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Views: 6 Author: Site Editor Publish Time: 2022-03-27 Origin: Site
An important part of any overpass is the valves. It is naive to believe that valves are installed only on water and gas pipelines, no. The scope of stop valves is extensive - oil, food, pulp and paper and others. Almost in any kind of industry, you can find overpasses that transport a liquid or gaseous medium, sometimes with foreign inclusions. Knife gate valves are an example of valves that are more common in industries than in water or gas pipelines. How is it different from other valves?
Brief description
The device and principle of operation of a knife gate valve
Body material and types of drives on knife gate valves
Design features of knife gate valves
Advantages and disadvantages of knife gate valves
Summing up
If conventional gate valves (type 30ch6br) feel great on water supply systems, then in other areas of industry, where heterogeneous inclusions are present in the transported medium, gate valves are most often used. The gate valve is distinguished by the very principle of operation - the locking device in it is a flat or wedge-shaped knife, which, descending perpendicular to the flow, cuts it, as it were, and closes it with tightness acceptable for the overpass. If foreign bodies are encountered in the path of the knife, it cuts them, which ordinary gate valves would not be able to handle. Also, the knife gate valve is characterized by a small building thickness, which allows installation even in confined spaces. Due to its structure, the knife gate valve is most often used in the following industries:
1) Oil
2) Pulp and paper
3) Mining
4) Chemical
5) Ventilation, sewerage systems, etc.
The working diameter of knife gate valves is from 50 mm to 2200 mm. Certain types of these valves are able to operate smoothly in conditions ranging from freezing temperatures to extreme 400 degrees Celsius. The operating pressure of knife gate valves is up to 60 atmospheres.
The main element is the body. It can be cast iron or steel, depending on the conditions of the transported medium. The knife itself is a pointed metal plate of small thickness. In the closed state, tightness is ensured by cuffs. Seals on the body and cover provide disassembly and maintenance of the valve mechanisms. The rod raises and lowers the knife with the help of torque. Nut, bushing and gasket movably connect the stem and handwheel. The yoke protects the nut-rod connection from temperature, pressure and other influences of the transported medium. The stem guard and cap protect the stem from external contamination when the valve is open. The retainer ring fixes the cuffs inside the valve body. On the body there are holes for bolts, through which the valve is tightly tightened during installation on the overpass through the flanges. This design scheme is almost constant for all types of knife gate valves, slight differences may occur with valves from different manufacturers. To close or open the damper, just turn the handwheel.
Most often, knife gate valves are made of steel (stainless steel, titanium and other alloys). But there may be products made of cast iron. It must be remembered that according to some performance characteristics, cast iron is inferior to steel - for example, it is more prone to splits under shock loads, and also does not tolerate negative temperatures. In any case, the valve body material is selected strictly from the industrial conditions in which the valve will operate.
Like all other types of valves, knife gate valves can be actuated in various ways:
1) Manual drive - using a lever or steering wheel;
2) Pneumatic drive - the process is carried out using compressed air;
3) Hydraulic drive - by means of hydraulic energy;
4) Electric drive - the shutter is driven by electric current.
The manual drive, although associated with the operator's physical exertion, is the most reliable and does not require the presence of certain external factors (such as electric current for an electric drive). It should be noted that any types of actuators, other than manual ones, can often be purchased separately from valves and significantly increase the cost of the entire structure (most often, actuators are even more expensive than the entire valve).
Depending on the tightness requirements, knife gate valves with one or two knives (gates) are distinguished.
The single-knife gate valve is characterized by high structural rigidity and simple design. Most often it is installed on pipelines with high pressure. The movable nut-rod connection is taken out of the body, which prevents contact of the working units with the transported aggressive medium.
Double-knife gate valves provide a very high tightness of the gate, but this is reflected in the cost of the gate valve for the consumer. Often, such valves are rarely used, since fittings with one knife in most cases provide the necessary sealing, and there is no point in increasing the cost of the assembly.
In the direction of movement of the transported flow, knife gate valves are divided into one-way and two-way. One-way gate valves are installed in pipelines where the flow of the medium is constant. Most often they are installed in sewers, water pipes. Double-sided knife gate valves are installed where a change in the movement of the medium can lead to insufficient sealing (one-way knife gate valves most often lose their tightness when the direction of the medium changes). Usually installed in the food and chemical industries.
1) Versatility - a sharpened knife easily cuts through almost any obstacle in the path of the wedge. This makes it possible to mount valves even in pipelines with bulk materials.
2) Short construction length - with a small length, the valves weigh less. This facilitates transportation and installation (especially noticeable on large diameters) and allows you to install the valve even in hard-to-reach places.
3) Simplicity of design and its maintainability - the valve consists of a small number of components, and its design allows repair and replacement of spare parts, which, in turn, increases the service life of the locking device.
4) The isolation of the valve itself from the transported medium also increases the service life of the mechanism and rubber cuffs.
5) Stem variability - depending on the environmental conditions and the project, the valve can be equipped with any type of actuator.
6) Possibility of use in vacuum systems - the knife can completely enter the body of the valve, thus achieving the maximum conditional passage and the minimum resistance of the residual gas;
Despite all the advantages, knife gate valves are not an ideal solution for many overpass nodes, but they have found application in their class.
Like any other device created by man, knife gate valves are not perfect, and have a number of design flaws:
1) In case of damage to rubber seals, repair is laborious and expensive, and involves temporary dismantling from the overpass;
2) Although compatible with various types of actuators, the process of opening / closing the damper takes longer;
3) High cost relative to conventional valves - this disadvantage is offset by a long service life with proper care.
It is cheaper to install ordinary 30ch6br type valves on water pipes with drinking and industrial water, especially since all water pipes often belong to the state budget, on which it is customary to save. If the transported medium is different from water, or it contains a significant amount of impurities that thicken the medium, it is more reliable to install a knife gate valve. They have good tightness, simplicity and uniqueness of design, as well as small dimensions. The price of knife gate valves ranges from cheap domestic and Chinese to expensive European ones. The simplicity and reliability of the valve design will allow you to achieve maximum service life with proper preventive care, and compatibility with various types of actuators makes it easier for the operator to
open / close the valve.
