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Maintenance methods and steps of main valves in power plants
Power plant valves are mainly used for cutting off or connecting pipeline media on various systems in power plants. Taking the valves of the internationally commonly used German brand WODE Power Plant as an example, the main valves used in thermal power generation can be divided into three categories according to their functions: shutdown, control, and safety protection. Such as power plant desulfurization system and ash removal system, suitable for the transportation of water, steam, ash slurry, dry powder, and other media. Compared to other valve products, power plant valves are characterized by high temperature and pressure, unique self sealing design, and the higher the pressure, the more reliable the sealing. Due to the special performance, technical characteristics, and working conditions, the product has also formed characteristics that other products cannot replace.
Security protection class
1、 Extraction check valve
Check valve is a valve used to prevent media backflow in pipelines and equipment. The steam extraction check valve is installed on each extraction pipe of the external heater of the steam extraction and regeneration system in the steam turbine unit. Its function is to automatically close the valve plate during load rejection of the unit, preventing the steam at the steam side of the heater and in the steam inlet pipe from flowing back into the turbine, causing overspeed of the turbine and avoiding accidents.
During normal operation of the unit, due to the effect of extraction pressure, the check valve plate remains in the normally open position. When the unit is shut down due to load rejection, the extraction pressure suddenly drops, and the valve plate quickly reseats against its own weight, closing the valve, thereby preventing the steam stored in the heater and pipeline from flowing back into the turbine, achieving the protection effect.
Preparations for the maintenance of the extraction check valve include the following:
After the equipment is out of service and the protection, power supply, and air source are cut off, loosen the air inlet and outlet connectors of the pneumatic control seat, remove the pin connecting the piston rod of the control seat and the check valve lever, and loosen the fixing bolts of the control seat before removing the control seat for quantitative placement. Then, proceed to the following work.
1. Disassembly of valve
(1) First, use a marker pen to mark the fit between the cover and the valve body, and then loosen the cover bolts to remove the cover.
(2) Remove the bearings, oil retaining rings, and levers on the connecting side of the lever shaft and the pneumatic control pedestal, and place them in a fixed position.
(3) Loosen the large seal cap nut and remove the large pressure ring.
(4) After making matching marks, loosen the fixing bolts of the large bracket, remove the large bracket, and place it in a fixed position.
(5) Remove the lever shaft and bushing and place them in position.
(6) Remove the small bracket and accessories on the rocker shaft side and place them in a fixed position.
(7) Loosen the fixing nut on the small pressure ring, remove the small pressure ring, and place it quantitatively.
(8) After making matching marks, loosen the fixing bolts on the flange cover, remove the flange cover, and place it in a fixed position.
(9) Remove the rocker arm and valve core and place them in a fixed position.
(10) Take out the rocker arm and valve core and place them in a fixed position.
2. Cleaning and inspection of valves
(1) Check the valve core and the valve line on the valve body. The valve line should be free of grooves, pores, and traces across the sealing surface that affect the sealing performance, and should be in full circumferential contact without interruption. If any defects affecting the sealing performance are found, grinding and other treatment should be carried out.
(2) Check the connection between the valve butterfly and the rocker arm. The gap between the adjusting gasket and the rocker arm should be between 1 and 1.2 mm, and should not be too large or too small. Otherwise, it should be adjusted. The fastening nut connection between the valve butterfly and the rocker arm should be firm, the positioned welding points should be complete and free of cracks, and the clearance between the valve butterfly shaft and the rocker arm ring should meet the requirements.
(3) Check that the clearance between the lever shaft, rocker arm shaft, large and small bushings, and the seeding arm meets the requirements. The surface of each shaft and the inner wall of the bushing should be smooth and clean without pits.
(4) Check that the packing in the large sealing cap and flange cover is complete, and replace it if damaged.
(5) Check that the sealing surfaces of the valve cover and valve body are complete and free of grooves, sand holes, and penetrating scratches that affect the sealing effect.
(6) Clean and inspect the sealing surface at the junction of the bracket, flange cover, and valve housing. Clean and shovel away the old gasket adhered to the sealing surface. The sealing surface shall be free of defects that affect the sealing effect.
