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35 As we will see in one of the case studies below, MHI (Mitsubishi) use a combination of air cooling, steam cooling open cycle and steam cooling closed cycle for their hottest airfoils. This then provides their customer base with a wide range of power developed values, all with essentially the same core geometry. Metallurgy Limits TIT The limiting factor to the maximum power to weight ratio a gas turbine can reach is the metallurgical tolerance of the alloys used in the hot section of the gas turbine. Ceramic coatings on the surfaces of the turbine airfoils can increase the peak temperatures these airfoils can tolerate, however ceramics have brittleness characteristics that have not been totally overcome yet. So the “ruling parameter” is turbine inlet temperature (TIT). TIT in turn is a function of the turbine flame/ firing temperature, compression ratio, mass flow, and centrifugal stress. So these factors limit size and ultimately, efficiency. A rough rule of thumb is that 55°C (100°F) increase in firing temperature gives a 10 to 13 percent power output increase and a 2 to 4 percent efficiency increase. The combustion chambers and the turbine first stage stationary nozzles and blades are therefore the most critical areas of the turbine that determine its power output and efficiency. Fuel Options Fuel selection also plays a major part in determining cost per fired hour, depending on its physical state and purity level. Natural gas is the most desirable fuel, as it takes least toll of the gas turbine’s component surfaces. Diesel oil (distillate) is a liquid fuel and also takes minimal (if not quite as good as natural gas’) toll of the gas turbine components. However, residual, also called “bunker” or crude oil is a viable fuel. Because of its high salt levels (sodium and potassium based), water washing is required. Also because of its Vanadium content, fuel treatment additives are required. The Vanadium salts that result take the Vanadium “out of solution” and the salts deposit on the surfaces of the turbine blades. The turbine can be washed, typically every 100 to 120 hours, and the salts are then removed. Were it not for the fuel treatment additives, the vanadium compounds that would form would form a hard coating on the turbine blades that could not be removed. For this entire system to work, TITs are kept down below 900 degrees Celsius. That TIT may be valid as base load and therefore part of a design or it may be run “derated” at the appropriate temperature until a “cleaner” fuel can be used. Aeroderivative versus Industrial Gas Turbines Industrial gas turbine compression ratios are in the order of 16:1 and aeroderivative (like their aeroengine parents) have compression ratios of about 30:1 and higher. About 50 percent of the total turbine power in any gas turbine is used to drive the compressor. Aero (and therefore aeroderivative) gas turbine designs have weight and size limitations depending on their mission profile. The minimized weight feature makes aeroderivatives highly suitable for offshore platform use, both in power generation and mechanical drive applications. Efficiency translates into fuel burn and this is a major and increasingly pertinent selling point. Rival OEMs in a specific engine size category vie for even 0.5% efficiency margin over their rivals. Design features that ultimately affect operator safety such as cooling air mass, are trimmed to the extent possible. Design engineers have been known to fight their management to get the pilots who fly their engines more cooling air, at the cost of efficiency. In severe service, such as aerobatic combat, that small margin of cooling air, can make the difference between the pilot getting home or not, especially if his engine is already severely stressed. War time conditions can and has included factors such as much heavier fuel than the aeroengines were designed for, being used. Such was the case with part of the Pegasus fleet (that power the VSTOL Harrier) during the Falklands war. In that particular case, the fleet survived the heavier fuel well, despite the fact that its TIT was higher than the industrial and marine engines that typically use the heavier fuel. Industrial gas turbines have none of the weight limitations imposed on their aero counterparts. Like the LM2500, General Electric’s (GE’s) 40 MW LM6000 is an aeroderivative based on GE’s CF6-80C2. The LM6000 has 40 percent simple-cycle efficiency and weighs 6 tons. If we consider GE’s Frame 9F, we note an output of about 200 MW with a weight of 400 tons. The contrast in power delivered to mass weight ratios between the aeroderivative and the industrial model is evident. Further the 9F is only about 34 percent efficient. High thermal efficiency (over 40 % on simple cycle and over 60 % on combined cycle are now common values for most new gas turbine systems) contributes to minimizing fuel burn and therefore minimizing environmental emissions. Even if an engine is “officially” an industrial engine, aero technology is likely to, at some point, contributed to its design. For instance, a contributor to V84.3 (Siemens Westinghouse) efficiency is the 15 stage compressor and 3 stage turbine which use aeroengine technology to optimizePDF Image | GAS TURBINES IN SIMPLE CYCLE COMBINED CYCLE APPLICATIONS
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