The working principle and function of gate valve:
Gate valves are cut-off valves, usually installed on pipelines with a diameter greater than 100mm, to cut off or connect the flow of media in the pipeline. Because the valve disc is in the form of a gate, it is commonly called a gate valve.
Gate valves have the advantages of labor-saving opening and closing and low flow resistance. However, the sealing surface is prone to wear and leakage, the opening stroke is large, and maintenance is difficult. The gate valve cannot be used as a regulating valve and must be in the fully open or fully closed position.
Working principle of gate valve:
When the gate valve is closed, the valve stem moves downward, relying on the gate valve sealing surface and the valve seat sealing surface to be highly smooth, smooth and consistent, and fit together to prevent the medium from flowing through, and rely on the top wedge to increase the sealing effect. Its closing member moves vertically along the centerline. There are many types of gate valves, which can be divided into wedge type and parallel type according to the type. Each type is divided into single gate and double gate.
Structure of Gate valve :
The gate valve body adopts self-sealing form. The connection method between the valve cover and the valve body is to use the upward pressure of the medium in the valve to compress the sealing packing to achieve the purpose of sealing. The gate valve sealing packing is sealed with high-pressure asbestos packing with copper wire.
Valve body materials are divided into cast iron, carbon steel, stainless steel and alloy steel according to the pressure and temperature of the pipeline medium. Generally installed in valves of superheated steam systems, t>450℃ and above, the valve body is made of stainless steel or alloy materials, such as boiler exhaust valves, etc. For valves installed in water supply systems or pipelines with medium temperature t≤450°C, the valve body material can be cast iron or carbon steel.
Maintenance of gate valve:
Valve disassembly:
1. Remove the fixing bolts of the upper frame of the valve cover. Unscrew the nuts of the four bolts on the lifting valve cover. Turn the valve stem nut counterclockwise to separate the valve frame from the valve body. Then use a lifting tool to lift the frame down. Put it to the appropriate location. The valve stem nut part needs to be disassembled and inspected.
2. Take out the retaining ring at the valve body sealing four-in-one ring, and use a special tool to press down the valve cover to create a gap between the valve cover and the four-in-one ring. Then take out the four-ring in sections. Finally, use a lifting tool to lift the valve cover, valve stem, and valve disc out of the valve body. Place it in the maintenance site, and be careful to prevent damage to the valve disc joint surface.
3. Clean the inside of the valve body, check the condition of the valve seat joint surface, and determine the maintenance method. Cover the disassembled valve with a special cover plate or cover and affix a seal.
4. Loosen the hinge bolts of the stuffing box on the valve cover. Loosen the packing gland and unscrew the valve stem.
5. Disassemble the upper and lower splints of the valve disc frame, take out the left and right valve discs, and keep their internal universal jacks and gaskets. Measure the total thickness of the gasket and record it.
Repair of various parts of gate valve:
1. The joint surface of the gate valve seat should be ground with special grinding tools (grinding gun, etc.). Grinding can use either abrasive sand or emery cloth. The method is also from coarse to fine, and finally polished.
2. The joint surface of the valve disc can be ground manually or with a grinding machine. If there are deep pits or grooves on the surface, it can be sent to a lathe or grinder for micro-machining, and then polished after all levels are leveled.
3. Clean the valve cover and sealing packing, and remove rust on the inner and outer walls of the packing pressure ring so that the pressure ring can be smoothly inserted into the upper part of the valve cover to facilitate the compression of the sealing packing.
4. Clean the packing inside the valve stem stuffing box, check whether the internal packing seat ring is intact, the gap between the inner hole and the stem should meet the requirements, and there should be no jamming between the outer ring and the inner wall of the stuffing box.
5. Clean the rust on the packing gland and pressure plate. The surfaces should be clean and intact. The gap between the inner hole of the gland and the cutting rod should meet the requirements, and there should be no jamming between the outer wall and the stuffing box, otherwise, it should be repaired.
6. Loosen the hinge bolt, and check that the threaded part is intact and the nut is complete. You can gently turn it to the root of the bolt by hand, and the pin should rotate flexibly.
7. Clean the rust on the surface of the valve stem, check whether it is bent, and straighten it if necessary. The trapezoidal thread part should be intact, without breakage or damage, and should be coated with lead powder after cleaning.
8. Clean the four-ring and the surface should be smooth. The flat surface shall not have burrs or curled edges.
9. Each fastening bolt should be cleaned, the nut should be complete and flexible, and the threaded part should be coated with lead powder.
10. Clean the valve stem nut and internal bearings:
① Remove the fixing screw between the valve stem nut lock nut and the housing, and unscrew the locking screw counterclockwise.
② Take out the valve stem nut, bearing, and disk spring, and clean them with kerosene. Check whether the bearing rotates flexibly and whether the disc spring has cracks.
③Clean the valve stem nut, check whether the internal bushing ladder thread is intact, and the fixing screws with the shell should be firm and reliable. The bushing wear should meet the requirements, otherwise it should be replaced.
④ Coat the bearing with butter and insert the valve stem nut. The disc springs are assembled as required and reinstalled in sequence. Finally, lock it with a locking nut and secure it firmly with a screw.
Assembly of Gate valve:
1. Install the qualified left and right valve discs on the valve stem clamp ring and fix them with the upper and lower splints. The universal top and adjusting gasket should be placed inside the valve stem according to the maintenance situation.
2. Insert the valve stem together with the valve disc into the valve seat for test and inspection. After the valve disc and the valve seat sealing surface are all in contact, it should be ensured that the valve disc sealing surface is higher than the valve seat sealing surface and meets the quality requirements. Otherwise, the universal jack should be adjusted. Adjust the thickness of the gasket until it is suitable, and seal it with a back-off pad to prevent it from falling off.
3. Clean the valve body, and wipe the valve seat and valve disc. Then put the valve stem together with the valve disc into the valve seat and replace the valve cover.
4. Install sealing packing in the self-sealing part of the valve cover as required. The packing specification and number of turns should meet the quality standards. The upper part of the packing should be pressed tightly with a pressure ring and finally closed with a cover plate.
5. Reinstall the four in sections one by one, use the retaining ring to prevent it from falling off, and tighten the nuts of the valve cover lifting bolts.
6. Fill the valve stem sealing stuffing box with the stuffing as required, insert the stuffing gland and pressure plate, and check them tightly with hinge screws.
7. Reinstall the valve cover frame, rotate the upper valve stem nut to make the frame fall on the valve body, and tighten it with connecting bolts to prevent it from falling off.
8. Install the valve electric drive device; the top screw at the connecting part should be tightened to prevent it from falling off, and manually test whether the door switch is flexible.
9. The valve label is clear, complete, and correct. Maintenance records must be complete and clear; and must be inspected and accepted as qualified.
10. The pipes and valves are fully insulated and the maintenance area is clean.