(7) All keys and keyways should be cleaned to ensure good connection.
3. Valve reassembly
Before reassembling the valve, the surfaces of each shaft and bushing should be vigorously wiped with molybdenum disulfide powder until they become bright. The reassembly should be carried out in the reverse order of disassembly according to the assembly marks. During the reassembly process, the following points should be noted:
(1) After the valve butterfly is connected to the rocker arm, the thickness of the adjusting gasket can be adjusted to ensure that the angle of the valve butterfly switch meets the angle required by the manufacturer. After that, the fastening nut should be firmly welded with the valve butterfly.
(2) When assembling the lever shaft, rocker arm shaft, and rocker arm, a gap of 1 to 1.20 mm should be left between the lever shaft and the rocker arm end, as well as between the adjusting gasket of the rocker arm shaft and the rocker arm end.
(3) When installing and tightening the bracket and flange cover connecting the outer side of the rocker shaft and lever shaft to the valve housing, it is necessary to keep the two shafts concentric to prevent the valve butterfly switch from becoming inactive or jammed.
(4) During the process of compressing the fillers at both ends, ensure that each shaft (lever shaft, rocker shaft) acts flexibly and does not get stuck.
(5) All sealing gaskets should be updated.
4. Disassembly of the control seat
(1) After making assembly marks between the upper cover and the cylinder block of the control base, as well as between the cylinder block and the base, 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 the two long fastening bolts at the same time or separately, both slowly (alternately when loosening them separately) until they are loosened, and place the bolts in place.
(3) Separate the upper cover and cylinder block of the control seat from the base, take out the piston, door lever, and spring, and place them in a fixed position.
(4) Remove the split pin and hexagon groove 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 is smooth.
Check the cylindrical compression spring and record the length of the spring in its free state for comparison with the previous (subsequent) overhaul.
Check that the piston rod and piston surface are smooth.
Check that the sealing surfaces at the joints of the upper cover, cylinder block, and base are intact and clean.
(6) Reassembly of the control seat. The reassembly of the control seat shall be carried out in the reverse order of disassembly, while paying attention to the following points:
Replace each sealing O-ring with a new one, and replace the temperature resistant rubber and O-ring seals;
Before reassembly, 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 pedestal is reinstalled, use compressed air to verify the empty stroke of the piston rod, which should meet the requirements of the manufacturer. The movement of the piston rod should be flexible and continuous, without jamming.
(7) Overall verification. Fix the control seat on the valve body and connect it to the lever of the valve. Connect the compressed air connector and conduct overall calibration. requirement:
After assembly with the extraction check valve, the stroke of the compressed air switch should meet the stroke specified by the manufacturer.
During the switching process, the overall action should be continuous and flexible without jamming.
The opening indication of the valve should be verified to be correct, and the action switch should be verified together with the thermal control. The display of opening and closing matches the actual position indication of the valve, and the calibration is completed.
2、 Drain valve
1. Introduction
The steam turbine drainage system is an important component of the steam turbine thermodynamic system, especially some important drainage valves such as the steam turbine body, main steam, reheat steam, high and intermediate pressure valves, and feed pump turbine fresh steam drainage. When the unit is started and warmed up, it is required to be able to drain water normally in a timely manner, while when the unit is in normal operation, it is also required that the valves be reliably isolated without leakage. If leakage occurs to these steam traps, the impact on the efficiency of the unit is very obvious.
In view of this, some important drain valves adopt imported valves, and their structural type is a pneumatic operated two-position stop valve. The stop valve has the advantages of simple structure, good sealing performance, and more convenient maintenance. The stop valve is equipped with a pneumatic control mechanism, which makes the valve operate quickly and react sensitively, and can better meet the requirements of unit automation.
2. Maintenance of drain valve
As the valve consists of two parts, the operating mechanism and the valve body, the disassembly is carried out in two steps.
(1) Removal of the control mechanism.
Remove the compressed air pipe intake connector, solenoid valve, action switch, and other peripheral components connected to the pneumatic control mechanism, and place them in a fixed position.
Remove the manual operating device of the control mechanism and place it in a fixed position.
Loosen all the spring compression bolts on the top of the operating mechanism.
