Any Application of a Gas Turbine could have a Great Deal to Offer End-Users in other Industrial Sectors

Power generation is often the least demanding application for a given gas turbine, unless it is used in variable load/peaking service. Mechanical drive units are more likely to experience load swings. One example would be turbines driving pumps that reinject (into the soil) varying volumes of seawater that accompany “mixed field” (oil, gas, and seawater deposits) oil and gas production.

Aircraft engine turbines may see varying stresses depending on their service. If, for instance, one considers an aerobatic squadron, one needs to be aware that the engines on the planes trying to stay a fixed distance from the wing tip of the formation’s leader may accumulate life cycle losses of twenty times that of the formation leader’s engines.

In other words, the variations in all parameters that pertain to a gas turbine’s overall life, component lives, or time between overhauls (TBOs) offer insight to gas turbine operators regardless of whether that turbine operates in “their” industry or not. Lessons learned in one sector of industry on gas turbine metallurgy and operating systems, such as controls or condition monitoring, can be applied in some way to other gas turbine applications.

As we will see in Chapter 2, gas turbine historians can argue that the aircraft gas turbine engine was developed well before its land-based counterpart, or vice versa. Different schools of thought will be presented on that subject so the reader can draw his or her own conclusions. What is undisputed, however, is that the gas turbine is also a versatile propulsion mechanism in terms of aviation applications.

Grid System

Figure 1–16 illustrates a typical grid system for distribution of electrical power, showing voltages at key points. The shaft power system must operate at a constant synchronous speed to deliver electrical power at fixed frequency via an alternator. Frequencies are usually 50 Hz or 60 Hz depending upon the country.

Until recently the trend was for grids to be supplied by a few large power stations. More flexible distributed power systems are now becoming popular, however, due largely to the gas turbine’s viability even at smaller sizes. Here electricity is generated locally to the consumers whether by a CHP (combined heat and power) plant, or to a lesser extent the mid-merit power stations described later. Excess power is exported to the grid.

To illustrate the levels of power that must be transported, a city of one million people may have a peak demand of up to 2 GW. The average and peak consumption in a home for a family of four is around 1 kW and 6 kW, respectively, excluding space heating.

Standby Generators (Items 1 and 2, Table 1–1)

Standby generators are employed for emergency use, where there may be a loss of main supply that cannot be tolerated. Examples include hospitals and other public buildings in areas such as Japan, which may be prone to earthquakes. The power generated is used locally and the units are not connected to the grid system. Usually the fuel type is diesel. Key requirements are predominantly driven by the low utilization, and are outlined below in order of importance:

The diesel engine is most popular, primarily due to the plethora of automotive and marine engines in the required power bracket, which reduces unit cost via high production volume. The gas turbine has made some inroads, particularly where weight and volume must be limited, for instance, if the standby generator is located on the roof of an office block with limited load-bearing capability.

Gas turbine engines in this sector are usually simple cycle, single spool rather than free power turbine engines. This is because the reduced number of components reduces unit cost and because part speed torque is unimportant. Following the start sequence the engine remains at synchronous speed to ensure constant electrical frequency. Centrifugal compressor systems with pressure ratios of 5:1–10:1 are employed to minimize unit cost, and because the efficiency of axial flow compressors at such low flow rates is poor. The turbine blades, and usually the nozzle guide vanes, are uncooled, leading to SOT (stator outlet temperature) levels of typically 1100–1250 K.

Major Shaft Power Producing Systems

These typically are:

Large-Scale CHP (Item 5, Table 1–1)

Here the waste heat is almost exclusively used to raise steam, which is then used in a large process application such as a paper mill, or for district heating. Again, the electricity generated may be used locally or exported to the grid. The importance of performance criteria to engine selection are as for small-scale CHP, except that emissions legislation is more severe at the larger engine size (Figure 1–17).

