article optical pyrometry

8/12/2019 Article Optical Pyrometry

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Improving Gas Turbine OperatingEfficiency using Optical Pyrometry

D. C. Amory, LAND Instruments International Ltd, Dronfield, UK,R. A. Hovan, LAND Instruments International Inc, Newtown, PA, USA.

Introduction

Gas turbine operating efficiency is affected not only by its effectiveness at turning its fuel intoelectrical power but also by its flexibility and availability. Traditional operating methods relyon indirectly measured and calculated parameters to define maintenance schedules andcontrol gas turbine operation

Optical pyrometers offer a major leap forward in the control and operation of industrial gasturbines, by continuously and accurately monitoring the temperature of individual bladeswithin the hot section of the gas turbine. Using this information it is possible to refine bladelife predictions, detect abnormal blade conditions, monitor blade coating condition and assessthe effect of abnormal operation. With this real time data the operator now has the ability toimprove gas turbine efficiency, and optimise maintenance scheduling, thus enabling minimumoperational costs

The Impact of Blade Temperature of Operating Efficiency

In order to generate power, gas turbines draw in atmospheric air, compress it within thecompressor stage, then heat it up by burning fuel in the combustor cans. As the air pressurein the turbine is kept constant, the hot air considerably increases its volume. The hot gasesare then allowed to escape through the exhaust of the turbine and during this part of thecycle, energy in the expanding gas is turned into mechanical power, turning the turbine shaft.This shaft can then be either directly coupled or geared to an electricity generator, therebyproducing electrical power.

Air Intake Compression Combustion Exhaust

TurbineShaft

Compressor Stage

CombustorCans Hot Blades

StationaryVanes

Exhaust

Fig. 1: Schematic View of a Typical Gas Turbine


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Because the gas turbine is a heat engine, the higher the temperature of combustion thegreater the expansion of the gases, therefore the greater the power produced to turn theturbine shaft. The combustion temperature, however cannot be allowed to exceed atemperature suitable for the design and materials of the hot parts of the turbine assembly(commonly called the hot gas path. The use of air-cooled and more recently thermal barriercoated turbine blades has allowed higher turbine inlet temperatures and consequently

enabled gas turbines to achieve higher thermal efficiencyRapid loss of structural integrity with temperature places great emphasis on the need to avoidoperating turbine blades beyond their thermal design limit. The traditional method ofaccomplishing this, based upon early turbines using uncooled blades, relies for itseffectiveness on the fact that all the blades in a row run at similar temperatures, therefore anaverage blade row temperature estimate is adequate.

Turbine control systems estimate average blade row temperature by inference from otherengine measurements, principally exhaust gas temperature. Uncertainty implicit in theindirectness of this approach (typically 10-15oC) requires that the turbine must be operatedbelow its optimum efficiency, by about 1%.

In principle, thermodynamic efficiency can be improved by increasing the firing temperature ofthe gas turbine. The introduction of cooled turbine blades has allowed modern gas turbinesto achieve improved thermodynamic efficiency through higher inlet gas temperature.Typically the cooled blades are exposed to gas temperatures of approximately 150

oC above

their material limit, and they rely upon unimpaired cooling flow for thermal protection. Thisdevelopment has introduced effects which are not detectable by the traditional method ofcontrolling blade temperature. Primarily; manufacturing variations in cooling passages cancause significant blade-to-blade temperature differences, and progressive oxidation of thecooling passages can cause the individual blades to run at progressively higher temperatures.These effects are is not reflected by a change in exhaust gas stream temperature

The most recent designs of gas turbine are utilising thermal barrier coated blades and insome instances steam in preference to air for cooling. This allows the gas stream to run evenhotter and therefore it is even more critical that cooling efficiency and coating integrity can bemonitored.

The operation and maintenance of modern engines can clearly benefit from a more detailed,direct and responsive measurement of blade temperature

Use of Optical Pyrometry on Gas Turbines

A technique which meets the needs for modern gas turbines is optical pyrometry. Thetechnique is not new - it was first used in the late 1960s - when its application waspredominantly for gas turbine research and development

1and also for in-flight top

temperature limiting on production fighter aircraft2. Over more recent years interest has grown

for continuous monitoring on industrial gas turbines, with many pyrometer installationsthroughout the world.

Power utility companies along with gas turbine manufacturers3are currently seeing many

benefits from these measurements. These include improved efficiency through turbine firingrate control, improved maintenance procedures with the development of blade lifemanagement programs, detection of blocked cooling channels in blades leading to preventionof blade failure and continuous monitoring of blade coating condition thereby reducing cost ofblade replacement.

