Instrument Air

Written by Jerry Ratzlaff. Posted in Instrumentation

Instrument air is used in a facility to operate valves and certain types of pumps.  Pneumatic actuators rely on instrument air for operation.  Some types of modulating valves require instrument air for throttling.  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.  

  • Abbreviated as AI

Consumption

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.  

Units

Volume and rates are described as:

  • nmch - normal meters cubed per hour
  • scfm - standard cubic feet per minute
  • fpm - feet per minute

Sizing

Sizing an instrument air system is different than sizing a piping system where there are constant flow rates. Instrument air systems should be designed to ensure the safe and consistent operation of the end devices.

When sizing an instrument air system, there are several different approaches. One approach would be tabulating all the instruments and devices that consume air. However, because so many systems in a plant are dependent on instrument air, it is far better to have to much air available than too little.

Another approach would be to size the system for all instruments consuming air at the same time. As a rule of thumb, a great starting point is assuming each end device requires 2 scfm. Add all the end devices up, account for future expansion and add 10% for leaks and contingency.