What are the structure type and use of the gate valve?

What is a gate valve?

The opening and closing parts (gate plate) are driven by the valve stem, along the valve seat (sealing surface) to do a linear lifting movement of the valve, called the gate valve.

What is the purpose of the gate valve?

Gate valve is a kind of cut valve, used to connect or cut the medium in the pipeline. Gate valve use range is wide, the current production of common gate valve performance parameters range is: nominal pressure PN1~760; nominal size DN15~1800;

 What kinds of gate valves do there are?

According to the structural form, the gate valve can be divided into the following types: elastic gate, rigid gate, parallel rigid gate, wedge rigid gate, double gate, single gate, fast opening and closing valve, shrink gate valve and so on.

What are the characteristics of the gate valve?

① Advantages of the gate valve

1. Fluid resistance is small. Because the internal medium channel of the gate valve body is straight through, the medium flows through the gate valve does not change its flow direction, so the fluid resistance is small.
2. The opening and closing resistance is small. Because when the gate valve opens and closes, the direction of the gate movement is perpendicular to the direction of the medium flow, and compared with the stop valve, the opening and closing of the gate valve is less energy saving.
3. The medium flow direction is not restricted. The medium can flow from any direction of both sides of the gate valve, which can achieve the purpose of use More applicable in lines where the flow direction of the medium may change.
4. The structure length is shorter. Because the gate plate of the gate valve is vertical " straight in the valve body, and the stop valve disc is horizontal in the valve body, so the structure length is shorter than the stop valve.
5. Good sealing performance. The erosion of the sealing surface is small in full opening.

② Disadvantages of the gate valve

1. The sealing surface is easy to damage. Gate plate and valve seat phase during opening and closing
2. Long opening and closing time, large height. Because the production of gate valve in the production of gate valve performance parameters are: the nominal pressure must be fully open or closed, the gate stroke is large, the opening needs a certain space, the appearance size is high.
3. Complex structure. More parts, manufacturing and repair is more difficult, the cost is higher than the stop valve.

 What structure is the gate valve?

The gate valve is mainly composed of valve body, valve cover or bracket, valve stem, valve stem nut, valve plate, valve seat, packing function, sealing packing, packing pressure cover and transmission device. As shown in Figure 1-1 is a typical flange-type connecting open bar wedge elastic single gate valve. For large diameter or high pressure gate valve, in order to reduce the opening and closing torque, the bypass valve (stop valve) can be connected in parallel on the inlet and exit pipe near the valve. When used, open the bypass valve first to reduce the pressure difference on both sides of the gate, and then open the gate valve. The nominal diameter of the bypass valve is not less than DN32.

Figure 1-1 Open rod wedge elastic single gate valve

① The valve body is the main body of the gate valve, which is directly connected with the pipe or (equipment), and constitutes the pressure parts of the medium circulation flow channel. It is an important part for installing the valve cover, placing the valve seat and connecting the pipe. The valve body to accommodate vertical and lifting movement of the disc gate, so the height of the valve body cavity is large. The shape of the valve body section mainly depends on the nominal pressure, such as the low pressure gate valve body can be designed flat to reduce the structural length; the middle cavity of the high pressure gate valve body is mostly designed to elliptical gradually round or round to improve its bearing capacity and reduce the wall thickness. The medium channel of the valve body is mostly circular section, for larger gate valve, in order to reduce the size of the gate, the opening and closing force and torque, can also use the form of the contraction, the contraction, will increase the valve fluid resistance, pressure drop and energy consumption increase, so the channel shrinkage ratio should not be too big, usually the diameter of the seat channel and nominal diameter ratio of 0.8~0.95, contraction channel bus to the centerline tilt Angle is not greater than 12°, see Figure 1-2.

Figure 1-2 Electric wedge double gate valve

The structure of the gate valve body determines the connection between the valve body and the pipe, and between the valve body and the valve cover. Valve body blank can be used in casting, forging, forging welding, casting welding and pipe plate welding. Casting valve body is generally used for DN50 diameter, forging valve body is generally used for DN5 diameter, forging welding valve body is used for the overall forging process is difficult, and for important occasions, casting welding valve is used for the overall casting can not meet the requirements, cast welding structure can be used.

② The bonnet is connected with the valve body and with the valve body constitutes the main pressure component of the pressure chamber, with a packing function. For medium and small diameter valves, the valve cover is equipped with supporting valve rod nuts or transmission gear parts.

③ The support is connected to the bonnet for supporting the valve stem nuts or transmission parts.

④ The valve stem is directly connected with the valve stem nut or transmission device, the light stem part and the filler form a sealing pair, can transfer the torque to play the role of opening and closing the gate plate, according to the position of the thread on the valve stem clear stem gate valve and dark stem gate valve.

1. Open bar gate valve: the valve stem does the lifting movement, the transmission thread in the gate valve in the outside of the body cavity. The lifting of the valve stem is achieved by the rotation of the valve stem nut, the valve rod nut can only rotate, and no up and down displacement, see Figure 11, is beneficial to the lubrication of the valve stem, the gate plate opening is clear, the valve stem thread and the valve stem nut do not contact with the medium, not affected by the medium temperature and corrosion, so it is widely used.
2. Dark rod gate valve: the valve valve with the valve rod is thread in the body cavity. Stem lifting is driven by rotating the stem stem nut, the valve rod can only rotate, without the up and down displacement, see figure 1-3, the height of the valve size is small, its opening and closing travel is difficult to control, need to increase the rod, valve rod thread and medium contact, affected by the medium temperature and corrosive, so suitable for non-corrosive medium and poor external environment conditions.

Figure 1-3 Dark rod wedge gate valve

⑤ Valve rod nut and valve rod thread group constitute the parts of the motion pair, can be directly connected with the transmission device, can transfer torque.

⑥ The transmission device can transmit electric power, pneumatic, hydraulic force and manpower directly to the stem or stem nut. In power plants, hand wheel, valve bonnet, transmission mechanism, connecting shaft and universal coupling are often used for long-distance drive.

⑦ Valve seat with rolling, welding, thread connection and other methods to fix the valve seat on the valve body and the sealing parts of the gate. The seat seal ring can directly weld the metal on the valve body to form the seal surface. For cast iron, austenitic stainless steel and copper alloy valves, the sealing surface can also be processed directly out on the valve body.

⑧ In the packing letter (packing box), the stop medium leaks along the valve stem.

⑨ The packing lid is pressed through bolts and nuts to press the packing to prevent the parts of the medium from leakage along the valve stem.

⑩ Gate valve is the opening and closing parts of the gate valve, the opening and closing of the gate valve and the sealing performance and life are mainly determined by the gate valve, it is the key pressure control parts of the gate valve. According to the structure of the gate form can be divided into two types,

1. Parallel gate valve: the two sealing surfaces of the gate are parallel to each other and perpendicular to the center line of the valve body channel. It is divided into parallel single gate and parallel double gate two kinds.
2. Wedge gate valve: the sealing surface and the vertical centerline of the gate are symmetrical into a certain inclination, called the wedge half angle. The size of the wedge half angle mainly depends on the temperature of the medium and the size of the diameter. Generally, the higher the temperature of the medium, the larger the diameter, the smaller the half angle of the wedge is. The wedge half angle of the common wedge gate is 2°52'and 5°. The wedge gate valve is divided into wedge single gate, elastic gate and wedge double gate.

The structure and characteristics of the gates are shown in Table 1-1.

Table 1-1 Structure and characteristics of the gate plate