Here gas turbines are used almost exclusively. High-grade heat is essential and the weight and volume of diesel engines prohibitive at these power outputs. Furthermore, the gas turbines used are often applicable to other markets, such as oil and gas and marine, which reduces unit cost. Aeroderivative gas turbines are the most common, though some heavyweight engines are used. Aeroderivatives usually employ the core from a large civil turbofan as a gas generator, with a custom designed free power turbine for industrial use. Heavyweight engines are designed specifically for industrial applications and as implied are far heavier than aeroderivatives, their low cost construction employing solid rotors, thick casings, etc.

The gas turbine configuration is usually a free power turbine. While this is not necessary for CHP applications, it is essential to also allow use in oil and gas and marine. Axial flow compressors are used exclusively with overall pressure ratios between 15:1 and 25:1. The aeroderivatives are at the top end of this range as this pressure ratio level results from a civil turbofan core. This pressure ratio is a compromise between that required for optimum CHP thermal efficiency of 20:1 and the 35:1 for optimum simple cycle efficiency. These values apply to the typical SOT of between 1450 K and 1550 K. Advanced cooling systems are employed for at least both the HP turbine first stage nozzle guide vanes and blades.

The Gas and Oil Pipeline System

Figure 1–18 shows the configuration of a natural gas pipeline system, in which gas is pumped from a well head to industrial and domestic consumers. Pipelines have diameters of typically 915 mm (36 in) to 1420 mm (56 in), and are usually underground. A notable exception is in permafrost areas where they must be raised to avoid melting the permafrost. These systems may extend over thousands of kilometers, with compression stations approximately every 200 km. For example, the trans-Canada pipeline runs from the Alberta province in Canada to the east coast of the United States. The power plant burns natural gas tapped off the pipeline, and drives a centrifugal compressor. Figure 1–18 also shows the typical flow rate of natural gas versus pumping power, and the pipeline compressor pressure ratio. For comparison, a family of four may consume up to 10 standard cubic meters per day in the winter period. A further use for gas turbines is to pump water into depleted natural gas fields to increase gas extraction.

Oil pipelines are less complex. Oil is pumped from a well head to a refinery, and occasionally distillate fuels are then pumped to large industrial users. Extracting oil from the well may involve pumping gas down to raise pressure and to force oil up the extraction pipe by bubbling gas through it.

Engine Requirements

The major power blocks required are around 6–10 MW, 15 MW, and 25–30 MW. These power levels are generally beyond the practical size for a high-speed diesel engine given the requirements outlined below, and hence gas turbines are used almost exclusively. In order of importance these requirements are:

For offshore well heads the engine must be located on a platform; hence the importance of low weight is amplified and low volume essential. While the gas or oil may have a high pressure as it comes out of the ground it invariably needs further pressurization to pipe it back onshore. The engine may drive the compression unit mechanically as described above, or sometimes via an electric motor. In the latter instance a CHP arrangement supplies power for other needs, such as electricity for the drill and heat for uses such as natural gas processing or space heating.

For the middle and higher power bracket, simple cycle, free power turbine aeroderivatives best meet the above criteria, and are used almost exclusively. Pressure ratios of 20:1–25:1 and SOT levels of 1450–1550 K are typical, leading to thermal efficiency levels in the mid to high thirties. Engines include the Cooper Rolls Coberra 6000 and the GE LM2500, which are also utilized in power generation applications. For the lower power bracket, both custom designed industrial engines such as the Solar Mars, and aeroderivatives such as the Allison 501, are used with lower pressure ratio and SOT levels leading to thermal efficiencies in the low thirties.

The Gas Turbine Versus Reciprocating Engines

The first gas turbine propelled automotive vehicle was the Rover JETI produced in the U.K. in 1950, the design team being led by Maurice Wilks and Frank Bell. The engine had a free power turbine and produced 150 kW from a simple cycle; the vehicle fuel consumption was 5.4 km/liter (15.2 mpg). Over the ensuing decades significant effort has been spent on automotive programs; however, the diesel and gasoline engines have continued to dominate, with the gas turbine only achieving a presence in specialist applications. The contributory issues are explored in this section, but there are three key reasons:

These disadvantages have mostly outweighed the benefits of the gas turbine, which are:

One other use for gas turbine engines has been in thrust-propelled vehicles for attempts on the world land speed record. In 1983 Richard Nobel’s “Thrust 2” achieved 1019 km/h (633 mph) using a Rolls Royce Avon turbojet. In 1997 his “Thrust SSC” piloted by Andrew Green exceeded the speed of sound, and set a new world land speed record of 1220 km/h (763 mph), using two Rolls Royce Spey turbofans.

