Abstract: This article provides a detailed description of the causes and measures taken for oil leakage in the bearing chamber of the YNKn300/200 electric feedwater pump booster pump through a specific example. The article analyzes the special causes of oil leakage in the bearing chamber, such as high lubricating oil pressure, large orifice diameter of the throttle orifice plate or missed installation of the throttle orifice plate during maintenance, excessive clearance caused by long-term wear of the labyrinth ring, blockage of the return oil hole on the labyrinth ring, unclogged or blocked return oil pipeline, and poor exhaust from the return oil tank. Specific solutions are proposed for the special causes of oil leakage in the bearing chamber.

【 Keywords 】 Pre pump; Sliding bearings; Oil blocking pan; Oil leakage; Structural improvement

Preface

The electric pump booster pump is extremely important in thermal power plants. Due to the water temperature at the inlet of the feed pump being close to the saturation temperature at this pressure, the working conditions are harsh and vaporization is prone to occur. To ensure the safe operation of the feed pump, a low-speed booster pump with a flow rate that matches the feed pump is generally configured for 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 and enters the feedwater pump, ensuring sufficient cavitation allowance for the boiler feedwater pump, thereby greatly improving the anti cavitation performance of the feedwater pump. The electric feedwater pump booster 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 feet below the centerline; The suction part is vertically upward with a flange, nominal diameter of 300mm, nominal pressure of 25Mpa, DIN2544; The protruding part is the flange facing vertically upwards, with a nominal diameter of 200mm and a nominal pressure of 25Mpa, according to DIN2544; The bearing consists of two radial bearings and a thrust bearing mounted on the non drive 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 the axial force; Lubrication is achieved through the use of oil from the hydraulic coupling or by equipping the shaft head oil pump; The steering appears counterclockwise from the driving end. There are many ways to address oil leakage and its solutions in the industry, such as checking for blockages or debris affecting the smooth flow of oil return pipes; Reduce the oil level in the coupling oil tank to ensure smooth oil flow in the return pipe; Reducing the size of the oil inlet orifice plate appropriately without affecting lubrication has reduced oil intake; Check if the oil stop is damaged and replace it with a new one if necessary; If possible, a smoke exhaust fan can be installed at the oil inlet of the coupling oil tank to create negative pressure inside the tank. Also: check each joint and re wrap the PTFE sealing tape; Check if the oil barrier is complete and pay attention to whether the clearance is too large; If forced lubrication, check if the inlet pressure is too high and if the return pipe is unobstructed; If it is fan cooling, check for oil pumping phenomenon, etc.

Main text:

In power plants, pump bearing oil leakage 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 sliding bearing oil leakage in the electric pump pre pump is an urgent task.

1. Structural characteristics of bearings

The supporting bearings at both ends of the pump adopt open type sliding bearings (Figure 1), and their structural characteristics are as follows:

1.1 Oil supply method

The lubrication method adopts external forced oil supply, that is, lubricating oil is directly injected into the bearing inlet through the pressure oil pipeline, and the oil volume is controlled by the throttle orifice plate installed on the oil pipeline to force the bearing 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 the small gap between the shaft and the labyrinth ring, and there are oil return holes between the labyrinth ring teeth. A very 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 function - During mechanical operation, if some friction parts do not receive proper lubrication, dry friction will occur. Practice has proven that the heat generated by dry friction in a short period of time is sufficient to melt metal, causing damage or even jamming of mechanical 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 the friction parts. The strength and toughness of the oil film are the key to its lubricating effect. (2) Cooling effect - When bearings rotate at high speeds, a large amount of heat is generated, 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 air, the carbon deposits formed by the combustion of mixed gases, the gel like substances generated by the oxidation of lubricating oil, and the metal shavings produced by friction between machine parts. These pollutants will adhere to the friction surface of the components, and if not cleaned, they will increase the wear and tear of the components. In addition, a large amount of glue can cause the piston ring to stick and jam, resulting in the inability of rotating equipment to operate normally. Therefore, it is necessary to clean these pollutants in a timely manner, and this cleaning process is completed by the circulation of lubricating oil in the body. (4) Sealing function - In rotating machinery, there is a certain clearance between each workpiece during fitting. This can ensure that there is no jamming between the various motion pairs. 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 - Moisture in the atmosphere and lubricating oil, as well as acidic gases generated, can cause corrosion and rusting of mechanical parts, thereby increasing damage to the friction surface. The oil film formed by lubricating oil on the surface of the parts can prevent direct contact between the parts and water and acidic gases, and prevent corrosion and rusting. (6) Eliminating Impact Load - Lubricating oil in the bearing clearance will alleviate the impact load on the mechanical parts, ensuring smooth operation and preventing direct metal contact, reducing wear.

