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 FLOW MEASUREMENT BASIC INFORMATION AND TUTORIALS

How to measure the flow in a gas turbine?

Gas flow through the compressor is measured by flow nozzles or other devices installed in the piping. Among the various devices are:

1. Orifice plates. Either the concentric orifice, eccentric orifice, or segmented orifice-type. Choice depends on the quality of the fluid handled.

2. Venturi tubes. These consist of a well-rounded convergent section at the entrance, a throat of constant diameter, and a divergent section. Their accuracy is high; however, installation, unless planned for in advance, is very difficult in the field.

3. ASME flow nozzles. These nozzles provide for accurate measurements. Their use is limited because they are not easily placed in a process plant; however, they are excellent for shop tests. Venturi meters and nozzles can handle about 60% more flow than orifice plates with varied pressure losses.

4. Elbow flow meters. The principle of centrifugal force at the bend is used to obtain the difference in pressure at the inside and outside of the elbow, which is then related to the discharge pressure.

5. Turbine flow meters. The principle of this flow meter is the computation of the revolutions of the turbine wheel in a given time frame.

Other techniques for measuring flow through the compressor include:

1. Calibrated pressure drops from the inlet flange to the eye of the first stage impeller in centrifugal compressors, when such data are available from the manufacturer.

2. A flow trace technique in which Freon is injected into the constream, and flight time between two detection points is measured. . Velocity traverse techniques must be used when, due to the configuration in piping, nozzles, or orifice plates, etc., cannot be used.

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

Nozzle arrangements for various applications vary considerably. For subcritical flow measurement at the outlet end, where nozzle differential pressure p is less than the barometric pressure, flow should be measured with impact tubes and manometers.

GAS TURBINE PERFORMANCE CODES BASIC INFORMATION AND TUTORIALS

Performance Codes used to Measure Turbine

Performance analysis is not only extremely important in determining overall performance of the cycle but also in determining life cycle considerations of various critical hot section components.

The following three ASME Test Codes govern the test of a Gas Turbine Power Plant:

ASME, Performance Test Code on Overall Plant Performance, ASME PTC 46 1996, American Society of Mechanical Engineers 1996 2.

ASME, Performance Test Code on Test Uncertainty: Instruments and Apparatus PTC 19.1, 1988
3. ASME, Performance Test Code on Gas Turbines, ASME PTC 22 1997, American Society of Mechanical Engineers 1997.

The ASME, Performance Test Code on Overall Plant Performance, ASME PTC 46, was designed to determine the performance of the entire heat cycle as an integrated system. This code provides explicit procedures for determination of power plant thermal performance and electrical output.

The ASME, Performance Test Code on Test Uncertainty: Instruments and Apparatus PTC 19.1 specifies procedures for evaluation of uncertainties in individual test measurements, arising from both random errors and systematic errors, and for the propagation of random and systematic uncertainties into the uncertainty of test results.

The various statistical terms involved are defined. The end result of a measurement uncertainty analysis is to provide numerical estimates of systematic uncertainties, random uncertainties, and the combination of these into a total uncertainty with an approximate confidence level.

This is especially very important when computing guarantees in plant output and plant efficiency.
The PTC 22 establishes a limit of uncertainty of each measurement required;

the overall uncertainty must then be calculated in accordance with the procedures defined in ASME PTC 19.1 Measurement Uncertainty. The code requires that the typical uncertainties be within a 1.1% for the Power Output, and 0.9% in the heat rate calculations.

It is very important that the post-test uncertainty analysis should also be performed to assure the parties that the actual test has met the requirement of the code.

The instrumentation will be calibrated as per the requirements of the test codes. All the instrumentation must be calibrated before a test and certified that they meet the code requirements. The ASME PTC 19 series outlines the governing requirements of all instrumentation for an ASME Performance Test to be within the governing band of uncertainty.

GAS TURBINE TEMPERATURE MEASUREMENTS BASIC INFORMATION AND TUTORIALS

Temperature Measurement Techniques of Gas Turbines

Temperature may be measured by any of the following instruments:

1. Mercury-in-glass thermometers

2. Thermocouples

3. Resistance thermometers

4. Thermometer wells

Thermocouples are the preferred type of instruments because of the simplicity in basic design and operation. They can attain a high level of accuracy, are suitable for remote reading, and are robust and relatively inexpensive.

Regardless of the temperature-measuring device to be used, on-site calibration of the entire measurement system is desirable. Usually, a two-point check can be made by employing frozen and boiling water. 

At the very least, all devices can be checked at a common temperature, preferably in the midrange of expected temperatures so that any deviant devices can be discarded. This check is particularly desirable for low-head machines where the temperature rise will be slight.

