Gas Turbines

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An Introduction To Gas Turbines

 

  • Overview

The gas turbine is an internal combustion engine. As with any internal combustion engine it must have:

An air supply

A fuel supply

A lubrication system

A starter system

A control system

A cooling system

An exhaust system

 

A 4 stroke or 2 stroke internal combustion engine follows a set pattern of inlet, compression ignition, and exhaust. This is called the Otto Cycle. The reciprocating piston effectively splits each phase of the cycle into separate parts. The gas turbine however follows the Brayton Cycle (Figure 1) and the whole process is a continuous one. See Figure 2.

 

 

Figure. 1

 

Air enters the compressor section, is compressed and passed to the combustion section where it is mixed with fuel and burned. This produces a large volume of hot gas that expands through the   turbine section. The turbine is attached to the compressor and drives it. The compressor in turn compresses more air, thus allowing more fuel to be burned, which in turn produces more hot gas. This in turn drives the turbine faster, which drives the compressor faster and so on until either no more air can me accommodated by the compressor,(Stone Walling) or no more fuel is available, or the whole unit just disintegrates due to the centrifugal forces and heat encountered.  That, in very simple terms is the gas turbine engine. Whereas the reciprocating engine can produce very large pressures within its closed cylinder, a gas turbine is effectively a tube open at both ends and with nothing to restrict gas flow in the middle. Hence by its very nature, it cannot be a high pressure engine. It is a high velocity engine and huge amounts of air pass through it at very high speed.

 

The gas turbine was originally developed in the 1940’s as a power source for aircraft. Since an aircraft engine has to carry not only its own weight, but that of the plane and payload and push the whole plane along at high speeds, it needs to have a very high power to weight ratio. An industrial engine goes nowhere and is used to drive such items as generators and compressors etc. It can therefore be built as heavy as one wishes within economical restraints. However the fundamental principles remain the same, the engine has only one basic moving part.

 

The gas turbine revolves at high rate. As a rule of thumb, the smaller the engine, the higher are its revolutions. Consequently, a gear box needs to be incorporated into the design of industrial units as the generally the RPM of the engine is far higher than required by the driven unit.

 

If we now regard the driven unit as a load, these loads come in two distinct types. The first is of the generator type. A generator is dynamically balanced and it takes very little effort to turn it when no power is being drawn from it.  Generators can generally be rotated by hand with very little effort. The gas turbine too, can be rotated with one finger. Both units offer very little rotational resistance when at rest. However, once the generator  is up to speed and the circuit breakers closed, current is drawn from the generator and this can be viewed as acting as a magnetic brake. Resistance to rotation is present. This resistance is only encountered when the generator is up to speed and the circuit completed. Up to the point when the circuit breakers are closed, there is very little resistance to rotation.

 

A compressor is another matter. This represents a fixed load when the unit is stationary.

Any attempt to rotate the gas turbine will require that the load to be rotated too. Therefore any starting system employed by the gas turbine will not only have to rotate the gas turbine but also the load too. This is rather like trying to start a car with the clutch and gearbox engaged.

Obviously some system had to be devised to separate the prime mover (gas turbine) from its load.  The systems used are know as the single shaft arrangement   and the split shaft arrangement and they are shown schematically in figure 2.

 

Figure.2  A Single Shaft Gas Turbine Arrangement

 

In the above arrangement one single shat connects prime mover (gas turbine) to both gear box and load. Any movement of any shaft will be transmitted to all other moving parts. This arrangement can only be used when the gas turbine is effectively under a no load condition at start up. This is suitable for power generation when using small alternators.

If however the gas turbine is under load in the start up condition a Split Shaft arrangement has to be utilized as in figure34. An example of this arrangement are Solar Saturn and Taurus generator sets.