The Gasoline Engine Versus the Diesel Engine

Figure 1–19 includes curves of thermal efficiency and torque versus part load power for the gasoline engine. Overall it has worse SFC than the diesel engine because to avoid pre-ignition its compression ratio is lower, typically 10:1, contrasting with 15:1–20:1 for diesels. Whereas the Otto cycle in a gasoline engine has combustion at constant volume, producing increased pressure, in a diesel engine it is at constant pressure. Both engines can be turbocharged to increase power by raising inlet density and hence air mass flow. The weight and size saving can be significant, though with some expense in terms of response time, due to turbo lag as the turbocharger spool accelerates.

The main advantage of the gasoline engine is that it has lower weight and volume, which approach those for the gas turbine at the 50 kW required for a typical family sedan. Hence gasoline engines are used where fast vehicle acceleration is essential, space is at a premium, and worsening of SFC is acceptable. Diesel engines dominate for applications such as trucks where fuel consumption is paramount due to high utilization, engine weight and volume relative to the vehicle are low, and high vehicle acceleration is not a priority.

Table 1–2 lists the major categories of the automotive vehicle and the engines used in them.

The Hybrid Electric Vehicle

Stringent emissions legislation is creating a niche market in certain parts of the world, such as California. Pure electric vehicles have limited performance and range before battery recharging is required, even those using the most advanced battery systems such as sodium sulfur. To overcome this, a heat engine may also be fitted in one of two possible hybrid configurations, currently the subject of many development studies.

Figure 1–20 describes the various modes in which the two hybrid electric vehicle configurations may operate. For a range extender there are two modes of operation. Mode A is that of a conventional electric vehicle, where the battery provides traction power to the wheel motors via power electronics. In mode B the heat engine drives a generator that provides power to charge the battery. In this application when operative the gas turbine runs at maximum power, where response is unimportant and the SFC difference versus piston engines is lowest.

For a full hybrid power plant, in mode B the engine may also drive the motor.

Major Classes of Marine Vessel

The major classes of marine vessel for which gas turbine engines are candidates are summarized in Table 1–3. Examples of actual vessels and the engines used are provided. The gas turbine competes with the diesel engine and nuclear power plant utilizing boilers and steam turbines. At the time of writing the oil-fired steam plant is becoming rare in new vessels, but remains in service.

Marine Vessel Propulsion Requirements

Figure 1–21 illustrates and quantifies the elements that comprise the total power requirement for vessel forward motion. A vessel moving through calm water creates two waveforms, one with a high water pressure at the bow, the other a reduced pressure at the stern. The energy to create this wave system is derived from the vessel via the wave making resistance. At high speed the wave resistance is dominant. Indeed for a given hull design a critical hull speed for wave making resistance is reached, where the vessel literally climbs a hill of water, with the propulsion thrust tilting upward, and it is uneconomical to go beyond this speed. The sinusoidal effect visible superimposed on the curve of wave making resistance is due to interactions of the bow and stern wave systems. The skin friction resistance, or friction form resistance, is also a major contributor to the total resistance. This is the friction between the hull and the water.

The pressure resistance, hydrodynamic drag, or form drag is due to flow separations of the water around the hull creating an adverse pressure field. Any resulting eddies or vortices are in addition to the waves created by the wave making resistance.

The air resistance due to the drag of the vessel above the water line contributes less than 5% of the total resistance. Often for low speed vessels little effort is spent in aerodynamic profiling.

