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Dynamic PressureMonitoring in Gas TurbinesAn Exploration into Gas Turbine Working Principles,Monitoring Techniques, and Cost Savings

Summary

The need to meet tougher emissions targets and to improve gasturbine reliability and performance means that engines undergomany improvement programs and run with ever-leaner fuel / airmixtures. These changes can lead to instabilities and excessivepressure pulsations that can result in mechanical failure.Piezoelectric sensor technology is available to measure thesedynamic pressures directly. Moreover, commercially availableon-line condition monitoring techniques can provide earlywarning of problems. The avoidance of mechanical repair costs,

downtime costs, and environmental fines can produce savingsmeasured in millions of dollars. This article examines thetechnology employed in the dynamic pressure measurementprocess.

Chris G. JamesSKF DymacAdriaan J. L. VerhageKEMA18 pagesNovember 2002

SKF Reliability Systems@ptitudeXchange

4141 Ruffin RoadSan Diego, CA 92123United Statestel. +1 858 244 2540fax +1 858 244 2555email: [email protected]: www.aptitudexchange.com

Use of this document is governed by the terms

and conditions contained in @ptitudeXchange.
mailto:[email protected]://www.aptitudexchange.com/http://www.aptitudexchange.com/mailto:[email protected]

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Dynamic Pressure Monitoring in Gas Turbines

Introduction......................................................................................................................................3

Overview of Gas Turbines...............................................................................................................3

Improvements in the Process....................................................................................................5

Common Problems ...................................................................................................................5

Reduction of Emissions ............................................................................................................6

Combustion Monitoring...................................................................................................................6

Costs/Benefits...........................................................................................................................6

Combustion Chambers..............................................................................................................6

The Dynamic Pressure Sensor..................................................................................................7

Indirect Method ........................................................................................................................8

Direct Method...........................................................................................................................8

Example Installation .................................................................................................................9

Signal Processing....................................................................................................................10

Spectral Banding.....................................................................................................................11

Case examples................................................................................................................................12

Process Conditions..................................................................................................................12

Cracking in Combustors .........................................................................................................12

Combustor Resonance #1 .......................................................................................................13

Combustor Resonance #2 .......................................................................................................14

Cardiograms...................................................................................................................................15

Visualization Tools.................................................................................................................16

Conclusion .....................................................................................................................................18

About Dymac and KEMA .............................................................................................................18

References......................................................................................................................................18

2002 SKF Reliability Systems All Rights Reserved 2

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Introduction

Combustion instabilities in industrial gasturbines can produce intolerably large pressurewaves, which lead to fatigue, detachment ofcomponents, and costly outages and repair.The measurement of dynamic pressureamplitudes within the combustion chambermay be used in condition monitoring analysesto detect and correct instabilities before theycause serious damage. However, the technicalchallenge in physically measuring dynamicpressure directly - within the very hightemperature environment of the gas turbinecombustion chamber - is a significant one.

This article examines the technologyemployed in the dynamic pressuremeasurement process, and reviews the meansand benefits of generating a "cardiogram" forthe gas turbine combustor during regularservice. First, a brief overview of the gasturbine process is covered to provide contextand explain terminology for readers unfamiliarwith gas turbine engineering.

Then, it covers the general background for theuse of Dynamic Pressure monitoring (alsocalled Pressure Pulsation), together with a cost/ benefit discussion to justify further

investigation. A discussion of themeasurement process leads to an illustrationof analysis of the measurements within thecomplete system of a gas turbine, using theanalysis software Flamebeat.

Overview of Gas Turbines

A simplified schematic of the basic gasturbine is shown in Figure 1.

A gas turbine is divided into two main

components: the Gas Generator (GG) andPower Turbine (PT). The GG is made up ofone or more spools, each consisting of acompressor, shaft, and turbine.

The fundamental thermodynamic cycle is likethat of any combustion engine - draw in air,compress it, mix it with fuel in a combustionchamber, ignite the compressed fuel / airmixture and exhaust the gas. Some of the

Figure 1. General diagram of a gas turbine. Airflow is from left to right. The general process is that the air is drawnin, compressed, mixed with fuel in a combustion chamber, ignited, and then the compressed fuel / air mixture is

exhausted through a turbine. The end result is energy that is used to drive or propel a machine.


