Sunday, January 20, 2013

CHAPTER 1 INTRODUCTION TO GAS TURBINES




1.1       GENERAL


All generators send out energy, in the form of electrical power, and they have to be given the equivalent mechanical energy.  This means that they have to be driven by an engine of some sort which derives its energy from fuel or some other natural source such as wind or water.  The engine which drives an electric generator is called a ‘prime mover and may take many forms.

In the early days steam-, gas- or oil-driven reciprocating engines were used.  Later, steam turbines became more general, especially in large power stations.  More recently gas turbines have come into use, especially on oil platforms where gas is produced as part of the production process usually in sufficient quantities to provide a source of fuel.

The modern form of oil engine, the diesel, is also much used, principally for standby or emergency plant when gas supplies to the gas turbines fail or are shut down.  On a platform, of course, it is necessary to bunker diesel fuel for these engines. (See Chapter 6.)

As the various forms of gas turbine may not be familiar to some, a brief description of this type of engine and how it evolved is given overleaf.  Firstly, however, it may be advantageous to recap the principles of operation of its predecessor, the steam turbine.

1.2       THE STEAM TURBINE


Fuel was burned under a boiler, whose water (shown blue) was turned into steam.  This steam, at high pressure and speed, hit the inclined blades of a turbine wheel (yellow) and drove it round.  In doing so it lost some of its pressure, but enough was left to drive a second wheel - and a third or fourth - on the same shaft.  Finally the steam was exhausted into a condenser, turned back into water and returned to the boiler to be reheated and used again.  This was the whole steam ‘cycle’ which is shown in Figure 1.1.


FIGURE 1.1
STEAM TURBINE SYSTEM

1.3       THE GAS TURBINE


Gas-turbine generators are found generally in offshore installations, where natural gas is available. Onshore installations normally have only small standby or emergency generators, and these are usually diesel driven. This chapter, therefore, applies to offshore installations only.



FIGURE 1.2
GAS TURBINE

The gas turbine works on a similar principle to that of the steam turbine except that there is no boiler or water: instead the fuel is burned in a combustion chamber at one end of the turbine where it produces a hot, high-pressure gas.  This gas, in trying to expand, causes a reaction on each row of blades on a rotor (shown yellow), expanding and cooling as it does so and driving the blades round to produce mechanical power.  By expanding in the confined volume of the turbine, the gas has to keep up and even increase its speed in order to pass through.  The principle of the gas turbine is shown in Figure 1.2.

It should be noted that the gas is hottest at the combustion chamber or inlet end.  As it expands in the turbine, it cools, and it should leave the exhaust end at a lower temperature, of the order of 650oC.  Many turbines have instruments to measure exhaust temperature.  If it is too high, it indicates some fault in the combustion, and the set is usually shut down to save the blades from damage.





FIGURE 1.3
COMPRESSOR

In order for the fuel to burn, oxygen, in the form of air, is needed, and it must be at high pressure in order to enter the combustion chamber; therefore an air compressor is fitted integrally with the turbine.  It is just like a turbine in reverse.  Air is drawn in at the larger diameter end by the inclined blades acting as a suction fan.  Once in, it is compressed by the blades of the compressor rotor (shown yellow) into a smaller volume, to be sucked in again by the next row of blades and compressed still further.  Each stage of compression causes the air to become hotter.  Eventually it emerges at the small diameter end as hot compressed air.  The principle of the gas-turbine compressor is shown in Figure 1.3.

To provide the power to compress the air, the compressor must be driven mechanically.  The turbine itself drives it.  The gas-turbine shaft is coupled to the compressor shaft and constitutes the complete gas-turbine assembly.  This looks like perpetual motion - which it is, provided that the fuel continues to be supplied.  The combined turbine and compressor unit is shown in Figure 1.4.