Test plans frequently are prepared on the assumption that a laboratory thermometer can replace an operating instrument in an existing thermometer well.

While this change may be satisfactory, the prudent tester needs to be aware that because of the propensity of thermowells to break off and perhaps enter the machine or cause a hazardous leak, their design is compromised such that true gas temperature determination is impossible. The compromise may be to make the well short and/or to make it thick-walled. 

In either event the mass of metal exposed to ambient temperature may exceed that exposed to the gas, resulting in significant error if the gas temperature is much different from the ambient temperature. High-pressure systems requiring thick-wall pipe are particularly susceptible to this fault. However, the use of a good heat-transfer fluid can minimize the error. 

The best gas temperature reading is attained by a calibrated fine-wire thermocouple with the junction directly exposed to the gas near the center of the flow. As deviations from this ideal are made, the potential for error is increased.

Inlet and discharge temperatures are the stagnation temperatures at the respective points and should be measured within an accuracy of 1 ~ (0.55 ~ When the velocity of the gas stream is more than 125 fps (36.6 mps), the velocity effect should be included in the temperature measurement with a total temperature probe. 

This probe is a thermocouple with its hot junction provided with a shielded cup. The cup opening points upstream. A trade-off has to be made in a field test situation where the gas is not clean.

GAS TURBINE PERFORMANCE TESTS BASIC INFORMATION AND TUTORIALS

How to conduct gas turbine performance tests.

The performance analysis of the new generation of gas turbines is complex and presents new problems, which have to be addressed. Performance acceptance tests, which are required to be conducted for contractual guarantees, require that the turbine be cleaned before the test.

The average commissioning time for the advanced gas turbine (G Type) units is longer than the F and FA Type units. This is usually due to the increased number of starts and trips during commissioning, due to a lot of fine tuning required for the DLN combustors, cooling systems, and complicated control systems, which increase the number of equivalent engine hours.

It is recommended that contractually the maximum number of equivalent engine hours be limited to about 600-800 hours regardless of the actual equivalent operating hours.

If this is not done then the power output will be corrected to a larger corrected output, reducing the actual power the plant will produce. There have been many cases of 2000 to 6500 equivalent operating hours recorded during commissioning, which in many cases amount to the power and heat rate being corrected by 2 to 5%. This affects the profitability of the plant.

The new units operate at very high turbine firing temperatures. Thus, variation in this firing temperature significantly affects the performance and life of the components in the hot section of the turbine.

The compressor pressure ratio is high which leads to a very narrow operation margin, thus making the turbine very susceptible to compressor fouling. The turbines are also very sensitive to back pressure exerted on them when used in combined cycle or cogeneration duty. The pressure drop through the air filter also results in major deterioration of the performance of the turbine.

If a life cycle analysis were conducted the new costs of a plant are about 7-10% of the life cycle costs. Maintenance costs are approximately 15-20% of the life cycle costs. Operating costs, which essentially consist of energy costs, make up the remainder, between 70-80% of the life cycle costs, of any major power plant.

Thus, performance evaluation of the turbine is one of the most important parameters in the operation of a plant. Total performance monitoring on- or off-line is important for the plant engineers to achieve their goals of:

1. Maintaining high availability of their machinery.
2. Minimizing degradation and maintaining operation near design efficiencies.
3. Diagnosing problems, and avoiding operating in regions, which could lead to serious malfunctions.
4. Extending time between inspections and overhauls.
5. Reducing life cycle costs.

To determine the deterioration in component performance and efficiency, the values must be corrected to a reference plane. These corrected measurements will be referenced to different reference planes depending upon the point, which is being investigated.

Corrected values can further be adjusted to a transposed design value to properly evaluate the deterioration of any given component. Transposed data points are very dependent on the characteristics of the component's performance curves.

To determine the characteristics of these curves, raw data points must be corrected and then plotted against representative nondimensional parameters. It is for this reason that we must evaluate the turbine train while its characteristics have not been altered due to component deterioration. If component data were available from the manufacturer, the task would be greatly reduced.

IGNITION THEORY BASIC INFORMATION AND TUTORIALS

What is ignition theory? How things ignite?

Most ignition theories are based on the idea that the transient ignition source, usually an electric spark, must supply sufficient energy to the combustible mixture to create a volume of hot gas that just satisfies the necessary and sufficient condition for propagation, namely, that the rate of heat generation just exceeds the rate of heat loss.

The work of Lewis et al. did much to clarify and improve knowledge of spark ignition in quiescent mixtures. The first major contribution to ignition theory for flowing mixtures was made by Swett who studied the influence on ignition energy of variations in pressure, velocity, equivalence ratio, and turbulence.

Swett’s theory is based on the ideas that (1) only a portion of the discharge length is important in the ignition process and (2) heat loss by thermal conduction is negligible compared with heat loss by eddy diffusion.

