An engine completing an outage may have tight blade
clearances in the turbine sections as well as new turbine
components and sealing (providing these parts were replaced).
Clearances will open up as the engine goes through several initial
thermal cycles. As shown, seals will also see most degradation during
those initial few thousand hours of operation.
This will cause the gas turbine to degrade. Once the gas
turbine is degraded it is not able to extract as much energy out of
the working fluid (mix of air and fuel) which causes the gas turbine
exhaust temperature to go up. The control curve “sees” the exhaust
temperature going up and will cause the engine to decrease the
firing temperature. If the firing temperature decreases, that’s a big
plus for the parts since now their lifecycle is extended. However, it is
detrimental to engine performance.
Figure 7 shows how decreased firing temperature due to engine
degradation affects an engine’s performance.
For the above plot, the Gas Turb model was maintained on the design
exhaust temperature control curve. Thus T4 is dropping as engine
heat rate* increases. Engine heat rate increase is modeled based on
turbine degradation only. Compressor degradation is not included.
As mentioned previously, because the turbine is not able
to extract as much work out of the working fluid, the exhaust
temperature goes up and T4 has to drop in order to stay on the
Figure 8 shows a plot of how an engine performance can be
optimized by adjusting the base-load control curve, so that an engine
operates at constant T4.
Blue and red lines are the same as from Figure 7. Purple and
green lines were developed by keeping the firing temperature at the
design level as the engine degrades.
In Figure 8, the purple line represents the unrecoverable and
expected engine degradation. The blue line represents power loss
due to both unrecoverable engine degradation and inability of the
control curve to adjust so that the engine is maintained at a constant
The benefit of monitoring cooling flows and trending firing
temperature is that when firing temperature drops, the control curve
can be adjusted and some of the gas turbine performance (and
hence output) may be re-gained.
An example of potential power recovery due to a control curve
adjustment is presented here.
That’s a potential recovery of 4MW at 8,000 hours. This value is
only an estimate because the heat rate deterioration is assumed to
be due to turbine degradation only. Compressor degradation is not
...Continued on page 54
Figure 6 – Long-Term Deterioration
of Power and Heat Rate Showing
Zone of Deterioration versus
Ref. “Performance Deterioration in
Industrial Gas Turbines.” Cyrus B.
Meher-Homji, Mustapha A. Chaker,
and Hatim M. Motiwala.
Figure 7 – Performance Behavior with Engine Degradation
Figure 8 – Impact of Maintaining Constant T4 through Engine
Table 1 – Sample Potential Recovery Calculation