3. General causes and measures taken for oil leakage in bearing chambers

We have summarized the causes of oil leakage in the bearing chamber based on the structure of the pump bearings and multiple maintenance and repairs.

3.1 High lubricating oil pressure

The sliding bearings (forced oil supply) of any rotating equipment have certain requirements for the oil supply pressure. If the oil pressure is too high, it is easy to cause an increase in the amount of incoming oil, poor oil return, and oil leakage in the bearing chamber oil stopper. 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 plate can be adjusted.

3.2 The diameter of the throttle orifice plate is too large or the throttle orifice plate is missing during maintenance

The aperture of the throttle orifice plate in the pipeline has been adjusted according to the required oil volume of the bearing during the installation and commissioning of the pump. Generally, the aperture size is 1-2mm. During the maintenance process, do not easily change its aperture size, which may cause a significant increase in the incoming oil volume and prevent the oil from being discharged in a timely manner. In addition, when reinstalling the oil pipeline after maintenance, be sure not to miss the installation of the throttle orifice plate, as this phenomenon often occurs during the maintenance process.

3.3 Long term wear and tear of the labyrinth ring causes excessive clearance

The 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 Blockage of the oil return hole on the labyrinth ring

Blockage of the oil return hole is also one of the main reasons for oil leakage in bearings. Due to the blockage of the oil return hole, a small amount of oil leakage cannot be returned to the bearing chamber through the oil return hole in a timely manner. Therefore, it is necessary to ensure the quality of lubricating oil and the cleanliness of the bearing room.

3.5 Oil return pipeline is not unobstructed or blocked

It is also common for the return oil pipeline to be blocked by foreign objects, which can cause the bearing chamber to be filled with oil due to poor return oil flow, resulting in oil leakage from the bearing chamber. So during the reassembly stage of the maintenance process, the oil pipeline must be cleaned thoroughly to prevent debris from remaining in it.

3.6 Poor smoke exhaust from the return oil tank

Due to the high return oil temperature of the oil system, a certain amount of oil fume is inevitably accumulated in the oil tank. If the exhaust is not smooth, a slight positive pressure is easily formed in the oil tank, causing poor return oil and leading to oil leakage in the bearing chamber.

4.Special causes and measures for oil leakage in bearing chambers

4.1 Further Cause Analysis

Excessive clearance between the bearing shells or poor scraping and grinding process of the tungsten surface of the bearing shells can cause grooves, deviations, etc., resulting in poor contact between the shaft and the bearing shells. When the pressure oil flows out from both sides of the bearing shells, it does not slowly overflow, but forms a jet shape. The distance between the two ends of the bearing shells and the labyrinth ring is also very close, and the jet shaped pressure oil directly splashes onto the labyrinth teeth. In this way, relying solely on the clearance seal of the labyrinth ring cannot seal such a large amount of pressure oil. In addition, the small oil return hole cannot return oil in a timely manner, ultimately causing oil leakage in the bearing chamber oil stopper.

After a maintenance of the unit, there was oil leakage in the bearing chamber of the pump. When dealing with this defect, we still had oil leakage after using the above-mentioned methods as usual. After disassembling and inspecting the equipment, it was found that the poor contact between the pump shaft and the bearing shell was the cause.

4.2 Improvement measures

For the special reason of oil leakage in the bearing chamber mentioned above, we have borrowed the bearing oil seal mode of rotating equipment such as R16K-550 coupler and DG400-140 feed pump; It is decided to install a simple oil baffle between the bearing shell and the labyrinth ring on the shaft by using a combination of oil baffle or labyrinth teeth and oil baffle (Figure 2).

The oil baffle is usually fixed to the shaft by tightening force or fastening screws. Some oil baffles also have a sealing ring between them and the shaft diameter, so theoretically there is no gap between them and the shaft. In this way, even if there is pressure oil spraying out 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. After installing a simple oil baffle, the pump bearing chamber no longer leaks oil.

5. Conclusion

For sliding bearings with external forced oil supply, if the labyrinth oil ring often leaks oil, it is recommended to analyze and solve the problem according to the above reasons. If traditional maintenance methods are ineffective, it is advisable to try adding an oil disc to effectively curb the oil leakage of the sliding bearing.

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