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A super duplex success story:from CalEnergy and Carolina Energy Solutions

Super duplex field welding technology - first employed in the cold North Sea is now hard at workin the arid California desert, thanks to a teamwork effort led by CalEnergy and WEC CarolinaEnergy Solutions. This challenging application involves geothermal power generation at CalEnergysSalton Sea complex. A leader in the renewable energy field, CalEnergy is a US based independentpower producer, providing over 340 megawatts to the California power grid. Similarly well known isCarolina Energy Solutions of Rock Hill, SC a renowned innovator of highly specialized field weldingand machining technology. Stainless Steel World first reported on this effort in the October, 2006issue. We revisited senior project engineer George Furmanski at CalEnergy Imperial Valleyengineering headquarters for an update on this intriguing application of super duplex stainlesssteel in a uniquely corrosive environment.

By John M. Griffin

The blistering California desert hasbecome a testing ground for advancedstainless steel alloys. It is here thatsuperheated geothermal fluid reservoirsare tapped to drive steam turbinegenerators, producing clean, reliableenergy for power hungry SouthernCalifornia.Externally, the ten Imperial ValleyCalEnergy generation plants appearquiet and efficient, collectivelyproducing 340 MW of net baseloadpower with an impressive 93% capacityfactor. Only upon closer inspection doesthe internal picture come into focus: anongoing battle to harness one of theworlds most aggressive naturallyoccurring geothermal fluids.

About WEC Carolina Energy SolutionsCarolina Energy Solutions is home to some of the worlds most advanced weldingtechnology. The real essence of the company, however, is a unique ability to harnessthis technology in truly demanding applications. CES routinely performs technicallyexacting welds under tight turnaround schedules and extremely tough field conditions.A division of the Westinghouse Electric Company Welding and Machining business,Carolina Energy field crews can be found at nuclear, fossil, and renewable jobsitesthroughout the world. CES trains highly certified welders at hands on instructioncenters, graduating over 100 craftsmen each year. WEC field machining, welding, andinspection equipment is designed and built in house, then stocked at strategic centersaround the country for quick deployment.Headquartered in Rock Hill, SC, the WEC Carolina Energy manufacturing facilityproduces highly engineered weldments and fabrications in support of field operations.Sister Westinghouse companies, PCI in LakeBluff, IL and WesDyne in Madison, PAprovide custom welding equipment, robotic inspection, and advanced NDEtechnologies. www.wecwam.com; tel: +1 803 980 3070.

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The concentration of chlorides, hydrogen sulfide, and carbon dioxide make this one of the mostabrasive, corrosive potpourris imaginable

Rising thousands of feet as causticsuperheated brine, this fluid is flashedto steam, which drives turbinegenerators year round. Released ofenergy, the brine is then reinjected backinto the underground geothermalreservoir.

A witchs cauldronEntering the borehole at roughly 300Cand 40 bar, the superheated brine is awitchs brew of two phase liquid/vaporand dispersed solids. The chemicalcombination of chlorides, hydrogensulfide, and carbon dioxide make thisone of the most abrasive, corrosivepotpourris imaginable. Typical samplestest at pH 4.5 5.7, with chlorideconcentrations of 106,000 167,000ppm, H2S of 10 50 ppm, and CO2 of800 4800 ppm. Dissolved solids

constitute 25% of the saturated brinesolution. Liquid phase silicaprecipitation occurs readily attemperatures below 205C makingdeposition and scaling a constantconcern.

Battling corrosionWhen George Furmanski arrived atCalEnergy in 2001, the battle againstcorrosion was well underway. At thatpoint, ongoing experiments withcement lined carbon steel pipe wereproducing mixed results. While anexcellent solution in steady statethermal conditions, the widely differingexpansion coefficients of steel andcement inevitably produced cracks andvoids particularly in the critical hightemperature/high pressure turbine feednetwork. Even with summer ambient

temperatures routinely reaching 50C,the thermal shock associated with dutycycling 245C feed lines was simply toogreat. And once a cement liner isbreached, the SA 106 carbon steel32mm wall x 76cm diameter pipematerial erodes at a rate of up to 1.3 mmper month, reducing effective servicelife to two years or less.

