With the many uses for o rings in various industries, it is important to choose the correct composite seal.

O-rings are acceptable for static sealing but inappropriate for dynamic sealing applications such as pumps and compressors, and high and low pressure applications. In these cases a single elastomer may be used with an X-ring profile to prevent twisting in rotary applications. However a composite seal design is often preferred.

A composite seal is a combination of two or more materials to produce a seal that offers the best attributes of both materials. For example, a T-shaped seal comprises an elastomer T-shaped component and two plastic back up rings. The seals are used in reciprocating piston, rod and rotary applications. The T-shaped design prevents the elastomer rolling and the plastic back-up rings, energised by the rubber elastomer, provide improved high pressure performance. Importantly the T-seals can be installed into standard O ring grooves. Other standard composite seal designs include spring reinforced seals, encapsulated seals, lip seals, energised lip seals and cap seals.

Where greater customisation is needed seal design engineering techniques based on popular CAD packages such as Solidworks, Catia, Pro Engineer and AutoCAD are employed. 3D modelling and analytical behaviour predictive tools such as finite element analysis (FEA) can then be used to explore ‘what if’ simulation scenarios to produce a 'right first time' design. Seal selection tools to assist the engineer are also available online. Temperature and chemical compatibility, groove dimensions for the most common o ring sizes, and physical performance datasheets enable selection of the most appropriate o ring seal.

As never before the design engineer is able to fine tune the performance of equipment rubber seals and gaskets, thanks to the development of new materials and improved design and manufacturing techniques.

Optimising sealing performance involves striking a balance between often competing properties: mechanical performance vs temperature resistance vs chemical resistance vs rapid gas decompression resistance and so on. The need for compromise is reducing all the time as weaknesses in existing elastomers are polymer-engineered out and new materials developed to meet extreme operating conditions are applied to more mainstream markets.

One such material is fluoroelastomer (FKM). FKM seals provide high temperature and chemical resistance to a broad range of chemicals and solvents. However the elastomer typically shows poor resistance to hot water and steam. By changing the way the polymer is manufactured radical improvements in water and steam resistance can be achieved. Within the FKM family of materials, there are differing ‘cure systems’ – the chemical cross-linking reaction (vulcanization) which occurs to join the polymer chains together. Common FKM sealing materials are of two types: co-polymer and terpolymer. The commonly used traditional, bisphenol-cured, co-polymer fluoroelastomers are cured by utilizing a ‘condensation’ reaction where, water is generated during the cure process. When the fluoroelastomer is exposed to high temperature water and steam environments, the cure is reversed, breaking down the cross-links of the material, leading to premature failure of the rubber seals.

Peroxide-cured, terpolymer fluoroelastomers, on the other hand, do not suffer the reverse condensation reaction. Instead, the peroxide cure system is a ‘free-radical’ reaction, and as such, provides superior water and steam resistance. This, and other new compounding techniques, is now enabling FKM elastomers to withstand steam cleaning regimes up to 200˚C.