Coke Drum Design

The global supply of crude isgetting heavier. In response,refiners are making their refineries

flexible enough to handle the changingfeedstock. These crudes are cheaper thansweet crudes due to their limitedprocessing capacity. One of the majoradditions and targets for upgrades aredelayed coking units, which process thebottom-of-barrel material into valuable,lighter products such as gasoline anddistillates. Adding a delayed coker unitto a refinery flow scheme provides atwo-fold financial incentive: cheaperfeedstock and a higher volume oflight products.

In several parts of the world, NorthAmerica being more severe, refiningcapacity is struggling to keep up withthe demand for products. Refiners aremaking strong efforts to improve thereliability and availability of processunits, with the ultimate goal ofdecreasing the frequency of turnaroundsand making process units safer.

In an environment where it is veryprofitable to keep refinery availability ashigh as possible, efforts are directedtoward process units and equipmentthat bear the most wear and tear. Also, itis financially preferable to run heaviercrudes, especially when a refiner has anexisting delayed coking unit. Butnothing is easy: delayed coking unitsrequire more maintenance than normalprocess units in a refinery, andincreasing the availability of this unithas a major impact on the long-termprofitability of a refiner.

Delayed coking processDelayed coking is a process in which aheavy residual feedstock is superheatedand then introduced into an insulated,vertically oriented cylindrical pressurevessel, commonly known as a cokedrum. Vapours are then removed to befurther refined into various petroleumbyproducts, leaving behind a high-density hydrocarbon residue referred toas petroleum coke. Petroleum coke is asolid coal-like substance that has beencommercially manufactured for nearly

80 years and is usedprimarily as a fuel or forthe production of anodesfor the aluminumindustry, electrodes forthe steel industry,graphite or similar carbon-based products. Thisresidue is then waterquenched not only toallow for its removal oncethe vessel has beendepressurised, but also tocool it to the point whereit will not self-ignite whenexposed to air. As part ofthe delayed cokingprocess, coke drumsundergo severe thermaland pressure cycling, asthe vessel is filled with hotproduct and subsequentlywater quenched aftercoke formation.

Due to the severe heating andquenching rates of this process, theuseful life of coke drums is much shorterthan that of other pressure vesselsoperating in non-cyclic conditions. Thisis because the severe operationalthermal cycling causes the plate and theweld to be stressed with each cycle and,due to their differential strengths, thedrum may bulge and eventually crack inthe vicinity of the circumferential weldseams. This leads to coke drumsexperiencing significant downtimethroughout their useful life to makeneeded repairs or partial shell replace-ments. It is a problem that has plaguedrefineries for decades.

Depending on the operating para-meters, type of coke produced, feedstockand other variables, the cycle time of agiven unit can be from as little as 14hours to more than 24 hours. Theindustry recognises that the shorter andmore severe the cycle, the sooner andmore pronounced the bulging andcracking will appear. To illustrate this,Figure 1 is a photo that was taken froma coke drum shell replacement project,illustrating the severity of the distortion.

In the last decade, the demand fordelayed coking capacity has beensteadily increasing. Experts attribute thistrend to refineries having to processmore lower-quality crudes than in thepast, as previously discussed. Thus, fewrefinery owners have the option ofincreasing cycle time, as they mustbalance drum distortion with the desireto meet rising throughput requirements.Instead, refiners must often resort toshortening the coking cycle, addingmore units or doing both.

Bulging and crackingphenomenon Weil and Rapasky1 (1958) identifiedradial bulging as a “re-occurringdifficulty” that existed in essentially alloperating coke drums of the time.Through extensive research carried outon a total of 16 coke drums erectedbetween 1938 and 1958, they identifieda radial growth varying from almostnegligible to as much as 0.3in per year.The rate of bulging was found to bedirectly attributable to the “quenchingportion” of the operating cycle. Theyalso recognised that the girth seams, dueto the higher yield strength of the weld

Coke drum design

Issues to consider for extending the turnaround schedule of a delayed cokingunit. Theory behind coke drum failure is discussed, with detailed solutions

centred on drum design and support structures

Coby W Stewart, Aaron M Stryk and Lee PresleyChicago Bridge & Iron

w w w. e p t q . c o m 00PPTTQQ Q3 2006

Figure 1 Coke drum shell bulging in early 1960s

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metal, tended to augment the bulging,causing a “constrained balloon shape”,as illustrated in Figure 2.

