Advanced Technologies of Preventive Maintenance For

Hitachi Review Vol. 51 (2002), No. 5 137

Advanced Technologies of Preventive Maintenance forThermal Power Plants

OVERVIEW: Although thermal power generating facilities are aging, flexibleoperation in response to changes in the demand for electrical power isrequired. To extend the life and manage the maintenance of those facilities,advanced preventive maintenance technology is important. For that reason,Hitachi has been grappling with nondestructive inspection, life assessmenttechnology, rationalization technology for scheduled inspections of theboilers, steam turbines, gas turbines and other machinery used in thermalpower generation to strengthen preventive maintenance and increase theefficiency of maintenance. Recently, we have been developing products andservices for advanced support for maintenance and preservation throughthe use of IT (information technology) and network technology, going beyondwhat has previously been available.

Kiyoshi Shimomura, Dr. Eng.

Hideaki Ishitoku

Shigeo Sakurai, Dr. Eng.

Fumiyuki Hirose

INTRODUCTIONIN recent years, startup and shutdown operations havebecome frequent in aging thermal power generationfacilities, and preventive maintenance technology hassteadily been increasing in importance from theviewpoints of extending the scheduled inspectioninterval and strengthening the independent managementof facilities. Furthermore, concerning preservation ofthe global environment as well, there is also a strongdemand for the reduction of CO2 emissions throughimproved generation efficiency and fuel conversion

in existing facilities.Under such circumstances, Hitachi is going beyond

the application of new technology that has beendeveloped for new power plants to old facilities, andhas taken up the challenge of developing our ownpreventive maintenance technology.

Together with these advanced technologies andproducts, Hitachi is making use of IT, which hasdeveloped rapidly in recent years, to provide globalservices that contribute to life-cycle cost minimizationfor power generation facilities (see Fig. 1).

Fig. 1—PreventiveMaintenanceSolutions for

Thermal PowerPlants Provided by

Hitachi.Hitachi is making

full use of IT toprovide the newest

preventivemaintenance systems

and services forthermal power

plants.

Administration

Central load dispatching office

Head office

Thermal power plants

Power demand adjustment

Power companies

Power generation

Thermal power plants

Power generation

Plant monitoring service

Remote Monitoring CenterAnswer Center

HITACHI Boiler

Turbine

Generator

Controller

Ordinary households

CALS: continuous acquisition and life-cycle support

Large-demand customers

CALS technology answering service

Information to customers (subscribers) via the Internet

Internet

Advanced Technologies of Preventive Maintenance for Thermal Power Plants 138

Here, we describe the latest technology for thepreventive maintenance of thermal power plants.

APPLICATION OF IT TO PREVENTIVEMAINTENANCE OF THERMAL POWERPLANTS

Hitachi is proceeding to add intelligence in a widerange of fields by making use of IT and the proprietaryknowledge we have accumulated in various industrialareas with the objective of being the customers’ “BestSolutions Partner” by offering new solutions.

In the field of preventive maintenance for thermalpower plants, too, we have taken up the challenge ofusing IT and network technology to provide servicesfor the support of advanced maintenance andpreservation that go beyond what has been availableto the customer in the past.

With the increasingly severe demands on theoperation of aging thermal power plants, typified byextension of the period for scheduled inspections andDSS (daily start-up and shut-down) operations, thereis increasing need to know the quantitative state of thefacilities, such as by monitoring operation at times oftransition and knowing the deterioration state ofequipment, in addition to the regular monitoring ofoperation. There is also a demand for the provision ofperformance evaluation technology for life-cycle costminimization, as well as the provision of operationmethods that use such technology to optimizeefficiency and the provision of operation datamanagement services. To meet that demand, Hitachiis developing services which make use of IT, such asplant monitoring services and engineering services.

As example of an engineering service, a means of

providing information over the Internet is illustratedin Fig. 2. In that service, subscribing customers canget speedy answers to technical questions via theInternet by directly accessing the Answer Center WebSite set up by the Thermal Power PreventiveMaintenance Department, a process that previouslyrequired waiting for written responses. It is alsopossible to access information on the latest operationand maintenance technology and improvements in realtime. This service can be provided to overseascustomers as well as to domestic power companies.

PREVENTIVE MAINTENANCETECHNOLOGY FOR THERMAL POWERPLANTS

In an environment of deregulation, there has beenlittle progress in the construction of new thermal powergeneration. Furthermore, load conditions have recentlybecome severe, including operation according tovarying load (middle load operation), etc., and thereis an accelerating trend of material damage. To copewith that problem, Hitachi is promoting thedevelopment and application of preventive maintenancetechnology for boilers, steam turbines, gas turbines,and other core technologies of thermal power plants,as described in the following sections.