Loosen the fastening bolts at the connection between the control mechanism and the valve rod, and move the fastening ring slightly upward.
Loosen the connecting bolts on the Huff lock block that secures the control seat to the valve body, remove the two lock blocks, and connect the two lock blocks with bolts before placing them in place.
Loosen the connecting nut between the valve lever and the operating mechanism with a wrench until the operating mechanism is disconnected from the valve body, and place the operating mechanism in a fixed position.
(2) Disassembly of the valve body.
Loosen and remove the fastening bolts on one side of the valve cover with an Allen wrench, screw the fastening bolts in from the threads on the other side, place an iron plate with a thickness of about 1 mm (diameter? 35~? 40) in the middle slot of the valve cover fastening position, tighten the fastening bolts with an Allen wrench, and further push the slot of the valve cover apart to increase its gap.
Use a special wrench (or rubber tap) to pull (or tap) the door cover and rotate it counterclockwise until it is removed and placed in a fixed position.
Gently tap the outside of the packing gland with a copper rod until it becomes loose, and take out the packing gland.
Take out the door rod and door core and place them in a fixed position.
(3) Check. After the valve is completely disassembled, all parts such as the sealing surface of the valve should be inspected.
Inspection of valve line on valve seat and valve core: visual inspection: the valve line seal should be in full contact, and there should be no defects such as grooves, blowing damage, etc. that affect the sealing.
The sealing surfaces between the valve stem, valve housing, and packing gland should be in good contact.
The inlet and outlet of the valve shall be free of foreign matters and impurities.
(4) Repair processing.
If the valve line is blown out, the valve line inside the valve core and valve seat should be treated differently: the valve line on the valve core should be turned out on the lathe at the original angle; The valve line on the valve seat is repaired and scraped with a special maintenance tool on the cemented carbide sealing surface inside the valve seat. The amount of each repair is about 0.05~0.08mm. The amount of repair and scraping is determined based on the actual situation of the valve line defect. After observing that the sealing surface is free of dents, finely grind the valve line with abrasive paste. After polishing the valve line, evenly apply blue oil (or red lead powder) to the sealing surface on the valve core, fix it with a special sleeve, and then send it into the valve seat for sealing contact test. The sealing surface should be in uniform circumferential contact without interruption.
If the packing in the stuffing box is non flexible graphite packing, all of them should be replaced with flexible graphite packing. The original flexible graphite filler should be inspected for damage, and if damaged, it should be renewed.
The sealing surface on the valve stem (at the contact with the packing) should be bright and free of rust. The self sealing surfaces inside the valve seat and the lower part of the packing box should be polished and clean.
The connection thread surface on the top of the door lever should be clean and the connection should be flexible.
The threaded connection of the packing pressing sleeve should be cleaned to make it move easily and freely to ensure that it can be easily adjusted during normal operation of the unit.
(5) Reassembly. After the cleaning and inspection work is completed, apply anti-seize paint to all connecting threads, and then reassemble the valve. Follow the reverse steps of the disassembly procedure for reassembly.
(6) Adjustments and precautions.
After the control mechanism is connected to the valve lever, the lower limit (closing direction) of the valve is initially adjusted. When screwing the door lever thread into the connecting nut of the control mechanism, connect compressed air directly to the air inlet of the control mechanism, switch the valve, and adjust the connecting nut until a gap of 3 to 5mm is left in the lower section of the connection between the control seat and the valve body after the valve is fully closed. At this point, the preliminary adjustment is completed (the Huff lock block is not installed at this time).
After the pneumatic control opens the valve and tightly connects the control pedestal and valve body, reinstall the Hada lock block and tighten the fastening bolts.
The pneumatic control switches the valve on and off, and adjusts the opening indicator of the valve to match the position of the valve pointer with the actual opening and closing position of the valve.
After installing peripheral equipment such as solenoid valves and action switches, adjust and verify the switch signal indication together with the thermal control discipline, confirm that the opening and closing directions are correct, and the adjustment is completed.
