Material

Of the multitude of available materials, metallic and non-metallic (such as plastic and ceramic) materials are preferred for centrifugal pumps, although the exact choice depends on the level of stress involved.

Metallic material

Metallic materials take the form of cast materials, forged or rolled materials, or materials in some other product form. Given the high level of freedom that is available in terms of design and the wide array of special alloy types, these remain the most frequently used design materials for pump casings and impellers.

The cast materials used in centrifugal pumps vary depending on the desired material composition and the mechanical and technological properties required. See Fig 1 Material

Plastics

Plastics are divided into four groups according to the ISO 1043-1 standard, based on how their mechanical response to temperature is.

Mechanical behaviour of the four plastic groups

• Thermoplastics:
  When subjected to heat, these materials can be deformed any number of times. Examples include polystyrene (PS), polyethylene (PE) and polyvinyl chloride (PVC).
• Elastomers:
  These are wide-mesh cross-linked high-polymer materials with rubber-like elastic properties in all temperature ranges (e.g. cross-linked natural rubber or polyisobutylene).
• Thermoplastic elastomers:
  These materials are wide-mesh cross-linked high-polymer materials that attain rubber-like elastic properties at temperatures above 20 °C, e.g. natural rubber crosslinked with more than 10 % sulphur, cross-linked polyethylene or high molecular weight polymethyl methacrylate (PMMA).
• Thermosets:
  These close-mesh cross-linked substances are cured high-polymer materials (e.g. polyester, epoxy or phenol formaldehyde resins).

Thermoplastics have a particularly important role to play as design materials for impellers and diffusers whereas elastomers are used for sealing elements or as coating materials.
See Fig. 2 Material

Ceramic materials

Ceramic materials are defined as materials that are non-metallic, inorganic and more than 30 % crystalline. They are used not only as state-ofthe-art engineering ceramics for sealing and bearing elements (see plain bearing) but also increasingly for other design elements, such as impeller wear rings and casing wear rings (see controlled gap seal) as well as impellers.

See Fig. 3 Material

This group of materials is characterised by its high mechanical, thermal and chemical stability. As well as being brittle and sensitive to tensile stress, these materials can only be formed to a limited extent and are expensive to process. Therefore, the designs have to take account of their properties.