A promising experimentThough technical solutions existed theoverriding issue was cost. For example,experiments had shown pricey Inconel625 pipe and fittings to be an excellentchoice in well head applications.Similarly, expensive titanium grade 29bore casing was well proven over anumber of years, whereas carbon steelcounterparts typically failed within 18months.Calling upon extensive experience withduplex and lean duplex alloys in hightemperature caustic environments, Mr.Furmanski moved quickly to installturbine feed test loops, in one casespecifying super duplex 2507 (ASTMA928 UNS 32750). The long term resultsof this choice were gratifying. Over a 50month test period, corrosion of less than0.1mm per year was measured,indicating a greatly extended servicelife. And super duplex 2507 thoughexpensive promised to be far more costeffective than the Inconel 625 pipingalternative also under consideration.When Stainless Steel World spoke withGeorge Furmanski in 2006, it seemedthat a solution for the problem offrequent and pricey pipelinereplacement was close at hand.

Corrosion test pipe specimens

About CalEnergyCalEnergys domestic geothermaloperations are a part of CE Generation,LLC, a limited liability company ownedby MidAmerican Energy HoldingsCompany and TransAlta (CE Gen)Investments USA Inc. MidAmericanEnergy Holdings Company specializesin the production of energy fromdiversified fuel sources includinggeothermal, natural gas, hydroelectric,wind, and coal. TransAlta is a powergeneration and wholesale marketingcompany that operates a highlycontracted portfolio of similarlydiversified assets in Canada, UnitedStates, Mexico, and Australia.CalEnergys geothermal operations inCalifornia produce 340 megawatts ofelectricity enough power to meet theenergy needs of 340,000 homes.www.calenergy.com

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About geothermal powerGeothermal power is a renewablesource of energy that uses the naturalheat of the earth to generateelectricity. The US Department ofEnergy is now fundingdemonstration projects to showcaseenhanced geothermal technologies.www.geothermal.id.doe.gov

Daunting obstaclesA number of obstacles still remained,however. The most daunting was a verypractical one: how could over ninekilometers of 2507 pipeline beeconomically assembled on site withoutdegrading corrosion resistance of thebase metal?Theres a good reason for this dilemma.While SDSS 2507 may be readily weldedusing similar 2507 filler, maintainingcorrect metallurgy within the heataffected zone requires strict adherenceto highly exacting weld procedures,including precise mechanical andchemical preparation, restrictiveinterpass temperature limits, and mostimportantly - demanding post weldheat treatment. In a factory setting,these requirements can be met as amatter of course. However,accomplishing them in the middle ofthe California desert is another matteraltogether.

Balancing actDuplex stainless steels are defined by amicrostructure containing both ferriteand austenite - hence the termduplex. Maintaining the delicatebalance of this ferrite/austenitemicrostructure in the weld heat affectedzone (HAZ) is critical to ensuring longservice life in highly corrosiveenvironments.Why specify super duplex? In anutshell, duplex stainless steels providesuperior resistance to stress corrosioncracking, together with higher overallstrength than standard and superaustenitic stainless steel. Super duplexstainless steels are characterized asalloys containing higher levels of Ni, Cr,Mo and N than standard duplexstainless steels. Some super duplexstainless steels are additionally alloyedwith tungsten.

Welding wizardryThe relative proportions of ferrite andaustenite in the weld deposit and heataffected zone closely depend upon fillercomposition and cooling rates. Rapidcooling rates result in higher levels offerrite, particularly when welding isundertaken using duplex stainless steel

filler metals. Practically, heat inputs inthe range of 0.5-1.5kJ/mm for superduplex, and 0.52.0kJ/mm for standardduplex stainless steels are employed; thisin conjunction with maximum interpasstemperatures of 100C and 150C,respectively. Interpass temperature isoften further restricted on thin wallsuper duplex materials to prevent theprecipitation of third phase intermetallics.In the quest for optimum corrosionresistance, a number of HAZcountermeasures are possible. One isclose specification of the inert shield gasrecipe. For example, a nitrogen richshielding gas acts to aid the hightemperature reformation of austenite inthe weld region. Nitrogen also tends toimprove erosion toughness, especiallyof the austenite phase.

