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POWER ENGINEER ING/JUNE 2003 • 38 www.power-eng.com

STEAM TURBINES

BY DOUGLAS J. SMITH IENG , SENIOR EDITOR

Since the late 19th century steam turbines have been used for the generation of electricity. A prototype steam turbine,designed by Charles Parsons in 1890, had 15 expansion

stages and was one of the first steam turbines to be connected toan electric generator for the generation of electricity. At the turn of the 20th century, steam turbines were designed for an inlet pres-sure of approximately 200 psi and a temperature of 500 F. Today,steam turbines are available with steam conditions of 4,350 psi,inlet temperatures of 1,100 F and reheat temperatures of 1,150 F.

Currently the U.S. has approximately 1,500 steam turbines with acapacity to produce 400,000 MW of electricity. However, an EPRI con-ducted survey has determined that turbines with the highest capacityhave an average age of 38 years. According to EPRI, the failure of com-ponents in high-pressure (HP) turbines is responsible for the greatest

loss of capacity and de-rating of the units. On the other hand, low-pres-sure (LP) turbines contribute to more forced outages, says EPRI.

Major incentives for upgrading fossil power plant HP and LPsteam turbines are to:

• Increase efficiency• Improve reliability• Reduce solid particle erosion

In nuclear power plants the elimination of stress cracking inLP steam turbines is a major reason for upgrading.

NEW  LP R OTOR  A DDS C APACITY 

Portland General Electric’s (PGE) Boardman coal-fired gen-erating plant entered commercial operation in 1980. The plant,

located 160 miles east of Portland, Oregon, burns primarily lowBtu, low sulfur Power River Basin coal. At present the plant is eco-nomically dispatched with load cycling in the spring and near fullload the rest of the year. The plant’s steam turbine, a tandem com-pound, HP/IP with two LP turbines, was supplied by Westing-house. PGE (65 percent) co-owns the facility with Idaho Power (10percent), Power Resources Cooperatives (10 percent) and GECapital (15 percent).

By the mid 1990s, PGE realized that increasing power demand in its service area, and the uncertainties with therestructuring of the industry, would require upgrading of their power plants, including Boardman. PGE understood that anyupgrade would only be feasible if it did not impact the environ-ment. As a result, PGE researched ways to increase the plant’scapacity without using additional fuel.

After conducting an engineering study in 1995, PGE con-cluded that upgrading the steam turbine would be the best solu-tion for increasing the unit’s capacity. After issuing a request for proposal, the utility received a variety of upgrade proposals,including the retrofitting of longer LP rotor blades to boostcapacity. However, because none of the proposals were econom-ically feasible compared to buying wholesale power from other sources, the project was put on hold.

Three years later, in 1998, Siemens Westinghouseapproached PGE and recommended a retrofit that would incor-

porate larger LP rotors with blades several inches longer than thestandard 25-inch last row blades. Although the blades would belonger, Siemens Westinghouse believed the new blades could bedesigned to fit into the steam turbine’s existing rotor shell, signifi-cantly reducing the cost of the upgrade.

PGE’s executive management hesitated in committing to anupgrade using untried technology. Siemens Westinghouse was soconfident of the new design, however, that they told PGE theywould be willing to share the risk of the upgrade. Under the pro-posal Siemens Westinghouse and PGE agreed to an incentiveschedule that included financial rebates to the utility. The pro-posal essentially guaranteed that the plant’s capacity wouldincrease by a minimum of 25 MW after the upgrade.

LP R OTOR  UPGRADEAlthough the manufacturer was convinced they could

increase the capacity of the steam turbine they still had to verifythat the generator could handle the increased capacity without anymodification. An analysis conducted by Siemens Westinghouse in1999, and an evaluation conducted by an independent consultanthired by PGE, confirmed that the existing generator could handlean increase in power without modifications.

Ultimately the last row of the LP turbine rotor was retrofittedwith 34-1/2 inch side entry blades with an interlocked integralshroud. With the new design the choke point moved from the LPturbine to the HP turbine, thus allowing the first blade row of theHP turbine to be eliminated. Blade rows two and three of the HPturbine were also replaced as part of the upgrade.

