Catalyst TubesIn Primary Reformer Furnaces

AH the evidence indicates that centrifugallycast HK-40 is the most reliable material forfabricating catalyst tubes for present am-monia reformers.

L. A. Zeis, and E. HeinzThe M. W. Kellogg Co., New York, N.Y.

IKE REFORMER FURNACE STANDS OUT AS THE HEARTof the present day ammonia plant. From the stand-point of on-stream reliability, the design of the packedcatalyst tubes is critical, and from an economic stand-point, the coil material and fabrication represent morethan half the cost of the reformer furnace radiantsection. In view of this, and M. W. Kellogg's consider-able experience in the ammonia industry, it is nowappropriate to re-evaluate the results of our practicesand improvements (1) in regard to the "state-of-the-art" of reformer tubes. Although the article is basedon our experience with reforming service, the prin-ciples considered are also common to all other designsof reformer furnace tubes.

A typical reformer tube is shown in Figure 1. Char-acteristically, all are vertical, have top inlets and bot-tom outlets, and are packed throughout their heatedlength with catalyst. Support of the tube may be byfixed supports or springs acting at the top or bottom,or a combination of both. The total vertical expansionof the tube must be accommodated by adequate flex-ibility in the inlet and outlet piping. This is accom-plished by providing expansion devices; i.e., pigtailsor pipe loops, or by utilizing complete flexible systemssupported by springs or counterweights.

Tube dimensions, which are based on process con-siderations and economics, normally range from 2.5 in.to 5 in. inside diameter with wall thicknesses up to 1 in.These dimensions are adequate for most temperatureand pressure conditions. Limitations are imposed onthe lower range of inside diameters by centrifugalcasting techniques. Tubes are centrifugally cast aus-tenitic materials since wrought materials do not havesufficient strength at temperature for present serviceconditions.

Mechanical design basisFurnace tubes are pressure parts which are sub-

jected to heat transfer through their walls. The designof a furnace tube must consider:

1. Allowable stress value for the specific materialas a function of temperature

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2. Methods to calculate all expected stresses3. Methods to calculate expected metal temperaturesFor the design of packed catalyst tubes in reformer

service, these considerations are interrelated and theadequacy of the final design is dependent on the valid-ity of the methods for each of these calculations. Theseverity and nature of the service makes it imperativethat conservative values be used to assure adequateand safe service.

The material selection, stress basis, stress, and tem-perature calculations have evolved through numerousphases to achieve their present status. A firm baseof more than ten years experience with cast catalysttubes now permits conclusions as to the validity ofthe methods and procedures presently being used.

The tube material selected as a generally acceptedindustry standard for current design temperatures iscentrifugally cast 25% chromium-20% nickel (HK-40) . Its selection is based on its superior resistanceto both rupture and deterioration at the elevated tem-peratures commonly encountered in reformer service.HK-40 is specified as 0.35% to 0.45% carbon. Thiscarbon content provides the optimum combination ofstrength and weldability. The same requirement isapplicable to the weldments made to connect the as-casttubes into longer tube assemblies.

Although HK-40 has the high carbon content re-quired of a high strength material, it develops lowambient temperature ductility upon aging. However,service experience has shown that low ductility is tol-erable in a system designed to minimize stresses atthese critical conditions.

More highly allffyed iron-chromium-nickel base

materials are sometimes used when warranted by thetemperature and pressure conditions. Their agedductility can be even less than that of HK-40. Forheat absorbing tubes, addition of tube wall thicknessdoes not proportionately increase resistance to pres-sure at temperature due to increase in temperaturegradient across the wall. Therefore, stronger materialsto withstand higher pressures are an industry require-ment if reforming conditions continue in their normalupward trend.

Most of these stronger materials are proprietaryalloys which have had limited reformer service ex-perience. In many of these alloys being considered,ductility at temperatures below operating metal tem-peratures is low, and careful consideration of thermaland mechanical stresses is required. There are un-known or unpredictable factors such as, gas-metalreactions, phase changes, mechanical properties, andfabrication problems, which will be determined onlyafter tests in operating furnaces. Manufacturers' datais helpful in preliminary screening, but the most reli-able judgment of new materials is derived from actualservice tests.

