In the case of a simple solenoid valve, many of the components, such as the plunger, return spring and seals are all exposed to the media. Therefore, information on the chemical nature of the media, its temperature and pressure, are all necessary to make an informed material selection.
If the production process involves liquid food products, for example, 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 data pertaining to this procedure are also required to ensure that there is no undue material degradation.
Factors such as mechanical loading, increased temperatures and chemical concentrations all have a bearing on the performance of individual components, which can be manufactured from a range of materials. Generally speaking, valve bodies are made either from brass or stainless steel, though some may also be constructed from a polyamide.
The type of valve that is appropriate to a particular application is governed by a number of factors but as a rule of thumb, plunger-type direct-acting valves are best suited to neutral and clean fluids, while pivoted armature valves offer improved reliability (thanks to a media separating membrane) for the control of corrosive, contaminated or aggressive fluids.
The range of options for sealing and separating membranes is quite extensive with each having benefits and drawbacks. The following are some of the more commonly used polymers, with brief descriptions of their properties.
PTFE (Polytetrafluoroethylene) is almost totally insoluble and chemically inert. It has a high temperature resistance and PTFE ball valve seats, because of their natural lubricity, require no lubrication. PTFE diaphragms and flange gaskets are used in the most severe chemical resistance applications.
EPDM (ethylene propylene diene monomer) is a synthetic rubber used as the standard seal material for many valves. It is the most economical choice of elastomer and has excellent chemical resistance to acids, alkalis and salts at temperatures of up to 90 degrees C. However, it is not suitable for use with oils, petrochemicals and concentrated acids.
NBR (Nitrile rubber) has a high chemical resistance to oil and petroleum but is weak when it comes to oxidizing media. Nitrile also has excellent abrasion resistance and is less expensive than FKM and FFKM (described below).
FKM and FFKM (Fluorinated elastomer and perfluoro-elastomers) are more expensive than neoprene and nitrile elastomers because they provide additional temperature and chemical resistance. Certain grades of FFKM can perform continuously at temperatures above 200 degrees C.
PPS (Polyphenyl sulphide) is a high performance thermoplastic used in many engineering applications. With the ability to perform above 200 degrees C and with resistance to acids and alkalis as well as abrasion, PPS is a suitable valve body material.
PVDF (Polyvinylidene fluoride) is resistant to solvents, acids and bases, making it an ideal material for the valve body where high temperature resistance is not required. A very flexible product, it can be injected, moulded and welded, allowing valve bodies to be more easily manufactured.
PEEK (Polyether ether ketone) has excellent mechanical and chemical properties but is susceptible to high concentrations of sulphuric and nitric acid. The excellent performance of this material must be weighed against its higher price, but it nonetheless provides superior qualities for valve bodies.
To help designers and maintenance engineers through the selection process, Bürkert has released a chemical resistance chart and a selection guide for solenoid valves. Click here to access all currently available guides and brochures.
Michael Hannig is product manager, solenoid valves, at Bürkert Fluid Control Systems