As a result of the restraint caused bythe weld seams, the base material tends tobecome thin and ultimately fails viathrough-wall cracking. The bulging ismost severe in the lower cylindricalportion of the vessel, usually 40–50ftabove the cone section. This section ofthe vessel experiences the highest quenchrates during the quench cycle andtypically has four to five circumferential

weld seams, depending on the width ofplate used for each shell course.

Through their studies, Weil andRapasky observed that high quenchingrates produced thermal gradients inexcess of 10°F per inch, while lowerquenching rates produced smallergradients, thus less bulging. To measurethis effect, they developed a “unitquench factor” (UQF, Table 1), which isthe ratio of water-quenching time inminutes to the coke yield per drum intons. Utilising the resulting data, theytheorised that when the UQF is greaterthan 0.5, the bulging is minimal, andwhen the UQF is greater than 0.8, thebulging is all but non-existent. The UQFis directly proportional to the rate of

water injection during the quench cycle— the slower and more controlled thequench is, the greater the UQF, whichtranslates into less bulging. However, aspreviously mentioned, fewer ownershave the option of longer cycle times. Tothe contrary, the trend is for evenshorter cycles, the result of which is ahigher UQF that will ultimately reducethe overall lifespan of the vessel.

In recent years, there have been anumber of studies undertaken thatsubstantiate the conclusions of Weil andRapasky: — Penso et al5 suggest that low cyclethermal fatigue is the most commonfailure mechanism of a coke drum. Theauthors state that thermal shock is themain mechanism for the initiation ofthese cracks. They further propose thatadditional analysis of the cooling cycleis warranted, since the cooling cycleaffects the severity of thermal shock — Livingston and Saunders4 report thatthe coke drums in their survey began tothrough-wall crack at roughly 2400cycles. The frequency of through-wallcracking increased — This evidence of initial andsubsequent cracking is further supportedby an independent 1996 API Survey thatconfirms the first through-wall cracks onconventional drums typically occur inthe 3000–5000 cycle range — Antalffy et al3 cite at least threeadditional examples of recent papersthat reference the bulging phenomena.He also cites that Boswell and Ferraro2

and others concluded that the highrelative strength of the circumferentialweld metal, as compared to the lowerstrength of the adjacent base metal, wasthe cause of the bulging.

As one can see, it is common industryknowledge — supported by severalthermal and stress analyses of variousoperating units — that the higherstrength of the weld metal in thecircumferential seams tends to have astiffening effect, which increases stressand leads to distortion and cracking. Itshould be noted that the longitudinalweld seams required to make the shellcourses were reported to be unaffectedby the thermal cycling, except wherethese seams intersected the circumfer-ential ones. In the bulge areas near thecircumferential seams, the longitudinalweld seams were also affected by thethinning effect and would be prone tocrack given enough thermal cycles.

Coke drum metallurgy andshell thickness To extend the useful life and reduce thedowntime of coke drums, severalmodifications have been made tostandard coke drum design metallurgy.Coke drums that were once constructedfrom mild steel have in recent yearsbeen constructed from low-alloy clad

w w w. e p t q . c o m00 PPTTQQ Q3 2006

Stage 1 Stage 2 Stage 3Onset ofbulging

Normalshape

Girth seamsbegin to form

Advancedbulged shape

A B C D

Figure 2 Coke drums displaying constrained balloon shape due to distortion

thk7/8

thk11/16

thk11/8

thk13/8

thk1

thk15/16

Figure 3 Coke drum vessel step reductionin thickness

Figure 4 Weld profile compoundsstiffening effect

Unit no. Water quenching Coke capacity, Unit quenching Relative severity of time, min tons factor, min bulging distortion

9 90 380 0.24 Severe8 100 370 0.27 Severe7 90 310 0.29 Severe1 140 180 0.78 Negligible5 135 170 0.80 Negligible2 150 170 0.88 Absent6 180 180 1.00 Absent

UQF =water – quenching time (minutes)

coke capacity (tons)

Comparison of quenching methods and growth on various coking

Table 1

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materials such as Carbon-1/2 Moly and11/4 Chrome-1/2 Moly, as well as 21/4Chrome-1 Moly and other higher alloys,typically with 410SS cladding. With theexception of 21/4 Chrome-1 Moly andother higher alloyed coke drums, thosemanufactured with low-Chrome alloymaterials have all failed in time.