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energy from the exhaust gas stream isabsorbed by the spool turbine, therebycompleting the engine cycle by turning thecompressor to draw in more air.

The power turbine, the rotation of whichconverts the thrust energy of the gas intorotational energy to turn the driven machine,absorbs the majority of the energy. Theremaining hot gas is exhausted to atmosphereor to another process.

Although the applications of gas turbines aremany, there are a certain number of commonaspects to allow for classifications. The first

possible classification is the mode of drive:

Direct drive from the power turbine to thedriven components.

Geared drive (speed reduction orincrease).

The next possible classification can be madeas a function of rotational speed:

Operation at constant speed - e.g. turbo-

alternators. Operation at variable speed - e.g. turbo-

pumps or turbo-compressors.

Finally there is the consideration of physicallocation:

On-shore - e.g. refineries, pipelines,power plant.

Offshore - e.g. export gas compressors.

On-board - e.g. main propulsion for ships.

The large variety of gas turbines available

have the above common class features.However, there is a broader grouping that ishelpful when considering vibration monitoringand condition monitoring of gas turbines. Thisbroader classification focuses on whether thegas turbine under consideration is an"Aeroderivative" or a "Heavy DutyIndustrial."

Aeroderivatives are light in construction -as they are derived from aircraft engines.

They lend themselves to offshore andshipboard applications. Aeroderivativeshave a vast array of possibleconfigurations, and operate in a powerrange broadly between 10 MW to 50 MW.

Heavy Duty Industrial range from 10 MWto well above 100 MW, and the heavyconstruction of this type of gas turbinenaturally predisposes them for stationaryapplications, such as onshore power

generation.The smaller units use rolling element bearings,the medium ones have a mix of rollingelement and journal (plain) bearings, and theheavy ones use almost exclusively journalbearings.

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Figure 2. This figure contains several key marker areas: 1 - area in which the rolling element or journal bearings

reside. 2 - the front end of the compressor section. 3 - cannular or annular combustion chambers. 4 - front end of

the turbine section. 5 exhaust chamber. 6 exhaust nozzle.

Improvements in the Process

The introduction of advanced heavy industrialgas turbines, such as GE Power System's

FRAME-type series, and Siemens-Westinghouse's W501G, has been far fromtrouble-free. In 1999-2000, a survey of 5000power plants in North America (byPOWERdat) found that the average capacityfactor for combined cycle plants ranged from42% to 52% in their first three months ofoperation. Over the years, gas turbinemanufacturers have achieved impressiveimprovements in output, thermal efficiencyand emissions by implementing a series of

modification programs, including:

Higher firing temperatures

Scaled-up rotors

Cooling schemes (steam cooling)

Use of exotic alloys

Cutting edge aerodynamics

Common Problems

The previous availability numbers show thatthese improvements have, in the early days at

least, frequently come at the expense of theoperations and maintenance budget, withproblems, such as:

Rotor vibration

"Humming"

Failed turbine blades and nozzles

Compressor disk cracking

Vibrations of front compressordiaphragms

Combustor high frequency dynamics

Manufacturers introduced combustor designimprovements to:

Reduce combustion oscillations

Reduce emissions


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Reduction of Emissions behave like, and are coupled to, acoustic andvibration phenomena, and may be measured asdynamic pressure. This leads us to the use ofcondition monitoring techniques using

dynamic pressure measurements to provideearly warning of conditions that can lead toexcessive pulsation amplitudes.

Environmental legislation across the globe isstriving for reduced emissions from allindustries. Harsh financial penalties will beincurred by utilities that do not meet the settargets. Proposed standard in the EuropeanUnion for NOx emissions from gas turbines

greater than 50 MW capacity is 43 g/GJ (50

mg/Nm3 in 15% O2, dry flue gas).Combustion Monitoring

Costs / Benefits

Many local authorities already impose thislimit as a daily averaged emission limit priorto granting permission for building newcombined cycle power generation units. Manyexisting utilities with gas turbine generationwere built to conform to previous state-

enforced limits of 65 g/GJ (75 mg/Nm3 in15% O2, dry flue gas) or greater. Thus, to

avoid future penalties, the utilities are facedwith implementing measures to reduce theirNOx emissions.