Retrofitting of the LP rotor and HP blading was completedduring a seven-week spring outage in 2000. Although the con-tract specified a complete ASME PTC 6 performance test oncethe units were returned to service, the test was cancelled after itwas determined that the unit was producing more power than thecontract required.

STEAM TURBINE UPGRADES IMPROVE RELIABILITY~

F I G U R E 1

I N C R E A S E I N P O W E R  

Source: Portland General Electric Co.

+45 MW 

600 

550 

500 100%   Inlet Pressure   103% 

     M     W

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POWER ENGINEER ING/JUNE 2003 • 40 www.power-eng.com

According to Janet Gulley, projectmanager, PGE, the upgrade has increasedthe capacity of the Boardman generatingplant from 555 MW to 585 MW when oper-ated at 100 percent inlet pressure. This hasbeen accomplished without an increase infuel consumption. Likewise, when the unitis operated at 103 percent pressure capac-ity, the load increases to 600 MW, Figure 1.

Increased revenue from 45 additionalMWs during a single summer period haspaid for cost of the retrofit, says Gulley. Theplant is now operating with a net plant heatrate of 9,800 Btu/kWh. A 12-month inspec-tion, carried out in 2001, showed very littledegradation of the blading.

LP R OTOR  R EPAIRS

Stress corrosion cracking (SCC)occurs in areas subjected to high stress

and when corrosive ions are present. Bladeroot fastenings, where corrosive com-pounds can accumulate between the bladeroot and the wheel, are very susceptible toSCC. As the fleet of U.S. steam tur-bines ages fossil and nuclear power plants are finding SCC inmany of their LP turbine rotors.

During a routine inspectionat Duke Energy’s California MossLanding power plant in 2000,SCC was found in the L-2 bladeattachment on the LP turbinerotor on Unit 6. A subsequent

inspection of Unit 7’s LP turbinealso found the same problem.Moss Landing’s units each have aGE single-flow HP turbine, an opposed-flow intermediate pressure (IP) turbineoperating at 3,600 rpm and two opposed-flow LP turbines operating at 1,800 rpm.

Because of the severity of the SCC, andthe need to return Unit 6 to service quickly,pressure plates were installed in the “A”rotor generator end L-2 stage and in the “B”rotor turbine end L-2 stage. Pressure platesare used to simulate the pressure drop of theblades that have been removed, thus pre-

venting overloading of downstream bladestages. The repair, which involved 50 per-cent of the L-2 stage, allowed the unit to beput back into service, but it reduced theunit’s capacity by 22 MW.

To devise a long term solution to theproblem of SCC in the LP turbines, DukeEnergy considered a number of options:

• Removal of the first stage blading• Weld repair• Installation of new monoblock rotors

• A complete upgrade of the steam path• Longshanking

After evaluating the different options,Duke Energy determined that the long-shank bucket design, with rotor modifica-tions, represented the most cost effective

and technically sound solution. Althoughlongshanking has been utilized on steamturbines in nuclear power plants and onsmaller fossil plants, the Moss Landing isbelieved to be the first large-scale fossilpower plant to use this design method.

PROBLEM R ESOLVED

Longshanking involves the machining of new blade attachments in the turbine’s exist-ing disk. Because the new dovetails aremachined at a lower radius on the wheel, anycracks and/or damaged material are removed.The new blades have extra material in the

skirt or shank area between the root and theairfoil section, thus restoring the working por-tion of the rotating blade to its original radialposition and location in the steam-path.

Duke Energy awarded the contract for the LP rotors on Units 6 and 7 to Turbo-Care. The contract included:

• Engineering and analysis of the orig-inal design L-2 blades.

• Design and manufacture of replace-ment longshank blades. (Thisincluded full rows of mixed stainless steel and titanium conventional long- shank blades for Units 6 and 7 and full rows of titanium ultra-longshank

blades for Unit 6).• Rotor inspection of Unit 7 (Unit 6 hadalready been inspected).

• On-site machining of the L-2 stage  wheels, blade installation and low speed balancing.

During a scheduled November-December outage in 2001, the LP rotors onUnit 6 were removed, machined and fittedwith new longshank blades. After low-speed balancing, the rotors were assem-

bled and the unit put back into operation.Upgrading the LP rotors on Unit 7 wascompleted during a two-month outage inJanuary-February, 2002.