Over alloyingSelection of filler metals is similarlycrucial. For field welding, typicalconsumables for duplex are similar incomposition to that of the basematerial, but may exhibit higher levelsof nickel to ensure an appropriate phasebalance (30-60% ferrite) in thedeposited weld metal.As an example, super duplex filler metalis often used for welding standardduplex stainless steels. Using such anover alloyed filler metal in thesecircumstances produces weld depositswith enhanced pitting corrosionresistance relative to standard welddeposits.

North Sea treasureFurther researching super duplex fieldwelding, George found that others hadfaced similar job site welding issues inthe North Sea. There, 2507 alloy borecasing selected for both high strengthand resistance to chloride stresscorrosion cracking must be weldedunder very tough offshore oil rig

conditions. Furmanski discovered that ametallurgical solution had beendeveloped which employed highlyoveralloyed fully austenitic nickel alloyfiller materials. The use of a fullyaustenitic nickel alloy weld depositremoves the requirement which existswith duplex stainless steel filler metalsto produce welds with balancedquantities of austenite and ferrite.The treasure chest? This discoveryopened a door to the possibility ofperforming optimum super duplexpipeline field welding without the needfor post weld heat treatment.Time hasproven the durability of North Sea fieldwelds. Properly executed, they exhibitexcellent corrosion resistance andsuperior mechanical properties. Thequestion: Would this magic work half aworld away?

One step forwardAfter further research andexperimentation, Furmanski settledupon a Special Metals Inco-Weld 686CPT filler material (AWS A5.14ERNiCrMo-14) as a possible technicalsolution. Nominal composition is 57%nickel, 21% chrome, 16% moly, and 4%tungsten. Very carefully employed, thisfiller will preserve 2507 corrosionresistance without requiring post weldannealing.As ever, cost control was paramount.With many kilometers of pipeline, thenext step was to find mass producedSDSS 2507 pipe and fittings of the correctmechanical tolerances, together with amethod of cost effectively field weldingthem using Inco-Weld 686 CPT.The first issue took a good deal of traveland effort to resolve. A solution waseventually found in Knesebeck,Germany at H. Butting GmbH. Onceinvolved, Butting engineers workedquickly to address production and fit upissues an important consideration

Heavy duty rim line-up clamps

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with tough, high tensile SDSS 2507piping.

A major stumbling blockField welding 2507 turned out to be aneven thornier problem, however. At first,George Furmanski searched for aqualified welding service with littlesuccess. Test samples and RFQs weresent out with poor response. Proposalsfell short as either too expensive, ortechnically unacceptable or both. Testsample welds routinely failed inspection.Enter Carolina Energy Solutions of RockHill, SC. Well known for precisiontechnical welding in nuclear and similarapplications, CES relished theCalEnergy challenge. First, CarolinaEnergy worked to develop a 2507base/686 CPT filler parameter set.Innovations included a 686 shield gasrecipe appropriate to field applications,together with a quick purge internalbacking gas fixture. Next came closeengineering of computerized powersupply and arc control characteristics,followed by in house weld specimeninspection and testing. The initialresult: CES weld samples passed allCalEnergy tests with flying colors.

The Desert RatsThe next hurdle proved a daunting one:How could these exacting welds beperformed economically and precisely while burdened with some of thetoughest field conditions under the sun?CES site management worked side-by-side with CalEnergy Desert Rats rolled

WEC Aggressive Equipment split frame portable lathe

up their sleeves and got to work. CESproject management worked side by sidewith CalEnergy, whilst simultaneouslyco-ordinating technical matters with theCES jobsite team of welding, machining,quality, and safety specialists. Along theway, field fixturing and toolingapproaches had to be developed forgroove prep and fit up an undertakingthat called upon the full scope of CESdesign and machining experience.

Field machiningSDSS 2507 is famously tough tomachine, and may be subject to workhardening and embrittlement if toolzone stresses and temperatures are notcarefully controlled. Additionally,fixturing must be simple and rugged,allowing rapid, precise set up underharsh field conditions. With hundredsof pipe and fitting junctions, machiningproductivity is equally important towork efficiency the trick is acombination of high cutting speeds andlow feed rates. With groove facesmachined, fit up concentricity isequally demanding. The solution was asplit frame WEC Aggressive Equipmentportable lathe, combined with heavyduty rim line-up clamps.

