Solenoid valves: some are more equal than others
06 May 2015
Throughout industry, process control relies heavily on the humble solenoid valve and its ability to perform both reliably and efficiently. Michael Hannig looks at the improvements in design that have allowed an increasing number of processes to rely on this essential device.
First, a bit of history: Bürkert achieved a landmark in valve technology in the 1950s with the world’s first plastic encapsulated solenoid coils. Since then, the company has continued to push the frontiers of design and functional innovation, and is probably well placed to comment - not just on the history - but on the present and future of solenoid valve technology.
The original concept of the solenoid valve - to provide simple on/off control - has, over the years, been developed and refined to allow it to be utilised in a wide range of applications, from water and gas handling to hygienic and microfluidic applications. Each of these has its own challenges and has led to a series of formats, both in terms of the mechanical components and the coils.
The vast majority of solenoid valves are equipped with single coils, and the simplest of all coil types is one that uses an enamelled copper wire wound on a coil bobbin. Bürkert coils are designed for continuous operation and are always coated with an insulating mass to ensure high electrical and mechanical protection. For applications involving ambient temperatures up to 250°C, anodised enamelled copper wires are used to ensure reliable operation.
A reduction in energy consumption has been achieved with the introduction of a double coil design. This feature two independent coils, one of high power to open the valve and a second, low power coil to provide the holding force. The switch-over is controlled by integrated electronics.
As many industries look to reduce the footprint of a variety of control processes, so there is a need to develop smaller control devices without compromising performance. Bürkert has addressed this particular requirement with the introduction of its TwinPower solenoid, which uses two parallel windings on the coil yoke. Integrated electronics switch the coils from parallel to series connection, reducing the holding power to a quarter of that required for valve opening.
The design of the coil and its operation has a direct effect on the performance of the valve. The latest energy saving designs enable the coil to be ‘over-excited’ in order to open the valve and, within half a second, the current is reduced by 95 percent, which is still sufficient to hold the valve in the open position. The cumulative effect of features such as these can have a significant impact on the energy required to facilitate process control. Increased reaction speed, energy saving and reduced size are all themes that are expected to continue to influence valve design into the future, as are more specialised applications.
If the production process involves liquid food products, the plastics and elastomers used should also conform to the local food and hygiene regulations. In addition, some processes require a cleaning cycle to be performed in between production processes and so the materials of construction of these devices must also resist any degradation that might otherwise be caused by steam or cleansing chemicals.
Some applications involve potentially explosive atmospheres and therefore any equipment operating in this environment must be appropriately ATEX. Similarly, components for other specialised applications such as cryogenic systems, must be designed to withstand the extreme temperature environment and continue to operate reliably.
Bürkert not only designs its solenoid valves for both standard and specialist applications, but also manufactures every component that goes into their assembly, including the machining of the valve body, the winding of the coil and the injection moulding of the coil encapsulation. In this way, the company maintains control over every aspect of the valve’s manufacture and construction and thus have the confidence to guarantee its performance.
The testing of individual components and complete valves is a continuous process, partly to ensure continued compliance with industrial standards and certification requirements, but also to ensure that quality standards are being maintained and the customer remains confident that devices will give reliable operation, even in harsh operating conditions.
On the face of it, selecting a solenoid valve might appear to be a simple enough procedure, but making the right choice requires knowledge of the process to be controlled as well as the materials of construction and designs that are available. By combining the knowledge of the process engineer with the fluid control experience of a manufacturer, customers will get the product that not only delivers long-term reliability, but also complies with the appropriate industry standards.
Michael Hannig is global product manager, solenoid valves at Bürkert Fluid Control Systems
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