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BOILER CONDITION ASSESSMENT AND RETROFIT PROGRAM AT

SIERRA PACIFIC POWER'S TRACY STATION, UNIT NO. 2by

Brent HigginbothamSierra Pacific Power Co.

andDaniel P. Hurley

andJames P. King

Riley Stoker CorporationABSTRACT

Ensuring that a twenty-three year old gas/oil f ired steam generator at Tracy Station, Unit No. 2wouldbe available to provide another twenty-years of service began with a complete boiler conditionassessmentprogram followed by a major alteration in the convection pass, penthouse and steelstructure.

This paper discusses the planning and results of the program used on a 700,000 lbs/hr, Riley steamgenerator built in 1963. The successful completion of the project, together with its cost, schedule, andoperating results are discussed herein.

The inspection, non-destructive testing and analytical tasks provided input for a condition assessmentreport on the boiler, and the feasibility of additional cyclic operation. Performance test data wascollected, evaluated and design parameters set which included a redesign and reconstruction of theconvection pass heating surfaces and their support systems.

INTRODUCTION AND BACKGROUND

Tracy Unit No. 2 is an 83 MW, gas and/oroil fired, non-reheat unit which has been incommercial operation since October 1, 1965.The Westinghouse turbine-generator is de-signed for 950F, 1313 psig, and 1-1/2" HgA.

The boiler is a Riley balanced draft, naturalcirculation unit, 700,000 lb.hr (750,000 lb/hr - 4hr. peak), capacity and 1400 psig, 950F designoutlet steam conditions. The boiler originallyhad tangent tube and tile construction with awelded casing. A side view of the original unit isshown in Figure 1.

As more efficient generation became avail-able and system demand changed, Tracy 2

became a peaking unit. The unit had accumu-lated almost 3000 start-up cycles. Since originalstart-up, the compact boiler design had causedsteam temperature control problems, especiallywhen gas fuel was fired. During its operatinglife, cyclic operation, high metal

temperatures, and high spray flows had takentheir toll.

Nevada's mining boom and populationgrowth had generated enough demand that it

was necessary to make the Tracy 2 unit adependable peaking energy source. The goals ofthe project were to provide the system with aunit capable of 3000 additional start-up cycles,reduce maintenance costs, and replace obsoletecontrol components. This resulted in the TracyUnit 2 overhaul consisting of:

Boiler restorationTurbine-generator overhaulBoiler feed and circulating water pumpoverhaulPump and fan motor maintenance

Control system replacementBurner management system replacement

Key milestones of the project schedule andother concurrent tasks are listed in Table 1.Table 2 summarizes project costs.

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TABLE 1 - PROJECT SCHEDULE

1988 PRE-OUTAGE WORK

2/88 Boiler Inspection and Assessment4/88 Boiler Performance Testing6/88 Budget and Schedule Preparation

Economic Justification of ProjectBoiler Bid Package. Released

8/88 Bids Due10/88 Award Boiler Work11/88 Architect Engineer Selection12/88 EPA Determination

1989 OUTAGE WORK

Milestones Concurrent Tasks

1/23-2/24 Asbestos Removal2/27-3/30 Boiler Demolition Burner Management System

Start of BoilerInstallation

Turbine Generator Overhaul4/1

6/15

ErectionBoiler Hydro

Pump OverhaulMotor Maintenance

7/5 Boilout Burner Register Linkage and

7/15 Steam BlowDrivesControl System Replacement

7/24 Main Steam Line Air Heater Baskets and Seals

7/25 Re-Hydro Economizer Minimum Flow 7/29 Turbine Roll Attemperator Piping

Heat TracingRe-insulation

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TABLE 2 - PROJECT COST

Initial WorkPerformance Testing

Condition AssessmentAsbestos Removal/ReinsulationTurbine Overhaul & Balance of Plant WorkBoiler Demolition & RestorationAir Preheater RepairsBurner RepairsControl System ImprovementsBurner Management SystemTechnical SupportPlant & Company Labor

PERCENT OFTOTAL COST

1 $ 120,0001 50,000

2 200,0009 800,0007 600,000

58 5,300,0002 200,0002 200,0006 500,0004 350,0005 430,0003 250.000

100 $9,000,000

INSPECTION ANDASSESSMENT PROGRAM

The first step in the boiler restorationprogram was to perform a thorough inspectionand assessment of the condition of the boiler.The on-site work activities consisted of visualand fiberoptic inspections of pressure parts,ultrasonic thickness testing of furnace water-wall, superheater and economizer tubes, mag-netic particle testing of selected header tube

nipple welds, and replication on higher stressregions of high temperature headers. Severaltube samples were removed for metallurgicallaboratory evaluation.

