Figure 19-1.. Data trending to predict maintenance sc.edules.
cleaning will be required. In thiscase， two parameters were monitored， butsince their rates differed， the cleaning was based on the first parameter toreach the critical point.However， using a trend of both temperature and pressure provides a cross check on the validity of the diagnostics.
The .as Turbine
The new gas turbines are the cornerstone of the rise of the combined cycleas the power source of the new millennium， and for many other drives forpetrochemical plants. The new gas turbines have a very high-pressureratio，a high-firing temperature， and in somecases， a reheat burner in the gas turbine. The gas turbines also have new dry low ..x combustors. The combination of all these components has dramatically increased the thermal efficiency of the gas turbine. The gas turbine since the early 1960s has gone from efficiencies as low 15-17% to efficiencies around 45%. This has beendue to the pressure ratio increase from around 7:1 to as high as30:1， and an increase in the firing temperature from about 800 0C to about 1350 0C. Withthese changes， we have also seen the efficiency of the major components in the gas turbine increase dramatically. The gas turbine compressor efficiency increased from around 78-87%; the combustor efficiency from about94-98%， and the turbine expander efficiency from about 84-92%.
The increase in compressor pressure ratio decreases the operating range of the compressor. The operating range of the compressor stretches from the surge line at the low flow end of the compressor speed line to the choke point
Pressure
Ratio
Flow Rate
Table 19-4 Effect of .arious .arameters on the Output and .eat .ate
.arameter .ower .eat .ate
.arameters
Change Output ... Change ...
Ambient temperature 200F (110C) -6.5
2 Ambient pressure 4 in. H2. 0.9 0.9
0.15 psi
10 mbar Ambient relative humidity 10% -0.0002 0.0005 Pressure drop in filter 1 in. WC -0.5 -0.3
25mm. WC Increase in gas turbine 1 in. WC back pressure 25mm. WC -0.25 -0.08
at the high flow end. As seen in Figure 19-18 the lower pressure speed linehas a larger operational range than the higher-pressure speed line. Therefore，the higher-pressure ratio compressors are subject to fouling， and can resultin surge problems or blade excitation problems， which lead to blade failure.
The drop in pressure ratio at the turbine inlet due to filter fouling amounts to a substantial loss in the turbine overall efficiency and the power produced.An increase in the pressure drop of about 1 in. (25 mm). WC， amounts to a drop of about 0.3% reduction in power. Table 19-4 shows the approximate changes that would occur for changes in ambient conditions; the fouling of the inlet filtration system and the increase in back pressure on the gas turbine in a combined cycle mode. These modes were selected because these are the most common changes that occur on a system in the field. It must be remembered that these are just approximations and will vary for individual power plants.
The gas turbine has to be operated at a constant speed since this is used forpower generation， and any slight variation in speed could result in majorproblems for the grid. Thus， the control of the load has to be by controllingthe fuel input， therefore the turbine firing temperature， and the inlet guidevaneposition， thus controlling the airflow. The effect of this is to try and maintain the exhaust temperature from the gas turbine at a relatively highvalue， especially in combined cycle or cogeneration plants， since this gas isused in the H.SG， and the effectiveness of the H.SG is dependent on maintaining this temperature.
The effect of compressor fouling is also very important on the overall performance of the gas turbine since it uses nearly 60% of the work gener-ated by the gas turbine.Therefore， a 1% drop in compressor efficiency equates to nearly a 0.5% in the gas turbine efficiency and about a 0.3% drop in the overall cycle efficiency. The cleaning of these blades by on line water washing is a very important operational requirement. In manyplants， this operational procedure has contributed literally hundreds of thousands of dollars to the bottom line of the plant. It has been the experience of many plants that washing using de-mineralized water is as effective as using a detergent in on-line water wash. The practice of using abrasive cleaning byinjecting walnutshells，rice， or spent catalyst is being suspended in most newplants. Where it isused， it must be carefully evaluated; rice for instance is a very poor abrasive since it shatters and tends to get into seals and bearings and into the lubrication system. Walnut shells should never be used since they tend to collect inside the H.SG system and in some cases have been noted to catch on fire. .n-line water washing is not the answer to all theproblems since after each wash the full power is not regained， therefore a time comes when the unit needs to be cleaned off-line. The time for off-line cleaning must be determined by calculating the loss of income in power as well as the cost of labor to do so and equate it against the extra energy costs.
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