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.
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.
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.
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.
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.
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 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-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.
Factors Affacting Lubrication
Circulation and Heating in the Presence of Air
Heat is generated within the bearings by friction and heat conduction along the shaft. Oil is broken into droplets while it is flowing. This allows greater exposure to air. During operation, oxidation (combination of the oil molecules with oxygen) occurs. Fine metal particles resulting, from wear or contamination and water act as a catalyst (enhance the rate) to oxidation. The viscosity of oil increases with oxidation. Insoluble oxidation products such as varnish and sludge may settle out on governor components, in bearings, heat exchangers, and strainers. Their accumulation will interfere with governor operation and oil flow to the bearings.
Water is the most prevalent contamination in turbine lubrication systems. Three common sources of water include:
- Leaking turbine and pump seals
- Condensation of humid air
- Water leaks in heat exchangers
Emulsion will form when the oil is mixed with water. The emulsion will separate quickly when the oil is new and clean. The water will settle in the reservoir where it can be removed by purification equipment. Oxidation or contamination of the oil will increase the tendency of the oil to emulsify. Emulsions can mix with insoluble oxidation products and dirt to form sludges. Water can combine with air to form red and black rust, which is similar in appearance to pipe scale. Particles of rust have the following effects:
- Act as catalysts that increase the rate of oil oxidation.
- Scratch the journals and cause excessive wear.
- Get entrained into the small clearances of the governing system. This will cause sluggish operation and, in extreme cases, disasters (due to slow operation of the governing valve).
Oil can become contaminated by air to form “bubbly” oil. This oil is compressible and can cause sponginess in hydraulic controls. It may reduce the load-carrying capability of oil films. Entrained air increases the rate of oxidation. An excessive amount of air can generate foaming in the reservoir or bearing housings.