Abstract: This article provides a detailed description of the reasons for oil leakage in the bearing chamber of the front pump of the YNKn300/200 electric water supply pump through a specific example, as well as the measures taken. It analyzes the high lubricating oil pressure, large orifice diameter of the throttle plate or missed installation of the throttle plate during maintenance, excessive clearance caused by long-term wear of the labyrinth ring, blockage of the return oil hole on the labyrinth ring, obstruction or blockage of the return oil pipeline, and poor exhaust of the return oil tank, And specific measures were proposed to address the special causes of oil leakage in the bearing chamber.
Keywords: Front pump; Sliding bearings; Oil baffle plate; Oil leakage; Structural improvement
Preface
The booster pump of electric pump is extremely important in Fossil fuel power station. Due to the fact that the water temperature at the inlet of the feed pump is close to the saturation temperature at this pressure, and the working conditions are harsh, it is easy to vaporize. To ensure the safe operation of the feed pump, it is generally equipped with a low-speed front pump with a flow rate that matches the feed pump. Due to the low speed and good anti cavitation performance of the booster pump, the feedwater from the deaerator is pressurized by the booster pump before entering the feedwater pump, ensuring sufficient cavitation margin required by the boiler feedwater pump, thereby greatly improving the anti cavitation performance of the feedwater pump. The front pump of the electric feed pump for Units 3 and 4 of Guangdong Zhuhai Jinwan Power Generation Co., Ltd. adopts the YNKn300/200 model from Shenyang Water Pump Co., Ltd. The structural type is a single stage double suction volute pump, with the pump foot below the centerline; The suction part is vertically upward with a flange, a nominal diameter of 300mm, a nominal pressure of 25Mpa, and DIN2544; The protruding part is the flange facing vertically upwards, with a nominal diameter of 200mm and a nominal pressure of 25Mpa, DIN2544; The bearings are two radial bearings and a thrust bearing installed on the non driving end; The shaft seal adopts a quick installation self circulating mechanical seal, and the thrust bearing at the non driving end is used to balance axial force; Lubrication is the oil from the Fluid coupling for lubrication or equipped with shaft head oil pump; The steering is seen as counterclockwise rotation from the driving end. There are many ways in the industry to deal with oil leakage and solutions, such as checking whether the return oil pipe is blocked or any debris that affects the smooth flow of the return oil; Reduce the oil level in the coupling oil tank to ensure smooth oil flow in the return pipe; Properly reducing the size of the oil inlet orifice plate without affecting lubrication has reduced oil inlet; Check if the oil baffle is damaged and replace it with a new one if necessary; If possible, a smoke exhaust fan can be installed at the oil injection port of the coupling oil tank to create negative pressure inside the oil tank. Also, check each joint and rewind the PTFE sealing tape; Check if the oil baffle is complete and pay attention to whether the gap is too large; If forced lubrication is applied, check whether the inlet pressure is too high and whether the return pipe is unobstructed; If it is fan cooling, check if there is any oil extraction phenomenon, etc.
Text
In power plants, oil leakage from pump bearings is one of the major hidden dangers that affect the environmental protection and equipment operation stability of the plant area. Therefore, for unit maintenance units, solving the problem of oil leakage from the sliding bearings of the electric pump booster pump is an urgent task.
1. Structural characteristics of bearings
The supporting bearings at both ends of the pump adopt split sliding bearings (Figure 1), and their structural characteristics are as follows:
1.1 Oil supply method
The lubrication method adopts external forced oil supply, which directly injects lubricating oil into the bearing oil inlet through the pressure oil pipeline. The oil quantity is controlled by the throttle orifice plate installed on the oil pipeline, and the bearing is forced to lubricate.
1.2 Sealing method
In order to prevent lubricating oil leakage, labyrinth rings are installed on both sides of the bearing body, sealed by a small gap between the shaft and the labyrinth ring, and there are oil return holes between the labyrinth ring teeth. A small amount of oil leakage returns to the bearing chamber through the oil holes.
2. The role of lubricating oil in bearing operation
The role of lubricating oil in bearings: (1) Lubrication - When machinery is running, if some friction parts are not properly lubricated, dry friction will occur. Practice has proven that the heat generated by dry friction in a short period of time is sufficient to melt the metal, causing damage or even jamming of the components. Therefore, it is necessary to provide good lubrication to the friction parts in the machinery. When lubricating oil flows to the friction area, it will adhere to the friction surface to form a layer of oil film, reducing the resistance between friction components. The strength and toughness of the oil film are the key to its lubrication effect. (2) Cooling effect - When bearings rotate at high speeds, they generate a large amount of heat, and lubricating oil can take away a lot of heat, playing a cooling role. (3) Washing function - During mechanical work, a lot of dirt is generated. Such as the sand and dust brought by inhaling the air, the carbon deposits formed after the combustion of the mixed gas, the colloidal substances generated after the oxidation of lubricating oil, and the metal chips generated by friction between machine parts, etc. These dirt will adhere to the friction surface of the machine parts, and if not cleaned, it will increase the wear of the machine parts. In addition, a large amount of gum can cause the piston ring to stick and stick, causing the rotating equipment to not operate properly. Therefore, it is necessary to clean these pollutants in a timely manner, and this cleaning process is completed by circulating lubricating oil within the body. (4) Sealing function - In rotating machinery, each workpiece has a certain fit gap when it fits. This can ensure that the Kinematic pair will not get stuck. But these gaps can cause poor sealing between workpieces. The oil film formed by lubricating oil in these gaps ensures the sealing of the entire system. (5) Rust prevention effect - The moisture in the atmosphere, lubricating oil, and the generated acidic gases can cause corrosion and rust to the components, thereby increasing the damage to the friction surface. The oil film formed by lubricating oil on the surface of the machine can prevent direct contact between the machine and water and acidic gases, and prevent corrosion and rusting. (6) Eliminating Impact Load - The lubricating oil in the bearing clearance will alleviate the impact load on the components, ensure smooth operation of the machinery, and prevent direct metal contact, reducing wear.