3、 Regulating valve
The regulating valve of WODE in Germany is used to regulate the flow of media in equipment and pipes. Its working principle is mainly to achieve the purpose of regulating the flow by changing the flow path area between the valve core and valve seat. The structure includes multi-level throttling, rotary cylinder type, sleeve plunger type, flat gate plate type, and cage type. Large thermal power plants have complex systems and high requirements for medium regulation. Therefore, regulating valves are widely used in steam and water systems of units. With the continuous improvement of unit automation, the requirements for regulating performance of regulating valves are also increasing.
1. Disassembly of valve
(1) Remove the compressed air pipe joints and solenoid valves connected to the pneumatic control mechanism.
(2) Loosen the nut connecting the actuating mechanism transmission lever and the valve lever, and unscrew the lever until it disengages from the transmission lever.
(3) Loosen the nut on the upper end of the valve cover control device bracket, lift out the pneumatic control device, and place it in a fixed position.
(4) Loosen the valve cover bolts, take out the cover together with the door lever, and place it in a fixed position.
(5) Loosen the gland bolts, pull out the door rod and core from the cover head, and place them in a fixed position.
(6) Take out the sleeve inside the valve body and the split valve line seat and sealing gasket on the valve seat, and place them in a fixed position.
2. Cleaning and inspection
(1) Check that the valve core and valve line on the valve seat are intact and free of defects that affect the sealing effect. If there are slight defects, use the method of grinding sand to eliminate them; If there are deep blowouts or pits, they should be repaired by lathe processing; If the valve line is seriously damaged and cannot be repaired, it should be replaced with spare parts.
(2) Check that the sealing section of the valve rod is smooth and clean.
(3) The window around the inspection sleeve should be free of oxide and other impurities accumulation and adhesion. If any, it should be cleaned.
(4) Check that the sealing surface between the valve line and the valve seat on the valve seat is smooth and complete, without dents, sand holes, and penetrating scratches that affect the sealing performance.
(5) Check that the sealing surface between the valve cover and valve body is complete.
(6) Check the inside of the valve body, especially the inner wall on the outlet side, to ensure there is no obvious blowing loss or thinning.
(7) Check the filler in the filler chamber and replace it if it is damaged.
(8) All sealing surfaces should be cleaned.
3. Reassembly
Follow the reverse steps of disassembly for reassembly. During reassembly, the following points should be noted:
(1) All sealing gaskets should be replaced with new ones.
(2) After reassembly of the sleeve, the sealing surface at the upper end should be basically kept at the same height as the sealing surface on the valve body to ensure the sealing effect of the cover and valve seat.
(3) When connecting the control seat to the valve body, the nut of the transmission valve lever should be tightened to facilitate adjustment.
4. Adjustments
After the valve is reinstalled in place, install peripheral accessories such as pneumatic connectors in place, and adjust the valve together with thermal control personnel.
(1) Connect compressed air, adjust the valve lever to the closed position, and then operate the control device to fully open the valve. Adjust the valve lever to the closed direction by another 2-3 mm. Then tighten the nut between the valve lever and the pneumatic control device.
(2) Adjust the valve to the closed position, adjust the opening indication of the valve to the zero position, and then operate the valve on and off several times to confirm that the valve has a full stroke switch and that the switching process is continuous and active. If there is no jamming, the limit adjustment is completed.
(3) Adjust the positioning plate of the action switch to ensure that the action of the action switch is correct when the valve is fully opened and closed.
4、 Butterfly valve of circulating water system
（1） Overview
Due to the large amount of cooling water required in thermal power plants, the pipe diameter of the circulating water system is also quite large. The nominal diameter of the circulating water main pipe for 300MW units is DN2600, and the nominal diameter of the circulating water inlet and outlet valves for the main condenser is DN1800. Due to its compact structure, small floor area, and good sealing performance, butterfly valves have become the opening and closing and regulating valves for low-pressure and large-diameter pipeline systems.
The DN1800 butterfly valve with the corresponding model DF47DH-1.0 in China has a working pressure of ≤ 1.0MPa and is suitable for water with a temperature of ≤ 50 ℃. The valve and pipeline are connected by flange, and a two-stage speed reduction device is adopted. The valve is arranged vertically. The valve butterfly valve mainly consists of a valve body with a positioning block, a butterfly plate, upper and lower valve shafts, and a transmission device.