The majority of tubes in service today have beenwelded assemblies of standard as-cast tubes. Cur-rently, there is some consideration of tubes which aremachined or honed on the inside surface to remove all,or the bulk, of the inside as-cast porosity. The advan-tages expected with a smooth defect free inside tubesurface are:

1. Freedom from mechanical notches and castingdefects

2. Lesser chance of deterioration because of lesser

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surface area, and fewer theoretically stagnant areas3. Control of dimensions, diameter and wall thick-

nessIn reformer services none of these expected advan-

tages are considered necessary for ensuring satisfac-tory service. Based on our experience with bored tubes,the additional expense of machined or bored tubes isnot justified. Approximately 10% of the tube in serv-ice, and of the reported failures described below, havebeen internally machined tubes. To our knowledge,there have been no reported tube failures attributableto inside porosity or inside surface roughness. Therate of deterioration, although theoretically affectedby surface area and roughness is much more dependenton temperature, gas ratios, and impurities. Roughnessappears to be a factor only in marginal conditions. Theexpected advantages of machined or honed tubesshould be balanced against an approximate additionalcost of $12/ft. for honing or machining in the U.S.

Allowable design stressesFor HK tubes in reformer service the stress to pro-

duce rupture in a specified time period is now consid-ered as a more accurate long time design stress cri-terion than a stress to produce a specified creep in aspecified time period at these imposed temperatures.The generally accepted stress criterion appears to be100% of the average stress to produce rupture in100,000 hours. This criterion has been recommendedand judged as a conservative method by some design-ers (2). However, we feel 75% of this stress repre-sents a conservative method.

M. W. Kellogg has established allowable stresses forcast 25 Cr-20 Ni (HK-40) based on Estruch's (3) pub-lished values for stress to produce rupture in 100,000hours. The stresses used are taken at the minimum ofthe scatter band of Estruch's values, which is consid-ered as 75% of the average values, Figure 2. Thismeans that, based on present knowledge, it is expectedthat all tubes in a furnace will last a minimum of

100,000 hours in service if the design temperaturesare maintained. This conservative approach is war-ranted in spite of the resulting increase in tube walland substantial additional cost.

Under operating conditions the tube is subjected topressure, weight, and thermal stresses. The wallthickness of the pressure part is based on the pressurestress acting in the circumferential direction, usuallyreferred to as "hoop" stress. The pressure stress act-ing in the axial direction is approximately one half ofthe hoop stress. The designer can choose from thefollowing methods to determine the pressure stressacting on the tube wall :

1. Bailey's Method2. Lame's Method3. Barlow's Formula4. ASME Section I Method5. ASA Method6. API Method7. Mean Diameter FormulaThe Mean Diameter Formula has had experimental

verification in that the calculated unaxial rupture datacorrelates with tube rupture life (4). The formulareasonably estimates the rupture life of tubes over awide range of material, tube size, thickness, and tem-peratures. For a typical reformer application, all ofthe above mentioned stress formulas will result in acalculated wall thickness within a ± 5% range. TheMean Diameter Formula, therefore, is judged as themost convenient and the best suited for present re-former tube design.

Weight stresses imposed on the coil are almost ex-clusively oriented in the axial direction. These stressesresult either from axial loads or from bending momentsacting perpendicular to the axis of the coil components.The weight stresses may be evaluated by conventionalbeam and column formula. The weight and pressurestresses, properly combined, must be less than theallowable design stress.

Thermal stresses are induced as the system tem-perature changes. Differential expansion of the sys-

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Figure 4. Longitudinal rupture ïn a HK-40 tube.

tern elements occur due to temperature differences ordifferent coefficients of expansions of the coil elements.The intent of the design is to recognize where thesedifferences develop and make provisions to keepstresses to a minimum. Since thermal stresses aretransitory, they are not combined with the weight andpressure stresses. In addition, radial thermal stressesare developed due to the temperature gradient throughthe wall and are a function of the thermal gradient,tube dimensions, and the tube physical and mechanicalproperties. Permissible thermal stress is governed bya combination of cold allowable stress, hot allowablestress, number of cycles anticipated, and an experiencefactor.

The ASA Piping Code formula for the permissiblethermal stress is 1.25 times the cold allowable designstress plus 0.25 times the hot allowable design stress.These permissible thermal stresses refer to thestresses due to thermal effects only, and are indepen-dent of pressure and weight stresses. The ASA ruleswere formulated primarily for ductile material, there-fore, their use in less ductile cast materials requiresmodification based upon experience and considerationof the operating conditions, combination of stress,location in the furnace, and susceptibility to upset orabnormal conditions.

Temperature determinationThe most critical aspect of catalyst tube design is

the determination of the design tube metal tempera-ture. At design temperatures the allowable stressesare very sensitive to temperature changes. As an ex-ample, a 50°F increase at 1,600°F could decrease theallowable stress 20%.

