Why Should You Choose Your Valves Carefully?

The next several blog posts are contributed by Philipp Wahl, Festo AG & Co. KG. Mr. Wahl introduces valve technologies, what we need to know about poppet valves, piston spool valves and the cartridge principle.  He will also give us a valve selection aid to help us determine which valve type suits us best.

Piston Spool Valves and Poppet Valves – A Technical Comparison

The increasing demands placed on valve technology in recent years have led to a formidable extension of the range of available technologies and valve types, models, and properties. The most commonly used are poppet valves and piston spool valves. Nowadays, there is hence a much wider choice of customized valve technology to successfully plan a project. However, choosing the right parts for the application at hand requires comprehensive knowledge and an accurate appraisal by the consulting engineer or technician.

Figure 1: Examples of valve technologies

An introduction to valve technologies

Poppet valves and piston spool valves are the most commonly used valves for controlling compressed air. Flat slide valves, rotary slide valves, piezo valves and other technologies tend not to be used as often. The chart in figure 1 offers an example of how the two common technologies can be subdivided. Among the piston spool valves, the valves with a cartridge seal are particularly interesting. This type of valve has lower leakage values and offers significantly higher operating pressures.

The poppet valve

The very simple construction and the option of using any material often makes the poppet valves more inexpensive than the piston spool valves. For instance, a 5/2 poppet valve (see figure 2) only requires 3 axial seals, but there are more advantages with this technology: the actuation strokes are small, which allows for shorter switching times. The axial sealing technology is resistant to soiling. Also, poppet valves do not require lubrication, which is an advantage in terms of chemical compatibility with lubricated compressed air.
The greatest drawback with poppet valves is the relationship between space and flow, due to constructional constraints. Poppet valves always offer a lower flow than slide valves, the constructional width being equal, when it comes to pressure-independent models. The reason for this is the greater structural demand – in pressure-dependent valves, the control pressure is dependent on the operating pressure.

Figure 2: Pressure-dependent poppet valve

This is because when the air flows through the valve, as seen in figure 2 from channel 1 to channel 2, the surface area D2 is larger than D1. Since the same operating pressure is exerted on both surfaces, an imbalance of forces occurs, which has to be compensated. Furthermore, it needs to be noted that this valve construction is not always without overlap. Depending on the design, a change of switching positions can lead to overflowing between the channels. This gives rise to unnecessary leakage and noise.

In our next blog Mr. Wahl will talk about the piston spool valve and the two categories that have evolved in this area.