Reformer Tools: Failure Mechanisms, InspectionMethods and Repair Techniques

Failure mechanisms in steam reformer tubes are reviewed including reformerdesign, catalyst type, and operating conditions. Inspection methods are presented as

well as techniques by which an operator assesses the remaining life expectancy of areformer tube. Options in the event of a tube f allure are also discussed.

B.J. Cromarty

ICI Katalco, Billingham, Cleveland TS23 1LB, England

ICI has been directly involved with SteamReformers at its Billingham, England, sitesince the 1950's. Since then, ICI has beenconcerned with all aspects of steam reformingtechnology: process design; mechanical design;catalysis; production; maintenance andrefurbishment. ICI has not only operatedmany types of reformer itself, it has alsogained a wealth of experience from the plantsof its licencees and catalyst customers.

On an ammonia plant, the steam reformervies with the synthesis loop for being themost complex and costly part of the plant.Within the reformer, the tubes operate atthe limits of temperature and pressure forthe materials concerned, and consequently,reformer tubes are expensive. The costof retubing an entire furnace can currentlycost around two to three million US Dollarsfor a "typical" 1000 tpd ammonia plant.Consequently, it is essential to ensurethat tube failure does not occur prematurely.

This paper will review the designprinciples of reformers, and the commonfailure mechanisms for reformer tubes.The emphasis of the paper, however, willbe on the operational aspects of reforming:

what can an operator do to reduce thelikelihood of tube failure?

- how can an operator monitor and predicttube life?

what options are available in the eventof tube failure?

Reference will be made throughout toreal examples.

It should be noted that the discussionwill be focused on reformer tubes. Distrib-ution systems, inlet and exit systems, pig-tails and headers have not been addressedexplicitly in this paper.

REFORMER DESIGN CONSIDERATIONS

There are three principal types ofconventional reformer design - namely,top-fired; side-fired; and terraced-wallfired - and within each of these, thereare a number of variants developed byparticular contractors. In all cases,however, the overall problem is to designa system in which reactant gases at 450-650°Cand 30-45 bar arrive through a pipe, aredistributed to several hundred verticaltubes filled with catalyst, heated to800-900°C, and then collected and deliveredto the next stage of the process. Thisimposes many mechanical problems - forexample, concerning the support of thereformer tubes; and the stressing of gasinlet and exit systems.

From the point of view of the reformertubes, it is necessary to have the differenttubes in the reformer operating with thesame temperature profile.

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Ibis can be achleved by balancing theheat and reactants flow into the differenttubes. Perhaps a key feature of reformerdesign with regard to tube failures is whetheror not individual tubes have small-boreinlet and outlet connections which maybe "nipped" in the event of a tube failure,allowing individual failed tubes to beisolated without the need to take the reformeroff-line. Where individual tubes cannotbe isolated on-line, then a single failurewill cause a plant shutdown, leading toproduction loss and full thermal cycling.

As is well-known, the high-temperaturematerials used in reformers undergo creep,which in time will lead to failure. Reformertubes are generally designed on the basisof an "expected life" criterion, or "rupture"criterion. The simplest way to presentrupture data for a given alloy is to plotstress (or log stress) against log timeat a given temperature. This is usuallydone on what are known as Larson-Millerdiagrams in which log stress is plottedagainst the Larson-Miller parameter P.This is specific to a given material, andis defined as

P = T(log(t)+K)/1000

where T = material temperaturet = timeK = material dependent constant.

As an example, Figure One shows theLarson-Miller diagram for a typical HK40.This shows the statistical correlation takenfrom a large number of measurements, togetherwith the 95% confidence limits.

Since P is a function of log time andtemperature, Figure One actually definesa surface if three axes (log stress; logtime; temperature) are considered: then,for any given stress, or pressure, a seriesof curves relating log time to temperaturecan be obtained, as shown in Figure Two.This illustrates very clearly the importanceof the tube wall temperature on tube life:for example, if the design life is ten years,then an increase in tube temperature of20°C would result in the reduction of thetube life by nearly a half. Severe overheat-ing to 1100-1200°C would result in tubelives of minutes. Clearly, the controlof the reformer tube wall temperature willbe critical to determining the actual lifeachieved by the reformer tube.

