Tube Wall Temperature Measurement inSteam Reformers

The theory and practice of tube wall temperature measurement are described. Acomparison of different methods on two types of reformer is given. A number ofcorrection methods are outlined, which allow an operator to correct "measured"

temperatures into "absolute" temperatures.

B.J. Cromarty and S.C. BeedleICI Katalco, Billingham, Cleveland TS23 1LB, England

INTRODUCTION

The tubes in a primary reformer furnaceoperate close to the limits of mater-ials technology in terms of the stressinduced as a result of very high temp-eratures, combined with large differ-ential pressures across the tube wall.Metal temperatures, typically around850°C (1470°F) for top-fired reformersand up to 900°C (1650°F) for side-firedor terraced-wall reformers, mean thatthe tubes undergo irreversible creepand therefore only have a limited lifebefore they fail.

Reformer tubes are usually designedon the basis of an expected life ofabout 10 years, during which time astatistical failure of roughly 5% ofthe total number of tubes in the fur-nace is regarded as typical, providingnormal operating conditions are main-tained .

Operation at tube wall temperaturessignificantly above design can result

in a rapid increase in the number oftube failures, since tube life is verysensitive to the absolute operatingtemperature of the tube. An increaseof only 20°C (36°F) in the tube metaltemperature can reduce the life of thetube by over 50%. This is particularlyimportant, bearing in mind that re-tubing of a typical 1000 te/day ammoniaplant primary reformer will cost some-where in the region of US$ 3 Million.

Clearly it is important to measuretube wall temperatures accurately inorder to prevent premature tube failureby overheating. At the same time, anoperator wants to avoid operating afurnace with tube temperatures so con-servative that the full capacity of thefurnace is not realised. Running areformer on the basis of measured tubewall temperatures which are too highcan artificially limit plant throughput.In favourable cases, a substantialincrease in rate could be achieved byoperating with more accurate tube walltemperatures. In addition, in hydrogen

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or methanol plants, firing the primaryreformer harder in order to reduce themethane slip can have a significanteconomic benefit. A reduction of 0.3%in methane slip (from 3.3 to 3.0 mol%dry) exit the primary reformer of atypical 1000 te/day methanol plant canresult in a net annual saving of approx-imately US$ 0.5 Million/year as aresult of increased carbon efficiencyin the methanol synthesis loop resultingin increased production. An operatormust therefore have confidence in thetemperatures measured.

Either prior to, or in conjunction withnon-destructive test (M)T) examinationsof tubes, the life fraction which hasexpired for each tube can be calculated.This needs a knowledge of the pressure,the tube wall temperature and the timeof exposure. Whilst such life fractioncalculations are of value in a relativesense, even if the tube wall tempera-ture readings are somewhat in error,clearly there is more value in such ananalysis if there is a higher degreeof confidence in the accuracy of thetube wall temperature measurements.This paper reviews the theory under-lying such measurements; reviews thetypes of devices currently used; anddescribes a comparison of the differentresults obtained with these instruments.Finally, the methodology used withinICI to obtain as accurate a tube walltemperature measurement as possible isdescribed.

THEORETICAL CONSIDERATIONS

A body at any temperature above absol-ute zero emits thermal radiation atall wavelengths. The temperature ofthe object may be inferred by measuringthis energy at a given wavelength usinga calibrated radiation thermometer.Radiated energy in the range 0.3 to20 microns wavelength is of sufficientmagnitude to be of interest; thiscovers the visible (0.4-0.7 microns)and the near infrared regions of thespectrum. A black body is defined asa body which absorbs all incident

radiation at all wavelengths withoutreflection or transmission. The radi-ation emitted by a black body is themaximum possible at a particular temp-erature. A black body radiates elec-tromagnetic energy according to atheoretically predictable spectraldistribution shown in Figure 1, knownas Planck's law. This describes theradiant energy emitted (Ipx ) of wave-length X by a monochromatic blacksurface at a temperature T.

EbÄ<T> =

where

[exp (C2/AT)-!]-1

(1)

h -k -c -X -T -

Cl =

Planck's constant (J.s)Boltzmann constant (J.K)Speed of light (m/s)Wavelength (microns)Temperature (K)2Tfhc2 = 3.743 x 108 W

C2 = hehe = 1.4387 x 104 micron.Kk

It is this relationship which allowsus to measure the emitted radiation(at given wavelength) and convert itto a temperature. Real bodies, how-ever, are not perfect black bodies.The ratio of the radiative energyemitted from a real surface (E\ ) ata given temperature T to that from ablack body at the same temperatureEJ.JX ) defines the spectral hemispher-ical emissivity of the surface (ex ).

= EX(T)(2)

EbX

The average of the spectral hemispher-ical emissivity over all wavelengthsis referred to as the hemisphericalemissivity or simply the emissivity (e)

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of a surface and is defined as follows:Eb (T) represents the radiation fluxemitted by a black body at temperature(T) over all wavelengths.

CO

= JL ÎEb (T) J

0

eX EbX (3)

Emissivity therefore, is a measure ofthat fraction of the total energyarriving at a surface that is absorbed.The remaining fraction defines thereflectivity and transmissivity of thesurface. The reflectivity (r) is thepercentage of total radiation reachinga body that is reflected without entry.Reflectivity is zero for a black bodyand near unity for a highly polishedsurface. Transmissivity (t) for metalsurfaces such as reformer tubes istaken as zero. The relationshipbetween these three parameters isdefined by Kirchhoff's expression asfollows :

e + r + t = l (4)

If t = O, as is the case for reformertubes, then:

l - e = r

The above may now be applied to areformer furnace. A radiation thermo-meter can be used to measure theradiated energy at a given wavelengthfrom the surface of a catalyst filledtube, and hence the temperature canbe calculated. However, the instrumentwill receive radiation from sourcesother than the target tube.

Consider a reformer tube at temperatureTt surrounded by a refractory wall atTw and flames at Tf. Radiation emittedby the wall and flames is reflectedfrom the surface of the target tube(since it is not a perfect black body)to the sensing element of the radiationthermometer. The thermometer thereforemeasures the combined radiation inten-sity emitted by and reflected from thetube, giving a measured tube temperatureTm. The thermometer operates over a

narrow band of wavelengths centred atX and measures a radiation intensityE(X >Tm), as shown in Figure 2.