Preventive Maintenance Technology for BoilersApplication of the latest development technology

The conventional techniques for nondestructiveinspection of boiler materials and parts include PT(liquid penetrant testing), MT (magnetic particletesting), and UT (ultrasonic testing), among others.These types of inspection, however, have problemssuch as:(1) difficulty of inspecting narrow gapped areas,(2) requiring a large amount of associated work, and(3) difficulty of lifetime diagnosis or can predictquantitative damage progress.

To cope with these problems, Hitachi has beendeveloping various kinds of scheduled inspectionrationalization technology to achieve lower-cost,timely inspection by reducing the preparation workand to achieve fast, highly accurate inspection byemploying advanced inspection equipment withoutdirect contact. The main diagnostic points anddiagnosis technology are shown in Fig. 3. Theserationalized technologies are already being applied inactual plants.

Of these technologies, ELFOSS UT (electronicfocus sector scan ultrasonic testing), which can

Fig. 2—Mechanism for Providing Technical Information via theInternet.Technical questions from customers can be answered quicklyand the latest information can be provided over the Internet.

Customer (contract holder) Hitachi Group Answer Center

Internet

• Rapid reply to technical questions• Provision of the latest operations and

maintenance technology• Provision of information on improvements


evaluate internal defects in the welds of main pipes, isdescribed below:

ELFOSS UT can evaluate defects contained inwelds and in narrow nozzle stubs by electronicfocusing and sector scanning techniques. A majorfeature of this device is that the scanning range of theUT probe is smaller than that used in the conventionalUT method, so the defect size and location can bedetermined accurately.

The results of ELFOSS UT inspection of a plate(100-mm thick) butt weld that contains an artificialdefect are shown in Fig. 4. The defect detected by thenondestructive method matches well with the defectas confirmed by cross-section examination. With suchan excellent defect detection capability, this equipmentis being applied to precise inspection of importantareas, such as the welds of main pipes.

Scheduled inspection rationalization technologyThe various types of nondestructive inspection

equipment shown in Fig. 3 are all capable ofquantitative evaluation of defects, and can also becalled rationalized technologies with respect toshortening inspection time for schedule inspections.In addition to such higher performance of theinspection equipment itself, rationalization technologysuch as for shortening the preparation time forinspection is also desired.

For example, in the furnace of coal-fired boilers,fused ash adheres to places such as the bends in theheat exchange pipes that are suspended from the topof the furnace. Such ash may grow to form hardclinkers. Because it is very dangerous if those clinkersfall inside the furnace, they should be removed before

installing the scaffolding and performing the inspectionand repair work at the time of scheduled inspections.The removal of the clinkers in this way requires muchtime and labor.

For that reason, we turned attention to WJ (waterjet) technology, and developed a special nozzle thatproduces several times the force of the conventionalnozzle. Using this nozzle reduces the time requiredfor clinker removal significantly, and the time fromthe shutdown of a plant to the beginning of inspectionand repair can be greatly reduced.

Preventive Maintenance Technology for SteamTurbinesLifetime diagnosis of steam turbines

For high-temperature components of aging steamturbines, accurate preventive maintenance and

Fig. 3—MainDiagnostic Pointsand DiagnosisTechnology.Hitachi hasdeveloped inspectionequipment that iscapable of non-destructive,quantitativeevaluation of all ofthe pressurecontainment parts ofthe boiler.

Fig. 4—ELFOSS UT Performance.The results of inspecting a sample butt weld that contains aninternal defect show that the defect size and location can bedetected accurately.

Main pipes

Drum CoolerHeader

Heat exchange tubes

Water-wall tubes

Burner

Fuel pipe

EMAT: electromagnetic acoustic transducer*: Preparation for inspection

Main pipes, headers and drums

• ELFOSS UT• Non-contact electromagnetic acoustic crack detector

• Cooler inspection equipment

• Internal UT tube inspection equipment• New internal UT tube inspection equipment• Support equipment for visual inspection of deep coil levels

• Ash erosion inspection equipment• Horizontal heat transfer tube inspection robot• Acoustic ash removal equipment

Attemperator Horizontal heat exchange tubes

Suspended heat exchange tubesWater-wall tubes

Insulated pipes

• Equipment for inspection of insulation from above

• Internal tube UT inspection equipment• Steam oxidation scale thickness detector• Local erosion thickness detector• Sling tube inspection equipment• EMAT crack detector• Clinker removal by water jet method*

• Water wall corrosion fatigue inspection equipment

• Corrosion fatigue detector• Elephant skin depth detector• Local erosion thickness detector• Water wall backside UT equipment

• Clinker removal by water jet method*

Measurement of an internal defect in a butt welded plate (100-mm thick)

Confirmation of detection accuracy by comparison with cross-section examination

Crack

Macroscopic photo of cross section

100

mm

Result of cross-section examination

UT result


extension of machine life is becoming established.On the other hand, for low-temperature

components, of which low-pressure turbines are atypical example, damage related to corrosion has beenappearing in recent years. In most of those cases,corrosion fatigue cracking or SCC (stress corrosioncracking) is seen in the severely corrosive phasetransition zone environment. To deal with theseproblems, we are establishing highly accurate lifetimeevaluation technology derived from an understandingof actual operation in service. That technology is basedon analysis of the vibratory response in grouped bladesin the low pressure stage and accumulating data onthe occurrence and growth of corrosion pitting in theactual environment. As a result, we are trying tomaintain the reliability of low-temperature parts in acorrosive environment by recommending the overhaulof low-pressure turbines that have been in service for20 years or more by removal of the bucket.