 

 Figure. 3  A Split Shaft Gas Turbine Arrangement

 

In the split shaft arrangement the load is separated from the prime mover. The power turbine shaft is connected to the gearbox and load, thus leaving the gas turbine (Hot Gas Generator) free to rotate independent of the load. In fact what we have now is an aerodynamic clutch arrangement. The hot gas generator can be started up under no load conditions and the power turbine will begin to rotate when sufficient hot gas is generated to overcome load and inertia. The hot gas generator is designed so that it will supply sufficient hot gas to drive the power turbine under any load conditions. If a feed back signal is then incorporated into the system, the hot gas generator will always supply the load requirements at the back end. An example of this arrangement is the Solar Titan Ses.

A further configuration is the twin spool, split shaft arrangement as shown in figure 4.

 

Figure.4  Split Shaft, Twin Spool Gas Turbine Arrangement

 In this arrangement the hot gas generator is mounted on the HP shaft and is free to rotate independent of both the LP shaft and load. This means that the HP shaft is connected to the starter system and is free to start up and rotate independent of any other shafts. The hot gasses produced will start up the LP shaft which will result in a first stage compression of the incoming air. As the hot gasses from the hot gas generator flow through the LP turbine. The load will start to rotate once sufficient hot gas passes through its power turbine. This system is really an aircraft engine configuration. Though the high bypass turbo fan, aero units usually employ a triple shaft arrangement. This configuration can be found in industrial uses and an example is the LB 1500 units. This actually uses a detuned GE aircraft engine with the third spool removed.

 

If you look at the inlet filters of any industrial gas turbine and then the size of the exhaust stack, you will begin to realize just how large is the volume of air passing through the unit when it is running. Only about 25% of that air is actually used for combustion purposes. This is called Primary Air. The remaining 75% mostly by passes the combustion chamber. Some does enter, but is used to position the flame so that it does not touch the walls of the combustor. If the flame did touch the walls, hot spots would rapidly occur and the wall would burn out at that point. This is called Secondary Air. The remainder and by far the largest percentage, mixes with the hot gasses and is heated itself in doing so. This is called Tertiary Air and it is this huge volume of rapidly expanding tertiary air that powers the turbine. The turbine has not only to supply sufficient power to turn the load, it also has to turn it’s own compressor. About 80% of all power produced by the turbine is used to turn its own compressor. This only leaves 20% less losses, for the load.

The whole fundamental principal of the gas turbine is one of a smooth flow of air through the engine. If for any reason that flow is disrupted, then the efficiency of the engine drops off. If you consider that the engine is at best 20% efficient, then even a 10% drop in compressor efficiency would translate as only having 12% power available to do any work. Hence the engine has lost nearly 50% of its efficiency. If you car has 4 spark plugs and one fails, you experience a lot more than a 25% reduction in available power. It is for this reason that the blades of the compressor and turbine require regular washing.

 

A general layout of a gas turbine can be seen in figure 5.

Figure 5 General Layout of a Gas Turbine

 

A further problem with gas turbines is that they are only really efficient when running at high speeds and under a constant load. When the engine is started the compressor begins to compress air. Unfortunately, the turbine at low speeds is far more inefficient than the compressor and it cannot handle the amount of air that wants to pass through the engine. If this were allowed to occur, then the pressure would build up between compressor and turbine until all flow ceased and attempted to reverse. Then the pressure would drop as the flow again reversed and passed out though the turbine. This is called Surge it is a highly dangerous condition and must be avoided. If not the cycle of stop, reverse, stop reverse could occur many times a second and the engine would go into catastrophic self destruction. For this reason the amount of air coming from the compressor during start up and shut down is strictly controlled. Various systems are employed to limit the amount of air allowed to enter the compressor (Inlet Guide Vanes) and bypass valves to divert air past the turbine into the exhaust.  See Figure 6 By Pass Valve  Thus the amount of heat energy available to the turbine section is reduced.

 

Figure. 6 Bleed Air Valve Solar System

The fuel supply is generally natural gas, but its condition and quality have to be strictly controlled. It must be free of dirt and liquids and at a set pressure. And temperature.

