Gate valve: gate valve is a kind of valve which is driven by the valve stem and moves up and down along the sealing surface of valve seat and guide rail. It mainly includes: valve body, valve cover, gate, stem, packing, gasket and operating device. Gate valve is divided into wedge gate valve and flat gate valve. Wedge gate valve is a widely used type of gate valve, which can be used in high temperature, high pressure, low temperature, large diameter, wear resistance and other conditions. Flat gate valve is mainly used for transporting natural gas and product oil. The so-called wedge gate valve refers to the wedge gate valve (thick at the top and thin at the bottom). Generally, for wedge gate valve, the rigid single gate valve (z41 or z61) shall be used when the "nominal size" DN ≤ 40; for "nominal size" DN ≥ 50, the elastic gate valve (Z40 or Z60) shall be used.
Advantages of gate valve:
1) low fluid resistance;
2) the torque required for opening and closing is small;
3) it can be used in the loop network pipeline with medium flowing in two directions, that is to say, the flow direction of medium is not limited;
When the valve is fully opened, the erosion of the sealing surface by the working medium is smaller than that of the stop valve;
5) simple structure and good manufacturing technology;
The length of the structure is short.
Because of the above advantages of gate valve, it is widely used. Generally, gate valve is used for pipeline with DN ≥ 50mm as cutting off medium.
The gate valve also has the following disadvantages:
1) the overall dimension and opening and closing height are large, and the installation space is large;
· 2) in the process of opening and closing, the sealing surface of the gate valve has relative friction and wear, and it is easy to cause abrasion even at high temperature;
3) generally, gate valves have two sealing pairs, which makes it difficult to process, grind and maintain;
4) long opening and closing time.
In the design of steel wedge gate valve, there were only two kinds of rigid single gate valve and double gate valve in the past. Rigid single gate valve is commonly known as "dead gate valve", which is represented by z41; double gate valve is to hang two single gates on the gate frame, which is represented by z42. The rigid single valve plate is not adjustable because the gate and the valve seats on both sides of the valve body are matched, so the "matching plate" precision is very high when assembling the valve. And for high and medium pressure large caliber gate valves, when the body cavity of the valve body is deformed under pressure or under high temperature, the rigid single gate itself has no compensation function, so the reliability of the seal is not good, and it is easy to cause internal leakage. Sometimes, in order to force its sealing, it is necessary to tighten the ram by increasing force, which often causes accidents such as "wedging" the ram or bending the valve stem. Due to the structure of double ram is not compact, the reliability is not good, the valve volume is large, the weight of the valve is heavier, resulting in the increase of user network load. In particular, the double ram is not suitable for high pressure, strong impact conditions and heavy oil, slurry, catalyst particles and other media.
In order to solve the shortcomings and disadvantages of single gate and double gate, a new structure - "elastic gate" was born in gate valve design, which is a significant technical progress in gate valve design. The so-called "elastic RAM" refers to the ram edge with "elastic groove", and its middle is still an integral structure (the elastic groove can be cut after welding, casting or forging). The elastic ram has both the advantages of single ram and double ram, but abandons their disadvantages. When the middle cavity of the elastic ram is pressed, the RAM will produce elastic deformation and cling to the valve seats on both sides of the valve body, so it can achieve reliable sealing. When the pressure is released, the elastic deformation of the ram disappears, so it is convenient to open, with small wear and long service life. It is especially suitable for gate valves with large diameter, high temperature and low temperature. It should be pointed out that: except for the special gate valve with special design, the gate valve is not allowed to be used for regulating the medium flow. Therefore, when opening, all the gate valves must be lifted above the channel. For high-speed flow medium, the vibration of the gate can be caused by the partial opening of the gate, and the vibration may damage the sealing surface of the gate and the valve seat, and the throttling will cause the gate to be eroded by the medium.