Programmable Logic Controllers & Human-Machine Interfaces

In this article RS Components looks at pneumatic pilot-operated control valves and examines how they work, what applications they suit and how to specify them. In the first instance we examine how they are deployed in safety applications.

Pilot operated pneumatic valves are used in a huge array of industrial applications, from simple on/off operations to those using multiple inlets and outlets for more complex logic-based control architectures. In many instances, their method of operation has remained the same for many years, with body and flow designs coupled with advances in materials providing the differentiating factors.

One area in which they are used very successfully is in processing environments, such as those found in the oil, gas and chemical industries. As well as offering a number of physical advantages over incumbent and traditional technologies, their method of operation is also safer from a fire risk perspective thanks to their use of a pneumatic infrastructure.

In just about every industrial and processing environment, over-pressure is a dangerous phenomenon that has to be managed very precisely and, in many cases, very 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 highly simplistic, deploying a spring to hold a valve closed until the pressure of the medium being contained is high enough to overcome 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 compress, 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. In today's modern industrial environment where real estate is at a premium and "clutter" is not tolerated, more suitable options are required.

This is where pilot-operated control/relief valves come into their own. By using the pressure of the fluid they are controlling against they can be much more compact; indeed they are positively diminutive in size compared to some similar-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 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 pressures are the same.

When the pilot valve experiences a set pressure level it opens to atmosphere, venting the small feed tube, which immediately removes the closing pressure on the cylinder, allowing the main system to vent. Once the pressure is below a certain safe set point, the pilot reseals and the system pressure once again closes the valve.

There are many benefits of using a valve of this design, including its smaller overall envelope and greater flexibility with regards to 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 from the mainstay of many pneumatic control solutions and designers are spoilt for choice when it comes to variants and suppliers. Leading suppliers include Asco Joucomatic, Bosch Rexroth, Crouzet, Legris, Norgren, Parker and SMC and when it comes to variants, one look at the RS website confirms that over 4,780 products are available from these suppliers and others.

However, the number of options available automatically helps to cut this huge range down to more manageable chunks, giving designers the freedom to narrow 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 manifolds/sub-bases – both on machine or remotely. The second consideration will be to look at material types for all parts of the valve as this will, to a certain degree, be determined by the application and its environment. As an example of whittling down the options, by simply choosing a manifold mounted, aluminium-bodied valve, the list of 4,780 variants is cut to just 31 products.

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 multiple options available including 2, 3, 4 and 5 way valves with various port configurations depending on the type of venting/supply required.

Connection types and operating pressures are next on the list, but as these are generally standard across international borders, chances are that there will be very little specialism and that 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 before hitting the realm of the specialist high-strength 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 resistance to aggressive substances – prolonging their life.

In all instances, as long as you know what you want the valve to achieve, the environment in which it is to operate, how it will be mounted and what supply parameters you have, selection can be extremely straightforward. All suppliers will be able to help with exact specifications and even specialist applications will often be readily considered, as chances are your specialist application is not unique. Suppliers such as RS are also able to provide 3D CAD models making the design process even more straightforward.