Created on:2021-07-19 13:47

Maintenance of circulating water system of 300MW Nuclear Power Unit_ Technical transformation scheme of butterfly valve

Maintenance of circulating water system of 300MW Nuclear Power Unit_ Technical transformation scheme of butterfly valve

Through the analysis, we determined the overhaul and technical transformation scheme of No.6 hydraulic butterfly valve. Due to the limitation of overhaul period, we decided to select and replace the main components of the oil supply system, and the focus of technical transformation was on the lifting hydraulic cylinder of the actuator.

① Technical transformation scheme of lifting hydraulic cylinder of actuator

a. The original two petal copper sealing bush was canceled, and the integral sealing sleeve was reprocessed, and the material was cast brass. The O-ring groove is machined on the sealing sleeve, and the O-ring is used to replace the original copper bush to achieve better sealing effect (Note: due to the limitation of the wall thickness of the sealing sleeve, the U-shaped piston sealing ring cannot be installed). Although the O-ring has the disadvantages of high movement resistance, poor low pressure sealing performance and short service life as a movable seal, it has good sealing performance under high pressure, and the number of working cycles of butterfly valve hydraulic cylinder is very few, so it is appropriate to select the O-ring seal. As the hydraulic cylinder is a high-pressure equipment, it is necessary to install a matching PTFE reinforced protection ring on both sides of the O-ring.

b. The inner wall of the outer cylinder and the inner wall of the large piston cylinder are polished to improve the surface finish of the inner cylinder; Higher finish can ensure the sealing effect of the O-ring on the piston and prolong the service life of the O-ring.

c. The interference fit is adopted between the copper sleeve and the boss of the piston body. The interference fit of the small piston is 0.08-0.12mm, and that of the large piston is 0.12-0.18mm. The heating assembly method is adopted for installation, and the locking cap ring is processed for locking.

d. The outer diameter of the copper sleeve and the size of the O-ring groove are finished on the lathe after the installation of the copper sleeve.

e. At the same time, an oil return main pipe with a diameter of 25 mm is installed on the system oil supply tank, and the joint is connected with the oil return main pipe by pressure hose, so that a small amount of hydraulic oil leaked from the large and small pistons through the O-ring can directly return to the oil tank.

f. A return pipe is also installed at the rear end of the actuator locking hydraulic cylinder to avoid corrosion and damage of the internal parts of the hydraulic cylinder due to the exposure of the exhaust port and the invasion of moisture. At the same time, the hydraulic oil leaking from the piston of the locking hydraulic cylinder can return to the oil tank.

g. The polyurethane material of V-shaped seals at large and small piston rod ends is replaced with nitrile rubber material to avoid the influence of long-term moisture aging and failure on the sealing effect of piston rod seals.

② Maintenance plan of hydraulic oil supply system

a. An accumulator is installed on the hydraulic oil supply system to stabilize the oil pressure when the system operates. The increase of accumulator volume can prolong the interval time of system pressure supplement, avoid the frequent action of hydraulic pump, and make the operation of hydraulic system more stable. At the same time, the nitrogen pressure in the accumulator should be detected regularly to ensure that the accumulator works normally.

b. The aging control components such as solenoid valve and pressure switch are selected and replaced. At the same time, the high-strength material (40Cr) is used to re process the valve seat and conduct heat treatment. After grinding the sealing line of relief valve head and seat, the setting value is calibrated again. If it is still unqualified, it is replaced to make the components meet the normal working conditions to meet the system requirements.

c. Rework and weld the system pipe joint to replace the original pipe joint, and cancel the red copper sealing gasket on the original system pipe joint. For the high pressure oil pipe joint, the sealing surface is processed with 0-shaped sealing ring groove, which is replaced by 0-shaped sealing ring; For the low pressure oil return pipe joint, the metal framework oil seal is added on the sealing surface to eliminate the leakage point on the original system pipeline.

d. Optimize the hydraulic system pipeline. On the premise of ensuring the connection size, the length of the hydraulic oil pipe should be shortened as far as possible to improve the response time of the hydraulic system; Reduce the bending point of hydraulic pipeline layout to avoid the resistance of hydraulic oil along the way; Reinforce the system pipeline again to avoid the vibration of pipeline under high oil pressure.

e. Check the filters on the inlet and system, clean and replace them to ensure that the system oil circuit is unobstructed.

Through technical renovation and maintenance, No. 6 hydraulic butterfly valve was put into normal operation, and all parameters met the requirements of the system.

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