Pump failure should be the exception not the norm
Published: 21 June, 2018
With bearing failures accounting for around 40% of all pump and rotating equipment malfunctions, 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 pump efficiency and minimise downtime by selecting the appropriate bearing housing seals, says Chris Carmody of AESSEAL.
Bearings failure is one of the most common causes of malfunction in pumps and other rotating equipment, leading to costly downtime and equipment repairs and replacement.
Of those failures, almost 50% 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 can be significant.
Research indicates that water contamination as low as 0.002% - that’s just a single drop of water in a typical bearings chamber - can reduce bearing life in some oils by as much as 48%. 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 - a dramatic decrease in mean time between failure (MTBF).
Reliability and cost efficiency are high on the agenda for most companies, so the logical response should be to look at the root cause of bearings contamination. And in most cases the culprit causing the problems is 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, due to surface contact with the rotating shaft, they can start to deteriorate almost immediately after installation and rapidly start to wear. This can leave it open to potential leakage into the bearings chamber, ultimately resulting in corrosion and bearings failure. In worst case scenarios the shaft or shaft sleeve become worn and damaged, requiring expensive repair or replacement.
It’s no secret among maintenance teams that lip seals are inefficient and have a relatively short life and that sealing solutions are available which significantly increase MTBF at little extra cost. Yet they continue to go through the familiar procedure of halting production to replace the inefficient lip seal with an identical spare part – allowing the whole cycle of leakage, contamination and failure to begin again.
This presents a clear anomaly in what are often otherwise perfectly efficient maintenance plans. And the solution is simple – break the lip seal habit and install labyrinth-design bearing protector seals. While lip seals are the weakest link in the operation and maintenance of rotating equipment, labyrinth bearing protector seals are the bedrock.
A reliability engineer who upgrades to protector seals will maximise the reliability of equipment, improve efficiency and achieve long term cost savings. The reliability of these seals is achieved by the facilitation of 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 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 lip seals are machined integrally into the bearing plate by the OEM, particularly in older equipment, 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. This in-place reparability is an important feature when considering ongoing maintenance costs.
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.
Lip seal breakdown and the resultant bearings and pump failure are accepted by some as a repeat task to be factored into an otherwise perfectly efficient maintenance plan. But there is no reason at all to accept this inefficient and uneconomic approach as ‘the norm’.
The higher initial cost of bearing protector seals is proven to be swiftly offset by the savings they achieve in terms of significantly improved MTBF and more efficient ongoing maintenance costs. This should make the case for upgrading not so much a matter of choice but of simple common sense.
Dr. Chris Carmody PhD, MSc BEng (Hons) is special products manager for AESSEAL, where he is responsible for development of high integrity sealing projects, including dry gas seals. He has 25 years of experience in the design of mechanical seals and is a named inventor on many of AESSEAL product designs. He sits on several different bodies, including the API692 Compressor Dry Gas Seal Committee.
For further information please visit: www.aesseal.com