 

The lubrication system with such a high speed engine is very critical. Frequently special synthetic oils are used. Great care should be exercised when handling such oils as they often contain cacogenic agents.

The lubrication system on a gas turbines differs greatly from that on a reciprocating engine is as much that the lube oil on a gas turbine never comes in contact with the products of combustion. Therefore the oil need not be changed so frequently as it does not collect carbon and sulfur. Since the engine does not move, the lube oil reservoir can be as large as one wishes. Thus heat can be accommodated through sheer bulk of lube oil employed and relatively small fin fan cooler heat exchangers used.

 

Starter systems can be pneumatic and frequently employ fuel gas as their power source. They can also be hydraulic or electric. However if they are electric, then provision has to be made for the starter to run at several different speeds to accommodate, purge, ignition, and run up to self sustaining speed. Typically a starter motor has to remain engaged up to 60% of normal running speed. If the normal  100% rating is say 25,000 Rpm, the starter motor would not disengage until a speed of 15,000RPM had been reached. Electric starters as favored by Solar sets have variable frequency starters so as to be able to run at speeds of around 4% 19% 30% and 60% of the normal operational speed.

 

Because the start up and running of a gas turbine is so critical and the speeds so high, only electronic controls are feasible. Manual or mechanical controls could never  operate with sufficient speed.

The cooling systems are split into two areas. Internal cooling which is achieved by compressor air and the lube oil and enclosure cooling. Industrial gas turbines are built on skids and the skids are located inside an enclosure. The enclosure has several functions.  It provides a weather proof location and it reduces the noise levels produced by the engine. It can also be automatically flooded with CO2 in the event of an engine fire. The enclosure has its own ventilation system that keeps the  engine cool and should there be an engine destruct, the enclosure will contain the debris. Because the air pressure in the enclosure is higher than that outside, no external gas leaks can access the hot engine and any gas leaks that might develop inside, are diluted and removed.

 

The exhaust system safety removes the spent hot gasses. Sometimes these hot gasses can be used in a waste heat recovery system.

Since certain ancillary items included in the package are placed on different sides on the engine it is important to adopt one system of location that is understood by all parties. This is called engine orientation. See Figure 5 (American system)

.

Figure. 6 Engine Orientation (American system)

 

 

  • The air System

Clean, filtered air is drawn into the front of the engine and passes via the Inlet Guide Vanes (IGV) These are partially closed upon start up and gradually open as the engine builds up speed. This gradual opening can be controlled by the Pressure Discharge Air (PCD) or hydraulically. The point where the air leaves the compressor section and just before it enters the combustor section  is called the PCD. It’s the highest pressure point in the engine. Even so, if it is not usually above 150 psig or10 barg. This pressure will gradually increase from ambient when the engine is stationary to maximum when the engine is at 100% RPM. It increases in proportion to compressor speed and can therefore be used as a signal to gradually open the IGV.  It is however subject to external temperature changes within the engine enclosure and since it is enclosed in a pipe, a temperature rise will show as an increase in volume. As the volume cannot increase in a fixed pipe, this temperature rise will be translated as an increase in pressure. A false reading could then occur. To prevent this on Solar sets the PCD pipe is insulated. A better system is to use  hydraulics. Once the engine begins to pick up speed, the lube oil pumps circulate the oil around the engine. Some of this oil is diverted and used as hydraulic oil to operate the engine geometry. IGV etc. Initially electric pumps do the work, but as the engine accelerates, its twin gear pumps take over. These are PD pumps and the faster the engine rotates, the faster the gear pumps turn. This is translated as pressure and liquids are non compressible. Hence we have an excellent signal medium that can be used to operate the IGV.

 

Figure. 7   Inlet Guide Vane

 

Many engines have Exit Guide Vanes too (EGV) They straighten the exiting compressed air.

