Seal the deal

Seals and gaskets that operate with industrial and process pumps have a tough life, especially those designed to protect bearings and rotating shafts. Yet, despite this difficult operating environment, seal or bearing failure can have dire consequences as contaminants have an opportunity to infiltrate both the bearing and the lubricant, with this having a profound impact on bearing service life.

In addition, should lubricant be lost from the bearing due to seal failure, dry running operation could lead to premature and rapid failure of the bearing. All this could be potentially expensive in terms of pump failure, repair or replacement, together with the associated down-time.

As with bearings themselves, the choice of bearing seal depends solely on the particular duty involved, with the obvious proviso that a bearing and its sealing system should be viewed holistically and not designed or specified individually without consideration of the other. Accordingly, manufacturers of bearings and seals typically supply matched components to simplify specification and use.

Components on duty

For general purpose duties, including those performed by ANSI (American National Standards Institute) class centrifugal pumps, dynamic radial shaft seals are often the most appropriate solution. Typically, these incorporate a steel or elastomer shell to which an elastomer lip seal is bonded, providing dynamic and static sealing against the pump shaft. In pump power frame applications, this sealing lip should always point towards the material to be retained i.e. the lubricant, so inwards to the bearing. Most sealing lips are of a nitrile rubber composition, but other materials are now available for use with synthetic or chemically aggressive lubricants.

For heavier duty applications such as those served by API (American Petroleum Institute) rated pumps or enhanced versions of ANSI pumps, more complex seals may be needed. Here you will often find labyrinth seals, also known as bearing isolators, being used. As the name hints, these seals feature a tortuous internal, non-contacting pathway between their static and dynamic elements. Installed correctly, these seals and their internal structure will keep contaminants out of the bearing and retain lubricant over a long service life.

Operating conditions

However, irrespective of seal design, all relevant operating conditions must be taken into account. Among the key parameters to be considered here are the surface speed of the shaft, temperature, pressure and surface finish preparation of the shaft and seal.

With higher than normal shaft speeds, the allowable pressure differential across the seal is reduced. Increased speed inevitably elevates frictional forces on the shaft, but as more of the lip seal surface is forced against the shaft, the pressure applied to the seal will increase. Pressure and speed, otherwise known as the PV factor, therefore have to be considered together and balanced against each other.

Each type of radial shaft seal will have specific surface speed limits but there are design variations that can mitigate the negative effects of higher speeds. These can include reducing the radial load of the seal tip, changing to a seal material capable of handling higher temperatures, changing the type of lubricant (or just its viscosity), or opting for a non-contacting labyrinth seal from the outset.

Similarly to surface speed limits, each type of seal material has an optimum temperature range within which it can operate effectively. Go beyond that optimum range and thermal stress can harden the elastomeric compound, causing it to crack. Indeed, this type of heat ageing is often the most common cause of failure in nitrile rubber seals over and above wear, and can be observed as subjective hardness or as a series of radial cracks. However, a seal’s thermal limit can be extended to meet almost any pump system requirements by upgrading the seal material to a thermopolymer or PTFE.

Another downside of running at high temperatures is the effect on the lubricant itself. Good quality, clean lubricants should have a life expectancy of many years running at 30°C, but some estimates have shown that each 10°C rise in temperature can halve operating life as the lubricant starts to lose its viscosity, ultimately beginning to ‘choke’ up and deposit solid particles into the bearing.

Additionally, for pump applications using radial shaft seals as the main pressure retention seal for pressurised seal cavities, manufacturers have developed particular lip profiles designed to resist deformation under pressure and moderate surface speeds. At higher speeds, the allowable pressure differential across the seal will decrease so the two parameters, pressure and speed, have to be balanced against each other – the PV factor. Indeed, some radial designs, principally using PTFE as a lip material, can operate with a PV in excess of 250,000 depending on the service life required.

Naturally, a smoother shaft surface will provide better sealing contact with a lip seal. For optimum radial lip seal performance and surface life, a surface finish of 0.20-0.43µm Ra (arithmetical mean roughness) is recommended with a machine lead of less than +/- 0.05 degrees. However, contact surfaces for labyrinth seals are considered less critical, with finishes suitable for static o-rings being classed as satisfactory.

Summary

It is widely accepted that radial rubber seals provide a cost effective and versatile means of protection for bearings in pumps. Working closely with an experienced and trusted manufacturer of bearings and seals from the outset and by adopting a holistic approach to seal specification can contribute significantly to the performance and service life of the entire system – ultimately allowing considerable improvements in productivity and profitability to be achieved.