1. Introduction to gate valves

1.1. Working principle and function of gate valves:

Gate valves belong to the category of cut-off valves, usually installed on pipes with a diameter greater than 100mm, to cut off or connect the flow of media in the pipe. Because the valve disc is in the gate type, it is generally called a gate valve. Gate valves have the advantages of labor-saving switching and low flow resistance. However, the sealing surface is prone to wear and leakage, the opening stroke is large, and maintenance is difficult. Gate valves cannot be used as regulating valves and must be in the fully open or fully closed position. The working principle is: when the gate valve is closed, the valve stem moves downward and relies on the gate valve sealing surface and the valve seat sealing surface to be highly smooth, flat and consistent, fit each other to prevent the flow of media, and rely on the top wedge to increase the sealing effect. Its closing piece moves vertically along the center line. 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.

1.2 Structure:

The gate valve body adopts a 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.

The gate valve structure is mainly composed of valve body, valve cover, frame, valve stem, left and right valve discs, packing sealing device, etc.

The valve body material is divided into carbon steel and alloy steel according to the pressure and temperature of the pipeline medium. Generally, the valve body is made of alloy material for valves installed in superheated steam systems, t＞450℃ or above, such as boiler exhaust valves. For valves installed in water supply systems or pipelines with medium temperature t≤450℃, the valve body material can be carbon steel.

Gate valves are generally installed in steam-water pipelines with DN≥100 mm. The nominal diameters of the gate valves in the WGZ1045/17.5-1 boiler in Zhangshan Phase I are DN300, DNl25 and DNl00.

2. Gate valve maintenance process

2.1 Valve disassembly:

2.1.1 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, and then use the lifting tool to lift the frame down and place it in a suitable position. The valve stem nut position is to be disassembled and inspected.

2.1.2 Take out the retaining ring at the valve body sealing four-way ring, press the valve cover down with a special tool to create a gap between the valve cover and the four-way ring. Then take out the four-way ring in sections. Finally, use the lifting tool to lift the valve cover together with the valve stem and valve disc out of the valve body. Place it at the maintenance site, and pay attention to prevent damage to the valve disc joint surface.

2.1.3 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 or cover, and affix the seal.

2.1.4 Loosen the hinge bolts of the stuffing box on the valve cover. The packing gland is loose, and the valve stem is screwed down.

2.1.5 Remove the upper and lower clamps of the valve disc frame, disassemble them, take out the left and right valve discs, and keep the internal universal top and gaskets. Measure the total thickness of the gasket and make a record.

2.2 Repair of valve components:

2.2.1 The joint surface of the gate valve seat should be ground with a special grinding tool (grinding gun, etc.). Grinding can be done with grinding sand or emery cloth. The method is also from coarse to fine, and finally polishing.

2.2.2 The joint surface of the valve disc can be ground by hand or grinding machine. If there are deep pits or grooves on the surface, it can be sent to a lathe or grinder for micro-processing, and polished after all are leveled.

2.2.3 Clean the valve cover and sealing packing, remove the 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, which is convenient for pressing the sealing packing.

2.2.4 Clean the packing in the valve stem stuffing box, check whether the internal packing seat ring is intact, the clearance between the inner hole and the stem should meet the requirements, and the outer ring and the inner wall of the stuffing box should not be stuck.

2.2.5 Clean the rust on the packing gland and the pressure plate, and the surface should be clean and intact. The clearance between the inner hole of the gland and the stem should meet the requirements, and the outer wall and the stuffing box should not be stuck, otherwise it should be repaired.

2.2.6 Loosen the hinge bolt, check that the threaded part should be intact and the nut is complete. You can lightly turn it to the root of the bolt by hand, and the pin should rotate flexibly.

2.2.7 Clean the rust on the surface of the valve stem, check for bending, and straighten it if necessary. The trapezoidal thread part should be intact, without broken threads and damage, and apply lead powder after cleaning.

2.2.8 Clean the four-in-one ring, and the surface should be smooth. There should be no burrs or curling on the plane.

2.2.9 Each fastening bolt should be cleaned, the nut should be complete and flexible, and the threaded part should be coated with lead powder.

2.2.10 Clean the stem nut and internal bearing:

① Remove the fixing screws of the stem nut locking nut and the housing, and unscrew the locking screw edge counterclockwise.

② Take out the stem nut, bearing, and disc spring, and clean them with kerosene. Check whether the bearing rotates flexibly and whether the disc spring has cracks.

③ Clean the stem nut, check whether the internal bushing ladder thread is intact, and the fixing screws with the housing should be firm and reliable. The bushing wear should meet the requirements, otherwise it should be replaced.

④ Apply butter to the bearing and insert it into the stem nut. Assemble the disc spring as required and reinstall it in sequence. Finally, lock it with the locking nut and fix it firmly with screws.

2.3 Assembly of gate valve:

2.3.1 Install the left and right valve discs that have been ground to the valve stem clamp ring and fix them with upper and lower clamps. The universal top and adjusting gaskets should be placed inside according to the inspection situation.