REFORMER TUBE FAILUEE MECHANISMS

From the previous section, it is clearthat tube failures are mainly as a result ofcreep, and that tube overheating is the primecause of this. There are, however, othermechanisms at work which can lead to tubefailure, such as stress corrosion cracking,longitudinal stress, and thermal shock. Inthis section, various design and operationalaspects will be considered, to see how theycan contribute to these failure mechanisms.

Tube Tops and Bottoms

It is easy to forget about the topsand bottoms of tubes: tube tops, however,do protrude above the furnace roof; andin some designs, tube bottoms have a coldcatalyst discharge end. Condensation (withassociated concentration of impurities incondensate on re-evaporation) can occur,leading to premature tube failure duepredominantly to stress corrosion cracking.Careful design of tube ends, and suitablestart-up and shutdown procedures to avoidthe dew-point of steam being reached, areneeded to prevent this problem. Althoughthis problem appeared to have receded, ithas reeently reappeared, and currently thereare several plants under investigation whichhave cracks at the tube tops.

Thermal Cycling

During the life of a reformer tube,it will experience a number of full thermaland pressure cycles caused by plant start-ups and shutdowns. The cumulative effectof these cycles can be very damaging, andlead to accelerated creep cracking. Thetube life is crudely related to the numberof cycles it has seen, and possibly alsothe tube wall thickness. Thick tubes, whichfor the sake of convenience may be definedas those in which the od:id ratio is greaterthan 1.35 (eg 17 mm wall thickness for a100 mm bore tube) are significantly lesstolerant to thermal cycling than thin tubes.Fortunately, the availability of strongeralloys in recent years leading to thinnertubes has reduced the significance of thisproblem in new plants.

Burners

The main objective of the burnerarrangement in any given furnace designis firstly to achieve an even heat fluxacross the furnace (and therefore uniform

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tube wall temperatures); and secondly, toachieve a vertical heat flux profile compatibleto that demanded by the reforming reactionstaking place within the tube. Uneven firingwill cause overheating in some tubes, andan inadequate heat flux to others, producinga range of product compositions and aninefficiënt reformer.

Let us assume that the burner systemhas been well-designed initially to giveeven firing across the reformer. It is nowessential that the operator maintains theburners properly. The most common problemis that of poorly-maintained burners leadingto flame impingement onto tubes. This willresult in local overheating of the tubes,and therefore reduced tube lives. However,there are other requirements: it is importantthat the design assumptions around the amountof heat delivery and flame shape are matched,and burner testing is essential.

As an illustration of the importance ofthe role of the burners, we can consider thisexample of a recent reformer tube failure.As is usually the case, therë is no singlecause of failure: rather, there is a cornb-ination of factors which ultimately leadto the failure of the tubes. In this example,however, it will be seen that the burnersare at the heart of the problem. This part-icular reformer is part of a small methanolplant, having only 72 tubes. Catastrophictube failure occurred in the late 1980's.After investigation, the sequence of eventsleading to the failure was found (in summary)to be:

i Serious burner problems on a significantnumber of burners: the burner quarlswere black, showing that the flame wasnot stabilised. Subsequently, manyburners were found to have erosion atthe tip, leading in the worst case toa hole in the burner tip. These burnerproblems gave rise to local overheating,which led to a small reformer tube leakfollowed by catastrophic failure ofa single tube. Burner problems of thistype have been noted on several plants,and can be easily rectified by the choiceof a suitable material for the burnertip.

ii This led to a plant trip. An attemptwas made to restart the plant immediatelyafter this trip. Now, the large leakon one tube resulted in reduced flowto the other tubes. This, coupled with

control of the reformer using unreliabletemperature measurements, gave riseto severe overheating of the furnacein general. Mdition of the naturalgas during the start-up led to quenchingof the reformer tubes, causing manytube failures, predominantly at uppertube weids. The plant shut down again,and had to be completely retübed.

Overfiring During Start-up

Large-scale steam reformers are suscept-ible to overfiring during plant start-updue to the inherent large thermal dead-time.Over the past decade, many of the catastrophicreformer failures have occurred during start-ups. It is important that during thiscritical operation, regular and frequentvisual inspections of the reformer tubesare made. The instrumentation on the reformeris primarily designed for monitoring andcontrolling operation at design flow-rates.During start-up, when flow-rates are wellbelow design, reliance on plant instrument-ation alone is dangerous. In particular,the reformer gas exit temperature is nota good indication of the temperatures inthe reformer.

