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Gas turbine engines
Gas turbines range in size from microturbines at < 50 hp (37.3 kW) to large industrial turbines of > 250,000 hp (190 kW). This page focuses on the gas turbine engine, the differences between types of turbines, and items to consider when they are applied as the prime mover.
As shown in Fig. 1 and Fig. 2, the “open” Brayton cycle is the thermodynamic cycle for all gas turbines. This cycle consists of:
- Adiabatic compression
- Constant pressure heating
- Adiabatic expansion
The gas turbine is made up of the following components:
- An air compressor
- A combustor
- A power turbine, which produces the power to drive the air compressor and the output shaft
Air enters the compressor inlet at ambient conditions (Point 1), is compressed (Point 2), and passes through the combustion system, where it is combined with fuel and “fired” to the maximum cycle temperature (Point 3). The heated air is expanded through the gas producer turbine section (between Points 3 and 5), where the energy of the working fluid is extracted to generate power for driving the compressor, and expanded through the power turbine to drive the load (Point 7). The air is then exhausted to the atmosphere. A starting system is used to get the air compressor up to sufficient speed to supply air for combustion with the fuel injected into the combustor. A turbine’s continuous-burning combustion cycle, combined with continuous rotation of the turbine rotor, allows virtually vibration-free operation, as well as fewer moving parts and wear points than other prime movers.
Design consideration and operation
Maximum cycle temperature, TRIT
The output power of a gas turbine may be increased by increasing the maximum cycle temperature. The maximum cycle temperature is designated TRIT, which stands for turbine rotor inlet temperature. API 616 defines rated firing temperature as the vendor’s calculated turbine inlet temperature (TIT) immediately upstream of the first-stage turbine rotor for continuous service at rated power output. TRIT is calculated immediately upstream of the first-stage turbine rotor and includes the calculated effects of cooling air and temperature drop across the first-stage stator vanes.
The output power of a gas turbine may also be increased by increasing the mass flow of air through the gas turbine. The geometry of the gas turbine, particularly the compressor, and the speed of the compressor dictate basic air mass flow. An increase in flow requires an increase in speed, which is limited to the maximum continuous running speed of any particular design. At a given speed, an increase in inlet air density increases air mass flow. Inlet air density increases directly with barometric pressure and inversely with ambient temperature.
The main parameters affecting output power are speed and TRIT for any given mechanical/aerodynamic design. Increasing any one of these parameters increases the output power capacity of the gas turbine. Speed and temperature may be dictated by the output power and heat rate desired within the constraints imposed by the following factors:
- Component life
- Technical feasibility
As the speed of a gas turbine increases, the centrifugal forces on the rotating components increase. These forces increase the stress on the rotating components, particularly the following:
- Blade attachment to the disk
Component materials have stress limits that are directly proportional to their speed limits and should not be exceeded. Thus, the maximum continuous speed of the rotating element is a function of:
- Rotor geometry
- Component material properties
- Safety design factors
It is the highest allowable speed for continuous operation.
One way to increase output power is to increase the fuel flow and therefore TRIT. As TRIT increases, hot section components operate at higher metal temperatures, which reduces the time between inspection (TBI) of the gas turbine. Because the life of hot section materials is limited by stress at high temperature, there are limitations on the maximum temperatures for a given TBI. Material life decreases rapidly at higher temperatures. TBI is a function of time at TRIT and the rate of TRIT change during transients such as startup. The creep or stress rupture limit is established by the material properties as a function of their stress level and operating temperature.
A rating point can be established for determining gas turbine performance for specified ambient conditions, duct losses, fuel, etc.
The International Standards Organization defines its standard conditions as:
- 1.013 bar
- 60% relative humidity with no losses
This has become a standard rating point for comparing turbines of various manufacturers and designs.
The site rating is a statement of the basic gas turbine performance under specific site conditions, including:
- Ambient temperature
- Duct pressure losses
- Emission controls
- Fuel composition
- Auxiliary power takeoff
- Compressor air extraction
- Output power level
For instance, an increase in ambient temperature reduces output power at a rate influenced by gas turbine design.
Inlet air temperature
Fig. 3 relates the following to inlet air temperature at optimum power turbine speed for an example gas turbine:
- Output power
- Fuel flow
- Exhaust temperature
- Exhaust flow
Increasing turbine efficiency
Most of the mechanical energy extracted from the gas stream by the turbine is required to drive the air compressor, with the remainder available to drive a mechanical load. The gas stream energy not extracted by the turbine is rejected to the atmosphere as heat.
