Instrument air is the mixture of N2 , O2 & very small amount of dust (below the required limit) used in the instrument to operate pneumatic valves, certain types of pumps, fans, some blowing down hoses.
Basics of Instrument Air System
The Instrument air in a plant is used to supply motive force for control valves & that keeps the plant in control and running. Instrument air is often specially dried to reduce the risk of condensation freezing-out in the small-bore piping.
To maintain the above situation in the plant Instrument air must be dried to remove any moisture and/or condensate for:
Protecting the instruments and control system from damage.
Obtaining exact readings through these instruments and control system.
So the instrument air supply should be of highest quality
Instrument Air Quality:
The quality of instrument air is what distinguishes it from a compressed or service air system. The quality of the air is important to ensure that instrumentation will function properly and reliably.
The most important parameters in specifying air quality are:
- Dew Point
- Oil Content
The Instrument Society of America sets quality standards for instrument air in ISA S7.3.
Different pieces of equipment consume different amounts of air. For example, a shutdown valve will consume air when it is being actuated. A throttling valve will have a constant bleed rate with additional consumption when the valve is modulating.
A diaphragm pump consumes air when it is being actuated. Instrument air is provided by a compressor and requires minimal treatment to ensure that the air is free of oil, water or particulate matter. This is usually accomplished with some type of filter regulator on the compressor outlet and a dryer.