Gate valve Repair
1.1. Working principle and function of the gate valve:
Gate valves are shut-off valves used in pipelines wider than 100 mm to control fluid flow. They get their name from the gate-shaped disc inside. These valves are simple to use and offer low resistance, but their sealing surfaces can wear out quickly, they need a long stroke to open, and maintenance can be challenging. Since they are not designed for flow regulation, always keep them fully open or fully closed to avoid damage and help them last longer. When closing, the valve stem moves down to press the disc against the seat for a tight seal. A thrust wedge helps with sealing as the disc lines up vertically. Gate valves come in two main types: wedge-type and parallel-type, and each can have either a single or double disc.
1.2 Structure:
The gate valve body uses a self-sealing design. The valve cover and body are sealed by the pressure of the fluid, which compresses the packing. This creates a secure seal. The packing contains high-pressure asbestos with copper wires for extra sealing.
The gate valve structure is primarily composed of the valve body, valve cover, frame, valve stem, left and right valve discs, packing sealing device, and other components.
Valve body materials are either carbon steel or alloy steel. The choice depends on the pressure and temperature of the pipeline medium. Valves in superheated steam systems operating above 450℃ typically utilize alloy bodies, such as those found in boiler exhaust valves. Valves in feedwater systems or those with medium temperatures at or below 450℃ can use carbon steel bodies.
Gate valves are generally installed on steam and water pipelines with DN≥100 mm. In the first phase, the gate valves have three nominal diameters: DN300, DN125, and DN100.
2. Repair technology of the gate valve
2.1 Valve disassembly:
Before beginning the disassembly process, ensure the valve is completely depressurized and isolated to prevent the risk of accidents. Make sure all personnel involved are equipped with proper personal protective equipment (PPE) such as gloves, helmets, and eye protection. Use correct lifting techniques to prevent injury. Now, proceed to remove the fixing bolts from the upper valve cover frame. Lift the valve cover and unscrew the nuts on all four bolts. Rotate the valve stem nut counterclockwise to disengage the valve frame from the valve body. Lower the frame using lifting tools and position it at an appropriate location. The valve stem nut assembly is now ready for disassembly and inspection.
Remove the retaining ring from the sealing four-ring assembly of the valve body. Use special tools to press down the valve cover. This creates space above the four-ring assembly. Remove the four-ring assembly in sections. Use lifting tools to extract the valve cover, valve stem, and disc from the valve body. Place the parts in a safe area. Protect the valve disc’s mating surfaces from damage.
2.1.3 Clean the interior of the valve body, check the condition of the valve seat joint surface, and determine the maintenance method. The disassembled valve shall be covered with a special cover plate or cover, and the sealing strip shall be attached.
2.1.4 Loosen the hinge bolt of the packing box on the valve cover. Loosen the packing gland and screw off the valve stem.
2.1.5 Remove the upper and lower clamping plates of the valve disc frame, remove the left and right valve discs, and keep the internal universal jacks and washers. Measure the total thickness of the washers and record the measurement.
Use grinding tools, such as a grinding gun, to smooth the joint surface of the valve seat (the area where the gate and seat touch). Start with a coarse abrasive and move to a finer abrasive, finishing with a polish for a smooth surface.
2.2 The valve disc joint surface can be ground by hand or with grinding tools. If there are deep pits or grooves on the surface, it can be sent to a lathe or a grinding machine for micro processing. After all, leveling and polishing are carried out.
Clean the valve cover and packing. Remove rust and dirt from the inner and outer walls of the packing ring. The packing ring should fit smoothly at the top of the valve cover for easy compression.
Clean the packing in the valve stem’s stuffing box. Check if the packing seat ring is intact. The gap between the inner hole and the valve stem should be correct. There should be no sticking between the outer ring and the wall of the packing box.
2.2.1 Remove the rust and dirt on the packing cover and pressure plate, and the surface should be clean and intact. The gap between the inner hole of the packing cover and the cutting rod should meet the requirements, and the outer wall and packing box should not be stuck; otherwise, it should be repaired.
