Types of valve bodies and their specifications
A wide range of materials is available to meet the many different service conditions in which valves may be required to operate. It is useful to consider the questions of material in relation to the function of the component, so that the material for the envelope of the valve (body and bonnet or cover) may be different from the trim (closing member, body seats, and stem). In this way, optimum economic life and performance can be obtained. The materials most frequently used for valve bodies and bonnets are bronze, cast iron, and steel and some consideration of each of these is given in the following. Trim materials are considered separately.
It should be understood that these notes can be only of a generally indicative nature and in making a specific selection it is necessary to pay regard to the manufacturer's specifications, national codes of practice, and local standards appropriate to the duty involved.
The great majority of valves supplied every year to the industry are made from nonferrous materials, chiefly 'bronze' or brass. Bronze in valve manufacture is usually an alloy of copper, tin, lead, and zinc. If a zinc-free bronze is required this must be specified. The physical strength, structural stability, and corrosion resistance of bronze in particular, makes it very suitable for a wide variety of applications in the mainstream of industrial requirements. Bronze also lends itself particularly well to economic large scale production of the smaller size of valves and, conveniently, the bulk of industrial applications is for valve sizes up to 100 mm. Although bronze valves are used mostly for applications at relatively modest temperatures, some grades of bronze are suitable for temperatures up to around 280 °C. At the other end of the temperature scale the attribute of not becoming embrittled at very low temperatures, possessed by most copper alloys, has led to bronzes being widely used for cryogenic services such as liquid oxygen and nitrogen where temperatures below - 180 °C prevail.
Cast Iron valves offer a considerable cost advantage and have a wide range of permissible service applications in practically every field of industry. They are commonly used on water, steam, oil, and gas services and find numerous applications in the handling of chemicals, dyestuffs, paints, textiles, and many other industrial products where a degree of iron contamination is of little or no consequence. The operating temperature range for cast iron valves is usually from ambient up to around 220 °C. Valves are also available in s.g. (spheroidal graphite) iron (also known as nodular or ductile iron), a form of cast iron in which the graphite is present substantially in nodular or spheroidal form instead of in flakes as in grey iron. This modification to the structure of the metal gives it mechanical properties superior to those of ordinary grey iron without detriment to its other characteristics. Consequently, valves made of s.g. iron can be used for operating pressures higher than those associated with grey iron valves.
Stainless steels containing about 18 per cent chromium and 8 per cent nickel, 18-8 austenitic stainless steels, are regularly used as body and bonnet materials for services at elevated and subzero temperatures and for highly corrosive conditions. The addition of molybdenum to the basic type 18-8 steel and a slight increase in nickel materially increases its corrosion resistant properties and valves made of 18-10-3 Mo steel are used extensively in the chemical industry for handling acetic acid, nitric acid, alkalis, bleaching solutions, food produces, fruit juices, sulphurous acid, tanning liquors, and many other industrial chemicals.
For use at elevated temperatures a further modification is made by the addition of niobium and this steel, known as 18-10-Nb, is suitable for temperatures up to 800 °C.
Austenitic stainless steels usually do not suffer embrittlement at extremely low temperatures, so valves in materials such as 18-8 and 18-10-3Mo are very suitable for operating on cryogenic services; instances are the handling of liquefied gases such as natural gas, methane, oxygen, and nitrogen.
Special Stainless Steels
Where conditions are too severe for the standard stainless steels the next group of interest is that of the more highly alloyed stainless steels. Probably the most common of these is '20' alloy, which contains about 29 per cent nickel and 20 per cent chromium with additions of molybdenum and copper. This alloy is extremely resistant to sulphuric acid over a wide range of concentrations and temperatures. In addition, it will handle phosphoric and acetic acids under most conditions, especially where chlorides or other impurities are present.
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 are 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), polypropylene polyvinylidene 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 a 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-around chemical resistance properties. The following materials are mainly used: polypropylene, polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (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.