Solenoid valves perform an essential task in the control of fluids and gases and the basic design has been refined over the years to allow them to be utilised in a wider range of applications. Modern fluid control systems present a number of challenges that require the correct design of solenoid to achieve a reliable and efficient control process. Kelly Booth, solenoid valve specialist at Bürkert UK, looks at the different valve types and their suitability to various applications.
On the face of it, selecting a solenoid valve appears to be a simple enough decision, however, making the right choice requires an in-depth knowledge of the process under control as well as expertise in the materials and designs available. With such a vast range of applications, it is important to match the correct design to each situation.
Direct acting
The first and most obvious characteristic of a solenoid valve is to determine whether it is direct acting or pilot operated. With a direct acting solenoid valve, the action of the solenoid plunger directly opens or closes the orifice. This type of solenoid is required when there is no line pressure to maintain the valve in the desired position.
A normally closed (NC) version will use a spring to keep the valve closed until the solenoid is energised and the coil will lift the plunger against the spring force to open the valve. Conversely, a normally open (NO) design uses the spring to keep the valve open allowing the media to flow until the solenoid is energised.
Pilot operated
In this design, there is a pilot and bleed orifice that uses line pressure to operate the main valve. When the solenoid is energized, the coil opens the pilot orifice and relieves pressure from the top of the valve piston or diaphragm to the outlet side of the valve. This results in an unbalanced pressure, lifting the piston or diaphragm off the main orifice. When the solenoid is de-energized, the pilot orifice is closed and full line pressure is applied to the top of the piston or diaphragm, which together with spring assistance, closes the valve.
Pilot operated solenoid valves require a minimum pressure differential across the valve to keep them open or closed and therefore they can only operate with a suitable upstream pressure differential and do not operate at zero pressure. Pilot operated solenoid valves can provide high flow rates at high pressures with lower power consumption.
Rockers, flippers and pivots
The armature design should be carefully considered with relation to the media that is being controlled. Pivoted armatures allow the use of a media separating diaphragm which enables the valve to be used in applications involving corrosive, contaminated or aggressive fluids. This style of valve can be easily mounted in a manifold and, unlike the plunger type, the 3/2-way version has all 3 ports in the same plane.
An alternative design is the rocker armature which is often used in microfluidic applications where small dead volumes and flush-ability are of prime importance. The compact design, combined with the small and efficient coil, enhance the suitability for aggressive or ultra-pure media. These attributes also make the design suitable as a pilot for pneumatic control valves in both hazardous and non-hazardous areas.
The flipper design uses a vertical armature that moves a flexible sealing system between two opposing seats. Bürkert has developed this principle for microfluidics with the possibility of using a valve that is only 4.5 mm wide. Again the compact design and encapsulated sealing make this type of solenoid valve suitable for applications involving both aggressive and ultra-pure media.
Chemical resistance
Selecting the most appropriate solenoid design for an application will ensure the most efficient process control; choosing the correct materials for the various components will ensure reliable service. Bürkert has considerable experience and expertise in testing and specifying both metallic and synthetic materials for solenoid components.
Based on the chemical composition and temperature of the media to be controlled, application engineers can provide the most suitable design of solenoid valve for any application. A wide range of materials can be used for the various seals and diaphragms within the valve and making use of the available advice can deliver the most cost effective solution. These include PVC-HT, PP/PE, PA, PTFE, PVDF, PPS and PEEK, depending on the acidity, alkalinity or temperature of the medium.
Specific applications and certifications
Many applications involve complicating factors such as high pressure, explosive atmospheres or special regulatory requirements. Bürkert aims to simplify the task of specifying solenoid valves for all applications, even those with special requirements. With designs capable of operating up to 250 bar it is still possible to select the most appropriate sealing material.
For applications involving potentially explosive atmospheres safety is of the utmost importance and therefore a number of solenoid valve designs are available with a range of certifications including ATEX. With over 25 years of experience in designing and manufacturing fluid control components for hazardous areas, Bürkert also participates in the various standards committees in order to keep up to date on the latest developments.
Conclusion
The solenoid valve remains an essential component of most fluid control systems and as such it needs to be properly specified if it is to deliver reliable and efficient service. Design and application engineers are always looking to improve existing designs and incorporate the latest technology for the benefit of the end user. Whether designing a new system or simply replacing an existing component, making use of the considerable expertise and experience available today is a sensible option.
