were ob-tained with low powder concentrationsand reduced magnetic field strength.Another result also seems to be important:MPT is estimated to give better resultswhen applied wet than when applied dry.Some firms, especially in the aircraft in-dustry, examine surface cracks with eddycurrent testing, but in our opinion this isdifficult when carried out in the field. Weoften find rough or irregularly shaped sur-

The technical ammo-nia process datesback to BASF atLudwigshafen, whereCarl Bosch built thefirst plant based onFritz Haber's recycleconcept. After severalyears of struggle, thefirst plant came onstream in 1913. Theimportance ofsuitable material wasrecognized during thisperiod and led to thefoundation of theBASF materials test'ing department, thefirst of its kind in achemical company.

The safety standardsand on-stream factorsof today's ammoniaplants could not havebeen achievedwithout the continu-ous development ofmaterials tastingmethods. During thisdevelopment, nonde-structive testing(NOT) became moreand more importantas the main directiveof materials testingShifted from in-vestigating damageto preventive Inspec-tion duringfabrication, assembly,and turnarounds.

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M A I N T E N A N C E

• Figure 4. Portable hardness testing equipment in operation.

Prior to:heat treafrrient

After neat treatment

-VYeM ràiga:-

DistarK» of :rneasurement j points: Aï cm

• Figure 5. On-line Vickers hardnessmeasurement of a weld seam.

faces, but the most disturbing factoris the tremendous change in the con-ductivity of weld seams.

Hardness measurements. Boththe internal structure and hardness ofa given material are important prop-erties of materials behavior. In par-ticular, the effectiveness of heat-treated welded seams can be checkedby hardness testing before and afterthe heat treatment. A special hard-ness-testing device, Figure 4, hasbeen proven to be effective. It is fixedat the surface to be checked with spe-cial clamping devices. The Vickersdiamond is mounted on a magnetoe-lastic system, which measures thedepth of the diamond's penetrationinto the material, and the result is dis-played on a digital instrument. It isclamped on the tubesheet of a heatexchanger. The hardness tester canalso be mounted on a manual x/y

scanner for measuring the hardnessalong a weld.

Figure 5 shows a continuous graphof the hardness across a circumferen-tial weld obtained with our testing de-vice. This hardness tester is also suitedto detecting diffusion processes at thesurface (for example, nitration of wallsin ammonia converters) that increasethe hardness.

Online replica technique. Themetallographic methods have alwaysbeen central to evaluating materialsconditions in chemical plants. Wehave carried out nondestructive, in-service metallography in chemicalplants by means of replica techniquessince the 1970s, and this method isnow one of the most importantmeans of monitoring the conditionof materials in ammonia plants.

The principle is shown in Figure6. The surface to be checked is first

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Figure 6. Replica technique.

• Figure 7. Nondestructive metal-lography by using replica techniquein the plant.

• Figure 8. Evaluating the metallography prepared surface by microscope.

We have carried out non-destructive, in-service

metallography inchemical plants by means

of replica techniquessince the 1970s.

prepared, Figure 7, and then in-spected with the help of a transport-able microscope, Figure 8. Finally,as shown in Figure 7, a replica thatgives an image of the surface's to-pography and displays the typicalfeatures of etched surfaces (grain andphase boundaries, cracks, pores, in-clusions, and precipitations) is fixed.

Acoustic inspection. In our opin-ion, acoustic emission (AE) is suita-

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M A I N T E N A N C E

• Figure 9.Severe

corrosion ofa vessel's

inner wall.

• Figure 10.Ultrasonic

image (D scan)of a corroded

wall of a vessel.

• Figure 11.Corrosion

profileobtained bydisplaying

only one lineofthe

completeultrasonic

image.

US Figure 12.Eddy current

testing of tubes byrotating probe.

ble for studying crack propagation orstress corrosion cracking (SCC) inthe laboratory, but it was not success-ful for field applications. Carryingout acoustic emission analysis in thefield is very complicated, althoughin principle it seems to be very sim-ple: transient ultrasonic signals arerecorded during a (mechanical) load.Up to now, we have not relied on theacoustic emission method carried outduring pressure tests of vessels madefrom steel; consequently, we do notapply the NOT methods for detect-ing cracks.

Wall-thickness measurementswith ultrasonic scanning probes arestill carried out manually point bypoint. The evaluation is done directlyafter the measurement. The follow-ing example illustrates an advancedultrasonic testing wall-thicknessmeasurement technique.

If the ultrasonic probe (sensor) ismounted at a scanning device, thewhole test surface can be inspectedcompletely without missing smallbut probably important areas. Com-bined with a computer, the measureddata can be taken, stored, and evalu-ated later. The data can also be dis-played as wall-thickness images.

Figure 9 shows a corroded ves-sel's wall, and Figure 10 shows theaccompanying ultrasonic image (Dscan) obtained by computer-con-trolled scanning from the other sideof the wall. This method makes theinner structure of the wall visiblefrom the outside.

Further evaluations can be madeby extracting wall-thickness profilesfrom the ultrasonic D scan, e.g., atpositions 1, 2, or 3 in Figure 10,where the thickness values can beread directly, as shown in Figure 11.Only a little material remains at thepositions 1 and 2.

Eddy current testing of heat ex-changer tubes The eddy currentmethod has proven reliable for de-tecting cracks, e.g., even for opera-tion-induced cracks at the inner sur-face of tubes. Special eddy currentprobes have been designed that can beadapted to different tube diameters.

