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Turbines are heavily relied on in many industries to generate power and transport gas for vital processes. The lubricant used in turbine (and compressor) systems is often in demand, with the demand currently increasing year on year.
The ability to maintain and analyze the lubricant oils within turbine systems is an important process, as it allows for a longer life-time and helps to keep down long-term costs.
The demand for lubricant fluids at the reservoir level is increasing, putting a strain (i.e. an increase) on system power outputs, working pressures, internal temperatures and oil service life. The higher demands also equate to an increase in long-term keep-clean performance, as well as a decrease in the working clearances.
To be used in turbines, the lubricants themselves are required to be both highly oxidative and thermally stable to be able to withstand the high operating temperature of the turbine system. In addition, the lubricants must be clean, i.e. free from contamination, to ensure longevity and efficiency.
If a lubricant within a turbine is contaminated, its primary functions of lubricating bearings and gears, and acting as a hydraulic medium for governors and valving control systems, will be hindered. This can lead to a less-effective turbine system.
The key issue with turbine oils is their affinity for forming a ‘sludge’ during the oxidative processes. Antioxidants are present in turbine oils, but the oxidization of the oil can cause the antioxidants within the oil to form a sludge. This is particularly true for amine-type antioxidants and these are commonly found in turbine oils.
The formation of the sludge within the turbine system can have disastrous effects on the whole system. The main example of the damage the sludge can cause is in the sticking of the servo valves. This means that the control valves will not open on demand.
In addition, general contamination of the oil can cause other issues. The main examples include accelerated wear, causing the governors and valves to operate erratically, plugging of the filters within the system and unplanned outages (which is very costly!).
The key to achieving a high operating efficiency with a turbine system is through regular maintenance of the oil and regular testing to ensure that high levels of oxidation are not present within the oil (alongside other contaminants).
So how do companies analyze turbine oils and maintain their efficiency?
Many companies provide different testing options and kits that can be utilized in-house to determine the level of oxidation in the turbine oil, analyze the particles in the oil and even detect the moisture content within the oil, the separation affinity with other mediums and how long the oil has left in terms of its usability.
Some of the common techniques utilized to analyze the turbine oils are: measuring the viscosity at different temperatures, elemental analysis, measuring the water content in the oil, calculating the overall acidity of the oil, identifying the number of magnetic particles within the oil, calculating the number of particles and particle size distribution within the oil, comparing the concentration of antioxidants present compared to a new/fresh oil, measuring the varnish potential based around the number of insoluble particulates in the oil, measuring the ability of the oil to produce a stable foam, analyzing how the oil releases entrained air, measuring the oils ability to separate from water and measuring the oxidative stability of the oil using rotary pressure vessels.
In all, the various kits and techniques can be used with different forms of quality control that help to maintain high efficiencies within the turbine system. Many tests can decide whether an oil is still fit for use or a replacement is required. Alongside regular testing and maintenance, the key to long-life turbine oil is through choosing an oil which is highly oxidative and resistant to antioxidant aggregation.
Sources and Further Reading