The drums manufactured from 21/4Chrome-1 Moly and other higher alloymaterials have been in service for only afew years and have not yet enduredenough cycles to demonstrate improvedreliability. The normal method ofdesigning the shell courses of coke drumsis to base the thickness of the material onthe specified design pressure. Typically,the pressure is specified as varyinglinearly from a minimum value at thetop of the vessel to a maximum at thebottom flange. Since overall vessel cost isa function of weight and thickness, thetendency is to design each shell coursefor the design pressure specified at thebottom of the course. As a result, thereare typically step reductions in thicknessfrom one shell course to the next, asshown in Figure 3. However, theresulting weld profile compounds thestiffening effect already present due tothe higher yield strength of the weldmetal, as shown in Figure 4.

Increasing vessel life Further effort to improving the reliabilityand lifespan of coke drums has focusedon developing or proposing measures tomitigate the stiffening effect of the weldseams. Most of these measures, several ofwhich have been incorporated intonumerous procurement specifications,were aimed at reducing the discontinuityeffect at the circumferential weld seam.The most common of these specifica-tions were as follows: — Decreasing the weld metal yieldstrength to be within a close percentageof base metal yield (ie, 0%, +10%) — Blend grinding the weld profile — Specifying higher alloy materials (ie,21/4 Chrome-1 Moly and higher) — Requiring more non-destructiveexaminations (NDE) and using morerestrictive NDE acceptance criteria thanthe construction code requires — Maintaining a uniform shell

thickness throughout the vessel — Specifying materials greater than 2inin thickness.

While most of these requirementshave technical merit, the resultingimprovement may only be incrementaland not necessarily practical. Forexample, attempting to narrow the yieldstrength mismatch between the weldmetal and base metal is difficult at bestdue to the many variables involved. Theactual yield strength of the base metalswhen compared to commerciallyavailable weld metals is typically morethan 10% lower (Table 2). In addition,the yield strength of base metals andweld metals varies with temperature,which may make the yield strengthdifferential greater than 10% at elevatedtemperatures. Also, controlling weldmetal yield strengths to a specificminimum/maximum in relation to theas-supplied base metal typically requiresthe use of weld processes that are notnecessarily productive or cost-effective.

Blend grinding the weld profiles,which reduces the geometric stress raisereffects near the weld joint, can extendthe service life of the weld joint and canbe cost-effective if properly specifiedand managed. However, eventual repairsto the weld joint are inevitable.

The use of uniform shell thicknessthroughout the critical area of the vesselhas an added material and labour cost.But, as previously mentioned, notkeeping a uniform thickness will increasethe stiffening effect of the metal used forwelding the circumferential weld joints.And while the specification of very thickand uniform shells will reduce the peakstresses caused by the thermal cyclessomewhat, it will not reduce the effectsof the circumferential seams acting asstiffeners during the quench cycle.

Higher alloys such as 21/4 Chrome-1Moly are thought to be able to resist thethermal cycling better because of theirhigher yield and better creep and creep-fatigue resistance. While these higheralloys may increase the useful life of thevessels and slow down bulging, most ofthe newer drums manufactured fromthis alloy have not yet seen enough cokeproduction cycles to determine if this isa major improvement. Plus, using these

alloys translates into increased costs. Some specifications are requiring

increased NDE testing to be performedand the acceptable flaw size reduced inan attempt to get more uniform weldjoint properties. This increases costswithout having any effect on the basicproblem of the circumferential weldsacting as stiffeners. There is also someevidence that excessive repairs duringmanufacturing may, in fact, reduceservice life by causing premature failures.

Elimination ofcircumferential weld seams When CB&I first approached the girthweld distortion and cracking problem,two observations were pivotal in theanalysis. One was the well-known factthat the higher strength of the weldmetal in the circumferential seams tendsto have a stiffening effect that increases


F. O. F.

ML

15–

23/11 861/1

Figure 5 Cylindrical shell sections of up to46ft help eliminate several circumferentialweld seams, increasing endurance underthe most severe thermal cycles.