Tube Support System, and Bowing

Almost all reformer tubes are toptensioned. This tensioning produceslongitudinal stress in the tube which mustbe added to the longitudinal stress causedby the pressure difference between the tubeinterior and exterior. Purther stress iscaused if the tubes are bowed, due forexample to differential heating betweenthe two sides of the same tube. The bendingstress produced is proportional to thedeflection from the vertical, and increaseswith the degree of top tensioning. If,therefore, tubes are bowed, then the sumof the combined stresses due to pressure,tensioning and bowing may be such that theallowable stress is exceeded, leading toshorter tube life. Since weids are frequentlyweaker than the parent material, the locationof weids on bowed tubes must be taken intoaccount. Figure Three shows the longitudinalstresses for a specific tube. In this case,it is clear that a weid in the region 40-50% down the tube would not be advisable.Nbte that this does not take into accountthe effect of any local overheating or hotspots in this region, which would have theeffect of significantly reducing the allowableweid stress.

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It is important to reinember that tubesexpand by around 100 to 150 mm between coldconditions, and when operating at full plantrate. It is therefore necessary to ensurethat movement of the tube on expansion isnot restricted, since this can result inbowing. For example, the roof and floorseals should not be packed so tightly asto lock the tube into position when cold,making it unable to expand.

Thermal Shock

Extreme thermal shock such as byquenching of the inside of the reformer tubeswill create both a high tensile stress onthe inside of the tubes, and reduced ductilityleading to sudden, deep cracking, or evenshattering of tubes. It is necessary toensure that water carryover, for examplefrom boilers, is not possible.

An example of such a suituation wasrelatively recently found in a large, modernWestern European plant. The ammonia plantconcerned is a 1350 tpd unit with a top-firedreformer. It was successfully commissioned,and shown to be capable of running well bothat and above flowsheet rates. However, afterless than a year in operation, a tube failed.This was followed by seven further tubefailures in the following 8 months. Onexamination, non-destructive testing (NDT)revealed widespread cracking of tubes, part-icularly at weids.

The tubes had generally failed bylongitudinal splitting. The tubes splitsome 4 to 5 feet down from the top of theroof, with the split being typically 2 feetin length. In all cases, cracks had origin-ated from inside the tube bore. Deep crazecracking was found to be common around thevicinity of the split. The cracks were allbrittle fractures, and were typical of thermaloverstressing. This form of failure canarise due to rapid heating of the outsideof the tubes or alternatively due to rapidcooling of the inside of the tubes. Thepresence of the craze cracking in the boreof the tube in this case suggested that thelatter mechanism was more likely. It appearedthat one or more thermal shocks occurred,since the cracks were arrested some 2-5 mmfrom the outer surface of the tube. However,creep of the remaining much reduced wallthickness rapidly led to final failure ofthe tube.

Extensive NDT was carried out. This

revealed that there were many more cracksin top weids than bottom weids, suggestingan ingress of cold fluid in the top ofthe tube. Purthermore there was a distinctivepattern of cracking in the reformer, withthe cracks increasing progressively fromRow A to Row K of the furnace. This provesthat the cracking had not originated duringtube fabrication, since this would haveresulted in a random distribution of cracking.

From the performance and appearanceof the reformer, there were no obvioussigns of catalyst deactivation. However,if the problem was indeed due to fluidquenching, then some evidence of an effecton the catalyst would have been expected.Analysis of the catalyst in the top ofthe reformer revealed the presence of 0.2%phosphorus as P$'g. Phosphorus is notpresent in new catalyst. An obvious sourceof phosphorus in this example was thephosphate doping of the boiler feedwater.Since the volatility of phosphate in steamis negligible, it could only have arrivedon the catalyst in solution form.

The weight of evidence thereforepointed at boiler water carryover intothe reformer. Since there was not massivebrittle failure of many tubes in one incident,it must be assumed that the quantity ofliquid getting through to the reformerin this case is small. The configurationof the steam system was such that waterfrom any source would go into the reactionsteam line and be carried forward to thereformer. Because of the short residencetime in the mixed feed preheat coil, completevaporisation is unlikely.