3 General causes of oil leakage in bearing chamber and measures taken
We have summarized the reasons for oil leakage in the bearing chamber from the bearing structure of the pump and multiple maintenance and repairs.
3.1 High lubricating oil pressure
The sliding bearings of any rotating equipment (forced oil supply) have certain requirements for oil supply pressure. If the oil pressure is too high, it is easy to cause an increase in the amount of oil coming in, and the return oil is not smooth, resulting in oil leakage from the bearing housing oil baffle. The inlet oil pressure of each bearing of the pump is designed to be (0.07-0.15MPa). If the oil pressure is higher than the upper limit, the diameter of the lubricating oil pressure control valve or throttle orifice can be adjusted.
3.2 Excessive orifice diameter or missed installation of throttle orifice during maintenance
The aperture of the throttle orifice plate in the pipeline has been adjusted according to the required oil amount for the bearing during pump installation and debugging. Generally, the aperture size is 1-2mm. During maintenance, do not easily change its aperture size, as it may cause a significant increase in incoming oil and prevent timely drainage of oil. In addition, when reinstalling the oil pipeline after maintenance, be sure not to miss the installation of the throttle orifice plate, as this phenomenon may occur during the maintenance process.
3.3 Long term wear of the labyrinth ring causes excessive clearance
Excessive radial clearance of the labyrinth ring is the main cause of oil leakage in the bearing chamber. Usually, the unilateral clearance between the labyrinth ring and the shaft is 1.5/1000-2/1000 of the shaft diameter. If the clearance exceeds the standard, the labyrinth ring must be repaired or replaced.
3.4 Clogged oil return hole on labyrinth ring
The blockage of the oil return hole is also one of the main reasons for oil leakage during bearing operation. Due to the blockage of the oil return hole, a small amount of oil leakage cannot be returned to the bearing chamber in a timely manner through the oil return hole, resulting in external leakage. Therefore, it is necessary to ensure that the lubricating oil quality is qualified and the cleanliness of the bearing chamber is maintained.
3.5 The return oil pipeline is not smooth or blocked
It is also common for the return oil pipeline to be blocked by foreign objects, as the return oil is not smooth, causing the bearing chamber to be full of oil and causing oil leakage in the bearing chamber. So during the reinstallation stage of the maintenance process, the oil pipeline must be cleaned to prevent debris from being left inside.
3.6 Poor smoke exhaust in the return oil tank
Due to the high return oil temperature of the oil system, it is inevitable that a certain amount of oil smoke will accumulate in the oil tank. If the exhaust is not smooth, micro positive pressure can easily form in the oil tank, causing poor return oil and causing oil leakage in the bearing chamber.
4 Special reasons and measures for oil leakage in bearing chamber
4.1 Further Cause Analysis
Excessive clearance between the bearing pads or poor scraping and grinding process of the tungsten surface of the bearing pads can cause grooves, deviations, etc., resulting in poor contact between the shaft and the bearing pads. When pressure oil flows out from both sides of the bearing pads, it does not slowly overflow, but forms a jet shape. The distance between the two ends of the bearing pads and the labyrinth ring is very close, and the jet shaped pressure oil directly splashes onto the labyrinth teeth. Therefore, relying solely on the gap sealing of the labyrinth ring, this large amount of pressure oil cannot be sealed, and the return oil hole is small, Unable to return oil in a timely manner, Z ultimately caused oil leakage from the bearing housing oil baffle.
After a unit maintenance, the bearing chamber of the pump experienced oil leakage. When dealing with this defect, we followed the conventional methods mentioned above and still found oil leakage. After disassembling and inspecting the equipment, it was found that it was caused by poor contact between the pump shaft and the bearing bush.
4.2 Improvement Measures
In response to the special reasons for oil leakage in the bearing chamber mentioned above, we have borrowed the bearing oil baffle sealing mode of rotating equipment such as R16K-550 coupler and DG400-140 feed pump; By using a combination of oil baffle or labyrinth teeth and oil baffle, it was decided to install a simple oil baffle between the bearing shell and labyrinth ring on the shaft (Figure 2).
The oil deflector is usually fixed to the shaft by tightening force or set screws. Some oil deflectors also have a sealing rubber ring between them and the shaft diameter, so there is theoretically no gap between them and the shaft. In this way, even if there is pressure oil spraying on both sides of the shaft after installing a simple oil baffle, it will directly spray onto the oil baffle and be blocked back into the bearing chamber, without directly spraying onto the labyrinth teeth. However, for a small amount of splashing oil, the labyrinth teeth can completely block and recover it through the oil return hole. Practice has proven that our analysis and measures taken were correct, and after installing a simple oil baffle, the bearing chamber of the pump no longer leaks oil.
Conclusion
For sliding bearings with external forced oil supply, if the labyrinth oil baffle ring often leaks oil, it is advisable to analyze and solve the above reasons. If traditional maintenance methods are ineffective, it is advisable to try adding an oil baffle plate to effectively contain the oil leakage of the sliding bearing.
Acknowledgement
During the process of writing this article, I received careful guidance and patient assistance from Comrade Wen Zhiqiang, the second mechanical inspection team of the Equipment Department of Zhuhai Jinwan Power Generation Co., Ltd. Thank you to Master Li Shenghui of Tuoqi Company. Every analysis and data in daily maintenance and repair cannot be separated from his meticulous implementation. Here, I would like to express my high respect and heartfelt gratitude to the leaders and colleagues who have provided selfless assistance and guidance.
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