Before discussing the technicalimplementation of condition monitoring ofcombustors, we first confirm the economicdriver for implementing such a system. The

cost of implementing an on-line conditionmonitoring system for combustion dynamicsvaries depending on the gas turbine quantityand type. The cost of dynamic pressuresensors, signal processing equipment, analysissoftware, and analysis expertise isapproximately $150,000 to $300,000 USDinitially, plus ongoing consultancy support.

Production of NOx occurs in the combustion

process of the gas turbine. The temperature ofthe flame is related to the amount of NOxgenerated: the lower the flame temperature,the lower the emissions. However, thethermodynamics of the engine cycle demandthe highest temperature possible to achievemaximum efficiency.

The cost of repairs to gas turbines cost morethan downtime and weigh heavily onmaintenance budgets. For example, the cost ofa single new turbine blade alone can be$30,000 USD. However, when one blade fails,it has consequential damage that takes manyothers with it.

The amount and quality of the fuel (usuallynatural gas) also influences the emissionslevel. The most common solution is acombination of a flat temperature profile with"lean" fuel mixtures to achieve the bestcompromise between efficiency and permitted

emissions. However, the drawback of thisapproach is flame instability. Flame instabilitycan produce pressure waves (or pulsations),which may lead to fatigue, and, if too large inamplitude, possible failure of combustionchamber components. The pulsations arecontinuously variable pressure amplitudes that

Combustion chamber failures can have similarimplications. A damaged chamber needs to bereplaced, and debris from a cracked chambercould enter the turbine and damage manyblades.

Net result is that a repair bill from any singleincident is frequently several million dollars.Avoiding such failures and omitting the costof downtime and possible fines from increasedemissions is an obvious financial motivator.


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Combustion Chambers

There are two possible combustion chamber(or combustor) geometry configurations

(Figure 3):

Annular

Cannular

An annular combustor is a hollow annulus (orring) wrapped around the circumference of theengine. A series of burners, located at equalintervals around the ring, mix fuel withcompressor air and introduce it into thecombustor, where it is ignited. The resultant

flame is a continuous ring. The annular hotgas flow is then channelled to the first stage ofthe turbine.

A can-type, or cannular configuration is aseries of individual, can-shaped combustorsplaced around the circumference of theengine. Each can has a series of burnerslocated on its cover. These mix fuel with

compressor air and introduce it into thecombustor, where it is ignited. The resultantflame is confined to the single can. The hotgas flow is then channelled to the first stage of

the turbine via a transition piece, where itjoins the flow from the other cans locatedaround the circumference.

The Dynamic Pressure Sensor

As with all measurement systems, the sensoris the critical factor for success, as the mostsophisticated and expensive processinghardware and software technology available ismade useless if the input signal is unreliableand inaccurate. The greatest obstacle in

actually measuring the dynamic pressure isachieving stable characteristics over the entiretemperature range present in the combustionchamber at the pick-up point (typically

400C). Two approaches are available today:

Indirect Measurement

Direct Measurement

Figure 3. The two basic configurations for a combustion chamber: annular and cannular. The method to achieve thecombustion varies slightly based upon the geometry of the chamber. (Insert from [1-3].)


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Figure 4. An illustration of the placement points for direct and indirect measurement probes for this specificconfiguration on a gas turbine. (Inserts [1-3] and [4].)

Indirect Method

Indirect Measurements involve fitting smallbleed tubes (or conduits) from the pick-uppoint to a pressure sensor located at a less

extreme temperature location. This permits theuse of freely available industrial pressuretransducers, which typically utilize strain-gauge technology in their construction, andcannot endure the extreme temperatures at thecombustor. The drawbacks are:

Reduction in sensitivity to pressurechanges owing to losses in the conduits.

Frequency-dependent attenuation of thesignal caused by the conduit length.

Possible conduit blockages caused bycondensation.

Distortions from acoustic resonance.

