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Considerations when selecting a lube oil include oxidative and thermal stability, viscosity, interactions with oil additives, and more.
Lube oil selection is extremely important for turbomachines. Mineral oils are frequently used, but the importance of the synthetic oil is increasing. A properly selected synthetic oil can offer superior performance, including resistance to high and low temperatures, a longer service life, and more.
The fundamentals of a successful lube oil application include:
PUMPS & COMPRESSORS
Some pumps and compressors are designed to function at high temperatures—temperatures exceeding 150oC. For these high-temp services, lube oil should have oxidative and thermal stability. Most compressors and pumps are best served by oils with ISO VG 32 or 46. Sometimes, ISO VG 68 or 100 is needed for relatively high temperatures. However, given the many different types of compressors and pumps, each manufacturer is likely to recommend only those oils that have been used successfully over long-term operations. This also pertains to other factors, such as service, ambient temperature, climate, etc.
Turbocompressors and centrifugal pumps typically use premium extreme-pressure lube oil—multi-purpose oils for a dependable performance over a wide range of temperatures and operating conditions.
When a compressor is driven by a steam turbine, a single lube oil skid is commonly used and the same lube oil is fed into all the turbomachines. In this case, make sure the lube oil selected is suitable for all the turbomachines it serves. This is a challenge because each piece of machinery has its own set of requirements and considerations, as do different lube oils.
IS A GEAR UNIT USED?
Often, a gear unit is used with a centrifugal pump, a centrifugal compressor, or other turbomachines in which the gear unit and turbomachine are lubed with the same oil. If this is the case, ensure the selected oil is well-suited for the gear unit, bearings, and other components in a gear-driven turbomachinery train. Synthetic oil formulated from a polyalkylene glycol (PAG) base oil, designed for anti-wear and high temperatures, is usually suitable for this setup. PAG oils are fully synthetic oils used with proper additives for each service.
Each turbomachine has a specific set of requirements and needs a properly selected lube oil to operate successfully. As a rough guideline, lube oils for turbomachines should have:
As a rule of thumb, lube oils with ISO VG 32, 46, and 68 are common in turbomachinery applications, such as centrifugal pumps and turbocompressors. The most common is ISO VG 46.
ABOUT THE AUTHOR
Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring, and reliability.