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Feature Valves & actuators

ilot operated pneumatic valves are used in an array of industrial applications, from simple on/off operations to those using multi- ple inlets and outlets for more complex logic-based control architectures. In many instances, their method of opera- tion has remained the same for many years, with body and flow designs and materials advances providing the dif- ferentiating factors.

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Heiko Luckhaupt, category marketing manager, Automation & Control, RS Components looks at pneumatic pilot-operated control valves and their deployment in safety applications

to cut off the flow from the pressure side. The sealing is achieved by the fact that the small-feed pressure is applied to a larger surface area behind the piston than that faced by the fluid under pressure; this means a greater force is applied on the reverse of the cylinder, even though the supply pres- sures are the same.

One area where they are used suc- cessfully is in processing environments found in the oil, gas and chemical industries. As well as offering a number of physical advantages over traditional technologies, their method of operation does not present such a fire risk thanks to their pneumatic infrastructure. In just about every industrial and processing environment, over-pressure is a dangerous phenomenon that has to be managed precisely and, in many cases, quickly. Even with generous safety factors built into pressure and storage vessels, pressures must not be allowed to go above certain installation specific levels. Traditionally, the role of managing pressure in vessels or pipes has been the job of pressure-relief valves. Basic pressure relief valves are simplistic, deploying a spring to hold a valve closed until the pressure of the medium being contained is high enough to over- come the force of the spring and open the valve, until the force of the spring is once again greater than the opposing pressure and able to close the valve. The closing force in relief valves is often adjustable to a certain degree, using variable spring compression, but this is not an exact science as over their lifetime the springs can relax or com- press, resulting in variable holding forces. Pressure relief valves are also fettered by the need to be physically large enough to resist higher pressures. Higher pressures mean bigger springs, which in turn mean bigger valve bodies, which in today’s modern indus- trial environments cannot be tolerated. This is where pilot-operated con- trol/relief valves come into their own. By using the pressure of the fluid they are controlling against they can be much more compact compared to simi- lar-performance relief valves. In operation, a small feed pipe is routed from the side of the system under pressure and this is sent via a control pilot to a piston, which is used

Process & Control NOVEMBER 2013

When the pilot valve experiences a set pressure level it opens to atmos- phere, venting the small feed tube, which immediately removes the clos- ing pressure on the cylinder, allowing the main system to vent. Once the pres- sure is below a certain safe set point, the pilot reseals and the system pres- sure once again closes the valve. There are many benefits of using a valve of this design, including its smaller overall envelope and flexible control options and remote mounting. It also provides a better seal as the system pressure will never quite reach the closing pressure as it is in direct correlation but always slightly lower. Pilot operated pneumatic control valves form the mainstay of many pneumatic control solutions and designers are spoilt for choice when it comes to variants and suppliers. The RS website alone offers 4,780 product variants from suppliers such as Asco Joucomatic, Bosch Rexroth, Crouzet, Legris, Norgren, Parker and SMC. However, the number of options available automatically helps to cut this range down to more manageable chunks, allowing designers to focus in on their exact requirements from a smaller choice of units.

In the first instance, the mounting solution is probably first on the list of options to consider as the valves can be installed either on their own or on man- ifolds/sub-bases – both on machine or

There are many benefits of using a pilot valve, includ- ing its smaller overall envelope, flexible control options and remote mounting

remotely. The second consideration is to look at material types for all parts of the valve as this will, to a certain degree, be determined by the applica- tion and its environment. As an exam- ple, simply choosing a manifold mounted, aluminium-bodied valve, the list of 4,780 variants is cut to just 31. Once you know that the mounting style and material choice is available, the next option is the primary function of the valve based on the number of ways (paths) and ports. There are multi- ple options available including 2, 3, 4 and 5 way valves with various port con- figurations depending on the type of venting/supply required.

Pilot operated pneumatic control valves form the mainstay of many pneumatic control solutions

Connection types and operating pres- sures are next on the list, but as these are generally standard, chances are that there will be very little specialism and standard sizes/pressures will suffice. The body materials give us some hints about specialist applications, with users given a choice of basic steel or aluminium metal construction through specialist alloys and plating options. We can then move into brass and brass/alloy combinations followed by specialist engineering plastics for very specific application requirements. Internal components can also play a large role in the functionality and longevity of valves. Spool and seal materials can affect speed of operation and sealing efficacy coupled with resis- tance to aggressive substances – pro- longing their life.

In all instances, as long as you know what you want the valve to achieve, the operating environment, how it will be mounted and what supply parameters you have, selection can be straightfor- ward. All suppliers will be able to help with exact specifications and even spe- cialist applications will often be readily considered. Suppliers such as RS are also able to provide 3D CAD models making the design process even more straightforward.

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