Base metal Type Tensile ksi Yield min. Typical weld Tensile ksi Yield min. (typ) ksi metal selection (typ) ksi

A516-70 Carbon steel 70–90 38 (46) EM12K 70–95 58 (61 ) A204 C Carbon-1/2 Moly 75–95 43 (52) EA2 70–95 58 (61) A387-11, CL2 11/4 Cr-1/2 Moly 75–100 45 (58) EB2 80–100 68 (75) A387-12, CL2 1 Cr-1/2 Moly 65–85 40 (50) EB2 80–100 68 (75) A387-22, CL2 21/4-1 Moly 75–100 45 (58) EB3 90–110 67 (80) 405 (clad) 13 Cr 60–90 25 (36) ERNiCr-3 80–95 40 (52) 410S (clad) 12 Cr 60–90 30 (36) ERNiCr-3 80–95 40 (52)

All weld metal and base metal in typical PWHT condition for material combinations. All weld metal data is from the SAW (submerged arc welding) process

Typical plate and weld metal properties6

Table 2

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stresses and leads to distortion andcracking. The other, based on previousresearch, was that the longitudinalwelds required to make the shell courseswere seemingly unaffected by thethermal cycling, except in the areaswhere those welds intersected thecircumferential ones.

After extensive research and analysis,CB&I concluded that the best solutionto the problem of girth seam distortionand cracking was to eliminate thecircumferential weld seams in the areaof concern. A method was developed forsuccessfully fabricating shell plates withthe long side oriented vertically. Thisprocess allows fabrication of cylindricalshell sections of up to 46ft without acircumferential weld seam. The steelmill manufacturing capability onlylimits plate size. Currently, the largestplates available are in the range of 46ft,depending on the specified thicknessand alloy. Depending on plate sizelimitations, up to five circumferentialweld seams can be eliminated, resultingin a cylindrical shell section that canendure the most severe thermal cycles(Figure 5).

In late 1997, CB&I launched anextensive investigation into thefeasibility of designing, fabricating and

erecting a vertical plate cokedrum. The resultingconclusion was that such avessel could be economicallyproduced, providing auniform thickness through-out the cylindrical portion ofthe vessel. This method isapplicable to new construc-tion (shop-built or field-erected) projects, as well asretrofit applications wherethe lower cone and top headsections of the vessels arereused.

Vertical plate coke drums Since 2000, CB&I has completed fourretrofit projects with the proprietaryvertical plate coke drum technology, allof which were finished at an equal orlower cost to the refiner than traditionalrepair methods, and were much lessexpensive than a full-vessel replacement.

The first successful installation of twodrums took place at a refinery on the USWest Coast in 2000. The refiner hadoriginally planned to replace two 20ft-plus sections of distorted and crackedshell on two of its coke drums, whichwould still have left circumferentialseams in the section of the drum wherethe bulging and cracking was mostpronounced. However, after reviewingthe vertical plate concept, the refinerdecided to modify its plan and replacedall but the upper 9ft of shell with twovertical plate courses reaching heights of23.5ft and 40ft respectively.

For this project, CB&I performed all ofthe turnaround activities itself, includingforming and welding the plates at one ofits fabrication shops, mobilising thecrane and all of the on-site materials, anderecting the vessels. To execute the actualshell replacement, CB&I cut the oldcircumferential sections and thenremoved them using a customised rail

system attached to the structure. Fromthere, the new vertical plate sectionswere set in place. The project includedreplacing a large nozzle in the top headand sections of the skirt support, alongwith post-weld heat treating andhydrostatic testing, all within a span of28 days. This vertical plate solution wasso successful that the refiner subse-quently contracted to replace the shellson an additional four coke drums.

In 2001, CB&I replaced the shellcourses on four coke drums for anotherrefiner that were originally fabricatedand installed by the company in1968–69. These vessels had reached theend of their useful life, and the cokerstructure required major modificationsto meet current building codes. CB&Irecommended replacing the existingcan sections on the coke drums withvertical plates. Total shell replacementwas approximately 65ft 8in in height.

