1q05_egt
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P R O D U C T U P D A T E
88 ORBIT [Vol.25 No.1 2005]
Gas turbine manufacturers and operators have long understood that exhaust gas temperature (EGT) pro-files contain valuable information regarding gas turbineperformance, combustion system operation, and hot gaspath component conditions. To provide access to thisinformation, we have recently enhanced the MachinePerformance application package of System 1® softwareto include EGT plots and information.
Measuring EGT Although the most direct gas temperature indicator of turbine power and turbine life is at the turbine inlet
section, temperatures there are usually too high to bemeasured with available instrumentation. Instead, theconsiderably cooler exhaust gas temperature is used,measured by placing thermocouples around the circum-ference of the turbine exhaust plenum. Thus, EGT isoften used for both turbine control and monitoring,since the gas turbine operates on a well-defined Braytonthermodynamic cycle, resulting in a consistent relation-
ship between turbine inlet and exhaust temperatures.EGT is a critical parameter to monitor for evaluating the condition of the turbine hot section and the com-bustion system. Combustion temperatures have a highly non-linear effect on the creep of turbine compo-nents, dramatically affecting their lives. A mere 50°Cincrease in turbine inlet temperature can reduce thecreep life of hot gas path components by an order of magnitude.
Since combustion is usually not uniform around thefiring section, monitoring the variance (or “spread”) of
the exhaust temperatures about the mean is just asimportant as monitoring the actual temperature. A largespread will result in the development of hot spots withinthe turbine and can lead to premature failure of turbinecomponents. An increasing temperature spread usually indicates a combustion system malfunction, such as a broken or clogged fuel nozzle.
Multiple Data Presentation Formats
To assist the operator in evaluating the EGT informa-tion, System 1 presents the data in three different
formats (Figure 1). The first format is a bargraph onthe left of the screen showing the direct temperaturemeasured by each thermocouple. This information isalso presented in a second format – a radial view on theright side of the screen. The third format is a bargraphin the center of the screen showing the temperaturevariance about the mean, allowing operators to quickly see any measurement locations with abnormally largedeviations.
Exhaust Gas Temperature CapabilitiesNow in System 1® Software
Patrick Hamilton, PE
Commercialization Manager, Performance Software
GE Energy
Ha Duong
Sr. Engineer, Performance Software
GE Energy
FIG. 1 | EGT INFORMATION IS PRESENTED IN
BOTH BARGRAPH AND RADIAL FORMATS,
SHOWING NOT ONLY DIRECT TEMPERATURE,
BUT ALSO TEMPERATURE SPREADS.
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P R O D U C T U P D A T E
[Vol.25 No.1 2005] ORBIT 89
Swirl Effect Compensation
As gas passes through the turbine it rotates. A volumeof gas exiting a single combustor at a specific instant will tend to rotate through the turbine as a cluster. Thus,the EGT pattern measured at any instant is directly related to the clusters leaving each combustor. In orderto relate a temperature variance to an individual com-bustor or group of combustors, this gas rotation or
“swirl” effect must be taken into account. The specificswirl angle is usually dependent on power and canbe determined from curves provided by the turbinemanufacturer. If the OEM-supplied data for swirl angleis available, the software displays it in the upper left-hand corner of the radial plot. It can also rotate theradial plot to account for the swirl effect, as shown inFigure 2, establishing a correct visual relationshipbetween the downstream temperature measurements
and the individual combustors. This is an especially powerful feature because it helps diagnosticians pin-point the specific combustor(s) with problems, ratherthan requiring an entire section to be inspected.
Summary
Because combustion temperature plays such a critical
role in determining the health of gas turbine compo-nents, continuous monitoring is essential. It not only allows operators to plan maintenance at optimal timesbased on actual component conditions, it also high-lights combustion problems that can degrade efficiency and waste fuel. The addition of exhaust gas tempera-ture plots to the System 1 Machine Performance appli-cation package provides operators with an easily understood visualization of this vital information.
FIG. 2 | EGT RADIAL PLOTS CAN USE OEM-SUPPLIED MODELS TO COMPENSATE FOR GAS “SWIRL” EFFECTS IN
THE TURBINE, ESTABLISHING A MORE DIRECT VISUAL RELATIONSHIP BETWEEN TEMPERATURE MEASUREMENTS
AND THEIR CORRESPONDING COMBUSTORS. THE LEFT SIDE OF THIS FIGURE SHOWS UNCOMPENSATED DATA.
THE RIGHT SIDE SHOWS THE SAME DATA ROTATED TO COMPENSATE FOR THE SWIRL EFFECT; IN THIS CASE,
58 DEGREES COUNTER-CLOCKWISE.