We have pneumatic control in a plant. For that two air instrument compressors are running. Now we need a modern control system. How can we measure how much air flow will required in proposed control system? How will we benefit from cost point side? How much air flow is required in a 2" diaphragm operated control valve?

 

Compressed air is expensive! And air usage is driven by more than just the valve size...

I wrote an article in Control magazine in 2002, that discussed minimizing the air flow requirements for control valves. Keep in mind that poorly tuned control loops can create excessive valve movement, requiring more air. The article shows some estimates of air flow and air cost for various valve/actuator combinations.

The article is no longer carried on Control magazine's web site, ControlGlobal.com, but you can view the original article at:
http://www.expertune.com/r2p.asp?f=AList&t=ValvesAir&l=articles/buckbee2002/buckbee.html

-George

 

The rule of thumb used to be 1 scfm per control valve or On/Off valve. This is a bit high for modern control valves. Look at the manual for a typical valve positioner it should give you the figure.
In my experience leakage through threaded fittings can be quite significant if galvanized piping is used for the air headers.

Roy

 

It seems that there are a number of questions here.
1. Is there a cost benefit to fitting a flowmeter?
2. How can a modern control system reduce costs
3. What is the typical useage of air for a 2" valve.

Answers
1. There is only a cost benefit when you fit a flow meter if you use the information that it provides to take some cost saving action. It is more likely you can use the flowmeter to evaluate a munber of pilot schemes.

2. Some of the changes that you could consider would be to correctly tune each control loop. as suggested earlier. This will reduce the cycling on vales and hence reduce the air useage.

You could also change any flapper nozzle, or other continious bleed valve positioners for piezo valve types. This will ensure that air is only vented when you deflate the valve diaphragm. Be carefull that this type of positioner needs clean, dry air.

3. Sorry, this is like asking how long is a piece of string. The answer in your case will depend on the stability of control, the type of valve positioner and the size of the actuator diaphragm. Too many variables to provide a sensible answer.

Hope this helps
Malvern