A lower shell course 46ft in height andan upper shell course 18ft 8in in heightwere fabricated. These courses comprisedeight shell plates, all of which wereformed in one of CB&I’s fabricationshops and then welded into two-plateassemblies. The assemblies were thenshipped to the refinery, where the on-site project team would first erect thelower and upper shell courses and thenset one on top of the other to completethe can section. In addition to replacingthe shell plates, braces were added to thestairwell and the drum was insulated.Working on a plant-wide, non-criticalturnaround basis, the can sections on allfour drums were replaced in 37 days.

The most recent vertical plate project,which involved the replacement of a35ft vertical section on a ten-year-oldcoke drum, was completed on aturnaround basis in 16 days. For thisproject, CB&I utilised its can sectionreplacement method. This particularvessel had experienced excessivebulging and cracking in 60% of itscircumferential welds. Its shell courseswere replaced with new 35ft-long platesarranged vertically in four sections.Braces were also added to the vesselstairwell and the drum was insulated.

Plate-by-plate replacement In many refineries, the area surroundingthe coking unit does not permit the useof a large-capacity crane to lift out theentire can section. To replace the shellson the four coke drums at the WestCoast refinery, for instance, the damagedcan sections had to be cut into smallervertical pieces, which were thenremoved plate by plate. Likewise, thenew vertical plates had to be lifted intoplace piece by piece, as shown in Figure6. While not as fast as replacing theentire can section, the plate-by-platereplacement method is still a safe, low-cost solution.


Figure 6 Plate-by-plate replacement method in progress

A

Upper skirt10 thkSA387-11 CL2

A

Elevation View A–A

Tan line

Hotbox

See note #1

Tetal

p p

o

ton

oD

tols

dlew

esab triks f

o m

otto

B

20–

10–

80–

26–

10–

1Lower skirt1 thk

SA516–70

11–

4/1

1 –51/2

1 –511/32

81/16

11/2 1/2

27.956°(Typ)

(Typ)

Stretchout of top plate

R=3

Figure 7 T-Rex skirt design

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Innovations in skirt design Another potential area of failure in cokedrums is the skirt-to-shell weldattachment. Since most coke drums havea knuckle transition from their cylindertop to a conical bottom, a skirt assemblyis used to support the vessel at its lowertangent line and provide uniformity ofsupport stresses in the structure. Sincethermal cycling is most severe near thebottom of the coke drum, wheretemperatures can reach up to 1000°F, theskirt and other attachment welds are justas prone to cracking and prematurestructural failure as the vessel wall.

Several alternative skirt designs havebeen developed to counter thesefailures. One such design is the T-Rexskirt (Figure 7), which is a culminationof best practices and lessons learnedfrom years of fabricating and repairingcoke drums. In service since the 1970s,this design has proven to be one of themost reliable skirts currently around.

Recently, a finite element analysis(FEA) was performed on the T-Rexdesign, the results of which werecompared to those for the conventionaldesign configuration. This is a transientthermal analysis to establish the modeltemperature profile over the load cycletime history. The results of the thermalload tests show that the T-Rexconfiguration has lower stress levels atall locations for the critical charge andquench thermal cycles. This appears tobe due to the longer hot-box length,which results in a more gradual thermalgradient and also moves the gradientlower on the skirt away from the weldedconnection. As such, the FEA evaluationshows that the T-Rex has substantiallyreduced stresses as compared to otherskirts currently in operation. Some ofthe features of the T-Rex design includesloped attachment welds that helpreduce stress-related failures and anattachment that covers the tangent weld(unless an elongated straight flange isutilised). The T-Rex design reducesinitial application costs and improveslong-term reliability over other designsthat have been evaluated.

Another design recently developed isthe Wrapper skirt (patent pending). Thisemploys a fabricated wrapper-type skirtthat conforms to the geometry of theupper cone, knuckle transition andlower cylinder to support the drumprimarily by bearing and frictionalforces rather than load bearing weldattachments (Figure 8). This detailprovides a more flexible connectionbetween the skirt and vessel thantraditional methods, which improvesthe fatigue resistance of the structure. Inaddition, the extended contact betweenthe drum and skirt lessens thermallyinduced stresses in both componentsby ensuring a uniform temperaturegradient between the two. Finally, the

elimination of the weld attachmentsubstantially reduces large pre-stressesfrom weld shrinkage, weld-inducedheat-affected zones and high localstresses in the structure, which likewiseimproves the overall fatigue resistanceof the structure and supporting skirt.