Development of scheduled inspection technology forimproving in efficiency

With the continuing relaxation of regulation, thereis an even stronger demand for improvement in theoperating efficiency of facilities as well as the loweringof maintenance costs. For those reasons, we areendeavoring to raise the efficiency of work involvedin the scheduled inspection of turbines and to developvarious types of technology for increasing efficiencyand conserving energy. The technology forrationalizing scheduled inspection work not onlyimproves that work directly, but also includestechnologies, such as facility diagnosis technology, thatminimizes that work and asset managementtechnology that in turn increases the efficiency ofmanaging the scheduled inspection data.(1) Rationalization technology for scheduledinspections

To shorten the scheduled inspection process, wehave developed tapered sleeve type bolting equipmentfor the bucket coupling and oil flushing reductionequipment. The tapered sleeve type bolting equipmentallows smooth bolt extraction, so the bolt looseningtime can be reduced to half a day per coupling.Previously, five to seven days were required to flushthe oil for cleaning the bearings and bearing box aftercompleting the assembly of the turbine and generator.The oil flushing reduction equipment, however,achieved results that fully satisfy the specified bearingoil cleanness requirement in only three days, as wellas reducing the test operation time.

(2) Facility diagnosis technologyFor facility diagnosis, we developed leak diagnosis

technology that makes use of AE (acoustic emission).Up to now, AE diagnosis has been applied toabnormality detection in rotating parts such as turbinesand pumps. Now we have developed technology forapplying this method to steam leaks in stop valves aswell. An application example is shown in Fig. 5.(3) Asset management technology

As an asset management technology, we havedeveloped a scheduled inspection record and historymanagement application system. Rapid and paperlessrecording of data from the time of measurement todata management is made possible by using a networkto connect the inspection site and the personsresponsible for data approval and management. Thisinspection recording application is linked to a historymanagement system, and serves to provide data forthe history management of assets. In this way, thevarious types of information from the inspections isdatabased for each asset so that the deterioration trendof each asset can be known. Furthermore, data miningis done easily by time series of the maintenance datafor each asset or by phenomenon. A sample window

Fig. 5—Mechanism for Detecting Steam Leaks in Stop Valvesand AE Waveform for a Leak.An example application of AE to stop valves and other staticparts.

Corrosion

AE emission AE sensorAE analyzer

Steam

AE waveform

No

leak

Am

plitu

de

Vol

tage

Vol

tage

Am

plitu

de

Lea

k

Result of frequency analysis

Time Frequency

Time Frequency


from the turbine history management application isshown in Fig. 6.

Introducing the various types of rationalizationequipment that have been developed can shorten thestandard process for a 600-MW turbine to about fivedays and a 10 to 15% saving in energy can be expected.

Preventive Maintenance Technology for GasTurbines

The load conditions are very severe for gas turbines,which use high-temperature combustion gases as theworking fluid. For that reason, differently from steamturbines, the hot gas path components that are placedin the path of the combustion gases have a relativelyshort operating time and are repaired repeatedly duringthe course of use.

Maintenance costs for repair, reconditioning andpart updating are a large proportion of the cost ofgenerating electricity, so the advancement ofpreventive maintenance technology is important interms of economy as well as reliability.

Lifetime management and repair of hot gas pathcomponents

The life of hot gas path components is shortenedby creep, abrasion, oxidation and other such damagethat is related to operating time and by damage thatoccurs as the result of repeated startups and shutdowns,such as low-cycle fatigue.

Accordingly, lifetime management employs theequivalent operating time method, which takes intoaccount the effects of the number of startups,

shutdowns and sudden changes in load, etc. as well asthe actual operating time. When the equivalentoperating time reaches the specified lifetime, thatcomponent is considered to have reached the end ofits operating life.

Concerning the hot gas path components, if theinspection result exceeds the judgement criteria, thecomponent is repaired. See Table 1 for a descriptionof the repairs.

Of those, the latest repair technology that iscurrently being used in actual plants is described below.

Gas turbine bucket recoating repairGas turbine buckets are subjected to severe

Fig. 6—Screen Images from theTurbine History ManagementApplication.Detailed information on each assetis easily grasped.

