Basics of Pneumatic instruments
Introduction
A pneumatic system is a system that uses compressed air to transmit and control
energy. Pneumatic systems are used in controlling train doors, automatic
production lines, mechanical clamps, etc
What are pneumatic systems used for?
The pneumatic systems used in the industry are usually driven by compressed air or compressed inert
gases. A centrally located, electrically powered compressor drives cylinders, air motors and other pneumatic
devices.
Examples of pneumatic systems and components
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Air brakes on buses and trucks
Air brakes on trains
Pneumatic actuator
Pneumatic cylinder
Pneumatic Launchers
Pneumatic mail systems
Pneumatic motor
Pressure regulator
Pressure sensor
Pressure switch
Air compressors
Disadvantages of pneumatic instruments
The disadvantages of pneumatic instruments are painfully obvious to anyone familiar with pneumatic and
electronic instruments.
 Sensitivity to vibration, temperature changes, mounting position and the like affect calibration
accuracy to a much greater extent for pneumatic instruments than electronic instruments.
 Compressed air is an expensive tool, much more expensive per watt-hour equivalent than electricity,
which makes the operational cost of pneumatic instruments much higher than the electronic one.
 One of the properties of pressurized air is like to always occupy the empty space and the air pressure
is maintained in hard work. Therefore we need a seal so that air does not leak. Seal leakage can
cause energy loss. Pneumatic equipment should be equipped with airtight equipment that
compressed air leaks in the system can be minimized.
 Pneumatic using open system, meaning that the air that has been used will be thrown out of the
system, the air comes out pretty loud and noisy so will cause noise, especially on the exhaust tract.
The fix is to put a silencer on each dump line.
 The installation cost of pneumatic instruments can also be quite high, given the need for special pipes
(stainless steel, copper or resistant plastic) to transport supply air and pneumatic signals to distant
locations. The volume of air tubes used to transmit pneumatic signals across distances acts as a low
pass filter, naturally dampening the response of the instrument and, therefore, reducing its ability to
respond quickly to changing process conditions .
 Pneumatic instruments can not be made “smart” as electronic instruments either.
 With all these disadvantages, one might wonder why pneumatic instruments are still used in modern
industry.
Part of the answer is a legacy. For an industrial facility built decades ago, it makes little sense to
replace instruments that still work well. The cost of labor to remove old pipes, install new conduits and
cables and configure new (expensive) electronic instruments is often not worth the benefits.
Advantages of pneumatic instruments
However, pneumatic instruments actually enjoy some definite technical advantages that ensure their
continued use in certain applications even in the 21st century.
 A decisive advantage is the intrinsic safety of pneumatic field instruments. Instruments that do not work
with electricity can not generate electrical sparks. This is very important in “classified” industrial
environments where there are gases, liquids, powders and explosive dusts.
 The pneumatic instruments also purge themselves. Its continuous purging of compressed air from the
ventilation holes in the relays and pneumatic nozzles acts as a natural clean purge for the interior of the
instrument, preventing the entry of dust and vapor from the outside with a slight positive pressure inside
the box of the instrument.
 Ease of power and speed transmission
 Ease of use
 Ease of maintenance
 Infinite availability of the source ;Air is the most important thing in the pneumatic system, and as we all
know, air is available in the world around us in unlimited quantities at all times and places
 It is not uncommon to find a pneumatic instrument mounted in the field with incrustations of corrosion
and dirt on the outside, but clean in the factory inside due to this continuous purge of clean air.
Pneumatic instruments mounted inside larger enclosures with other devices tend to protect them all by
providing a positive pressure air purge for the entire enclosure.
 Some pneumatic instruments can also work in environments of high temperature and high radiation that
would damage electronic instruments. Although it is often possible to “harden” electronic field
instruments in such harsh conditions, pneumatic instruments are virtually immune in nature.
 An interesting feature of pneumatic instruments is that they can work with compressed gases other than
air. This is an advantage in remote natural gas installations, where natural gas is sometimes used as a
source of pneumatic “power” for the instruments. As long as there is compressed natural gas in the
pipeline to measure and control, the instruments will operate. No air compressor or power source is
needed in these installations. What is needed, however, is good filtering equipment to prevent
contaminants in natural gas (dirt, debris, liquids) from causing problems within the sensitive
mechanisms of the instrument.
Air compressor basics – types & accessories
What is an air compressor?
A compressor is a machine that compresses air or another type of gas from a low inlet pressure (usually
atmospheric pressure) to a higher desired pressure level. The compressor increases the air pressure by
reducing its volume.
The pneumatic control systems operate with a supply of compressed air, which must be available in
sufficient quantity and at a pressure that suits the capacity of the system. The useful life of a pneumatic
system depends largely on the preparation of the compressed air. Thus, the compressor is an important
device in industries.
There are different types of compressors, classified based on the principle of operation, the number of
stages included to compress air and highest pressure developed.
Types of the compressor based on working principle:
A reciprocating compressor compresses the air in a cylinder, against the cylinder head by a
reciprocating piston. A reciprocating compressor can single acting and double acting and single stage and
multi-stage compressors. Single acting compressor utilize piston connected to the crankshaft by connecting
rod and compressed air will be at piston side only. Double acting piston compress on both side.
Here shows a multistage compressor, there are three cylinders acting to compress the air.
Rotary Screw air compressor:
A rotary screw air compressor utilizes two meshing helical shaped rotors to compress the air. As the rotor turns, air is
compressed by advancing the helix. The rotor may be either oil flooded or dry.
The dry rotor compressor requires the use of synchronization gears to maintain proper separation between the rotor. The
oil in the oil-filled compressor lubricates and seals the rotor and acts as a refrigerant to remove heat from the compressor.
The rotary screw compressor has smaller moving parts than the reciprocating compressor and provides a smooth, almost
pulse-free supply of air. Little maintenance can be achieved in the field by plant personnel.
Main accessories of compressors:
Inlet filter:
Inlet filter prevents dust and other particles from entering the compressor.
Aftercooler:
Aftercoolers are implemented in the discharge line to lower the compressed air discharge temperature.
Separator:
Separators are used to remove entrained liquid from compressed air. The most common type of separators
is impingement, centrifugal and cyclone types. The separator should be equipped with trap and drain.
Traps:
Traps collect liquid that has to removed from the air by separation and condensation and release it.
Dryer:
Dryers are used when dryer air is required than can be provided by an aftercooler system.
Pressure regulating valve:
Pressure regulating valves are used to supply small volumes of air to various pneumatic equipment.
Pressure relief valves:
In every compressor, there will a pressure relief valve for safety. The relief valve should be set to open at no
higher than 10% above the maximum working pressure and periodically checked for proper operation.