Air Flow Requirement for Pneumatic Instruments


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How much air flow would be required for an installation having 'n' nos of Pneumatic Controllers & Transmitters, 'm' nos of Control Valves & 'p' nos of Loader Valves...

Currently the installation is operated using the available process gas, but we are planning to have a dedicated Instrument Air Compressors now. So we wish to know how much is the required air flow to cater to the needs of all Control Valves, SDVs, Loader Valves, Pneumatic Transmitters and Pneumatic Controllers.

Is there any exact or approximate formula available to be able to know the Air Flow Requirements??

Thank you very much...
That all depends on the number of instruments, the valve use, (size, proportional, or on-off and how active), vendor literature is a starting point,of knowledge of previous plants of the same design.

Don't forget instrument air is a lot different than simply compressed or shop air.

Inst air is dehydrated so as to avoid water build up, and not lubricated like heavy use shop air....both which result in high maintenance for you instrumentation.


If you take the air consumption of each device per cycle and multiply that number by the number of cycles in an hour you will be pretty close. Make sure to add at least a 50% overhead.
At one time we used to allow 1 cfm per instrument e.g. 1cfm per control valve. If instrument air is distributed in galvanized pipe, much of it disappears in leaks from threaded fittings, especially 2" and over.
Do you currently measure the volume of process gas used to supply instruments & valves? Of if not, could you add a temporary flowmeter/totaliser? With typical gas usage figures you should be able to convert to air using relative density figures?