Maintenance and quality standards of gate valves are shown as follows:
| Items | Parts for maintenance | Key points of the process | Quality requirements |
| 1 | Visual inspection of the valve body | 1. Remove dirt and heat preservation. 2. Check the surface of the valve body for defects such as double skin, cracks, and sand holes. |
There are no double skin, cracks, and sand holes on the surface of the valve body. |
| 2 | Disassembly of valves | 1. Keep a record before disassembly. 2. The valve should be in the open state when it is disassembled. 3. Pay attention to the sequences of disassembly and assembly. 4. Do not damage parts. 5. Clean the removed bolts and parts. 6. Perform a spectral inspection of the internal parts of the alloy steel valves. |
1. Bolts and parts should be in a good condition. 2. The internal parts of the alloy steel valve have passed the spectral inspection. |
| 3 | Inspection and repair of valve stems | 1. Clean up the dirt on the surface of the valve stem and check the valve stem for defects. 2. Straighten or replace the valve stem if necessary. 3. Perform nitriding treatment on the surface based on the situation. |
1. The curvature of the stem is not more than 1% of the full length of the stem, and the out-of-roundness is less than 0.05mm. 2. The valve stem should be smooth, free of dots, scratches, and cracks. The uniform pitting depth of the contact part of the valve stem and packing is not more than 0.3mm, and there is no defect in the other parts. 3. The thread of the valve stem is intact and should be replaced when the wear exceeds 1/3 of the original thickness. |
| 4 | Inspection and repair of valve plates, valve seats, and valve bodies | 1. Check the valve plate, valve seat and valve body for cracks, grooves and other defects. 2. Use red lead powder to check the goodness of fit of the sealing surface, and determine the repair process and grinding method according to the inspection. 3. Polish the joint surface of the valve body and self-sealing gasket, and replace the detachable valve seat that cannot be repaired. 4. Check whether the joint surface of the valve seat and valve body is firm. |
1. There are no cracks or grooves on the valve plate, valve seat, and valve body. 2. The sealing surface should be straight; the roughness of the sealing surface should be less than 0.1µm; the radial goodness of fit should not be less than 80%, and the circumference of the sealing surface should be in uniform contact. 3. There is no foreign matter and other defects inside the valve body. 4. The joint surface of the valve body and self-sealing gasket is smooth without grooves. 5. The valve body is firmly combined with the valve seat, and there is no looseness. |
| 5 | Inspection and repair of the valve bonnet | 1. Clean the stuffing box and polish the inner wall of the stuffing box, packing gland and seat ring. 2. Polish the joint surface of the valve bonnet and sealing gasket. |
1. The inner wall of the stuffing box, gland of the stuffing box and seat ring should be smooth and clean. 2. The joint surface of the valve bonnet and sealing gasket should be smooth and clean. |
| 6 | Inspection and repair of the bracket |
1. Clean the thrust bearing and check whether the bearing is worn, corroded or broken.
2. Check the stem nut on the bracket.
3. Check whether the bracket is damaged. 4. Polish the joint surface of the valve body. |
1. The bearing should meet the requirements, or it must be replaced. 2. The stem's nut is intact. 3. There is no damage to the bracket. 4. The connecting surface of the valve body is smooth. |
| 7 | Repair of gaskets | 1. Polish the gasket. 2. Check the material and hardness of the gasket. |
1. The gaskets are smooth and not corrosive. They have a uniform thickness, no damage, deformation, and cracks. 2. The material and hardness of the gasket should meet the requirements. |
| 8 | Assembly of valves | 1. When the valve is assembled, the valve should be in an open state. 2. The valve should be assembled in sequences. 3. Replenish the lubricant. 4. Replace the packing. 5. Adjust the contact area between the gate and valve seat. 6. Install the sealing ring in sequences. 7. Evenly tighten all parts. 8. Check the gaps of all parts. |
1. When the valve is closed, the center line of the valve plate should be higher than that of the valve seat. 2. The valve stem is firmly connected to the valve plate, and the valve stem is in good agreement. 3. The gap between the gasket and valve body bonnet is between 0.1 and 0.3 mm. 4. The gap between the valve stem and gland is from 0.1 to 0.3 mm. 5. The gap between the packing and gland is between 0.1 and 0.15 mm. 6. The gap between the valve stem and seat ring is between 0.1 and 0.2 mm. 7. The gap between the seat ring and the stuffing box is from 0.1 to 0.15 mm. 8. The accessories and signs are complete, and the valve body is well insulated. |
| 9 | Opening and closing test | Check the opening and closing indication and check the opening and closing conditions. | There is no jamming or virtual travel of the valve in the whole process of opening and closing. |
| 10 | Replacing with a new valve | 1. Conduct disassembly inspection and necessary spectral inspection of the valve. 2. Perform 100% flaw detection of welding. 3. Carried out the water pressure test if necessary. |
1. All parts should be in a good condition, and the material and quality of gate valves should meet the requirements. 2. The welding should be qualified. 3. There is no leakage on each joint surface and sealing surface during the hydraulic test. |
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