The second phase of the inspection andassessment program comprised the in-houseanalytical and reporting tasks. The analyticalwork consisted of a review of maintenance andoperating records, and flexibility analyses ofselected tube connector and seal designs,component supports, and piping systems. Alsothermal transient, fatigue and creep analysis of

critical pressure components. The results of theanalytical tasks were factored into a conditionassessment together with the inspectionfindings and metallurgical results. This waspresented in the form of life expended to date.Riley prepared a final report, which provided

accumulated damage values for critical compo-nents and presented recommendations forrepair, replacement, modification and monitor-ing items. Budgetary pricing and schedules forthe implementation of such items were includ-ed in the final report.1. Assessment Program Conclusions:

The final report concluded that the Tracy

Unit No.2 boiler was in reasonably goodcondition considering the twenty-two years ofessentially cyclic operation. Based on the visualinspection, there were a number of problemareas where the physical condition haddeteriorated. These included extensive damagedue to overheating of casing and insulation onthe side and rear convection walls, sagging ofthe high temperature super-heater elements,bowing of the economizer elements, internalcracking in the economizer inlet header andcracking of the airheater support members. The

metallurgical analysis confirmed that the hightemperature super-heater tube sample wascompletely spheroidized indicating long termexposure to elevated temperatures.

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The analysis of the economizer tube samplerevealed the presence of severe oxygen pitting.The thermal transient and fatigue analyses forcomponents of the economizer inlet headerresulted in a fatigue cumulative damage factorof 2.6 for the header/tube junction. Since a

fatigue value of 1.0 conservatively indicates theend of useful life, this component was conclud-ed to be in distress, as confirmed by the inter-nal bore hole and ligament cracking, found viathe fiberoptic inspections.2. Assessment Program Recommendations

The program recommendations based onthe inspection and assessment findings, and inline with a postulated additional 20 years ofoperation, involved removal and replacementof the economizer and superheater with com-ponents redesigned to meet the requirementsof cyclic duty. These recommendations aresummarized below:

Replace the convection pass tube and tilewalls with membrane wall construction toeliminate the excessive maintenance in thisarea.

Replace the high temperature superheaterand upgrade to stringer tube supports.Replace the secondary superheater inletand outlet headers. Upgrade the outletheader to SA335-P22 material.

Replace the primary superheater and up-grade to stringer tube supports. Upgradethe outlet header to SA335-P11 material.Replace the crossover piping and attemp-erator.

Replace the economizer and inlet header.Upgrade the economizer to a stringer tubesupport for the primary and secondarysuperheaters. Locate the inlet headeroutside the convection pass to reducethermal shocking during hot restarts.

Design an economizer trickle feed systemfor supplying flow to the economizer duringstartup to prevent steaming and reducethermal shocking.

Design a turbine bypass system to provideadequate steam flow through the super-heater during startup.

Schedule an inspection of the airheaterduring the next annual outage.

Remove all insulation from expansion jointsfor inspection and repair of corrosion andcracks. Replace expansion joints within fiveyears.

Replace the existing burner register assem-blies with fixed vane, shrouded registers.

Most of these recommendations wereincluded in the specifications proposed for theboiler restoration work packages.PERFORMANCE TESTING PROGRAM

During the inspection and assessmentprogram, a review of plant records and Rileyservice files indicated that the boiler hadoperated with higher than design flue gastemperatures, and that attemperation require-ments had far exceeded the original predic-tions. Riley recommended that a boiler per-formance testing program be accomplished.This program was conducted in April of 1988.The tests were done for oil and gas firing, atMCR and reduced load cases, and were under-taken primarily to determine the furnace exit

gas temperature (FEGT) and the performanceof the convection pass heating surfaces.