The temperature profile of the catalyst tube is pre-dicted by a mathematical model (5) which has been

proven capable of describing the performance of react-ing systems, and of exploring a wide range of potentialoperating conditions. It can take into account such fac-tors as steam reforming reactions, reaction kinetics,heat transfer, concentrations, pressure, temperature,and burner patterns. For the design operating condi-tions and an assumed tube wall thickness, the modelpredicts inside and outside tube wall temperature atincrements along the length of the tube. For use inthe Mean Diameter Formula, the design temperatureis considered the maximum arithmetic mean wall tem-perature plus an allowance for abnormal conditions.

Abnormal conditions, in which flow is restricted oruneven through the catalyst, or non-uniform catalystactivity results in localized overheating, must be con-sidered. The tubes are able to withstand localized tem-peratures above design for short periods, but extendedoperation under over-temperature conditions will re-sult in reduced service life. Precautions for the opera-tion of furnaces with "hot spots" on the tubes arecontained in operating instructions. The service lifeof the tubes is directly related to the activity andcharacteristics of the catalyst. Unfortunately, a dis-cussion of this subject is beyond the scope of thisarticle.

Cast tube fabricationThe critical points of tube fabrication are :1. Control of tube tolerances2. Removal of inner porosity at the weld3. Tests and inspection to assure that specified me-

chanical properties and chemical composition are ob-tained

4. Procedures for the welding of the as-cast tubesegments into long tube assemblies

While detailed specifications are essential, most ofthe compliance with the critical requirements in points1 through 3 are based on the cast tube vendors pro-prietary know-how and experience. For this reason,the sources of cast tube are limited to a relatively fewfoundries who have adequately demonstrated theirability. Deviations from the specified quality have beendetected in isolated cases after tubes have been ac-cepted and put into service, but these deviations arerare, and in only two cases known to us has a foundrytube defect been the primary cause of failure.

The welding of the as-cast tube segments poses noparticular problem providing the procedure is ade-quately defined and properly qualified. The acceptedprocedure normally used is the manual electric arcprocess with the inert gas arc process used for theroot pass. Automatic welding has also been qualifiedfor weld-out. Details of weld bevels should includeremoval of inner porosity which would interfere withwelding. Final weldments should be free of notchesand excessive ripples, and should have a controlledinner protrusion and exterior reinforcement. Atten-tion to these points prevents the possibility of "notcheffects" and possible "stress raisers". Examination ofthe interior surfaces of welds is by optical methodsand liquid penetrant testing of the root pass. Finishedwelds are random radiographed. Initially, welds join-ing cast lengths were completely radiographed, how-ever, results of this complete radiography, and serviceexperience, indicate that complete radiography is notnecessary. None of the tube failures described belowwere attributable to weld defects or weld quality. Theinlet connections to the reformer tube are made out-side of the firebox; and fabrication of the assembly inthe unfired areas is done in accordance with normal

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piping practice. Cast tubes are preferably not extendedoutside of the furnace heated zone due to the lowductility of the material at ambient temperatures andthe possibility of stress corrosion due to condensateaccumulations.

DeteriorationIn addition to the required strength at elevated tem-

peratures, reformer tubes must have resistance todeterioration both from products of combustion (ex-ternally) and process gases (internally). Gas-metaland corrosion reactions considered as possible deteri-oration mechanisms at temperatures between ambientand 1,800°F are: carburization, decarburization, oxi-dation, sulfidation, salt attack, stress corrosion, nitrid-ing, and intergranular attack.

These reactions will all be considered with referenceto the standard HK-40 tube in steam-methane reformerservice. Reactions of other alloys and other processescan be anticipated based on their particular compo-nents.

Although not listed as deterioration, the most sig-nificant change in properties of HK-40 is the loweringof ductility due to carbide precipitation which takesplace as a result of exposure to furnace temperatures.As a measure of the change which takes place, tensiletest elongation drops from over 10% in the as-cast con-dition to 1% or 2% after a short time in service.

The following conditions have been found in tubeexposed to normal reforming conditions: carburiza-tion, decarburization, oxidation, and nitriding (of theoutside surface). Under expected conditions of metaltemperature, furnace firing, and gas composition, therate at which these reactions occur in HK-40 tubes issuch that there is no significant change in tube lifeor serviceability. Under upset conditions, high tem-peratures or contaminants in the process stream orfuel, the normally innocuous reaction rates may beaccelerated to allow early failures of tubes.

Carburization is an increase of carbon content ofthe tube material by diffusion of carbon into the tube.The source of this contaminent can be solid carbon(from soot laydown) or carbon diffusing directly fromthe methane feed. Carburization can lower ductility,especially at room temperature, as well as lower oxida-tion and corrosion resistance.

