Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/108300
COVER STORY three bearing protector seals on the first machine had to take place during a scheduled plant shutdown. With no detailed drawings of the bearing housings available, the exact installation geometry could only be finalized after dismantling the small turbine seen in Figure 2. One of the main problems was the short outboard length — less than 0.25 inch was available due to the presence of steam deflectors and oil flingers. But the manufacturer's engineers were able to modify the advanced design to fit into the existing groove of the original equipment manufacturer's labyrinth seals. Delivery was made within one week of taking measurements of the steam turbine and bearing housings, and the turbine has been running flawlessly for years. The point is that highly cost-effective equipment upgrades are possible at hundreds of refineries. However, superior bearing protector products for use in steam turbines must be purposefully developed. Compared with standard products typically used in pumps, the type described here offers important advantages, such as being suitable for high temperatures, incorporating Aflas O-rings as the standard elastomer, providing extra axial clearance to accommodate thermal expansion and utilizing high-temperature graphite gaskets. With these advantages in mind, there should no longer be any reason for water intrusion into the bearing housings of process pumps and small steam turbine drivers at reliability-focused facilities. When running a pump, you might ultimately achieve an operating temperature that allows a certain thick oil to flow nicely, but what if the initial operating temperature at startup is quite low and the oil will not flow freely? That's what often happens when someone buys a standard "multi-purpose Figure 4. A conventional lip seal (top) oil" where a superior synthetic versus a modern rotating labyrinth bearing housing protector seal would be a far better choice. (bottom). (Ref. AESSEAL Inc.) Of course, these and similar issues are avoided with pure oilmist systems. These systems eliminate much of the human element and are less maintenance-intensive than traditional pumps and drivers lubricated with vulnerable oil rings and constant-level lubricators. Oil Mist Represents Best Available Technology As stated previously, thick oils can be quite difficult to apply with the oil rings that are typically supplied with API process pumps. These oils are more easily and reliably applied as an oil mist. Neither oil rings nor constant-level lubricators are used in pumps and drivers connected to plant-wide oil-mist systems. 14 | January - February 2013 | www.machinerylubrication.com Oil mist is an atomized amount of oil carried or suspended in a volume of pressurized dry air. The oil mist — actually a ratio of one volume of oil suspended or carried in 200,000 volumes of clean, dry air — moves in a piping system. The point of origin is usually a simple mixing valve (the oil-mist generator) connected to a header pipe. Branch lines often feed hundreds of rolling elements in pumps and drivers connected to the header. At standstill, or while on standby, pump and driver bearings are preserved by the surrounding oil mist, which exists in the bearing housing space at a pressure just barely higher than ambient. These pump and driver bearings are lubricated from the time when atomized oil globules join to become larger oil droplets. This combining begins whenever the equipment shafts rotate, which is when small globules get to contact each other and start coating the bearing elements. There are also plant-wide oil-distribution systems whereby liquid oil (not an oil/air mixture) is pressurized and injected through spray nozzles into the pump bearings. These oil-spray systems are not to be confused with the more economical oil-mist systems. However, both oil-mist and oil-spray applications can take credit for lower frictional losses (see Figures 5 and 6) and should be taken into account when doing cost-justification analyses. Using High-Film-Strength Synthetic Lubricants Good lubrication practices include choosing the right oil, taking proper care of it and changing it before bearings are harmed. Improvements in lubricant quality can only be achieved by utilizing oils with superior lubricating properties. These would be premium synthetics. Yet even among prominent synthetic lubricants, oil performance can vary greatly based on the amount and composition of additives in the oil. For process pump bearing lubrication, at least one company combines synthetic base oils including polyalphaolefin (PAO) and dibasic ester base stocks with advanced additive chemistry to realize greater film strength. Numerous incidents have been documented where advanced lubrication technology has significantly improved pump reliability. In the majority of cases, advanced lube technology with its often more favorable (lower) coefficient of friction results in reduced bearing operating temperatures.