Steam Turbine Lubricating oil Characteristics

Mostly turbine of steam power plant may have its rotational 3000 rpm to produce required power. Due to friction the temperature of bearing, rotor may increase which, combined with other factors, may leads to the failure of bearing hence produce a serious damage to turbine and plant. Plant stability, production depends on the turbine and turbine stability depends on bearing life as bearing will always be damage first. This causes the shutdown of power plant and hence gives a loss of money. Lubricant has vast effect on the life of bearing that’s why much care should be given while selecting it. We are going to be familiar with some of the properties, a lubricant should have.

Viscosity

The journal and thrust bearings of steam turbines require lubrication. Oils having higher viscosity provide a greater margin of safety in the bearings. However, its friction losses are high. In high-speed turbines, the heat generation becomes significant. Most oils used in this service have International Organization for Standardization (ISO) viscosity grade 32. Higher viscosity is used in some applications, ISO viscosity grade 46 (41.4 to 50.6 cSt at 40°C).
Higher-viscosity oils are used for geared turbines to provide adequate lubrication for the gears. Most of these systems use oils of ISO viscosity grade 68 (61.2 to 74.8 cSt at 40°C). Some geared turbines cool the oil in a heat exchanger before delivering it to the gears. The increase in viscosity provides better protection for the gears.

Load-Carrying Ability

Steam turbines normally use mineral oils. Boundary lubrication conditions occur in turbines not equipped with lifts. Wear will occur under these conditions unless lubricants with enhanced film strength are used. The higher viscosity of cool oil provides the increase in load-carrying ability of the oil films needed during start-up. Additives are also frequently used in turbine oil to improve the film strength.

Oxidation Stability

The ability to resist oxidation is the most important characteristic of turbine oils. This property is important from the standpoint of retention of viscosity (resistance to the formation of sludges, deposits, and corrosive oil oxyacids) and retention of the ability to separate water, resist foam, and release entrained air.

Protection Against Rusting

Rust inhibitors are required from turbine oils to improve their ability to protect against rusting of ferrous surfaces. These inhibitors “plate out” on metal surfaces to resist the penetration of water.

Water-Separating Ability

New mineral oils usually resist emulsification when there is water ingress. Any emulsion formed breaks quickly. Some additives such as rust inhibitors increase the tendency of an oil to emulsify. Thus, additives should be selected carefully to ensure that the oil has good water-separating ability.

Foam Resistance

Turbine oils usually contain defoamants to reduce the foaming tendency. Since oxidation increases the tendency to foam, good oxidation stability is essential to maintain good resistance to foaming.

Entrained-Air Release

Entrained air can cause sponginess and delayed or erratic response. Some additives are known to degrade the ability of the oil to release entrained air. Thus, the additives selected for turbine oil should not reduce its ability to release air.

Fire Resistance

Fire-resistant fluids (FRFs) are normally used in electrohydraulic governor control systems due to high pressures (up to 3000 psi). Phosphate esters or blends of phosphate esters and chlorinated hydrocarbons are normally used. These systems are extremely sensitive to the presence of solid contaminants. Considerable attention should be paid to the filtration of the oil.