Check-ups keep pumps working

Centrifugal pumps must sometimes perform under arduous conditions, and the risk of premature failure can be minimised by good maintenance procedures.

Paying particular attention to bearings, lubrication and seals, can help sustain the pumps’ service life and avoid costly replacements or rebuilds.

Pump bearings take quite a load during their service life not only supporting the hydraulic loads imposed on the pump impeller and shaft, but also supporting thrust loads from couplings and drive systems, while minimising friction forces on the pump shaft.

Users know that radial ball bearings on pumps can fail suddenly if the correct installation and maintenance programmes are not in place. Such premature failure can be attributed to incorrect lubrication of the bearing. Some industry studies in fact point to nearly a third of all pump-bearing failures being due to poor lubrication.

Pump bearings have to be selected to suit the application but generally they all have to fulfil that basic requirement of enabling smooth, efficient shaft rotation. Lubrication is essential to this task and bearing seals are essential to efficient lubrication.

Whether provided by oil / grease, lubrication supplies the necessary separating film between the rolling elements of a bearing to prevent metal-to-metal contact and reduce friction, while at the same time inhibiting wear and corrosion. Protecting the quality of that lubrication is the vital role played by bearing seals in maintaining pump performance.

If a pump bearing seal fails, a two-way door is opened up to operating problems. Lubricant can be lost from the bearing through the failed seal, while flowing the other way into the bearing can be contaminants such as water and solid matter. The latter can be exacerbated in aggressive and dirty environments.

The choice of bearing seal depends 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.

Bearings and seals suppliers, such as SKF, supply matched components to simplify specification and use. For general duties, dynamic radial shaft seals are the preferred choice.

In pump power frame applications, this sealing lip should point towards the material to be retained –the lubricant — so inwards to the bearing. Most sealing lips are of a nitrile rubber composition, but other materials are available for use with synthetic or chemically aggressive lubricants.

More complex seals may be needed on heavier duty applications such as those served by API-rated pumps or enhanced versions of ANSI pumps. Typical of this complexity are labyrinth seals, or bearing isolators.

As the name indicates, 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 contamination out of the bearing and retain lubricant over a long service life.

Irrespective of seal design, however, users must take into account all relevant operating conditions. Among key parameters to be considered are surface speed, temperature, pressure and surface finish preparation of shaft and seal.

With higher than normal shaft speeds, the allowable pressure differential across the seal is reduced. Increased speed clearly increases 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 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.

Each seal material has an optimum range of temperature in which to operate. Too high and the elastomer seal can harden and crack. Upgrading to a thermopolymer or PTFE-like compound can extend a seal’s operating range.

Good quality, clean lubricants should last years running at 30ºC, but some estimates have shown that each 10ºC rise in temperature can halve operating life as the lubricant starts to lose its viscosity, ultimately beginning to “coke” up and deposit solid particles into the bearing.

With so many parameters and operating conditions to consider, assessing seal life can be difficult. It has to be accepted, that by their very nature they will have a shorter operating life than the components they actually protect. But many premature bearing failures will be prevented by replacing seals at the first signs of wear or leakage.

The best practice is to have a rigorous inspection and replacement programme in place for pump bearing seals, one best designed in consultation with experienced bearing and seal manufacturers who can share their application knowledge to arrive at the best solution.

Phil Burge is marketing manager at SKF. T: 01582 496433