Decarburization is the removal of carbon from thetube material by reaction with the steam and products

Table 1. Known failures in cast 25 Cr-20 Ni tube(HK-40) in high pressure reforming service.

Tube Life Numberat Failure of Type of(months) Failures Failures Cause

7 1 Creep-Rupture Overheating8 1 Fracture Cast Defect

11 1 Creep-Rupture Overheating12 1 Creep-Rupture Unknown18 7 Creep-Rupture Unknown25 1 Creep-Rupture Unknown30 1 Creep-Rupture Unknown30 4 Corrosion39 2 Creep-Rupture Overheating40 3 Creep-Rupture Overheating44 2 Creep-Rupture Overheating

Total 24

Figure 5. Salt attack at Inconel weld in Incoloy 800.(below: original condition, above: after attack)

of gas decomposition. For each set of conditions (tem-perature and gas ratios) through the length of thetube, there is an equilibrium carbon content at thetube surface. With sufficiently long time and withoutmechanical interference, the theoretical equilibriumsurface carbon content will be reached by means ofcarburization or decarburization. Decarburization canresult in a surface layer with lower carbon contentcompared to the matrix carbon content. Although thelow carbon surface layer is theoretically weaker thanthe base material with normal carbon content, decar-burized layers normally found in reformer tubes areonly a few thousandths of an inch thick and have nomeasurable effects on tube strength.

Oxidation (or scaling) occurs when the oxidizingpotential from C02 or H20 (internally), or C02, H20,and 02 (externally) is greater than that which pro-duces decarburization. The normal protective oxidecoating on tube surfaces, primarily chromium oxides,limits oxidation of underlying base metal. If the coat-ing is broken, or if fresh metal surfaces are exposed,oxidation of chromium carbides at grain boundaries,and of iron rich matrix material, can take place.Depending on physical form and composition, oxidesmay or may not be protective. In some cases, inter-granular oxidation is self limiting in that the oxidesformed prevent further oxidation. In other cases,stresses and strains can fracture the oxide build upand thus allow further oxidation. It is this latter typeof progressive intergranular attack which accompa-nies stress-rupture failures.

Nitriding is the formation of metal nitrides on ex-ternal tube surfaces by reaction with the nitrogen inthe furnace atmosphere. Nitrides at low temperaturesare hard and brittle and are sometimes used for wearresistant surfaces on mechanical equipment. In re-former tube service, the nitriding reaction with HK-40 is slow and the presence of nitrides is usually anindication of overheating. No failures of reformertubes due to nitriding are known.

Other elevated temperature reactions which, bycomparison with the gas-metal reactions describedabove, are not expected and cannot be tolerated, aresulfidation, salt attack, and metallic contamination.These types of attack are similar in that the contam-inant can react with or dissolve normally protectiveconstituents of the tubes. The attack then proceedseither by allowing accelerated carburization and oxida-tion, or by a self renewing continuation of the corrod-ing reaction where the corrosion product decomposesallowing a continuing penetration. These types of

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attack, depending on a foreign material, usually asolid agent, are normally spotty in occurrence, and bycomparison with adjacent unaffected material, aresometimes referred to as "catastrophic".

Stress corrosion cracking by aqueous contaminantsis theoretically possible and has been reported inwrought components and cast tubes where condensatecollected. This has not been a problem where the designis such that condensate cannot collect.

Although there is an imposing list of mechanismswhich can be described as deterioration of reformertubes, they fall into two simple classifications :

1. Those which occur in normal operation and arenot harmful—carburization, decarburization, oxida-tion, and nitriding (outside surface).

2. Those which occur only when contaminants areintroduced—sulfidation, salt attack, liquid metal attack,stress corrosion.

Those in the first class can be serious if temperatureor process is out of control, but in normal operationthey are not a problem. Those in the second class areserious but can be prevented by maintaining the clean-liness of the furnace, equipment, and process streams.This cleanliness is vital during construction, start up,operation, shut down and especially during mainten-ance and turnaround operations.

There have been research results which indicate thatvariations in tube chemical analysis, grain structure,and surface conditions can affect carburization andoxidation. If process (temperature or gas composition)changes require greater carburization resistance,these controls can be applied.

ExperienceThe following shows our own experience with nearly

9,000 tube assemblies in 32 ammonia reformer fur-naces operating over 450 lbs./sq. in. gauge pressure.

