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The capabilities of new coatings are initially evaluated in the laboratory on specially designed rainbow rotor test rigs to determine their corrosion resistance and effect on mechanical properties.
Another area of research is the development of techniques to ensure that the application of the coatings are extremely even. The external depositionsource can be electron beam vapor deposition， sputtering， plasmaspray，cladding， or any number of other techniques. The technique for application of overlay coatings which appears to have the most promise is high-velocityplasma. For this technique， powder particles of the desired coating composi-tion are accelerated through a plasma field to velocities as high as three times the speed of sound. The impact of the powder onto the workpiece results in a much stronger bond between the coating and workpiece than can beachieved by using conventional subsonic plasma spray deposition. In addition， much higher coating densities can be achieved using the high-velocity plasma.
One company has developed and patented a ""detonation gun'' to use for coating application.Basically， the gun detonates a metered mixture ofoxygen，acetylene， and particles of the desired coating material and throws them at supersonic velocities at the workpiece surface. The workpiece itselfremains at quite low temperatures， so its metallurgical properties are not modified.
Bibliography
Bernstein，H.L.， ""High Temperature Coatings for Industrial Gas TurbineUsers，''Proceedings of the 28th TurbomachinerySymposium， Texas A.M University; p. 179; 1999.
BernstienH.L.， ""Materials Issues for Users of GasTurbines，'' Proceedings of the27th Texas A.M TurbomachinerySymposium， (1998).
Lavoie，R.， andMcMordie，B.G.， ""Measuring Surface Finish of Compressor Airfoils Protected by Environmentally ResistantCoatings，'' 30th AnnualAerospace/Airline Plating and Metal FinishingForum， April 1994.
McMordie，B.G.， ""Impact of Smooth Coatings on the Efficiency of ModernTurbomachinery，'' 2000 Aerospace/Airline Plating . Metal Finishing ForumCincinnati，Ohio， March 2000.
Schilke，P.认.，""Advanced Gas Turbine Materials andCoatings，'' 39th GETurbine State-of the-Art TechnologySeminar， August 1996.认arnesB.M.， and HampsonL.M.， ""Extending the Service Life of Gas TurbineHardware，'' ASME 2000-GT-559.认ood，M.I.， ""Developments in Blade Coatings: Extending the Life of Blades.Reducing LifetimeCosts.，'' CCGTGeneration， March1999， IIR Ltd.
12
Fuels
The gas turbine"s major advantage has been its inherent fuel flexibility. Fuel candidates encompass the entire spectrum from gases to solids. Gas-eous fuels traditionally include naturalgas， processgas， low-Btu coalgas， and vaporized fuel oil gas. ""Process gas"" is a broad term used to describe gasformed by some industrial process. Process gases include refinerygas， pro-ducergas， coke oven gas， and blast furnace gas among others. Natural gas is the fuel of choice and is usually the basis on which performance for a gasturbine is compared， since it is a clean fuel fostering longer machine life.
Vaporized fuel oil gas behaves very closely to natural gas because it provides high performance with a minimum reduction of component life. About 40% of the turbine power installed operates on liquid fuels. Liquid fuels can vary from light volatile naphtha through kerosene to the heavy viscous residuals. The classes of liquid fuels and their requirements are shown in Table 12-1.
The light distillates are equal to natural gas as afuel， and between lightdistillatesandnaturalgasfuels， 90% ofinstalledunitscanbecounted. Caremust be taken in handling liquid fuels to avoid contamination， and the very light distillates like naphtha require special concern in the design of fuelsystems because of their high volatility. Generally， a fuel tank ofthe floating head type with no area for vaporization is employed. The heavy true dis-tillates like #2 distillate oil can be considered the standard fuel. The truedistillate fuel is a good turbine fuel; however， because trace elements ofvanadium， sodium， potassium，lead， and calcium are found in thefuel， the fuel has to be treated. The corrosive effect of sodium and vanadium is very detrimental to the life of a turbine.
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