Gas Turbine Engineering Handbook 2 Episode 15 doc

ASME, Performance Test Code on Overall Plant Performance, ASMEPTC 46 1996, American Society of Mechanical Engineers 1996 2.. ASME, Performance Test Code on Gas Turbines, ASME PTC 22 1997

4 Bearing failure The symptoms of bearing problems for a turbine arethe same as for a compressor.

5 Cooling air failure Problems associated with the blade cooling systemmay be detected by an increase in the pressure drop in the cooling line

6 Turbine maintenance This should be based on ``equivalent enginetime,'' which is the function of temperature, type of fuel used, andnumber of starts Figure 19-21 shows the correction that can beapplied to running hours for intermittent-duty units with high-start/stop operation

Turbine Efficiency

1 With the current high cost of fuel, very significant savings can beachieved by monitoring equipment operating efficiencies and correct-ing for operational inefficiencies Some of these operational inefficien-cies may be very simple to correct, such as washing or cleaning of thecompressor on a gas turbine unit In other cases, it may be necessary

to develop a load-distribution program that achieves maximum all efficiency of the plant equipment for a given load demand

over-Figure 19-21 Equivalent engine time in the turbine section

2 Figure 19-22 shows the significant dollar cost penalties that occurwhen operating a turbine at a very small percentage efficiency degrada-tion.

3 Table 19-8 shows a load-distribution program for an 87.5-MW powerstation of steam turbines and gas turbines The selection of equipmentand their loading for the most efficient operation can be programmedwhen the efficiency of individual units are monitored The program

Figure 19-22 Savings versus efficiency

selects the units that should be operated to provide the powerloaddemand at the maximum overall efficiency of the combination ofunits.

Mechanical Problem DiagnosticsThe advent of new, more reliable, and sensitive vibration instrumentationsuch as the eddy-current sensor and the accelerometer coupled with modern

Table 19-8 Load SharingProgram Description of Utility Plant Units Unit # Design

MW TurbineType Design Output PointEfficiency at

Total Output Supplied = 30.00 MW Total Output Supplied = 50.00 MW Units not working ˆ 1 4 9 0 Units not working ˆ 1 4 0 0

Maximum Overall Efficiency ˆ 27:04 Maximum Overall Efficiency ˆ 25:02

Power Demands ˆ MW (Maximum demand ˆ 87:5)

technology analysis equipment (the real-time vibration spectrum analyzerand low-cost computers) gives the mechanical engineer very powerful aids inachieving machinery diagnostics.

A chart for vibration diagnosis is presented in Table 19-9 While this is ageneral criterion or rough guideline for diagnosis of mechanical problems, itcan be developed into a very powerful diagnostic system when specificproblems and their associated frequency domain vibration spectra are

Table 19-9 Vibration Diagnosis Usual Predominant Frequency* Cause of Vibration

Running frequency at 0±40% Loose assembly of bearing liner,

bearing casing, or casing and support Loose rotor shrink fits

Friction-induced whirl Thrust bearing damage Running frequency at 40 ±50% Bearing-support excitation

Loose assembly of bearing liner, bearing case, or casing and support Oil whirl

Resonant whirl Clearance induced vibration

Rotor bow Lost rotor parts Casing distortion Foundation distortion Misalignment Piping forces Journal & bearing eccentricity Bearing damage

Rotor-bearing system critical Coupling critical

Structural resonances Thrust-bearing damage

Pressure pulsations Vibration transmission Gear inaccuracy Valve vibration

Blade passage

*Occurs in most cases predominantly at this frequency; harmonics may or may not exist.

logged and correlated in a computerized system With the extensive memorycapability of the computer system, case histories can be recalled and efficientdiagnostics achieved.

