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Maintenance method and steps of power plant extraction steam check valve 2020-03-30


Check valve is a valve used to prevent the medium from flowing backward in pipes and equipment. Extraction check valves are installed on each extraction pipe of the external heater of the extraction steam heating system in the steam turbine unit. Its role is to automatically close the valve plate when the unit is dumped to prevent the steam side of the heater and the inlet pipe The steam is returned to the turbine, causing the turbine to overspeed and avoid accidents.

 

The extraction check valve is closed when the unit is started. When the heaters of all levels are randomly started, the pneumatic control seat of the extraction check valve is fed with air. The piston rod drives the swing arm to rotate the check valve sleeve to the valve. The limit of the plate shaft opens. At this time, the valve plate is in a free state. The valve plate is opened by the pressure of the extraction pipe, and the steam enters the heater through the valve.

 

During normal operation of the unit, due to the effect of the extraction pressure, the check valve plate is kept in the normally open position. When the unit shuts down during load rejection, the extraction pressure drops sharply, and the valve plate quickly returns to its seat by its own weight, closing the valve, thereby preventing the stored steam in the heater and the pipeline from returning to the turbine to achieve protection.

 

 The preparatory work for the extraction steam check valve has the following contents:

 

After the equipment is out of service, protection and power supply and air supply are cut off, loosen the air inlet and outlet joints of the pneumatic control seat, remove the pins connecting the piston rod of the control seat and the check valve lever, and release the control seat. After fixing the bolt, remove the control seat and place it quantitatively. Then, proceed as follows.

 

1, valve disintegration

 

(1) First, mark the matching of the cap and the valve body with a marker pen, and then loosen the cap bolt and remove the cap.

 

(2) Remove the bearing, oil retaining ring and lever on the side of the lever shaft connected with the air control console, and place it on a fixed position.

 

(3) Loosen the nut of the large seal cover and remove the large pressure ring.

 

(4) After making the matching marks, loosen the large bracket fixing bolts, remove the large bracket, and place it in a fixed position.

 

(5) Remove the lever shaft and bushing, and place it.

 

(6) Remove the small bracket and accessories on the shaft side of the rocker arm, and place them.

 

(7) Loosen the fixing nut on the small pressure ring, remove the small pressure ring, and place it quantitatively.

 

8 (8) After making the matching mark, loosen the fixing bolt on the flange cover, remove the flange cover, and place it in a fixed position.

 

9 (9) Remove the rocker arm and valve core and place them in a fixed position.

 

10 (10) Take out the rocker arm and valve core, and place them.

 

2, cleaning and inspection of the valve

 

(1) Check the valve core and valve line on the valve body. There should be no grooves, air holes and traces across the sealing surface that affect the sealing performance, and the contact should be continuous throughout the entire circumference. If defects affecting the sealing performance are found, grinding and other treatments should be performed.

 

(2) Check the connection between the valve butterfly and the rocker arm. The gap between the adjusting pad and the rocker arm should be between 1mm and 1.2mm. It should not be too large or too small, otherwise adjustment should be made. The connection between the valve nut and the rocker arm's fastening nut should be firm, the positioning welding point should be complete and free of cracks, and the gap between the valve butterfly axis and the rocker arm ring should meet the requirements.

 

(3) Check that the clearance between the lever shaft, rocker shaft and the large and small bushings and sowing arms should meet the requirements. The surface of each shaft and the inner wall of the bushings should be smooth and free of pits.

 

(4) Check that the packing in the large seal cover and the flange cover is complete, and if there is damage, replace it.

 

(5) Check that the sealing surfaces of the valve head and the valve body are complete, and that there are no grooves, trachoma and penetrating scratches that affect the sealing effect.

 

6 (6) Clean and inspect the sealing surfaces where the brackets, flange covers and valve casings are jointed together. The old gaskets on the sealing surfaces should be cleaned and removed. The sealing surface should be free of defects that affect the sealing effect.

 

(7) All keys and keyways should be cleaned, and they should be well connected.

 

3. Reinstallation of the valve

 

Before reinstalling the valve, the surface of each shaft and bushing should be rubbed hard with molybdenum disulfide powder until it shines. Reassembly should be carried out in the reverse order of disassembly according to the assembly mark. Pay attention to the following points during the reassembly process:

 

(1) After the valve butterfly is connected to the rocker arm, the thickness of the adjustment gasket can be adjusted to ensure that the turning angle of the valve butterfly switch reaches the angle required by the manufacturer. After that, the set nut should be firmly welded to the valve butterfly.

 

(2) When assembling the lever shaft, the rocker shaft and the rocker arm, there should be a gap of 1 ~ 1.20mm between the lever shaft and the rocker arm end, the adjustment pad of the rocker shaft and the rocker arm end.

 

(3) When the bracket and flange cover connected to the valve housing outside the rocker shaft and lever shaft are tightened, care must be taken to keep the two shafts concentric to prevent the valve butterfly switch from becoming inactive or stuck.

 

(4) In the process of packing filling at both ends, each axis (lever axis, rocker axis) should be kept active and not jammed.

 

(5) All gaskets should be updated.

 

4. Disassembly of the console

 

(1) Mark the assembly of the upper cover of the control seat and the cylinder block, and between the cylinder block and the base, then remove the short bolts from the fastening studs of the upper cylinder and the base, and place them in a fixed position.

 

(2) Loosen the nuts of two long fastening bolts at the same time or separately, and it is required to perform slowly (alternately when loosening separately), until they are loosened, set 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, the door lever and the spring, and place them in a fixed position.

 

(4) Remove the split pin and hexagonal slot nut to separate the piston rod from the piston and place them in a fixed position.

 

(5) Inspection and cleaning of the control seat.

 

Check that the inner wall of the piston cylinder should be smooth.

 

Check the cylindrical compression spring and record the length of the spring in its free state for comparison with the previous (rear) maintenance.

 

Check that the piston rod and piston surface are smooth.

 

Check that the sealing surfaces where the upper cover, cylinder block and base are connected are intact and cleaned.

 

(6) Replacement of the control seat. The installation and replacement of the control seat is performed in the reverse step of disassembly, and the following points should be noted:

 

The sealing O-rings are replaced with new ones, and the temperature-resistant rubber and sealing rings are replaced;

 

 The inner wall of the cylinder and the surface of the piston should be coated with silicone grease to ensure lubrication;

 

After the air control console is installed, the compressed air is used to verify the empty stroke of the piston rod, which should meet the requirements of the manufacturer. The piston rod should be active and continuous without jamming.

 

(7) Overall verification. The control seat is fixed on the valve body, and is connected with the valve lever, and the compressed air joint is connected for overall verification. Claim:

 

After assembly with the steam extraction check valve, the stroke of the compressed air switch should comply with the stroke specified by the manufacturer.

 

During the switching process, the overall action should be continuously active without jamming.

 

The valve opening indication should be calibrated correctly, and the action switch should be calibrated together with the thermal control. The display of opening and closing is in accordance with the actual position indication of the valve, and the verification is completed.


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