Placing the transducer closer to the pick-uppoint, and cooling it with water jacket devicesreduces the conduit losses, but addscomplexity, and hence reduces reliability.

Therefore, the most effective solution fordynamic pressure is theDirect Measurement.

Direct Method

To date, only apiezoelectric crystal device isproven to withstand the high temperatures ofthe combustion process and provide a reliablemeasurement system. The piezoelectric(quartz) crystal itself can physically withstandtemperatures well in excess of those required,and still retain its piezoelectric properties. Apiezoelectric crystal produces an electricalcharge proportional to the mechanical forceexerted upon it. Hence a pressure transducerexploits the simple principle of:

Pressure = Force/Area.

The pressure to be measured is applied to thecrystal over a known cross-sectional area. Theresultant electrical charge is then proportionalto the pressure. This is measured in pico-coulombs per millibar (pC/mbar). Theelectrical charge is then carried a few metersfrom the sensor to a charge-amplifier, where itis converted to a usable electrical voltage or

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current. Current transmission (A/mbar) tothe signal processing system is preferred tominimize interference and provide longercable runs.

The crystal and sensor body have mass, and insome applications the normal mechanicalvibration of the combustor causes the crystalto respond to Newton's Second Law:Force = Mass x Acceleration.

The crystal experiences a force produced byacceleration, in addition to that from thepressure. In this case it is necessary tocompensate for vibration by placing in-line, a

second crystal with a known mass. Thispiezoelectric accelerometer permits thevibration component to be subtracted, leavingonly the pressure output.

Other design factors are thermal expansion ofthe metal components and their joints. Theseinvolve the use of exotic alloys, such asINCONEL, which, although costly anddifficult to machine and weld, have farsuperior thermal tolerances to steels, and arein common use in the aerospace industry. Theend design is a sensor consisting of onlyquartz crystals and high temperature alloys -no moving parts, temperature sensitivecomponents, joints, and very robust. Thisdesign is more reliable and accurate than theindirect method.

Example Installation

The pictures in Figure 6 illustrate a retrofit ona large GE FRAME machine. The engine is a

cannular design, and therefore requires onedynamic pressure sensor per combustor: inthis case, 18 sensors. One or two pressuresensors are required for dynamic pressuremonitoring of annular combustors.

Figure 5. An illustration of the piezoelectric makeup of

the direct measurement sensor. (Insert [4].)

Figure 6. An installation of dynamic pressure sensorson a cannular combustion chamber. [1-3].

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Signal Processing

Data must be obtained from the pressuresensors, casing vibration sensors(accelerometers), and vital process inputsfrom the gas turbine (GT) to get a completepicture of machine health.

Pressure sensors may already be present andused as part of the GT's control logic. In thiscase, access to the raw dynamic signal isnecessary for condition monitoring purposes.If pressure sensors are not present, they can beeasily retrofitted, as "blanked-off" locationflanges are usually present from the enginetest-bed procedures.

Acceleration sensors are already present onthe engine for unbalance and casing vibrationmonitoring. These also may be used as part ofthe GT's control logic. Once again, it isnecessary to gain access to the raw dynamicsignal for condition monitoring analysis.

The sensor support of both pressure andacceleration may be provided on the engine byseparate analog monitoring systems. Figure 7

illustrates the unification of these systems intoa modern digital system, which providessensor support and conditioning, together with

condition monitoring processing and trip(protection) logic for both vibration anddynamic pressure. The monitor systemdigitizes the incoming signals (ADC) and

performs Fast Fourier Transform (FFT)processing to obtain frequency spectra ofbothvibration and dynamic pressure. For machineshutdown (protection) purposes theRootMean Square (RMS) value over a number offrequency bands are processed and comparedto trip alarms. Alternatively, peak values areused for shutdown.

The FFT processing must occur independentlyof the processing for trip logic in order to

ensure integrity of the shutdown process.Multiple sensors must be processed in parallel.Shutdown requires parallel processing in orderto capture every trip signal, and conditionmonitoring requires simultaneous data in orderto compare time-stamps with GT control data.

