How to choose the right valve for optimised machine performance
08 May 2018
Increased production demands in recent years have led to a need for more accurate, reliable and durable components. Ironically, as the range of available technologies and pneumatic valve types, models, and properties expands to meet these requirements, specifiers, machine builders, OEMs and plant engineers can find the choice daunting.
In this article Andy Parker-Bates, product manager at Festo gives a brief overview of the main valve technologies, their operation and respective pros and cons.
Choosing the right valve for an application is crucial to ensure the efficiency, safety and longevity of critical plant and equipment. Understanding the internal design of the valve type and its relative pros and cons is vital to ensure that plant and equipment is operating at optimum levels. The most commonly used valves for controlling compressed air are poppet valves and piston spool valves.
The poppet valve
Poppet valves are simple in design, using a spring to push the face of a disc (the poppet) down on its seat. They typically allow gas or vapour to flow in one direction but can be energised to flow in reverse. Because of their simple construction they are often more cost effective than piston spool valves.
There are many advantages with this technology. The actuation strokes are small, which allows for faster switching times, and they are particularly well-suited to sorting applications in industrial environments where contaminated air may be present. In addition, they do not require lubrication, which is an advantage in terms of eliminating any potential problems with chemical compatibility due to lubricated compressed air.
The greatest drawback with poppet valves is the relationship between space and flow, due to constructional constraints. Pressure-independent models always offer a lower flow than slide valves, for a given width.
Piston spool valves
A spool valve features seals along its surface. By actuating the valve, the spool shifts, causing the seals to travel down the bore and opening the ports to allow for airflow. For systems that have a flow of 6.3 l/s or less, spool valves are most commonly used to direct flow.
The actuation forces are smaller than in poppet valves. This is because the valves do not have to be switched against any forces generated by the operating pressure. However, there are some drawbacks. For example, the strokes for switching operating positions are longer than with poppet valves. These longer strokes and the placement of several pistons mean that the spool valves generally have to be longer.
The sealing technology of piston spool valves is also more challenging and two categories have evolved in this area:
1: Hard-sealing systems
Hard-sealing systems have no soft rubber sealing element and are therefore more durable. As such, the valves are immediately ready for full dynamic use even after longer periods of inactivity. However, these systems have an air gap, so there is always a certain leakage. Furthermore, their flow capacity is lower than that of other valves with the same constructional dimensions.
The greatest challenge with the hard-sealing system is that the air gap surrounding the moving piston must not be larger than a few micrometres – any more than this and there will be too much air leakage and the piston will be ineffective. Even the smallest deformation in the housing can damage sleeves and affect the life and leakage values of the valve.
2: Soft-sealing systems
In soft-sealing systems, rubber gaskets (O-rings or moulded elastomer seals) are used. One point to note with this option is that the rubber gaskets may be worn down quickly when in contact with the channel edge – the so-called control edge. To prevent this, the seals can be mounted directly in the valve housing, but this results in a couple of disadvantages. First, the recesses required are difficult to produce and, second, the Bernoulli effect will cause the seal to be pulled out at a pressure of approximately eight bar or higher. This again causes greater wear on the seal.
One solution to this problem is to use the cartridge principle whereby the seals are retained in recesses in the metal cage housing. The advantage with this is that the seal will not be pulled out of its position, even at an operating pressure of up to 16 bar, resulting in a significantly longer valve life. In addition, the valves can be used for vacuum operations without any problems.
Which valve type suits you best?
In summary, poppet valves are lower cost and offer good switching times, but provide lower flow and can be noisier than other alternatives. Hard-sealed spool valves will experience some leakage and lower flow rates but are more durable and do not stick after long periods of inactivity. Soft-sealed spool valves have traditionally been relatively expensive due to seal wear. However, the incorporation of a cartridge which holds the gaskets and sealing rings is now fairly standard and this greatly reduces wear and increases working life.
If an engineer is looking for a valve with low leakage, the option of vacuum and ejector pulse should be considered. Where higher operating pressures are required, the most suitable option is probably a piston spool valve with cartridge sealing technology.
Valve operation is just one aspect that engineers need to take into consideration when selecting the best valve for their application. Other factors include the environment, temperature and humidity the valve will be exposed to, connectivity, mounting and correct sizing of valves. To help engineers designing and specifying pneumatic systems Festo has a number of guides available, including a series of short videos and top tips. For more in-depth information on valve selection download Festo’s free white paper from www.festo.co.uk/stars-valves.
Festo has extended its Blue Star core pneumatics range with the addition, within its VUVS valves range, of a new poppet operated valve. Available in standard or ATEX-certified versions, the VUVS poppet valve offers OEMs, machine builders and specifiers even more choice of high quality, reliable pneumatic components at a competitive price. Festo’s VUVS poppet valves offer up to 58% faster switching times than their spool valve equivalents, making them ideal for applications where responsiveness is a key consideration. There is also less friction during operation, which means the VUVS poppet valve delivers a more consistent performance and repeatable switching times.
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