The conditions of turbine oil operation in a turbine generator are heavy due to a range of unfavorable factors.
Heating of oil in the presence of air causes intensive oxidation and other changes in the oil. Due to evaporation of volatile fractions, viscosity grows, emulsification stability decreases etc. The heating of oil mostly occurs in the bearings of the turbine, to 35-55°С. The oil is heated due to friction of the bearing in the oil film and partially due to conductivity of heat along the shaft from the hotter parts of the rotor.
The temperature of oil going out of the bearing is measured in the output line, offering an indication of the temperature conditions in the bearing. However, a relatively low temperature at the outlet does not preclude local overheating of oil due to bearing construction flaws, faulty manufacturing or incorrect assembly. This is especially true for support bearings, where segments may carry different loads. Such local overheatings promote aging of oil, because oxidation increases sharply in temperature above 75÷80°С.
Oil heating may also occur in bearing housings from contact with hot surfaces, heated from the outside by hot steam or by heat from the turbine enclosure. Heating also occurs in the regulation system: the servos and oil lines located near hot turbine surfaces and steam ducts.
Spraying of oil by rotating components of the turbine
All rotating parts, such as couplings, gears, shaft ridges, centrifugal governor etc cause spraying of oil in the bearing and speed governor housings. The surface area of the spray’s contact with air, which is always present in the housing, is significant, and the oil becomes mixed with air and oxidized. The high velocities of the flying oil droplets is also a factor in this.
Air in the bearing housings is continuously replaced by the suction through the gap on the shaft due to slightly lower pressure in the housing. The lower pressure can be explained by the ejection in the oil output lines. The spraying problem is especially apparent in movable couplings with forced lubrication. Therefore, to decrease the oxidation of oil, such couplings are covered with metal jackets to reduce oil spraying and ventilation. The jackets are also installed in fixed couplings to reduce the circulation of air in the housing and limit oil oxidation.
To prevent the axial outflow of oil from the housing, oil baffles are especially efficient.
Air in the oil
The air is present in the oil as bubbles of varying size and as dissolved gas. Air is always captured by the oil when intensive oil and air mixing takes places and in oil output lines, where the oil does not cover the entire cross section of the tube and air suction occurs.
When oil saturated with air passes through the main oil pump, the bubbles experience rapid compression. The temperature of the air in the bubbles grown sharply. The rate of the process does not allow the air to dissipate the heat, and the compression process is adiabatic. The emitted heat, despite the low absolute value and short duration, plays a significant role in promoting oil oxidation. After the pump, the compressed bubbles gradually dissolve, while the contaminants in the air (dust, ash, steam etc) contaminate the oil.
Aging of oil due to the presence of air is especially apparent in large turbines, where the pressure of oil after the main oil pump is high, leading to significant increase of air temperature in the bubbles with the consequences detailed above.
Water and condensing steam
The main source of contamination of oil with water in turbines of older designs (without steam exhaust from labyrinth seals) is the steam from the seals sucked into the bearing housing. The rate of water contamination depends to a high degree on the condition of the shaft seals and the distance between the bearing and turbine enclosures. Another source of water is malfunction of steam isolation of auxiliary turbine oil pump. Water also enters the oil from the air due to condensation of vapor and through oil coolers.
The oil in supply pumps may become contaminated with water due to water leaks through the pump’s seals.
Contamination with water is especially dangerous if caused by contact with hot steam. In this case, not only is the oil contaminated with water, but is also heated, accelerating aging. The resulting low molecular acids affect the metal parts in contact with the oil. Water promotes the formation of sludge on the surfaces of the oil tank and oil ducts. Sludge can clog openings in the dispensers in the input lines and cause overheating or even melting of the bearing. Sludge in the regulation systems can disrupt normal operation of slides, bearing boxes and other components of the system.
Entry of hot steam into the oil also causes formation of oil-water emulsion. In this case, the contact area between oil and water grows, making it easier for low molecular acids to dissolve in water. The emulsion may enter the lubrication and regulation system and significantly hinder their operation.
Circulating in the system, the oil is in constant contact with metals, such as cast iron, steel, bronze, babbitt etc, also increasing the rate of oil oxidation. The effects of corrosion due to contact of acid and metals also remain in the oil. Some metals catalyze turbine oil oxidation.
All these adverse factors accelerate aging of oil, that is, the deterioration of performance of oil due to physical and chemical changes.
The indications of aging are:
- viscosity increases;
- acid number increases;
- flashpoint decreases;
- acidic reaction of water extract;
- sludge and solid impurities appear;
- oil becomes turbid.
The intensity of aging depends on the initial oil quality, oil handling procedures, turbine and oil system design. Aging oil can still be used, if:
- acid number is below 0.5 mg KOH/g;
- tha change of viscosity does not exceed 25%;
- flashpoint reduction less than 10°С;
- water extract reaction neutral;
- the oil is transparent and contains no visible water or sludge.
If any of the above criteria is not met and the quality of the oil cannot be restored with the turbine running, it must be replaced as soon as possible.
Oil quality control
Thorough and systematic control of oil quality is the most important condition of successful oil handling.
There are two types of quality control for oil in use: onsite control and abbreviated analysis.
if the quality of the oil in use is deteriorating abnormally fast, the time between analyses should be reduced. Quality tests are performed on an alternative schedule.
The oil coming into the power plant must be tested for compliance with all quality requirements. If one or more parameters are outside of the standards for new oil, the batch of oil should be sent back. Oil analysis is also performed before filling oil tanks of steam turbines. Oil in reserve is tested at least once every three years.