Blade life monitoring programs use optical pyrometers to continuously measure individualblade temperatures in order to better predict crack growth and therefore the life of the gasturbine blades. An effective blade life management system

4can ultimately lead to better

maintenance scheduling of the hot gas path components and therefore reduce plant operatingcosts. This trend to improving maintenance scheduling procedures is being adopted not onlyby power utilities but also by turbine manufacturers to use optical pyrometer temperature datato determine condition of the turbine blades5. This condition-based maintenance method has

been proposed to replace interval based maintenance regimes.


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Continuous measurement is also important when running the engine beyond its normaloperating limits, such as when over-firing. Interval based maintenance methods take littleaccount of this type of operation, with potentially serious consequences.

Using the close relationship between blade temperature and turbine inlet temperature, opticalpyrometry can provide increased control of the firing rate by improved measurement of theaverage blade temperature

6. While optical pyrometers have been used for many years on

both industrial and military jet engines as part of the control system they have yet to beuniversally adopted for use on industrial gas turbines in this way.

Blade cooling introduces blade life limiting processes, such as oxidation blockage of coolingchannels, which are not detectable using traditional gas turbine instrumentation methods.Using an optical pyrometer however, an overheated (or over heating) blade can be quicklydetected, enabling corrective action to be taken to prevent unnecessary and expensiveengine damage. Output from an optical pyrometer system installed by EPRI on a GeneralElectric (GE) MS7001F gas turbine (Figure 2), clearly shows one hot and two warm blades.Continuous blade temperature monitoring using pyrometry ultimately led to improvements inthe blade manufacturing and quality control processes7

As operation at over temperature conditions can lead to damage or even engine failure in themulti-million dollar range, blade cooling integrity is critical in modern gas turbines, as stresscreep life is a strong function of the material temperature

8. Figures 3 and 4 clearly shows an

over-heated stage 1 turbine blade that would not have been detected by any other monitoringmethod. This data was taken from an optical pyrometer system installed on a GeneralElectric 7F gas turbine owned by Potomac Electric Power Corporation (PEPCO). From dayone of operation the system detected blades in the first stage of the gas turbine that were

running significantly hotter than the rest. This particular blade developed a crack after aboutone year of peaking operation. Optical pyrometry saved PEPCO over$2million

9as the

overheated blade was detected and action was taken before failure occurred.

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Blade Number

BladeTemperature/'F

Fig. 2: Optical Pyrometer Trace from GE MS7001F Gas Turbine


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BladeTemperature(degF)

Fig. 4: Close up of Hot Blade at PEPCO

Fig. 3: Output From Optical Pyrometry System Showing Over-Heated Blade

Blade Number

Tempera

ture

/oC


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All modern gas turbine blades are coated; some with an anticorrosion coating, other moremodern designs use a thermal barrier coating (TBC). Loss of coating, especially TBC, cansignificantly reduce the life of the blade, therefore the ability to monitor coating erosion canenable the user to prevent irreparable damage to the blades.

Figures 5 and 6 show coating erosion on gas turbines blades. In both of these cases thisdegradation was detected using an optical pyrometer. The pyrometer measured a gradualincrease in the difference between the blade hottest and the blade average temperature, thiswas found to correlate with loss of blade coating. Detecting erosion at its onset enables theplant operator to have the blades recoated rather than being forced to replace them withsignificant cost savings as a result.

Optical Pyrometer Systems

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POWERCHANNELS

Mounting Kit Pyrometer

Turbine mountedspool piece

KP/PSU

Keyphasorsignal

Additional outputfor use by turbinecontrol system

Data Acquisition System

Plant Network Server

Desktop PC

EthernetLink

EthernetLink

Personal Workstation runningdata analysis software

DAS accepting multiplepyrometer inputs

+V

-V

KEY PHASORPULSE

POWERSUPPLY

Fig. 7: Typical Pyrometer System Configuration

Fig. 6: An Overheated Gas Turbine BladeFig. 5: Blade Coating Erosion

Coating Erosion


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A typical optical pyrometer system illustrated in Figure 7. The pyrometer monitors thetemperature at a number of points across the blade. Multiple pyrometers can be used to givemeasurements from different areas of the blade. In the same way several blade rows can be

monitored by a single systemThe pyrometer requires a direct line of sight on to the gas turbine blade in order to measureits temperature. The required engine penetration necessary (Figure 8) is typically designed,and installed by the engine manufacturer. many gas turbines already have an enginepenetration available from the gas turbine manufacturer (Table 1)