FIGURE 1.4
SINGLE SHAFT GAS TURBINE SET

In practice something like 80% of the power developed in the turbine from the combustion is needed to drive the compressor, leaving only about 20% ‘payload’ to drive the load.  It can be seen that, if there is a drop of only about 5% in the combustion efficiency, so needing 85% of the output to drive the compressor, the effect is to reduce the payload from 20% to 15% - an effective reduction of load drive of 25%.  Gas turbines are therefore very sensitive to combustion control.

In the gas turbine on an oil platform the power developed by the gas turbine, less that part of it needed to drive the compressor, is used to drive the load, which may be a machine such as a generator, air compressor or pump.

One of the great advantages of the gas turbine over other forms of prime mover is its high power/weight ratio.

An important point to note is that, unlike other types of engine, the gas turbine needs to take in up to 70 times the amount of air actually needed for combustion.  The excess is for cooling.  This means that gas turbines have very large intake ducting, usually provided with screens and filters to prevent the entry of sea birds and other sizeable particles.  In freezing weather the screens can become iced up and restrict the flow of air.  Therefore, anti-icing equipment and blow-in doors are often provided (see para. 1.13).

1.4       SINGLE SHAFT AND TWO SHAFT TURBINES


The type of gas turbine shown in Figure 1.4 is known as a ‘single-shaft’ type - that is, the power turbine, compressor and driven load are on a single, common shaft.  The power delivered by the power turbine is divided between the compressor (about 70% to 80%) and the driven load (about 20% to 30%).

In some larger gas turbines the arrangement is different.  A standard aircraft-type jet engine may be used, as shown in Figure 1.5, where the compressor turbine is only large enough to drive the compressor itself, with no driven load.  But the exhaust gas which, in an aircraft, would go straight to jet is ducted to the input of a further power turbine on a separate shaft; its rotor is shown blue.  This drives the load, and usually at a speed different from that of the compressor turbine.

This is known as a ‘two-shaft’ gas turbine and has the advantage that it can be used with an existing proved aero gas-turbine design with only minor modifications to the jet end.  The power turbine is a completely separate design which need not even be in line with the compressor (though it usually is).  The complete aero compressor/compressor-turbine unit is known as the ‘gas generator’, and the separate load-drive unit as the ‘power turbine’.

In a single-shaft gas turbine the power turbine is usually coupled to the driven load (a generator or compressor) through a gearbox.  The compressor and power turbine therefore run at the same fixed speed, which is the generator speed multiplied by the gear ratio.  In a two-shaft turbine the compressor and the power turbine can, and do, run at different speeds.  The power turbine is coupled to the generator and runs at governed speed, but the compressor speed varies with the loading.  At light load it will be idling, but as loading increases it increases its own speed up to full load, when it will generally be running much faster than the power turbine.





FIGURE 1.5
TWO SHAFT GAS TURBINE SET

1.5       FUEL


Like any other internal combustion engine, a gas turbine can burn gas or liquid fuel (usually diesel oil).  Some turbines are designed for single-fuel burning - that is, for gas only or liquid only - whereas others may have been adapted for ‘dual fuel’; they may be set to run on either fuel or, in some gas turbines, on a mixture of both.

On oil platforms where gas is available the turbines will be for gas only or dual fuel.  If dual, they will normally run on gas, with liquid fuel as a fall-back if gas pressure should fail.  If this happens, the changeover from gas to liquid is automatic; the turbine does not stop, but an alarm is given.  When gas pressure is restored the change back must sometimes be done by hand in slow time, but on some sets the change back is automatic provided that ‘Gas Fuel’ had been selected originally and the fuel selector switch had not been moved.

There are exceptions to the arrangements described above.  In some installations there is no automatic changeover, even from gas to liquid fuel.

Gas turbines can operate on a variety of fuels which range from crude oil (with some derating of the turbine) through to fuel gas.

1.6       SPEED CONTROL


The turbine speed is always controlled by a governor.  In single-shaft sets the governor controls the shaft speed, but in two-shaft sets it controls only the power turbine speed - and so the speed of the driven load.  The gas-generator shaft is free to take up its own speed, depending on the load, as explained below (see also Figures 1.6 and 1.7).