The characteristics of the dynamic pressurespectrum in particular, are significantlyinfluenced by engine operational conditions.So, it is essential to merge Data CollectionSystem (DCS) data with dynamic pressuredata to provide all the necessary inputs foreffective diagnostics.


Figure 7. Overall Set-up of the Monitoring System.

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This is done at a software database level by acondition monitoring system server computer,where data is imported from the DCS bystandard network protocols and unified in a

single display and analysis software package,which may be used remotely by a network. Inthis example, the software FlameBeat, atrademarked product by KEMA, is shown.

Spectral Banding

The FFT of the pressure signal shows distinctpeaks spread across a number of bands in thespectrum. The RMS value of these bands isprocessed continuously. Experience showsthat strong acoustic oscillations at particular

frequencies are precursors to trips. Thus, theRMS values can be used to trip automaticallyor provide warning to an operator to takeaction, such as change the load on themachine, switch on air pre-heaters, or changethe excess air ratio of a group of burners.However, condition monitoring analysis mayoccur at less frequent intervals. Pressurepulsation spectra are averaged typically over 5

minute intervals if no events occur. If there isan event - an alarm breach in any specificband, a power change, or a trip, then datastorage increases to approximately once per

second. All FFTs are date-stamped.

The spectra in Figure 8 show data from anannular combustor. There are a large numberof peaks in the pressure spectrum, which arerelated to the ring geometry and acousticstanding waves. As the load and temperaturesincrease, the peaks shift in frequency. For

example, at 58 MW, peak Number5 indicatedby the vertical red line, is at approximately375 Hz, at 70 MW at 385 Hz, and at 81 MW

at 400 Hz. Thus, for a 23 MW increase inload, we see an approximate 23 Hz shift infrequency. In order to detect a frequencychange the digital FFT line resolution must beat least 1 Hz, or 1600 lines if Fmax = 1000

Hz. Therefore, to determine whether thesespectra are "normal" it is necessary to knowthe load and temperatures from the DCS wheneach spectrum was captured.

Figure 8. The right side of this figure illustrates spectral data containing peaks that are related to pressure levels in

the annular combustion chamber. These peaks indicate pressure in the system related to the ring geometry andacoustic standing waves produced in the turbine. As the load and temperature increase in the system, the waves shift

and intensify. The left side of this figure shows the software program FlameBeat that helps to display the

information in a useful way that can be understood. (Data from [1-3].)


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Figure 9. Trending process conditions in a turbine can help diagnose potential harmful changes in the system [1-3].

Case examples

Process Conditions

Pulsation spectra change when the quality ofthe fuel and air is varied. Figure 9 shows howthe various parameters relate to time, or areplotted against time. Load is constant, but achange in gas calorific value produces achange in the amplitudes seen in a selectedfrequency band.

However, the changes in the pulsation bandsmay also be caused by a crack in thecombustion chamber, a leaking gasket, or amalfunction in the burners or fuel supply.Hence, tracking calorific value, airtemperature, and humidity is needed todistinguish a potential fault from normalchanges.

Cracking in Combustors

A crack in the liner material or transition piece

of a combustor allows cooler compressor airto leak into the combustion chamber. This hastwo consequences:

1. Leaking cool air influences the flow in thecombustion chamber, increasing the turbulent

background of the pressure spectrum, and

shifts the frequency of certain oscillations. Thestrength of these effects is determined by the

direction of the crack with respect to the gas

flow direction inside the chamber. The

following spectrum, Figure 10, shows discretepeaks in the pressure spectrum caused by a

crack perpendicular to the flow. The same

spectrum after repair illustrates thedifference. A crack parallel to the flow would

have a less prominent influence.

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2. The outlet temperature distribution will

witness a deviation, as the leaking air willcool some of the thermocouples. If deviations

in the temperature profile and pulsation

spectra continue to correlate over time, thereis a good chance that a crack is present. Howquickly the deviations have been developing,

when compared to historical monitored data,

will signal whether it is a fast or slow-growingfissure, and hence influence the decision as to

when to stop the machine to make a repair.

Should the crack develop to a size wherematerial fracture occurs, then the

consequential damage is not confined to the

combustor alone but will travel downstream to

the turbine stages, increasing the repair billexponentially.