Both of these ideas were fully confirmed in subsequent experiments carried out by Ballal and Lefebvre on ignition in flowing mixtures. Unfortunately, Swett’s treatment of turbulence is very limited and much of his experimental data are suspect for reason.

Gaseous Mixtures
Ballal and Lefebvre analyzed the processes governing the rate of heat generation in an incipient spark kernel and the rate of heat loss by thermal conduction and turbulent diffusion. They conclude that, for the spark kernel to survive and propagate unaided throughout a gaseous mixture, its minimum dimension should always exceed the quenching distance.

Heterogeneous Mixtures
All the evidence obtained in the studies of Subba Rao, Rao, and Lefebvre on the ignition of flowing mixtures of fuel drops and air serves to suggest that passage of the spark creates a kernel in which high gas temperatures are attained, partly from the energy supplied in the spark, but also from the heat liberated by the evaporation and rapid combustion of the smallest fuel drops.

GAS TURBINE COMBUSTOR REQUIREMENTS BASIC INFORMATION AND TUTORIALS

What is a gas turbine combustor?

A gas turbine combustor must satisfy a wide range of requirements whose relative importance varies among engine types. However, the basic requirements of all combustors may be listed as follows:

1. High-combustion efficiency (i.e., the fuel should be completely burned so that all its chemical energy is liberated as heat)

2. Reliable and smooth ignition, both on the ground (especially at very low ambient temperatures) and, in the case of aircraft engines, after a flameout at high altitude

3. Wide stability limits (i.e., the flame should stay alight over wide ranges of pressure and air/fuel ratio)

4. Low pressure loss

5. An outlet temperature distribution (pattern factor) that is tailored to maximize the lives of the turbine blades and nozzle guide vanes

6. Low emissions of smoke and gaseous pollutant species

7. Freedom from pressure pulsations and other manifestations of combustion- induced instability

8. Size and shape compatible with engine envelope

9. Design for minimum cost and ease of manufacturing

10. Maintainability

11. Durability

12. Petroleum, synthetic, and biomass-based multifuel capability.

For aircraft engines, size and weight are important considerations, whereas for industrial engines more emphasis is placed on other items, such as long operating life and multifuel capability. For all types of engines, the requirements of low fuel consumption and low pollutant emissions are paramount.

GAS TURBINES CATEGORIES BASIC INFORMATION AND TUTORIALS

Categories of Gas Turbines defined.
What are the different types of gas turbines?

The simple-cycle gas turbine is classified into five broad groups:

1. 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%.

2. Aircraft-Derivative Gas Turbines 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%.

3. Industrial Type-Gas Turbines.

These vary in range from about 2.5 MW- 15 MW. This type of turbine is 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 MW-2.5 MW. They often have centrifugal compressors and radial inflow turbines. Efficiencies in the simple cycle applications vary from 15-25%.

5. Micro-Turbines. 

These turbines are in the range from 20 kW-350 kW. The growth of these turbines has been dramatic from the late 1990s, as there is an upsurge in the distributed generation market.

INDUSTRIAL TYPE GAS TURBINES BASIC INFORMATION AND TUTORIALS

What are industrial type gas turbines?

Industrial Type Gas Turbines are medium-range gas turbines and usually rated between 5-15 MW. These units are similar in design to the large heavy-duty gas turbines; their casing is thicker than the aero-derivative casing but thinner than the industrial gas turbines.

They usually are split-shaft designs that are efficient in part load operations. Efficiency is achieved by letting the gasifier section (the section which produces the hot gas) operate at maximum efficiency while the power turbine operates over a great range of speeds.

The compressor is usually a 10-16 stage subsonic axial compressor, which produces a pressure ratio from about 5:1-15:1. Most American designs use can-annular (about 5-10 combustor cans mounted in a circular ring) or annular-type combustors.

Most European designs use side combustors and have lower turbine inlet temperatures compared to their American counterparts. Figure below shows an Industrial Type Gas Turbine.

The gasifier turbine is usually a 2-3 stage axial turbine with an air-cooled first-stage nozzle and blade. The power turbine is usually a single- or two-stage axial-flow turbine. The medium-range turbines are used on offshore platforms and are finding increasing use in petrochemical plants.

The straight simple-cycle turbine is low in efficiency, but by using regenerators to consume exhaust gases, these efficiencies can be greatly improved. In process plants this exhaust gas is used to produce steam. The combined cycle (air-steam) cogeneration plant has very high efficiencies and is the trend of the future.

These gas turbines have in many cases regenerators or recuperators to enhance the efficiency of these turbines. Figure below shows such a new recuperated gas turbine design, which has an efficiency of 38%.

