How to Perform Routine Maintenance of Valves?

Leakage of the Valve Body and the Bonnet
 
Reason:
1. The casting quality of iron castings is not high, and there are defects such as sand holes, loose structure and slag inclusion on the valve body and bonnet body.
2. Freezing cracking
3. Poor welding, including slag inclusion, lack of welding, stress crack and other defects
4. Cast iron valves are damaged after being hit by heavy objects.
 
Maintenance method:
1. Improve the casting quality, and carry out strength tests in strict accordance with regulations before installation.
2. For valves with temperatures below 0℃, heat preservation or mixing should be carried out. Stagnant water should be excluded from the valves that are out of use.
3. The welding seam of the valve body and the valve cover connected by welding shall be carried out according to the relevant welding operation rules. The flaw detection and strength test shall be carried out after welding.
4. It is forbidden to push and put heavy objects on valves. It is not allowed to hit cast iron and nonmetal valves with hammers. The installation of large-diameter valves should be supported.
 
Leakage at the Packing (the proportion of the valve leakage that happened at the packing is the largest)
 
Reason:
1. The packing is not selected correctly, and it is not resistant to the corrosion of the medium, high pressure or vacuum, high temperature or low temperature of valves.
2. The packing is not installed correctly, and there are some defects such as replacing the big one with the small one, poor spiral coil joints, upside tightening and downside loosening.
3. The packing has expired, aged and lost its elasticity.
4. The accuracy of the valve stem is not high, and there are defects such as bending, corrosion and wear.
5. The number of packing turns is insufficient, and the gland is not pressed.
6. The gland, bolts and other parts are damaged, which makes the gland unable to be pressed.
7. Improper operation, excessive force, etc.
8. The gland is skewed, and the gap between the gland and the valve stem is too small or too large, resulting in valve stem wear and packing damage.
 
Maintenance method:
1. Materials and types of fillers should be selected according to working conditions.
2. Install the packing correctly according to relevant regulations, and the packing should be pressed one by one. The joint should be at the temperature of 30℃ or 45℃.
3. Packing with too long service life, aging and damage should be replaced in time.
4. After the valve stem is bent and worn, it should be straightened and repaired, and the damaged valve stem should be replaced in time.
5. Packing should be installed according to the specified number of turns. The gland should be tightened symmetrically and evenly, and it should have a pre-tightening clearance of more than 5mm.
6. Damaged glands, bolts and other parts should be repaired or replaced in time.
7. Comply with the operating rules. Except for the impact handwheel, it should be operated with constant 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, it should be appropriately increased. The clearance between the cover and the valve stem is too large that it should be replaced.
 
Leakage of Sealing Surface

Reason:
1. The sealing surface is ground uneven, so it can't form a tight line.
2. The center of the joint between the valve stem and the closing part hangs in the air, worn and not straight.
3. The valve stem is bent or improperly assembled, which makes the closing part skew or out of position.
4. Improper selection of sealing surface materials or failure to select valves according to working conditions.
 
Maintenance method:
1. According to the working conditions, correctly choose the materials and types of the top gasket.
2. Careful adjustment and smooth operation
3. Screw the bolts evenly and symmetrically. Use a torque wrench when necessary, and the pre-tightening force should meet the requirements, not too large or too small. There should be a certain pre-tightening clearance at the flange and threaded connection.
4. The gasket assembly should be aligned in the middle, with uniform stress. The gasket is not allowed to overlap and use double gaskets.
5. With corrosion of the static sealing surface, damage processing, poor processing quality, it should be repaired, ground, inspected in coloring, which makes the static sealing surface meet the relevant requirements.
6. Attention should be paid to cleaning when installing gaskets. The sealing surface should be cleaned with kerosene, and gaskets should not fall to the ground.
 
Leakage at the Joint of the Sealing Ring

Reason:
1. The sealing ring is lax.
2. The surfacing quality is poor when the sealing ring is welded with the body.
3. The connecting thread, the screw and the pressing ring of the sealing ring are loose.
4. The sealing ring is corroded due to connection.
 
Maintenance method:
1. The rolling seal should be perfused with adhesive and rolls fixed again.
2. The sealing ring should be repaired again according to the welding specification. When the surfacing cannot be repaired, the original surfacing and processing should be removed.
3. Remove the screws; press the ring for cleaning; replace the damaged parts; grind the sealing surface with the connecting seat, and reassemble. Parts with large corrosion damage can be repaired by welding, bonding and other methods.
4. The connection surface of the sealing ring is corroded, which can be repaired by grinding, bonding and other methods. If it cannot be repaired, the sealing ring should be replaced.
 
Leakage Caused by the Falling-off of the Closing Part

Reason:
1. Poor operation causes the closing part to jam, and the joint is damaged and broken.
2. The closing part is not firmly connected, and it loosens its strength and falls off.
3. The material of the selected connector is wrong, and it can't stand the corrosion of medium and mechanical wear.
 
Maintenance method:
1. Operate the valve in the correct way. The force we use to close the valve can't be too hard. The valve opening can't exceed the top dead center. After the valve fully opens, the handwheel should be reversed a little.
2. The connection between the closing part and the valve stem should be firm, and there should be non-return parts at the threaded connection.
3. Fasteners used to connect the closing parts to the valve stem should withstand the corrosion of medium and have certain mechanical strength and wear resistance.