1. The working principle and function of the gate valve
Gate valves are cut-off valves used to cut off or connect the medium in the pipe, which are usually installed on pipes with a diameter greater than 100mm. The disc is shaped like a wedge. The gate valve has the advantages of easy opening and closing and low flow resistance. However, the sealing surface is easy to wear and leak; the opening stroke is great, and the maintenance is difficult. The gate valve cannot be used as a regulating valve and must be in the fully open or fully closed position. The working principle of the gate valve is: when the gate valve is closed, the valve stem moves downward depending on the sealing surface of the gate valve and valve seat being highly smooth and matching with each other to prevent the medium from flowing and relying on the top wedge to increase the sealing effect. The closing part moves along the vertical direction of the center line. There are many types of gate valves, and they can be divided into the wedge type and parallel type. Each type is divided into a single wedge and a double wedge.
2. Structure of gate valves
The valve body of the gate valve adopts a self-sealing form. The connection between the bonnet and valve body: use the upward pressure of the medium in the valve to make the sealing packing to be compressed to achieve the purpose of sealing. The packing of the gate valve is sealed with high-pressure asbestos packing with copper wire. The gate valve is mainly composed of valve bodies, valve bonnets, valve stems, left and right valve discs, and packing seal devices. According to the pressure and temperature of the medium in the pipeline, the materials used for valve bodies are divided into two types: carbon steel and alloy steel. Generally, valves installed at the superheated steam system with a temperature over 450℃ should be made from alloy materials, for example, boiler exhaust valves. As for valves installed in water supply systems or pipelines with a medium temperature smaller than or equal to 450°C, the material for the valve body can be carbon steel. Gate valves are generally installed on steam and water pipes with DN greater than or equal to 100mm.
3. Maintenance standards for gate valves
Valve bodies
- The valve body should be free of defects such as sand holes, cracks and scouring, and they should be dealt with in time if they are found.
- There should be no debris in the valve body and pipeline, and the inlet and outlet should be unblocked.
- Ensure reliable sealing and no leakage for the plug at the bottom of the valve body.
Valve stems
- The curvature of the valve stem should not be greater than one 1000th of its full length. Otherwise, it should be straightened or replaced.
- The trapezoidal thread part of the valve stem should be intact, free from breakage, snapping and other defects, and the amount of wear should not be greater than a third of the thickness of the trapezoidal thread.
- The surface is smooth, free of rust scale, and the contact part with the packing must not have flaky corrosion and surface delamination. The depth of the uniform corrosion point greater than or equal to 0.25 mm should be replaced with a new one. The degree of finish should be guaranteed to be above ▽6.
- The connecting thread should be intact and the pins should be fixed reliably.
- After the stem is connected with the stem nut, it should rotate flexibly without jamming in the whole process, and the threads should be applied with lead powder for protection.
Packing seal
- The pressure and temperature of the packing used should meet the requirements of the valve medium, and the product should be provided with a certificate or necessary test appraisal.
- Specifications of packing should meet the size requirements for the packing box, and it should not be replaced by packing that is too big or too small. The height of the packing should meet the requirements for the valve size, and a thermal tight margin should be reserved.
- The interface of the packing should be cut into an oblique shape with an angle of 45°. The interface of each circle should be staggered by 90° to 180°; the length of the filler after cutting should be appropriate, and there should be no gap or overlap at the interface in the packing box.
- The packing seat ring and packing gland should be intact and free of rust and dirt. The packing box should be clean and smooth. The gap between the stem and seat ring should be from 0.1 to 0.3 mm, and the maximum should not exceed 0.5 mm. The gap among the packing gland, periphery of the seat ring and inner wall of the packing box is from 0.2 to 0.3 mm, and the maximum is not over 0.5 mm.
- After the hinge bolts are tightened, the pressing plate should remain flat and evenly tightened. The inner hole of the packing gland and pressure plate should be consistent with the peripheral clearance of the valve stem. The packing gland in the packing box should be one-third of the height of the packing box.
- The sealing surface of the valve disc and valve seat after maintenance should be free of spots and grooves, and the contact part should occupy more than two-thirds of the width of the valve disc. The surface finish should reach ▽10 or above.
- Assemble and test the valve disc. After the disc is inserted into the valve seat, the valve core should be 5 to 7 mm higher than the valve seat to ensure sealing.
- When assembling the left and right discs, ensure that the self adjustment is flexible, and the anti-falling device should be intact and reliable.
Stem nuts
- The inner bushing screw thread should be intact, which can be fixed to the shell reliably without looseness.
- All bearing components should be intact and flexible to rotate. There are no flaws, such as cracks and rust on the surface of the inner and outer bushing and steel ball.
- The disc spring should be free of cracks and deformation. Otherwise, it should be replaced with a new one.
- The fixing screws on the surface of the lock nut should not be loose. The stem nut rotates flexibly, and there should be some axial clearance but not more than 0.35 mm.
Next: The Analysis of Low-pressure Sealing of Swing Check Valves (Part One)
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