Directional-control air valves are essential building blocks of pneumatic circuits. But there is a wide variation among different valves, ranging from the number of positions, flow paths, and number of ports they offer to how they are actuated, mounted, and controlled.

Based on our experience over the years designing pneumatic systems, here are some guidelines for specifying valves that give the best performance and efficiency in a given application.

Sizing basics
One key consideration is how much flow a valve can deliver to an actuator. This is usually rated in terms of coefficient of velocity, or Cv. It is generally used to compare flows of different valves: the higher the Cv, the greater the flow.

To match a valve and cylinder, the following equation gives the Cv (valve flow) required for operating a given air cylinder within a specific time interval.
Cv = (A × S × a × Cf) ÷ (t × 29)
where A is cylinder piston area
(π × r2), in.2;
S is cylinder stroke, in.,
t is time, sec,
a is a pressure drop constant,
and Cf is compression factor. These last two are listed in the accompanying “Sizing factors” table.

 These constants are factors in the valve-sizing equation. Use a at ΔP = 5 psi for most applications; at 2 psi for critical applications; and at 10 psi to save money and mounting space.

Another method for selecting valves is to use the “Valve sizing” chart. It indexes valve Cv against cylinder bore size and gives the resulting cylinder speed in inches of stroke per second. It assumes a pressure of 80 psi and ΔP = 80%.

To account for various losses in all pneumatic systems, experts generally recommend oversizing the valve by at least 25%.

And keep in mind that many other factors contribute to the performance of a cylinder. These include: quantity and type of fittings leading to the cylinder, tube length and capacity, cylinder operating load, and air pressure.

 Shuttle valves receive air from either of two inputs and direct it to a single outlet.

You can attempt to calculate Cvs for every component and place a value on the other contributing factors. But it is often more practical, not to mention faster, to follow a valve manufacturer’s guidelines when sizing valves.

For example, a sizing table for various Mead air valves (available at http://bit.ly/hp0312mead) relates valves to cylinder bore sizes between 0.75 and 6 in. The cylinder operating speed resulting from the use of each valve at 80 psi is rated in general terms as high-, average-, or low-speed operation for single- and double-acting cylinders. If no rating is shown, the valve is considered unsuitable for use with that particular bore size. To determine the suitability of valves not listed in the table, you can compare the Cv of the valve in question with the one nearest it on the table for reference. And, of course, it’s a good idea to consult with the manufacturer’s application engineers for additional information and guidance.

Cv and scfm
It is sometimes helpful to convert Cv into scfm (standard cubic feet per minute) and conversely, scfm into Cv. Although Cv represents flow capacity at all pressures, scfm represents flow at a specific air pressure and temperature — sea level and 70° F. Therefore, the “Converting Cv to scfm” chart covers a range of pressures.

 This chart shows valve flow and cylinder bore and gives cylinder speed in in./sec of stroke. It assumes 80 psi pressure and a ΔP of 80%.

To obtain scfm at a specific pressure, divide the valve Cv by the appropriate factor shown in the chart. For example, output in scfm of a valve with a Cv of 0.48, operating at 100 psi, is:
0.48 ÷ 0.0177 = 27 scfm.

To convert scfm into Cv, simply reverse the process and multiply scfm by the factor.