The working principle is: the motor drives the valve rod to rotate, and then drives the valve shaft and butterfly valve to rotate within 90 ° relative to the valve body, achieving the purpose of controlling flow and opening and closing.
Due to the fact that most of the isolation valves of the circulating water mains are arranged in outdoor valve wells, the working environment in the valve wells is very poor, and there is too much water and gas, which causes the control circuit of the electric device to fail, making the valve limit ineffective, and causing the valve to switch excessively. In practical use, the electric device should be replaced with an outdoor type to ensure reliable switching and adjustment of the valve in harsh environments.
（2） Maintenance of circulating water butterfly valve
Before the system is isolated, the appearance of the valve can be inspected: first, clean up the water stored in the valve well. Then, check the tightness of the connecting flange of the valve (at this time, the pipeline should be filled with water and under pressure), and the flange of the valve should be tightly connected without leakage.
After the system is reliably isolated, the valve can be serviced. The maintenance steps are as follows:
(1) First, confirm that the water in the pipeline has been drained.
(2) Open the valve electrically to the fully open position and pull off the power supply.
(3) Personnel enter the pipeline through the condenser manhole door (sufficient lighting should be ensured) to inspect the sealing surface of the valve. At this time, there should be no special person outside the manhole to monitor and contact in a timely manner. Do not leave. The lighting in the pipeline should use 12V portable lights or explosion-proof power supply to ensure the safety of personnel.
(4) If the valve sealing ring is found to be seriously damaged, the sealing ring should be replaced. First, prepare spare parts for the same type of sealing ring, then remove the bead screws of the original sealing ring, remove the bead, take out the damaged sealing ring, install the spare sealing ring in place, and reinstall the sealing ring bead and bolts.
After checking that the sealing ring is undamaged and the valve is well sealed, power on the valve and verify the high and low limits of the valve. At this time, personnel should stay in the pipeline to observe the action of the valve limit (especially the low limit). If the valve limit is inaccurate, it should be recalibrated.
(5) Check the limit of the valve. After the switch is normal, personnel shall evacuate and take out tools, the manhole door shall be reinstalled, and the valve inspection shall be completed.
5、 Vacuum butterfly valve
（1） Overview
The exhaust steam of the 300MW feed pump turbine is directly connected to the condenser of the main engine through the exhaust pipe. To ensure reliable isolation of the feed pump turbine during the operation of the main engine, the exhaust pipe is equipped with an isolation valve. Due to the large diameter of the exhaust pipe and the vacuum state, the electric vacuum butterfly valve is selected.
Due to the importance of its location, it is required that the sealing performance of the valve be good, otherwise the vacuum of the unit will decrease and the economy will decrease.
The operation steps for adding water are as follows:
(1) After confirming that the exhaust valve is closed in place, open the exhaust valve on the upper part of the valve.
(2) Open the sealing water inlet valve at the lower part of the valve and add water to the chambers of the two sealing valves. When the exhaust pipe overflows, close the exhaust valve to maintain sealing water pressure in the chamber.
（2） Maintenance of vacuum butterfly valve
After the unit has been shut down and the valve meets the maintenance conditions, the maintenance work on the vacuum butterfly valve can be started. The maintenance steps are as follows:
(1) Open the manhole door beside the valve.
(2) Open the valve electrically to the fully opened position.
(3) The relevant personnel entered the pipeline to inspect the valve under the condition of ensuring sufficient lighting. At this time, there should be a dedicated person outside the manhole to monitor and contact at any time, and they should not leave. If a portable lamp is used, the voltage should be guaranteed to be 12V, and the portable lamp transformer should be placed outside the pipe.
(4) Check the double sealing surface of the valve and the sealing ring on the valve plate. If the sealing ring is seriously damaged during the use of the valve and affects the sealing of the valve, replace the spare sealing ring in a timely manner.
(5) After checking the integrity of the valve sealing, calibrate the valve opening and closing stroke and high and low limits. At this time, the personnel in the pipeline should carefully monitor and check to confirm that the normal limit action of the electric valve switch is correct. After that, take out the tools, appliances, and other items in the pipeline, evacuate the personnel, and reinstall the manhole door.
