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ary vanes and rotating blades. The origi-nal unit gave a nominal output of120MW, with 32.5% simple cycle effi-ciency and a maintenance cycle based on 25,000 Equivalent Operating Hours(EOH). Today, with the latest blading andimprovements to the combustion systemit is anticipated that the efficiency willincrease to ~34.3% and the output to ~165MW, generating an additional 20 million dollars per year of revenue(based on 6500 hours and price of70$/MWh).

Sulzer engineers worked on a redesignof the parts that would provide increasedcapacity or allow for extended mainte-nance intervals. To assist the owner theparts can be operated at either 1060°C or1075°C with a different ‘cost of mainte-nance’ and different unit output based onthe chosen firing temperature. Thisresults in the owner being able to opti-mize their costs and revenues based onmarket conditions. The uprate requiredthree major changes compared to theprevious design.

Improved coolingThe easiest way to improve the perform-ance of a blade or vane during operationis to increase the cooling air flow to thepart. However, this can significantlyreduce output or increase NOx emissions(nitrogen oxide, an air pollutant) of theunit as the air is no longer available forcombustion. The toughest issue is there-

fore to make best use of the air flow thatexists by incorporating new airflowdesigns to the cooled blades.

New materialsOver time new materials have becomeavailable and generally accepted for usein industrial gas turbines. New castingtechnologies have also helped because itis now possible to cast large parts thatpreviously would have had to have beenforged. This allows materials with bettercreep and oxidization resistance to beused.

Better coating systemsAt Sulzer Turbo Services it has beenfound that the most effective way to coolthe parts is to apply a preparatoryMCrAlY-TBC system. The TBC, or ther-mal barrier coating, is a ceramic layerthat insulates the metal from the full tem-perature of the hot gas path. The applica-tion of a TBC coating system allows twodifferent benefits to be provided:• Extension of maintenance interval

without increase in output (e.g.,at constant firing temperature)

• Increase in capacity and efficiency byraising the firing temperature butwith the original maintenanceintervalIn our case, a preparatory MCrAlY-

TBC system was also applied to theblades to improve the external oxidationand corrosion resistance to help ensure

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PROJECT 1Uprates to a gas turbineIt is normal for a gas turbine during thelifetime of the model to increase its per-formance both in terms of efficiency andoutput. The easiest way to improve bothis to simply increase the firing tempera-ture of the gas turbine. However, withoutadditional changes to the components,this will increase the costs. The simplerule of thumb is that for every 10 °Cincrease in firing temperature, the lifetime of the parts is reduced by 50%.

To combat what would be a loss of lifeand increase maintenance costs, SulzerTurbo Services has been working on newand improved components for a 120 MWgas turbine, which will provide numer-ous operational benefits to the owner.

The full gas path is shown in the draw-ing and consists of four rows of station-

By using modern technologies, Sulzer Turbo Services has been able to develop uniquesolutions to support the owner’s operational needs. This approach is applicable toboth steam and gas turbines and can provide sustainable competitive advantages. The article looks at three completed projects.

CAD view of the hot gas path section.

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the parts were refurbishable followingthe planned inspections. Additionalinternal coatings were applied to reducethe internal oxidation of the base materi-al that can lead to internal cracking.

PROJECT 2Steam turbine with higher efficiencyWhile operating at low load, the steamturbine of a Finnish paper mill suffereda bearing failure. Stray electrical cur-rents caused the babbitted lining of thebearing to fail. As a consequence, clear-ance between the rotating and station-ary elements was lost, causing bladedamage and the rotor to distort or bow.The turbine is was 59% reaction tur-bine, designed for 50Hz operation. The

design incorporated 44 stages, and eachstage had a relatively low-pressuredrop. Due to the low-pressure drop perstage, the manufacturer designed manyidentical stages, which was commonpractice in the past. This had the effectof limiting the efficiency of the turbine.The opportunity to produce both aneconomical repair of the turbine and anincrease in the internal efficiency wasidentified. All the rotor stages that wereseverely damaged were disassembled.In total, about 6000 blades were disas-sembled with 3000 replaced by reverse-engineered blades. Stages that had tobe reverse-engineered were equippedwith blades that had an optimizedgeometry.

Rotor repairsCorrection of the damage to the rotor androtating components necessitated a seriesof operations including specialized weld-ing techniques and heat treatment. Theseprocesses were selected in part toimprove operational durability throughcorrosion and erosion resistance. Wherethe existing design held the potential forimprovement, the dimensions were ana-lyzed and selected improvements madeto reduce operating stresses.

After high-speed balancing, the rotorwas installed on site. The turbine wasput into service and achieved full powerwithout problems. Steam data showedthat the internal efficiency of this tur-bine, after the straightening and repair

Sulzer Turbo Services expertise helps to extend the operational lifetime and the performance of steam and gas turbine as well as combined-cycle power stations.

Photo: BP

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was 2% better than before the incident,providing a payback to the repair with-in two years.

PROJECT 3Steam turbine with more outputRe-rating a steam turbine may some-times require more than just the replace-ment of the turbine blades and thediaphragms. This was the case when anowner of a process plant approachedSulzer Turbo Services Rotterdam aboutthe potential upgrade of a waste heatrecovery steam turbine in a chemicalplant in Germany. The owner had re-con-figured the operational process of theplant and because of this now had 25%more steam available. The current steamturbine was rated at 7 MW and wasunable to accept the additional steam.One simple solution was to replace thesteam turbine with a new steam turbine,however this was viewed by the owner

as a very expensive option. Even worse,the lead time for a replacement unit wasover 24 months. Another solution wouldhave been to dump the steam and wasteboth energy and money.

Feasibility studyThe requirements of the owner were verydemanding. The first phase of the projectwas to complete a feasibility study thatwould lead, if successful, to a new designof the rotor/casing configuration andalso ensure that the generator and otherauxiliaries would be able to deal with the additional output. Using a Compu-tational Fluid Dynamics (CFD) program,the flow path conditions were calculatedat the original design parameters andthen with the additional steam flow. Thesteam path re-design process was vali-dated with the CFD, allowing the designphase to be completed with confidencethat the customer required boundary

conditions would be met. Based on thereport the owner decided to go aheadwith the upgrade.

Design work and repairThe steam path had been designed; how-ever, it was still necessary to understandhow the new design would interact withthe casing. A computer program wasagain used to allow the designs to becompleted, including the modificationsto the casing highlighting the fit up of theold and new components. A new rotorwas then forged, machined and bal-anced. The modifications to the casingwere made and an insert ring rolled intothe original casing, allowing the originalbut modified casing to be re-used.

Once fully bladed, the rotor wasinstalled into the casing and all of thetolerances checked. With the casingsclosed, the unit was then returned to the chemical plant, re-installed and re-commissioned. The unit operated withinthe acceptable limits and was able tocope with the additional steam, helpingthe owner to operate the chemical plantmore efficiently than before.

The rotor fitting within the casing. The blades being installed in the new rotor.

Shaun WestSulzer Turbo ServicesZürcherstrasse 128400 WinterthurSwitzerlandPhone +41 52 262 34 44shaun.west@sulzer.com

Mach numberplots from the CFD analysis of the steam path flow.

Steam turbine rotor installed within the casing.

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