Manufacturers suggest you consider four main criteria in selecting a pneumatic directional control valve: valve function, type of valve actuation, installation arrangement, and the flow capacity (or valve size) needed for the actuator. The first three are pretty well defined by the application. The fourth — valve size — also is defined by the application, but it takes some work to dig out an accurate figure.
Most pneumatic system designers know that the main configurations (how the valve’s internal pathways are connected) for directional control valves are 2/2-, 3/2-, 5/2-, and 5/3- way. The first number represents the number of ports, and the second number deontes the number of discrete positions. Likewise, designers can easily determine whether actuation will be manual or mechanical, by solenoid, by pilot pressure, or by a combination of solenoid and pilot pressure.
Installation options cover a little more ground, but most encompass stand alone (in-line or sub-base) or manifold mounting. Wiring can be plug-in or hardwired, and serial communication makes wiring simpler, faster, and much less expensive.
Avoid pressure dropIn conjunction with the first three main criteria, the most important decision — from the viewpoint of economical operation — is matching the valve’s flow capacity to the application requirements. Air flow usually is measured in cubic feet or liters per minute. A component’s flow coefficient (a unit-less number designated as CV) is an indication of the amount of resistance to flow that the component presents. Flow coefficients for air valves are available from their manufacturers. All devices that conduct air resist flow to some degree. As a result, Air valves abound in versatility as air flows through a restriction, its pressure drops. The less resistance, the lower the pressure drop, also called ΔP. The amount of pressure drop across the restriction increases exponentially as flow increases.
Any pneumatic device — even a fitting or run of tubing — will affect the flow rate in a system. In rapidly cycling applications, a few extra inches of tubing or the wrong fitting can mean the difference between a circuit that works as designed and one that misses the mark. For this reason, valve flow ratings alone cannot predict the flow rate through a system or branch. All components must be considered.
Size doens’t matterIn the past, sizing a valve simply involved matching the port size of an actuator to the valve. Today, however, it no longer is even suggested as the way to size an air valve. Technological advancements now allow physically smaller valves to pass much greater flows, so port size has become even less significant.
Smaller valves tend to have several advantages over larger valves. In general, these include:
- faster shifting,
- less leakage,
- lower power consumption because smaller solenoids can move lighter internal parts,
- greater mounting flexibility because valve footprints are smaller, and
- lower cost.
But small size isn’t the only advancement of today’s air valves. Serial communication, innovative designs, and proprietary configurations give designers a much wider variety of products to choose from . We present a small sampling of this variety on gthis and following pages.