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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