Data Retrieval

In addition to being valuable as a diagnostic and analysis tool, adata retrieval program also provides an extremely flexible method of datastorage and recovery By careful design of a health monitoring system, anengineer or technician can compare the present operation of a unit withthe operation of the same machine, or of another machine, under similarconditions in the past This can be done by selecting one or several limit-ing parameters and defining the other parameters that are to be displayedwhen the limiting parameters are met This eliminates the necessity

of sifting through large amounts of data A few examples of how this system

is used are:

1 Retrieval by time In this mode, the computer retrieves data takenduring a specified time period, thus enabling the user to evaluate theperiod of interest

2 Retrieval by ambient temperature The failure of a gas turbine mayoccur during an unusually hot or cold period, and the operator maywish to determine how his unit functioned at this temperature in thepast

3 Retrieval by turbine exhaust temperature The exhaust temperature can

be an important parameter in failure investigations An analysis ofthis parameter in failure investigations An analysis of this parametercan verify the existence of a problem with either the combustor orturbine

4 Retrieval by vibration levels Inspection of data provided by this modecan be useful in determining compressor fouling, compressor or tur-bine blade failure, nozzle bowing, uneven combustion, and bearingproblems

5 Retrieval by output power In this mode, the user should input theoutput power range of interest and thus obtain only data applying tothat particular power setting In this manner, he has only to considerthe pertinent data to pinpoint the problem areas

6 Retrieval by two or more limiting parameters By retrieving data withlimits on several parameters, the data can be evaluated and will beeven further reduced Diagnostic criteria can then be developed

1 The monitoring of turbomachinery mechanical characteristics, such

as vibrations, has been applied extensively over the past decade Theadvent of the accelerometer and the real-time vibration spectrumanalyzer has required a computer to match and utilize the extensiveanalysis and diagnostic capability of these instruments

2 The high cost for machinery replacements and downtime makesmachinery operational reliability very important; however, with thecurrent and projected increases in fuel costs, aerothermal monitoringhas become very important Aerothermal monitoring can provide notmerely increased operational efficiency for turbomachinery but, whencombined with mechanical monitoring, it provides an overall, moreeffective system than one that monitors only the mechanical functions

or aerothermal functions

3 While there had been concern about the reliability of computer tems, they are currently receiving wide acceptance and are fast repla-cing analog systems

sys-4 The systematized application of modern technology (instrumentation,both mechanical and aerothermal and low-cost computers) andturbo-machinery engineering experience will result in the developmentand application of cost-effective systems

BibliographyASME, Gas Turbine Control and Protection Systems, B133.4 Published: 1978(Reaffirmed year: 1997)

Boyce, M.P., Gabriles, G.A., Meher-Homji, C.B., Lakshminarasimha, A.N.,and Meher-Homji, F.J., ``Case Studies in Turbomachinery Operation andMaintenance Using Condition Monitoring,'' Proceeding of the 22nd Turbo-

Boyce, M.P., and Herrera, G., ``Health Evaluation of Turbine Engines going Automated FAA Type Cyclic Testing,'' Presented at the SAE Inter-

Paper No 932633

Boyce, M.P., Gabriles, G.A., and Meher-Homji, C.B., ``Enhancing SystemAvailability and Performance in Combined Cycle Power Plants by the Use

of Condition Monitoring,'' European Conference and Exhibition

Boyce, M.P., ``Control and Monitoring an Integrated Approach,'' Middle East

Boyce, M.P., ``Improving Performance with Condition Monitoring''ÐPower

Boyce, M.P., and Venema, J., ``Condition Monitoring and Control Center'',Power Gen Europe in Madrid, Spain, June 1997

Boyce, M.P., and Cox, W.M., ``Condition Monitoring Management-Strategy'',The Intelligent Software Systems in Inspection and Life Management ofPower and Process Plants in Paris, France, August 1997

Boyce, M.P., ``How to Identify and Correct Efficiency Losses Through ModelingPlant Thermodynamics,'' Proceedings of the CCGT Generation Power Con-ference, London, United Kingdom, March, 1999

Boyce, M.P., ``Condition Monitoring of Combined Cycle Power Plants,'' Asian

Meher-Homji, C.B., Boyce, M.P., Lakshminarasimha, A.N., Whitten, J.A., andMeher-Homji, F.J., ``Condition Monitoring and Diagnostic Approaches forAdvanced Gas Turbines,'' Proceedings of ASME Cogen Turbo Power 1993,7th Congress and Exposition on Gas Turbines in Cogeneration and Utility,Sponsored by ASME in participation of BEAMA, IGTI-Vol 8 Bournemouth,