It was recommended that a catchpotbe installed in the reaction steam line.When this was done, the tube failure problemwas resolved. It is worth noting thatin this case, the tube walls were relativelythick: this will of course make the problemof cracking due to thermal shock moresevere. More modern designs, using thinnertube walls, should not be as prone to thistype of problem.

Catalyst Charging

The way that the catalyst is chargedinto the reformer tubes will have an impacton the performance of the reformer in twoways. Firstly, it is important that thevariation in catalyst pressure drop betweendifferent tubes of catalyst achieves a

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permitted tolerance. Differences in catalystpressure drop between tubes will resultin differences in flow between tubes, andthis in turn will result in differencesin tube wall temperature. The results fora typical top-fired furnace are shown inFigure Four: it can be seen that packingtubes to within +10% of the average catalystpressure drop will result in a temperaturevariation of around 12°C. From Figure Twoit is clear that this could result insignificantly different tube lives. Clearly,the more uniformly the catalyst can be packedinto the reformer tubes, the better. Withreasonable care and a systematic approach,a tube-to-tube pressure drop variation of+3% is achievable in a reasonable timescale.ICI has developed a portable catalyst pressuredrop measurement instrument, which canquickly and accurately measure the pressuredrop of catalyst in each tube.

Secondly, packing within a given tubemust be uniform - that is, there shouldbe no voids. Clearly, if there is no catalystpresent, then there is no reaction to removethe heat from the tubes. This will resultin localised "hot spots" on a reformer tube.

Care should be taken to allow for theeffect of tube expansion. Sufficientcatalyst must be charged into the reformertube when cold to make sure that whenoperating, and therefore hot, the catalystdoes not settle down so far as to exposéspace at the top of the reformer tube.

Catalyst Performance

Several aspects of catalyst performancecan lead to overheating of reformer tubes.

i Initial Cfaoice of Catalyst Shape.Simplistically, the greater the overallactivity of the catalyst, the coolerthe tubes will be. In recent years,there has been a move away from conven-tional reforming catalyst rings tomore complex shapes, which have higheroverall activity. This will lead tolower tube wall temperatures underidentical operating conditions. Itis important to note that the optimis-ation of such a new shape is not simplyone of increasing the geometrie surfacearea (gsa) of the catalyst, and therebyincreasing its activity. In many cases,the overall activity is limited bythe heat transfer process from theinside tube wall to the bulk gas/catalyst

surface interface. It is important,therefore, to improve the heat transferproperties of the packed bed of catalystinside the reformer tube, as wellas increase the gsa. The effect ofchanging from ICI rings to ICI 4-holeoptimised shape under identical operatingconditions is shown in Figure Five.In this example, one of ICI's top-fired furnaces in Fjigland, an overallreduction in the maximum tube walltemperature (about a third of theway down the tube) of 20°C is possible.It should be noted, however, thatin many cases where operators havemoved from ring catalyst to optimisedshapes, they have taken advantageof this reduction in tube wall temp-erature by uprating their throughput.In many cases, an extra 10-15% ratecan be achieved before reaching thetube wall temperature limit previouslyseen with rings.

ii Stability of Catalyst. Obviouslythe stability of the catalyst activityis important. When a new, activecatalyst is installed, a desired methaneslip can be obtained at a relativelylow maximum tube wall temperature.As the catalyst ages, the activitywill decline, and the methane slipwill increase. Initially, this canbe brought back to the desired levelby increasing the reformer gas exittemperature. This will also increasethe tube wall temperature, however,JQ this can only be done until thetube wall temperature limit is reached.Operation beyond this point will onlybe possible either at high methaneslip levels or at excessively hightube wall temperatures. If this isto be avoided either the plantthroughput must be reduced, or thecatalyst must be changed. FigureSix shows comparative data, takenfrom an actual operating natural gas,heavily loaded top-fired ammonia plant,comparing two commercially-availablealkalised catalysts - one with goodstability, the other with poor stability.

iii Catalyst Deactivation. Just as catalystaging will reduce catalyst activityand lead to high tube wall temperature,so other deactivation mechanisms canlead to the same effect. The twomost common are poisoning, and carbonformation.

