Plastics and Elastomers Plastics and elastomers are used in valve construction where corrosion resistance and cleanliness are required and also for the quantity production of valves and components by moulding where the finished parts can be made accurately, with good appearance and surface finish.
The mechanical properties and range of temperature operation of these materials is less than that of most metals but with proper selection and design this is not a serious disadvantage for many applications.
Complete valves, usually of the ball, plug, or diaphragm type, are moulded in plastics, the following being the more common materials used: polyvinyl chloride-unplasticized (PVC), chlorinated polyvinyl chloride (CPVC), polypropylenepolyvinylidene fluoride (PVDF).
These materials have excellent resistance to a wide range of chemicals and many compounds are approved for use on foodstuffs. Such valves are used in plastics pipework systems where connection can be made by solvent or fusion welding as well as with threaded or flanged joints.
The upper temperature limit of these materials varies from about 60 °C for PVC to 150 °C for PVDF. Actual operating temperatures and pressures are closely related and will vary with the valve size. A variety of plastics is used for certain structural valve components, such as bonnets, handwheels and spindles. The following are the more common materials used: polypropylene, acrylonitrile butadiene styrene, nylon, polycarbonate, polyphenylene oxide, acetal.
Many of these materials possess good tensile strength and toughness and their properties may be enhanced by the addition of fillers, such as glass fibre. The use of plastics is generally restricted to relatively small valve components and excludes applications involving extremes of temperature.
Plastics are used for lining valve bodies where it is required to maintain the full working pressure of the valve and utilize the corrosion resistance and temperature properties of the plastics. Linings are usually moulded into the valve body and the body itself forms the outer part of a moulding tool.
Plastics linings are applied to several different valve types, including butterfly, diaphragm, ball, and plug valves.
Valve body plastics linings are usually limited to those materials having the best all-round chemical resistance properties. The following materials are mainly used: polypropylene, polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), ethylene tetrafluorethylene (ETFE), perfluoro alkoxy (PFA).
The fluoropolymers are resistant to a very wide range of chemicals and their upper temperature limit may be as high as 200 °C. Elastomers are also used to a large extent for valve body linings. These may be moulded in much the same way as plastics but a common method of application, especially for large valves, is to take a sheet of the elastomer and 'tailor' it to the inside of the valve body.
A wide range of elastomers is used in this way and is most suitable for many chemical services as well as having good abrasion resistance. Among the materials commonly used are: natural rubbers, styrene rubbers, polyurethane rubbers, ethylene propylene rubbers, butyl rubbers, nitrile rubbers, neoprene rubbers.
Plastics and elastomers are used as valve coatings for decorative purposes as well as corrosion resistance. These are applied using spray or dip coating techniques. On external surfaces, epoxy and nylon coatings give good appearance and corrosion resistance. These materials are also applied to internal valve surfaces but for better corrosion resistance, the fluoropolymer coatings such as PVDF and ethylene chlorotrifluoroethylene (ECTFE) are used. Many of the elastomers already described are applied to valve components in a paint form using a suitable solvent.
Although the thickness of these coatings is limited, continuous layers are applied and a high degree of corrosion protection is achieved. It will be seen that there is a large number of plastics and elastomeric materials available for use in valve construction by the methods described.
It will be seen that there is a large number of plastics and elastomeric materials available for use in valve construction by the methods described. Although some indication of their properties has been given, the suitability of a given material for valve service depends upon a number of factors. Reference should always be made to manufacturers' literature and recommendations.