There are usually two possible forms of water in steam turbine oil: dissolved water and free water. Dissolved water is water that is evenly distributed in the oil. Under the operating conditions of the power plant, the maximum concentration of water dissolved in the oil is about 150 mg/L (the specific value is related to the oil composition, temperature, additives, etc.). Free water is the water that is present in the oil in the form of tiny water droplets. These small water droplets have impurities and the particle size is very small, which is sometimes difficult to separate from the oil. Lubricating oil containing free water is cloudy in appearance, while lubricating oil containing only dissolved water is transparent in appearance. Due to the strong mixing of the power plant steam turbine tank system, only the water that exceeds the solubility will exist in the form of free water. That is to say, if the oil contains free water, the appearance is turbid and the water content is all above 150mg/L. The hazards of steam turbine oil containing more water are mainly manifested in the following aspects:
(1) Oil emulsification may occur, which makes the adhesion of the lubricating oil not good, the adhesion of the oil to the friction surface is not enough, the oil film is damaged, and the rotor journal is damaged. Dry friction may occur with the bearing bush of the bearing, causing the bush to burn, the unit vibrates strongly, or even destroys the machine.
(2) The water content of the steam turbine oil will accelerate the oxidation of the steam turbine oil, increase the acid value, and generate more oxidation precipitates, thereby prolonging the demulsification time of the steam turbine and deteriorating the demulsification performance of the oil.
(3) The water content of steam turbine oil will rust the carbon steel pipes and components in the oil system, causing jamming and reducing the sensitivity of the action. In severe cases, it will also cause the adjustment and security devices to refuse to move, resulting in the occurrence of overspeed accidents of the unit .
(4) The corrosion products produced by the water content of steam turbine oil are an important source of particulate matter in the oil. The water content of the steam turbine oil will make the particle size of the oil easily exceed the standard, the filter element of the oil filter will be replaced frequently, and the workload of operation and maintenance will be large. In order to reduce the harm of steam turbine oil water content, GBT 7596-2008 issued by my country's GBT 7596-2008 power plant steam turbine oil quality standards stipulate that the water content of lubricating oil should be less than 100mg/L.
When steam turbine oil contains water, we must first work hard from the source of the leak, try to find the leak point, analyze the cause, and reduce or eliminate the leak. However, the water leakage phenomenon of many units is a long-term reality, especially the stubborn disease of some old units, which is limited by many conditions and is difficult to solve. When the water leakage is difficult to change, using appropriate dehydration equipment to remove the water leaking into the turbine oil system is an effective way to solve the large water content in the oil. At present, there are three main technologies suitable for steam turbine oil system dehydration: coalescence separation, centrifugal separation, and vacuum dehydration.
The dehydration principle of coalescence separation is to use the surface tension difference between oil and water to separate small water beads into large ones. Since the concentration of lubricating oil that can dissolve moisture is about 150mg/L (the specific value is related to the oil composition, temperature, additives, etc.), the coalescence separation oil filter usually can only treat moisture to about 180mg/L, which is impossible Reduced to below the national standard value of 100mg/L. The advantage of coalescing separation oil filter is simple structure, no heating is required during dehydration. When the free water content is high, the dehydration speed is fast, the operation is convenient, and the reliability is high, so it is widely used in the power industry.
The centrifugal oil filter uses the difference in the specific gravity of water droplets and oil to accelerate the sedimentation of water droplets under high-speed conditions for separation. Because the formation of high speed conditions requires large power and is prone to vibration, the usual centrifuge is also equipped with auxiliary heating to increase the temperature of the oil and reduce the viscosity of the oil. Therefore, the operating power of the centrifugal oil filter machine is compared Big. When the free water content is high, the dehydration speed of the centrifugal oil filter is relatively fast. Similar to coalescence and separation oil filters, centrifugal oil filters can only remove free water, but not dissolved water, and cannot make the water content in the oil reach national standards.
Vacuum Oil Purifiers are commonly used today. Most of them use vacuum tanks for dehydration. The principle is to introduce the oil to be treated into the vacuum tank, and the volatile gases (including water vapor) in the oil volatilize to the vacuum tank. In the process, the degassed oil collects to the bottom of the vacuum tank under the action of gravity. Vacuum oil filters generally run continuously. Therefore, a vacuum pump is used to continuously extract gas from the upper end of the vacuum tank, and a pump is used to extract degassed oil from the bottom of the vacuum tank. The higher the vacuum degree in the vacuum tank, the faster the degassing process and the better the effect. The vacuum oil filter can remove free water and dissolved water, so it can reduce the water content in the oil to a very low level, which can actually reach about 10mg/L. Generally speaking, the lubricating oil system can control the water content in the oil at 80mg/L.
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