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If the reforraer catalyst is poisoned,the tube appearance can go from mildlyblotchy ("hot spots" or "giraffe necking")to severely hot-banded. Some poisons,such as arsenic, are irreversible,and the catalyst must be renewed.Other poisons, such as sulphur, arereversible: the sulphur can be removedfrom the catalyst by steaming. Ofcourse, the source of the sulphur mustbe identified and removed, otherwisethe sulphur poisoning will just recur.

Carbon formation can occur when thereformer feedstock contains a significant"tail" of heavier hydrocarbon, or whenthe reformer, even operating on a veryclean natural gas, is very heavilyloaded. Carbon is formed by thermalcracking of the hydrocarbon. Sincethis is a thermal effect, the criticaltemperature to consider is the insidetube wall temperature, since at anypoint down the tube, this will be thehottest surface seen by the gas. Forany reformer and feedstock, this canbe calculated, and will, of course,be linked to the outside tube walltemperature. From the kinetics ofthe carbon formation and removal react-ions, the carbon formation zone canbe defined as a temperature function:if the inside tube wall temperatureexceeds this carbon formation temperaturelimit, then cracking will occur, andcarbon will be deposited onto the catal-yst. This will result in severe deact-ivation of the catalyst, giving riseto immediate hot bands, which if un-treated, will lead to hot tubes, blockedtubes, and potentially, tube failure.

The best way to deal with a systemwhich is predicted to be carbon-formingis to use a lightly alkalised catalystin the top half of the reformer tube.The alkali enhances the carbon removalreactions, and effectively moves thecarbon formation temperature up sign-ificantly. For especially severe cases,there is a further benefit to be hadby using an optimised shape ratherthan a ring, since this will reducethe tube wall temperature. These effectsare shown in Figure Seven.

The "Wind-down" Effect

If a hot tube or hot spots develop,then it may often happen that the local

firing is reduced, to help lower the tubetemperature. In order to maintain theoverall production rate, however, it isthen deemed necessary to increase the generallevel of firing. This has been known tolead to more hot spots - so the local firingis reduced, and the general firing increased,as before. This process can lead to avicious circle, ending with many damagedtubes, and reduced overall firing efficiency.It is probably advisable to live with theslight loss of efficiency caused by NOTincreasing the general level of firingin the first place.

INSPECTION MEIHODS AM) MONITORING TECHNIQUES

Several methods of NDT are available.As a general observation, none of themshould be used in isolation: the best app-roach is to use several techniques, andinterpret the results with care and attentionto detail.

A useful first approach in an examinationto confirm tube condition is by visualexamination, and tube diameter measurement.Obviously, baseline measurements must betaken when the tubes are new. Furthermore,readings must be taken at defined pointsdown the tube, since there can be a variationin tube od of up to 2 mm in a new tube.Locations should be chosen in the regionwhere the highest tube wall temperaturesare expected. Readings of the tube diameterat these defined points taken periodicallycan give a reasonably good indication ofcreep damage. The increase in diameterat failure is often of the order of 1%.Accordingly, once the diameter growthreaches 0.7% or so, then this should beregarded as significant, and the tube con-cerned should be carefully considered byother techniques to confirm these findings.Clearly, a danger with diameter measurementsat defined locations down the tube is thatthe location may not correspond to thehottest region of the tube. Inspectorsneed to be aware of the measured tube walltemperature profile down the tube, andrelate this to the diameter measurementlocations. Modern cast high temperaturealloys may have little ductility, and insome cases, failure can occur before anysignificant increase in diameter is iden-tified.

For examination of butt weids forcracking, the only really satisfactorymethod is radiography. However, the

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technlque requires some expertise, and invol-ves some practical difficulties in that itis slow, and limits access to other partsof the furnace whilst it is being used.

Ultrasonic attenuation methods fordetecting creep damage in the parent materialhave been widely available for many years.If used correctly, they are a powerful inspec-tion technique for identifying local areasof creep. However, on many occasions, soundtubes may be identified as defective, andthe technique should not be used in isolation.Perhaps a better approach is to record resultsas a function of time, and monitor for det-erioration, rather than for absolute sizingof defects.

