The harm of the water cut of the turbine oil -- the oil filter
|The water in the turbine oil usually has two possible forms of existence: dissolved water and free water. Dissolved water is the water that is evenly distributed in the oil. Under the operation condition of power plant, the maximum concentration of water dissolved in oil is about 150mg/L (the specific value is related to the composition, temperature and additives of oil). Free water is the water that exists in oil in the form of tiny droplets. These small droplets are small and sometimes difficult to separate from oil due to impurities. The lubricating oil containing free water is cloudy, and the lubricating oil that contains only dissolved water is transparent. Because of the strong mixing of the turbine oil tank system in the power plant, 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 above 150mg/L.
The harm of the water cut of the turbine oil
(1) oil emulsion may occur, which makes the adhesion of the lubricating oil not good. The adhesion of the oil to the friction surface is not enough. If the oil film is damaged, the rotor journal may be dry friction with the bearing bush of the bearing, so that the bearing bush will burn down, the unit will vibrate strongly, or even destroy it.
(2) water content of turbine oil will accelerate the oxidation of turbine oil, increase the acid value and generate more oxidizing precipitates, thereby prolonging the anti emulsifying time of steam turbine and deteriorating the emulsifying property of oil.
(3) water content in turbine oil will cause corrosion of carbon steel pipes and components in the oil system, resulting in jamming and reducing the sensitivity of movement. When severe, it will also cause the regulation and safety devices to refuse to move, resulting in the overspeed accident of the unit.
(4) the corrosion product produced by the water cut of the turbine oil is an important source of the particles in the oil. The water cut of the turbine oil will make the particle size of the oil easy to exceed the standard, the filter core of the oil filter is replaced frequently and the operation and maintenance work is great.
In order to reduce the harm of turbine oil water content, the quality standard of turbine oil in operation of GBT 7596-2008 power plant issued in China is stipulated. The water content in lubricating oil is less than 100mg/L.
Possible countermeasures analysis
The turbine oil is hydrated, first of all, we should work hard from the source of the leakage, try to find out the leakage point as far as possible, analyze the cause, and reduce the leakage. However, the leakage phenomenon of many generating units is a long-standing reality, especially for some old units, which is difficult to solve due to the limitations of many conditions.
It is an effective way to solve the problem of large water content in the oil system by using suitable dewatering equipment to drain the water out of the turbine oil system when the leakage is difficult to change.
At present, there are three main techniques suitable for the dehydration of turbine oil system: coalescence separation, centrifugal separation and vacuum dehydration.
(1) coalescence separation type oil filter
The principle of dehydration of coalescence separation is to separate the small water beads from the large water beads by using the difference of surface tension between oil and water. Since the concentration of water can be dissolved by water is about 150mg/L (the specific value is related to the composition, temperature and additives of oil). Therefore, the coalescence separation oil filter usually can only treat the moisture to about 180mg/L, and it is impossible to reduce it to below the national standard value of 100mg/L.
The advantages of coalescence separation oil filter are simple structure, no need to heat when dehydrating. When the free water content is high, the dewatering speed is fast, the operation is convenient and the reliability is high, so it is widely applied in the electric power industry.
(2) centrifugal oil filter
The centrifugal oil filter is separated by the use of the difference in the proportion of water droplets and oil to accelerate the settlement of water droplets at high speed. Because the formation of high speed conditions requires large power and easy to generate vibration, the usual centrifuge also has auxiliary heating to improve the oil temperature and reduce the viscosity of the oil, so the centrifugal oil filter has a relatively large running power.
When the free water content is high, the dewatering speed of the centrifugal oil filter is also faster. Like the coalescence separation oil filter, centrifugal oil filter can only remove free water, but not remove dissolved water, and cannot make the water content in the oil meet the national standard.
(3) vacuum dehydrating oil filter
Ordinary vacuum oil filter commonly used machine mostly adopts the vacuum tank to dehydration, the principle is to be processed into oil vacuum tank, oil volatile gases (including water vapor) to evaporate into the vacuum tank, degassing after oil accumulation to the bottom of the vacuum tank under the action of gravity. The vacuum oil filter is generally running continuously, so the vacuum pump is used to extract gas continuously from the upper end of the vacuum tank, and the oil after degassing is pumped from the bottom of the vacuum tank by the pump. The higher the vacuum degree in the vacuum tank, the faster the degassing treatment is, the better the effect is.
The vacuum filtration function removes the free water and removes the dissolved water, so it can reduce the water content in the oil to a very low level, and it can achieve about 10mg/L. Generally speaking, the oil system can control the water content of the oil in the 80mg/L.
At present, the vacuum oil filter is the only technology that can make the water content of the oil system with a small amount of water leak up to the standard.
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