Closure leakage of knife-type gate valves:
Stuffing box leakage is usually called external leakage, while closure leakage is called internal leakage. Closure leakage is inside the valve and is difficult to detect. Closure leakage can be divided into two categories: one is sealing surface leakage; the other is sealing root leakage. Causes of leakage include:
1. Improperly polished sealing surfaces of imported pneumatic knife-type gate valves;
2. Loose fit between the sealing ring and the valve seat and stem;
3. Insecure connection between the stem and valve stem;
4. Bent or twisted valve stem, causing misalignment of the upper and lower closing elements;
5. Closing too quickly, resulting in poor contact or damage to the sealing surfaces;
6. Improper material selection, unable to withstand the corrosive effects of the medium;
7. Using imported globe valves or gate valves for regulating purposes, resulting in sealing surfaces unable to withstand the impact of high-speed media flow;
8. Certain media gradually cool after the inlet valve is closed, causing cracks in the sealing surface and erosion;
9. Threaded connections between some sealing rings and the valve seat and stem can easily create oxygen concentration cells, leading to corrosion and loosening;
10. Impurities such as welding slag, rust, and dust can become embedded, or mechanical parts in the production system can become loose and block the valve core, preventing the knife-type gate valve from closing tightly.
Leakage in the stuffing box of imported electric knife-type gate valves:
1. The packing is not compatible with the corrosiveness, temperature, and pressure of the working medium;
2. Improper packing method, especially screwing in a whole piece of packing as a spare, is most likely to cause leakage;
3. Inadequate machining precision or surface finish of the valve stem, or ovality or scoring;
4. Pitting corrosion or rusting of the valve stem due to lack of protection from the elements;
5. Bent valve stem;
6. The packing has aged due to long-term use.
7. Excessive force.
Imported Knife Gate Valve Stem Lift Failure:
1. Thread damage caused by excessive operation;
2. Lack of lubricant or lubricant failure;
3. Bent or twisted valve stem;
4. Inadequate surface finish;
5. Inaccurate fit tolerances, overtightening;
6. Tilted valve stem nut;
7. Improper material selection; for example, the valve stem and stem nut are made of the same material, which can easily cause seizure;
8. Thread corrosion caused by corrosive media (this applies to concealed-stem valves or valves with stems at the bottom);
9. Inadequate protection for outdoor valves, resulting in the stem threads becoming contaminated with dust and sand, or rusting due to rain, dew, frost, and snow.
Other:
1. Valve body cracking: This is usually caused by freezing. In cold weather, knife gate valves should be insulated and heated.
2. Handwheel damage: This is caused by impact or forceful operation with a long lever. This can be avoided with the operator or other relevant personnel's attention.
3. Packing gland breakage: Uneven force applied when tightening the packing, or a defective gland. When tightening the packing, the screws must be rotated symmetrically, avoiding any deviation. During manufacturing, attention must be paid not only to large and critical components, but also to minor components like the gland, as this can affect performance.
4. Failure of the valve stem and gate connection: Knife-type gate valves often use a rectangular stem head connected to a T-slot on the gate. The T-slot is sometimes left unfinished, causing the stem head to wear more quickly. This is primarily addressed through manufacturing. However, the user can also re-process the T-slot to achieve a certain level of finish.