Turbine lubricants: meeting the latest challenges

Paul Boughton
The latest methodologies in tribology - the science of lubricants - is now helping to develop new technology that can deliver trouble-free service and improved efficiency levels for turbine lubrication. Ronald Bakker reports.

Energy prices, emissions legislation and the drive for economic recovery means that power generation operations are under more pressure than ever to improve performance, minimise downtime and reduce operating costs.

Power generation in all its forms requires outstanding lubrication of the turbines and associated equipment to ensure long, trouble-free operational lifetimes, putting stringent demands on the performance of the lubricants. There are, of course, already a number of high performance lubricants on the market today, many of which use the latest technology to help increase performance and efficiency. Back in the laboratory, however, new scientific methodologies in tribology are being developed. These are being used to identify new opportunities to further improve performance, elevate efficiency levels and address specific wear or deposit issues, even for those turbines operating under the most severe conditions. It is these new methodologies that are driving the next generation of lubricants.

Today's economic climate provides the backdrop for much of this development work; with energy prices at historically high levels and environmental considerations increasingly important, lubricants are a component in the overall value chain and can have an important and positive impact on efficiency, emissions and productivity.

The latest scientific techniques employ wear modelling and simulation tests, 'smart' laboratory screening and extensive performance prediction models - all of this is designed to simulate the performance of lubricants in the real world. Through testing, tribologists can build a more detailed understanding of the physical characteristics and demands placed on modern-day turbine oils, in turn shaping product development pathways. Here, I outline two examples of these techniques in action.[Page Break]

Efficiency and friction

Lubrication efficiency is largely dependent on the ability of the system and lubricant to form and maintain an oil film of adequate thickness - this can be affected by a number of factors including friction, viscosity, load, sliding speed and temperature. By optimising various factors such as lubricant viscosity, lubricant viscosity-pressure coefficient (the viscosity behaviour of the oil at high pressure), and surface active additives in the formulation, it is possible to increase lubrication efficiency.

Five to 10 years ago, most mineral-based turbine oils were made using Group I base oils. Today, the majority of modern turbine oils are now based on more hydro-processed Group II and Group III oils in order to meet higher performance requirements.

However, not all base oils created are equal, and this is where being able to accurately model the friction coefficient comes into play. To do this, Shell tribologists have been using a traction measurement instrument called a mini traction machine - MTM - (Fig. 1). This is a ball on a rotating disk that can be used to compare the friction properties of different fully-formulated turbine oils.

As one might expect, Shell studies indicate that at various load and speed levels, Group III base oil lubricants consistently generate less friction than their Group II counterparts; this confirms that using higher quality, more refined and advanced base oil composition, supported by the latest additive technology, can lead to lower friction coefficients to improve lubrication efficiency. However, more significant is that not all Group III turbine oils give a similar friction reduction. In graph 2, oils A and B are blended using the same additive chemistry, but different types of Group III base oils. Oil B gives lower friction at all loads and speeds tested, clearly indicating that significant differences in friction behaviour between addivated Group III base oils are possible. The data gathered in tests such as this is being used in further modelling aimed at predicting, and in turn enhancing, the performance of new lubricants.[Page Break]

Deposit formation

In recent years, turbine users have reported lube oil 'varnishing' in their systems. This varnish can appear as a thin, orange, brown or black film deposit occurring on the interior of lubricant systems and represents a significant performance issue. Varnish formation in servo-valves can cause the valves to stick or seize, leading to unit alarms, trips or fail-to-starts. Another costly concern is formation of varnish on thrust or journal bearings, causing increased wear rates and accelerated oil degradation. Other problems caused by varnish include reduction of cooler performance, increased bulk oil temperatures and prematurely plugged filters and strainers.

The causes of varnish formation are many and varied and still the subject of active research. One of the factors, however, includes extreme oxidative stresses from high operating temperatures and catalytic wear metals. Although mentioned in many manufacturer specifications, none of the industry oxidation tests provide a clear indication of an oil's deposit-forming tendency in the field. These tests are designed to assess the suitability of a lubricant for use as a turbine oil and give an indication of expected life, but not to accurately predict varnish formation in service.

This led Shell Lubricants' technology department to develop a screening test that could be used to indicate an oil's deposit-forming tendency - a modified Wolf Strip test (ex DIN 51392) - see Fig. 2 - which can evaluate resistance to deposit formation when exposed to high temperatures, air and catalytic metals. Compared to conventional laboratory oxidation tests, this screening method is comparable with the field performance of high quality turbine oils and makes it possible to improve the longer-term deposit-forming tendency of the next generation of turbine oils.

Ronald Bakker is Senior Product Application Specialist, Shell Global Solutions, The Hague, Netherlands. www.shell.com/home/content/lubes

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