Instrument Air

Instrument air, abbreviated AI, is compressed air that is specifically used for various industrial instrumentation and control purposes.  In industrial settings, especially in sectors like petrochemical, chemical, and power plants, there are numerous instruments and control devices that rely on compressed air to operate effectively.

Instrument air is dry and free of contaminants, such as oil and water vapor, to ensure the proper functioning of sensitive pneumatic instruments.  These instruments include control valves, actuators, transmitters, certain types of pumps, and various sensors that play crucial roles in monitoring and controlling industrial processes.  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.

The production of instrument air involves compressing atmospheric air using compressors and then treating it to remove moisture, oil, and other impurities.  The resulting instrument air is typically delivered at a specific pressure and quality standard to meet the requirements of the instruments it serves.  Maintaining a clean and dry supply of instrument air is essential to prevent damage to sensitive equipment and ensure the accuracy and reliability of process control systems in industrial facilities.  Regular monitoring and maintenance of the instrument air system are necessary to guarantee its proper functioning and adherence to quality standards

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. 

Sizing Instrument Air

Sizing an instrument air system is different than sizing a piping system where there are constant flow rates.  Sizing an instrument air system involves determining the necessary capacity and pressure requirements to meet the demands of various pneumatic instruments and control devices within an industrial facility. Here are the general steps involved in sizing an instrument air system:

Identify Instrumentation Requirements  -  Create a comprehensive list of all pneumatic instruments and control devices that require instrument air to operate.  This includes control valves, transmitters, actuators, and any other pneumatic equipment.  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.
Determine Air Consumption  -  Identify the air consumption of each instrument in standard cubic feet per minute (SCFM) or other relevant units.  This information is typically available in the instrument data sheets.
Estimate Peak Air Demand  -  Calculate the total peak air demand by summing up the individual air consumption values of all instruments that may operate simultaneously.
Consider Redundancy and Future Expansion  -  Account for redundancy and any potential future expansion in the system.  It's a good practice to design the system with some extra capacity to accommodate additional instruments or account for maintenance periods.
Specify Air Pressure Requirements  -  Determine the required pressure level for the instrument air system.  This is usually based on the highest pressure required by any of the instruments in the system.
Calculate Total Air System Capacity  -  Based on the peak air demand and required pressure level, calculate the total capacity needed for the instrument air compressors.  Ensure that the compressors selected can meet the peak demand while maintaining the required pressure.
Select Compressors  -  Choose appropriately sized compressors based on the calculated capacity. Consider factors such as compressor efficiency, turndown capabilities, and any other specific requirements of the facility.
Design Air Distribution System  -  Design the distribution system that delivers the instrument air to various points in the facility.  Consider factors such as pipe sizing, pressure drop, and the layout of the air distribution network.
Include Dryers and Filtration  -  Integrate air dryers and filtration systems to remove moisture, oil, and contaminants from the instrument air to meet the required quality standards.
Perform System Checks  -  Conduct checks and simulations to ensure that the instrument air system meets the demands of the instruments under different operating conditions.

It's crucial to work with engineering standards, guidelines, and the recommendations of equipment manufacturers during the sizing process to ensure a reliable and efficient instrument air system.  Additionally, consulting with experienced engineers or specialists in compressed air systems can be beneficial in achieving an optimal design.

Units

Volume and rates are described as:

• acfm - actual cubic feet per minute.  This is scfm adjusted for actual pressure and temperatures. 
• scfm - standard cubic feet per minute