In the recuperative cycle, also called a regenerative cycle, the compressor discharge air is preheated in a heat exchanger or recuperator, the heat source of which is the gas turbine exhaust. The energy transferred from the exhaust reduces the amount of energy that must be added by the fuel. In Fig. 4, the fuel savings is represented by the shaded area under 2 to 2′. The three primary designs used in stationary recuperators are the:
- Plate fin
- Shell and tube
- Primary surface
Adding a steam bottoming cycle to the Brayton cycle uses the exhaust heat to produce additional horsepower, which can be used in a common load, as shown in Fig. 5, or for a separate load. The shaded area represents the additional energy input.
Air inlet system
Inlet Air Filtration. The quality of air entering the gas turbine is a very important design consideration. Turbine efficiency will decrease over time because of deposits building up on the turbine internal flow path and rotating blades. This buildup results in increased maintenance and fuel consumption. Selecting and maintaining the proper inlet air filtration system for the specific site conditions will affect the rate of decrease of efficiency over time.
It is critical to minimize the pressure drop of the air passing through the: Inlet ducting Inlet air filter Inlet silencer (see Noise Attenuation below)
Pressure loss on the atmospheric air entering the turbine greatly affects the performance of the gas turbine.
The noise produced by a gas turbine is primarily in the higher-frequency ranges, which are not transmitted as far as the lower-frequency noises produced by slower-speed prime movers such as reciprocating engines. Most high-frequency noise produced by the turbine is generated in the air inlet, with a smaller amount coming from the exhaust. The sources of noise and method of attenuation are as follows:
The inlet silencer should be specifically designed to the noise profile of the gas turbine and the site requirements. This silencer is installed in the air inlet ducting between the air filter and the turbine air compressor inlet.
The exhaust silencer should be specifically designed to the noise profile of the gas turbine and the site requirements. The exhaust stack height in conjunction with the silencer is an important consideration. Discharging the hot exhaust gases as high as practical reduces the measurable noise at ground level plus has the added benefit of reducing the chance of recirculation of the hot exhaust back into the air inlet. Pressure loss (backpressure) on the exhaust of the turbine greatly affects the performance of the gas turbine.
Casing/gear box/driven equipment
Sound-attenuating enclosure(s) can be installed directly over the equipment such as skid-mounted walk-in enclosures or a building containing the equipment insulated to meet the requirements or both.
The most common method of cooling the oil is the use of air exchanger/fan coolers. These generate fan noise that can be controlled with fan tip speed. The use of shell and tube water coolers can be noise-efficient if the cooling media is available.
Types of gas turbines
Turbine designs can be differentiated by:
- Type of duty
- Combustor types
- Shaft configuration
- Degree of packaging
Types of duty
Aircraft turbine engines
Aircraft turbine engines or jet engines are designed with highly sophisticated construction for light weight specifically for powering aircraft. These designs require maximum horsepower or thrust with minimum weight and maximum fuel efficiency. Aircraft turbines have roller bearings and high firing temperatures requiring exotic metallurgy. They can be operated on a limited variation of fuels. When a jet engine is used in an industrial application, it must be coupled with an independent power turbine to produce shaft power.
Heavy industrial gas turbine engines
The basic design parameters for heavy industrial gas turbine engines evolved from industrial steam turbines that have slower speeds, heavy rotors, and larger cases than jet engines to ensure longer life. These gas turbines are capable of burning the widest range of liquid or gas fuels.
Light industrial gas turbine engines
The basic design parameters and technology used in aircraft turbines can be combined with some of the design aspects of heavy industrial gas turbines to produce a lighter-weight industrial turbine with a life approaching that of a heavy industrial gas turbine. These engines are called light industrial gas turbine engines.
Radial or annular combustor
This combustor surrounds the gas turbine rotating parts and is integral to the engine casing (Fig. 6). Aircraft turbines and light industrial gas turbines use this design.
This is a single- or multi-combustion system that is separated from the rotating turbine as external combustion cans (Fig. 7). Designs using this type of combustor can burn a wider range of fuels.
The gas turbine can have either a single-shaft or a two-shaft design. The single-shaft design consists of one shaft connecting the air compressor, gas producer turbine, and power turbine as one rotating element (Fig. 1). This design is best suited for constant-speed applications such as driving electric generators for a constant frequency.
