Causes of valve leakage and maintenance methods

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In the daily use of valves, various kinds of malfunctions often occur. Leakage from the valve body and cover is one of these malfunctions. What causes this phenomenon? Are there any other faults that need attention? Part 1: Leakage from the valve body and bonnet
1. The casting quality of cast iron components is poor; there are defects such as sand holes, porous structures, and inclusions on the valve body and bonnet ； 2. Cryogenic cracking ； 3. Poor welding; defects such as slag inclusions, incomplete welding, and stress cracks are present ； 4. The cast iron valve was damaged after being struck by a heavy object. Maintenance method 1: Improve casting quality; prior to installation, conduct strength tests strictly in accordance with regulations ； 2. For valves exposed to temperatures at or below 0°C, thermal insulation or heating should be applied; for valves that are out of service, any accumulated water must be drained ； 3. The welds between the valve body and the valve cover, which are formed by welding, shall be carried out in accordance with the relevant welding procedures; after welding, flaw detection and strength testing shall also be conducted ； 4. It is prohibited to place heavy objects on valves; hammering cast-iron and non-metallic valves with a hammer is not allowed. Large-diameter valves should be installed on supports. Part 2: Leaks at the packing area
1. Incorrect selection of packing: it cannot resist corrosion from the medium, nor withstand high pressure or vacuum, as well as high or low temperatures in valve applications ； 2. Improper packing installation; defects include using a smaller size in place of a larger one, poor spiral winding joints, and being tight on top but loose at the bottom ； 3. The packing has exceeded its service life; it has aged and lost its elasticity ； 4. The valve stem lacks precision and has defects such as bending, corrosion, and wear ； 5. Insufficient number of packing turns; the gland is not tightly secured ； 6. The gland, bolts, and other components are damaged, preventing the gland from being compressed properly ； 7. Improper operation, excessive force, etc ； 8. The gland is misaligned; the gap between the gland and the valve stem is either too small or too large, resulting in wear of the valve stem and damage to the packing. Maintenance Method 1: The material and type of filler should be selected based on the operating conditions ； 2. Install the packing correctly in accordance with relevant regulations; the packing rings should be placed and compressed one by one, and the joints should be at an angle of 30° or 45°℃ ； 3. Packing that has been in use for too long, is aged, or damaged should be replaced in a timely manner ； 4. If the valve stem is bent or worn, it should be straightened and repaired; in cases of severe damage, it should be replaced promptly ； 5. The packing should be installed according to the specified number of turns; the gland should be tightened symmetrically and evenly. There should be a pre-tightening clearance of at least 5 mm for the compression sleeve ； 6. Damaged gland packs, bolts, and other components should be repaired or replaced in a timely manner ； 7. Follow the operating procedures; except for impact-type handwheels, operate them at a steady speed with normal force ； 8. The gland bolts should be tightened evenly and symmetrically; if the gap between the gland and the valve stem is too small, this gap should be increased appropriately ； The gap between the gland and the valve stem is too large; it should be replaced. Part 3 Reasons for leakage at the sealing surface: 1. The sealing surface is not properly ground, preventing the formation of a tight seal ； 2. The tip at the connection between the valve stem and the closing element is suspended, misaligned, or worn ； 3. The valve stem is bent or not properly assembled, causing the closing element to be misaligned or out of center ； 4. The quality of the sealing surface material was improperly selected, or the valve was not chosen based on the operating conditions. Maintenance Method 1: Select the material and type of shimming pads appropriately based on the operating conditions ； 2. Careful adjustment and smooth operation ； 3. The bolts should be tightened evenly and symmetrically; a torque wrench should be used if necessary. The preload must meet the specified requirements and should neither be too high nor too low. There should be a certain pre-tightening gap at flange and threaded connections ； 4. The gaskets should be aligned properly when assembled to ensure even distribution of stress; overlapping gaskets or the use of multiple gaskets is not allowed ； 5. In cases where the static sealing surface is corroded, damaged, or poorly processed with substandard processing quality, repairs and grinding should be carried out. A dye penetrant inspection must also be conducted to ensure that the static sealing surface meets the relevant requirements ； 6. When installing the gasket, attention should be paid to keeping it clean; the sealing surface should be cleaned with kerosene, and the gasket should not touch the ground. Part 4 Causes of leakage at the seal joint
1. Inadequate rolling of the seal ring ； 2. The sealing ring is welded to the body; the surfacing weld quality is poor ； 3. Loose sealing ring connection threads, screws, and retaining rings ； 4. Corrosion due to connection of the sealing ring. Maintenance method 1: For leaks at the sealed rolling area, apply adhesive and then roll to secure it ； 2. The sealing ring should be rewelded without disassembly in accordance with the welding specifications. When re-welding is not possible at the surfacing area, the original surfacing and machining must be removed ； 3. Remove the screws and retaining rings for cleaning, replace the damaged components, grind the mating surfaces of the seal and the mounting base, and reassemble. For components that have suffered significant corrosion damage, they can be repaired using methods such as welding or bonding ； 4. If the connection surface of the seal ring is corroded, it can be repaired by methods such as grinding or bonding; if repair is not possible, the seal ring should be replaced. Part 5 Reasons for Leakage Caused by the Detachment of the Closing Element: 1. Improper operation, which causes the closing element to get stuck and the connections to become damaged or broken ； 2. The closure components are not securely connected; they become loose and fall off ； 3. The material of the connectors selected is inappropriate; it cannot withstand corrosion from the medium and mechanical wear. Maintenance method 1: Operate correctly – do not use excessive force when closing the valve, and do not open it beyond its upper dead center. After the valve is fully open, turn the handwheel slightly in the opposite direction ； 2. The connection between the closing element and the valve stem must be secure, and a backstop should be provided at the threaded connection ； 3. The fasteners used to connect the closure member and the valve stem must be resistant to corrosion by the medium, and must possess certain mechanical strength and wear resistance.