“Availability and reliability have a very major impact on the plant’s economy. Reliability is essential in that when the power is needed it must be there. When the power is not available, it must be generated or purchased and can be very costly to the operation of a plant. Planned outages are scheduled for non-peak periods. Peak periods are when the majority of the income is generated; as usually, there are various tiers of pricing depending on the demand. Many power purchase agreements have clauses, which contain capacity payments, thus making plant availability critical in the economics of the plant. Reliability of a plant depends on many parameters, such as the type of fuel, the preventive maintenance programs, the operating mode, the control systems, and the firing temperatures. To achieve a high availability and reliability factor, the designer must keep in mind many factors. Some of the more important considerations which govern the design are: blade and shaft stresses, blade loadings, material integrity, auxiliary systems, and control systems. The high temperatures required for high efficiencies have a disastrous effect on turbine blade life. Proper cooling must be provided to achieve blade metal temperatures between 1000 ◦F (537 ◦C) and 1300 ◦F (704 ◦C), below the levels of the onset of hot corrosion. Thus, the right type of cooling systems with proper blade coatings and materials are needed to ensure the high reliability of a turbine. Serviceability is an important part of any design, because fast turnarounds result in high availability to a turbine and reduce maintenance and operations costs. Service can be accomplished by providing proper checks such as monitoring of exhaust temperature, shaft vibration, and surge. Moreover, the designer should incorporate borescope ports for fast visual checks of hot parts in the system. Split casings for fast disassembly, field balancing ports for easy access to the balance planes, and combustor cans that can be easily disassembled without removing the entire hot section, are some of the many ways that afford ease of service.
Ease of installation and commissioning is another reason for gas turbine use. A gas turbine unit can be tested and packaged at the factory. Use of a unit should be carefully planned so as to cause as few start cycles as possible. Frequent start-ups and shutdowns at commissioning greatly reduce the life of a unit.
Environmental considerations are critical in the design of any system. The system’s impact on the environment must be within legal limits and thus must be addressed by the designer carefully. Combustors are the most critical component, and great care must be taken to design them to provide low smoke and low NOx output. The high temperatures result in increasing the NOx emissions from the gas turbines. This resulted in initially attacking the NOx problem by injecting water or steam into the combustor. The next stage was the development of dry low NOx combustors. The development of new dry low NOx combustors has been a very critical component in reducing the NOx output, as the gas turbine’s firing temperature is increased. The new low NOx combustors increase the number of fuel nozzle and the complexity of the control algorithms.
Lowering the inlet velocities and providing proper inlet silencers can reduce air noise. Considerable work by NASA on compressor casings has greatly reduced noise. Auxiliary systems and control systems must be designed carefully, since they are often responsible for the downtime in many units. Lubrication systems, one of the critical auxiliary systems, must be designed with a backup system and should be
as close to failure-proof design as possible. The advanced gas turbines are all digitally controlled and incorporate online condition monitoring to some extent. The addition of new online monitoring requires new instrumentation. Control systems provide acceleration-time and temperature-time controls for start-ups as well as control various antisurge valves. At operating speeds, they must regulate fuel supply and monitor vibrations, temperatures, and pressures throughout the entire range.
Flexibility of service and fuels are the criteria that enhance a turbine system, but they are not necessary for every application. The energy shortage makes it closer to its operating point and thus it operate at higher efficiencies. This flexibility may entail a two-shaft design incorporating a power turbine, which is separate and not connected to the gasifier unit. Multiple fuel applications are now in greater demand, especially where there may be a shortage of various fuels at different times of the year.
Categories of Gas Turbines
The simple-cycle gas Turbine is classified into the following six broad groups:
1. Frame type heavy-duty gas turbines. The frame units are the large power generation units ranging from 3 to 480MW in a simple-cycle configuration, with efficiencies ranging from 30% to 48%.
2. Aircraft-derivative gas turbines. Aeroderivatives, as the name indicates, are power generation units that have origin in the aerospace industry as the prime mover of aircraft. These units have been adapted to the electrical generation industry by removing the bypass fans and adding a power turbine at their exhaust. The power of these units ranges from about 2.5 to 50MW. The efficiencies of these units can range from 35% to 45%.
3. Industrial-type gas turbines. These turbines vary in range from about 2.5 to 15MW. These are used extensively in many petrochemical plants for compressor drive trains. The efficiencies of these units are in the low 30s.
4. Small gas turbines. These gas turbines are in the range from about 0.5 to 2.5MW. They often have centrifugal compressors and radial-inflow turbines. The efficiencies of the simple-cycle applications vary from 15% to 25%.
5. Microturbines. These turbines are in the range from 20 to 350kW. The growth of these turbines has been dramatic from the late 1990s, as there is an upsurge in the distributed generation market.
6. Vehicular gas turbines. These turbines have ranged from 300 to 1,500 HP. The first vehicular turbine was built in 1954 by Chrysler Corporation and followed by the Ford Motor Company’s truck engine. The only vehicular turbine that has been very successful has been the gas turbine used in US Army Abrams Tank.
Figure 1-11 shows the distribution by manufacturers, total dollars of a $60-billion market, and the types of gas turbines that are sold throughout the world based on the data as given in the Turbomachinery 2010 Handbook. In this figure, GE is by far the largest supplier having about 49% of the market. Figure 1-12 shows the distribution by manufacturers and the number of turbines manufactured by each manufacturer; this figure shows that Solar Turbines Inc. holds the first place followed by GE.” türkçe çevirisi