The above four resistances comprise the naked resistance. The appendage resistance must then be added to evaluate the total resistance. This is the loss incurred by rudders, bilge keels, propellers, etc., and is less than 10% of the total resistance.

Traditionally these resistances for a vessel design are evaluated by model testing in a water tank and then using non-dimensional groups to scale up the resulting formulae and coefficients to the actual vessel size. This process is complex. The power requirement approximates to a cube law versus vessel velocity for displacement hulls, which support weight by simple buoyancy. Resistance is also dependent on vessel displacement (i.e., weight). Power approaches a square law for semi-planing hulls, which produce lift hydrodynamically.

Engine Load Characteristics

Ship engines drive either a conventional propeller or a waterjet via a gearbox. The latter consists of an enclosed pump that sends a jet of water rearwards. Since both devices pump incompressible water, power versus shaft speed adheres closely to a fluid dynamics cube law. For a propeller the vessel speed determines the shaft speed; in the absence of propeller blade slippage these are uniquely related. As the number of engines driving changes, engine operation moves between different possible cube laws, with slippage likely for fewer propellers driving. In the engine concept design phase cube laws are a reasonable assumption, but at the earliest opportunity the law(s) for the actual propeller or waterjet should be obtained from the manufacturer. Variable pitch propellers are often employed, which mainly affect the lowest speed characteristics.

Also shown are the resulting characteristics of engine speed versus ship speed. The cube laws for power versus ship speed, and power versus engine output speed combine to make engine output speed directly proportional to ship speed. With one engine driving, however, engine output speed is almost 20% higher than with two to achieve a given ship speed, hence propeller speed, due to slippage of the loaded propeller. These multiple load characteristics must be considered when designing a gas turbine for a multi-engine vessel.

CODAG, CODOG, COGAG, and CODLAG Propulsion Systems

Figure 1–22 presents schematic diagrams of the above systems.

Hovercraft (Items 1 and 2, Table 1–3)

Hovercraft use a fan to maintain pressure under side skirts to hover above the water surface and air propellers to provide thrust for propulsion. The ratio of power required for propulsion to that for hovering is between 5:1 and 10:1. These vessels are designed for high speed. They are generally used in applications such as commercial passenger ferries or landing craft where the vessel spends most of its time at maximum speed. The key power plant criteria in order of importance are:

The output power is below 10 MW, hence a high-speed diesel engine is practical. The diesel engine has the best high power SFC but the gas turbine the lowest weight. Consequently both power plant types are used. The gas turbine configuration is simple cycle for minimum weight, with a free power turbine. Typically SOT levels of 1250 K are employed with pressure ratios of around 7:1, the low values partly reflecting the predominance of older engine designs in current applications.

The Flight Mission and Aircraft Thrust Requirements

The major phases of a flight mission are takeoff, climb, cruise, descent, and landing. For military aircraft combat must also be considered, and all aircraft must turn, albeit briefly. By definition, the relationship between drag and equivalent airspeed is independent of altitude. At low and medium altitudes considerable excess thrust beyond aircraft drag is available.

During takeoff, high excess thrust is available for acceleration. Typical takeoff velocity and distance for a fighter are 140 kt (0.21 Mach number) and 1.2 km, respectively. Corresponding values for a civil aircraft are up to 180 kt (0.27 Mach number) and 31 km. Takeoff is a key flight condition for engine design, with usually the highest SOT.

In order to climb, additional upwards force is required. This is achieved by maintaining a high angle of attack to increase lift coefficient. The resulting increased drag is overcome by increasing thrust, excess being available beyond that required for steady flight. Also, a component of the thrust is directed vertically. The excess thrust available at low altitudes provides a high rate of climb, typically 500 m/min for a subsonic transport, and up to 8000 m/min for a fighter. Flight speed during climb is initially at a fixed level of equivalent air speed due to airframe structural considerations (maintaining constant dynamic pressure), and then at the limiting flight Mach number for airframe aerodynamics once achieved. At the top of climb the maximum engine thrust is just equal to the aircraft drag. This is a key sizing condition for the engine, with highest referred speeds and hence referred airflow. It is not the highest SOT, due to the lower ambient temperature.