2.2.2 Loosen the hinge bolt, check that the thread part is intact and the nut is complete, rotate the bolt to the root with a light hand, and the pin shaft should turn smoothly.
2.2.3 Clean the rust and dirt on the surface of the valve stem, check whether it is bent or not, and straighten it if necessary. The trapezoidal thread part should be intact without breakage or damage, and be coated with lead powder after cleaning.
2.2.4 The four-sided ring shall be cleaned up, and the surface shall be smooth. There shall be no burrs or rolled edges on the plane.
2.2.5 All fastening bolts should be cleaned, nuts should be intact and flexible, and lead powder should be applied to the thread part.
2.2.6 Cleaning of valve stem nut and internal bearing:
① Remove the locking nut and fixing screw of the valve stem nut from the shell, and turn off the locking edge in a counterclockwise direction.
② Remove the valve stem nut, bearing, disc spring, and clean with kerosene. Check whether the bearing is rotating smoothly and inspect the disc spring for any cracks.
Clean the valve stem nut
③, check whether the trapezoidal thread of the internal bushing is intact, and ensure the fixing screw is securely fastened to the shell. The wear of the bushing should meet the requirements; otherwise, it should be replaced.
④ Apply grease to the bearing and fit it into the valve stem nut. The disc spring is assembled as required and reinstalled in sequence. Finally, lock it with a locking nut and secure it firmly with screws.
2.3 Assembly of gate valve:
2.3.1 The qualified left and right valve blades shall be installed on the valve stem clamping ring and fixed with upper and lower clamps. The universal top and adjustment shim shall be placed inside according to the maintenance situation.
2.3.2 Insert the valve stem together with the valve disc into the valve seat for test inspection. After the valve disc and the sealing surface of the valve seat are fully in contact, the sealing surface of the valve disc should be guaranteed to be higher than the sealing surface of the valve seat and meet the quality requirements. Otherwise, adjust the thickness of the gasket at the universal top until it is appropriate, and seal the anti-loose pad to prevent loss.
2.3.3 Clean the valve body, and wipe the valve seat and valve disc clean. Then put the valve stem and valve disc into the valve seat, and install the valve cover.
2.3.4 Seal packing shall be installed in the self-sealing part of the valve cover according to the requirements. The specification and number of packing shall meet the quality standards. The packing shall be pressed tightly with a compression ring at the top and finally closed with a cover plate.
2.3.5 Install the four-sided ring segments one by one, and use a retaining ring to hold the anti-loosening. Tighten the nut of the valve cover lifting bolt.
2.3.6 Fill the packing box, which encloses the valve stem, with sealing material (packing) as specified. Attach the packing cover and pressure plate, then secure everything in place with the hinge screw.
2.3.7 Mount the valve cover frame, rotate the upper valve stem nut to make the frame fall on the valve body, and tighten the anti-loosening with the connecting bolt.
2.3.8 Install the electric drive device of the valve; tighten the set screw at the connection part to prevent disengagement, and manually test whether the valve is flexible or not.
2.3.9 The valve marking plate is clear, intact, and correct. The maintenance record is complete and clear, and has passed the acceptance inspection.
2.3.10 The pipeline and valve insulation is complete, and the maintenance site is clean.
3. Gate valve repair quality standard
3.1 Valve body:
3.1.1 The valve body shall be free from sand holes, cracks, erosion, and other defects, which shall be dealt with in a timely manner after discovery.
3.1.2 There should be no debris in the valve body and pipeline, and the inlet and outlet should be smooth.
3.1.3 The plug at the bottom of the valve body shall ensure reliable sealing and no leakage phenomenon.
3.2 Valve stem:
3.2.1 The bending degree of the valve stem shall not be more than 1/1000 of the full length; otherwise, it should be straightened or replaced.
3.2.2 The trapezoidal thread part of the valve stem shall be intact, without defects such as broken or bitten thread, and the wear shall not be more than 1/3 of the thickness of the trapezoidal thread.
