PUMPS, VALVES & ACTUATORS FEATURE TAKE A BREAK FROM THE NORM
Bearing failures account for around 40% of all pump and rotating equipment malfunctions, and are often accepted as simply part of the deal. But there is a low-cost way to maximise pump efficiency and minimise downtime by selecting the appropriate mechanical seals, says Chris Carmody of AESSEAL
B
earing failure is one of the most common and costly causes of
malfunction for pumps and other rotating equipment. Almost half of all those bearings failures are the result of contamination of the lubrication oil by water, process fluid or particles of dust and dirt. And the impact on efficient operations of even the minutest ingress into the bearings chamber can be extremely costly. Water contamination as low as 20 parts
per million – equivalent to a single drop of water in a typical bearings chamber - can reduce bearings life in some oils by as much as 48%, according to research. For example, an oil-lubricated 45mm radial bearing running at constant load and speed under ultra-clean conditions (nc=1) 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. This constitutes a dramatic decrease in mean time between failure (MTBF). With reliability and cost efficiency of
paramount importance to most companies, the logical response should therefore be to look to the root cause of bearings contamination. And in most cases the finger of blame points squarely at the conventional lip seal. Lip seals literally 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 and they rapidly start to wear. This leads to the lubricant leaking into the bearings chamber, and ultimately causes corrosion and breakdown of the bearings. In worst case scenarios, the shaft itself becomes worn and damaged, requiring expensive repair or replacement. It’s a well known fact that lip seals are inefficient and have a relatively short life,
and that sealing solutions are readily available which increase MTBF at little extra cost. But through force of habit many maintenance teams simply go through the motions of halting production, replacing the inefficient lip seal with an identical spare part – and the whole cycle of leakage, contamination and failure begins again. So, how to remedy this clear anomaly in an otherwise perfectly efficient maintenance plan? The solution lies in installing labyrinth-design bearing protector seals. These are the bedrock of the efficient operation and maintenance of rotating equipment. There are several factors to consider
when selecting a bearing protector seal to maximise efficiency. For 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. Advanced labyrinth bearing protector
seals are available which incorporate dynamic lift technology to facilitate the breathing cycle whilst preventing the ingress of contaminating dust and moisture. This involves using the
With the risk of leakage all but eliminated through the use of bearing protector seals, reliability engineers can make other cost-effective decisions, for example by upgrading to the more efficient synthetic oil for bearing lubrication
Dr. Chris Carmody PhD, MSc BEng (Hons) is special products manager for AESSEAL. He is responsible for development of high integrity sealing projects, including dry gas seals and sits on several different bodies, including the API692 Compressor Dry Gas Seal Committee
A deteriorated seal can cause bearing contamination and subsequent failure
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 resilient elastomer material which exhibits near- zero wear, further enhancing the lifespan of the seal. Many lip seals, particularly in older
equipment, are machined integrally into the bearing plate by the OEM, adding to downtime for maintenance and replacement, while modern bearing protector seals can be retrofitted onto shafts. In-place repair is another important feature, though they require minimal maintenance and will last until the bearings reach the end of their natural lifespan. There is no reason at all to accept lip
seal breakdown and the resultant bearings and pump failure as an element to be factored into an otherwise perfectly efficient maintenance plan.
The cost of bearing protector seals might
be slightly higher compared to lip seals but the savings they achieve in terms of improved MTBF, associated shaft repairs and reduced ongoing maintenance makes the case for upgrading a matter of both financial and operational common sense.
AESSEAL
www.aesseal.co.uk/en
/ PROCESS&CONTROL
PROCESS & CONTROL | JUNE 2018 29
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