Burkert Fluid Control Systems sheds some light on the correct selection of valves for use in any process control system.
The type of valve, particularly the material used in its construction, is an important consideration for ensuring the performance and reliability of a process control system. Bürkert examines the valve selection process, with a focus on the chemical resistance of various components.
Valves are selected based on their end application. For instance, a valve in a brewery can be very different to the one used in a pharmaceutical application. One of the critical aspects of valve design is the chemical resistance properties of the valve housing and the gasket materials in relation to the substances it is expected to control. A long and trouble-free service can only be expected if the correct materials have been selected in the design process.
Many of the components of a simple solenoid valve such as the plunger, return spring and seals are all exposed to the media. Therefore, information on the chemical compounds, temperature and pressure are necessary to make an informed material selection. If the production process involves liquid food products, the plastics and elastomers used should also conform to the local food and hygiene regulations. Some processes may require a cleaning cycle to be performed in-between production processes, which requires data that ensures 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. Valve bodies generally are made from brass or stainless steel with some from Polyamide.
When selecting the type of valve most appropriate for any application, keep in mind that plunger-type direct acting valves are best suited to neutral and clean fluids while pivoted armature valves offer improved reliability by employing a media separating membrane to control corrosive, contaminated or aggressive fluids.
Material choices are also available for sealing and separating membranes with each having benefits and drawbacks. Chemical compound, temperature, pressure and cost all combine to influence this design decision.
Common compounds include:
PTFE (Polytetrafluoroethylene) is almost totally insoluble and chemically inert with high temperature resistance. PTFE ball valve seats require no lubrication due to their natural lubricity. PTFE diaphragms and flange gaskets are used in extremely severe chemical resistance applications.
EPDM (Ethylene Propylene Terpolymer), a synthetic rubber used as the standard seal material for many valves offers the most economical choice among elastomers and has excellent chemical resistance to acids, alkalis, salts and many others at temperatures up to 90°C. However, it is not suitable for use with oils, petrochemicals and concentrated acids.
NBR (Nitrile rubber) offers high chemical resistance to oil and petroleum but is weak in oxidising media such as acids. Nitrile also has excellent abrasion resistance and is less expensive than FKM and FFKM.
FKM (Fluorinated elastomer) and perfluoro-elastomers (FFKM) 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°C.
PPS (Polyphenyl sulphide), a high performance thermoplastic used in many engineering applications performs above 200°C and offers resistance to acids and alkalis as well as abrasion, making it suitable for use as 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 easier manufacturing of the valve bodies.
PEEK (Polyether ether ketone) has excellent mechanical and chemical properties but is susceptible to high concentrations of sulphuric and nitric acid. More expensive than other materials due to its improved performance, PEEK provides superior qualities for valve bodies.
Burkert Fluid Control Systems offers a wide range of German engineered and manufactured solenoid valves, process valves, pneumatic actuation, instruments, microfluidics, mass flow controllers and accessories and accessories.