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Heavy Fuels
With heavy fuels， the ambient temperature and the fuel type must be considered.Even at warm environmental temperatures， the high viscosity of the residual could require fuel preheating or blending. If the unit isplanned for operation in extremely cold regions， the heavier distillates could become too viscous. Fuel system requirements limit viscosity to 20 centi-stokes at the fuel nozzles.
Fuel system fouling is related to the amount of water and sediment in the fuel. A by-product of fuel washing is the desludging of the fuel. Washing ridsthe fuel of those undesirable constituents that causeclogging， deposition， and corrosion in the fuel system. The last part of treatment is filtration just prior to entering the turbine. Washed fuel should have less than .025% bottom sediment and water.
Frequently， no visible smoke and no carbon deposition are design para-meters. Smoke is anenvironmentalconcern， while excessive carbon can impair the fuel spray quality and cause higher liner temperatures due to the increased radiation emissivity of the carbon particles as compared to the surrounding gases. Smoke and carbon are a fuel-related property. The hydrogen saturation influences smoke and free carbon. The less-saturated fuels like benzene (C6H6) tend to be smokers; the better fuels like methane (CH4) are saturated hydrocarbons. This effect is shown in Figure 12-8. Boiling temperature is a function of molecular weight. Heavier molecules tend to boil at a higher temperature. Since a less-saturated molecule willweigh more (higher molecular weight)， one can expect residuals and heavy distillates to be smokers. This expectation is founded in practice. The designsolution pioneered by General Electric on its LM2500， which has an annularcombustor as shown in Figure12-9， was to increase flow and swirl through the dome surrounding the fuel injector. The increased flow helped to avoid rich pockets and promoted good mixing. The axial swirler achieved a no-smoke condition and reduced liner temperature.

Figure 12-8. Effect of hydrogen saturation in primary f.ow on smoke.

Figure 12-9. Cross section of an annu.ar combustor showing high dome f.ow configuration. (Courtesy of Genera. E.ectric Company..
Special consideration must be afforded to the combustion chamber walls. Low-grade fuels tend to release a higher amount of their energy as thermalradiation instead of heat. This energy release， coupled with the large dia-meter of the single can and the formation of carbon deposits， can lead to an over-heating problem on the liner. One vendor advocates the use of metallic tiles as combustor liners. The tiles hook into the wall in slots provided for them. The tiles have fine-pitched fins cast on the back. The fins form a double-wall structure by bridging the gap between the flame-tube wall andthe tile. This annulus is fed byair， thus providing a strong cooling action. The standard sheet metal design was abandoned due to warpage.
Cleaning of Turbine Components
A fuel treatment system will effectively eliminate corrosion as a majorproblem， but the ash in the fuel plus the added magnesium does cause deposits in the turbine. Intermittent operation of 100 hours or less offersno problem， since the character of the deposit is such that most of it shedsupon refiring， and no special cleaning is required. However， the deposit does not reach a steady-state value with continuous operation and gradually plugs the first-stage nozzle area at a rate of between 5% and 12% per 100 hours.Thus，at present， residual oil use is limited to applications wherecontinuous operation of more than1，000 hours is not required.
If the need exists to increase running time between shutdowns， the turbine can be cleaned by the injection of a mild abrasive into the combustionsystem. Abrasives include walnut shells，rice， and spent catalyst. Rice is avery poor abrasive， since it tends to shatter into small pieces. Usually， a 10% maximum blockage of the first-stage nozzle is tolerated before abrasive cleaning is initiated. Abrasive cleaning will restore 20-40% of the lost power by removing 50% of the deposits. If the frequency of abrasive injection becomes unacceptable and cannot prevent the nozzle blockage from becom-ing more than 10%， water washing becomes necessary. Water or solvent washing can effectively restore 100% of the lost power. A typical operating plot is shown in Figure 12-10.
　
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