Gas Turbine

Performance Test

IntroductionThe performance analysis of the new generation of gas turbines arecomplex and presents new problems, which have to be addressed The newunits operate at very high turbine firing temperatures Thus, variation in thisfiring temperature significantly affects the performance and life of the com-ponents in the hot section of the turbine The compressor pressure ratio ishigh which leads to a very narrow operation margin, thus making the tur-bine very susceptible to compressor fouling The turbines are also very sensi-tive to backpressure exerted on them when used in combined cycle orcogeneration duty The pressure drop through the air filter also results inmajor deterioration of the performance of the turbine

If a life cycle analysis were conducted the new costs of a plant are about

of the life cycle costs Operating costs, which essentially consist of energy

major power plant Thus, performance evaluation of the turbine is one of themost important parameter in the operation of a plant

Total performance monitoring on or off line is important for the plantengineers to achieve their goals of:

1 Maintaining high availability of their machinery

2 Minimize degradation and maintain operation near design efficiencies

3 Diagnose problems, and avoid operating in regions, which could lead

to serious malfunctions

692

4 Extend time between inspections and overhauls.

5 Reduce life cycle costs

To determine the deterioration in component performance and efficiency,the values must be corrected to a reference plane These corrected measure-ments will be referenced to different reference planes depending upon thepoint, which is being investigated Corrected values can further be adjusted

to a transposed design value to properly evaluate the deterioration of anygiven component Transposed data points are very dependent on the char-acteristics of the components performance curves To determine the charac-teristics of these curves, raw data points must be corrected and then plottedagainst representative nondimensional parameters It is for this reason that

we must evaluate the turbine train while its characteristics have not beenaltered due to component deterioration If component data were availablefrom the manufacturer, the task would be greatly reduced

Performance CodesPerformance analysis is not only extremely important in determiningoverall performance of the cycle but in also determining life cycle considera-tions of various critical hot section components

In this chapter, a detailed technique with all the major equations ing a Gas Turbine Power Plant are presented based on the various ASMETest Codes The following five ASME Test Codes govern the test of aGas Turbine Power Plant:

govern-1 ASME, Performance Test Code on Overall Plant Performance, ASMEPTC 46 1996, American Society of Mechanical Engineers 1996

2 ASME, Performance Test Code on Test Uncertainty: Instruments andApparatus PTC 19.1, 1988

3 ASME, Performance Test Code on Gas Turbines, ASME PTC 22 1997,American Society of Mechanical Engineers 1997

The ASME, Performance Test Code on Overall Plant Performance,ASME PTC 46, was designed to determine the performance of the entireheat cycle as an integrated system This code provides explicit procedures todetermination of power plant thermal performance and electrical output.The ASME, Performance Test Code on Test Uncertainty: Instrumentsand Apparatus PTC 19.1 specifies procedures for evaluation of uncertainties

in individual test measurements, arising form both random errors and

systematic errors, and for the propagation of random and systematic tainties into the uncertainty of a test results The various statistical termsinvolved are defined The end result of a measurement uncertainty analysis is

to provide numerical estimates of systematic uncertainties, random tainties, and the combination of these into a total uncertainty with anapproximate confidence level This is especially very important when com-puting guarantees in plant output and plant efficiency

uncer-The PTC 22 establishes a limit of uncertainty of each measurementrequired; the overall uncertainty must then be calculated in accordance withthe procedures defined in ASME PTC 19.1 Measurement Uncertainty Thecode requires that the typical uncertainties be within a 1.1% for the PowerOutput, and 0.9% in the heat rate calculations It is very important that thepost-test uncertainty analysis should be also performed to assure the partiesthat the actual test has met the requirement of the code

The instrumentation will be calibrated as per the requirements of the testcodes All the instrumentation must be calibrated before a test and certifiedthat they meet the code requirements The ASME PTC 19 series outlines thegoverning requirements of all instrumentation for an ASME PerformanceTest to be within the governing band of uncertainty

Table 20-1 is a very short abstract of the test measurement requirementsfor the performance tests; the ASME PTC 19 series should be the finalgoverning document:

Flow StraightenersMinimum lengths of straight pipe are required for flow-measuring devicesand for certain pressure measurements Flow straighteners and/or equalizersshould be used in the vicinity of throttle valves and elbows, as shown inFigure 20-1

Table 20-1 Instrumentation Accuracy

Temperature below 200  F (93.3  C) 0.5  F (0.27  C) Temperature above 200  F (93.3  C) 1.0  F (0.56  C)

Pressure Measurement

The following types of instruments are used to make pressure ments:

measure-1 Bourdon tube gauges

2 Dead-weight gauges (used for calibration purposes only)

Figure 20-1 Flow equalizers and straighteners (Power Test Code 10, sors and Exhausters, American Society of Mechanical Engineers, 1965.)