2.3.2 Insert the valve stem and valve disc into the valve seat for test inspection. After the valve disc and the valve seat sealing surface are fully in contact, the valve disc sealing surface should be higher than the valve seat sealing surface and meet the quality requirements. Otherwise, the thickness of the gasket at the universal top should be adjusted until it is suitable, and the stop gasket should be used to seal it to prevent it from falling off.

2.3.3 Clean the valve body, wipe the valve seat and valve disc. Then put the valve stem and valve disc into the valve seat and install the valve cover.

2.3.4 Install sealing packing on the self-sealing part of the valve cover as required. The packing specifications and number of rings should meet the quality standards. The upper part of the packing is pressed with a pressure ring and finally closed with a cover plate.

2.3.5 Reassemble the four-ring in sections, and use the retaining ring to prevent it from falling off, and tighten the nut of the valve cover lifting bolt.

2.3.6 Fill the valve stem sealing stuffing box with packing as required, insert the material gland and pressure plate, and tighten it with hinge screws.

2.3.7 Reassemble 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.

2.3.8 Reassemble the valve electric drive device; the top screw of the connection part should be tightened to prevent it from falling off, and manually test whether the valve switch is flexible.

2.3.9 The valve nameplate is clear, intact and correct. The maintenance records are complete and clear; and they have been accepted and qualified.

2.3.10 The pipeline and valve insulation are complete, and the maintenance site is clean.

3. Gate valve maintenance quality standards

3.1 Valve body:

3.1.1 The valve body should be free of defects such as sand holes, cracks and erosion, and should be handled in time after discovery.

3.1.2 There should be no debris in the valve body and pipeline, and the inlet and outlet should be unobstructed.

3.1.3 The plug at the bottom of the valve body should ensure reliable sealing and no leakage.

3.2 Valve stem:

3.2.1 The bending degree of the valve stem should not be greater than 1/1000 of the total length, otherwise it should be straightened or replaced.

3.2.2 The trapezoidal thread part of the valve stem should be intact, without defects such as broken buckles and biting buckles, and the wear should not be greater than 1/3 of the thickness of the trapezoidal thread.

3.2.3 The surface should be smooth and free of rust. There should be no flaky corrosion and surface delamination at the contact part with the packing seal. The uniform corrosion point depth of ≥0.25 mm should be replaced. The finish should be guaranteed to be above ▽6.

3.2.4 The connecting thread should be intact and the pin should be fixed reliably.

3.2.5 The combination of the felling rod and the felling rod nut should be flexible, without jamming during the full stroke, and the thread should be coated with lead powder for lubrication and protection.

3.3 Packing seal:

3.3.1 The packing pressure and temperature used should meet the requirements of the valve medium. The product should be accompanied by a certificate of conformity or undergo necessary test and identification.

3.3.2 The packing specifications should meet the requirements of the sealing box size. Packings that are too large or too small should not be used instead. The packing height should meet the valve size requirements, and thermal tightening margin should be left.

3.3.3 The packing interface should be cut into an oblique shape with an angle of 45°. The interfaces of each circle should be staggered by 90°-180°. The length of the packing after cutting should be appropriate. There should be no gap or overlap at the interface when it is placed in the packing box.

3.3.4 The packing seat ring and packing gland should be intact and free of rust. The stuffing box should be clean and smooth. The gap between the gate rod and the seat ring should be 0.1-0.3 mm, with a maximum of no more than 0.5 mm. The gap between the packing gland, the outer periphery of the seat ring and the inner wall of the stuffing box should be 0.2-0.3 mm, with a maximum of no more than 0.5 mm.

3.3.5 After the hinge bolts are tightened, the pressure plate should remain flat and the tightening force should be uniform. The inner hole of the packing gland and the clearance around the valve stem should be consistent. The packing gland should be pressed into the packing chamber to 1/3 of its height.

3.4 Sealing surface:

3.4.1 The sealing surface of the valve disc and valve seat after inspection should be free of spots and grooves, and the contact part should account for more than 2/3 of the valve disc width, and the surface finish should reach ▽10 or more.

3.4.2 When assembling the test valve disc, the valve core should be 5-7 mm higher than the valve seat after the valve disc is inserted into the valve seat to ensure tight closure.

3.4.3 When assembling the left and right valve discs, the self-adjustment should be flexible, and the anti-drop device should be intact and reliable. 3.5 Stem nut:

3.5.1 The internal bushing thread should be intact, without broken or random buckles, and the fixing with the shell should be reliable and not loose.

3.5.2 All bearing components should be intact and rotate flexibly. There should be no cracks, rust, heavy skin and other defects on the surface of the inner and outer sleeves and steel balls.

3.5.3 The disc spring should be free of cracks and deformation, otherwise it should be replaced. 3.5.4 The fixing screws on the surface of the locking nut must not be loose. The valve stem nut rotates flexibly and ensures that there is an axial clearance of no more than 0.35 mm.