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Pump Bearing Distress & Seal Failure It has been known for years that bearing malfunction often precedes mechanical seal failure in centrifugal process pumps. Statistical information to that effect has recently been published in a technical paper; it facilitates assessing the benefits of sound remedial action. The data of interest were presented at the Mechanical Sealing Technology Seminar, IMechE, London under the title “Mechanical Seal Reliability - What Realistically can be Achieved”1. The presentation reviewed 11,000 mechanical seal failures from 148 different reliability contract and alliance plant sites over a two-year period. It concluded that 13% of the seal failures were attributable to lack of effective corrective and preventive maintenance of the pump bearings. These and associated findings are graphically represented in Figure 1.
The author’s statistics (Figure 1) showed 13% of total mechanical seal failures are the consequence of distress that originated with a bearing problem. In his narrative, the author expressed the belief that fitting modern labyrinth seals would eliminate these problems. Costs Associated with Bearing Failures Over the years, pump users and manufacturers have collectively estimated that 33% of all pump failures are due to bearing distress. Incorporating feedback from many pump users, bearing manufacturers generally attribute roughly half of these failure incidents to airborne (atmospheric) debris and water vapor entering the bearing housings of process pumps2. Yet, it is widely known that contaminants can be excluded with suitably configured bearing housing seals. Modern bearing housing seals, also called bearing protector seals (Figure 2), lead to fewer failures and achieve rapid payback. This is especially true when industry research corroborates that lube oil contamination is rampant in process pumps used in the Hydrocarbon Processing Industry3.
However, even while limiting one’s focus on the reported statistic that 13% of all mechanical seal failures are preceded by bearing failures, the projected benefits of advanced bearing protector seals will be highly attractive. In all cases, avoiding bearing failures leads to fewer lost opportunity events (production losses). Failure avoidance also limits outlays for general maintenance, repair parts, and cost of labor. The 13% statistic indicates that one in every seven or eight mechanical seals will fail because of inadequate pump bearing protection. (Note that one would thus have to take suitable preventive action on eight bearing housings to preclude the one-in-eight bearing failure that would harm a mechanical seal.) How does this Affect the Hydrocarbon Processing Industry? Various studies estimate the world market for mechanical seals at $3,200 million/year. This means that each year mechanical seals worth $416 million must be replaced because of pump bearing failures. The same market studies show that 40% of the mechanical seal market supply goes to the Hydrocarbon Processing industry, an estimated $400 million/year via the pump OEM’s and $800 million/year directly to end users. So, in essence, the Hydrocarbon Processing industry is needlessly spending $156 million each year on mechanical seals, and these costs could be easily avoided through the correct use of modern bearing housing seals. On an individual plant level it may be reasonably assumed that the installed cost of the average API cartridge mechanical seal is $4,000. Also, one might assume that a typical hydrocarbon processing facility has 2,500 pumps with an MTBF (mean-time-between failures) of four years. In this instance, the plant would spend $2.5 million/year on mechanical seals of which 13% or $325,000/year could be saved by the use of modern bearing housing seals. Bearing Housing Seals: Cost vs. Payback It is quite reasonable to assume that the average pump shaft size employed in the Hydrocarbon Processing industry is 2.250” (60 mm). For such a pump, a modern API 610 (10th Edition-compliant) non-contacting bearing housing seal with an integral static moisture shut-off valve (Fig. 2) would cost $147. A similar assumption places the cost of an equivalent and presently installed pump lip seal at $45 each. Therefore, the upgrade cost (incremental cost) from previously two lip seals to now two advanced rotating labyrinth seals is, on average, $204 per pump. To then upgrade eight 2.250”/60mm average shafted pumps to advanced rotating labyrinth-type bearing protector seals will cost $1,632. Returning to our earlier points and data reviews, a mechanical seal in hydrocarbon service costs $4,000. According to the Stephen Flood study and the cost data submitted by a well-respected manufacturer of both mechanical seals and advanced bearing protector seals, the rotating labyrinth seals of Fig. 2 will pay for themselves within a 3 - 6 month period, based solely on the mechanical seal failure reduction realized. Considering the other costs of equipment failure---including lost production---modern bearing protector seals are a winning proposition and should always be included in the reliability professional’s “upgrade toolkit.” Heinz Bloch graduated from the New Jersey Institute of Technology with B.S. and M.S. degrees in Mechanical Engineering. His professional career included long-term assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. He has written or co-authored 17 comprehensive books on failure analysis, failure avoidance, compressors, steam turbines, oil mist lubrication and practical lubrication for industrial facilities. Heinz can be contacted at hpbloch@mchsi.com Alan Roddis is an honors graduate with a degree in Mechanical Engineering. As the Engineering Director of UK-based AESSEAL plc, he holds numerous patents relating to fluid sealing and related products. Founded 23 years ago, his company now has over 1000 employees and has distinguished itself as one of the most innovative manufacturers in the business. In his present position, Alan Roddis has engineering and design responsibilities that encompass AESSEAL branches and/or representation in dozens of countries throughout the world. He is a frequent contributor to IMechE and other engineering societies. Alan can be contacted at alan.roddis@aesseal.co.uk References: 1. Flood, Stephen; “Mechanical Seal Reliability-What Realistically can be Achieved,” IMechE Mechanical Sealing Technology Seminar, London, UK, April 2007
2. Bloch, Heinz P. and Alan Budris; “Pump User’s Handbook: Life Extension”, (2006) Fairmont Publishing Company, Lilburn, GA 30047
3. Adams, V., R. Barry Erickson, Bill
Needelman, Michael D. Smith; “Field Investigation of Bearing
Housing Oil Cleanliness,” Proceedings of the 13th International
Pump User’s Symposium, Texas A&M University, Houston, TX 1996
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