Compres-deadweight tester in their normal operating range When selecting a pressuregauge, it is important to see that the measure value is above midpoint on thescale.

Differential pressures and subatmospheric pressures should be measured

by manometers with a fluid that is chemically stable when in contact withthe test gas Mercury traps should be used where necessary to prevent themanometer fluid from entering the process piping Errors in these instru-ments should not exceed 0.25%

A common failure in pressure measurement is the uncertainty of theconfiguration of static-pressure taps penetration through the pipe wall.This failure is another early-planning concern, since proper taps are easy

to provide prior to placing the machine in service, but inspection of thetaps after operation has commenced is a luxury rarely afforded the testteam

Another pitfall in pressure measurement, particularly important in flowmeasurement, is the potential for liquids in gauge lines All too often gaugelines coming from overhead pipes have no provision for maintaining aliquid-free status, even though the flowing fluid may be condensible atgauge-line temperatures

Calibration of the pressure-measuring device presents another pitfall fortest crews All too often a test is conducted through the field calculation stepbefore bad data reveals that gauges, possibly with too large a minimumincrement, were removed from the shipping carton and installed, relying onthe vendor's calibration On-site calibration of all instruments is alwaysgood insurance against a bad test

Frequently, new machines are put into service with a ``startup screen'' inthe compressor inlet piping to guard against the inevitable weld slag andconstruction debris that will remain in a new or rebuilt piping system afterconstruction Regardless of the age of the installation, care must be exercised

to ensure that measurements defining suction or discharge conditions are notinfluenced by such devices

Inlet and discharge pressures are defined as the stagnation pressures at theinlet and discharge, which are the sum of static and velocity pressures at thecorresponding points Static pressures should be measured at four stations inthe same plane of the pipe as shown in the piping arrangements Velocitypressure, when less than 5% of the pressure rise, can be computed by theformula

Pvˆ2g…Vav†2

…Vav†2

where Vav is the ratio of measured volume flow rate to the cross-sectionalarea of the pipe.

When the velocity pressure is more than 5% of the pressure rise, it should

be determined by a pitot-tube traverse of two stations For each station, thetraverse consists of 10 readings at positions representing equal areas of the

Barometric pressure should be measured at the test site at 30-minuteintervals during the test

Figure 20-2 Traverse points in pipe (Power Test Code 10, Compressors andExhausters, American Society of Mechanical Engineers, 1965.)

Test plans frequently are prepared on the assumption that a laboratorythermometer can replace an operating instrument in an existing thermo-meter well While this change may be satisfactory, the prudent tester needs to

be aware that because of the propensity of thermowells to break off andperhaps enter the machine or cause a hazardous leak, their design is com-promised such that true gas temperature determination is impossible Thecompromise may be to make the well short and/or to make it thick-walled

In either event the mass of metal exposed to ambient temperature mayexceed that exposed to the gas, resulting in significant error if the gastemperature is much different from the ambient High-pressure systemsrequiring thick-wall pipe are particularly susceptible to this fault However,the use of a good heat-transfer fluid can minimize the error The best gastemperature reading is attained by a calibrated fine-wire thermocouple withthe junction directly exposed to the gas near the center of the flow Asdeviations from this ideal are made, the potential for error is increased.Inlet and discharge temperatures are the stagnation temperatures at

the velocity effect should be included in the temperature measurement with atotal temperature probe This probe is a thermocouple with its hot junctionprovided with a shielded cup The cup opening points upstream A trade-offhas to be made in a field test situation where the gas is not clean

3 ASME flow nozzle These nozzles provide for accurate measurements.Their use is limited because they are not easily placed in a processplant; however, they are excellent for shop tests Venturi meters andnozzles can handle about 60% more flow than orifice plates withvaried pressure losses.