Aircraft usually cruise at high altitude because here the true air speed achieved for the given level of equivalent air speed is significantly higher, and because engine fuel consumption is minimized by the correspondingly lower thrust requirement. The choice of cruise altitude is complex, and depends on engine size required to achieve the altitude, the true air speed, and range demanded by the market sector. The optimum altitude for cruise generally increases with the required level of flight Mach number. Over a long period at cruise required thrust may reduce by 20% of that at the top of climb, due to the reduction in aircraft fuel weight. Some aircraft therefore climb gradually as weight reduces, known as cruise-climb.

During descent the engines are throttled back to a flight idle rating and the aircraft angle of attack reduced. Both these effects reduce lift, and the flight direction is below horizontal. A component of the weight now acts in the direction of travel, supplementing the engine thrust to overcome drag. With zero engine thrust this would be gliding.

Turning requires centripetal force, provided by banking the aircraft to point the wings’ lift radially inwards. To support the weight the overall lift must be increased, hence also the thrust as drag thereby increases.

The approach for landing is on a glide slope of approximately 3°, with a high angle of attack and flaps set to reduce aircraft speed as far as possible to give the required lift. Typically landing speeds are between 120 kt (0.18 Mach number) and 140 kt (0.21 Mach number). Landing distances are substantially less than those required for takeoff, as deceleration due to reverse thrust or brakes and spoilers is faster than the takeoff acceleration. Most turbofan propelled aircraft employ engines with a reverse thrust capability, where the bypass air is diverted forward using either louvers in the nacelle or rearward clamshell doors.

Afterburning or reheat is often employed for fighter aircraft and supersonic transport. Fighters generally use it only for short durations due to the high fuel consumption. These are specific maneuvers such as takeoff, transition to supersonic flight, combat, or at extreme corners of the operational envelope. Generally supersonic transports such as Concorde use it for takeoff and supersonic transition.

Engine Configuration Selection for a Required Flight Regime

The key parameters of interest are:

The gas turbine engine achieves adequate acceleration times of around 5 seconds for civil engines and 4 seconds for military, so this does not give it or a piston engine any competitive advantage.

Range factor is the most commonly used parameter to assess the suitability of engine configurations for a required flight mission. It is the ratio of the weight of fuel and engine to the engine net thrust less pod drag for a range and flight speed. Clearly a low value of range factor is better. Figure 1–23 presents range factor versus flight Mach number for ranges of 1000 km and 8000 km, and a number of engine configurations including a piston engine.

It is immediately apparent why the gas turbine so readily replaced the piston engine for most aircraft propulsion: the latter is only in contention at low Mach numbers, below around 0.3. This is primarily because propulsion power requirements increase rapidly with Mach number. The weight and frontal area of a piston engine increase far more rapidly with output power than they do for the gas turbine. The immense importance of these factors at high flight speed is quantified by the range factor diagram.

Above 0.3 Mach number the weight and frontal area considerations mean the turboprop takes over from the piston engine as the optimum power plant. It has better fuel consumption than a turbojet or turbofan, due to a high propulsive efficiency, achieving thrust by a high mass flow of air from the propeller at low jet velocity. Above 0.6 Mach number the turboprop in turn becomes uncompetitive, due mainly to higher weight and frontal area. In addition, high propeller tip speeds required are a difficult mechanical design issue, and the high tip relative Mach numbers create extreme noise.

Above 0.6 Mach number the turbofan and turbojet compete, the optimum choice depending on the application. The turbofan has a better SFC than the turbojet, but at the expense of worse specific thrust and hence weight and frontal area. Increasing bypass ratio provides the following engine tradeoffs:

The high bypass ratio engine is most competitive at flight Mach numbers of approximately 0.8, whereas at 2.2 Mach number the ideal bypass ratio is less than 1 and a turbojet becomes increasingly competitive.

