Instrument Air Consumption

[doomster]

Are the pressure rating of control valves in a system additive?
Like in a plant, I have 5 control valves at 2 bars minimum operating pressure, at most three valves will be working together. Is it safe to say that the minimum consumption of instrument air will be at 6 bars?

 

[pierreick]

Hi,
P*V/T =constant should be used to validate your hypothesis !

Some figures about compressed air consumption .
Pierre

 

[georgeverghese]

Control valves dont consume pressure, they consume volumetric rates of air. Typically stated in scfm ( standard cubic feet/hr) or sm3/hr(std. m3/hr) or some other units of volume per unit of time. The pressure of the the air is usually stated as a supply pressure range (say 3barg min to 8barg max) or something similar. At lower pressures, the control valve will fail to produce the required torque.

 

[doomster]

Thank you Pierreick,

Okay thank you for the info georgeverghese, i will rephrase my question. If I have 3 control valves at 2 bars operating pressure, will be the pressure needed to open the three valves will result to 6 bars?
Since we have issues in our plant that valves dont open as desired with the given conditions. We have suspected that there's an insufficient supply of instrument air. We've planned to have air receiver tanks as a solution.
Now the challenge will be sizing the receiver to have enough supply of instrument air for the said control valves.

Thank you in advance for your response.

 

[LittleInch]

Doomster,

It's always best to draw a picture, sketch or a diagram to avoid confusion.

At the moment we know virtually nothing, including if these three control valves (regulators?) are in series or parallel.

when you say "2 bars minimum operating Pressure", what do you mean? Are they trying to give you a maximum pressure of 2 bar and failing to do that?

Are they trying to maintain 2 bar as a back pressure and hence if they don't see 2 bar they are closed?

Or do they need 2 bar inlet to operate and control to some other pressure?

Unless these valves open and close over time, then an air receiver won't do you much good as the air will rapidly become exhausted and it's really lack of air supply is your key issue.

But if these are in parallel, no, you don't need 6 bar, but I for one can't understand your system or the issue you are facing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
to support your request 2 additional documents :
Good luck
Pierre

 

[pierreick]

hi,
one more document from Atlas copco.
Good luck
Pierre

 

[bimr]

Thank you Pierreick,

Okay thank you for the info georgeverghese, i will rephrase my question. If I have 3 control valves at 2 bars operating pressure, will be the pressure needed to open the three valves will result to 6 bars?
Since we have issues in our plant that valves dont open as desired with the given conditions. We have suspected that there's an insufficient supply of instrument air. We've planned to have air receiver tanks as a solution.
Now the challenge will be sizing the receiver to have enough supply of instrument air for the said control valves.

Thank you in advance for your response.

No, you don't need 6 bars based on your circumstances, but 2 bars pressure is too low. Instrument air is generally specified to be available at 7-8 bar pressure.

Observe the pressure in your instrument air system while operating the valves. If the instrument air pressure drops while you are operating the valves, then you don't have an adequate instrument air supply.

Many of the automated instruments in process industry are often operated using instrument air or pneumatic signals. The instrument air is generally required to be available at 7-8 bar pressure. Within the instrument air supply system, atmospheric air supply is filtered, compressed, dried and cooled to be used for instrument signals.

 

[georgeverghese]

In most IA systems, normal op pressure to control valves is 6-7barg, but they are expected to operate with IA pressure as low at 50psig(3.5barg). Check the instrument datasheets for these control valves for this detail. Agreed, 2barg is too low. In correctly designed air systems, the critical dry IA header (going to plant instruments essential for process control) is configured to be separate from the general, non critical wet utility air header. And a self operated backpressure control valve is placed on the utility air header to maintain IA header pressure at no less than 5-6barg. Buffer vessels on each of both headers will help to smoothen out header pressures resulting from any intermittent surge flows.