According to TurboCare, the major advantage of longshanking is that it pro-vides full restoration of the steam-path with-out any de-rating of the unit. Refurbishingthe LP rotor through machining andinstalling new longshank blades has low-ered the stresses in areas subjected to SCCon the Moss Landing’s LP turbines. Theend result is increased reliability and avail-ability and the restoration of the lost capac-ity from the temporary pressure plate repair.

NUCLEAR  PLANT

COMPLETES UPGRADE

On April 21, 2003, PPL returned Unit2 of the Susquehanna nuclear plant to ser-

vice after a routine refueling outage. Dur-ing the refueling outage Susquehanna alsoupgraded the HP and three LP steam tur-bines. The plant, located seven miles

northeast of Berwick, Pa, has twoboiling water reactors with a totalcapacity of 2,248 MW. Unit 1 wentinto commercial operation in 1983and Unit 2 in 1985.

During the first four years of commercial operation Susque-hanna experienced problems withdisc cracking on the LP turbinerotors. As a result the plant

replaced the original disc type LPturbine rotors with monoblockrotors. The new rotors had a six-

year inspection interval that was eventu-ally extended to eight years.

Although the plant resolved the disc-cracking problem, subsequent problemsdeveloped with both dovetail cracking andinner casing erosion. Dovetails, where theblades are attached to the rotor, are subjectto stress corrosion cracking at the rim of the turbine’s disc.

Because of the continual problems of dovetail cracking and inner casing erosion

with the plant’s LP turbines and the needto increase unit performance and reliabil-ity, PPL Susquehanna LLC approached anumber of vendors in November 1999 tobid on retrofitting new LP turbines. Eventhough there were no specific problemswith the HP turbines, the plant requestedthat the vendors include upgrading of theHP turbines in their bid.

Siemens Westinghouse replaced theoriginal seven-stage rotors on Unit 2’s

STEAM TURBINES

 Boardman’s Steam Turbine Generator. Photo courtesy of Port-land General Electric.

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three LP turbines with new nine-stage LPturbine rotors with integrally shrouded sta-tionary and freestanding LP rotating sec-tion blades. The last stage of the LP tur-bine has been fitted with 46-inch blades.

To mitigate moisture erosion, theleading edges of the LP freestanding rotat-ing blades are flame hardened. Likewise,improving the flow distribution of the lastrotating stage has been accomplished byinstalling forward leaning last stage sta-tionary blades. In addition, slots in the last

stage stationary blade ring will help toreduce moisture erosion in this area duringoperation. The retrofitted turbines weredesigned to fit into the existing turbinebearings, and except for some minor mod-ifications, all of the existing auxiliary sys-tems were re-used.

As mentioned previously all of theoriginal LP turbine casings at Susque-hanna were subjected to increasing erosionof the carbon steel inner casing compo-nents. After evaluating inner casing

upgrade options, includingan all stainless steel inner casing construction, theutility selected a hybridinner casing design withupgraded materials in thehigh velocity areas.

Upgrading of the HP

turbine included a newsolid mono-block rotor design, the installation of astainless steel inner casingand the installation of bladecarriers in the existingouter casing. In addition,advanced blading and inlet“L” seals between the inner 

and outer casing were installed. Similarly,the existing seven-stage design wasreplaced with 12 blade stages.

According to John Bartos, senior engineer, PPL Susquehanna LLC, theNuclear Regulatory Commission (NRC)has approved an increase between inspec-tions for the LP turbine design retrofittedat the Susquehanna plant to 10 years.

Although Unit 2 has only recentlybeen returned to service, it is expectedthat the retrofit with advanced disc LPand the upgrading of the HP full reactionturbine will provide the reliability andperformance improvements specified inthe contract. Likewise, with extendedinspection intervals and increased relia-bility, maintenance costs will also bereduced.

Since being put back into operation

the capacity of Unit 2 has increased, saysHerbert D. Woodeshick, special assistantto the president of Susquehanna. A post-test to accurately determine the perfor-mance of the unit is scheduled for theend of May 2003. The results of this testwill be compared with a pre-test con-ducted prior to the turbine upgrade tosatisfy the contract. p  

STEAM TURBINES

 Installing upgraded LP rotor. Photo courtesy of PortlandGeneral Electric.