The term "regenerative heat exchanger" is used for this system in which the heat transfer between two streams is affected by the exposure of a third medium alternately to the two flows. (The heat flows successively into and out of the third medium, which undergoes a cyclic temperature.)

In a recuperative heat exchanger each element of heat-transferring surface has a constant temperature and, by arranging the gas paths in contraflow, the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions.

This optimum temperature distribution can be achieved ideally in a contraflow regenerator and approached very closely in a cross-flow regenerator.

GAS TURBINE DESIGN CONSIDERATIONS

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.

Got any info about turbines? Feel free to leave your comments in the comment sections.

AIRBLAST ATOMIZER GAS TURBINE COMBUSTION BASIC INFORMATION

This atomizer employs a simple concept whereby fuel at low pressure is arranged to flow over a lip located in a high-velocity airstream. As the fuel flows over the lip it is atomized by the air, which then enters the combustion zone carrying the fuel droplets along with it.

Minimum drop sizes are obtained by using designs that provide maximum physical contact between the air and the liquid. In particular, it is important to ensure that the liquid sheet formed at the atomizing lip is subjected to high-velocity air on both sides.

This not only gives optimum atomization, but also prevents fuel from depositing on solid surfaces.

The airblast atomizer has some very significant advantages in its application to gas turbine combustors. For example, the fuel distribution is dictated mainly by the airflow pattern, and hence the outlet temperature traverse is fairly insensitive to changes in fuel flow.

Combustion is characterized by the absence of soot formation, resulting in relatively cool liner walls and a minimum of exhaust smoke. As another advantage, the component parts are protected from overheating by the air (at compressor outlet temperature) flowing over them.

The major practical disadvantages are rather narrow stability limits and poor atomization quality at startup, owing to the low air velocity through the atomizer. Both these problems can be solved (albeit at the expense of a more complicated fuel system) by combining the airblast atomizer with a pilot pressure-swirl atomizer.

By this means, the merits of the pressure-swirl atomizer at low fuel flows, namely, easy lightup and wide stability limits, are combined with all the virtues of airblast atomization (notably a soot-free exhaust) at high-fuel flow rates.

CARNOT CYCLE (IDEAL POWER PLANT PERFORMANCE) BASIC INFORMATION

The second law of thermodynamics may be used to show that a cyclic heat power plant (or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle called the Carnot cycle for a given (maximum) temperature of supply (T-) and given (minimum) temperature of heat rejection (Tmin)

Such a Carnot power plant receives all its heat (QB) at the maximum temperature @.e. TB = Tmm) and rejects all its heat (QA) at the minimum temperature (i.e. TA = Tmin) the other processes are reversible and adiabatic and therefore isentropic. Its thermal efficiency is

Clearly raising T,, and lowering Thn will lead to higher Carnot efficiency.

The Carnot engine (or cyclic power plant) is a useful hypothetical device in the study of the thermodynamics of gas turbine cycles, for it provides a measure of the best performance that can be achieved under the given boundary conditions of temperature.

It has three features which give it maximum thermal efficiency:

(i) all processes involved are reversible;

(ii) all heat is supplied at the maximum (specified) temperature (T-);

(iii) all heat is rejected at the lowest (specified) temperature (Tmin). 

In his search for high efficiency, the designer of a gas turbine power plant will attempt to emulate these features of the Carnot cycle.

THEORETICAL HEAT CYCLE BASIC INFORMATION AND TUTORIALS

In the original patent by Rudolf Diesel the diesel engine operated on the diesel cycle in which the heat was added at constant pressure. This was achieved by the blast injection principle.

Today the term is universally used to describe any reciprocating engine in which the heat induced by compressing air in the cylinders ignites a finely atomized spray of fuel.

This means that the theoretical cycle on which the modern diesel engine works is better represented by the dual or mixed cycle, diagrammatically illustrated in Figure 1.1. The area of the diagram, to a suitable scale, represents the work done on the piston during one cycle.

Starting from point C, the air is compressed adiabatically to a point D. Fuel injection begins at D, and heat is added to the cycle partly at constant volume as shown by vertical line DP, and partly at constant pressure, as shown by horizontal line PE.

At the point E expansion begins. This proceeds adiabatically to point F when the heat is rejected to exhaust at constant volume as shown by vertical line FC.

The ideal efficiency of this cycle (i.e. of the hypothetical indicator diagram) is about 55–60 per cent: that is to say, about 40–45 per cent of the heat supplied is lost to the exhaust. Since the compression and expansion strokes are assumed to be adiabatic, and friction is disregarded, there is no loss to coolant or ambient.

For a four-stroke engine the exhaust and suction strokes are shown by the horizontal line at C, and this has no effect on the cycle