Qualities of a Good Pneumatic Valve
Compressed air is an essential energy source throughout industry. Driving actuators and grippers, compressed air powers automation of every kind. It’s responsible for assembly, transport, filling and packing operations in electronics, food and beverage, medical device manufacturing and more.
Compressed air, a.k.a. pneumatic power, is distributed and delivered through valves and piping to operate cylinders that deliver motion. When it’s time to carry out industrial machine repair work, the pneumatic components are some of the first components to replace. Choosing those replacements is often a challenge though: there are a host of manufacturers and it can be difficult to know what to look for. Here’s our advice.
Basics of Pneumatic Valves
Valves come in innumerable types but that most commonly used in pneumatic applications is the directional spool valve. This comprises a cylinder with stepped diameters that slides axially in a bore. The bore runs through the center of a housing which contains multiple ports. Air is supplied to one or more or these ports and the position of the spool determines which port it leaves through.
Sealing between spool and bore is achieved either by machining both to very tight tolerances or by fitting a sealing element to the periphery of the larger or raised diameters on the spool.
While manually-operated valves may be used in some situations, most valves in industrial machinery are solenoid-controlled. In these the solenoid responds to signals issued by a PLC. Some valves may have two solenoids, (one each end of the spool,) to enable accurate movement and allow stopping midway between the end travel positions.
Solenoid valves are either direct-acting, where the coil moves the spool, or pilot-actuated. In a pilot-actuated valve the solenoid enables air to flow in a way that moves the spool. Pilot-actuated valves are often smaller and more cost-effective.
The best valve for an application is one that most closely matches the requirements. The first points to consider are:
1. What is the valve controlling?
Some mechanisms need only a single movement with an immediate return while others need to move and stay in that position. Others may require a midpoint position. Safety should be a major consideration: if power to the valve is cut should the motion continue or stop?
2. What flow is needed?
Flow is expressed in cubic feet per minute (CFM) or as a volume coefficient, (Cv). The diameter of the ports on the valve body play a big part in the flow rate achieved.
Getting the right flow is key to achieving the acceleration and velocity the cylinder needs in the application. An alternative to calculating the required flow through the valve is to discuss the requirements with a valve specialist.
3. How will it be controlled?
Most valves encountered during an industrial machine repair will be solenoid-controlled with signals sent from a PLC. (A double solenoid valve needs two PLC lines.) Safety-critical applications need special consideration.
Installation is closely linked to control method. Typically, industrial pneumatic valves are either standalone, (in-line or with a sub base) or joined into a manifold.
Qualities of a Good Pneumatic Valve
Number of ports is the main point to consider. Directional spool valves come with two, three or five ports. Also, determine whether the valve should be normally open or normally closed.
Industrial equipment valves are usually solenoid-operated. Direct acting valves are still used but most applications have gone over to pilot actuation. Some manually-operated valves are used for guards, turntables and similar devices.
Valve bodies can be either metal or plastic. Stainless steel, brass and aluminum are common metal choices while PEEK and PPS are the plastics of choice. Spools are usually either aluminum or stainless.
All directional spool valves will leak slightly, unless specified as bubble-tight. ‘O’ rings or other elastomeric seals on the spool typically last 10 to 20 million cycles but can be damaged as they move over port openings The alternative is where bore and spool lands are precision ground and finished for an extremely tight clearance. Such valves, (where body and spool are both metal,) can last much longer.
Consider Function and Life
Whether replacing a valve as part of an industrial machine repair or designing and building new equipment, the best valve is one that’s matched to the application. Flow, port sizes and port numbers are key considerations for optimal performance. Where life is of concern, look for valve spools with ground components (commonly referred to as “spool and sleeve” design) rather than seals.
Ask JH Foster for Advice on Pneumatic Valves
JH Foster specializes in industrial equipment of all types and our experts have a deep understanding of compressed air and pneumatic systems. Whether you’re repairing, upgrading or building new, they can advise on the best valves for your pneumatic equipment and machinery. Contact us to start that conversation.