 

2.0 The Compressor Section

The compressor rotor is rotated at fairly high speed by the turbine. Air is continuously drawn into the inlet of the compressor, and accelerated rearward by the rotor blades. The high velocity air then flows onto the stator blades, where the velocity of the air is reduced and converted into a small pressure rise.  This process continues across each stage of the compressor gradually increasing the pressure from inlet to outlet. Look at figure 7. The air enters from the left hand side and passes through the IGV. Then onto the first ring of rotor blades where it is compressed a small amount. The air cannot get out of the rear of the rotor section because its exit is blocked by the stator blade. It cannot escape until the rotor has turned sufficiently for it to pass into the stator blade area. Here its velocity is reduced and its pressure rises. Again its exit is blocked by the next ring of rotor blades and it has to wait until it the rotor turns and allows it to escape the stator ring and into the next stage of compression. As the rotor is constantly turning, the process is a smooth and continuous one. See figures 8.

 

Figure. 8 IGV, Rotor and Stator Blades

 

The rotor blades are attached to the rotor and rotate. The stator blades are stationary and are fixed to the compressor housing. Each stage causes a slight rise in pressure and also a rise in temperature. The more stages that there are, the greater the pressure rise. A Solar Mars set has 14 stages of compression.

 

3.0 COMBUSTION CHAMBER

The purpose of the combustion chamber, which is a difficult task, is to burn large quantities of hydrocarbon fuels, supplied by the burner, mixed with even larger quantities of compressed air, fed from the compressor, and releasing the heat in such a manner that the air is expanded and accelerated rearwards so as to give a smooth flow of uniformly heated gas to the turbine assembly under all operating conditions. This task must be accomplished with the minimum loss in pressure, and also the maximum amount of heat release within the limited space available. In this way the chemical energy in the fuel is converted into heat energy in the gas stream. Within the combustion chamber, we consider that it is split up into three separate zone. These are known as (1) Primary (2) Secondary   (3) Cooling or tertiary zones. See figure.9

 

The fuel and part of the air enter the combustion chamber via the primary zone, where they are mixed together in the correct ratio: approx. 15 parts of air to 1 part of fuel (that is an air/.fuel ratio approx. 15:1 usually calculated by weight). They are ignited and rapid combustion takes place.      The heat of the combustion gases reaches a temperature in the region of about 2000 deg.C. Since this temperature is well in excess of the safe operating level of the turbine, the remaining parts of the air enter the combustion chamber via the secondary and cooling zones, therefore gradually reducing the gas temperature down to the acceptable level of the turbine blading.

 

In order to achieve the acceptable temperature the minimum air/fuel ratio can be approx. 60:1.

 

Figure. 9 Combustion Chamber

 Some combustion chambers are annular (doughnut) in shape whilst others are of the flame can configuration. Ruston Gas Turbines are frequently of the flame can type. These systems need cross firing connections so that should one can fail it will be relit from another. The heat balance is critical in these units to prevent hot spots forming on the turbine. See figure 10.

 

Figure. 10 Flame Can Type Combustion Chamber

 

4.0 Fuel Gas System

 

A typical gas fuel system has the following basic equipment.

Fuel gas pressure regulator

Fuel gas  de mister

Main fuel gas solenoid

Pilot gas solenoid

Emergency slam shut  off valve

Gas fuel valve

Emergency manual shut of valve

Pressure gauge and switch

Fuel gas burners.

The function of the system is to provide a constant supply of clean dry gas to the fuel valve which can then supply variable pressures and quantities to the combustion chamber to enable-starting, running, and load changes.

 

FUEL GAS REGULATOR

A pressure setting valve that is used to ensure a constant supply (volume) of gas at the turbine manufacturers designed pressure.

 

GAS DE MISTER

A filter that is capable of removing any condensate from the supply gas prior to its use for combustion.

MAIN GAS SOLENOID

An electrically operated fuel open and shut valve used for isolation.

