Pressure /Flow control on Air Line


Thread Starter

B Wills

I want to control the air pressure and the flow rate on a pressurized air line. The air pressure comes from a train of reciprocating compressors feeding in to an air receiver. It is planned to put a a PT , PIC and a pressure control valve on the oulet of air receiver for precise pressure and then a little further downstream to put an FT, FIC and a flow control valve for flow rate regulation.

What are the pros and cons of such a scheme especially looking at the possibility of the PV and the FV always trying to balance or pull against each others demands ? The line presssure desired is about 40 psi but upstream of the receiver is about 100psi. The line size on which the valves are to be installed is about 10 inches.

James Fountas

Assumption #1: usage of air changes with process and equipment in use.

It probably won't work, depending on your objective. You can control pressure or flow but not both in most situations. If you control pressure and flow to fixed set-points, your usage is fixed. The only place your pressure will be at the setpoint is between the two valves.

The two control loops will interact and possible create problems depending on dynamics and the desired stability.

What is your objective?

James Fountas
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From your email, maintaining downstream pressure of 40 psi seems to be the real variable to be maintained. As a utility, the air compressor and
drier must be sized to meet the routine demand. Air receiver gives few precious minutes to bring an orderly shutdown of plant through Control and air operated shutdown valves. Flow control on a utility line (air, steam, Nitrogen) is not generally done. You may consider flow meter in air line with High alarm if you expect sudden demands and sort out such issues by mutually agreed operating procedure. Classic case of FIC on instrument air line is to measure Instrument air flow and throttle plant air flow (for hose stations and other miscellaneous applications) being less critical than instrument air.

Hope this helps.

You can do it ONLY if you have a higher range any one differential. That is if your priority is more on flow, then set your pressure range tolerence broader to say 40 to 50 PSI. You also must include a quick exhaust in the air line to release the excess pressure.Your pressure transducer should be located after the flow transducer

There is nothing wrong in giving a try. I ever tried this once before and I could maintain only one parameter well than the other.

best of luck
Hope this helps.