Created on:2021-07-19 13:47

Maintenance of circulating water system of 300MW Nuclear Power Unit_ Cause analysis of butterfly valve problems

Maintenance of circulating water system of 300MW Nuclear Power Unit_ Cause analysis of butterfly valve problems

Through the test and inspection, it can be determined that the main reason for the abnormal operation of the hydraulic system is the serious leakage of the piston rod seal of the lifting hydraulic cylinder when the actuator is opened. In the past maintenance work, only the internal seal of the hydraulic cylinder was replaced. Although the leakage phenomenon was temporarily alleviated, the same failure occurred after a period of operation. The main reason is that the design and manufacture of the hydraulic cylinder structure are not perfect, and the leakage can not be completely eliminated without technical transformation.

No.6 hydraulic butterfly valve lifting hydraulic cylinder is a single acting, telescopic hydraulic cylinder, that is, the upper part of the piston unilateral oil, large and small two-stage telescopic piston structure. The big and small pistons are sealed with two split copper pads, which form an integral sealing ring after assembly to realize sealing, and the copper pads are easy to replace after wear. Compared with the single-stage piston hydraulic cylinder sealed with U-shaped piston seal, it has the advantage that it can achieve a larger working stroke with a shorter installation size. But the disadvantages are also obvious. Firstly, the sealing effect of the two petal split copper bush is far less than that of the U-shaped piston sealing ring made of NBR. The characteristic of U-shaped piston seal ring is that under the pressure of high-pressure medium, its sealing lip opens to the outside and closely fits with the sealing surface. After high-pressure compression, the sealing ability of the sealing lines on both sides of the lip increases with the increase of medium pressure. It is an ideal seal for high-pressure components in hydraulic system. NBR material has excellent toughness, elasticity and grease resistance, which can make the U-ring bear great hydraulic load and achieve high sealing effect. However, due to a certain fit clearance between the sealing copper bush and the cylinder block, and the copper material has no elastic compression, the pressure oil is easy to leak from the copper bush to the bottom of the piston, and the oil return interface is not designed in the lower chamber of the piston, resulting in the leakage of the piston rod seal under high pressure. At the same time, without oil return pipe, the air in the hydraulic cylinder can not be eliminated in time, resulting in large vibration of the system when opening the valve. The two-stage piston structure also results in the larger diameter of piston and piston rod and the increase of the number of piston rod seals. The piston rod seal and dust removal ring are made of polyurethane with high hardness and high friction resistance; At the same time, when the butterfly valve is installed in the well pit, the humidity is relatively high, and the polyurethane material is easy to be aged and decomposed in case of water for a long time, which also affects the sealing effect. Moreover, because the manufacturing process is rough, the machining accuracy of equipment parts is not enough, the assembly process is not up to standard, etc., are the factors that cause the system to work abnormally.

During the disassembly and inspection of the components of the oil supply system, it is found that the main reasons for the internal leakage and pressure maintenance of the system are as follows.

① The valve seat of pump outlet and system check valve deviates, resulting in poor sealing. Check valve structure: the valve head is a steel ball, the back is equipped with a return spring, and the valve seat is a drilled plate. Because the valve head and the valve seat are not guided and positioned, the impact position of the valve head and the valve seat is offset and the hardness of the valve seat material is not enough in the process of opening and closing, which leads to the deviation of the valve seat sealing line, which directly causes the pressure of the hydraulic system can not be maintained, and the hydraulic pump frequently starts to supplement the pressure.

② The relief valve acts too early to release pressure. When checking the setting value of the overflow valve at the pump outlet, it is found that the action value should be 18Mpa. When the actual pressure just reaches 4 ~ 5MPa, the overflow valve has begun to relieve pressure; When the pressure reaches about 9 MPa, the leakage of relief valve increases obviously. After adjustment, the action value is still low, indicating that the sealing between the valve head and the valve seat is not tight.

③ The increase of internal clearance of solenoid valve components such as reversing, pressure supplement and unlocking, aging of seals, lax sealing and internal leakage can cause the failure of hydraulic system pressure maintenance and frequent pressure supplement. At the same time, the pressure switch, solenoid valve and other models on the system are older, which are prone to set value drift, action failure, etc. they should be timely selected and replaced. The hydraulic system is equipped with a pressure accumulator, but because the working volume of the accumulator is not enough, and the internal gallbladder is not filled with enough nitrogen pressure, it is easy to cause the system pressure fluctuation.

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