4 Elbow flow meters The principle of centrifugal force at the bend isused to obtain the difference in pressure at the inside and outside ofthe elbow, which is then related to the discharge pressure

5 Turbine flow meters The principle of this flow meter is the tion of the revolutions of the turbine wheel in a given time frame.Other techniques for measuring flow through the compressor include:

computa-1 Calibrated pressure drops from the inlet flange to the eye of thefirststage impeller in centrifugal compressors, when such data is avail-able from the manufacturer

2 A flow trace technique in which Freon is injected into the constream,and flight time between two detection points is measured

3 Velocity traverse techniques must be used when, due to the uration in piping, nozzles, or orifice plates, etc., cannot be used.These techniques have been described previously in the pressure measure-ment section Usually, one of the flow-measuring devices and the requiredinstrumentation is incorporated as a part of the plant piping The choice oftechnique depends on the allowable pressure drop, flow type, accuracyrequired, and cost

config-Nozzle arrangements for various applications vary considerably Forsubcritical flow measurement at the outlet end, where nozzle differentialpressure p is less than the barometric pressure, flow should be measuredwith impact tubes and manometers as shown in Figure 20-3

For critical measurement, where the drop p is more than the barometricpressure, flow should be measured with static-pressure taps upstream fromthe nozzle as illustrated in Figure 20-4 For exhaust measurements, differ-ential pressure is measured at two static taps located downstream from thenozzle at the inlet as shown in Figure 20-5.

Gas Turbine TestBefore starting any performance test the gas turbine shall be run untilstable conditions have been established Stability conditions will be achieved

10 D MINIMUM

NOZZLE TEMPERATURE

2 - MEASURING STATIONS SPACED 90 DEG.

NOTE d NOT MORE THAN 6 D FOR ANY NOZZLE ARRANGEMENT FLOW EQUALIZER AND STRAIGHTENER (FIG 17 D)

D

D

APPROXIMATELY (23°)

d

6 D

NOZZLE PRESSURE ONE IMPACT TUBE FOR d < 5°

TWO IMPACT TUBES FOR d > 5°

THROTTLE VALVE

NOT GREATER THAN20d

Figure 20-3 Flow nozzle for subcritical flow (Power Test Code 10, Compressorsand Exhausters, American Society of Mechanical Engineers, 1965.)

NOZZLE TEMPERATURE

2 - MEASURING STATIONS SPACED 90 DEG.

NOZZLE PRESSURE

2 - MEASURING STATIONS SPACED 90 DEG.

BY B.T GAGES OR HG MANOMETER

6D

10 D MINIMUM

D d

Figure 20-4 Flow nozzle, for critical flow (Power Test Code 10, Compressors andExhausters, American Society of Mechanical Engineers, 1965.)

D 2

Figure 20-5 Nozzle for exhausters (Power Test Code 10, Compressors andExhausters, American Society of Mechanical Engineers, 1965.)

when continuous monitoring indicates the readings have been within the

performance test will be run as much as possible to the design test conditions

as specified in the contract The maximum permissible variation in a test runshall not vary from the computed average for that operating conditionduring the complete run by more than the values specified in Table 20-2 Ifoperation conditions vary during any test run vary by more than the pre-scribed values in Table 20-2 than the results of that test run shall bediscarded The test run should not exceed 30 minutes and during that timethe interval between readings should not exceed 10 minutes There should

be three to four test runs performed, which then could be averaged to get thefinal guarantee test points

Correction factors are also provided in ASME PTC Test Code-46 Thecorrection factors for ambient temperature, ambient pressure, and relativehumidity are presented in this chapter

The equations and performance parameters for all the major components

of a power train must be corrected for ambient conditions and certainparameters must be further corrected to design conditions to accuratelycompute the degradation Therefore, to fully compute the performance, anddegradation of the plant and all its components, the actual, corrected, andtransposed to reference conditions of critical parameters must be computed.The overall plant needs the following parameters to be computed Themost important two parameters from an economic point of view arethe computation of the power delivered and the fuel consumed to deliver

Table 20-2 Maximum Permissible Variation in Test Conditions Variables Variation of Any StationDuring the Test Run

Inlet air temperature 4:0  F ( 2:2  C) Heat valveÐgaseous fuel per unit volume 1%

PressureÐgaseous fuel as supplied to engine 1%

Absolute exhaust back pressure at engine 0:5%

Absolute inlet air pressure at engine 0:5%

Coolant temperatureÐoutlet [Note (2)] 5:0  F ( 2:8  C) Coolant temperatureÐrise [Note (2)] 5:0  F ( 2:8  C) Turbine control temperature [Note (3)] 5:0  F ( 2:8  C)

the power The following are the parameters that need to be computed tofully understand the macro picture of the plant.

