How to choose seals for aggressive applications

Andrew Kownacki, Technical Director at Ceetak, looks at the information required for seal selection, and identifies some products that are ideal for harsh environments.

It has traditionally been difficult to specify effective and reliable seals to meet the needs of harsh manufacturing or process applications.

By their very nature, applications that involve extreme temperatures and pressures, aggressive media or environmental conditions can place considerable wear or stress on conventional plastic or rubber seals and diaphragms.

To date, the choice of suitable materials for seals in demanding applications has been relatively limited.

As a result, those products that have been available have often cost considerably more than those used in mainstream sectors of industry, while the need to formulate seal materials to withstand particular process media has frequently involved a compromise between technical performance, operating life and simplicity of replacement.

One example of a sealing compound that was, until recently, only suitable for relatively benign environments is polyurethane.

Polyurethane is an extremely adaptable and effective sealing material and has been widely used in many applications where less aggressive media are involved.

Although there is a wide range of polyurethane sealing materials available, including those based on formulations such as diphenylmethane diisocyanate (MDI), diphenyldiisocyanate (TODI) and p-pphenylenediisocyanate (PPDI), these can be adversely affected by extremely acidic or alkaline solutions, and by extremes of temperature and mechanical stresses.

Now, however, a new generation of polyurethane materials with superior levels of abrasion resistance are gradually being introduced.

These have the potential to extend the application of relatively low cost but high performance seals into areas of industry that previously required extremely specialised materials.

For example, the latest PPDI-based compounds offer an operating temperature range that exceeds conventional materials by up to +20C, while being capable of withstanding temperatures as low as -45C, making them suitable for use in chemical engineering, automotive and mobile hydraulics and in process and plant applications.

Unlike conventional polyurethanes, seals and diaphragms produced from the latest high performance compounds are also compatible with process media across the entire pH range.

In particular, they offer superior chemical resistance to hydrolysis in strong alkaline fluids and concentrated acidic solutions, enabling them to retain their essential characteristics in processes with alcohol/water and ethylene-glycol/water mixtures.

Similarly, these new compounds are suitable for areas where lubricants based on polyalphaolefines, silicones and perfluorinated polyethers are used, and in demanding applications such as those in the food sector, where they replace traditional sealing materials in order to meet increasing stringent regulations.

In addition, Teflon-based PTFE (polytetrafluoroethylene) seals, which rely on spring or O-ring energisers to counteract their characteristic rigidity, are increasingly being superseded by a generation of cost-effective polyurethane materials.

These can easily be moulded into standard profiles and seals, yet offer effective chemical and mechanical characteristics, while being far simpler to fit and more reliable in service.

Further innovations in polymer compounds have included the development of chemically resistant seal, O-ring and diaphragm materials, which has been achieved by increasing the fluorination of the fluorocarbon polymer chains.

This results in a significant improvement in the compression set, while maintaining high levels of chemical stability; as a result, these materials can function in temperatures ranging from -25 to +260C and ensure minimal deformation under changing temperature conditions to provide a more consistent sealing force and minimise the possibility of leakage over longer periods.

Although O-ring seals are widely used in industrial and automotive applications their standard compound formulation has remained largely unchanged for many years.

Now, however, to meet the needs of increased automation, reduced assembly times and to eliminate damage during installation, a new generation of pre treated O-rings is available.

These are manufactured in a selection of NBR, HNBR, EPDM, FKM and FVMQ compounds, that have been specially treated with a polymer film that dramatically reduces frictional resistance without affecting the physical or chemical properties of the original compounds.

As a result, installation and assembly times are significantly shortened, while the operating life of seals used in abrasive couplings and fittings is extended.

This new generation of O-rings are suitable for use in both manual assembly operations and automatic feeding systems.

In addition, it is possible to elongate the pre treated seals by up to 150% without causing the 5um antifriction coating to fracture.

It should not be overlooked that although PTFE is being replaced by polyurethane in certain applications, the material still offers excellent performance in areas where high temperatures or aggressive media are encountered, such as the chemical, pharmaceutical, food and offshore industries.

Indeed, diaphragms and bellows manufactured from the latest PTFE materials are capable of withstanding temperatures up to 260C and prolonged exposure to most chemicals and process media.

Perhaps as importantly, they offer excellent dimensional stability, are generally resistant to stress fracturing and do not become brittle as a result of ageing.

These characteristics make them suitable for controlling or separating the flow of aggressive media in valves and pumps, as spacers in equipment such as heat exchangers, or to provide flexible connections in pipe work.

Until recently, harsh applications meant that seals and O-rings had to be selected from a relatively narrow range of materials.

Now, however, due to an ongoing commitment to research and development combined with application experience from suppliers such as Ceetak, the range of options is significantly larger, giving OEMs, equipment manufacturers and end users far greater choice, with the ability to make significant improvements to system performance, functionality and reliability but at reduced installation and operating cost.

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