Above about 2.0 Mach number the specific thrust of the ramjet becomes even better than that of a turbojet; however, it has poorer specific fuel consumption. The impact of this on range factor is shown on Figure 1–23. The low engine frontal area and weight resulting from the high specific thrust dominates at low range and high Mach number where the ramjet becomes the most competitive power plant. Also applications to date requiring this flight regime have been missiles and hence the lower unit cost of the ramjet is beneficial. For turbojet mechanical integrity the compressor delivery temperature must be kept to below approximately 950 K, hence above 2.5 flight Mach number there is very little room for compressor temperature rise.

The other possible power plant is a rocket, which is beyond the scope of this discussion.

Shaft-Powered Aircraft—Turboprops and Turboshafts

This section describes the requirements of shaft-powered aircraft, while the next section covers thrust propelled aircraft. The term turboprops usually refers to gas turbine engines that provide shaft power to drive a propeller for fixed wing aircraft propulsion. Those providing power for a rotary wing aircraft, or helicopter, are referred to as turboshafts.

Major Classes of Shaft Powered Aircraft

Table 1–4 presents the major classes of shaft powered aircraft together with examples of actual aircraft and the engines utilized. Figure 1–25 presents key and interesting characteristics of these aircraft classes using the items from Table 1–4. Aircraft takeoff weight, range, maximum speed, and number of seats are plotted versus required power.

TABLE 1–4

Major Categories of Turboprop/Turboshaft Aircraft

Item	Aircraft Type	Examples of Aircraft	Engines Utilized	Total Shaft Power (KW)
1	Light aircraft, piston engines	Piper Warrior II Beech Bonanza	1 × TL 0320-133G flat twin 1 × TC 10 520 BB flat 6	120–220
2	Business/executive Turboprop	Piper Cheyenne 400 Cessna Caravan Dornier 228-100	2 × Garrett TPE331 1 × PW PT6A-114 2 × Garrett TPE331	500–1200
3	Commuter/regional Transport turboprop	BAE Jetstream 41 Shorts 330 BAe ATP Fokker 50	2 × Garrett TPE331 2 × PW PT6A-45R 2 × PW 126A 2 × PW 125B	1800–4000
4	Light helicopter, piston engines	Robinson R22 Schweizer 300C	1 × TL 0-32-132C flat 4 1 × TL H10-360-DIA	120–170
5	Light helicopter, turboshaft engines	Bell-Jetranger III Bell 406	1 × Allison 250-C20J 1 × Allison 250-C30R	300–500
6	Multirole medium helicopter	Sikorsky S-70A (Black Hawk) Westland/Augusta EH101	2 × GE T700-700 3 × GE T700-401A, or 3 × RR/TM RTM322	2300–3500
7	Heavy lift helicopter	Sikorsky H53E Boeing Chinook CH-47	3 × GE T64-416 2 × TLT55-712	6500 10,000

TL =Textron Lycoming; GE = General Electric; TC = Teledyne Continental;
RR = Rolls Royce; PW = Pratt & Whitney; BAe = British Aerospace
To convert kW to hp multiply by 1.341.

(Source: Rolls Royce.)

Thrust Propelled Aircraft—Turbofans, Turbojets, and Ramjets

Auxiliary Power Units (APUs)

Aircraft APUs have normally fulfilled several functions in an aircraft, namely:

These functions give an aircraft self sufficiency when on the ground. In addition an APU will be required to fire up at altitude in case of main engine flame out, to power electrical systems—vital for fly by wire aircraft—and if at low flight Mach number to provide crank assistance to help restart the engines.

Until recently, new developments have been rare, but APU sophistication is now increasing to match that of recent aircraft, where APU operation is becoming less intermittent. For civil applications APU requirements may now include operation in all regions of the flight envelope, and for military aircraft advanced systems with start times as low as a second. A current typical start time is around 6 seconds at 15,000 m. There are occasional studies on the benefits of permanent running power units that avoid compromising the design of the propulsion engines by power and bleed offtake.

Historically the main requirements for APUs have been: