Directional control valves are the traffic police of pneumatic systems, charged with diverting, directing, and reversing airflow to manage the operation of actuators. A fluid power designer must first understand valve functions, flow rates (and the resulting sizes), and the construction materials best suited to their application before sending out requests for quotes. This designer’s guide provides you with the insight you need to appropriately select the DCVs best suited to your application and machine.
Circuit design comes first
Most pneumatic systems start with a sketch and subsequent schematic preparation in CAD software, where designers add actuators, pressure valves, flow controls, accessory components, and, of course, directional valves. The nature of an actuator’s operation varies based on the valve(s) used to control for and aft operation in linear actuators, or the rotational direction of motors.
Functions with a simple on/off operation, such as a drill motor, require nothing more than a two-way, two-position valve to start and stop the drilling. The astute rookie may quip, “But Josh, the air is only going in one direction. Isn’t it a one-way, two-position valve?” Indeed, you’d be correct as far as operation is concerned, but back in the olden days of fluid power, they didn’t predict or care that valves may only flow unidirectionally. Instead, a “way” is simply defined as the number of ports used by the valve.
It’s also important to mention you don’t have to describe every directional valve within your bills of materials as “two-way, two-position.” Simply by abbreviating to 2/2, we can continue to describe the remaining popular options as 3/2, 4/2, 5/2, and also 5/2 and 5/3. If you’re a hydraulic professional whose toes have yet dipped into pneumatic waters, there may be confusion surrounding five ports, which is no typo.
Most pneumatic systems do not port their exhaust back to a central location unless for “clean” purposes in semiconductor, pharmaceuticals, and food & beverage. Instead, the exhaust port vents directly to atmosphere, requiring one or two extra such ports depending on valve construction. The five most common configurations of spools for pneumatics are listed below:
The 2/2 valve is the simplest and most reliable, but also offers a versatile list of functions. From a directional perspective, it could actually be used to operate a single-acting cylinder that uses springs or gravity to retract, so long as the air supply line is removed to allow retraction. But these valves are also often used as accessories, such as blow-off valves to eliminate trapped pressure or pilot signals to control other valves.
Refer to Figure 2 to see how a 3/2 valve offers more typical directional-valve functionality, allowing air to flow from port 2 to port 3 in its rest position while blocking the passage through port 1. Then, when shifted, the valve blocks air at port 3 while flowing from port 1 to port 2. I’ve also labelled one envelope with P for Pressure, E for Exhaust, and WP for Working Pressure so you can better imagine how air will flow through the valve. Unlike the 2/2 valve, this one is perfectly suited for a single-acting cylinder application, where it remains at rest until the valve is activated, sending air through port 2 to extend. This valve also works well with a spring-retract cylinder
*Sidebar: I challenge you to draw a circuit that replaces one 5/3 valve with two 3/2 valves.
The 4/2 valve looks very much like the 3/2 valve but has an additional work port, which will be labelled as port 4. Pneumatic valves respect a rule of the ISO 5555-1/2 code that specifies the pressure port as 1, the exhaust ports as 3 and 5, leaving us with 2 and 4 for the work ports. Regardless, the 4/2 valve is somewhat rare in pneumatics, at least these days, because the single exhaust chamber experiences backpressure. As well, designers have less flexibility with the 4/2 design compared to the 5/2 versions, which can use meter-out flow control valves with different settings at the 3 and 5 ports. They’re still available, but likely in older designs.
The 5-ported valves are popular for cylinder applications that require an inactive center condition, where they must be devoid of pressure. All ports are blocked, and air cylinders (or even motors for that matter) can be held in place mid-stroke. In Figure 3, I show the 5/3 valve with closed center. Note that some manufacturers offer various spools that provide functional options for designers, such as the exhaust center that maintains an open path between ports 4 and 5 and ports 2 and 3, which can also be called a float center. Pressure center valves (port 1 open to port 4 and port 2 in center condition) help keep double-rod cylinder applications stable despite load-induced pressure and discourage piston leakage.
You’ll notice in Figure 3 that the 5/3 valve has air-pilot arrows, indicating it’s pilot-operated only. The vast majority of valves are pilot-operated, and for good reason — pilot operation allows easier and faster shifting while reducing the power of the coils, which in many cases may be only a few watts. Because pilot-operated pneumatic valves are easy to manufacture and have no downside, you’ll have a harder time finding direct-acting valves. Also, some PLCs can drive pneumatic valve coils directly without relays. Also standard are the 12- and 14-port number designations, as shown here.
Determining size
Once you’ve built your circuit and selected the valves and their spools, you’ll need to choose the valve size that suits your application. Sizing pneumatic valves is unique because air is a compressible fluid; special attention must be paid to velocity, cycle time, acceleration, and energy consumption. All manufacturers publish a valve’s flow resistance, designated as its Cv value.
Cv literally represents a valve’s flow coefficient, which is the number of US gallons per minute of water that will pass through the valve (at 60°F) with only 1 psi of pressure drop. Of course, water is not compressible while air is, so use the valve’s Cv as a rule of thumb, but also refer to the manufacturer’s flow curves, which might show cfm at a given pressure drop. Fluid dynamics are complex, so watch out for valves with a high Cv and an unimpressive pressure drop curve, because you want the opposite. Understand that a higher Cv is better, and if manufacturers don’t publish their flow curves, it doesn’t hurt to jump up if your flow rate would be marginal.
You’re probably wondering what marginal flow rate looks like, so let’s get into the math. Sorry, not sorry. Let’s select a relatively small cylinder that is 2 in. bore and 8 in. stroke, which extends at 8 in./sec, and then plug in our available shop air pressure of 90 psi. Let’s first calculate the cylinder piston area:
A = Area in square inches
Π = Pi
r = Radius of the piston
You math geniuses ran this one in your head and know that our area is equal to pi, something you only see with 2 in. bore cylinders, but let’s round down to 3.14. Because we’re going to use cfm (cubic feet per minute) in our calculations, we need to run the numbers from the equation we used from A Designer’s Guide to Pneumatic Cylinders.
CFM = Cubic feet per minute
A = Area of the piston in square inches
V = Velocity in inches per second
N = 60 seconds in a minute
1728 = Cubic inches within a cubic foot
Both N and the 1728 are required because we’re mixing units, but the result is a modest 0.87 ft3/min of air required to achieve 8 in./sec velocity. Again, remember we could add another 25-50% to accommodate acceleration, so let’s call it about 1.5 cfm. We can always add a metering valve if it’s too fast, but if we don’t have enough airflow, it’s difficult to find more later.
Okay, now is where it gets fun because we can finally calculate the Cv we need for our application. This formula is as precise as you’ll ever need as a fluid power designer and is worth understanding. There are reasons we include all these variables, but let’s break it down. In this formula, Q is Standard Cubic Feet Per Minute, which factors in absolute pressure of 14.7 psg along with 35% humidity. Yes, I know you have a dryer, but just run with it unless you have a hygrometer in your air lines. We also include specific gravity, but, luckily for us, G = 1 in this case, which is the gas-specific convention. And of course, both temperature and pressure are expressed in absolute values:
Cv = Flow coefficient
Q = scfm
T = Absolute temperature (460°F + ambient)
G = Specific gravity (we’ll use 1)
ΔP = Pressure drop (relative pressure)
P1 = Inlet pressure (absolute pressure)
Pa = Atmospheric pressure
Okay, this is the last formula, and it’s not as scary as it looks. We’re going to assume 68°F room temperature at sea level (14.7 psi), with 10 psi pressure drop and use our 90 psi shop compressor. You can work through it now, but if you didn’t, the answer is a Cv of only about 0.04, which would make for a tiny valve. The good news is that even the smallest miniature directional valve will provide more than enough flow and do so efficiently.
Selecting your valve design
Now that we know the size of our valve, let’s choose an appropriate design for our system. In general, designers can select from inline valve bodies, manifold mount, and other modular designs. The stand-alone inline body is a cheap and cheerful method for controlling pneumatic actuators, requiring only a few fittings to connect the workline tubes and an electrical connection for the solenoid (Figure 4).
Regarding inline valves, there’s no commonly used standard today, nor is it required, since they’re not subplate-mounted to match any particular port configuration or dimensions. They’re compact, easy to mount, and available in myriad sizes ranging from 1/8 to 1-1/2 in. NPT, depending on the manufacturer. And of course, the same selection of spools, coils, and voltages is available, with even more options than the ISO valve standards. Cams, rollers, or treadles are options for inline valves; explore manufacturers’ catalogs to see which works best for your application. I should mention the NAMUR-style valve, which resembles an inline valve but has manifold ports with O-rings on one side, allowing direct mounting to manifolds or custom actuators.
The dominant valve series worldwide, and not just recently, either, are the various ISO valve standards. The venerable ISO 5599-1/2 uses a visually distinct rectangular mounting pad with seven oval ports in a staggered layout (also in Figure 4). Available in various sizes, these modular valves offer a dizzying array of features and options, entirely customizable on the spot from your local pneumatics distributor.
Manufacturers offer standard manifolds and modular ones, so you can use just a single station with an inlet and outlet section for plumbing, or bolt together many to power an entire circuit. The manifold plates come with various options to customize your circuit, such as throttle plates, pressure-shutoff plates, or built-in regulators. The options are nearly endless.
The similar, but more compact and slim, ISO 15407-1 offers similar versatility as the 5599-1/2. Their thinner manifold sections still contain the offset oval ports, but with fewer and smaller size options. With their smaller size comes lower flow, but the larger sizes (1/4 in. G or NPT ports) still offer respectable flow. Again, like the ISO 5599-1/2, expect catalogues to contain hundreds of pages, offering solutions for every industry in every market. You may even find some manufacturers who provide manifold options for mounting both ISO 15407 and 5599 on the same assembly.
There are other soft standards that arose from manufacturers copying each other, which are more equivalents than like-for-like replacements. These could be metal-bodied poppet valves, aluminum safety blocking valves with big, red knobs, or miniature-style valves nearly small enough to fit in your wallet. Any innovative product from the past sixty years that saw market success was fair game. And of course, there are straight-up knockoffs of the major player’s valves, even down to the correct part number.
As a designer, you have the luxury of a library of products from various manufacturers at your fingertips, from standards to customs and from tiny to massive. There is a spool for every actuator, a coil for every control system, and a list of features long enough to suit even the most demanding application.
