Turbo Tips: Lube Oil Types & Selection for Turbomachines

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.

FUNDAMENTALS

The fundamentals of a successful lube oil application include:

• The lube oil behavior in different operating conditions: Specifically, different expected temperatures such as ambient temperature during start-up and operating temperatures. For example, the ambient temperature on a cold winter morning versus a warm summer afternoon. The same lube oil is expected to work properly at high operating temperatures when the turbomachine reaches the peak load. In this case, a synthetic oil with a high viscosity index (VI) and small changes in viscosity with temperature changes is needed.
• The effects of lube oil additives: Oil additives are added to lube oils for superior characteristics and performance. However, it is extremely important to make sure the intended additives work properly in combination with the base oil. Take into consideration any possible side effects or complications concerning additives, e.g., additive depletion over time, etc.
• ISO oil grades: ISO viscosity grades (VG) 32-100 are primarily used for turbomachines. Often, ISO VG 150-400 have been used in gear units and high-temperature turbomachinery applications. The oils of interest for turbomachines are typically ISO VG 32, 46, 68, and 100.

PUMPS & COMPRESSORS

Some pumps and compressors are designed to function at high temperatures—temperatures exceeding 150^oC. 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.

KEY CONSIDERATIONS

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:

• Superb oxidative and thermal stability, especially for medium- and high-temperature turbomachines
• High VI means smaller changes in fluid viscosity upon changes in temperature, particularly when a turbomachine is experiencing different temperatures in varying modes of operation, such as start-up, alternative operating, etc.
• Relatively low pour points for an easy cold-temperature start-up
• Excellent lubricity for enhanced resistance to friction and wear
• Extreme pressure characteristics
• Resistant to mechanical breakdown
• High resistance to sludge and varnish formation
• Non-corrosive and stain-resistant
• Compatibility with elastomers and coatings, particularly the sealing components, gear unit internal paint, and other components in contact with the lube oil.

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.