Determining flow capacity is a critical factor in valve selection.
Directional-control valves are common in pneumatic systems. They control compressed-air flow to cylinders, rotary actuators, grippers, and other mechanisms in packaging, handling, assembly, and countless other applications.
|Properly sizing the directional valve ensures adequate cylinder reaction times. |
Selecting and sizing a directional-control valve involves several criteria -- function, actuation, installation, and flow. Here's a closer look at factors that affect valve performance.
Function refers to the number of ports and the number of different ways a valve can operate. In other words, how the valve's internal pathways connect. The four main functions for directional-control valves are 2/2 (two-port, two-way), 3/2, 5/2, and 5/3.
All 2/2 valves have a pressure-supply port and a working port, and are either normally closed (NC) or normally open (NO). In an NC valve, the ports do not connect and permit flow until the valve actuates. NO valves block flow when the valve actuates. They are typically used in blow-off applications or to drive vacuum nozzles.
3/2 valves have a supply, working, and exhaust port, again in NO or NC versions. An NC valve, for instance, has working and exhaust ports connected and supply blocked. When actuated, it connects supply and working ports and blocks the exhaust. The valves are commonly used to exhaust downstream pressure through the valve. For example, they can control single-acting actuators that must exhaust air to atmosphere to return to the start position.
5/2 valves have a supply port, two working ports, and two exhaust ports. They most commonly control double-acting actuators, so the working ports connect to the actuator and exhaust ports usually vent to atmosphere. One working port always connects to the pressure-supply port and the other working port always connects to an exhaust port.
5/3 valves, like the 5/2 versions, also have a supply, two working, and two exhaust ports, but 5/3 valves have an additional spring-centered neutral position. When in neutral, both working ports may be:
Blocked, so no ports are connected.
Pressurized, with the supply port connected to both working ports and the exhaust ports blocked.
Exhausted, so working ports connect to exhaust ports and the supply port is blocked.
|Pneumatic directional valves, such as the Festo CPE 10, control compressed-air flow in countless industrial applications.|
Actuation method determines how a valve turns on and off.
Solenoid actuators come in direct-acting and pilot-assist versions. In direct-acting solenoid valves, the solenoid plunger is also the valve poppet or it connects directly to the poppet or spool. Because solenoid size must increase with pressure and flow capacity, large valves often generate substantial heat. As a result, some direct-acting solenoid valves are not rated for continuous duty. They are typically used in applications that require low-flow switching at high speeds.
Pilot-assist valves, on the other hand, have no mechanical linkage between the solenoid plunger and main-valve spool. Instead, a small direct-acting solenoid valve sends pilot air to an internal piston or diaphragm which, in turn, shifts the main spool. This significantly reduces solenoid size, energy consumption, and heat generation versus direct-acting solenoid valves. Most industrial solenoid-operated valves use this design.
Mechanical actuation relies on a lever, pushbutton, or knob to shift a valve. Manually actuated valves are often used when a machine requires human intervention, or when a machine needs pneumatic output to start another process.
Air-piloted actuation encompasses valves that shift by air pressure alone. External manual or solenoid valves supply compressed air to the directional valve's pilot ports. They are often used in applications where safety concerns rule out solenoid valves.
The two main installation options are stand-alone and manifold-mounted valves.
Stand-alone valves include in-line and subbase mounted versions. All ports are part of an in-line valve's body, while a subbase valve has all ports on the subbase, not the valve.
In-line valves are often used where mounting a valve near the actuator improves performance, or where a machine has a few widely scattered actuators. Subbase mounted units simplify repair because plumbing to the subbase need not be disturbed when replacing a valve. They are especially attractive on systems using rigid piping.
Manifold-mounted units are banks of valves that share common exhaust and supply channels. This method reduces the number of individual connectors and tubing when an application requires a large number of valves.
Newer versions integrate electrical and electronic devices into the manifold assembly. Examples include valve manifolds that have plug-in solenoids with all electrical connections routed to a single multipin connector, thus simplifying installation. And system wiring is drastically reduced by fieldbus manifolds that connect many solenoids to a remote PLC using a two-wire high-speed data cable.
After selecting the type of valve, flow capacity becomes the most important selection criterion. Flow capacity indicates the amount of resistance a valve presents to a pneumatic circuit, and is typically measured as volume coefficient (Cv) or in liters per minute. All devices that conduct air resist flow to some degree, and pressure drop across a device will increase with flow. Less resistance means a smaller pressure drop.
Note that any device, fitting, or run of tubing can affect the system flow rate. In time-critical applications, a few extra inches of tubing or the wrong fitting can mean the difference between a circuit that works and one that does not. For this reason, valve ratings alone cannot predict the flow rate through a system branch.
In the past, common practice was to match the port sizes of actuators and valves. Experts no longer recommend this method because today's valves are smaller yet have greater flow capacity than their counterparts of a few years ago. Smaller valves tend to switch quicker, cost less, and consume less power because they use smaller solenoids.
Thus, selecting a valve means calculating the flow required to move an actuator within an allotted time. For U.S. units,
As an example, consider a double-acting cylinder with a 25-mm bore and 100-mm stroke. Rod diameter is 10 mm, air pressure is 6 bar, and pressure drop across the valve is 0.25 bar. The application requires the cylinder to extend in 0.25 sec and return in 0.2 sec. The goal is to determine the necessary valve Cv.
First calculate areas and volumes on the extend side of the cylinder. Ae = ¶(d/2)2 = 490.87 mm2.
Ve = AeL = 49,087 mm3 = 0.049 l
Areas and volumes on the retract side are
Ar = ¶((d/2)2 - (dr/2)2) = 412.33 mm2.
Vr = ArL = 41, 233 mm3 = 0.041 l
Second, calculate the compression factor, Cf = (6 + 1)/1 = 7
Third, calculate the flow rate required to extend and retract the cylinder. Qe = (VeCf)/te = 1.372
Qr = (VrCf)/tr = 1.439
Finally, calculate the Cv necessary to extend
For this example, the valve must have at least a Cv = 0.158 for the extend stroke and Cv = 0.166 to retract within the system's time requirements. A valve with the exact specific flows for both the extend and the retract most likely does not exist, so select a valve with a larger Cv. One with a Cv = 0.200 should suffice. A larger valve also takes into account restrictions cause by fittings and tubing which can effect reaction time. If the larger valve moves the cylinder too fast, flow controls can reduce flow rate to the cylinder. n