3.2.3 The surface shall be smooth and free from rust and scale, and there shall be no flake corrosion or surface delamination at the contact parts with the packing seal. If the depth of uniform corrosion exceeds 0.25mm, it shall be replaced. The gloss shall be guaranteed to be above ▽6.
3.2.4 The connecting thread should be in good condition, and the pin should be fixed reliably.
3.2.5 After the pole and the nut are combined, it should rotate smoothly without sticking in the whole stroke, and the thread should be coated with lead powder for lubrication and protection.
3.3 Packing seal:
3.3.1 The pressure and temperature of the filler shall meet the requirements of the valve medium, and the product shall be accompanied by a certificate of conformity or necessary test and appraisal.
3.3.2 The packing specification shall meet the size requirements of the sealing box, and shall not be replaced by a size that is too large. The packing height shall meet the valve’s size requirements, and a heat-tightening allowance shall be left. be left.
3.3.3 The filler interface shall be cut at an angle of 45°, and the interface of each circle shall be offset by 90°-180°. The length of the cut filler shall be appropriate, and there shall be no gap or superposition at the interface when put into the filler box.
3.3.4 The packing seat ring and packing gland shall be in good condition without rust and scale, the packing box shall be clean and smooth, the gap between the door rod and the seat ring shall be 0.1-0.3mm, the maximum shall not exceed 0.5mm, and the gap between the outer periphery of the packing gland and the seat ring and the inner wall of the packing box shall be 0.2-0.3mm, the maximum shall not exceed 0.5mm.
3.3.5 After the hinge bolt is tightened, the pressure plate should be kept flat, and the tight force should be uniform. When filling the gland, the inner hole of the pressure plate should be consistent with the gap around the valve stem. When the gland is pressed into the packing chamber, its height dimension should be l/3.
3.4 Sealing surface:
3.4.1 After maintenance, the sealing surface of the valve disc and valve seat shall be free of spots and grooves, the contact part shall occupy more than 2/3 of the width of the valve disc opening, and the surface finish shall reach ▽10 or more.
3.4.2 Assemble the test valve disc. After inserting the valve disc into the valve seat, ensure that the valve core is 5-7 mm higher than the valve seat to ensure the closure.
3.4.3 When assembling the left and right valve discs, ensure that the self-adjusting mechanism is flexible and the anti-loose device is in good and reliable condition. 3.5 Valve stem nut:
3.5.1 The internal bushing thread shall be intact, without broken thread or random thread, and the fixation with the shell shall be reliable, with no loosening phenomenon.
3.5.2 All bearing parts shall be in good condition and flexible in rotation. There shall be no defects such as cracks, rust, turbidity, or heavy skin on the surface of inner and outer sleeves and steel balls.
3.5.3 The coil spring shall be free of cracks and deformation; otherwise, it shall be replaced. 3.5.4 The surface fixing screw of the locking nut shall not be loose. The valve stem nut shall rotate freely, ensuring that the axial clearance is not greater than 0.35mm.
