HIGH PRESSURE PNEUMATIC SYSTEM
General:
The use of a high pressure compressed-air system is normally seen in piston
engine aircraft to operate an its services and this usually represents a saving
in weight compared to a hydraulic system, since the operating medium is freely
available, no return lines are necessary, and pipes can be of smaller diameter.
Systems having operating pressures of up to (3,500 lbf/in2) are in
use, and provide for the rapid operation of services when this is required.
However, this compressed air is generally not suitable for the operation of
large capacity components, leaks can be difficult to trace and the results of
pipeline or component failure can be very serious.
which also use a high-pressure
pneumatic system, however, and there are many aircraft which use pneumatic
power for the emergency operation of essential services; the latter type of
system is usually designed for ground-charging only.
Supply sources:
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Figure 1.1: Pneumatic supply
source in a high pressure pneumatic system layout.
As said earlier, high pressure
pneumatic system uses high pressure bottles as a storage cylinder that receives
pressurized pneumatic from independent compressor, as shown in Figure 1.1, or, from an engine driven
compressor as shown in Figures 1.2, 1.3.
Figures also illustrate essential
components required for operation, distribution and control of pneumatic.
1.5.3
Typical high pressure pneumatic systems: The
system illustrated in Figure 1.2 is
a pneumatic system contains two separate power circuits, each of which is
supplied by a four-stage compressor driven from the gearbox of one main engine,
and a common delivery pipe to the high-pressure storage bottles and system
services. A multi-stage cooler attached to each compressor cools the air between
each of the compression stages, and a means is provided for off loading the
compressor when the system is not being used.
Air is drawn
through an inlet filter into each compressor, and is discharged through an
oil-and-water trap, a chemical dehydrator, a filter and a non-return valve, to
the main storage bottle and system. Overall
control of main system pressure is provided by means of a pressure regulator,
but pressure relief valves are included to prevent excessive pressures in the
system, which may be caused by regulator failure or by an increase in
temperature in the pipelines and components. Pressure reducing valves are used
to reduce the pressure supplied to some components.
A storage
bottle for the emergency system is pressurized through a non-return valve from
the main system supply, and maintains an adequate supply of compressed air to
enable the landing gear and flaps to be lowered, and the brakes to be applied a
sufficient number of times to ensure a safe
landing.
Isolation
valves are fitted to enable servicing and maintenance to be carried out without
the need to release all air from the system, and pressure gauges are provided
to indicate the air pressure in the main and emergency storage bottles.
Figure 1.3
illustrates another typical full pneumatic system as is used on a popular
European-built twin-engine commuter transport airplane. Each of the two
compressors is a four-stage piston-type pump driven from the accessory gearbox
of the two turboprop engines. Air is taken into the first stage through an air
duct and is compressed, then passed successively to the other three stages. The
discharge air from the fourth stage is routed through an intercooler and a
bleed valve to the unloading valve. The bleed valve is kept closed by engine
oil pressure and, in the event of a loss of the engine lubricating oil, the
valve will open and relieve the pump of any load.
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Figure 1.2: A high pressure pneumatic
system layout
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The unloading valve maintains pressure in the
system between 2,900 and 3,300 psi. When the pressure rises to 3,300 psi, a
check valve traps it and dumps the output of the pump overboard. When the
system pressure drops to 2,900 psi, the output of the pump is directed back
into the system.
A shuttle valve in the line between the compressor
and the main system makes it possible to charge the system from a ground
source. When the pressure from the external source is higher than that of the compressor, as it is when the engine is
not running, the shuttle slides over and isolates the compressor.
Moisture in a compressed air system will condense
and freeze when the pressure of the air is dropped for actuation and, for this
reason, every bit of water must be removed from the air. A separator collects
the water that is in the air on a
baffle and holds it until the system is shut down. When the inlet pressure to
the separator drops below 450 psi, a drain valve opens and all of the
accumulated water is blown overboard. An electric heater prevents the water
collected in the separator from freezing.
After the air leaves the moisture separator with
about 98% of its water removed, it passes through a desiccant, or chemical
dryer, to remove the last traces of moisture.
The air before it enters the actual operating
system is filtered through a 10-micron sintered metal filter, and when we
realize that the lower level or visibility with the naked eye is about 40'
microns, we see that this provides really clean air to the system.
A back pressure valve is installed in the right
engine nacelle. This is essentially a pressure relief valve in the supply line
that does not open until the pressure from the compressor or ground 'charging
system is above 1,700 psi and this assures that the moisture separator will
operate most efficiently. If you should want to operate the system from an
external source of less than 1,700 psi, it can be connected into the left side
where there is no back pressure valve.
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Figure 1.3: A typical HP pneumatic system used on a
twin-engine turboprop airplane
The majority of the components in this system
operate with pressure of 1,000 psi, so a pressure reducing valve is installed
between the isolation valve and the supply manifold for normal operation of
the landing gear, passenger door, drag brake, propeller brake, and nose wheel
steering. This valve not only reduces the pressure to 1,000 psi, but it also serves as a backup pressure
relief valve.
The emergency system stores compressed air under
the full system pressure of 3,300 psi and supplies it for landing gear
emergency extension.
Emergency Backup System: All aircraft with retractable
landing gear must have some method of assuring that the gear will move down and
lock in the event of failure of the main extension system. One of the simplest
ways of lowering and locking a hydraulically actuated landing gear is by using
compressed air or nitrogen stored in an emergency cylinder. The gear selector
is placed in the gear down position to provide a path for the fluid to leave
the actuator and return into the reservoir. Compressed air is then released
from the emergency cylinder, and it enters the actuator through a shuttle
valve. This valve is moved over by air pressure to close off the hydraulic
system so no air can enter it. The air pressure is sufficient to lower and lock
the landing gear against the flight loads.
Emergency operation of the brakes is also achieved
in many airplanes by the use of compressed air. When the pilot is sure he has
no hydraulic pressure to the brakes, he can rotate the pneumatic brake handle
located on the left instrument panel. Clockwise rotation of this handle
increases the brake pressure, and when the handle is held stationary, the
pressure is constant. Nitrogen pressure released by this control handles forces
hydraulic fluid in the transfer tube into the main wheel brakes through shuttle
valves. When the brake handle is rotated counter clockwise, pressure is
released and the nitrogen is exhausted overboard.
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Figure 1.4: Emergency brake
actuating system for a large jet transport airplane
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Figure 1.5:
Emergency brake control handle located on the left instrument panel of a jet
transport airplane
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