TABLE 1. Hot Gas Path Component RepairsExamples of the latest gas turbine repair technologies aredescribed in this table.

Component Description of Repair

Static blade of turbine

Turbine bucket

Combustion liner

Transition piece

• Welding repair of cracks (build-up) Brazing and diffusion bonding

• Creep deformation repair• Recoating

• Tip welding build-up• Recoating

• Welding repair of cracked parts• Replacement of spring seals, collars, etc.• Recoating

• Welding repair of cracks and abrasion• Creep deformation repair• Recoating

History menu window

Corrosion was seen in the 1993 scheduled inspection; continuous monitoring is being done.

Details (case record)


conditions of centrifugal force and high thermal stressload in the high-temperature and high-pressurecombustion gases. For that reason, Ni-based super-alloys are used and an oxidation resistance coating isapplied to the bucket surface to protect the basematerial. With long-term use of the buckets, the coatingsuffers damage by deterioration, so it is necessary tostrip the old coating and recoat the buckets before thebase material is damaged. Measures are also takenagainst erosion of the bucket tips by high-temperatureoxidation. Hot tearing from welding heat flux usuallyoccurs in Ni-based super-alloys with high Ti or Alcontent, so it is a difficult material to weld. In recentyears, however, low-current welding methods havebeen developed, so repair by build-up welding hasbecome practical for the low-stress bucket tips.

Gas turbine nozzle blade diffusion brazing repairGas turbine nozzle blades suffer thermal fatigue

cracking due the thermal stress caused by start-up andshut-down. Those cracks must be repaired by weldingduring the course of nozzle blade use. There is atendency for multiple thermal stress cracks to appearover a wide area of the nozzle blades, so repairing themtakes a great deal of time and effort. Furthermore,deformation of the nozzle blades occurs in proportionto the amount of heat applied during the welding.Diffusion brazing repair avoids that problem by usinga brazing filler metal that has about the samecomposition as the base metal, but with a low meltingpoint metal added. This method involves cleaning thesurfaces of the cracks, flowing the filler metal into thecracks and then diffusing the filler metal into the blademetal with a heat treatment. This method reduces laborfor the repair and prevents deformation of the blades.By using both welding repair and diffusion brazingrepair, we are reducing labor and extending the life ofnozzle blades.

CONCLUSIONSWe have described the development situation for

IT application technology and the latest technologyrelevant to boilers, steam turbines, gas turbines in thefield of preventive maintenance for thermal powerplants, as well as how Hitachi is solving problemsrelated to this technology.

Hitachi will continue to propose IT services,

ABOUT THE AUTHORS

Kiyoshi ShimomuraJoined Hitachi, Ltd. in 1980, and now works at theThermal and Hydroelectric Power Technology

Headquarters, the Thermal and Hydroelectric Power

Business Department of the Power and IndustrialSystems. He is currently working on the promotion of

preventive maintenance for thermal power plants. Dr.

Shimomura is a member of the Japan Society ofMechanical Engineers (JSME) and Gas Turbine

Society of Japan, and can be reached by e-mail at

[email protected].

Hideaki IshitokuJoined Babcock-Hitachi K.K. in 1981, and now worksat the Project Integration Department of the Thermal

Power Technology Headquarters at Kure Business

Office. He is currently working on projects related topreventive maintenance. Mr. Ishitoku is a member of

the Thermal and Nuclear Power Engineering Society,

and can be reached by e-mail [email protected].

Shigeo SakuraiJoined Hitachi, Ltd. in 1977, and now works at the

Hitachi Production Headquarters, the Thermal and

Hydroelectric Power Business Department of thePower and Industrial Systems. He is currently

engaged in the development of preventive

maintenance technology for steam turbines and gasturbines. Dr. Sakurai is a member of the JSME, the

Society of Materials Science, Japan, and the Japan

Institute of Metals, and can be reached by e-mail [email protected].

Fumiyuki HiroseJoined Hitachi, Ltd. in 1973, and now works at the

Hitachi Production Headquarters, the Thermal and

Hydroelectric Power Business Department of thePower and Industrial Systems. He is currently

working on preventive maintenance for thermal

power plants. Mr. Hirose can be reached by e-mail [email protected].

including Internet application technology, and coretechnology services.

REFERENCES(1) S. Sakurai et al., “Gas Turbine Preventive Maintenance

Technology for Reliable Maintenance and Extended Life,”Hitachi Hyoron 79, pp. 271-274 (Mar. 1997) in Japanese.

(2) “Preventive Maintenance and Lifetime Diagnosis Technologyfor Thermal Power Plants/Preventive Maintenance at Hitachi,”Thermal and Nuclear Power Generation, No. 530, Vol. 51, pp.1583-1587, 1599-1602 (2000) in Japanese.

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Advanced Technologies of Preventive Maintenance For