Solenoid valves:  An enduring design

Solenoid valves are frequently-used control elements in fluidics. Their role can be to shut-off, release, dose, distribute or mix fluids or gases, all of which can pose a wide range of requirements.

In the 1950s Bürkert developed a plastic-moulded body version of the solenoid valve. Chemically resistive encapsulation protects the electrical system from harmful influences such as dust, dirt and moisture. This was followed by a universal modular valve system, which allows efficient use of space when constructing more complex installations.

Design principles

Over 90% of all solenoid valves have single coils, the simplest of all coil types, which use an enamelled copper wire wound on a coil form. These coils are designed for continuous operation and are usually coated with an insulating mass to ensure high electrical and mechanical protection. For applications involving ambient temperatures to 250°C, it is important to ensure that the coils can withstand these conditions and so anodised enamelled copper wires may be used to ensure reliable operation.

Development of the basic design principles has led to reduced energy consumption with the introduction of the direct acting double coil design. This uses two independent windings with high and low power. The high power coil opens the valve and then integrated electronics switch over to the low power coil to provide the holding force. This twin coil approach provides sufficient power to open the valve, but then uses 75% less power to hold the valve in position.

The design of the coil and its operation has a direct effect on the performance of the valve. The latest energy saving designs, even in single coil conventional formats, can allow the coil to be overexcited to open the valve and within half a second, the current is reduced by 95%, which is still sufficient to hold the valve in the open position. The cumulative effect of this type of feature can have a significant impact on the energy required for process control.

Valve anatomy

Essentially solenoid valves are split between direct acting and pilot-operated. The former relies solely on the power of the magnetic coil to make or break the seal with the seat of the valve. In normally closed configurations, the coil is energised and the valve opens until the voltage is removed and the spring pressure closes the valve. The pressure and flow capabilities of these valves are determined by the power of the solenoid.

A pilot operated solenoid valve uses the pressure differential between the inlet and the outlet to assist the solenoid coil and spring in opening and closing the valve. This design concept can be applied to various styles of solenoid valve to accommodate a wide range of applications where higher pressures and flows are required.

Selection process

The type of solenoid valve most appropriate for each application is governed by a number of factors. Many of the components, such as the plunger, return spring and seals are all exposed to the media and therefore information on the chemical compounds, temperature and pressure are all necessary to make an informed material selection.

In the case of solenoid valve design, the general rule-of-thumb is: Plunger-type direct acting solenoid valves are best suited to neutral and clean fluids, while pivoted armature solenoid valves employ a media-separating membrane and are therefore suitable for controlling corrosive, contaminated or aggressive fluids.

If the production process involves liquid food products, the plastics and elastomers used should conform to the local food and hygiene regulations. Some processes require a cleaning cycle to be performed in between production processes and so the data is also required for this procedure to ensure there is no undue material degradation caused by steam or cleansing chemicals.

On the face of it, selecting a solenoid valve appears to be a simple decision; however it requires an in-depth knowledge of the process under control as well as expertise in the materials and designs available. By combining the knowledge of the process engineer with that of a company such as Bürkert that has fluid control experience, customers can be assured of a high quality product that will deliver long-term reliability and comply with appropriate industry standards.