Orbital machine weldingAfter lab tests were successfullycompleted, Carolina Energy establisheda gas tungsten arc welding procedureemploying manual root passes andorbital machine fill welding. Withproductivity and quality control as theprincipal drivers, this procedure wasrefined through extensive on site fieldtesting.Manual GTAW root passes allowedhighly skilled CES welders toaccommodate the gap and offsetvariables inevitable with cross countrypipeline installations. Clean up andinspection completed this step,enabling the Carolina Energy orbitalmachine crews to quickly set up for fillpasses.During procedure development,numerous obstacles confronted the

Carolina Energy Solutions orbital machine welding

About George Furmanski

George Furmanski received amasters degree in metallurgy atthe Academy of Mining andMetallurgy in Cracow, Poland in1970. After a brief spell at SkawinaAluminium Smelter as a plantengineer, he joined Chemadex as adesign engineer working onprojects related to environmentalengineering and pyrometallurgicalplants.In 1981, Mr. Furmanski went toAustralia to work on uraniumprojects for Wright Engineers inSydney. In 1983 he joined BHPEngineering Australia, where hewas involved in numeroushydrometallurgical projects that in1997 brought him the Rio TintoAward of Excellence in ChemicalEngineering for outstandingachievement. This award was givefor his participation in the designof the Electrolytic ManganeseDioxide Plant where lean duplex2304 and duplex 2205 were usedfor the first time in hot sulphuricleaching. His long-terminvolvement in chloridemetallurgy was finally tested atCalEnergy, where he has beenworking since 2001 ondemonstration projects related tothe selection of materials forconstruction for high chloride,high-temperature geothermal brineapplications.

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Desert Rats. With perseverance andteamwork, practical solutions wereeventually found. Desert night/daytemperatures routinely swing through16C, making moisture condensation anissue - even in ultra low humidityconditions. The solution: thermalblanketing of pipeline junctions. Gritladen dust storms whip daily over theSalton Sea. The solution: portablesheltering for operators, shield gas, andarc control. Critical interpasstemperatures can vary widely as thedesert sun rapidly heats exposed pipesections. The solution? Interactive weldcontrol parameters tied to base metaltemperature rise.

About the authorJohn Griffin received a bachelors degree in industrialengineering at Cal Poly San Luis Obispo in 1981,followed by a masters at Duke University in 1993. Hisbackground includes the development and applicationof industrial control and servo systems. At GE andFanuc, John was instrumental to the introduction ofmodern microprocessors to computer numerical controland programmable logic systems. Working withmachine designers and builders, John further developeddigital servo system applications in concert with thesetechnologies. After executive management stints with

Square D Schneider, Kollmorgen Motion Technologies, and InterrollCorporation, Mr Griffin cofounded EnergyPath Corporation in 2001.EnergyPath provides technical and financial decision support services toenergy industry OEMs and power producers worldwide. www.nrgpath.com;tel: +1 252 675 1936.

Super duplex 2507 turbine feed pipeline.

Radiographic NDETo meet ASME B31.1, all project fieldwelds must be 100% inspected. Toeconomically accomplish this underharsh field conditions, CES relied uponlong experience with non destructiveexamination (NDE). In this case, aradiographic telemetry (RT) approachwas employed. After development andrefinement, the final inspectionprocedure was easily accomplished incadence with machining, fit up, rootpass, and fill operations.

Success at last!Working closely together, CarolinaEnergy Solutions and CalEnergy havesuccessfully tackled a really toughapplication. The results are impressive:over nine kilometers of SDSS 2507production feed pipeline now glitter inthe desert sun, silently powering morethan a quarter million California homes

and businesses with clean, renewablegeothermal energy.Thanks to innovation, perseverance, andteamwork, CalEnergy has accomplisheda major goal: 15 years plus of troublefree production turbine feed pipelineservice life. Importantly, the practicalapplication of this unique stainless alloynow encompasses one of the moststrategically important fieldsimaginable: expanding the use of ourprecious geothermal energy resources.

Looking forwardWith this breakthrough in hand,CalEnergy now envisions a majorcapacity expansion at Salton Sea ninenew geothermal wells generating acombined 495 MW. Current plans call forconstruction to begin in 2011, with newcapacity coming on line through 2020.And - as you might imagine - superduplex 2507 is specified throughout.