1 Overall plant system

2 Gross unit heat rate

a Net unit heat rate

b Gross output

c Net output

d Auxiliary power

Gas TurbineThe ASME, Performance Test Code on Gas Turbines, ASME PTC 22examines the overall performance of the gas turbine The ASME PTC 22only examines the overall turbine and many turbines in the field are betterinstrumented for computation of the detail characteristics of the gas turbine.Figure 20-6 shows the desired location of the measurement points for a fullyinstrumented turbine The following are the various computations required

to calculate the gas turbine overall performance based on the code:

1 Gas turbine overall computation

2 Gas turbine output

3 Inlet air flow

Figure 20-6 Gas turbine suggested measurement points

4 First stage nozzle cooling flow rate

5 Total cooling flow rate

6 Heat rate

7 Expander efficiency

8 Gas turbine efficiency

9 Exhaust flue gas flow

10 Specific heat of exhaust flue gas

To further analyze, the gas turbine must be examined in its four majorcategories:

1 Air inlet filter

2 Compressor

3 Combustor

4 Expander turbineAir Inlet Filter Module

Loss computation, this enables the operator to ensure that the filters areclean and that no additional losses than necessary reduce the performance ofthe gas turbine The following parameters are necessary to monitor the filter:

1 Time to replace each stage of filters

2 Filter plugged index to monitor the condition of each stages of filters

3 Inlet duct air leakCompressor Module

The compressor of a gas turbine is one of the most important components of

turbine Thus fouling of the compressor can cause large losses in power andefficiency for the gas turbine Furthermore, the fouling of the compressor alsocreates surge problems, which not only affects the performance of the com-pressor but also creates bearing problems and flame-outs The following aresome of the major characteristics that need to be calculated:

Overall Parameters of the Compressor

1 Efficiency

2 Surge map

3 Compressor power consumption

4 Compressor fouling index

5 Compressor deterioration index

6 Humidity effects on the fouling

7 Stage deteriorationCompressor Losses These losses are divided into two sections:

1 Controllable Losses Losses, which can be controlled by the action ofthe operator such as:

a Compressor fouling

b Inlet pressure drop

2 Uncontrollable Losses Losses, which cannot be controlled by theoperator such as:

On-line Wash This wash is done by many plants as the pressure dropdecreases by more than 2% Some plants do it on a daily basis The water forthese washes must be treated

Off-line Wash Figure 20-7 shows that on-line water wash will not returnthe power to normal thus after a number of these washes, an off-line waterwash must be planned This is a very expensive maintenance program andmust be fully evaluated before it is undertaken Chapter 12 deals with thevarious washes in detail

Combustor Module

The calculation of the firing temperature is one of the most importantcalculations in the combined cycle performance computation The temper-ature is computed using two techniques (1) Fuel Heat Rate (2) Power Balance.The following are the important parameters that need to be computed:

1 Combustor efficiency

2 Deterioration of combustor

3 Turbine inlet temperature (first stage nozzle inlet temperature)

4 Flash back monitor (for dry low NOx combustors)

5 Specific fuel consumption

Expander Module

The calculation of the turbine expander module depends whether or notthis is a single shaft gas turbine or a multiple shaft gas turbine In aero-derivative turbines, there are usually two or more shafts In the latestaero-derivative turbines, there are usually two compressor sections, the LPcompressor section, and the HP compressor section This means that theturbine has three shafts; the third shaft is the power shaft The turbines thatdrive the compressor section are known as the gasifier turbines, and theturbine, which drives the generator, is the power turbine The gasifier turbineproduces the work to drive the compressor

The parameters which must be computed are:

