Sizing Valves

One of the parameters to describe pneumatic valves is called “CV”.  CV is an expression that describes the flow characteristics of the valve.  A higher value of CV indicates a higher flowrate for a given pressure drop across the valve.  To drive a given pneumatic actuator, some flowrate is required.  If the valve chosen has a CV value that is too low, the actuator may not perform as desired because of too much pressure drop across the valve at the consumption rate of the actuator.

There are many good web references that go in to the technical details of CV calculations.  In practice, manufacturers typically offer a few sizes of valves.  Unless you are designing a highly optimized system, the vendor that is selling you the actuators will be able to get you into the correct valve family.  Typical actuators found in industrial automation systems fall well under a valve CV of 1.0, and most valve manufacturers have a valve series in that flow range.

As a rule of thumb, most valve manufacturers make a series of valves somewhere in the 0.8 to 1.2 range that will work very well in many cases.  I would estimate that more than 90 percent of the valves we have used are in the 1CV range.  This is probably because most of the cylinders we see fall in the bore range of 1 to 3 inches.  I would typically not question a 1CV valve size when I see cylinders in that range.  On smaller cylinders, I might consider a smaller series of valves if all of the valves in the system could be smaller.  On cylinders above a 3 inch bore, I would take the time to figure out whether a larger CV valve is necessary.

The upshot is, we see a lot of 1CV valves, and they usually work.  A 1CV valve may be gross overkill for a small bore cylinder, but the only ill effect of it will be on the budget.  The same 1CV valve might work well, even on pretty large bore cylinders, unless speed is critical.  The cases where a 1CV valve is going to be too small are usually easy to spot, and the vendor selling the actuators will probably swerve you in the right direction.

It is much more likely to see an actuator undersized than a valve, in my experience.


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