Prior to undertaking such metallurgicaland NDT examinations, it may be useful totry and highlight those areas in the reformerwhere there may be concerns, and where suchfurther examination may be required. Thismay be done by calculating the life fractionwhich has expired for each tube. This canbe done with a knowledge of the pressure,the tube wall temperature and the tJuneexposure. The pressure at the tube inletis usually fairly constant and known. Regulartube wall temperature measurements can becarried out, allowing the peak metal temper-ature to be plotted against time on linefor each tube. Naturally, care must be takento ensure that as accurate a measure of thetube wall temperature as possible is used.A simple, uncorrected reading from an optical(infra-red) pyrometer will tend to read highby up to 50-100°C for a top-fired furnace,and 30-50°C for a terraced-wall type furnace.

The methodology can be described asfollows:

i Calculate the hoop stress in the tubesusing

ii

wherepdjjt

2t

= hoop stress, (N.ram" )= tube inlet pressure (N.mm"2)= mean of tube id/od (mm)= tube minimum sound wallthickness (mm)

11 Using the appropriate Larson-Millerdiagram, calculate the Larson-Millerparameter, P. Note that this is doneusing the mean creep rupture curve.

IV

For each tube, plot the maximum tubewall temperature against time on line.Divide the temperature into 10°C bands,and calculate the time each tube hasspent at each 10°C temperature band, n.

For each tube, for each 10°C temperatureinterval, calculate the time to rupture,N, using

P = T x IQ~* (K + log N)

where P = Larson-Miller parameterT = maximum tube wall temp (K)K = material constantN = time to rupture (h)

Having now calculated an actual exposuretime, n, and a calculated rupture time,N, for each 10°C temperature band,for each tube, the life fraction expired,F, can be calculated using Robinson'sRule.

N, N2 N3

Failure occurs when F = 1.

As an example, data from one cell ofICI's terraced wall reformer is shown inFigure Eight. The tubes were installedin 1972. The monitoring exercise was doneon three occasions. There are two signif-icant points to note: firstly, the tubesat each end of the cell had consistentlyhigher fractions of expired life. Thisis probably as a consequence of the id fanconfiguration, compounded by the end walleffects. Secondly, there was a markedincrease in the rate of deterioration between1982 and 1983. This was subsequentlyattributed to significant increases in plantrate (and therefore firing). This analysisenabled priorities to be set for tubereplacement. Note that whilst the absoluteaccuracy of the expired life fraction Fcalculated may be questioned, the comparativevalues do indicate those tubes which aremost vulnerable to failure.

The above approach does require reason-ably detailed tube wall temperature datafor each tube as a function of time. Analternate technique, when only a small amountof data is available, is to préparé a familyof statistical failure curves for the peaktube wall temperatures above and below the

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expected or design furnace figure. TheInformation needed to préparé these curvesat each temperature is the mean stress torupture data for the tube material, and the95% confidence limits. By assuming a normaldistribution through these points, the percen-tage of tubes failing at a given stress inperiods before and after the mean life canbe calculated. Then, as tube failures occur,a line showing the actual number of failures,or percentage of failures, can be superimposed.

A real example of this approach is shownin Figure Nine. It can be seen that up toabout 8 years life, the reformer tubes wereon course for a life of well over 10 years,and were behaving as if the mean maximumtube wall temperature was about 860°C. Thisin fact was very close to the design temper-ature. A significant change in behaviourwas seen between 8 and 10 years, when thefailure rate increased, and the tube walltemperature curve appeared to be some 20°Chigher. This was attributed to a changein the nature of the feedstock, leading tohigher tube wall temperatures, and re-emphasises the role that the optimum choiceof catalyst could have in such circumstances.At about 10 years, the failure rate accelerateddramatically, with a mean maximum tube walltemperature of 900°C or more. On subsequentinvestigation, it was found that changeshad been made to the way the furnace wasbeing fired, in an attempt to improve theoverall plant performance. By changing thefiring pattern, it was possible to reducethe failure rate and allow the reformer tooperate until a planned retube date wasreached.

OPTIONS FOLLOWING A TUBE FAILURE

Following a single tube failure, ratherthan a catastrophic failure of all or mosttubes, it may be pertinent to consider whatthe repair or replacement options are. Insome cases, such as the small reformerdescribed in Section 3.3, the failure ofa single tube can significantly alter theflow pattern of reactants through the remain-ing tubes. This must always be evaluatedcarefully. On a larger, 1000 tpd ammoniaplant, with several hundred tubes, the lossof one tube may not significantly alter theflow through the remaining tubes. If thisis the case, and if the reformer tube failureis such that it does not generate a flamewhich impinges on neighbouring tubes, thenit may in fact be possible to continue runningeven with the split tube. However, in most

cases, it would be preferable to eitherreplace the tube, or isolate the tube ifthis is possible.

ICI has developed a device which allowsreformer tubes to be isolated with thefurnace on-line, by squeezing flat the inletand outlet pigtails - hence the name,"pigtail nipper". This device, which ispatented, is supplied to customers througha simple licence agreement, which coversthe supply of the know-how necessary forthe safe and efficiënt use of the pigtailnipper. As more and more tubes on a givenreformer are nipped, however, the flow patternthrough the remaining tubes will becomeincreasingly disrupted. Also, the dutyon the catalyst will become increasinglysevere - for example, if 10% of the reformertubes are nipped, then the remaining 90%of the catalyst volume will have to copewith the 100% flowrate - an effective "uprate"from the catalyst point of view. Both ofthese effects need to be evaluated carefully,since they can both result in significantlyhigher tube wall temperatures for theremaining tubes.

In cases where cracks in tubes havebeen detected, rather than the tubes havingfailed, repairs may be carried out. Themost important fact about weiding of castalloy tubes in service is to ensure thatthe weid metal is allowed to contract duringsolidification. Great care is needed inboth setting up for weiding and duringweiding. Practical trials have shown thatit is possible to repair the most severelydamaged material for some short term life,where either production requirements orunavailability of replacement materialsdictates.

CONCLUSIONS

Reformer tubes undergo creep, and aredesigned on the basis that the failuremechanism will be creep rupture. In orderto minimise the likelihood of this, careshould be taken to avoid overheating thetubes. A number of factors can lead tooverheating:

burner maldistribution (mismatch ofheat-release and reactants flow)burner maloperationoverfiring during start-upcatalyst factors - charging

- choice of shape- stability

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- deactivation.

An operator has the ability to influencesome or all of these factors, to maximisetube life or reliability.

Other failure mechanisms must not beoverlooked:

stress corrosion cracklngthermal cyclingthermal shock * B.J. Cromartybending stress.

Whilst an operator may have less controlover these parameters than over those leadingto overheating, nonetheless some degree ofmonitoring and control is possible. A rangeof inspection methods and monitoringtechniques is available which, if used withcare and discretion, can assist the operatorin determining where failure will be mostlikely, and in developing a suitable tubereplacement strategy. Some limited repairsare possible to get some short term lifeif necessary.

It must be emphasised, however, thatthe prediction of reformer tube life is byno means an exact science. A detailedunderstanding of the many different factorsinvolved coupled with a sound appreciationof plant performance, is needed to minimiselosses due to tube failures.

ACKNOWLEDGEMENTS

The author wishes to acknowledge thewealth of information on these topics madefreely available to him from members ofICI Katalco, ICI Engineering Department,Teesside Operations, and various customersof ICI Katalco. Particular thanks are dueto:

B StirlingR KeilP J NicholsonN MackenzieJ M TruscottJ R ByresP Farnell

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100,000

1,00016 17 18 19 20 21 22 23 24 25 26

P(LARSON-MILLER PARAMETER)

Figure 1. Larson-Miller diagram: results of 170rupture tests on a typical HK40.

50

800° 900° 1000° 1100°

MEAN TEMP. ACROSS TUBE WALL ('C)

1200'

Figure 2. Influence of internal pressure and tubewall temperature on iife for typical reformer tubemade of a typical HK40.

oT

Z 20Q

Assumptions

TubeLength 17.5 minternal" diameter 104mrnWall thidtness 8.4mmInterna! pressure 24.6 bar

Tube tap. tensionJng .10.0%Bowing 1 tube diameterWeid strength 80% of tube

FRACTION DOWN TUBE

1.1 -,

1.05-

1.0 -

0.95-

910

- 900

- 890

- 880

870-15 -10 -5 +5 +10 +15 +20

Relativa pressure drop (%)

Figure 4. Effect of catalyst pressure dropvariation on tube wall temperature.