As shown in Figure 12, the eddy

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l SA »..V ¥^ < H S<a * «*ï . s? . J * * JA i •ïfil <»ê *i*Js SI -, - "' !ï »--- ; - .

; s' * ï v* | f ||i.> *!**?* |* »*i w N '

B* * •

H-*' ^ ^ '^< '?|; | ï |'* ||-s« ^ ,, "***^*f^4llil*ï "'s " ?*& ^ "" %

.% _ ; ;,<;, ̂ L-

The eddy currentmethod has proven to

be reliable for thedetection of cracks, e.g.,

even for operation-induced cracks at the

inner surface of tubes.

Literature CitedL Madhavan, S., S. Y. Sathe,

"Inspection of Ammonia Plants, "M. W. Kellogg Co., Houston, TX.

current coil rotates at high speed inthe probe's housing, which is driveninto the inspected tube. The signalsare transferred and stored in a com-puter. Figure 13 shows the shape ofa branched stress corrosion crack asan eddy current image based on anx axis of 100 mm tube length and ay axis of 360° circumference.

In conclusionNearly all known NDT techniquesare applied in our ammonia plants.Some of the testing techniques aretailored to the requirements of ourplants. Because of the increasing de-mands regarding the safety of theplants, new testing techniques are socomplex that only computer-aidedinspection systems are used. Newtechniques cannot, however, replaceexperienced inspectors. They canhelp the inspectors by collecting dataand displaying them as images, andthe images are a good basis for help-ing the inspector to give reliable testresults. But inspectors still must relyon their own experience. E13

107

• Figure 13. Eddycurrent image ofthe shape of cracksinside of a tube.

M, DRESSEL— Director ofengineering, maintenance,Utilities, fir« protection, and*nf«ty for »ASF/She«, a Jointventure, lie holds « mastersdegr«« in mechanicalengineering from the Uttïv.of Munich. Since joiningBASF In 1974, he has serveda« head of maintenance andengineering worldwide InCentral Engineering,» and aslit ad of an engineering andmaintenance group inLudwigshafen/Rhein.

G. HEINKE- Director of theMaterials Engineering Dept.of BASF, he studied mechan-ical and materials engineer-ing at Darmstadt TechnicalUniv. where he earned hi«doctorate.Since Joining thecompany« he has beeninvolved in materials testingin the fields of metals andpiasti««, including fiber»reinforced materials.

U J* STf INHOPF-non-

destructive testing activitiesof the BASF MaterialsEngineering Dept. Includinga group developing newNOT technique«, lie studiedmaterials engineering atDarmstadt Technical Univ.He is a member of severalGerman standards workinggroups (DIN) involving non-destructive testing and ischairman of the ultrasonicworking group of DIN.

Âbdul Hanan, P.T.Pupuk, Indonesia: Could the acousticemission testing be used on equipment during operation orjust for detecting crack propagation during pressure testing?Steinhoff: Some say the test is quite good, and the otherssay it is not. If there is a special problem for one particularpiece of equipment, for which one can perform tests beforeputting it in service, I am sure you can use this technique.But, if this technique is used after five or ten years ofoperation without any preparation before going in service,then you may have to pay.Anders Nielsen, Haldor Topsoe: Mr. Steinhoff, I thinkalso the question was, "Can you use acoustic emission onequipment in operation?"Steinhoff: We placed this acoustic emission in about tenof our plants, and we have not been successful. So, I shouldsay no.Hanan: In some plants, they use acoustic emission testingfor crack monitoring of ammonia tanks. Is this reliable?Steinhoff: I don't know.Hanan: How prevalent is the use of eddy current testing onmagnetic material? So far the use of eddy current testing ismostly on nonmagnetic material.Steinhoff: I think eddy current testing is the mostsensitive method: there are many nonrelevant signals that canmask relevant signals from discontinuities. This methodshould not be used for testing weld seams, since theirconductivities vary drastically, and a major problem occurswhen this method is used on steel. We succeeded only intesting thin tubes, but not thicker tubes.Max Appl, BASF: I would like to illustrate Mr.Steinhoff s statement about the reliability (or unreliability)of acoustic emission testing. Some of you might rememberwe had a paper in Denver on the inspection of ouratmospheric ammonia storage tank at BASF Chemicals in

Seal Sands in the UK. This tank underwent acousticemission testing several times without indicating anydefects, but when we inspected the tank internally, we foundextensive stress corrosion cracking.Mike Conley, DuPont: The comments made aboutacoustic emission are very interesting. DuPont has usedacoustic emission since 1973 on hundreds of vessels, and it'san important part of our integrity program. I'll be presentinga paper later that deals with it in some detail. Your pointsare well taken that it does not measure the size of defects. Itdoes not do a lot of things, but it's a tool that can work ifapplied properly.K. R. Krishnaswami, Madras Fertilizer, India: I wouldlike to ask a question on the testing of tubesheet welds in thesynthesis-loop high-pressure heat exchangers. This tube-to-tubesheet weld leaks at very minimal quantities. Theammonia leaks out into the cooling water or the boilerfeedwater depending on the case. While fouling occurs in theformer, can the stream get contaminated in the latter? Whatis the extent or magnitude of a leak that can be detected byyour equipment and method? In other words, if you applythis new test, is it possible to show that this leak is somany liters?Steinhoff: The method of spot-radiographing tube totubesheet welds was developed to increase the integrity of aheat exchanger by testing the quality of the welds. It is notpossible to apply this method to get a leakage rate.Krishnaswami: What is the thickest tube/tubesheet thatcan be tested?Steinhoff: About 70 mm in diameter and about 4.5 mm inmaximum thickness.

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