The Overhaul of Check Valves in Power Plants

A check valve is a valve used to prevent backflow of media in pipes and equipment. The steam extraction check valve is set on each extraction pipe of the external heater of the extraction steam regenerative system in the steam turbine unit. Its function is to automatically close the valve plate when the unit rejects the load to prevent the steam in the steam side of the heater and the steam inlet pipe from returning to the steam turbine, avoiding the steam turbine to overspeed and accidents.

The extraction check valve is closed when the unit is started. When all the heaters start randomly, the air control seat of the extraction check valve takes in air; the piston rod drives the rocker arm to rotate. The limit of the plate shaft is opened, and the valve plate is in a free state at this time. The valve plate is pushed and opened by the pressure of the steam extraction pipe, and the steam enters the heater through the valve. During the normal operation of the unit, the check valve plate remains in the normally open position due to the action of the extraction steam pressure. When the unit is load-rejected and shut down, the extraction steam pressure drops sharply, and the valve plate quickly returns to its seat by its weight; the valve is closed, thereby preventing the steam stored in the heater and the pipeline from returning to the steam turbine and achieving the function of protection. The preparations before the maintenance of the extraction check valve are as follows:

After the equipment is out of service and power supply and air source are cut off, loosen the air inlet and outlet joints of the air control seat; remove the pin connecting the piston rod of the control seat with the check valve lever, and loosen the fixed bolts of the control seat and disassemble the control seat. Place it quantitatively. Then, do the following work:

The disassembly of the valve

Loosen the valve bonnet's bolts and remove the valve bonnet.
Disassemble the bearing, oil retaining ring and lever on the connecting side of the lever shaft and the pneumatic control seat, and place them in a fixed position.
Loosen the large sealing cover nut and remove the large pressure ring.
After making the matching mark, loosen the fixing bolts of the large bracket; remove the large bracket, and place it in a fixed position.
Remove the lever shaft and bushing, and place them in a fixed position.
Remove the small bracket and accessories on the side of the rocker shaft and place them in a fixed position.
Loosen the fixing nut on the small pressure ring; remove the small pressure ring, and place it quantitatively.
After making the matching mark, loosen the fixing bolts on the flange cover; remove the flange cover, and place it in a fixed position.
Disassemble the rocker arm and valve core, and place them in a fixed position.
Take out the rocker arm and valve core and place them in a fixed position.

2. Cleaning and inspection of the valve

Check the valve line on the valve core and valve body. There should be no grooves, air holes and traces across the sealing surface that affect the sealing performance on the valve line, and the contact should be uninterrupted throughout the circumference. If defects affecting the sealing performance are found, grinding and other treatments should be carried out.
Check the connection between the valve butterfly and the rocker arm. The gap between the gasket and the rocker arm should be between 1 and 1.2 mm, which should not be too big or too small. Otherwise, it should be adjusted. The tightening nuts connecting the valve butterfly and rocker arm should be firm. The positioning welding point should be complete and free of cracks, and the gap between the valve butterfly shaft and the rocker arm ring should meet the requirements.
Check that the clearance between the lever shaft or the rocker shaft and the large and small bushings and the seeding arm should meet the requirements, and the surface of each shaft and the inner wall surface of the bushing should be smooth and free of pits.
Check the packing in the large sealing cover and the flange cover, and they should be complete. They should be replaced if damaged.
Check whether the sealing surface of the valve bonnet and valve body is complete, and there should be no grooves, sand holes and penetration scratches that affect the sealing effect.
Clean the sealing surface at the connecting part of the inspection bracket, the flange cover and the valve shell, and the old gaskets adhering to the sealing surface should be cleaned and shoveled away. The sealing surface should be free of defects that affect the sealing effect.
All keys and keyways should be cleaned and the connection should be good.

3. The installation of the valve
Before the valve is installed, the surfaces of each shaft and bushing should be rubbed with molybdenum disulfide powder until they are shiny. The assembly should be carried out in the reverse order of disassembly according to the assembly marks. In the assembly process, the following points should be paid attention to:

After the valve butterfly is connected with the rocker arm, the thickness of the adjusting gasket can be adjusted to ensure that the rotation angle of the opening and closing of valve butterfly reaches the angle required by the manufacturer. After that, the fixing nut should be spot welded with the valve butterfly firmly.
When the lever shaft, the rocker arm shaft and the rocker arm are assembled, there should be a clearance of 1 to 1.20 mm between the lever shaft and the rocker arm end, or the adjusting washer of the rocker arm shaft and the rocker arm end.
When installing and tightening the bracket and flange cover connected to the valve shell on the outside of the rocker shaft and lever shaft, pay attention to keeping the two shafts concentric to prevent the valve butterfly not being opened or closed well.
In the process of pressing the packing at both ends, ensure that each shaft (lever shafts or rocker arm shafts) moves smoothly and does not jam.
All gaskets should be renewed.

4. Disassembly of the control seat
(1) After marking the assembly between the upper cover of the control seat and the cylinder block, and between the cylinder block and the base, remove the short bolts in the fastening studs between the upper cylinder and the base, and place them in a fixed position.
(2) Loosen the nuts of the two long fastening bolts at the same time or separately. The process should be slow. Place the bolts in a fixed position.
(3) Separate the upper cover of the control seat, the cylinder block and the base; take out the piston, door rod and spring, and place them in a fixed position.
(4) Remove the cotter pin and the hexagonal grooved nut; separate the piston rod from the piston, and place them in a fixed position.
(5) Inspect and clean the control seat.
(6) Reinstall the control seat. The assembly of the control seat is carried out in the reverse steps of disassembly, and the following points should be paid attention to:

Replace each O-ring with a new one, and replace it with the temperature-resistant rubber sealing ring.
The inner wall of the cylinder and the surface of the piston should be coated with silicone grease to ensure lubrication.
After the pneumatic control seat is installed, use compressed air to check the empty stroke of the piston rod, which should meet the requirements of the manufacturer, and the movement of the piston rod should be smooth.

(7) Overall verification: Fix the control seat on the valve body and connect it with the lever of the valve. Connect the compressed air connector, and carry out the overall verification. The requirements are as follows:

After being assembled with the extraction check valve, the opening and closing stroke of the compressed air should conform to that specified by the manufacturer.
In the opening and closing process, the overall action should be continuous.
The opening indication of the valve should be verified correctly, and the action switch should be verified together with the thermal control. The display of ON and OFF matches the actual position indication of the valve, and the calibration is over.

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Teresa

Teresa is a skilled author specializing in industrial technical articles with over eight years of experience. She has a deep understanding of manufacturing processes, material science, and technological advancements. Her work includes detailed analyses, process optimization techniques, and quality control methods that aim to enhance production efficiency and product quality across various industries. Teresa's articles are well-researched, clear, and informative, making complex industrial concepts accessible to professionals and stakeholders.