Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/108300
COVER STORY • Superior synthetics achieve high film strength through proprietary additives, so there can be significant differences in the performance of two lubricants of the same viscosity and base stocks. Only one might be suitable for the highest reliability services. • The notion that one oil type or viscosity suits all applications is rarely correct. Similarly, no fixed or particular oil ring geometry is ideally suited for all oil types and viscosities. Customdesigned oil rings may be required to work with the thicker oils at certain high shaft peripheral velocities. Although synthetic lubricants cost more than mineral oils at the point of initial purchase, rigorous and all-encompassing cost justifications will often show relatively short payback periods. Combining extended bearing life and extended drain intervals results in better payback. This is made possible by keeping contaminants away from the lubricant. Therefore, upgrading to the best-available bearing protector seals and implementing plant-wide oil-mist lubrication are two principal strategies adopted by world-class plants. Protect the Lubricant for Longer Bearing Life Air and the lubricant take up whatever housing space is not actually used by the bearings. For bearings to survive, solid particles and water intrusion must be avoided. For this reason, proper bearing housing protection seals are essential for keeping the oil clean. Reliability-focused users are mindful of the fact that nonoptimized bearing housing protector designs can hurt oil cleanliness by shedding slivers of sealing components. Keeping the oil clean is the first order of business if extended oil replacement intervals are the goal. In turn, achieving extended oil replacement intervals often makes it economical to use superiorquality synthetic lubricants. In combination, advanced bearing protector seals and synthetic lubricants create an environment conducive to long bearing life. Common sense and statistical evidence point to greatly reduced downtime risk and demonstrable maintenance cost avoidance credits. Because synthetic lubricants are more expensive than mineral oils, some users cling to mineral oils for their process pumps. They also may employ inadequate bearing housing seals because their only concern is the initial purchase price. Wear-prone seals include lip seals and also certain rotating labyrinth seals. Seals to avoid are those that allow a rotating O-ring to contact the sharp edges of an O-ring groove, or O-ring grooves that are wide enough to prevent such contact but that enable copious amounts of contaminants to enter the bearing housing. While even small machine manufacturers recognize the need to limit both contaminant ingress and oil leakage, inexpensive lip seals are found in some pumps and drivers to keep initial cost low. Yet lip seals typically last only about 2,000 operating hours (three months). When lip seals are too tight, they cause shaft wear and in some cases lubricant discoloration known as "black oil." Once lip seals have worn and no longer seal tightly, oil is lost through leakage, or contaminants find their way into bearing housings. This fact is recognized by the API-610 standard for process pumps, which disallows lip seals and calls for either rotating labyrinth-style or contacting face bearing housing protector seals. 12 | January - February 2013 | www.machinerylubrication.com Figure 2. This cross-section view shows a small steam turbine driver for process pumps. (Ref. Worthington-Turbodyne) Small steam turbines often suffer from steam leakage at both drive and governor-end sealing glands. Each bearing housing is located adjacent to one of these two glands, which contain carbon rings. As soon as the internally split carbon rings start to wear, high-pressure and high-velocity leakage steam finds its way into the bearing housings. Traditional labyrinth seals have proven ineffective in many such cases, and only solidly engineered bearing protector seals are effective in blocking leakage steam intrusion. The bearing housing protector seal in Figure 3 was designed for steam turbines. It incorporates a small- and large-diameter dynamic O-ring. This bearing protector seal is highly stable and not likely to wobble on the shaft; it is also field-repairable. With sufficient shaft rotational speed, one of the rotating ("dynamic") O-rings is flung outward and away from the larger O-ring. The larger cross-section O-ring is then free to move axially, and a micro-gap opens up. When the turbine is stopped, the outer of the two dynamic O-rings will move back to its standstill position. At standstill, the outer O-ring contracts and touches the larger cross-section O-ring. In this design, the larger cross-section O-ring touches a relatively large contoured area. Because contact pressure equals force divided by area, a good design aims for low pressure. In outdated configurations, contact with the sharp edges of an O-ring groove risks O-ring damage, and slivers of O-ring material can end up contaminating the lube oil. Upgrade to a Better Bearing Environment Fortunately, concerns as to the time it might take to upgrade to advanced bearing protector seals have been alleviated. Modern products fit in the space previously taken up by lip seals. In 2009, when a Dutch refinery asked for the installation of the bearing protector seal shown in Figure 3 for one of its steam turbines, no modificaFigure 3. This cross-sectioned half-view tions were allowed on illustrates an advanced bearing housing the existing equipprotector seal for small steam turbines. (Ref. AESSEAL Inc.) ment. Installation of