In various types of valves, the gate valve is used more widely. Gate valve refers to the valve that closes the gate along the vertical direction of the channel axis to open or close the valve. In the pipeline, it is mainly used to connect or cut off the medium, that is, fully open or fully closed. General gate valve can not be used as a throttle, it can be used for high temperature and high pressure, can also be used for low temperature and low pressure, and can be used for a variety of different media. Gate valve is generally not used in the conveyor belt particles, viscous, easy scarring medium pipeline. The gate valve has the following advantages: ① the fluid resistance is relatively small, safe and reliable; ② the torque required for opening or closing is small; ③ can be used in the ring network pipeline where the medium flows in two directions, that is to say, the flow direction of the medium is not restricted; ④ when fully open, the sealing surface is less eroded by the working medium than the stop valve; ⑤ the shape is relatively simple and easy to cast and process; ⑥ the structure length is relatively small.

Because the gate valve has many advantages, it is widely used. Usually nominal size DN≥ 50mm pipeline as a device to cut off the media are selected gate valve, even in some small diameter pipeline (such as DN15 ~ 40mm) still retain a part of the gate valve.

Gate valves also have some shortcomings, mainly: ①The external dimensions and opening height are large, and the installation space required is also large; ②During the opening and closing process, there is relative friction between the sealing surfaces, and the wear is large, which is easy to cause scratches. Phenomenon; ③General gate valves have two sealing pairs, which adds some difficulties to processing, grinding and maintenance; ④Opening and closing time is long.

Gate valves have a variety of different structural forms, the main difference is the use of different structural forms of sealing elements. Gate valves are generally classified into two different types, I .e., parallel gate valves and wedge gate valves, depending on the structure of the sealing element. According to the structure of the valve stem, it is also divided into rotating lifting rod (open rod) and rotating rod (dark rod) gate valves.

1. Parallel gate valve

The two sealing surfaces of the parallel gate valve are perpendicular to the pipeline axis, that is, the two sealing surfaces are parallel to each other, as shown in Figure 2-1.

In parallel gate valves, the structure with thrust wedge is the most common. Figure 2-1a and B are parallel gate valves with a double-sided thrust wedge located in the middle of the two gate plates, which are used for gate valves with low pressure, medium and small nominal sizes (DN50 ~ 400mm). Figure 2-1c is a parallel gate valve with a spring between two gates. The spring can produce the necessary pre-tightening force, which is conducive to the sealing of the gate. Figure 2-4 shows that the two rams are connected by V-shaped concave and convex. After closing, the ram with convex V-shaped is in contact with the bottom surface of the valve body. When closing again, the ram is opened to generate sealing force. In addition, there are parallel gate valves with mechanical devices (such as levers, screw mechanisms, etc.), parallel gate valves with only one sealing pair and parallel gate valves with only one sealing pair when opened, and parallel gate valves with single gate or double gate plates with floating sleeve valve seats. These structures are currently used only under special working conditions.

In the process of oil and natural gas exploitation and the pipeline of oil, natural gas and refined oil, the single-plate or double-plate parallel gate valve with floating valve seat is widely used, as shown in Figure 2-2, Figure 2-3 and Figure 2-4. This kind of parallel gate valve has small fluid resistance, reliable sealing performance and long service life. It can clean pipelines through pigging balls and is suitable for pipelines with large, high parameters and automation requirements.

2. Wedge gate valve

The two sealing surfaces of the wedge gate valve are at an angle to the pipeline axis, that is, the two sealing surfaces are wedge-shaped gate valves, as shown in Figure 2-5. Such wedge-shaped rams consist of one or two rams. The size of the wedge gate angle (2 φ) has an important influence on the performance of the wedge gate valve. When the angle is small, the force required to close the gate valve is reduced. However, when the pipeline is deformed due to temperature changes, the wedge gate The possibility of being wedged in the valve body increases.