In single-shaft turbines the governor controls speed by regulating the gas control valve or the liquid fuel valve.  In two-shaft sets the governor itself is driven from the power turbine shaft but regulates the fuel input to the gas generator.  This runs at such a speed as to provide just enough gas to the power turbine to keep it at its correct speed.  Thus, as load increases, the gas generator speeds up, but the power turbine stays at constant speed.  The skilled operator can detect load changes by the note of a two-shaft machine, but not with a single-shaft.

Speed control is discussed in detail in Chapter 5.




FIGURE 1.6
SINGLE SHAFT GAS TURBINE CONTROL




FIGURE 1.7
TWO SHAFT GAS TURBINE CONTROL

1.7       STARTING


Gas turbines must be started by an external starter.  This may be electric, air-motor or even a diesel engine. For some sets a separate small gas turbine is used, which itself is electrically started.  Electric starting requires a separate battery and charger.

Because in single-shaft machines not only the compressor and turbine but also the driven load (usually a generator) and gearbox must all be started together, the starting unit must be relatively heavy, and this precludes electric start, with battery, on any but the smallest machines.  On the other hand in a two-shaft set the starter has only to spin up the compressor/turbine unit of the gas generator, so that the starter need only be quite small and is suited to an electric motor.

On some of the largest sets the d.c. power for the starting motor is taken not from a battery but is rectified from the set’s general a.c. supplies.  This requires that auxiliary a.c. supplies for the turbine set shall be available before the set can be started.

When the start button is pressed, but before the gas-turbine shaft actually begins to move, automatic circuits put into action a sequence programme which normally includes starting a lubrication pump to pre-lubricate the turbine, gearbox and generator bearings. In some machines the turbine hood is also purged with air to remove any gas present.  When the lubricating oil pressure has reached a certain level, the start motor is actuated and begins to rotate and accelerate the turbine shaft (in the case of a two-shaft turbine, the gas-generator shaft only).  When it reaches a certain speed, usually about 20%, fuel is admitted to the combustion chamber which is by now receiving some air from the compressor.  Automatic ignition by spark-plug and torch follows, and the hot burning gas passes to the power turbine in the case of a single-shaft set, or to the compressor turbine of a two-shaft set.

The turbine gradually takes over the job of driving the compressor, and the starting motor steadily becomes off-loaded.  When its load falls to a predetermined level, the electric start motor is switched off, or the mechanical start motor unclutched and stopped.  The ignition is also switched off.  As the set runs up, a mechanically driven lubricating oil pump begins to deliver lubrication to the bearings. When its pressure reaches a certain level, the electrically driven pump stops automatically.

The turbine is now self-sustaining, and the speed continues to build up until it comes under governor control and settles at its correct level.  In a two-shaft set the gas-generator speed continues to build up, and the hot burning gas from it passes on to the main power turbine, which then starts to move by itself without any mechanical starting.  Its speed too builds up until it comes under governor control, when the fuel to the gas generator is cut back and it settles down to its no-load or ‘idling’ speed.  The power turbine, however, is now running at its controlled speed.

1.8       STOPPING


To stop the set, fuel is simply cut off, and the set runs down steadily.  Part of the stopping sequence includes the starting of the electrically driven lubricating oil pump so that bearing lubrication continues as the set runs down and the mechanical lubricating oil pump becomes less effective.  The electric pump continues to run for some time after the set has actually stopped.

Some larger sets have a hydraulic ratchet arrangement which slowly turns the turbine rotor, after stopping, for up to 24 hours to prevent ‘bowing’ of the rotor due to uneven cooling.

All sets are arranged so that, when the ‘Stop’ button is pressed or when a shutdown signal is given for any other reason, the associated generator supply breaker is tripped, if not already open, to off-load the turbine.  Sometimes automatic provision is made to off-load the set gradually (except with an emergency stop) before the supply breaker actually trips.