 

PILOT GAS SOLENOID

An electrically operated fuel on /off valve used to supply ignition fuel if system fitted.

 

EMERGENCY SLAM SHUT OFF VALVE

This valve is an open and closed valve and if all electrical power was lost would shut off the gas to the turbine.

 

GAS FUEL VALVE

The throttle valve. Its function is to supply fuel to the fuel gas burners at variable pressures and quantities as required by changing operating conditions.

 

THE BASIC SYSTEM (GAS)

 Gas is supplied to the regulator.    Pressure and volume are set.  The gas travels to the

de-mister where  any liquid is removed.  The dry gas is passed through the solenoid valve and the slam shut valve to the fuel (throttle) valve.       This governor controlled valve then supplies the correct amount  of gas to suit engine running need to the fuel gas burners located in the combustion chambers.

 

Figure. 11      Ruston Fuel Supply System

  

5.0 Turbine section

In the turbine section, the hot gasses flow over the turbine blades and expand. They give up their energy to the turbine and it rotates due to two forces. Reaction and impact

See figures No

 

Figure. 12

 

 

Figure. 13 The Two Forces Acting Upon A Turbine Blade

 

6.0 Lubrication Systems.

 Lubrication is important in any machine, but vitally so in an expensive and high speed machine such as a gas turbine. Provision has to be made to prelube the unit up to a certain pressure and also to ensure that an adequate lube oil supply is available in the event of a power failure. Some systems use a run down tank. This is where when the unit starts up an electric pump then pumps oil to a storage tank located at a high level. If the power fails and the fuel supply is cut off, the unit will begin to spool down. At some point the mechanical pumps will no longer be able to supply the lubrication demands. At this point the pressure in the oil tank due its head, will take over automatically and gravity will supply the necessary lube oil pressure. Other units depend upon a 24volt emergency supply, DC lube oil pump. Most Gas turbines also require some post lubrication to help in cooling down the hot unit evenly. Some units use external electric lube oil pumps under normal running conditions and some use a combination of electric and mechanical. The electric pumps handling the low pressure needs during start up and shut down and the engine driven PD pumps handling the HP requirements at speed. Figure    Is the schematic drawing of a typical Solar Set lubrication system showing the major lubrication lines.

 

Figure. 14    Major lubrication lines. Solar Unit

The diagram also indicates the oil cooling system and the hydraulics. This is the same oil but is used to configure the engine geometry. That is IGV and any other items such as variable stator blades that are used to prevent surge occurring during start up and shut down.

 

7.0 Starting Systems

 

As previously mentioned, there are many different types of system used for the initial start of an engine. However, they all must follow the same rule that the started must have a range of speeds. This is because every gas turbine has a set starting sequence. Various things happen at certain speeds and within set time limits. If not, the unit will shut down and abort the start up sequence. Prior to the unit being allowed to move to start up, a series of self checks and purging will take place. A typical start sequence would be.

 

  1. fuel supply and valve check
  2. Geometry correct
  3. Starter engages and turns the unit for a set time at about 19%
  4. Lube oil and emergency supply pumps are self checked.
  5. After the purge time is completed Ignition sequence is initiated and power is supplied to the igniter.
  6. Fuel gas is admitted into the unit and a check is seen that a flame is established
  7. Starter accelerates to 30% and the fuel loader meets demands and the flame is self sustaining. Igniter is shut down
  8. Starter accelerates to 60% and the fuel loader meets demands. Starter is shut down
  9. Engine ramps up to 100% Fuel supply is now met by both 100% open fuel loader and a butterfly valve on the fuel gas supply and throttle.
  10. Lube oil is now supplied by the engine driven pumps and all electric pumps shut down
  11. Engine speed is now controlled by load demands. Except in the case of a generator that has to keep the alternator turning at a fixed speed. In this case more fuel is supplied to meet the load and maintain RPM as a constant. Figure 15 Shows a typical starting graph.

 

Figure 15 Typical Start Up Graph