E(X,Tm) = e E(X,Tt) + (1-e) [E(A,TW)

E(\,Tf) ] (5)

Radiation view factors are geometryfactors: they define the fraction ofthe radiative energy leaving one sur-face that strikes another directly.The above equation is derived froma model in which the target tube isassumed to be surrounded by refractoryand flames both of uniform temperature.The tube sees only radiation from therefractory and flames. This eliminatesrefractory and flame emissivities andview factors from the equation.

From the above expression it becomesclear that in order to measure accur-ately the true tube wall temperature,then the tube emissivity and a measureof the background radiation is required.If these factors are ignored, theuncorrected measured tube temperaturewill be significantly in error.

Ignoring the effects of emissivity andbackground radiation will generallyresult in the instrument reading highif the background is hotter than thetarget and vice versa for a coolerbackground. A further problem, termedsight path effects, is that the furnacegas between the target tube and mon-itoring instrument may interfere withthe emitted radiation from the tube.The effects of emissivity, backgroundradiation and sight path, together withsome other effects, are discussed inthe next section.

FURNACE TEMPERATURE PROBLEMS

Target Emissivity

Emissivity varies widely for different

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materials, and also for the same mater-ial at different temperatures and withtime on-line. In addition, surfaceconditions such as roughness and scaledeposits can result in a significantvariation in emissivity over a smallarea of target surface.

The most effective method to minimisesuch an error is actually to measurethe emissivity directly. This is nowpossible by the use of laser pyrometers.However, there are practical difficul-ties, which result in concerns aboutthe measurement obtained in this way.Measurements by ICI in typical steamreforming furnaces have shown that therange of tube emissivities is generallyconfined to narrow band, in a givenreformer. Traditionally, a reformertube was assumed to have a constantemissivity of 0.85. Measurementscarried out using a laser pyrometershowed that in the top-fired furnacestudied, the tube emissivity variedbetween 0.96 - 0.98; and in theterraced-wall furnace (in which thetubes are several years older) between0.90-0.94. These results, however,are believed to be high, and will bediscussed later.

Reflected Radiation

Conventional pyrometers cannot disting-uish between radiation emitted by thetarget tube and radiation emitted bythe walls of the furnace, luminousflames or near-by tubes and reflectedfrom the surface of the target tube.All non-contact radiation pyrometersin fact measure the sum of the two, thereflected radiation superimposingitself on the thermal self-emission ofthe target, masking its true value.The most effective contact method oftube wall temperature measurement isthe gold cup pyrometer. This eliminatesall reflected radiation by forming anenclosure around the target spot. Inthis way no correction for reflectedradiation is required. In addition,since the internals of the gold cuppyrometer approximate well to a black

body, the emissivity can be taken asunity. As a result, this method repre-sents the most accurate way of measuringtube wall temperatures.

To illustrate the magnitude of theerrors caused by emissivity and back-ground radiation, consider a reformertube at a temperature of 900°C (1650°F),surrounded by a hotter refractory wall.The temperature of the target is meas-ured using an optical pyrometer operat-ing at a wavelength of 0.90 microns.Figure 3 shows the theoretical differ-ence between the measured tube temper-ature (which assumes an emissivity of1) and the true tube temperature, asderived from Planck's expression(Equation 1).

This shows that both actual targetemissivity and target minus walldifferential temperature have a signi-ficant effect on the size of the meas-urement error. A larger differentialtemperature will give a greater errorwhich is compounded by an actualemissivity somewhat less than unity.The magnitude of the error caused bybackground radiation is generallydominant when compared with the errorintroduced by assuming the emissivityto be unity. This observation is part-icularly relevant to terraced-wallreformer furnaces where the wallscan be up to 200°C (360°F) hotter thanthe tubes. With an emissivity of 0.85,the theoretical error in measured tubewall temperature would be 60°C (108°F)with a differential temperature of200°C (360°F).

Sight Path Effects

This phenomenon refers to absorptionand emission of radiation by theintervening furnace gases between the

66

target and the instrument. In partic-ular, this is due to carbon dioxideand water molecules. There is a wealthof published data showing that hightemperature combustion gases are part-ially opaque over large portions of theinfrared spectrum. In terms of mean-ingful temperature measurement, it isessential to choose a pyrometer withan operating wavelength at which thefurnace atmosphere is effectivelytransparent to the radiation.

Operating windows are available at 0.9,1.25, 2.2 and 3.9 microns which incombination with infrared filtertechnology allows narrow band operationto become practical. A wider wave-length band gives greater sensitivitybut loses accuracy as the degree offurnace gas interaction increases.Many primary reformer furnaces operateon natural gas firing in which thereis little interaction from the fluegas at wavelengths shorter than aboutone micron. Laser pyrometers arerestricted to a maximum wavelength of1.55 microns whereas optical pyrometermanufacturers prefer longer wavelengthsto minimise sight path effects. Figure4 shows furnace gas opacity variationwith wavelength and identifies somesuitable operating wavelengths whichare used by various pyrometers.

Other Effects

Flames

Flames show appreciable luminosity inthe visible region of the spectrum(1 micron) and tend to affect adverselythe readings of pyrometers operatingat this wavelength. Longer wavelengths,typically 3.9 microns, are favouredby many optical pyrometer manufacturers,since flames become increasingly trans-parent at higher wavelengths.

ICI has had experience with severaloptical pyrometers on its steam

reformer furnaces. The Land Cyclops39, which uses a fixed narrow spectralband centred at 3.9 +_ 0.031 microns,gave a reliable performance in theflame region, whereas the Land Cyclops52, operating over a much wider spect-ral band of wavelengths 0.8 to 1.1microns, showed relatively poor per-formance and reliability in flame.

Solid Particles

Firing a naphtha fuel can result inthe presence of solid particles inthe furnace gases. In general, thisis not a significant problem when mea-suring tube wall temperatures usingradiation pyrometers. A glowingsolid particle in the field of viewof the pyrometer can distort a singlereading; however, multiple readingsshould overcome such problems.

Air Ingress

Reformer furnace boxes are operated atpressures marginally below atmospheric;consequently when a viewing port dooris opened to allow a tube temperatureto be measured, cold air will flowinto the furnace. This will cool thetubes closest to the opening bytypically 20-30°C (36-5A°F). Thisobviously depends on the proximityof the tube to the viewing port, andthe length of time that the viewingport door is left open. Clearly thisfactor dictates that the instrumentmust be simple and quick to operatein order to minimise such errors.Glass-covered viewing ports offer apossible solution but tend to createanother problem since they can distortthe radiation measured by the pyrometer.With careful analysis and choice ofglass this can also be overcome.

Target Identification

On top-fired furnaces, when lookingin through a viewing port at a givenrow of reformer tubes, it oftenbecomes difficult to identify a part-icular tube of interest in the dist-ance. A system whereby a number is

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clearly painted onto each tube can helpto overcome this problem. Havingidentified the tube, it is then oftenuseful to take measurements at a num-ber of points down the tube. This canbe facilitated by painting rings onthe tube at the desired distances downthe tube. Painting two narrow ringsclose together (about 1 foot apart)allows the operator to take tempera-ture readings between the rings. Suchpairs of rings can be painted at anumber of points at known distancesdown the tube. Ordinary white mattemulsion paint can be used for thispurpose.

Geometry Effects

Ideally a pyrometer should "see" rad-iation only from the target if anaccurate temperature measurement isto be obtained. Line-of-sight accessrestrictions and pyrometer opticallimitations both contribute to measure-ment errors termed "geometry effects"due to the layout of the reformer.The magnitude of this effect is relatedto tube diameter, pitch, length oftube row and the angle of the pyrometerto the tubes whilst taking readings.

Optical pyrometers have measuringangles of generally 0.33°. For atypical furnace geometry, at distancesgreater than about 5m (16 ft) into thefurnace a whole single tube cannot beseen, as it will be obscured by neigh-bouring tubes. Radiation from theseneighbouring tubes will be picked-upby the instrument and mask the truetemperature of the target tube.

In addition, optical pyrometers haveviewing angles of 7-9°. Therefore,at distances greater than 1m (3 ft)into the furnace more than one tubewill be in the field of view. Thiscreates problems for the operator oftargeting the correct tube. At dist-ances greater than 10m (33 ft) intothe furnace, individual tubes arevirtually impossible to target adequate-ly.

METHODS OF TUBE WALL TEMPERATUREMEASUREMENT

There is a very wide range of instru-ments available to measure the temper-ature of tubes in a primary reformerfurnace, each with their own particularadvantages and disadvantages. Theprevious section has described thevarious factors that hinder accuratetemperature measurement. In thissection, several of the most widespreadfurnace temperature measuring instru-ments will be described in terms oftheir hardware and the techniqueswith which they seek to overcome thepotential difficulties. The majorcategories are shown in Table 1.

High temperature measuring instrumentscan be divided into two basic categor-ies, contact and non-contact methods.For contact methods the sensing elementis placed directly on the surface ofthe target. Non-contact methods mea-sure infrared radiation of a particu-lar wavelength from the target throughthe furnace gases. They includedevices such as optical and laser pyro-meters .

A further sub-division of contactmethods can be drawn between radiationthermometers (such as the gold cup)and surface thermocouples. Thermo-couples are welded to the target tubeand measure temperature by conductionwhereas radiation thermometers (astheir name suggests) measure the radia-tion emanating from the target surface.

Surface Thermocouples

Surface thermocouples are currentlythe most widespread method of contin-uous tube wall temperature measurementin reformer furnaces. When correctlyinstalled, they can give an accurateindication of the actual tube walltemperature. Installation requires

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a slot to be cut in the tube wall intowhich the thermocouple is located andsecured by either peening over thetube metal or by spray welding.

Two major disadvantages are inherentin this method: firstly, under typicalfurnace conditions, thermocouples havea short and unpredictable life oftypically 6-12 months and are verydifficult to replace; and secondly,the slotting process can significantlyweaken the tube wall. Thermocouplescan be located anywhere within a fur-nace, thus avoiding the 'line-of-sight'access restrictions associated withother techniques.

The potentially more serious problemof a weakened tube wall caused bycutting a slot in the tube can beovercome by spray welding the thermo-couples to the outside surface of thetube. This technique has the draw-back in that it will not provide anaccurate indication of the tube walltemperature at much above 600°C(1110°F). Above this temperature, thethermocouple is influenced by the hotflue gas and radiation emanating fromthe surface of the furnace box refrac-tory lining and will tend to give areading higher than the actual temp-erature, since the thermocouple actsas a heat transfer fin on the surfaceof the tube.

Gold Cup Pyrometer

The Land gold cup pyrometer was orig-inally developed for measuring thesurface temperature of hot steelingots emerging from rolling mills.The surface of the ingots may becovered by scale and other impurities,causing variations in surface emissi-vity such that conventional infraredpyrometers (with a constant emissivityassumed) were found to be inaccurate.The gold cup pyrometer consists of asilicon cell infrared radiation detec-tor which is located behind a fluoritewindow at the crown of a gold-platedhemisphere approximately 2.5 cm(1 inch) in diameter, ICI uses a

version of this pyrometer mounted ona 3.3m (11 ft) long water-cooled probe.

In practice, the probe is inserted intothe furnace and the cup placed accur-ately on the tube axis, thus eliminat-ing all incident radiation from sur-faces and flames surrounding thetarget, and simultaneously removing theeffects due to the intervening furnacegases. On applying the cup, the temp-erature of the tube surface will beginto fall: thus the millivolt signalfrom the detector must be read as soonas contact is made.

Multiple reflections between the hottube surface and the hemisphere createconditions approximating to a blackbody enclosure where measurement ofthe radiation intensity allows directdetermination of the surface temp-erature without prior knowledge ofthe target emissivity. The pyrometermust be applied squarely to the surfaceof the tube; if not, the pyrometerwill be able to 'see' radiation fromrefractory or flame and the black bodyassumption will no longer apply,resulting in an abnormally high temp-erature reading.

The instrument is calibrated in ablack body furnace against platinum/platinum-rhodium thermocouples beforeand after surveys. The measurablerange is 625-1200°C (1160-2190°F) andunder good furnace conditions theaccuracy of this instrument is betterthan 10°C at 800°C (18°F at 1470°F).The gold cup virtually eliminatesthe effect of surface emissivity with-in the range 0.6-1.0, and below 0.6the error is small.

The major disadvantage of the goldcup pyrometer is its unsuitability forcontinuous measurements and itsrequirement for sustained effort bya practised team of two operators.The number of tubes that can be survey-ed is limited by line-of-sight accessand the location of available viewingports. Tubes 2-2.5m (6-8 ft) fromthe inner face of the wall represent

69

the practical limit for precise handlingof the probe.

Infrared Optical Pyrometer

Infrared optical pyrometers are gener-ally hand-held units which are sightedon the target by means of an eye-piece,and a lens system which focuses radia-tive energy onto the sensing element.The major advantage of these instrumentsis their simplicity and the speed atwhich readings can be taken.

As with all non-contact temperaturemeasurement techniques, the instrumentrelies on measuring the infrared radia-tion from the target. In practice,such pyrometers measure the totalradiation from the target and the sur-rounding hotter surfaces. These instru-ments cannot differentiate betweenradiation emitted by or reflected fromthe target. They must therefore becorrected accordingly. Correctionmethods will be discussed in more detailin a subsequent section.

In order to overcome the problem of theintervening furnace gases, pyrometerstypically measure incident radiationover a narrow band of wavelengthswhere the furnace gas is effectivelytransparent. Such windows most commonlyused by optical pyrometer manufacturersare centred at 0.9 and more recently3.9 microns.

Radiation reflected from the target asa result of a temperature differentialbetween target and surroundings canseriously distort the pyrometer read-ing. In order to minimise this error,it is necessary to make a correctionbased on an average of the surroundingsurface temperatures. Background temp-eratures even in the same furnace canvary enormously with position. As aresult, finding an average backgroundtemperature not only introduces errorbut is also operator sensitive.

Optical pyrometers are unable to measurethe target emissivity directly. Whenmeasuring a target temperature, the

emissivity compensation control shouldbe switched off as otherwise it maygive misleading results due to reflec-ted radiation effects. The instrumentthen assumes a target emissivity ofunity and afterwards the reading canbe corrected manually for emissivityassuming a constant target emissivityof for example 0.85.

Laser Pyrometer

Classic infrared pyrometers are passivedevices, in that they receive energyfrom a target which is modified by amanual emissivity selection to givea target temperature. Under theseconditions, it is assumed that noother energy source is present and thetarget has uniform temperature andemissivity. This is very rarely thecase in practical applications. As thedistance from the target (R) increases,the target size increases (R2), whilethe collected energy per unit areadecreases (1/R2); thus, the two effectscancel out and the radiation receivedby the instrument appears to beindependent of the distance from thetarget.

Laser pyrometers incorporate thepassive characteristics of conventionalinfrared pyrometers together with anemissivity-determining feature achievedby an active reflectometer technique.A low powered pulsed laser of knownenergy is fired at the target and thereturn signal is detected by the sameoptics as with a conventional infraredsignal. The return signal is ameasure of the target reflectivity,from which the instrument can inprinciple determine the target emiss-ivity. The laser energy is dispersed(1/R2) on the return path and for thisreason the distance to the target mustbe known. The Pyrolaser in particularhas a very sophisticated optical systemwhich provides ranging to an accuracyof +0.5%. However, for this to beeffective in practice requires thatthe emitted laser beam impact thetarget at right-angles: this conditioncannot be met with reformer tubes, and

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hence there is some concern about theaccuracy of the measured emissivity.This error in the measurement of thereturn signal may be the cause of highreadings of the target emissivity,which in turn will result in a highestimate of the target temperature.

The measured radiation includes thetotal target and background radiation.As with all non-contact methods, toconvert this to the true target temp-erature requires that compensationfor background radiation be made.This is achieved in two ways. Firstly,a representative measure of backgroundradiation intensity is obtained bytaking a few temperature readings inthe region close to the target, whichare then averaged and stored in themicroprocessor. Secondly, the verysmall cone angle of the viewingsystem gives high spatial selectivity;ie, only radiation emanating from thetarget within this cone is detectedby the pyrometer.

This second point is particularly use-ful when trying to measure a hot spoton a tube. This resolution allowsthe user to focus on the hot spot,eliminating cooler adjacent tubes fromthe field of view. Typically, ameasuring field 0.3° gives a targetdiameter of 5mm (0.2 ins) at a 1m (3ft)range.

In summary, the laser pyrometer inprinciple can be referred to as a1pyro-reflectometer' which can measuretrue surface temperatures by combiningbackground temperature measurementswith the direct measurement of thesurface emissivity. Current instru-ments operate at a wavelength of 0.9microns, thus avoiding effects due tothe intervening furnace gases. Use ofthis wavelength limits the lowestmeasurable temperature to approximately600°C (1110°F), since thermal radiationdecreases rapidly with temperature atshorter wavelengths. The upper temp-erature limit at this wavelength is1500°C (2730°F).

The laser pyrometer is a hand-held de-vice and is simple to use, but as withmost non-contact methods it is restric-ted by line-of-sight access. With auseful range of 20 cm - 10 m (8 ins -33 ft) it can cover most tubes withina typical primary reformer furnace box.The laser pyrometer has been in usesince early 1987 in refinery/chemicalplant furnaces, ceramic kilns, weldingon high alloy steels and foundry uses.

Disappearing Filament Pyrometer

The disappearing filament pyrometeris a hand-held optical device whichalthough dated is still used in manyapplications. The target is viewedthrough a lens system. When a temp-erature measurement is to be taken, atungsten filament is superimposed onthe target image. The current throughthe filament is altered either manuallyor via a microcomputer until thebrightness of the filament is the sameas that of the target. When a bright-ness match is obtained the filamentappears to merge with or "disappear"into the image of the hot target. Thefilament current is calibrated interms of temperature.

The disappearing filament pyrometer hasa measured temperature range of 800-3000°C (1470-5430°F). ICI operatingexperience has shown that temperaturesbelow 850°C (1560°F) are difficultto measure. This type of instrumentis losing favour due to the availabilityof direct reading instruments such asinfrared optical pyrometers whicheliminate the operator sensitivity in-volved in matching the colour tempera-tures of target and filament.

CORRECTION METHODS

The main sources of error have beendiscussed previously: they are targetemissivity, background radiation andsight path effects. Use of a Gold Cuppyrometer overcomes these problems.Typically, however, optical pyrometersare used, which need to be corrected

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for these effects. The problem ofsight path effects is only effectivelydealt with by the optimum selectionof the instrument wavelength, andlittle further can be done to improveon the accuracy of a given instrument.Sight path effects can be thought ofas a form of radiative "interference",caused by interaction of the furnacegases. The furnace gases can not onlyabsorb radiation from the target butare hotter than the target tubes them-selves and are radiating energy intheir own right. Radiation emittedby the furnace gases will distort themeasured target temperature, whichwill tend to read high. The greaterthe distance between target and meas-uring instrument, the more significantthis effect becomes. However, witha correctly chosen wavelength andnarrow band, this effect is minimisedbut can still be significant. Inthis respect, both the Land Cyclops 39and the Pyrolaser pyrometer are pre-ferred to the Land Cyclops 52, asshown in the table below:

Instrument Spectral View MeasurBand -ing -ing(microns) Angle Angle

Cyclops 52 0.8-1.1 9 0.33Cyclops 39 3.9+0.031 9 0.33Pyrolaser 0.865 +_ 7 0.33

0.015

Correction for background effectscan be made as follows. The Planck'slaw expression (equation 1) can beintegrated over all wavelengths togive the Stefan-Boltzmann Law for ablack body:

CD

Eb (T) = j Ebx (T) d\

(6)

where < is the Stefan-Boltzmannconstant (5.67 x 10~8 W/m

For a real or "grey" body, theemissivity e must be included:

E (T) = e é T4

This can be substituted into equation5, which on re-arranging gives

rp „

- (l-e) 4 (7)

where Tt - true temperature (K)

- measured temperature (K)

T'w - averaged backgroundtemperature (K)

In order to be used, accurate values ofbackground temperature(s) and tubeemissivity are required. In equation(7), T'w refers to an averaged back-ground temperature, combining Tw andTf as referred to in equation (5).

Averaged Background Temperature (T'w)

Up to 20 "background" temperaturesshould be measured with the pyrometeremissivity set at unity. An averageof these is then taken as follows:for N background temperatures,

'w = fTwl4N

\ (8)

Readings Tw , TW2> etc should includemeasurements of wall and flame. How-ever, the practical difficulties ofmeasuring truly representative back-ground temperatures are more severe.

The overriding principle is to selecta point on the target tube and toimagine what the target will "see":this will be flame, adjacent tubes orrefractory, or both. A suitablybalanced set of "background" temper-atures should then be taken. Ofcourse, these readings will be specificfor each target. The following exam-ples illustrate the principle involved.

Figure 5 shows a schematic reformerlayout. The background correctionrequired will depend on the locationof the target tube.

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iii Tube C

Tube A

The bulk of the background radia-tion seen by the tube emanatesfrom the refractory wall directlyopposite the tube. In addition,radiation will emanate fromadjacent tubes and also refractorysurfaces either high up in thefurnace in the flame regionwhere the refractory walls arevery hot or from the fluegasextraction tunnels at the bottomof the reformer. This is deter-mined by the height at which thetube wall temperature is beingmeasured. Background readingsshould be taken as follows:

10-12 readings from refractorywall directly opposite tubes (al).

4-5 readings from refractory wallin combustion/tunnels regions.

4-5 readings from adjacent tubes(a2).

ii Tube B

This tube is located in the cor-ner of the furnace and is facedby an expanse of refractory wall.More background refractory temp-eratures should be measured atthe expense of fewer adjacenttube temperatures:

6-8 readings from west refractorywall (bl).

6-8 readings from north refractorywall (b2).

4-5 readings from refractory wallin combustion/tunnels regions.

2-3 readings from adjacent tubes(b3).

This tube is located in the centreof the furnace and cannot see therefractory walls. The dominantbackground radiation source is fromthe tube row directly opposite thetarget tube:

10-12 readings from tubes in rowopposite target tube (cl)

4-5 readings from refractory wallin combustion/tunnels regions.

4-5 readings from adjacent tubes(c2).

Effective Tube Emissivity

The values of emissivity obtained bylaser pyrometers appear to be high.This is presumably due to the fact thatthe incident laser beam is not at right-angles to the target tube. A high valueof emissivity will lead to an inadequatecorrection for background radiation,resulting in an overestimate of the truetube wall temperature.

The approach adopted by ICI involvesthe use of a gold cup pyrometer inconjunction with an IR pyrometer. Thegold cup pyrometer, when operatedcorrectly, will give the true targettemperature, since background radiationand sight path effects are virtuallyeliminated, and the black box conditionswithin the gold cup/target eliminatethe need for knowledge of the targetemissivity. Hence, a reading from agold cup instrument can be taken asbeing accurate. These readings can beused to calibrate the IR pyrometer.The steps involved are:

i measure the target temperatureTm using an IR pyrometer withthe emissivity set at 1.0.

11 measure the background temperatureT'w as described above using theIR pyrometer with the emissivityset at 1.0.

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iii measure the "true" temperatureof the target, Tt, using a goldcup pyrometer.

IV

v

VI

from equation (7), a value forthe emissivity can be obtained.

the above procedure is repeatedfor several tubes at variouspoints in the reformer. A spreadof emissivity values may be found:an averaged value of the emiss-ivity for the reformer is thusobtained.

the temperatures of all tubescan now be measured using the IRpyrometer. Background readingsare also taken as describedpreviously. The measured valuesTm are corrected for backgroundand emissivity by equation (7),using the average reformer emiss-ivity as calculated above.

In this way, it is possible to obtainmore accurate tube wall temperatures.Without such corrections, measuredtemperatures are significantly greaterthan the true temperatures, as willbe shown in the following case studies.It should be noted that the emissivitymeasured in this way will be specificto the reformer concerned.

CASE STUDIES

Comparative Study

Two steam reformers on the ICI site atBillingham were selected for a fullprimary reformer tube wall profilesurvey, a 352 tube top-fired Ammoniaplant reformer of ICI/Humphreys andGlasgow design and a terraced-wallMethanol plant reformer of FosterWheeler design. This has two reform-ers, each with two cells, each con-taining 140 tubes, giving 560 tubes intotal. Each reformer has a numberof glass-free viewing ports with slid-ing covers.

In the case of the ammonia plant, thegold cup pyrometer could measure only

six tubes per row of 44, whilst in themethanol plant one cell of 140 tubeswas surveyed in which the temperaturesfrom 30 tubes were measured. Eachtube was measured in turn with a goldcup pyrometer, Land Cyclops 52 opticalinfrared pyrometer and a Pyrolaserlaser pyrometer. The Cyclops 52 wasused to measure tube temperature withthe emissivity set at unity.

Several readings of refractory temper-atures were taken in the region ofthe target tubes using the Cyclops 52.These would be used later in order tocorrect the tube temperatures measuredby this instrument for backgroundradiation.

Similarly, several background temper-atures were taken as above using thePyrolaser. Here, the correction ismade automatically in the micro-processor incorporated within theinstrument. As the Pyrolaser measuresemissivity directly then the tempera-ture displayed by the instrument isin principle fully corrected for bothbackground radiation and target emiss-ivity.

The averaged results are shown inTable 2. They show both the uncorrec-ted and corrected tube wall tempera-tures with the Cyclops 52, whilst forthe laser pyrometer the given temper-ature has been automatically correctedfor emissivity and background radia-tion. The measured target emissivityis also given. All of these measuredtemperatures are expressed relative tothe gold cup pyrometer which is takenas the base reading. All temperaturereadings on the reformers were takenin regions of the furnace where flameswere not in the field of view of theinstrument.

74

When carrying out a comparison ofinstruments, it is inevitable thatsome differences will occur due to

instruments being operator sensitive;time delays between readings; how longthe viewing port doors were left open;etc. Whilst bearing these difficultiesin mind, examination of the readingsfrom the surveys of both reformersreveals a very clear trend in thetemperature differences between instru-ments. This allows several importantconclusions to be drawn from this work.

Comparing all measured temperaturesrelative to the gold cup, it is clearthat there is little differencebetween temperatures measured usingthe Pyrolaser and those uncorrectedtemperatures measured using theCyclops 52. Generally, the Pyrolaseris 10°C (18°F) closer to the goldcup temperature on both reformers,which suggests that measuring theemissivity of a reformer tube with thelaser pyrometer and correcting forbackground radiation with this measuredemissivity has had only a very smallimpact on the final accuracy of thetube wall temperature measurement.

In steam reformer furnaces where tubeemissivity is generally high (in excessof 0.80), there appears to be verylittle benefit to be gained by measur-ing emissivity directly using a laserpyrometer.

Correcting the Cyclops 52 reading forbackground radiation and assumed tubeemissivity as described in Section 5.2has brought the revised tube tempera-ture in line with the gold cup for thetop-fired reformer. This representsa marked improvement over the Pyro-laser. In the case of the terraced-wall reformer, the manual correctionhas had a similar impact on the accur-acy of the Cyclops measurement.

Terraced-wall reformers inherentlyhave a much wider variation in refrac-tory wall temperatures than an equiva-lent top-fired reformer, since theburners are much closer to the wall;consequently, the correction forbackground radiation by taking an

average of several wall temperaturesbecomes a poorer representation of theradiation the target tube actuallyreceives. Therefore, background temp-erature measurements should be madefor each tube individually to obtainthe best results.

The variation in tube wall temperaturesmeasured in the terraced-wall reformer(70-100°C; 125-180°F) is roughly doublethat of the top-fired reformer (30-50°C;54-90°F). Experience gained by ICI inoperating both types of reformer hasconfirmed this to be normal. As ageneral rule, large single box top-fired reformers tend to give moreeven tube temperature profiles thanmulti-cellular terraced-wall reformers.

A phenomenon which ICI has observedin its terraced-wall reformers andwhich was apparent in the results ofthis survey was the undulating temper-ature profile along the length of atube row, termed 'camel humps'. Peaksand troughs of tube temperatures occuralong the tube row. The peaks corres-pond to the position of the fluegasextraction fans which draw hot fluegasover the tubes nearest to the fanscreating regions of relatively hottubes. This is shown in Figure 6,which also shows that the relativereadings of both the IR and laser pyro-meters follow the gold cup readingsquite well. Thus for comparativework, these pyrometers will be useful,but if absolute temperatures are need-ed, then these IR and laser readingsneed to be corrected as described.

The cumbersome nature of the gold cuppyrometer has been well reported.The number of tubes in a typical re-former that can be surveyed is only asmall fraction of the total. Hand-heldoptical pyrometers are lightweightinstruments that are very simple touse. The newer laser technology offers

75

some further facilities. The mostsignificant improvement offered by thePyrolaser over the Cyclops from apurely practical viewpoint is theinclusion of a data-logger. Data-loggers are also available for Cyclopsoptical instruments which simply plugin to the instrument via a flexiblecable.

ICI Katalco Customer Queries

In the course of dealing with itscusomters, ICI Katalco is often askedto assist in the measurement and/orinterpretation of tube wall tempera-tures. A recent case study illustr-ates some of the observations commonlyseen. Tube wall temperatures on thislarge top-fired steam reformer weremeasured, using both a Cyclops 52pyrometer, and a laser pyrometer.The customer wanted to know:

i why different temperatures wereseen for the same tube whenviewed from different portsaround the furnace, using thesame IR pyrometer (Cyclops 52).

ii why the Cyclops 52 measuredhigher temperatures than thelaser pyrometer.

iii why the temperatures measuredat the base of the reformer wereso high, and whether this is anaccurate measurement or not.

Different temperatures for the sametarget tube

The tube temperatures were measuredusing a Cyclops 52 pyrometer (emiss-ivity set at 1.0; no backgroundcorrection) from different viewingports; specifically, east side portswhere the target tubes are 2-3m (6 -10 ft) from the measuring pyrometer;and north/south ports, where thedistance is about 20 m (66 ft). Asample of the results obtained isshown below in Table 3.

These discrepancies arise from the com-bination of sight path and backgroundeffects. The instrument used, a Cyclops52 pyrometer, has a relatively broadwavelength band. At short distances,sight path effects will be small; however, at longer distances of 20 m(66 ft), the field of view diameter is0.12 m (5 ins), and so some backgroundradiation will be seen, leading tohigher measured temperatures.

Cyclops 52 versus Laser Pyrometerreadings

Some typical results are shown belowin Table 4:

At the close distance of 2-3m (6-10ft),little difference between the Cyclopsand Laser pyrometers would be expected.The effects of background would bethe same for both pyrometers; the meas-uring angles of both are such that ineach case the target fills the tube;and the error from emissivity is small.At the longer distances, however, thesight path effects as describedpreviously will start to have an influ-ence. These will affect the Cyclops 52much more than the laser pyrometer,since the Cyclops operates over a muchwider waveband than the Pyrolaser.The uncorrected Cyclops will thereforeread higher at long distances than thelaser pyrometer.

Note that the laser pyrometer readingsas taken here will not be as accurateas Gold Cup readings.

76

High temperatures at the base of theReformer

Tube wall temperature profiles weretaken, using the Cyclops 52 pyrometer.The uncorrected results are shown inFigure 7, together with the calculatedtube wall temperature as predictedby ICI Katalco's reformer simulationprogram. As mentioned in previoussections, the Cyclops readings need tobe corrected for background radiationand emissivity. Background effectswill be particularly severe in thebottom quarter of the tube. In orderto measure the tube temperature inthis region, the operator must lookdown from the viewing port at an angleof about 45° into the base of thereformer. The flue gas extractionsystem ("tunnels") are extremely hotrefractory, and as such, backgroundeffects will be particularly importantin this region. ICI Katalco's approachof using a Gold Cup pyrometer to cal-culate the effective emissivity, andthen correct for background effects,would be recommended. However, inthis case, the customer did not havea gold cup pyrometer.

Although only an approximation, inthese circumstances, it is possible touse ICI Katalco's reformer simulationprograms to assist in the correctionof the tube wall temperature readings.The furnace simulation program calcu-lates the background refractory temp-erature, shown in Figure 8.

If this is taken as the backgroundtemperature, and an emissivity for aknown similar large top-fired reformeris taken, then the correction equationgiven previously can be used, result-ing in the corrected profile shown in

Figure 9. Whilst this is of necessityan approximate correction, it clearlyillustrates that the uncorrected read-ings, particularly at the bottom ofthe tube, are significantly higherthan the true temperature. The un-corrected measured readings of 910-930°C (1676-1706°F) are likely to be50°C (90°F) high; the true temperaturesare likely to be 860-880°C (1580-1616°F)

CONCLUSIONS

Accurate tube wall temperature measure-ment is a useful way of diagnosingplant problems before they manifestthemselves in terms of large numbersof failed or damaged tubes. If a sens-ible method of monitoring tube temper-atures is devised, economic benefitssuch as increased process yields andlower furnace maintenance costs couldbe realised, which would substantiallyoutweigh the cost of the instrument.The tube wall temperature work carriedout on the ICI steam reformers atBillingham highlighted the problemsfacing an operator trying accuratelyto measure the temperature of reformertubes with the pyrometer instrumentscurrently available. In particular,unknown target emissivity and back-ground radiation effects can give riseto significant inaccuracies in theinstrument reading. The principalconclusions which can be drawn are asfollows :

1 Under normal operating conditions,the tube wall temperature measuredusing a laser pyrometer (whichautomatically measures emissivityand corrects for backgroundradiation) was only a marginalimprovement over that measuredusing an optical pyrometer uncor-rected for background radiationwith a constant emissivity setat unity. Both instruments werereading 30-40°C (54-72°F) higherthan the reference tube temper-

77

-ature measured using a gold cuppyrometer (which is assumed tobe the true temperature).

This very small difference inmeasured temperatures betweenuncorrected optical and laserinstruments was apparent inall results from the ICI survey.

An important effect is that ofradiation emitted by the refrac-tory lining of the furnace andreflected from the surface ofthe target to the measuringinstrument, masking the truetarget temperature. The maindifficulty in compensating forthis effect is in obtaininga realistic estimate of thebackground radiation intensity.

Both the optical and laserpyrometers use the same methodto estimate the backgroundradiation which involves takingan average of several refractorytemperatures near the targettube. This provides a substan-tial correction to the measured(uncorrected) temperature.

The laser pyrometer measuresemissivity directly; however,it appears to overestimate theemissivity.

The best results are obtainedby taking the temperaturesmeasured using an IR pyrometerand manually correcting forbackground radiation and tubeemissivity using a simplecorrelation with Gold Cupmeasurements. When takingfield measurements using anoptical pyrometer, the emissi-vity setting should be unity.

Top-fired reformer furnacesgenerally have more even walltemperatures. This factormakes the correction for back-ground radiation easier andmore reliable.

In the operation of steam refor-mers, it is very often the trendin tube wall temperatures ratherthan the absolute temperaturethat is important; an upturn inthe tube wall temperature trendmay indicate a process problemdeveloping. For this type ofdata collection, a simpleoptical pyrometer calibratedand used in a sensible and re-peatable manner will provide aneffective low cost system withwhich to monitor the generalperformance of a furnace.

4 At the moment, the only satis-factory method by which tubetemperatures can be measuredwithin 10°C (18°F) is by a goldcup pyrometer, since this avoidsthe reflected radiation problemwhich plagues non-contact instr-uments. However, it is not ageneral solution to the problemof monitoring tube wall tempera-tures throughout a furnace.

At this stage in its development,laser pyrometer technology does notappear to offer a significant improve-ment in steam reformer tube wall mon-itoring ability over establishedoptical pyrometers due to the unre-solved weakness in all non-contactpyrometer instruments in correctingfor background radiation. Where thelaser pyrometer will come into itsown is in applications where thebackground temperatures are relative-ly even and the target has a lowemissivity, (less than 0.8) where theemissivity correction then becomessignificant. Under such conditions,the laser pyrometer will then becomea very valuable and flexible methodwith which to measure surface temp-eratures. It should be noted thatgood results have been achieved bylaser pyrometers in steam and hydro-cracker units, where tube emissivitiesare usually low (less than 0.85).

