Body Mateerials
One of the most essential reqruirements for a valve body is that it is able to withstand pressure, temperature and corrosion. in addition, the production of the body should be affordable and conform to all relevant regulations.
Metals
Cast materials dominate
Cast materials are very important for butterfly valves because the production process enables design engineers to design the most favourable valve body and to reduce the weight at the same time. The chosen body material should have a high elongation before breaking. This material property ensures that a pressure-stressed body can expand with increasing pressure loads expand with increasing pressure loads within its material-specific limits before bursting abruptly.
A simple cast iron with a laminar graphite structure (e.g. 0.6025, also known as GG25) is the cheapest iron material for the valve body with an elongation at break of only 1%. In the current regulations, this material therefore has a higher margin of safety factors than, for example, cast steel which leads to a greater wall thickness of the body design. That is why simple cast iron should never be the first choice for high pressure requirements. Basic cast iron can only be used for non-critical applications at low pressure. Ductile cast iron has an elongation at break of 15% and is mostly used in soft-seated body valves. Cast steel of the type 1.0619 (ASTM 216 WCB) or stainless steel casting of the type 1.4408 (ASTM CFSM) is mostly used in eccentric butterfly valves (Show Table). This type of steel can also be used for concentric butterflyvalves in highly corrosive atmospheres or for low-temperature applications. Another important aspect is the temperature
| Material | Material Number | EN Standard | Elongation at break | Price in %; grey cast iron corresponds to 100 % (empirical values) | Application for Valve body |
|---|---|---|---|---|---|
| Grey cast iron | 0.0625/ASTM A48 (408) | EN 1563 | 1% | 100 | e.g. concentric valves for uncritical applications to PS = max. 1.6 MPa |
| Ductile iron | 5.3106/ASTM A536 | EN 1563 | 15% | 140 | e.g. concentric valves to PS = max. 2.5 MPa |
| Cast Bronze | CC333G/UNS C95800 | EN 1982 | 13% | 1100 | Critical media such as sea water |
| Cast steel | 1.0619/ASTM 216 WCB | EN 10213 | 22% | 220 | General Applications |
| Stainless steel | 1.4408/ASTM CF8M | EN 10213 | 30% | 560 | General Applications |
| Duplex cast steel | 1.4469/UNS J93494 | EN 10213 | 22% | 900 | Higher corrosive requirements |
behaviour of the body material. In this case there is an important disparity between cheap cast iron and cast steel mate Cast steel materials are superior to the cheaper cast iron materials. Grey cast iron is not suitable for low temperatures below 10°C (14°F), whereas ductile cast iron of the type (Low-Temperature LT) or cast steel can be used for low temperature appli cations. Both materials are subjected to the most demanding tests compared to normal testing. As an example, the Charpy impact test, also known as the Charpy v-notch test, is a standardised high strain-rate test which determines the amount of energy absorbed by a material during fracture.
The chemical resistance of cast iron mater ials is relatively low. That is why cast iron ma terials are usually used for lined valves. The liner (sleeve) has the function to protect the body against media influences. This also applies for cast steel materials. Due to its higher strength properties, this material is usually used in eccen tric valves in which the medium acts directly on the unprotected body.
Corrosion protection by stainless steel
Steel and cast steel materials are usually used at high pressures and low temperatures. Bodies made of cast iron and cast steel are generally coated because of their low corrosion protec tion. In contrast to the mentioned materials, stainless steel and stainless steel castings have a very good corrosion protection. A coating is not always necessary. Steel and stainless steel are standard materials for high-performance but terfly valves (eccentric valves). For special appli cations, non-ferrous metals such as bronze or titanium can be used as a body material.
For foundries, it is essential to have a certifi cation to PED (Pressure Equipment Directive) to produce pressure-stressed components. Cast iron, cast steel and stainless steel castings are subject to the harmonised European standards and regulations. In addition, there is a common regu lation observed by global chemical companies (PAS 1085- Public Available Specification). In this specification, the technical delivery terms for valves are defined more precisely than in the European regulations.
Plastics
Most commontly used materials
The use of butterfly valves with plastic bodies depends on the medium being transported in the system. These valves usually have a centrally mounted disc and a shaft seal. Commonly used plastics for the body are PVC-U (hard polyvinyl chloride) (Fig. 10), PP (polypropylene) or PVDF (polyvi nylidene fluoride). Valves with plastic bodies usu ally have lower permissible operating pressures of 0.6 to a maximum of 1.0 MPa (6 to 10 bar/87 to 145 PSI) than valves with metallic bodies. Themaximum permissible operating temperatures depend on the plastic and are between -30°C to +120°C (-22 to 248°F). In some cases, glass fibres are used to increase the strength of plastic.
Advantages of plastic bodies
Plastic bodies are often used in systems that are fitted with plastic pipes. These have been adapted to suit the standards that are valid for pipeline and flange systems. A good advantage of plastic bodies is their corrosion resistance against external and galvanic corrosion, ich may some times appear between metal pipes and metal butterfly valves. Compared to metallic or metal lic-coated butterfly valve bodies, plastic bodies are less sensitive to external mechanical damage.To increase the chemical resistance against the operating medium, it is possible to use an additional lining.