Number Numberof Tube of Reported

Years Service Assemblies Failures0-1 1,922* 01-2 964* 02-3 4,454 123-4 1,416 124-5 210 0

8,966 24"Thicker tubes with 75% of average stress to rupture in 100,000 hours.

Of this total of 8,966 tubes, the failures are less than0.3%, Table 1. Earlier experience with tubes operatingat lower pressures and of other materials has not beenincluded in the preceding summary. This experiencehas been adequately covered elsewhere (6).

It has been estimated by one cast tube manufacturerthat the failure rate has been % of 1 % on a productionof 45,000 tube assemblies over a 12 year period. Serv-ice experience should definitely improve with the re-finement of operating techniques. The adoption ofconservative stress values, previously mentioned, isanother important factor in improving the experiencerecord.

Known tube failures fall into several classes. Mostof those reported are stress rupture failures associ-ated with overheating. As experience and test dataaccumulate it has become possible to predict the be-havior of HK-40 tubes. Knowing two of the three vari-ables of stress, temperature, and time enables predic-tion of the third, Figure 3. Overheating is generalizeddue to catalyst deterioration or restricted gas flow.

Failure initiates at grain boundaries and proceedsthrough the wall. As fresh metal surfaces are exposed,oxidation proceeds along the fracture path. As over-heating continues, the primary fracture penetratesthrough the wall, while numerous parallel secondarygrain boundary fissures are evident. Fracture surfacesare filled with magnetic oxides and there is very littleevidence of ductility in the failure. In tubes, thesefailures are nearly always longitudinal in directionbecause the maximum primary stress is the pressurestress, Figure 4.

Failure of tubes due primarily to an unexpectedgas-metal or corrosion mechanism are rare. Many ofthe conditions listed under Deterioration are acceler-ated by overheating and are frequently found in tubeswhich have failed by stress rupture. In most of thesecases the deterioration is an interesting side phenom-enon which has not contributed to the failure.

Notable exceptions were tube failures concluded tohave been caused by carry over of sulfur containingcompounds (7). Similar attack has taken place in andon Incoloy 800 due to water treating salts, Figure 5.Elevated temperature accelerated the combined car-burization, decarburization and, molten salt attack.

in summaryWe have shown the design, fabrication and material

selection considerations applied to centrifugally castHK-40 reformer furnace tubes. Service experience hasshown that they are reliable engineering componentswhose behavior is predictable. This successful experi-ence, (less than 0.3% failures from all causes), verifiesthat a conservative design approach has been appliedto minimize failures. Based on our analysis of theisolated failures, we conclude that properly utilizedHK-40 centrifugally casting continues to be the mostreliable choice for present ammonia reformer condi-tions. #

Literature cited1. Jacobowitz, J. L., and L. A. Zeis, "Safety in Air and Ammonia

Plants," AIChE technical manual, 11, 6 (1969).2. Tielroy, J., "Aspects of Alloy Selection and Use of Materials in

Reformer Furnaces," NACE Conference Paper No. 97, Houston, Tex.3. Estruch, B., "High Temperature Alloys for Use in Reformer Fur-

naces," from "Materials Technology in Steam Reforming Processes,"C. Edelenau, ed., Pergamon Press, London (1966).

4. Carlson, W.. B., and D. Duval, Engineering, 193, (June 22, 1962).5. Heineman.'H., L. Friend, and A. Gamero, "New Developments in

Steam Hydrocarbon Reforming," Proc. : Seventh World PetroleumCongress, 5, (1967).

6. Ciuffreda, A. R., and B. E. Hopkinson, "Survey of Corrosion Problemsin Reformer Hydrogen Plants," API Preprint No. 13-68, 48, 33rdMid-Year Meeting (May 15, 1968).

7. Fitzharris, J. J., and D. B. Bird, Material Protection, 7, No. 9, (Sep-tember, 1968).

L. A. Zeis is senior staff consultant withthe M. W. Kellogg Co. in the area of ma-terials engineering. He is a graduate ofPurdue University and has been withNordstrom Valve and Mobil Oil Compa-nies. He is Chairman of the FabricationSection of the United States Delegationto the International Standards Organiza-tion, Boiler and Pressure Vessel Groupand is a member of USAS B 31.3 Sec-tion Committee & Code for Refinery

Piping. He is a registered Professional Engineer in New York,New Jersey and Texas.

E. Heinz is the furnace mechanical sec-tion engineer at the M. W. KelloggCompany, being responsible for the me-chanical design of reformer furnaces.He has received MCE from PolytechnicInstitute of Brooklyn in 1953. He is amember of ASCE and holds a Profes-sional Engineers License in New York.

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