FAQs on Turbine Oil Issues

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1. What determines the speed control oil pressure and lubricating oil pressure of a steam turbine? The control system of a steam turbine typically uses oil to transmit signals and as power to drive the actuators, which in turn open and close the control valves and the main steam valve. To ensure rapid and sensitive adjustment, a certain speed control oil pressure must be maintained. The commonly used speed control oil pressures for turbines include 0.4–0.5 MPa, 1.2–1.4 MPa, 1.8–2.0 MPa, etc. Generally speaking, a high oil pressure enables the control system to operate with high sensitivity, and reduces the size of the hydraulic actuators and throttle mechanisms. The speed control oil pressure has reached as high as 4.0 MPa in some cases. However, excessive oil pressure can lead to oil leakage and fire. The lubricating oil pressure in a turbine is calculated during design based on factors such as the rotor’s weight, rotational speed, the structure of the bearing shells, and the viscosity of the lubricating oil, in order to ensure that a proper oil film is formed between the shaft journals and the bearing shells, as well as to provide sufficient oil for cooling purposes. Therefore, the lubricating oil pressure in turbines is typically set at 0.12–0.15 MPa. Excessively high lubricating oil pressure can cause oil leakage from the oil seals and bearing vibration. Low oil pressure results in a poor formation of the oil film, and may even cause loss of oil supply that damages the bearing shells. 2. Why is a smoke exhaust fan installed in the turbine oil tank? The function of an exhaust fan installed in the fuel tank is to remove gases and water vapor from the tank. This prevents water vapor from condensing in the fuel tank on one hand ； On the other hand, it keeps the pressure in the tank at or below atmospheric pressure, allowing the bearing return oil to flow smoothly into the tank. Conversely, if the fuel tank is airtight, a large amount of gas and water vapor will accumulate inside the tank, creating positive pressure that can affect the oil return to the bearings; it also makes it easy for water to accumulate in the fuel tank. Range hoods also help to remove harmful gases, preventing the oil quality from deteriorating. 3. Why is a drain pipe installed at the bottom of the fuel tank? During the operation of the turbine, water ends up in the turbine oil due to reasons such as excessive steam leakage from the shaft seals, poor drainage from the steam-driven oil pumps, and excessive leakage at the water inlet flanges of the water-cooled generator rotor. Once these water-containing oils return to the fuel tank, due to the higher density of water, the water separates from the oil and settles at the bottom of the tank. Removing this water in a timely manner prevents the water that has already been separated from mixing back with the oil, thereby avoiding a deterioration in the quality of the oil. That’s why there are drain pipes at the bottom of the fuel tank. 4. What are the hazards of degraded turbine oil quality? The quality of turbine oil is closely related to the proper operation of the turbine. The deterioration of the oil quality alters the properties of the lubricant as well as the strength of the oil film, resulting in inadequate lubrication of various lubricated parts; this in turn causes the bearing materials to melt and get damaged ； It can also cause the components of the control system to corrode and rust, leading to jamming and serious consequences such as the failure of the control system and protective devices to function properly. Therefore, it is essential to pay attention to the supervision of the quality of turbine oil. 5. What are the quality indicators for turbine oil? There are many indicators for the quality of turbine oil, with the main ones being viscosity, acid value, acidity and alkalinity reactions, emulsification resistance, and flash point. In addition, transparency, freezing point temperature, and mechanical impurities are also criteria for assessing oil quality. 6. What is the viscosity of turbine oil? What is the viscosity index? Viscosity is the standard for determining the blending and thinning of turbine oil. High viscosity means the oil is thick and does not flow easily ； Low viscosity means the oil is thin and flows easily. Viscosity is measured in Eng units, with the viscosity of commonly used turbine oils ranging from 2.9 to 4.3 Eng units. Viscosity has a significant impact on the lubrication performance of bearings; too high viscosity can cause the bearings to overheat, while too low viscosity can lead to the breakdown of the oil film. When the oil quality deteriorates, its viscosity increases. 7. What is the acid number of turbine oil? What is an acid-base reaction? The acid value indicates the amount of acids present in the oil. It is calculated based on the number of milligrams of potassium hydroxide required per gram of oil to neutralize it. The acid value of the new turbine oil should not exceed 0.04 KOH mg/g of oil. When the oil quality deteriorates, the acid value rises rapidly. The acidity-alkalinity reaction refers to whether the oil is acidic or alkaline. Good turbine oil should be neutral in nature. 8. What is anti-emulsification degree? What is the flash point? Anti-emulsification degree is the ability of an oil to separate rapidly from water, and it is expressed by the time required for separation. The good emulsification resistance of turbine oil should be no more than 8 minutes; when organic acids are present in the oil, this emulsification resistance decreases. The flash point refers to the temperature at which, when turbine oil is heated to a certain degree, part of the oil turns into gas and can catch fire upon being exposed to flame; this temperature is known as the flash point (also called the ignition point). Since the temperatures in turbines are very high, the flash point cannot be too low. A good turbine oil should have a flash point of no less than 180°C. When the oil quality deteriorates, its flash point decreases. 9. Why must vent holes and steam pipes be installed on the turbine bearing cover? Generally, a negative pressure exists inside bearings, usually due to the suction effect of the oil flowing out of them. Due to the negative pressure formed inside the bearing, steam is drawn in along with water droplets, which then condense. To prevent negative pressure from forming inside the bearing, ventilation holes or vents are provided on the bearing cover to connect with the atmosphere. On the other hand, providing an air vent on the bearing cover can also serve to remove the vapor generated by the turbine oil in the bearings due to heating, thereby preventing the pressure inside the bearing housing from exceeding atmospheric pressure. During operation, care should be taken to keep the ventilation holes unobstructed to prevent blockages. The vent hole on the front bearing cover of a turbine in a factory became blocked, causing combustible gases to accumulate inside the bearing housing; these gases were ignited by electrical sparks within the bearing housing, resulting in an explosion of the front bearing housing. 10. What determines the capacity of the main fuel tank? What is the circulation rate of turbine oil? The oil storage capacity of the turbine’s main oil tank is determined by the size of the oil system, and it should be sufficient to meet the oil requirements of the lubrication and control systems. The larger the engine, the more oil is required for the regulation and lubrication systems. The larger the fuel tank capacity, too. The circulation ratio of turbine oil is equal to the ratio of the output volume of the main oil pump per hour to the total oil volume in the tank, and it should generally be less than 12. If the circulation rate is too high, the turbine oil stays in the tank for a short time, leaving no time for air and moisture to be separated, which causes the quality of the oil to deteriorate rapidly and shortens its service life.