Gate valve refers to the closed parts (wedge) moves along the central line of the vertical direction. Gate valves are mainly used for cutting in the pipeline. Gate valve is used widely, especially most of DN≥50mm cutting devices, sometimes small diameter cutting devices also adopt gate valves.
I. Advantages of Gate Valves
1. Fluid resistance is very low.
2. It is small that the external force for opening and closing.
3. The flow direction is unrestricted.
4. When fully opened, the erosion of gate valves’
sealing surface is much less than globe valves’.
5. The shape is relatively simple, and the casting procedure is better.
of Gate Valves
1. Both the dimension and the opening height are large. The space required for installation is also large.
2. In the process of opening and closing, there is relative friction between the sealing surfaces, which is liable to cause scuffing.
3. Gate valve generally has two sealing surface, processing, grinding and maintenance to increase some difficulties.
of Gate Valves
1. Wedge Structure
1） Parallel gate valve: the sealing surface is parallel to the vertical central line, that is, the two sealing surfaces are parallel to each other. Thrust wedge structures are most common in parallel gate valves, that is, double-sided thrust voussoir is located between the two gates. Also, the spring could be located between the two gates, the spring can produce the tightening force which is useful to the sealing of gates. This kind of gate valves is usually used in low pressure and small size（DN40—300mm） condition.
gate valve: the sealing surface is in a certain angle from the vertical
centreline, i.e., the two sealing surfaces are wedge shaped gate valves. The inclined angle of the sealing
surface generally includes 2°52´, 3°30´, 5°, 8°, 10°etc., and the size
of the angle mainly depends on the temperature of the medium. In general, the
temperature is higher, the angle should be taken greater to reduce the
possibility of wedging when the temperature is changed.
In the wedge gate valve, there are single wedge gate valve, double wedge gate valve and flexible wedge gate valve. Single wedge gate valve is characterised by simple structure and reliable use, but the precision of sealing surface angle is higher, and processing and maintenance are more difficult, it is very easily wedged when the temperature changed. Double wedge gate valves are usually used in water and steam medium lines. Its advantages are the precision of sealing surface angle is low, and it is not easy to wedge when the temperature changed. Also, if the sealing surface weared, the gaskets can be added to apply the compensation. But this kind of structure has too many assemblings, it is easy to bond in the viscous medium and influence the sealing performance. It is much more important that the upper and lower baffles are easy to rust when used for a long time, and the wedge is easy to fall off. Flexible wedge gate valve, it has the advantages of simple structure and reliable operation same as the single wedge gate valve, and also it can produce deviation micro elastic deformation of the cover sealing surface angle in the process, which improves the process and now widely adopted.
2. Stem Structure
1）O S&Y Gate Valve：Stem nuts are located on the valve bonnet or yoke. When opening and closing the wedge, rotates the stem nut to achieve the lifting of the stem. This structure is beneficial to the lubrication of the valve stem, and the degree of opening is obvious, so it is widely used.
2）Inside Screw Gate Valve： The stem nut is in the valve body and also directly contacts with the medium. When opening and closing the wedge, rotates the stem nut to achieve. The advantages of this structure include that the height of the gate valve is not changed, so the installation space is small, and also suitable for large size or limited installation space. The structure shall be equipped with an opening and closing indicator to indicate the degree of opening and closing. The disadvantage of this structure is that stem threads are not only lubricated, but also subject to erosion by media and are easily damaged. The disadvantage of this structure is that stem screws are not easily lubricated and damaged, also subject to erosion by media.
Shrinkage of Gate Valve
The internal diameter of a gate valve is different (usually the diameter of valve seat is smaller than the flange joint), it is called path contraction. The path contraction can reduce the size of parts and the force of opening and closing, and also enlarge the application scope of parts. But the fluid resistance loss is increased when the path is contraction. In some departments under certain operating conditions (such as the petroleum pipeline), the valves with the path contraction are not allowed. This is to reduce the resistance loss of the pipeline, on the other hand, in order to avoid the obstacles for the mechanical cleaning pipeline after the path is contracted.
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