1 Expander efficiency

2 Fouled expander parameter

3 Eroded turbine nozzle monitor parameter

4 Expander power produced

5 Deterioration monitor parameter

6 Plugged turbine nozzle monitor parameter

TIME

Off-line Crank Wash

Figure 20-7 Compressor water wash characteristics

Life Cycle Consideration of Various Critical Hot Section ComponentsThe life expectancy of most hot section parts is dependent on variousparameters and is usually measured in terms of equivalent engine hours Thefollowing are some of the major parameters that effect the equivalent enginehours in most machinery, especially gas turbines:

1 Type of fuel

2 Firing temperature

3 Materials stress and strain properties

4 Effectiveness of cooling systems

5 Number of starts

6 Number of trips

7 Expander Losses

a Controllable losses1) Firing temperature2) Back pressure3) Turbine fouling (combustion deposits)

b Uncontrollable (degradation) Losses1) Turbine ageing (increasing clearances)

Performance Curves

It is very important to form a base line for the entire power plant This wouldenable the operator to determine if the section of the plant is operating belowdesign conditions The following performance curves should be obtained eitherfrom the manufacturer or during acceptance testing so that the in-depth study

of the parameters and their interdependency with each other can be defined:

1 Gas turbine compressor inlet bell-mouth pressure differential versusair flow rate

2 Gas turbine output versus compressor inlet temperature

3 Heat rate versus compressor inlet temperature

4 Fuel consumption versus compressor inlet temperature

5 Exhaust temperature versus compressor inlet temperature

6 Exhaust flow versus compressor inlet temperature

com-pressor inlet temperature

8 Gas turbine generator power output and heat rate correction as result

of water injection

9 Effect of water injection on generator output as a function of pressor inlet temperature

com-10 Effect of water injection rate on heat rate as a function of sor inlet temperature

compres-11 Ambient humidity corrections to generator output and heat rate

12 Power factor correction

13 Losses in generation due to fuel restriction resulting in operationalconstraints (e.g temperature spread, problems on fuel stroke valve,etc.)

Performance ComputationsThis section deals with the equations, and techniques used to compute andsimulate the various performance and mechanical parameters for the gasturbine power plant The goals have been to be able to operate the entirepower plant at its maximum design efficiency, and at the maximum powerthat can be obtained by the turbine without degrading the hot section life.Gas turbine power adjustments in a utility application require that themechanical speed must remain constant due to unacceptable consequences

of frequency fluctuations The control is obtained by IGV adjustments toreduce the flow at off-design loads and to maintain the high exhaust gastemperature

The gas turbine efficiency drops off quickly at part load as would beexpected, as the gas turbine is very dependent on turbine firing temperatureand mass flow of the incoming air The gas turbine heat rate increasesrapidly at part load conditions

The plant overall power and the heat rate are very dependent on the inletconditions as seen in Figure 20-8, which is based on a typical gas turbineplant The effect of temperature is the most critical component in theambient condition variations of temperature, pressure, and humidity.General Governing Equations

The four fundamental equations, which govern the properties of thecombined cycle are the equation of state, conservation of mass, momentumand energy equations

Equation of stateP

R

which can also be written as:

(con-Conservation of mass

Momentum equation for a caloricaly and thermally perfect gas, and one

in which the radial and axial velocities do not contribute to the forces

follows (Euler Turbine Equation):

Ambient Temperature (C)

0.995 1 1.005 1.01 1.015 1.02 1.025

Plant Power(%) Plant Heat Rate(%)

Figure 20-8 Plant conditions as a function of inlet ambient temperature

Energy equation for a caloricaly and thermally perfect gas the Work (W)can be written as follows:

where
U is the change in the internal energy,
PV is the change in the flowenergy,
KE is the change in kinetic energy, and
PE is the change inPotential Energy The total enthalpy is given by the following relationship:

neglecting the changes in potential energy (
PE) and heat losses due to

In the gas turbine (Brayton cycle), the compression and expansion cesses are adiabatic and isentropic processes Thus, for an isentropic adia-

pro-C p

constant pressure and volume respectively and can be written as:

values for air and products of combustion (400% theoretical air) are given inAppendix B It is important to note that the pressure measured can be eitherTotal or Static however, only Total Temperature can be measured Therelationship between total and static conditions for pressure and tempera-ture are as follows: