Bearings failure is such a common cause of rotating equipment malfunction it’s easy to accept downtime for repair and replacement as simply part of the deal. But there is a quick-gain, low-cost way to maximise efficiency and minimise downtime by selecting the appropriate bearing housing seals says Chris Carmody
Bearings failures account for roughly 40 per cent of all rotating equipment outages in a plant. Of those bearings failures, almost 50 per cent are the result of contamination of the lubrication oil, most commonly by water, process fluid or particles of dust and dirt.
This ingress of moisture or particles into the bearings chamber might be minute, but the impact on efficient operations is anything but.
Research indicates that water contamination as low as 0.002 per cent – that’s 20 parts per million or just a single drop of water in a typical bearings chamber – can reduce bearing life in some oils by as much as 48 per cent.
Take another example of an oil-lubricated 45 mm radial bearing running at constant load and speed. Under ultraclean conditions (nc = 1), that bearing has been calculated to complete 15,250 operating hours. Introduce contaminated conditions where nc = 0.02 and its operational life plummets to just 287 hours – a dramatic decrease in mean time between failure (MTBF).
With most companies keeping a close eye on reliability and the bottom line, it makes sense therefore to look to the root cause of bearings contamination – and in most cases the culprit is the conventional lip seal. These seal the space between the stationary and rotating parts of rotary equipment and contain the lubricant that in theory protects it from contamination. However, surface contact with the rotating shaft means lips seals start to deteriorate almost immediately following installation. They rapidly wear, allow leakage into the bearings chamber, and ultimately cause corrosion and breakdown of the bearings. Another costly outcome of this surface contact is damage to the shaft itself.
It’s not uncommon for traditional lip seals to have a short life. It’s a long acknowledged fact that they are inefficient and that sealing solutions are readily available which significantly increase MTBF at little extra cost. But force of habit means many maintenance teams simply accept the routine of halting production, plucking another inefficient lip seal off the shelf – and allowing the whole cycle of leakage, contamination and failure to begin again.
Yet there is no reason at all to accept their breakdown and resultant bearings failure as an element to be factored into an otherwise perfectly efficient maintenance plan.
The solution lies in installing labyrinth-design bearing protector seals. Where lip seals are the weakest link in the operation and maintenance of rotating equipment, labyrinth bearing protector seals are the bedrock.
A reliability engineer looking to maximise the efficiency of both equipment and operations should consider a number of factors when selecting a bearing protector seal.
If bearing seals are to work to optimum efficiency they must facilitate the ‘breathing cycle’ required by rotating equipment, allowing the oil/air mixture to move through the bearing seal out into the atmosphere when it heats and expands with the rotating equipment, and then sucking air from the external atmosphere back into the bearings housing as it cools.
The most advanced labyrinth bearing protection seals incorporate dynamic lift technology to facilitate the breathing cycle whilst preventing the ingress of contaminating dust and moisture. This involves using the centrifugal force of rotating equipment to open a temporary micro gap, allowing expansion of the oil air mixture in the bearing housing and allowing the equipment to ‘breathe’. When the equipment stops rotating the micro gap immediately closes forming a perfect seal against potential contaminants.
The seal’s integral, self-adjusting axially energised shut-off O-ring is made from a highly resilient elastomer material which exhibits near-zero wear, further enhancing the lifespan of the seal.
While many seals, particularly in older equipment, are machined integrally into the bearing plate by the OEM, the most efficient bearing protector seals can be retrofitted onto shafts in place of the previously used lip seals. Once fitted they require minimal maintenance and will last until the bearings reach the end of their natural lifespan. An important feature when considering ongoing maintenance costs is the in-place reparability offered by quality protector seals.
With the risk of leakage all but eliminated, reliability engineers can be confident of making other cost-effective decisions, for example by upgrading to the more efficient synthetic oil for bearing lubrication.
The cost of bearing protector seals might be slightly higher compared to lip seals but the savings they achieve in terms of significantly improved MTBF, associated shaft repairs and reduced ongoing maintenance makes the case for upgrading a matter of simple common sense.
Dr Chris Carmody is special products manager for AESSEAL, where he is responsible for development of high integrity sealing projects, including dry gas seals. AESSEAL is one of the world’s leading specialists in the design and manufacture of mechanical seals, bearing protectors, seal support systems and gland packing. AESSEAL has 230 locations worldwide, supplying customers in 104 countries.
For further information please visit: www.aesseal.co.uk/en