Packaging these skirt designs with thepreviously discussed vertical plateproprietary technology provides acomprehensive vessel solution that notonly improves the lifespan of a refinery’sdelayed coking vessels, but also saves theowner significant repair and replacementcosts. These designs can be used duringretrofit projects such as coke drum repairor replacement, as well as forincorporation into new unit designs.

Economics of vertical platecoke drumsWhile the installation of vertical platetechnology will reduce the capitalrefinery owners will need for repairingand replacing their vessels, it is too earlyto quantify the actual observed benefit,since the existing vertical plate cokedrums in service have not seen enoughcoke production cycles. However, thecost and benefits associated with thetechnology can be demonstrated using alifecycle value assessment (LCVA)methodology, which establishes a truecost of ownership. This tool, which wasdeveloped by the Pembina Institute, is avalue assessment model that allowsowners to combine initial Capex and 20years’ Opex for both conventionaldrums and the vertical plate cokedrums. The methodology also considersthe direct financial losses arising fromunplanned outages of the drums, andalso applies net present value (NPV)principles to further assess the expectedcost savings. A rigorous NPV calculationcan demonstrate the excellent value ofthese innovations. However, for thepurposes of this discussion, a simplifiedLCVA model is utilised as follows:

Conventionally designed drumsCapex The initial Capex outlays for fourvertical plate coke drums are assumed tobe $26.1 million. In this example, thedrums come at an initial Capexpremium of $2.175 million compared toconventional drum designs. Thus,Capex for conventional drums isassumed to be $23.9 million. Opex At an average of 14 hours per unitcycles, each conventional drum inoperation will go through 313 cycles peryear, which translates into 6260 cyclesover a 20-year period. As previouslymentioned, Livingston and Saundersreported that the coke drums in theirsurvey began to through-wall crack atroughly 2400 cycles, while anindependent 1996 API Survey confirmedthat the first through-wall cracks onconventional drums typically occur in

the 3000–5000 cycle range (Figure 3).Based on these findings, no fewer than11 cracks per drum may reasonably bepredicted during the 20-year period.

For simplicity, it is assumed thatplanned outages deal with 30% of thecracks that will appear (three cracks), sothe remaining eight cracks willsignificantly dictate more expensiveunplanned refinery stoppages of thecoke drums, which is where the Opexcost is greatest. In this example, costs aredetermined for a planned turnaround,and then a 30% uplift is applied forunplanned events. It is assumed thateach stream-to-stream unplannedoutage will last an average of six days.Thus, to remedy a through-wall crack, itwill cost (in 2005 dollars): — Costs of a single crack repair (20workers * six days * two 10-hour shifts *$85 per hour) + (materials, scaffolding/equipment, insulation, repairs and soon) = minimum $225 000 — Costs of crack repairs over a 20-yearperiod Three planned events * ($225 000)+ eight unplanned events * ($225 000 *1.3) = $3 million in repair costs for eachdrum, or $12.1 million for all four drums. Direct financial losses The greatestimpact occurs when one considers thelost revenues caused by the afore-mentioned cracking events. In addition,there are hazardous plant risks that canaccompany the cracking. For the eightunplanned refinery stoppages that willoccur over the 20-year period, if theexample drum pair has a capacity of 25 000bbl/day and the product is valuedat 85% of the WTI value (an average of$50 per bbl): — Direct financial losses over a 20-yearperiod Eight unplanned events/drumpair * six days for each event * 25 000bbl/day * (0.85 * $50) = $51 million inlost revenues for each drum pair, or$102 million for all four drums.


Support

Coke drumshell

Coke drumshell

R3

H2

H1

R2

R1

Figure 8 Wrapper design

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Vertical plate coke drumsCapex As previously mentioned, theinitial Capex outlays for four verticalplate coke drums is assumed to be $26.1million. Opex The vertical plate technology isexpected to defer cracking substantially.Since 75% of the critical circumferentialweld seam footage has been eliminated,we can assume there will be 75% fewercracks. Thus, the Opex for shell crackingon a vertical plate coke drum may be 11* 0.25 = three events over the 20-yearperiod, of which 30%, or one, of theevents will be planned and theremaining two events will beunplanned. Using the same analysis aswith the conventionally designed cokedrums over the same 20-year period: — Costs of crack repairs over a 20-yearperiod One planned event * ($225 000) +two unplanned events * ($225 000 * 1.3)= $810 000 in repair costs for each drum,or $3.2 million for all four drums. Direct financial losses In terms of lostrevenues caused by cracking, the sameassumptions apply from above, but foronly two unplanned events. Thus: — Direct financial losses over a 20-yearperiod Two unplanned events/drum pair* six days for each event * 25 000bbl/day* (0.85 * $50) = $12.7 million in lostrevenues for each drum pair, or $25.5million for all four drums.