78

Notation

~ Spectral blackbody emissivepower at temperature T(W/m2 micron)

E^ (T) - Spectral radiation fluxemitted from a real surfaceat temperature T(W/m2 micron)

EJ.J (T) - Radiation flux emitted by ablack body at temperatureT over all wavelengths(W/m2)

T

X

h

e

r

t

W

a

- Absolute temperature (Kelvin)

- Wavelength (micron)

- Planck's constant(6.6256 x 10-34 j. 3)

- Boltzmann constant(1.38054 x 10-23 J.HÜ

- Speed of light in a vacuum(2.9979 x 108 m/s)

- Spectral emissivity

- Emissivity

- Reflectivity

- Transmissivity

- True target temperature(Kelvin)

- Measured target temperature(Kelvin)

- Flame temperature (Kelvin)

- Wall temperature (Kelvin)

- Combine background (Walland flame) temperature(Kelvin)

- Stefan-Boltzmann constant(5.67 x 10-8w/m2.K4)

- Conversion factor in Planck'slaw = 2 tY he2

= 3.743 x 108 W.micronVm2

- Conversion factor in Planck'slaw = he

k

= 1.4387 x 104 micron.K

ACKNOWLEDGEMENTS

The authors wish to acknowledge theconsiderable time and effort put intothe reformer tube surveys by thefollowing:

I Shakespeare - ICI FertilisersK Proctor - ICI FertilisersD Bramley - ICI Engineering

The authors are grateful for the loanof a Pyrolaser pyrometer, and theassistance provided by Ed Matthewsof the Pyrometer Instrument Company,Inc, New Jersey, USA.

Pyrolaser is a trademark of thePyrometer Instrument Company, manufac-tured in the USA under license fromExxon Research and Engineering Company.

The Minolta Land Cyclops pyrometersare distributed by Land Infra-redLtd, Sheffield, UK, and are manufac-tured by Land Instruments, Inc, USA.

79

DISCUSSION

Ross Brunson, United Catalysts: Do yourecommend that you make corrections for tubeswhich are at the outer walls compared with those inthe middle of the reformer?Cromarty: The method that we adopted was tomeasure the emissivity in the way I described.That can only be done using the gold cuppyrometer and therefore only on tubes close to thewall. The tubes in the middle have a differentbackground correction than those around the edgeof the reformer.Brunson: Would you use the wall at the end of therow as your basis?Cromarty: For the tubes in the middle you wouldnot include any reflection from walls, only fromsurrounding tubes.Bernard J. Grotz, Brown and Root Braun: Whatis the diameter of the gold cup, and do you expectthat the shading of the gold cup would cause themeasured temperature to read low?Cromarty: The diameter of the gold cup is about25.4 mm, and as soon as you contact the gold cupon the tube it begins to cool it down. There is atechnique to take readings as a function of time andwork back to contact time.

Grotz: In the past we have used a target made oftube material with a thermocouple imbedded in itand attached to a 50.8 mm diameter pipe withcooling air flowing through. The target is insertedinto the reformer or pyrolysis furnace. By shootingthe target with an infrared pyrometer and readingthe thermocouple output at the same time, acalibration curve can be developed for the infraredpyrometer. The temperature of the target can bevaried by adjusting the flow of cooling air.Cromarty: Did you correct for background aswell?Grotz: No, there is no need to, because you canlocate the target next to the tube that you wish tomeasure the temperature of.Luc Guns, BASF: Do you have experience inestimating remaining life of reformer tubes usingthese improved measurements?Cromarty: We have only been using thesemeasurements for 12-18 months, so they haven'tbeen applied to estimating remaining tube life.

80

Table 1. Methods of Tube Wail Temperature Measurement

Method

Contact

Non-contact

Instrument

Surface thermocouple

Gold cup pyrometer

Infrared optical pyrometer

Disappearing filament pyrometer

Laser pyrometer

Table 2. Results of Comparison on Two ReformersEmissivity

Top-firedLaserUncorrected Cyclops 52Corrected Cyclops 52

Side-firedLaserUncorrected Cyclops 52Corrected Cyclops 52

(* relative to Gold Cup pyrometer)

Temperature Error*('C) ('F)

0.971.000.82

0.921.000.74

+26+32+ 2

+41+50+ 16

+47+58+ 4

+74+90+29

Table 3. Measurements of the Same Tube with the Same Instrument

Row

1324

Tube

52525252

Tube

gast

890892882887

Wall Temoeri

South_

-912902

iture Measured f*C)

North

910900--

Difference

2083015

Table 4. Measurement of the Same Tube with Different Instruments

Row Tube

440-52

Measured Temperature ( C)

Cyclops 52* Laser

882930

880890

Target Distance(m) (ft)

2-3 6-1010-15 33-50

(* emissivity set at 1.0, no background corrections).

81

I '°7

! 10=5•>1#

^e•° à

UJ 10

f ,o>(D

Black Bodytempérature.

3 000 (1667) IT"!

1500(833) .--•MB^^^Bmi««

1 000 (555) - -

O l l 2Violet Red-*| |<— Visible region

6 8

Wavelength, X / micron

Figure 1. Emissive characteristics of ablack body.

5-I

jj K»

(0.9 micron pyrometer)

Différence In wall and target temperature (Beg C)

OJt 0.85

Targe! EmtssWty

Figure 3. Effect of emissivity anddifferentia! furnace temperature on thetheoretical measured target temperature.

Target tubeTt Flame T.

Refractory WallTU,

Figure 2. Effect of reflected radiation fromtarget surroundings.

0.5

0.4

0.1

1i

- %S fS co,8 .-. 1"ï

1 AAII jnl: y | :,-

avel

engt

h

: S

|

0>

\ 8\ 1

UCO,

:

A " H/ *° V

Sight Palh: 3m

Temperature: 1350 'Cpp CO;: 0.08

pp H£> 0 08

2 3 4 5 6 7

Wavelength, X / micron

8 9 10

Figure 4. Sight path effects - furnace gasopacity.

82

NORTH

Figure 5. Background temperaturemeasurement.

060

840 -

Î""I 000 -

860 -

I 840 -

820 -

800

9 tt 13 15 17

Tub* Humbw21 23 25 27

Figure 6. Comparative readings from

terraced-wall reformer.

83

MO -

G"

! 1c„

Uncorrocted

Calculated

—r~0.2 0.4 0.6

Fraction down Tube

Figure 7. Uncorrected IR pyrometer tube

wall temperature profile.

Tube Wall

—I—0.2

T~

0.4

—r-0.6

Fraction down Tube

Figure 8. Calculated tube wall and

refractory profiles.

Uncorrected

Corrected

_„-ïï-"- Calculated

Fraction down Tube

Figure 9. Corrected IR pyrometer tube wall

temperature profile.

84

B.J. Cromarty S.C. Beedle

85