The results of the performance testing pro-gram indicated the following: At Ioads above 60% MCR, the FEGT was

higher than originally predicted for both oiland gas firing. At loads below this capacity,the FEGT was lower than predicted on gasfiring.

The steam temperatures leaving the primarysuperheater were lower than predicted for

both fuels. Even so, the superheaterattemperator requirements were consider-ably greater than predicted. This indicatedthat the primary superheater was not per-forming as well as predicted, and the sec-

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ondary superheater was exceeding predictedperformance.

The water temperatures leaving the econo-mizer were below saturation, but greaterthan expected for both fuels. Conversely,the recorded flue gas temperatures leavingthe economizer were less than originallyexpected.

The results of the boiler testing programconfirmed the findings of the condition assess-ment program. These results were included inthe boiler restoration bid package, forming thebasis for the redesign of the economizer andsuperheater.

BOILER RESTORATION

After completion of the condition assess-ment, a project budget and schedule wereprepared. Project justification required com-parison with alternative sources of power.

Replacing the superheater and economizer toutilize the remaining life of the unit was, by far,the most attractive alternative.

A specification for the removal, redesignand replacement of the economizer and super-heater was prepared by SPPCo and Stone &Webster Engineering Corporation (SWEC). Tomake use of the remaining life of the plant, theboiler had to be restored.

Riley Stoker Corporation was awarded boththe design and erection contracts on acompetitive bid basis. The table below pre-sents the design changes incorporated in thenew economizer and superheater. Figure 2shows the changes in the design of the re-stored unit.

TABLE 3 - SUMM ARY OF DESIGN CHANGES

Original Unit Restored Unit

Tube Supports End Supports Stringer TubeSupport Lugs Inconel, 304SS 50Ni/50Cr, InconelWall Construction Tube and Tile Welded MembraneConvection Pass Sidewalls-Economizer Side & Rear wallsWalls Rear Wall Superheater SuperheaterTube materials Code Allowable Upgraded ThroughoutEconomizer Bare Tube, Staggered H-Fin, in lineEconomizer inlet In Gas Stream Outside Gas StreamAttemperation Single Nozzle Dual Nozzles in Each

of Two Crossover PipesHeader Outlet Legs Rigid Flexible

1. DESIGN AND FABRICATION

Project schedule dictated that Riley com-plete its contract within two hundred eightycalendar days of which one hundred twenty-nine days were used to design and fabricate

material. Long lead items such as tubing andheader materials were ordered within the firstfifteen days to ensure fabricated componentswould be delivered on site by April, 1989.

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Three hundred fifty thousand dollars wasexpended completing engineering and draftingof structural steel, boiler design, stress analysisand boiler controls.a. Structural Steel Design

Riley's structural redesign added forty tonsof steel to compensate for the added weight ofsootblowers, welded wall construction, refrac-tory , heavier buckstays and seismic guidesneeded to meet specified NFPA85G coderequirements, and changed load points.

Long retract sootblowers replaced theexisting IK sootblowers mounted on the rearwall. New sootblowers were installed on oneside of the unit that required platforms beextended and columns reinforced to supportthe added load.

Existing tangent tube design was changed toa welded wall configuration with refractorypoured seal boxes used at element penetra-tions. Seal boxes were completely welded to thetubes with flexible tube connections providedon both sides. A poured refractory wallreplaced an existing tile baffle at the bottom ofthe screen between furnace and convectivesurfaces to eliminate side lanes and gas shortcircuiting. The added weight was supporteddown through the columns to the foundation.

Implosion protection was factored into thenew design which changed the physical size ofthe buckstays. Larger seismic guides wereadded to the new alteration.

Structural design and detailing wascompleted at Riley then bid to steel fabricators.Fabricated members were sand-blasted andspray painted to save time during erection.New design components matched existingdesign.b. Convection Pass Alterations

Performance tests had confirmed thatFEGT exceeded predicted levels in the con-vection pass of the boiler. Riley redesigned theconvection pass heating surfaces to meet theworst service conditions. A top supported

stringer tube system was used to suspendsuperheater and economizer elements ratherthan use conventional wall supports. Also,steam cooled lug materials, supporting indivi-dual elements, were upgraded to assure ex-tended support life.