1.9       PROTECTION


The turbine (as distinct from the electric generator) has a number of protective devices to guard against malfunction both during starting and while running.  During starting each stage of the sequence is monitored to ensure that it is completed within a certain time; if it is not so completed, the start is ‘aborted’, and the set, if already moving, is stopped and the appropriate alarm given.

During running other protection operates, including such obvious things as overspeed, excessive vibration, loss of lubricating oil pressure, high lubricating oil temperature and excess exhaust temperature (which indicates a combustion fault).  All these and other malfunctions shut the set down (having also tripped the generator breaker) and give the appropriate alarm.

Alarms are visual and audible.  The visual alarms are grouped into ‘annunciator’ lamp boxes on the control board, each lamp window being annotated with the fault it announces.  At the onset of a fault the lamp flashes and a buzzer sounds.  When the ‘Accept’ button is pressed the buzzer stops and the lamp burns steadily.  It does not go out however until the fault has been cleared; even then a ‘Reset’ button must be pressed before the set can be started again.  If a turbine stops during starting or shuts down during running, it should be possible for the operator to diagnose from the lamp indications the cause of the trouble.

On some sets the starting sequence lamps, which merely monitor the starting stages but do not indicate a malfunction, are segregated from the fault lamps.  On most platforms the turbine malfunction alarms are repeated in the Electrical Control Room, though they are usually grouped to reduce the number of lamps there.

One of the most damaging things that can happen to a turbine is failure of lubrication.  This can cause bearings to fail at high speed with probable catastrophic damage and danger to personnel.  To guard against this all sets have not only the mechanically driven oil pumps and the a.c. electrically driven pumps for starting pre-lubrication, but also a d.c.-driven emergency pump fed from a battery, which cuts in automatically on failure of lubricating oil pressure.  This is a vital piece of equipment, and it must be regularly tested to prove its proper functioning.

1.10     CONTROL


With every gas turbine there is a Local Control Panel, usually adjoining the control panel for the generator.  The turbine control panel has instruments to indicate speed, temperatures and pressures at various points and fuel pressure, as well as controls for starting and stopping, for fuel selection and for setting the speed.  There are a number of lamps in an annunciator panel to indicate malfunction, and others to indicate successful completion of each starting sequence step.  On some larger sets the governor control occupies a complete panel on its own.

1.11     SPEED CONTROL


Control of speed by automatic governor is dealt with in Chapter 5.

1.12     WASHING


Air pollution, especially salt, can cause encrustation of the compressor blades, distorting their aerodynamic form and reducing the efficiency of compression.  This shows up as higher exhaust temperature.

When this situation occurs (and high exhaust temperature indication is a pointer) the compressor must be ‘washed’.  Turbine manufacturers make arrangements for this to be done at reduced turbine speed using water, with or without detergent, or solid abrasives such as ground walnut husks or bran.  The two methods are sometimes known as ‘Crank Soak’ (liquids) and ‘Abrasive Cleaning’ (solids).

Details of the methods are given in the manufacturers’ Operations or Maintenance Manuals.

1.13     ANTI-ICING


In freezing weather the air intake filter screens can become iced up; in this state they can severely restrict the intake of air and cause serious combustion problems.

One way of dealing with this is to duct warm air from the engine to the area of the screens.  On some makes of turbine this is taken from the exhaust ducting; on others it is bled off the later stages of the turbine compressor.

Blocked screens, whether due to icing or other causes, can bring about problems, and immediate steps should be taken to clear them. One feature which assists the air flow in the short term is the ‘blow-in’ door. This is a door, usually on the side of the intake and downstream of the screens, which is loosely hinged and is just kept closed by gravity.  If the screens become blocked, the differential pressure across them increases, and the lower pressure inside the ducting sucks the door open, so allowing air to bypass the screens until action to clear them can be taken. The opening of a blow-in door gives an alarm to the operator.  The door usually has a de-icing heater to prevent its becoming iced up.
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