TECH SPOTLIGHT Is All Bearing Surface Damage Troublesome?
In his first article in a two-part series, exploring bearing corrosion in offshore environments, Chris Johnson, Managing
Director of bearing specialist SMB Bearings, explains why not all bearing surface damage should cause alarm and future headaches for offshore oil and gas plant managers.
Bearing surface damage can take many forms, from spalls in the raceways, wear from ineffective lubrication, corrosion due to harsh chemicals to false brinelling marks caused by static vibration. While surface distress can result in problematic symptoms, not all external bearing flaws point to compromised internal machine performance.
Corrosion is a naturally occurring phenomenon and a common form of bearing surface damage that offshore oil and gas plant managers must contend with. There are ten primary forms of corrosion, but bearing corrosion usually falls into two broad categories — moisture corrosion or frictional corrosion. The former is environment specific, but can appear on any component of the bearing, creating an alarming oxide layer as a result of a chemical reaction with a metal surface.
For example, in offshore mining, bearings are often exposed to moisture or mild alkalinity due to their contact with sea water. Mild corrosion may result in light surface stains, but in more serious cases, it can lead to etching on the surface of the bearing, resulting in flakes of rusted material entering the raceway. For this reason, corrosion is often known as the natural enemy of bearings.
Corrosion isn’t just visually alarming; it can also significantly impact a business’ finances. According to the IMPACT study conducted by NACE International, the world’s leading corrosion control organisation, it has been estimated that 15- 35 per cent of annual corrosion could have been saved if optimum corrosion management practices were followed. This equates to
savings between US$375 and $875 billion annually on a global basis.
The Enemy? It’s impossible to ignore the significance of corrosion costs, however corrosion resistance must be considered alongside other operating requirements such as bearing longevity and load.
Consider this as an example. A drilling machine is required to operate with precision but must also operate in unforgiving conditions. Due to the extreme environment of oil and gas rigs, corrosion resistant bearings would be recommended. If a design engineer were to opt for a highly corrosion resistant bearing fabricated from polyether ether ketone (PEEK), this would stop the corrosion in its tracks, but the precision of the machine would be compromised. In this scenario opting for a high precision stainless steel bearing with superior roundness while allowing some superficial corrosion may be preferable.
When assessing the suitability and quality of bearings, it is important to look beyond the external aesthetic. Corrosion control is just one performance requirement, which doesn’t necessarily equate to poor performance or affect the bearing’s internal rollability.
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www.sosmagazine.biz September 2020
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