Engine & Turbine Technology
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). Tis is a ball on a rotating disk that can be used to compare the friction properties of different fully-formulated turbine oils.
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. Te 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.
Deposit formation In recent years, turbine users have reported lube oil ‘varnishing’ in their systems. Tis 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. Te 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. Tese 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. Tis led Shell Lubricants’ technology department
Fig. 2. Modified Wolf Strip test evaluates resistance to deposit formation.
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
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/
www.engineerlive.com 39
“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.”
Ronald Bakker, Senior Product
Application Specialist, Shell Global Solutions
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