Gas Turbines -Application and Experience

Abstract

Abstract Gas turbine engines, which convert chemical energy to mechanical energy, have become increasingly popular for industrial uses during the past few years. Oil and gas are used as fuel for the turbine. As the demand for turbine engines grows, new markets for oil and gas products will be created. A description of the basic theory, construction and performance of the gas turbine engine, with emphasis on oil and gas field applications, is presented. Introduction In recent years, the gas turbine engine has captured a significant portion of the industrial prime mover market. The "1967 Annual Gas Turbine Catalog" lists a total of over 35 million industrial gas turbine horsepower now in use. The total was only 24 million hp in 1966. The population of industrial gas turbine horsepower increased 50 percent in 1 year - proof of the increasing acceptance of the industrial gas turbine engine. Gas turbines convert chemical energy to mechanical energy. Natural gas and petroleum products are the source of chemical energy for gas turbines. In a larger sense, the oil and gas industry is interested in every turbine sold. As turbine technology improves, new markets will be created for oil and gas products. More directly, the oil and gas industry itself provides a great part of the existing turbine market. Basic theory, construction, performance and application of the modern gas turbine is reviewed. Oil and gas field applications are emphasized. Basic Theory The principles of the gas turbine are simple, and conclusions based on knowledge of these principles define performance. Although gas turbine theory was patented over 150 years ago, metallurgy and hardware has hindered development. Commercial industrial turbine engines were not available until the last few decades. The simple cycle turbine has three major parts: air compression, combustion and power recovery. Fig. 1 shows a gas turbine cut-away. Air enters an axial compressor and is compressed to a ratio of about 6:1 (75 psig at sea level). Instead of an axial compressor, centrifugal compressor could be used that would provide a lower first cost, but lower efficiency. The air flow then enters the combustor where fuel is added and burned in the air stream. There is no further increase in pressure during combustion; however, temperature increases considerably. The hot gas stream then expands through power turbine wheels that recover mechanical energy from the gas flow. These wheels are on a common shaft with the air compressor, thereby driving it. Additional power is taken from the same shaft to drive the external load. Gas turbines are thermally rated machines. If the fuel into the turbine is sufficient to burn all of the air compressed (stochiometric combustion), the power turbine blading and nozzles will be continuously immersed in a flow of about 3,500F gas. Present metallurgy has not produced commercial blading material that can withstand the necessary forces at this temperature and provide long life. Industrial gas turbines presently limit power turbine blade and nozzle temperature to about 1,450F or less, in order to allow long life of hot parts. To maintain this temperature level at the power turbine blades, only about a fifth of the air compressed is burned. Thus, relatively large air flow and compressor horsepower are required. A 1,000-air turbine contains a 2,000-hp air compressor. The power turbine wheels develop 3,000 hp, but only 1,000 hp is available for the external driven load. Also, since only one-fifth of the air compressed is burned, the exhaust has high oxygen content, When this exhaust heat can be commercially used, over-all thermal efficiency is extremely attractive. Because turbines are thermally rated and the bulk of the air compressed is used only as a mass for cooling, several other performance characteristics are known. Turbines operating above sea level use less dense air, and the cooling air mass rate is reduced. This reduces allowable rate of fuel input; horsepower must be de-rated in proportion to ambient pressure. Also, as ambient temperature decreases, the turbine air becomes denser and colder and allows an increase in rate of fuel input. Therefore, available turbine horsepower increases as ambient temperature decreases. JPT P. 833ˆ