What Are The Design Considerations For Gas Turbines?
The gas turbine is the best suited prime mover when the needs at hand such as capital cost, time from planning to completion, maintenance costs, and fuel costs are considered. The gas turbine has the lowest maintenance and capital cost of any major prime mover.
It also has the fastest completion time to full operation of any plant. Its disadvantage was its high heat rate but this has been addressed and the new turbines are among the most efficient types of prime movers. The combination of plant cycles further increases the efficiencies to the low 60s.
The design of any gas turbine must meet essential criteria based on operational considerations. Chief among these criteria are:
1. High efficiency
2. High reliability and thus high availability
3. Ease of service
4. Ease of installation and commission
5. Conformance with environmental standards
6. Incorporation of auxiliary and control systems, which have a high degree of reliability
7. Flexibility to meet various service and fuel needs
A look at each of these criteria will enable the user to get a better understanding of the requirements. The two factors, which most affect high turbine efficiencies, are pressure ratios and temperature. The axial-flow compressor, which produces the high-pressure gas in the turbine, has seen dramatic change as the gas turbine pressure ratio has increased from 7:1 to 40:1.
The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine firing temperature. The increase in the pressure ratio increases the overall efficiency at a given temperature, however increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency. It should also be noted that the very high pressure ratios tend to reduce the operating range of the turbine compressor.
This causes the turbine compressor to be much more intolerant to dirt build-up in the inlet air filter and on the compressor blades and creates large drops in cycle efficiency and performance. In some cases, it can lead to compressor surge, which in turn can lead to a flameout, or even serious damage and failure of the compressor blades and the radial and thrust bearings of the gas turbine.
The effect of firing temperature is very predominant.for every 100 .F (55.5 .C) increase in temperature, the work output increases approximately 10% and gives about a 1..% increase in efficiency. Higher-pressure ratios and turbine inlet temperatures improve efficiencies on the simple-cycle gas turbine. Figure 1-6 shows a simple-cycle gas turbine performance map as a function of pressure ratio and turbine inlet temperature.
Another way to achieve higher efficiencies is with regenerators. Figure 1-7 shows the effects of pressure ratio and temperatures on efficiencies and work for a regenerative cycle. The effect of pressure ratio for this cycle is opposite to that experienced in the simple cycle.
Regenerators can increase efficiency as much as 15.20% at todayfs operating temperatures. The optimum pressure ratios are about 20:1 for a regenerative system compared to 40:1 for the simple-cycle at todayfs higher turbine inlet temperatures that are starting to approach 3000 .F (1649 .C).
High availability and reliability are the most important parameters in the design of a gas turbine. The availability of a power plant is the percent of time the plant is available to generate power in any given period. The reliability of the plant is the percentage of time between planned overhauls.
The basic definition of the availability of a power plant is defined as
A = (P − S − F)/P (1-1)
where:
P = Period of time, hours, usually this is assumed as one year, which amounts
to 8760 hours
S = Scheduled outage hours for planned maintenance
F = Forced outage hours or unplanned outage due to repair.
The basic definition of the reliability of a power plant is defined as
R = (P − F)/P (1-2)
Availability and reliability have a very major impact on the plant 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 in the operation of a plant.
Planned outages are scheduled for nonpeak 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, since fast turnarounds result in high availability to a turbine and reduces m
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 NOx emissions from the gas turbines. This resulted in initially attacking the NOx problem by injecting water or steam in 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 firing temperature is increased. The new low NOx combustors increase the number of fuel nozzles 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 as possible. The advanced gas turbines are all digitally controlled and incorporate on-line condition monitoring to some extent.
The addition of new on-line monitoring requires new instrumentation. Control systems provide acceleration-time and temperature-time controls for startups as well as control various anti-surge 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 criteria, which enhance a turbine system, but they are not necessary for every application. The energy shortage requires turbines to be operated at their maximum efficiency. 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 various fuels may be in shortage at different times of the year maintenance and operations costs.
Service can be accomplished by providing proper checks such as exhaust temperature monitoring, shaft vibration monitoring, and surge monitoring. Also, 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, which can be easily disassembled without removing the entire hot section, are some of the many ways that afford the 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 startups and shutdowns at commissioning greatly reduce the life of a unit.
No comments:
Post a Comment