The stringer support system required thatsteam flow be maintained. This wasaccomplished by a minimum flow system whichroutes a portion of the feedwater from theeconomizer outlet to the condenser. Controlinterlocks prevent reverse flow from the steamdrum which could drain the drum andcontaminate the condenser hot well.

Also, a small turbine bypass system wasadded to the unit. The turbine bypass enablesSPPCo to avoid multiple starts on the boiler

feed pumps, better match steam and turbinetemperatures on warm and hot re-starts,provide flow through the superheater andrestrict drum swell during start-up.

The amount of superheater and economizersurface required to meet the desired outletconditions was determined by analysis of thefurnace exit gas temperatures. Temperatureprofiles were calculated for both natural gasand residual fuel oil over the whole steam loadrange. SPPCo specified seven possibleoperating ranges where actual test data was

collected.

A heat balance across the convectionsurfaces was modeled by Riley. Boiler effi-ciency was calculated and fuel and gas weightgenerated. After establishing an FEGT for eachof the tests, a computer model of the furnacewas developed in order to determine theexpected FEGT at any operating condition.The amount of primary and secondarysuperheater heating surface required toachieve the desired outlet conditions wasdetermined through the use of heat transfercomputer programs. SPPCo required that theunit be capable of achieving full temperature of950F at 350,000 pph when firing natural gas.It was this condition that determined the exactamount of surface required. Desuper-

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heater spray is used to control outlet tempera-ture at higher loads.c. Desuperh eater Spray

The steam temperature control systemprovided is a two-path parallel flow system that

incorporates a "crossover" of the superheatedsteam to promote better side-to-side distribu-tion. The two-path system has the advantageof decreasing steam side pressure drop whileincreasing steam piping length to ensureadequate spray water evaporation before thesteam enters the secondary superheater.Increased spray water turndown capability ineach of the flow paths was achieved by incor-porating two spray nozzles. The first spraynozzle is a low capacity, variable orifice nozzlethat provides excellent spray water atomization

at low spray flows. In series with the firstnozzle is a high capacity, fixed orifice spraynozzle with a separate spray water controlvalve. The two nozzle arrangement providessteam temperature control over the load rangewith turndown capabilities of 20 to 30:1 orbetter, depending on the control schemeincorporated into the system.d. Economizer Selection

Once the proper heating surfaces had beenestablished for the primary and secondarysuperheaters, an economizer design was select-ed to decrease the temperature entering theairheater to as low as possible for improvedunit efficiency, while maintaining an outletwater temperature with sufficient margin belowsaturation temperature to ensure economizersteaming would not occur at any operatingcondition. Due to space limitations it wasdecided to use an extended surface (finned)economizer.e. Material Selections

After establishing the required heating

surface to achieve the desired outletconditions, the proper wall thickness andmaterial grades were established for eachpressure part component.

Heat transfer calculations were performedat various loads with different excess air toestablish the optimum operating condition formaterial selection. As a further check, theFEGT for the maximum continuous rating of700,000 lb/hr., when firing natural gas, was

increased by 200F and heat transfer calcula-tions were performed under this condition.Appropriate margins were added to the steamside temperature to account for tube bundleflow unbalance.f. Riley Fabrication

Except for the purchased economizersurface, all components were fabricated atRiley's facility in Erie, PA. Frequent shopvisits were made by SPPCo personnel andtheir representatives to monitor progress and

confirm that fabrication conformed with de-sign.

Modular components were shop assembledto reduce the time required to complete erec-tion. Riley sequenced fabricated material toassist field construction. A roof module wasassembled with economizer outlet headers andstringer tubes attached. Membranes werewelded and roof seals completed in the shop.This module comprised the entire width of theunit. When the module was hung, the roofabove the convection pass was complete.

Furnace roof tubes were fabricated to closethe roof above the superheater outlet andfurnace screen tube penetrations above thefurnace area.2. CONSTRUCTION

Riley's scope of work included demolitionand re-construction of the convection passheating surfaces during five months of the sixmonth outage. Construction work represented$2.4 million of the $5.3 million dollars expend-ed for boiler restoration. Riley averaged

twenty-eight men per shift for one hundredfive available days, each shift working ten hourdays, five days each week, which assuredcompleting the work ahead of schedule.

