Although we commonly use current to relay
information, some critical process measurements use compressed air to
transmit information from one point to another, an example of this can be a
petroleum refinery. Pneumatic instruments find use in some applications
that won’t work well with say 4-20 mA current signals due to safety concerns.
Pneumatic Instruments still find wide application in industry, although it is
increasingly rare to encounter completely pneumatic control loops.
One of the
most common applications for pneumatic control system components is control
valve actuation. Not only is compressed air used to create the actuation force
in many control valve mechanisms, it is still often the signal medium employed
to command the valve’s position. In most cases this pneumatic signal originates
from a device called an I/P transducer or current-to-pressure converter, taking
a 4-20 mA control signal from the output of an electronic controller and
translating that information as a pneumatic 3-15 PSI signal to the control valve positioner or the actuator.
Below is an example of Pressure Transmitter being applied in Pneumatic instrumentation:
Let's now look at the advantages and disadvantages
of Pneumatic Instruments.
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Disadvantages of Pneumatic Instruments
Include:
- Sensitivity to vibration, changes in temperature and mounting position which may affect the calibration accuracy to a far greater degree than electronic instruments.
- Compressed air is an expensive utility which is much more expensive per equivalent watt-hour than electricity which makes the operational cost of pneumatic instruments far greater than electronic. The installed cost of pneumatic instruments can be quite high as well given the need for special material i.e. Stainless steel, Copper, or Tough plastic tubes to carry air and pneumatic signals to distant locations.
- The volume of air tubes used to convey pneumatic signals over distances act as a low-pass filter, naturally damping the instrument’s response and thereby reducing its ability to respond quickly to changing process conditions.
So with the above disadvantages, why are the pneumatic instruments still
in use today?
The main reason may be due to legacy hence
facilities using these pneumatic instruments and have them in good work
conditions, won’t see the need to replace them since in most cases the cost of
labor to remove old tubing, install new conduit and configure new (expensive)
electronic instruments is often not worth the benefits.
You can also read: Pneumatic Signal Transmission
Advantages of pneumatic Instruments
include:
- Intrinsic Safety of pneumatic field instruments. Instruments that do not run on electricity cannot generate electrical sparks. This is of utmost importance in classified industrial environments where explosive gases, liquids, dusts and powders exist.
- Pneumatic instruments are also self-purging. The continual bleeding of compressed air from vent ports in pneumatic relays and nozzles acts as a natural clean-air purge for the inside of the instrument, preventing the intrusion of dust and vapor from the outside with a slight positive pressure inside of the instrument case. 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 function in high-temperature and high-radiation environments that would damage electronic instruments.
- Pneumatic instruments can also operate on compressed gases besides air. This is an advantage in remote natural gas installations, where the natural gas itself is sometimes uses as a source of pneumatic ”power” for instruments. So long as there is compressed natural gas in the pipeline to measure and to control, the instruments will operate. No air compressor or electrical power source is needed in these installations. All you need is a good filtering equipment to prevent contaminants in the natural gas (Dirt, liquids, Debris) from causing problems within the sensitive instrument mechanisms.