In an annular combustion chamber the effectof cracking is "smeared" over the completering and is more difficult to detect. In acannular combustion chamber, burner failure,stagnation of the gas supply, or leakinggaskets profoundly change the pressurespectrum. Damage caused by fatigue cyclingreveals itself in both temperature and pressure

spectra.

Combustor Resonance #1

As discussed previously, a drawback ofoperating combustors with lean mixtures toreduce NOx emissions is the occurrence of

instabilities. These can invoke audiblehumming. At large amplitudes the hummingcauses components to resonate, resulting infatigue damage and failure.

The combustion noise is generated by naturalturbulence, thermodynamic fluctuations, dueto imperfect fuel / air mixing, self-excitedoscillations of the flame surface, and forcedoscillations caused by the swirl.

Buckling of liners without leakage of air ispossible in cans with thin liners (Figure 11).The temperature distribution in this case willnot be influenced significantly as there is nocool air to affect the thermocouple array.

However, sudden changes in frequency and ofparticular acoustic oscillations in theamplitude pressure spectrum give earlywarning of such a problem.

Figure 10. The increase of background level in the pressure spectrum caused by turbulence, due to a crack

perpendicular to the flow of the air. The discrete peak is due to interference (50 Hz and higher harmonics) [1-3]. 2002 SKF Reliability Systems All Rights Reserved 13

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Figure 11. A picture of the buckled combustor on the left. On the right is the finite element analysis of thecombustor. The finite element analysis showed that the triangular modes of the can produce a natural frequency

around 142 Hz [1-3].

Combustor Resonance #2

An on-line monitoring system fitted on a largegas turbine with cannular combustorsexhibiting humming under certain load andmix conditions showed a strong peak in theacoustical spectrum at 150 Hz.

A discarded flame shield (liner) was analyzedby KEMA, and impulse excitations (BumpTests) produced a distinct ringing at 185 Hzthat lasted for over 10 seconds (Figure 12).Higher resolution FFT revealed this to be twopeaks: 183 Hz and 187 Hz. Additionalmeasurements determined that these peakswere related to standing waves with differentphase, and the phase difference was caused bytriangular distortion (buckling) of the used testpiece. Despite this, the 185 Hz excited natural

frequency remained distant from the 150 Hzmeasured in operation.

However, the tests were conducted at roomtemperature (20C). The working combustorruns at 900C. Elasticity, and hence the naturalfrequency, change with temperature.Calculations were performed using Young'sModulus at 20C and 900C, and a 20% shiftwas predicted, bringing the 185 Hz at room

temperature close to the measured 150 Hz inoperation. A finite-element analysis predictedthat the triangular modes of the can produce anatural frequency, temperature compensated,around 142 Hz (Figure 11). Assumptions inthe computational model accounted for thedifference between that and the measured 150Hz.

The end result of this exercise was a confidentprediction that presence of the 150 Hz

components during on-line monitoring couldresult in liner buckling.

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Figure 12. Analysis of the discarded flame shield, conducted by KEMA, showed ringing of the shield at 185 Hz and

resonance at 150Hz. The increase in the natural frequency during operation caused the buckling [1-3].

Cardiograms

As can be seen from the previous examples,there are many symptoms that can be used topredict a problem in advance, but the nature ofthe symptoms changes with load, fuel, enginedesign, operational regime, etc. In this respectvirtually every engine installation is different.Although there are some commonalities ofsymptoms from one identical engine design tothe next, the truth is that interpretation of thenature of the flame instabilities is a learningprocess, communicated to the analyst bydynamic pressure spectra. After months ofobservation with changing speeds, starts,

stops, and occasional trips, the correlationbetween observed pulsation signatures andother turbine parameters become known.

Optimal (reference) spectra under differentconditions can then be defined, and oftenconfirmed by before and after incidents.

Statistical analyses can also be employed tofurther define and add confidence to thespectral bands set for alarm and/or trip. Theend result is a cardiogram (Figure 13), of ahealthy machine (red), which can be used as abaseline to compare against the current(green).

