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Analysis and Treatment of Butterfly Valve Closing Fault

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1 Overview of faulty butterfly valve

Qinshan Hydropower Station adopts PDF130-WZ-280 butterfly valve, which is a horizontal double-plate valve structure.butterfly valveThe final assembly includes valve body, valve, working main seal, bearing, rotating arm, servomotor, locking device, weight and bypass valve, etc. The body of the butterfly valve adopts the overall structure of casting and welding, and the contact between the valve body and the working sealing ring is made of stainless steel, which prolongs the service life of the sealing ring and improves the sealing performance.valve sealLocated on the downstream side, the sealing surface adopts a slope structure, so it has good sealing performance. There are 8 anchor bolts at the bottom of the valve body for fixing the valve body, which can withstand the upward thrust of the servomotor and the upward force of the moving water acting on the valve, but cannot bear the water thrust when the valve is fully closed. In order to increase the self-closing torque of the valve when the moving water is closed, the valve and the valve body are designed as offset, and the geometric center and the rotation center are offset by 50mm, which increases the reliability of closing.valveThe main working seal is made of solid special-shaped rubber placed around the valve, and fastened on the valve with a pressure ring and a pressure screw. In order to ensure that the seal ring can be adjusted and positioned after adjustment, a locking screw is provided to fix it. The valve shaft is made of 40Cr forged steel material, and the contact with the bearing and the shaft end seal is protected by chrome plating, and the valve is fixed with a cylindrical pin. The bearing is composed of a steel sleeve and a copper tile. The copper tile is made of cast aluminum bronze (ZCuAl1OFe3). The shaft end seal adopts a U-shaped support ring seal ring. The sealing force is adjustable and easy to replace.

The valve is equipped with two straight cylinder swing servomotors with a diameter of 200mm. The piston rod of the servomotor is connected to the valve shaft through a rotating arm, and the surface of the piston rod is chrome-plated. The seal between the piston and the cylinder body adopts O-ring and PTFE combined seal ring; the servomotor cylinder is equipped with a two-stage closing device and a throttling buffer structure, and the outside is connected by a high-pressure hose.

Control System. The opening of the butterfly valve adopts the oil supply of the FYZ valve hydraulic control device. After pushing the servomotor to fully open the valve and the weight, the spindle is locked by hydraulic pressure. The oil source of the spindle lock comes from the hydraulic device of the governor. When the unit has an accident or needs to be shut down normally, the oil source of the switch lock spindle is disengaged, and the butterfly valve relies on its own two weights (3.5 t each) and hydraulic action to fully close the valve to cut off the water flow and protect the unit. .

2 Symptoms

The butterfly valve of Qinshan Hydropower Station was put into operation in March 2000 and operated until March 2002. It was found that when the butterfly valve was closed to about 95% of the full stroke, the butterfly valve arm (weight) stopped falling, and the butterfly valve It cannot reach the fully closed position by itself. This contradicts that the butterfly valve can only be in two positions of fully open or fully closed. The position of the butterfly valve is in an indeterminate state, resulting in a large amount of water leakage and strong noise, which brings a greater safety hazard to the maintenance and operation of the hydropower station.

3 Reason Analysis

3.1 Oil pipeline

This phenomenon may be due to the poor oil discharge of the oil return line, which caused the oil in the oil chamber on the closing side of the servomotor to not be completely drained. As a result, the piston of the servomotor could not fully return to the fully closed position, resulting in the failure of the butterfly valve arm to reach the maximum position. stroke, causing the butterfly valve to close less than the fully closed position.

In this regard, check the operating oil pipeline, and disassemble and check the oil valve on the pipeline to ensure that the operating oil pipeline is unimpeded.
3.2 Structure of servomotor

In order to reduce the speed and pressure at which the servomotor piston impacts the oil bottom of the servomotor during the closing process, a two-stage closing device, namely a throttling buffer structure, is provided in the operating servomotor cylinder.

Clean the throttle buffer structure of the servomotor and adjust the throttle valve of the servomotor, increase the opening of the throttle valve, increase the flow of oil return, and reduce the pressure difference between the front and rear of the throttle valve, which is beneficial to the servomotor The drop of the piston. At the same time, the inspection found that the chrome-plated layer on the surface of the servomotor piston rod was intact, and there was no sign of rust and jam.

After using the above two methods, there is no obvious effect, indicating that the reason why the butterfly valve cannot reach the fully closed position has nothing to do with the operating system (operating pipeline and servomotor device).

3.3 The torque required to close the butterfly valve during the closing process of the butterfly valve is less than the frictional resistance torque at the bearing

In order to prevent sedimentation from hindering the opening of the valve,butterfly valveThe lower edge is closed against the flow, and the hydrodynamic moment M always tends to close the valve. The component p of the hydrodynamic pressure on the valve along the direction of water flow is along the direction of water flow, and the direction of vertical component P1 is upward.

When the valve is closed, the water is completely still and there is hydrostatic pressure on the valve. Due to the position difference between the upper and lower valves, the action center of the hydrostatic pressure is below the center line of the valve. In this way, for the valve of the horizontal axis butterfly valve, a hydrostatic moment is generated to make the valve rotate.

