Knife gate valve is also known as slurry knife gate valve, mud knife gate valve, slide gate valve, blade gate valve. Knife gate valve is one kind of special gate valve, whose valve blade-disc moves up and down perpendicular to the direction of the medium flow. Knife gate valves are widely used for mediums like cement, slurry, paper pulp, wood pulp, slag, coal slime, dust, fiber, chemical treatment sewage, waste water and other medias.
Knife gate valve types
According to different sealing structure, knife gate valves can be divided into resilient seated soft sealing knife gate valve, metal to metal hard sealing knife gate valve, uni-directional sealing knife gate valve and bi directional knife gate valve.
With different body material used, knife gate valves can be divided into cast iron knife gate valve, ductile iron GGG40 knife gate valve, carbon steel knife gate valve, stainless steel knife gate valve.
With different operating device used, knife gate valves can be divided into manual hand wheel operated knife gate valve, manual chain wheel knife gate valve, electric actuated knife gate valve, pneumatic cylinder operated knife gate valve,hydraulic actuated knife gate valve.
With different end connect, knife gate valve can be divided into wafer type knife gate valve, wafer lug type knife gate valve, flange end knife gate valve.
Those various types of knife gate valves are widely used in our pipelines, while sometimes,the knife gate valve may have leaking issues or they cannot work well, its better we can understand why that knife gate valve is leaking or failed, and how to fix them, so there is no need to replace them.
1.The closing part of the knife gate valve is leaking.
Usually the leakage of the stuffing box is called the external leakage, and the leakage of the closing part is called the internal leakage. The leakage of the closing part is inside the knife gate valve, and is not easy to find. The leakage of the closing part can be divided into two types: one is the leakage of the sealing surface; the other is the leakage of the root of the sealing part.
There are some reasons to cause the leaking:
1.1.The sealing surface of the knife gate valve is not well ground;
1.2.The sealing ring of the knife gate valve is not tightly matched with the valve seat and the valve bonnet;
1.3.The connection between the valve bonnet and the valve stem is not firm;
1.4.The valve stem is bent and twisted, so that the upper and lower closing parts are not aligned;
1.5.The closing of the knife gate valve is too fast, the sealing surface of the knife gate valve is not in good contact or has already been damaged;
1.6.The improper selection of materials cannot withstand the corrosion of the medium, like the selection of body material, disc material, stem material,sealing surface material,etc.
1.7.The globe valves and gate valves are used for adjustment, and the sealing surface cannot withstand the impact of high speed flowing medium;
1.8.For some media, after the knife gate valve is closed, it will gradually cool down, causing fine cracks on the sealing surface and erosion will also occur;
1.9.Some sealing rings of knife gate valve are connected with valve seat and valve bonnet by thread, which is easy to produce oxygen concentration battery, corroded and loose;
1.10.Impurities such as welding slag, rust, dust etc are embedded or mechanical parts fall off in the production systems to block the valve trim, so that the knife gate valve cannot be closed tightly.
2.The stuffing box of the knife gate valve is leaking.
2.1.The packing of the knife gate valve is not compatible with the corrosiveness, temperature and pressure of the working medium;
2.2.The filling method of the packing is wrong, especially the whole packing is screwed in for spare, which is most likely to cause leakage;
2.3.The processing accuracy of surface finish of the valve stem of the knife gate valve is not enough, or there is ovality, or there are nicks;
2.4.The valve stem of the knife gate valve has been pitted or rusted due to lack of protection in the open air;
2.5.The stem of the knife gate valve is bent;
2.6.The packing has been aged for too long;
2.7.The operation of the knife gate valve is too aggressive;
3.The valve stem of the knife gate valve fails to rise up and fall down:
3.1.The thread of the valve stem of knife gate valve is damaged due to excessive operation;
3.2.There is lack of lubricant or lubricant fails to work on the stem of knife gate valve;
3.3.The stem of the knife gate valve has been bent or twisted;
3.4.The surface finish of the valve stem of the knife gate valve is not enough;
3.5.The matching tolerance is incorrect, and the bite is too tight;
3.6.The stem nut of the knife gate valve is tilted;
3.7.The material selection is improper; for example, the valve stem and the valve stem nut are made of the same material, which is easy to bite;
3.8.The thread wave is corroded by medium(for the non rising stem knife gate valve or the knife gate valve with valve stem at the bottom);
3.9.There is lack of protection for the knife gate valve in the open air, the thread of the valve stem of the knife gate valve is covered with dust and sand, or is corroded by rain, dew, frost,snow,etc;
4.Other reasons to cause the failure of the knife gate valve:
4.1.The cracking of the valve body of knife gate valve: generally caused by freezing. When the weather is clod, the knife gate valve should have thermal insulation and heat insulation measures.
4.2.Damage to the hand wheel; which may be caused by impact or violent operation of the long lever. As long as the operator or other relevant personnel is careful, it can be avoided;
4.3.The packing gland of the knife gate valve is broken: the force is uneven when the packing is pressed or the gland is defective. To compress the packing, the screws should be rotated symmetrically without deviation. When the knife gate valve is under manufacturing, we should not only pay attention to those large items,but also pay attention to those small items like gland, otherwise, the knife gate valve cannot work well.
4.4.The connection between the valve stem and the valve plate fails: the knife gate valve uses a rectangular head of the valve stem to connect with the T-shaped groove of the gate plate. Sometimes the T-shaped groove is not processed, so the rectangular head of the valve stem wears quickly, which can be improved during manufacturing. However, the user can also carry out supplementary processing on the T-shaped groove, so that it can have a certain degree of smoothness.
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