AIRCRAFT DERIVATIVE GAS TURBINES BASIC INFORMATION AND TUTORIALS

Aircraft-Derivative Gas Turbines - What Is It?

Aero-derivative. As the name indicates, these are power generation units, which originated 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. These units range in power from 2.5 MW to about 50 MW. The efficiencies of these units can range from 35–45%.

Aero-derivative gas turbines consist of two basic components: an aircraft derivative gas generator, and a free-power turbine. The gas generator serves as a producer of gas energy or gas horsepower. The gas generator is derived from an aircraft engine modified to burn industrial fuels. Design innovations are usually incorporated to ensure the required long-life characteristics in the ground based environment.

In case of fan jet designs, the fan is removed and a couple of stages of compression are added in front of the existing low-pressure compressor. The axial-flow compressor in many cases is divided into two sections: a low pressure compressor followed by a high-pressure compressor.

In those cases, there are usually a high-pressure turbine and a low-pressure turbine, which drive the corresponding sections of the compressor. The shafts are usually concentric thus the speeds of the high-pressure and low-pressure sections can be optimized.

In this case, the power turbine is separate and is not mechanically coupled; the only connection is via an aerodynamic coupling. In these cases, the turbines have three shafts, all operating at independent speeds. The gas generator serves to raise combustion gas products to conditions of around 45–75 psi (3–5 Bar) and temperatures of 1300–1700 ◦F (704–927 ◦C) at the exhaust flange.

Figure 1-10 shows a cross section of an aero-derivative engine.

Both the Power Industry and the petrochemical industries use the aircraft-type turbine. The Power Industry uses these units in a combined cycle mode for power generation especially in remote areas where the power requirements are less than 100 MW.

The petrochemical industry uses these types of turbines on offshore platforms especially for gas re-injection, and as power plants for these offshore platforms, mostly due to their compactness and the ability to be easily replaced and then sent out to be repaired.

The aero-derivative gas turbine also is used widely by gas transmission companies and petrochemical plants, especially for many variable speed mechanical drives. These turbines are also used as main drives for Destroyers and Cruise Ships. The benefits of the aero-derivative gas turbines are:

1. Favorable installation cost. The equipment involved is of a size and weight that it can be packaged and tested as a complete unit within the manufacturer’s plant. Generally, the package will include either a generator or a driven pipeline compressor and all auxiliaries and control panels specified by the user. Immediate installation at the job site is facilitated by factory matching and debugging.

2. Adaptation to remote control. Users strive to reduce operating costs by automation of their systems. Many new offshore and pipeline applications today are designed for remote unattended operation of the compression equipment.

Jet gas turbine equipment lends itself to automatic control, as auxiliary systems are not complex, water cooling is not required (cooling by oil-to-air exchanges), and the starting device (gas expansion motor) requires little energy and is reliable. Safety devices and instrumentation adapt readily for purposes of remote control and monitoring the performance of the equipment.

3. Maintenance concept. The off-site maintenance plan fits in well with these systems where minimum operating personnel and unattended stations are the objectives. Technicians conduct minor running adjustments and perform instrument calibrations.

Otherwise, the aero-derivative gas turbine runs without inspection until monitoring equipment indicates distress or sudden performance change. This plan calls for the removal of the gasifier section (the aero-engine) and sending it back to the factory for repair while another unit is installed.

The power turbine does not usually have problems since its inlet temperature is much lower. Downtime due to the removal and replacement of the Gasifier turbine is about eight hours.

FRAME TYPE HEAVY DUTY GAS TURBINES BASIC INFORMATION AND TUTORIALS

Frame Type Heavy-Duty Gas Turbines - What is it?

Frame Type Heavy-Duty Gas Turbines. The frame units are the large power generation units ranging from 3 MW to 480 MW in a simple cycle configuration, with efficiencies ranging from 30–46%.

These gas turbines were designed shortly after World War II and introduced to the market in the early 1950s. The early heavy-duty gas turbine design was largely an extension of steam turbine design.

Restrictions of weight and space were not important factors for these ground-based units, and so the design characteristics included heavy-wall casings split on horizontal centerlines, sleeve bearings, large-diameter combustors, thick airfoil sections for blades and stators, and large frontal areas.

The overall pressure ratio of these units varied from 5:1 for the earlier units to 35:1 for the units in present-day service. Turbine inlet temperatures have been increased and run as high as 2500 ◦F (1371 ◦C) on some of these units. This makes the gas turbine one of the most efficient prime movers on the market today reaching efficiencies of 50%.

Projected temperatures approach 3000 ◦F (1649 ◦C) and, if achieved, would make the gas turbine even a more efficient unit. The Advanced Gas Turbine Programs sponsored by the U.S. Department of Energy has these high temperatures as one of its goals.

To achieve these high temperatures, steam cooling is being used in the latest designs to achieve the goals of maintaining blade metal temperatures below 1300 ◦F (704 ◦C) and prevent hot corrosion problems.

The industrial heavy-duty gas turbines employ axial-flow compressors and turbines. The industrial turbine consists of a 15–17 stage axial-flow compressor, with multiple can-annular combustors each connected to the other by cross-over tubes.

The cross-over tubes help propagate the flames from one combustor can to all the other chambers and also assure an equalization of the pressure between each combustor chamber.

The earlier industrial European designs have single stage side combustors. The new European designs do not use the side combustor in most of their newer designs. The newer European designs have can-annular or annular combustors since side (silo type) combustors had a tendency to distort the casing.

Figure 1-8 is a cross-sectional representation of the GE Industrial Type Gas Turbine, with can-annular combustors, and Figure 1-9 is a crosssectional representation of the Siemens Silo Type Combustor Gas Turbine. The turbine expander consists of a 2–4-stage axial-flow turbine, which drives both the axial-flow compressor and the generator.

The large frontal areas of these units reduce the inlet velocities, thus reducing air noise. The pressure rise in each compressor stage is reduced, creating a large, stable operating zone.

The auxiliary modules used on most of these units have gone through considerable hours of testing and are heavy-duty pumps and motors. The advantages of the heavy-duty gas turbines are their long life, high availability, and slightly higher overall efficiencies.

The noise level from this type of turbine is considerably less than an aircraft-type turbine. The heavy duty gas turbine’s largest customers are the electrical utilities, and independent power producers. Since the 1990s the industrial turbines have been the bulwarks of most combined cycle power plants.

The latest frame type units introduced are 480-MW units using steam cooling in the combined cycle mode, enabling the firing temperatures to reach 2600 ◦F (1427 ◦C). This enables efficiency in the combined cycle mode to reach 60% plus.

GAS TURBINE DESIGN CONSIDERATIONS BASIC INFORMATION AND TUTORIALS

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 todayfs operating temperatures. The optimum pressure ratios are about 20:1 for a regenerative system compared to 40:1 for the simple-cycle at todayfs 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.