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Intermittent operation and peak-load operation lead to more problems and reduce the life of many of the hot parts. Peak-load units account for only about 20% of the units above the 10 MW range. The smaller units (between 1 and 10 MW) are usually operated as standby power generators or for compressor drives mostly on offshore platforms. Small-unit repair costsaverage about.55，000-.100，000. These units are very rarely used for peak-ing services.
Fuel nozzles clog easily and can comeloose， creating a large flow offuel， which ignites and can lead to severe burning problems. A number of suchcases has occurred in dual-fuel units. In most of thesecases， the fuel nozzleloosens and comes off. Because of design constraints， the nozzle does not gothrough the turbine， but allows a very large amount of fuel to enter the combustor can. This fuel is then transported to the transition piece andtoward the first-stage nozzles. The first-stage nozzles act as flame holders，causing the fuel to ignite and create a largeflame， which burns out the first-stage nozzle and rotor blades. Figure 21-15 shows the burnt first-stage rotor blades. Note how evenly the blades have been burned. It is obvious from these photographs that the flame was angled and that the first-stage nozzle acted as a flame holder. Figure 21-16 shows the damage on the first-stagenozzle， which acted as a flame holder. Note that the retainer disc has been melted due to the intense heat.
Another common combustion problem concerns the crossover tubes. Cross-over tubes are used in can-annular combustors to assure combustion in all chambers and to equalize pressure. Many times the flow of hot gasesthrough the crossover tubes is increased due to blocked fuel nozzles， which can lead to tube failures as shown in Figure 21-1..
Liner cracks can be caused by liquids in the fuel or blockednozzles， whichcan create hot sections in the liner as seen in Figure 21-18. In many turbines， the injection of steam or water in the combustor can has been used to meet

Figure 21-15. .urnt first-stage turbine blades. .ote evenness of burn.

Figure 21-1.. .urnt first-stage no..le. Figure 21-17. Damaged crossover tubes.

NOx emission requirements. This injection of steam reduces the temperaturein the hot section， thus reducing the amount of NOx produced. Whensprayed through the fuelnozzle， this steam can impinge on theliner， thuscreating a temperature gradient， which can lead to cracks. Steam injection-whether it is required for NOx control or for extra power (5% steam by weight will produce 12% more work and increase efficiency a few percent)-must inject steam into the compressor diffuser to be safe and effective. This process will allow the steam to be fully mixed with the air before it enters thecombustor， reducing the incidence of liner failures due to steam injection.
Nozzle bowing of the first-stage turbine nozzle is another common pro-blem. Bowing can be caused by uneven combustion or loss of cooling air to the nozzle and can decrease turbine efficiency by changing the air velocity and angles leaving the nozzles. Another problem with turbine nozzles occurs with the second or downstream nozzles. This problem is due to liquids entrained in the fuels or ignition failure at startup. Liquid hydrocarbons are entrained in the fuel impinge onthe turbine， causing hot spots and leading to cracked blades. Ignition failure at startup can lead to an accumu-lation of the fuel in pockets. When combustion finally occurs， it creates an

Figure 21-18. Cracks in a combustor liner.
explosion and.or fire where the fuel is trapped. This trapping of the fuel occurs in areas where the velocity is lower and the blades act as flame holders.Thus， the second-stage nozzles are an ideal candidate for this problem. The problem can be avoided by purging the fuel after a failure ofthe turbine to ignite. This function can be automatic or manual..sually， a five-minute interval is required and at least five times the total air volume must be changed before another startup can be attempted.
　
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