The two-shaft design has the air compressor and gas producer on one shaft and the power turbine on a second independent shaft. This design provides the speed flexibility needed to cover a wider performance map of the driven equipment more efficiently. This allows the gas producer to operate at the speed necessary to develop the horsepower required by the driven equipment such as centrifugal compressors or pumps. Fig. 6 shows a cutaway view of a typical two-shaft gas turbine. Major components include the compressor, combustion system, gas producer turbine, and power turbine. This design includes a two-stage gas producer turbine and a two-stage power turbine.
Degree of packaging
The norm for most gas turbines used in industry consists of incorporating the gas turbine into a base frame/skid with all the components required for the basic operational unit. This includes such systems as the:
- Start system
- Fuel system
- Lubrication system
- Local controls
- In some cases the gear box and driven equipment
Additional operationally required systems are all generally separate pre-engineered packaged systems that can be provided and customized by the turbine manufacturer. Included in this category are systems such as:
- Air inlet filtration/silencing
- Oil coolers
- Remote control systems
- Sound-attenuated enclosures
- Exhaust silencers
Deterioration of the atmosphere by gaseous pollutants is an important environmental issue. The gas turbine by basic cycle design gives a cleaner combustion and produces a lower level of pollutant compared with other prime movers, which is a major advantage. The gas turbine pollutants that typically are regulated are:
- Oxides of nitrogen
- Carbon monoxide
- Unburned hydrocarbons
- Sulfur dioxide
The solution to some, but not all, of these pollution problems lies within the gas turbine combustor. A brief discussion follows.
Oxides of nitrogen (NOx)
Only two of the seven oxides of nitrogen are regulated: NO and NO2, referred to collectively as NOx. Almost all emission concerns involving prime movers relate to NOx production and NOx controls. The gas turbine is relatively clean compared with other prime movers. For example, gas turbines burning natural gas generally produce 4 to 12 times less NOx per unit of power than reciprocating engines produce. However, NOx is the major factor in permitting gas turbine installations.
Carbon monoxide (CO)
CO is also at a very low level in turbine exhaust because of the excess air in the combustion process. Therefore, it is usually not a problem. However, in some areas where the ambient level of CO is extremely high or when water injection is being used for NOx control in the gas turbine, CO may be a factor in obtaining permits.
Unburned hydrocarbons (UHC)
Unlike reciprocating engines that produce a significant amount of UHC, gas turbines produce a low amount of UHC because the large amount of excess air involved in the gas turbine combustion process completely combusts almost all the hydrocarbons. Consequently, UHC emissions are rarely a significant factor in obtaining environmental permits for gas turbines.
No particulate measuring techniques have been perfected that produce meaningful results on gas turbine exhausts. This is rarely a factor in obtaining permits for gas turbines when clean fuels are burned in the gas turbine.
Sulfur dioxide (SO2)
Almost all fuel-burning equipment, including gas turbines, converts all the sulfur contained in the fuel to SO2. This makes SO2 a fuel problem rather than a problem associated with the characteristics of the turbine. The only effective way to control SO2 is by limiting the amount of sulfur contained in the fuel or by removing the SO2 from the exhaust gases by means of a wet scrubbing process.
The need to meet or surpass the emission standards set by federal, state, and local codes has required industrial gas turbine manufacturers to develop cleaner-burning turbines. Dry emission systems have been developed with lean-premix fuel injectors, special combustion technology, and controls for reducing emissions of NOx and CO by creating lower maximum flame temperatures and more complete oxidation of hydrocarbon fuels. All industrial gas turbine manufactures have dry low emission products. The performance varies with the individual product because of differences in combustor design.
These lean-burn systems reduce the formation of NOx and CO to very low levels, thus making it unnecessary to use expensive high-maintenance catalytic converters to eliminate NOx and CO after they are formed. In extreme high-attainment areas, it may be necessary with some gas turbines to use selective catalytic converters to further reduce the level of NOx and CO. The fuel of choice for the gas turbine is clean dry natural gas, which produces the cleanest exhaust.
Gas turbines have most of the heat loss from the cycle going out the exhaust. This heat can be recovered and used to increase the overall thermal efficiency of the fuel burned. The most common method of exhaust heat use is in the production of steam.
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