Above: HP steam turbine with the upper casing removed.
Below: Low pressure steam turbine undergoing inspection.
Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.7 Turbine Oil >> 7.7.2 Steam Turbines
Steam Turbine Oil
Steam Turbine oils have lower temperature requirements than gas turbine oils. Steam turbine oils closely resemble hydraulic fluids and the same viscosity grade of turbine oil is often approved for use in hydraulic equipment.
The aim of monitoring steam turbine oil is to establish that the oil retains its key functions and that oil treatment systems are maintaining contaminants at acceptable low levels. Oxidation tends to continue at a slow and progressive rate. Events such as water leakage at coolers and introduction of wrong grades of oil into the system do not happen quite so predictably.
The key features of steam turbine oil are superior oxidation resistance, rust/corrosion protection and good water shedding properties. Because steam turbine oils routinely carry out their function in a ‘wet’ environment, it is vital that steam turbine oil additives have very good hydraulic stability (i.e. are not degraded by the presence of water).
Steam turbines may be applied as main propulsion machinery for power generation and for pumping applications. All of this equipment can be regarded as critical with regard to the need for periodic monitoring of the oil. In the case of steam turbine main propulsion machinery, large oil changes are used and a very long service oil life is expected. These systems generally have associated coolers, filters/coalescers and centrifugal cleaning equipment.
ISO VG 32 & 46 grades are usually used for equipment without gearing, while an ISO VG 68 grade will normally be used for geared equipment or where higher temperatures are found. Where steam turbine oil is also used as a gear lubricant in an associated geared transmission system, it is normal to apply either a heavier grade (typically ISO VCT 68) or alternatively a lower viscosity grade (typically ISO VG 46) having enhanced wear protection. Such oils will usually be identified by an EP suffix.
Steam turbine oils operate within a ‘wet’ environment and there will invariably be some water present. For any given system, the amount of water present will stabilise at equilibrium level when the water input and output rates are the same. The water output rate will be determined by the characteristics of the oil and associated water removal systems, settling tanks, coalescers and/or centrifuges. When a system is operating normally and without additional water input via leakage, any gradual increase in water content that is evident may be due to deterioration of water shedding characteristics of the oil itself and/or reduced efficiency of the associated treatment system. Water coalescer filters may have their water removal efficiency reduced by deposition of contaminant on the active surface of the coalescer.
In newly commissioned systems, corrosion protection coatings may contaminate an oil and seriously degrade the water shedding ability. If this is suspected, then send a sample for lab analysis.