The main reasons for leakages of valves and their measures are as follows:
Leakages caused by falling off of the closing parts
Reasons for falling off of the closing parts
- The operation is inappropriate, so that the closing part is stuck or exceeds the top dead center; the connection part is damaged and broken.
- The connection of the closing part is not firm, and it is loose and falls off.
- The material of the connector is incorrect, and it cannot withstand the corrosion of the medium and the mechanical wear.
Maintenance
- Operate the valve correctly. Do not use too great force to close the valve. The valve should not be opened by exceeding the top dead center. After the valve is fully opened, the handwheel should be turned reversely a little.
- The connection between the closing part and the valve stem should be firm, and there should be a stopper at the threaded connection.
- The fasteners used to connect the closing part; the valve stem should withstand the corrosion of the medium and have certain mechanical strength and wear resistance.
Leakages caused by the packing
The leakage caused by the packings accounts for the largest proportion of the outside leakage of valves.
Reasons for leakages caused by the packing
- The packing is not selected correctly, and it is not resistant to the corrosion of the medium, high pressure, vacuum, high temperatures and low temperatures.
- The packing is installed incorrectly, and there are mistakes such as replacing the large packing with the small one, poor tightness of spiral coil, tightening at upper parts and loosening at lower parts.
- The packing has exceeded its service life, is aged and loses its elasticity.
- The accuracy of the valve stem is not high, and there are defects such as bending, corrosion, wear, etc.
- The number of packing turns is insufficient, and the gland is not pressed tightly.
- The gland, bolts, and other parts are damaged, so that the gland cannot be compressed;
- The operation is improper, and the force is too great.
- The gland is skewed, and the gap between the gland and the valve stem is too small or too big, resulting in wear of the valve stem and damage to the packing.
Maintenance
- The material and type of packing should be selected according to the working conditions.
- According to the relevant regulations, the packing should be installed correctly. The packing should be placed and compressed one by one.
- Packing that has been used for a long time, aged and damaged should be replaced in time.
- After the valve stem is bent and worn, it should be straightened and repaired; if the damage is serious, it should be replaced in time.
- The packing should be installed according to the specified number of turns. The gland should be tightened symmetrically and evenly, and the pressing sleeve should have a pre-tightening clearance of more than 5mm.
- Damaged glands, bolts and other components should be repaired or replaced in time.
- The operating procedures should be followed, except for the impact handwheel, the handwheel should be operated at a constant speed and normal force.
- The gland bolts should be tightened evenly and symmetrically. If the gap between the gland and the valve stem is too small, the gap should be appropriately increased; if the gap between the gland and the valve stem is too big, there should be replacement.
Leakages caused by sealing surfaces
Reasons for leakages caused by sealing surfaces
- The sealing surface is unevenly ground and the valve can't be sealed well.
- The top center of the connection between the valve stem and the closing part is suspended, tilted, inclined or worn.
- The valve stem is bent or improperly assembled, so that the closing part is skewed or out of alignment.
- The quality of the sealing surface's material is improperly selected or the valve is not selected according to the working conditions.
Maintenance
- The material and type of the gasket should be correctly selected according to the working conditions.
- Careful adjustment and smooth operation should be conducted.
- The bolts should be tightened evenly and symmetrically, and a torque wrench should be used if necessary. The pre-tightening force should meet the requirements and should not be too great or too small. There should be a certain pre-tightening clearance between the flange and the threaded connection.
- The gasket assembly should be aligned at the center, and the force should be uniform. The gasket is not allowed to be overlapped and double gaskets are not allowed.
- If the static sealing surface is corroded or damaged, and the processing quality is not high, it should be repaired, ground, and checked by penetrant inspection to make the static sealing surface meet the relevant requirements.
- Pay attention to cleaning when installing the gasket; the sealing surface should be cleaned with kerosene, and the gasket should not be put on the ground.
Leakages caused by the connecting part of the sealing ring
- The sealing ring is not pressed welled
- The surfacing quality of sealing rings and bodies is poor.
- The connecting threads, screws and pressure rings of the sealing ring are loose.
- The sealing ring is corroded.
Maintenance
- The leakage at the sealing and rolling place should be injected with adhesive and then fixed by rolling.
- The sealing ring should be re-welded according to the welding specification. If the surfacing welding cannot be repaired, the original surfacing welding and processing shall be removed.
- Remove and clean the screws and press ring. Replace the damaged parts; grind the sealing surface of the seal and the connection seat, and reassemble. For parts with great corrosion, welding, bonding and other methods can be used.
- The connecting surface of the sealing ring is corroded and can be repaired by grinding, bonding and other methods. If it cannot be repaired, the sealing ring should be replaced.
Leakages caused by bodies and bonnets
Reasons for leakages caused by bodies and bonnets
- The casting quality of iron castings is not high, and the valve body and valve bonnet have defects such as sand holes, loose structures, and slag inclusions.
- Cracking happens due to cold weather.
- The welding is poor. There are defects such as slag inclusions, non welding, stress cracks, etc.
- The cast iron valve is damaged after being hit by heavy objects.
Maintenance
- Improve the casting quality, and carry out the strength test in strict accordance with the regulations before installation.
- For valves with temperatures of 0° and below 0°, heat preservation should be carried out. The valves that are not used should be drained.
- The welding seam of the valve body and the bonnet should be carried out according to the relevant welding operation regulations, and the flaw detection and strength test should be carried out after welding.
- It is forbidden to place heavy objects on the valve, and it is not allowed to hit cast iron and non-metallic valves with a hand hammer. Brackets should be provided for the installation of large-diameter valves.