Based on the LCVA, it can be seen thatthe lifecycle Opex and lost revenue costsover a 20-year period for fourconventionally designed coke drumstotal about $114 million, compared withabout $29 million for coke drums usingvertical plate technology. Thus, verticalplate coke drums can save refineryowners approximately $85 million overthe 20-year period, not to mention theadded advantages of greater plant safety.Note that these calculations areconservative, as downtime in a delayedcoker unit, as in any key processing unit,

affects the overall throughput of therefinery. Also, without a rigorousinspection and maintenance schedule,failures can occur at inopportune timessuch as the summer driving season whenproduct values are at their peak. Hence,the direct financial losses due to outagesare likely to be greater than described inthis discussion, and the correspondingbenefits of vertical plate technology arelikely to be that much higher.

Future outlook Since 2000, CB&I has completed fourvertical plate projects, bringing its totalnumber of vertical plate coke drums inservice to 11. These projects haveincluded the full circumferential cansection replacement of shells rangingfrom 35–73ft, as well as new lowerknuckle and skirt assemblies. As far asimproving the reliability and lifespan ofcoke drums go, as previously mentionednone of the vessels have been in servicelong enough to allow quantification ofthe actual improvement. However, thevertical plate coke drums installed atthe West Coast refinery in 2000 haveseen about six years of service and areapproaching 2000-plus cycles with noevidence of any unusual distortion.

Currently, four more drums are undercontract. The first project is for aCanadian refiner and includes two newdrums of slightly larger dimensions to beinstalled during a turnaround later thisyear. This project is unique in that thedrums are being fabricated in threesegments. The segments are then to betransported to the site and installedthrough the side of the structure duringthe outage while seams are being weldedin place. The second project is for twodrums being erected on site and installedon the “table top” for final assembly.

As feedstocks become heavier and theneed for coking capacity continues torise, refiners will need more than ever to

have coke drums that provide greaterreliability than traditionally possible.Whether it is a retrofit application, a fullvessel replacement or part of a newprocess unit, vertical plate coke drumswill be able to endure the most severethermal cycles and outlast any conven-tionally designed vessel. Implementinga T-Rex or wrapper design for thesupport is expected to further improvethe life of these vessels.

This article is based on a paper (AM-06-48)presented at the NPRA Annual Meeting, SaltLake City, Utah, 20 March 2006.

References1 Weil N A, Rapasky F S, Experience with

vessels of delayed — coking units, API 23rdMidyear Meeting, 1958.

2 Boswell R S, Ferraro T, Remaining lifeevaluation of coke drums, Plant Engineering,Design and Responsibility Symposium,Energy Engineering Conference, 1997.

3 Antalffy L P, Malek D W, Pfeifer J A, StewartC W, Grimsley B, Shockley R, Innovationsin delayed coking coke drum design, ASME,1999.

4 Livingston B, Saunders K L, Coke drumsfailure prediction evaluation usingprobabilistic techniques, ASME, 1998.

5 Penso J A, Lattarulo Y M, Seijas A J, Torres J,Howden D, Tsai C L, Understanding failuremechanisms to improve reliability of cokedrums, ASME, 1999.

6 ASME Section II parts A & C, plus Plate andElectrode Manufacturers brochures andCB&I data.

Coby W Stewart is business developmentmanager, Chicago Bridge & Iron, TheWoodlands, Texas, USA. Email: [email protected] M Stryk is global marketingmanager, Chicago Bridge & Iron, TheWoodlands, Texas, USA. Email: [email protected] Presley is product and engineeringmanager, pressure vessels and specialstructures, Chicago Bridge & Iron, TheWoodlands, Texas, USA. Email: [email protected]


acs

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Coke Drum Design