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TABLE 4 - CONSTRUCTION STATISTICS

ManhoursPercent of

Total1.

Demolition 9,000 15

2. Structural Steel 4,200 73. Hanging Pressure Parts 21,000 354. Install Sootblowers 1,500 2.55. Welding Pressure Parts 13,200 226. Testing Support 1,100 27. Work Extras 10,000 16.5Total Riley Construction Hours 60,000 100

Riley sub-contracted insulation, lagging,painting, and electrical work.

Riley established an erection scheme thatwas rigidly adhered to throughout the workperiod. Few schedule changes were necessary.Good dialogue between the Owner's imple-mentors, plant personnel and Riley's contractteam resolved issues before costly mistakeswere made.

Sierra Pacific planned other outage workaround Riley's boiler restoration schedule.This outage included a turbine overhaul,asbestos abatement work, and a burner man-

agement and boiler controls replacement. Acritical path schedule was utilized that identi-fied milestone dates and all interface points.

Sierra Pacific set up a construction manage-ment and plant maintenance staff supported byStone and Webster field personnel that moni-tored boiler, turbine, burner, air heater, con-trols and balance of plant work.

Riley completed boiler demolition early.This work involved removal of all superheaterand economizer surface in the convection pass

between the economizer feed pipe and thehigh temperature superheater outlet pipe.Structural steel, suspension level steel, head-ers, partial roof panels and side and rear wallpanels were removed. The Figure 3 photo

graph shows the condition of the secondarysuperheater just prior to its removal.

Sootblowing equipment was removed fromthe rear wall and later replaced with longretract blowers on the sidewall. All existingheader drain piping and burner register driveswere removed.

The existing structure had not been de-signed to support the added weight of newboiler components, so the structure was modi-fied from the suspension level down to thefoundation. Load changes and specifiedstructural load calculations, code up-grades,

and increased component weights controlledthe early work sequencing. Structural rein-forcement had to be completed before moreweight was added to the unit.

After the structural reinforcement work wascompleted new beams and hanger rods wereinstalled at the suspension level. Then the newboiler components were installed beginningwith all headers above the new furnace roof,then roof panels and loose tubes, side wall andrear wall. Access was limited to two pathsfrom the rear of the unit, so sequencing ofmaterial flow was critical throughout theconstruction phase.

Alignment of the walls and roof tubes hadto be finished and membrane welding com-pleted before proceeding. Roof, side walls and

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furnace screen work was sequenced before thehigh and low temperature superheater andeconomizer elements could be hung out in theconvection pass cavity. Figure 4 shows theinstallation of superheater and economizer ele-ments in the convection pass cavity. A window

was cut in the new rear wall through which allremaining materials were fed.

New primary and secondary superheaterheaders, crossover piping and economizer inletand outlet headers were installed outside of thegas stream and then connected to new surfaceelements.

Riley completed over three thousand fieldwelds in less than one month. Riley performedseparate hydrostatic tests on old and newsystems to avoid subjecting the existing steam

drum and furnace to the 150% of maximumallowable working pressure hydro test requiredfor the new components.

Isolation of the old and new componentswas achieved by using hydro plugs. Weldstying the old and new components were sub-jected to 100% examination by radiographyfollowed by a hydro test at operating pressure.

Work on new spray piping, economizerminimum flow piping and by-pass piping sys-tems was implemented. Existing gas and oilburners were rebuilt and new airheater basketsand seals installed.

Riley completed construction and unittesting earlier than scheduled. Unit start up andtesting began in late June of 1988 and extendedthrough July. The new boiler and existingfurnace went through a chemical boilout andsteam blows in order to safeguard theoverhauled turbine.OPERATION SINCE MODIFICATIONS

After thirteen months in service, the unit isfulfilling its role as a reliable peaking unit forSPPCo. To date the unit has been through 168cycles, operated 2906 hours, with an averagecapacity factor of 15%. Unit load has variedfrom off line to minimum load to maxi-mumload as required through economic dispatch.

The operation of Tracy Unit 2 since theoverhaul has been very satisfactory.

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