TOP F1RED REFORMER

R1NGS

O 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

Distance down Tube (m)

Figure 5. Effect of shaped catalyst on tube walltemperature profile.

i *IS :ft "*"" '™A X"

,/' V'Poorstability

Good Stabillty

400 500 «Days on Line

700 800 900 1,000

Figure 3. Longitudinal stress in a bowed reformer Figure 6. Effect of catalyst stability on tube walltube. temperature.

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TOP-FIRED REFORMER

O

TOP0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

FRACTION DOWN TUBE BOTTOM

Figure 7. Carbon formation.LIFE - Years

10 20 30 40 50 60 70 80 90 100 110 120 130 140

TUBE NUMBER

Figure 9. Comparison of actual tube failure rateand statistical predictions.

Figure 8. Example of expired life fraction chart.

DISCUSSION

Carol Arm Baer, Chevron Research & Technology,Richmond, CA: I have a question about your tube isolationprocess. We call that pigtail pinching. Do you pinch pigtailswhite the plant is still in operation or do you shut the furnacedown? Do you have Ml shut down? What temperature limitsdo you have on the pigtails during pinching?Cromarty: The exact circumstances vary from situation tosituation, but normally pigtail nipping or pinching is donewhen the plant is fully on-line.Baer: Do you pull the natural gas feed at all or continue fullthrough?Cromarty: It can be done while you are running. That is theway it would normally be done.Takaaki Mohri , Chiyoda Corporation, Yokohama,

Japan: Your life fraction calculation seems to be not includingthe thermal stress caused by a thermal gradiënt through thetube wall thickness. So, why you do not include such asignificant effect for tube life?Cromarty: The tubes are designed on the basis of creep; themechanism that will ultimately lead to tube failure is creep.So, that is the basis we use for monitoring. This techniquelooks at steady operating conditions: thermal stress is animportant factor with regard to thermal cycling, but that is nottaken into account in this case. The object is to identify whichtubes are "aging" faster than others: any thermal effects due tocycling will apply to ALL tubes, and so will not affect theanalysis.Max Appl, BASF AG, Ludwigshafen, Germany: Did you

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up to now measure and record tube wall temperatures only withside-fired furnaces or did you investigate also top-fired furnaces?I hardly can imagine it with a top-fired furnace with often morethan 360 tubes. It would be a difficult task to address each tubeproperly. I understand that you are recording and charting eachindividual tube. Could you please comment on this problera?Cromarty: Of course, your observation is quite correct. In aside-fired furnace, you generally have verygood access and youcan identify each individual tube very clearly. In a top-firedreformer, that is much more difficult. In that situation, quiteoften it is possible to group tubes together into regions and doit on the basis of a region, a block of tubes, if you will.

S.R. Ghosh, Kribhco, Surat, India: A similar question fortube skin temperature measurement. We have applied methodslike optical pyrometer, infrared thermography and also thenewer generation cyclops. We also try to measure temperaureof individual tubes. We have 504 of them in each furnace. Wemeasure it every week. But the problem is of temperaturevariation and accuracy of measurement by individual methods.

We are always in doubt as we find a difference of 20-30° Cfrom one method to another. Further, we are never sure of whatis the emissivity factor we should be using. So, in regard tothe statistical model or all the models that you are talkingabout, if the measurement of temperature is not accurate, we

are not able to really judge the life of it by our aboveexperience.Cromarty: What you say is to some extent correct. However,it is possible if you use a consistent measurement technique tointerpret the results on a comparative basis. You can look atthe results in year one, year three, year five, and see howthings are changing in that sense. So, even though you maynot be getting the absolute tube wall temperatures, there isstill value in using the technique. I would also say that acomparison of the methods that are available for measuringtube wall temperatures is a topic for a paper that I would liketo submit for next year. That is a study that we at ICI haverecently concluded, and clearly the different methods do lead todifferent temperatures. Infrared pyrometers will give readingsthat are, generally speaking, higher than the absolutetemperature. However, there are techniques available to correctthe readings that you get with these kind of instruments andbring them very much more in line with the absolutetemperature of the tube. But, that is a topic that would takeperhaps another half an hour to go through.Ghosh: But, is this a laser based one?Cromarty: No, even the laser instruments need to becorrected for background radiation. Although the laserinstrument gives you an emissivity value, in fact that is only asmall part of the total correction that needs to be applied.

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