4. Gate valve maintenance technology and quality standard
order number | Maintenance projects | Technological points | quality requirement |
| 1 | Valve body appearance inspection | 1. Remove dirt and remove insulation 2. Check whether there are defects such as heavy skin, cracks and sand holes on the surface of the valve body | No heavy skin, cracks and sand holes on the surface of the valve body |
| 2 | Valve disassembly | 1. Make coordination records before disassembly. 2. The valve is open when disassembling. 3. Pay attention to the disassembly order. 4. Do not damage parts. 5. Clean the removed bolts and parts. 6. Perform spectral review on the internal parts of alloy steel valves | 1. Bolts and parts should be in good condition. 2. The internal parts of alloy steel valve pass the spectral inspection. |
| 3 | Check and repair valve rod | 1. Clean the dirt on the surface of the valve stem and check the defects of the valve stem. 2. Calibrate or replace when necessary. 3. Surface nitriding treatment according to the situation. | 1. The bending degree of the valve stem shall not be greater than 1% of the total length of the valve stem, and the roundness shall be less than 0.05mm. 2. The valve stem shall be smooth, without pockmarks, scratches and cracks. The uniform pitting depth of the contact part between the valve stem and packing shall not be greater than 0.3mm, and there shall be no defects in other parts. 3. The valve stem thread is in good condition. It should be replaced when the wear exceeds 1/3 of the original thickness. |
| 4 | Check and repair of valve plate, valve seat and valve body | 1. Check whether there are cracks, grooves and other defects in the valve plate, valve seat and valve body. 2. Check the conformity of the sealing surface with red cinnabar powder, and determine the repair process and grinding method according to the inspection situation. 3. Polish the joint surface between the valve body and the self-sealing gasket, and replace the detachable valve seat that cannot be repaired. 4. Check whether the valve seat and valve body bonding surface is firm. | 1. Valve plate, valve seat and valve body have no cracks and grooves. 2. The sealing surface shall be straight, the roughness of the sealing surface shall be less than 0.1µm, the radial conformity shall not be lower than 80%, and the sealing surface shall be evenly contacted around without any break. 3. There is no foreign matter or other defects inside the valve body. 4. The valve body and the self-sealing gasket have smooth surface without groove. 5, the valve body and valve seat are firmly combined, no loosening phenomenon. |
| 5 | Check and repair of valve cover | 1. Clean the packing box and polish the inner wall of the packing box, packing cover and seat ring. 2. Polish the joint surface between the valve cover and the sealing ring. | 1. The inner wall of the packing box, the packing box cover and the seat ring are smooth. 2. The valve cover and sealing gasket mating surface is flat and smooth. |
| 6 | Inspection and repair of stents | Clean the thrust bearing and check for wear, rust and breakage. 2. Check the valve stem nut on the bracket. 3. Check whether the bracket is damaged. 4. Polish the valve body joint surface. | 1. The quality of the bearing meets the requirements, otherwise it must be replaced. 2. Valve stem nut is in good condition. 3. No damage to the bracket. 4. The valve body joint surface is flat. |
| 7 | Repair of four-hole ring (six-hole ring), washer, etc | 1. Polish the four rings and washers. 2. Check the material and hardness of the four-sided ring. | 1. The four-sided ring and washer are smooth and free from rust. The thickness of the four-sided ring is uniform, without damage or deformation. The washer is free from deformation, cracks and other defects. 2. The material and hardness of the four-ring ring meet the requirements. |
| 8 | Assembly of valves | 1. The valve shall be in the open state when assembled. 2. Assemble in accordance with the sequence of cooperation. 3. Add lubricant. 4. Replace the packing. 5. Adjust the contact area between gate and valve seat. 6. Install the four rings in order. 7. Tighten all parts of the fasteners evenly. 8. Check the clearance of each part | 1. When the valve is closed, the center line of the valve plate should be higher than the center of the valve seat. 2. The valve stem is firmly connected with the valve plate, and the valve stem fits well. 3. The gap between the washer and the valve body and cover is 0.1－0.3mm. 4. The gap between the valve stem and the cover is 0.1－0.3mm. 5. The gap between filler and cover is 0.1－0.15mm. 6. The gap between the valve stem and the seat ring is 0.1－0.2mm. 7. The gap between the seat and the packing box is 0.1－0.15mm. 8. Accessories and signs are complete, and the valve body is well insulated. |
| 9 | Switching test | Check the switching indicator and check the switching condition | The valve has no sticking and false stroke during full stroke |
| 10 | Replacement of new valve | 1. Disassembly inspection of valves and necessary spectral inspection 2. 100% flaw detection of weld joints. 3. Pressure test when necessary. | 1. All parts are in good condition, and the material and gate valve quality meet the requirements. 2. The welding joint is qualified. 3. No leakage at all joints and sealing surfaces during hydraulic test. |
