Instrument air requirements

[hianbotech]

Hi:
Anyone knows of how to estimate instrument air consumption by pneumatic instrument?
Normaly, during engeneering phase instrument air requirements are estimate as 0.072 kmol/h for each consumer of instrument air, such as valve positioner on control valves loop or 0. 144 kmol/h for actuator displace volume on on-off control service. For evaluating an existing air instrument system, i am wonder if there is any criteria or formule to get more accurate figures per each consumer, for example, as a function of valve or actuator size.
I already contact instrument vendor and their answer has not been satisfactory.
Thanks for your help.

 

[freda]

Air consumption should be specified in the instrument vendors liturature. Air consumption will differ significantly between different instruments.

 

[jsummerfield]

Identify all of the air consumers. My first assumption would be that your need is for an electronic plant with few if any pneumatic transmitters or controllers. If the process is regulatory the valve use the data for the valve transducer. Shutdown valves use almost no air during normal operation and during a trip. You need to consider the time required to recharge the valves during startup. Some equipment may use instrument air to purge enclosures. Look at these requirements carefully. If any equipment uses a Vorex cooler you must include this very high volume.

Too big is not as bad as too small.

John