The variety of influencing factors means thatregular analysis and review of the cardiogramby qualified staff is very important to ensure

an effective system. However, advances inautomated expert systems and neural networkshold the promise of a fully automated andaccurate diagnostic system.


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Figure 13. Example of several cardiograms conducted on a gas turbine. The red line of the graphic is the healthy

turbine or baseline. The green overlay is the current state of the turbine. One cardiogram shows that the baseline

does not match the current turbine status, indicating a possible problem in combustor number 13 [1-3].

Visualization Tools

An effective cardiogram is not just a singlespectrum display with baseline plot, as thereare a variety of spectra to track over differentbandwidths and the multiplicity of processparameters that need to be taken into account.This places a heavy strain on the analyst tospot the differences. Therefore, an importantpart of any display software, such as

FlameBeat is the provision of visual tools toimprove diagnostics and problem flagging.The images in Figure 14 illustrate examples.The simpler the presentation of the data, theeasier it is for the analyst to spot problems.One solution is to condense the variousparameters into various trends (Figure 15).

P-Trends: Where a peak or averaged value ofall dynamic pressure plots is trended, and anyincreases are related to turbulence factors.This flags increases in pulsations, hopefullybefore the levels reach trip conditions (Figure10).

F-Trends: Where peak frequencies aretracked, and any significant load andtemperature adjusted change is most likelyfrom a natural frequency shift, such as amechanical deformation, or in more severecases a crack.

T-Trends: Where peak or average exhausttemperatures are tracked, and any significantchange is most likely from a crack or burnermalfunction.

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Figure 14. Several different parameters that are trended and monitored in a software package called FlameBeat.

These various parameters are trended because they assist in determining the health of the turbine [1-3].

Figure 15. Case Example of P, F, T Trends [1-3].

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Conclusion

The need to meet tougher emissions targetsand to improve gas turbine reliability andperformance means that engines undergomany improvement programs and run withever-leaner fuel / air mixtures. These changescan lead to instabilities and excessive pressurepulsations that can result in mechanicalfailure. Piezoelectric sensor technology isavailable to measure these dynamic pressuresdirectly. Moreover, commercially availableon-line condition monitoring techniques canprovide early warning of problems. Advancedanalysis software like FlameBeat visualizesthe results for operators and maintenancepersonnel. The avoidance of mechanical repaircosts, downtime costs, and environmentalfines can produce savings measured inmillions of dollars.

About Dymac and KEMA

SKF's Dynamic Monitoring Analysis andControl (DYMAC) offers total systemintegration by bringing advanced conditionmonitoring and protection systems into a

plant-wide control platform.http://www.dymac.com/

KEMA delivers professional consultancyservices to gas turbine users worldwide.http://www.kema-gasturbines.com/

The authors acknowledge Vibro-Meter SA forpermission to reprint the sensor pictures.

References

[1] Predictive Diagnostics for CombustionChamber Damage and Flame Instabilities inGas Turbines" by Adriaan J.L. Verhage,Heino J. Jansen, and Paul M.P. Stevens ofKEMA, Arnhem, Netherlands.

[2] Predictive Diagnostics for CombustionChamber Damage and Flame Instabilities inGas Turbines" by Adriaan J.L. Verhage,Heino J. Jansen, and H. Spiele of KEMA,Arnhem, Netherlands. ECOS 2000Conference, Twente, Netherlands.

[3] Flamebeat - Combustor Diagnosis for

Gas Turbines" - by Heino J. Jansen of KEMA,Arnhem, Netherlands. SKF TechnicalPresentation, 2002.

[4] Dynamic Pressure Monitoring Systems" -Technical Application Note 461.008 fromVibro-Meter S.A, Switzerland, 1999.

[5] Raising the Reliability of Advanced GasTurbines" by Robert Swanekamp, PowerEngineering Magazine, March/April 2002.

[6] Dynamic Pressure Monitoring in GasTurbines" - Technical Application Note,DM3020-EN from DYMAC, SKF, 2002.Available at the @ptitudeXchange site.

http://www.dymac.com/http://www.kema-gasturbines.com/http://www.kema-gasturbines.com/http://www.dymac.com/

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