Turbines: What Goes Around Comes Around

Dr. Robert M. Gresham, Contributing Editor | TLT Lubrication Fundamentals June 2015

These hard-working machines play a vitally important role in industry and transportation.
 

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KEY CONCEPTS
• The three main types of turbines are hydro, steam and gas.
• Hydro turbines are driven by water and connected to an electricity generator to produce electricity.
• Industrial steam turbines have multiple uses, usually to drive compressors, turbo-alternators, blowers, pumps and the like.

I MAKE NO CLAIM TO BEING AN EXPERT IN TURBO-MACHINERY, but these gadgets have always intrigued me. I find them, as a class of machine, to be interesting. Most of STLE’s education courses and practical papers tend to gloss over lubrication of turbo-machinery.

So here it goes.

In the world of turbines, the main types are hydro, steam and gas. Well, I guess you should also include wind turbines, but they are a bit different than the three above. While they would be a special case of a gas (air) turbine, to me they are a big wind-driven propeller hooked up to a gear box that is, in turn, hooked up to a generator or alternator.

Hydro turbines are driven by water (a little more substantial than air, yet fundamentally similar to wind turbines) and connected to an electricity generator or, more properly, turbo-alternator to produce electricity. These turbines are on the low end in terms of application requirements, not that they are insignificant. Thus, the lubricating oil sees temperatures in the oil sump on the order of 40-60 C (104-140 F) with peak temperatures in the circulation system to maybe 70-95 C (158-203 F). 

Speeds are also moderate around 50-600 rpm. The oil also must lubricate guide vanes and control system components. The main lubrication issues for these kinds of turbines are water contamination and service life. Thus, mineral oil-based R&O oils that have good water demulsibility are needed. The long service life is desirable to reduce maintenance costs and down time. These are important due to the physical location of these turbines and to the need for uninterrupted electrical power output.


A diagram of a micro-hydro generator.


The different parts of a generator and turbine.

Industrial steam turbines have multiple uses, usually to drive compressors, turbo-alternators, blowers, pumps and the like. In compressor applications, steam turbines and the compressor itself are similar in that they are both turbo-machines. As such, they can behave similarly in terms of speed versus output. Thus, steam turbines are excellent for variable-speed turbo-compressors. Steam turbines can take on many different configurations, analysis of which goes beyond the scope of this article. However, all multistage steam turbines require cool, clean oil supplied to their journal bearings.


The different parts of an industrial steam turbine.


An industrial steam turbine in a facility.

This oil is supplied from several types of systems, but the key is the oil must be delivered at the right flow rate, pressure and temperature. Often the driven equipment, such as the compressor, also is lubricated with the same oil, and again at the right level of cleanliness, flow, pressure and temperature. Further, additional equipment might be lubricated by this same system, such as various control valves. In general terms, steam turbines can operate up to speeds of 3,000 rpm, oil sumps around 40-70 C (104-158 F) with peak or hot spot temperatures as high as 150 C (302 F). 

Clearly, aside from particulate contamination, which is always a problem, steam turbines are subject to the effects of water and steam. Typically turbine oils for steam turbines must be primarily resistant to rust and oxidation, so called R&O oils. Additionally, turbine oils may contain antiwear and EP additives. Further, they should exhibit good demulsibility of water and be low foaming. 

Interestingly, when one thinks about the highly formulated automotive oils (10%-20% additives), turbine oils by contrast are typically formulated with not much more than 1% additives. Therefore the base oils are important. They can vary from Group II, III, III+ or IV. The higher-refined oils tend to have more inherent oxidation stability, but, especially in the old days, they also tended to exhibit more varnish formation, likely because the less-refined oils tended to solubilize varnish rather than letting it form deposits. More modern formulations, along with effective filtration, generally manage excessive varnish formation or should. Such oils typically have a viscosity index in the range of 95-100 so that the viscosity doesn’t vary too much from start up to peak temperatures.


General Electric gas turbine.

Industrial gas turbines, which are basically land-based jet engines (windmills notwithstanding), have some similarities to steam turbines except that steam is replaced with hot combustion gases. Thus, water contamination is not so much a problem with gas turbines. However, sump temperatures can range from 50-100 C (122-212 F) with hot spot peaks up to 280 C (536 F). Speeds are also higher on the order of 3-7,000 rpm. Therefore the performance requirements of the lubricating oil are necessarily higher while lubricating oils based on hydrotreated base stocks are used for the more stringent applications. 

Clearly gas turbines are a good application for synthetics lubricants, such as polyalphaolefins and ester-based oils. Many of the larger, higher-performing industrial gas turbines often use an aircraft engine core and require similar lubricants. For more advanced aircraft jet engines, which really are just higher performing gas turbines, we have to kick it up a notch. These turbines use primarily ester-based lubricants for their wide temperature range capabilities, such as those described by military specifications Mil-L-7808 and Mil-L-23699. Or, for even higher-temperature resistance with operating temperatures as high as 300 C (572 F), polyphenyl ether lubricants as described in Mil-L-87100 could be used. But these are not for the financially faint of heart, and the polyphenyl ethers have poor low-temperature performance, which is not so critical for an industrial gas turbine but can be a showstopper for an airplane.

Clearly turbo-machinery, like the applications described above, affects our everyday lives in many ways. Keeping them going around allows us to go around.


Bob Gresham is STLE’s director of professional development. You can reach him at rgresham@stle.org.