Presently we have a pneumatic control system. All transmitters, controllers and transmitting signals are working with air. For that we are running two instrument air compressors. Now we want to upgrade present control system to DCS, where control valves and I/P converter will be operated in pneumatic only. For these jobs how much air will be required? How we calculate this? Our aim is to stop one instrument air compressor. Can we be able to this?

 

You will need to figure out how much air you are saving. Each of your instruments should have a consumption rate.

To figure out a compressor size I normally allow 1 cfm per control valve and I expect the rest of the loop would be about the same (2 cfm per loop).

Air leaks can have a significant impact, particularly threaded galvanized pipe so the more of that you can eliminate the better.
If you really want to save air check out some of the new positioners that don't operate on flapper nozzle. Some use hardly any air when the valve is stationary.

It might be an interesting exercise to do before and after study. If you don't have a flowmeter perhaps you could total the compressor run hours for a week.

Let us know how you get on.

Regards,
Roy