Manufacturer Gas Turbine Type

General Electric 6B, 7B, 7EA, 7F, 7FA, 7FA+E, 9E, 9F, 9FA

Seimens Westinghouse V64.3, V94.2, V94.3a, V84.3a

Alstom Power GT11, GT13E2, GT26, GT24

Table 1: Gas turbine Penetration Designs

A number of holes are drilled in the internal boss allowing a small volume of compressordischarge air to flow down the penetration assembly when the gas turbine is operating. Thisflow creates a purging system that inhibits contamination of the inboard surface of the sightglass. Experience has shown that this system can keep the optical surfaces of the equipmentclean for period of over a year.

The pyrometer is mounted to this penetration assembly on the gas turbine and directly viewsthe rotating blades through a pressure-proof sight glass assembly (Figure 9)

Mounting Flange

Removeable Sight tube

Gas Turbine Outer Casing

Gas Turbine Inner CasingInternal Boss

Vanes Blades

Fig. 8: Typical Gas Turbine Penetration


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The infrared energy from a small area on the blade (as defined by the pyrometer opticalsystem) is collected by the pyrometer and is converted into an electrical signal that isproportional to the temperature of the area defined. Signal processing within the pyrometerprovides outputs to the plant operator that can be used for both detailed blade temperatureanalysis as well as outputs (e.g. average and hottest temperature) that are suitable for use bythe turbine control system.

Using a high speed data acquisition system along with the turbine shaft once per revolutionsignal (Keyphasor signal), temperature data from all the blades can be displayed, storedand recalled allowing changes to be easily detected

ConclusionsOptical pyrometry can provide a gas turbine operator with a great deal of information aboutthe temperature of the gas turbine blades. This information has proved invaluable for refiningmaintenance procedures and detecting potentially serious problems, therefore reducing costsand improving plant overall efficiency. In addition to this, by using pyrometry as a controlparameter, the thermodynamic efficiency of the turbine can potentially be improved hencesaving even more money.

Plant efficiency is a combination of the thermodynamic efficiency as well as the cost ofoperating the plant and therefore includes the cost of maintenance, the turbine reliability, theability to operate at varying load and for flexible time periods. Optical pyrometry hasdemonstrated that it can be used to provide the operator with information that allows them tomake better decisions.

Optical pyrometry suitable for use on a gas turbine is now readily commercially available withone such system already installed on over 40 gas turbines throughout the world

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Engine MountedSpool Piece

Mounting Kit Pyrometer Optic Head

DustSeal

PressureWindow

Sighting AdjustmentScrews

Fig. 9: Optical Pyrometer Installation


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References

1Turbine Pyrometry A Equipment Manufacturers View T.G.R. Beynon, ASME 81-GT-136

2

A Radiation Pyrometer Designed for In-Flight Measurements of Turbine BladeTemperatures R. Barber, SAE 690432, 1969

3SPEEDTRONIC Mark VI Turbine Control System (GER-4193A)- Walter Barker, MichaelCronin, GE Power Systems, Schenectady USA4Seimens Model V84.2/V94.2 Combustion Turbine Blade Life Management System J.D

Wilson, Powergen PLC Nottingham, UK, G Touchton, EPRI Palo Alto, USA, and F vanZeveren, Laborelec, Linlebeek, Belgium5United States Patent 6,579,005 Utilization of Pyrometer Data to Detect Oxidation -

Michael Ingallinera, General Electric Company, Schenectady, USA

6Infrared Thermometry for Control and Monitoring of Industrial Gas Turbines (ASME 86-GT-

267) P. J. Kirby, Land Turbine Sensors, R.E. Zachary, F. Ruiz, Dow Chemical, Plaquemine

Louisiana USA. Presented at ASME Dusseldorf, Germany, June 19867Advanced Gas Turbine Guidelines: Hot Gas Path Parts Condition and Remaining Life

Assessment: Durability Surveillance at Potomac Electric Power Companys Station H EPRIReport TR-104101

8Blade Temperature Monitoring for Improved Turbine Operation and Maintenance D.C

Amory, Dr P.J. Kirby, Land Instruments International Inc, Bristol, PA, USA. Presented atAmerican Industrial & Power Gas Turbine Operation & Maintenance Conference, Houston, TxUSA, February 1997

9Thirty-One Innovative Uses of EPRI Products EPRI Fossil Plant News

10Land Turbine Sentry Installation list Land Instruments International Ltd

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