In the Qinshan Hydropower Station, the butterfly valve cannot be closed normally when it is close to the fully closed position.

existvalveIn operation, there is friction force and friction torque at each rotating pin shaft of the operating mechanism. Among them, the friction force and torque between the valve shaft and the bearing bush are the main ones, and their values ​​occupy the vast majority of the total friction force and friction torque.

According to the design working conditions, the friction coefficient between the valve shaft and cast aluminum bronze is generally taken as μ=0.15, so when the higher water head is 121.5m, the butterfly valve can be automatically closed by its own weight. The necessary condition for the phenomenon that the butterfly valve cannot be automatically closed is μ>0.27. Therefore the following conclusions can be drawn:

After 2 years of operation, the #1 and #2 butterfly valves of Qinshan Hydropower Station failed to close to the fully closed position to varying degrees when they were closed. According to our analysis, there are two reasons for this situation:

(1) After more than 2 years of operation of the butterfly valve, sedimentation may occur between the valve shaft and the bearing, resulting in a decrease in the gap between the valve shaft and the bearing and an increase in the friction factor of the bearing. During the closing process of the valve, when the valve is close to the fully closed position, a pressure difference is formed between the upstream and downstream of the valve, and this pressure difference will generate a torque on the valve shaft that hinders closing. This resistance torque is proportional to the friction factor of the bearing. Proportional, when the resistance torque is greater than the valve closing torque, the above phenomenon will appear.

(2) After the bearing has been in operation for a period of time, the lubricating surface of the bearing may be corroded, the bearing is worn, and the friction factor increases, which will also cause the above phenomenon.

4 processing method

(1) Change the operating procedure for closing the butterfly valve. While closing the butterfly valve, open the bypass valve, and then close the bypass valve after closing the valve. In this way, the front and rear water pressures of the butterfly valve can always be balanced during the closing process, eliminating the pressure difference between the upstream and downstream of the butterfly valve during the closing process, greatly reducing the frictional resistance during closing, and making the closing of the valve smooth. It can reduce the wear of the lubricating surface of the bearing and increase the service life of the butterfly valve bearing.

(2) Four M30 screw holes are reserved on the sides of the two heavy hammers currently configured, and some steel plates can be added to the heavy hammers, and then fastened with screws. In this way, the quality of the hammer is increased, thereby increasing the moment of the hammer when the butterfly valve is closed, so that the butterfly valve can be closed smoothly when the friction coefficient of the bearing increases.

Among the above two methods, if the second method is used for modification, the butterfly valve can be closed by itself relatively smoothly for a period of time, but after a long time of operation, with the precipitation of sediment between the valve shaft and the bearing and the bearing The further wear of the lubricating surface and the increase of the friction factor will cause the friction resistance to increase, and the butterfly valve may not be able to close by itself, and even need to be replaced with a new bearing. Therefore, considering the difficulty of dealing with the above problems and the safe and economical operation of the power station, the method of modifying the butterfly valve closing process is adopted, as shown in Figure 1 (the gray in the figure is the part of the improved process).

5 Practical application

After the butterfly valve closing process of Qinshan Hydropower Station has been improved, the bypass valve is opened at the same time as it is closed, and the valve is filled with water. The butterfly valve is closed when there is no pressure difference between the upstream and downstream. During this process, the hydrostatic moment acting on the valve is basically eliminated, and at the same time, the frictional moment of the valve shaft is greatly reduced. Practice shows that the improved butterfly valve operates stably, closes normally by itself, and has a good closing effect.

At the same time, when the unit is overspeed, the butterfly valve needs to be closed to prevent the unit from overspeed because the main pressure distribution valve of the governor refuses to move. In this way, in order to prevent the speed of the unit from increasing due to the opening of the bypass valve when the butterfly valve is closed, an oil pipe is connected to the accident solenoid valve of the governor in the action oil pipeline of the overspeed protection device. When the overspeed protection device operates, the accident solenoid valve also operates, and the main pressure distribution valve of the governor quickly moves toward the closing side to close the guide vane and prevent the unit from flying away.

The butterfly valve using this closing process, due to the balance of the front and rear hydraulic pressure of the butterfly valve when closing, is less hydrostatic pressure, which greatly reduces the friction torque of the valve shaft, and also reduces the wear of the main shaft and bearing of the butterfly valve, and prolongs the life of the main shaft of the butterfly valve. And the maintenance cycle of the bearing, and also ensure the stable operation of the butterfly valve, and ensure the safe and economical operation of the hydropower station.

If the butterfly valve is used in a hydropower station with a large amount of sediment, it is better not to use this kind of butterfly valve with a self-closing device, because the butterfly valve with a self-closing device will run for a long time. The precipitation between the bearings and the wear of the lubricating surface of the bearing, the increase of the friction factor of the bearing will cause the frictional resistance to increase, and the butterfly valve will eventually not close by itself. This will increase the number of overhauls of the butterfly valve and reduce the service life of the butterfly valve, which is not conducive to the safe, economical and stable operation of the hydropower station.

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