Instrument Air

Intro to Instrument and Plant Air

Author

Matt Anderson
August 04, 2023

Instrument air is under-appreciated. Serious upsets will occur if the instrument air system is unreliable.

The majority of control valves in a refinery use instrument air to control valve position via an actuator. Valves can be designed either as “air-to-open” or “air-to-close”. What “air-to-open” means is that air pressure must be supplied to the valve to increase the valve opening. The consequence of this design is that in the case of instrument air failure, the valve will fail to the closed position (fail-closed valve). The reverse is true for the “air-to-close” type valve.

Besides the obvious of maintaining the instrument air header pressure, how can we define the qualify of instrument air?

Fortunately, there is an industry standard, ANSI / ISA 7.0.0-1996, which includes four elements of instrument air quality. [1]

  1. Dew Point

    Dew point measured at the outlet of the air dryers should be at least 18 degF lower than the coldest ambient temperature to which the instrument air system is exposed.

  2. Particle Size

    40-um (micrometer) particle size is the maximum to maintain instrumentation reliability

  3. Lubricant Content

    If you have heard about “oil-free” air, this is the same concept. Lube oil should not be part of the instrument air system.

  4. Contaminants

    Instrument air should be air, and not include other vapors or gases. This depends mostly on the siting of the air inlet of the air compressors.

Works Cited