Gate valve refers to a valve in which the closing member (gate) moves in the vertical direction of the centerline of the passage. The gate valve
can only be used to cut off fully open and fully closed in the pipeline, and cannot be used for adjustment and throttling. Gate valve is a kind of valve with a wide range of use. Generally, it is used for the cut-off device with a diameter of DN50, and sometimes a gate valve is also used for the cut-off device with a small diameter. The gate valve is used as a cut-off medium. When fully open, the entire flow is straight, and the pressure loss of the medium is minimal at this time. Gate valves are usually suitable for conditions that do not require frequent opening and closing, and keep the gate fully open or fully closed. Not intended for use as regulation or throttling. For high-speed flowing media, the gate can cause vibration of the gate when it is partially opened, 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. From the structural form, the main difference is the form of the sealing element used. According to the form of the sealing element, the gate valve is often divided into several different types, such as: wedge gate valve, parallel gate valve, parallel double gate valve, wedge double gate gate valve, etc. The most common forms are the wedge gate valve and the parallel gate valve. The gate has two sealing surfaces. The two sealing surfaces of the most commonly used wedge gate valve form a wedge shape. The wedge angle varies with the valve parameters, usually 5 degrees. The gate of the wedge gate valve can be made into a whole, called a rigid gate; it can also be made into a gate that can produce a small amount of deformation to improve its manufacturability and make up for the deviation of the sealing surface angle during processing. The plate is called an elastic gate. When the gate valve is closed, the sealing surface can be sealed only by the medium pressure, that is, relying on the medium pressure to press the sealing surface of the gate to the valve seat on the other side to ensure the sealing of the sealing surface, which is self-sealing. Most gate valves are forcibly sealed, that is, when the valve is closed, the gate must be forced against the valve seat by external force to ensure the tightness of the sealing surface. The gate of the gate valve moves linearly with the valve stem, which is called a lift-rod gate valve (also called an open-rod gate valve). Usually, there are trapezoidal threads on the lift rod. Through the nut at the top of the valve and the guide groove on the valve body, the rotary motion is changed into a linear motion, that is, the operating torque is changed into an operating thrust. When the valve is opened, when the lift height of the gate is equal to 1:1 times the diameter of the valve, the fluid channel is completely unobstructed, but this position cannot be monitored during operation. In actual use, the apex of the valve stem is used as a sign, that is, the position where it cannot be opened, as its fully open position. In order to take into account the lock-up phenomenon caused by temperature changes, usually open to the top position, and then rewind 1/2 to 1 turn, as the fully open valve position. Therefore, the fully open position of the valve is determined according to the position of the gate (ie, the stroke). Advantages of gate valve: (1) Small fluid resistance Because the medium channel inside the gate valve body is straight, the medium does not change its flow direction when it flows through the gate valve, so the fluid resistance is small. (2) The opening and closing torque is small, and the opening and closing is more labor-saving because the gate movement direction is perpendicular to the medium flow direction when the gate valve is opened and closed. Compared with the globe valve, the opening and closing of the gate valve is more labor-saving. (3) The flow direction of the medium is not restricted, and the medium can flow in any direction on both sides of the gate valve without disturbing the flow or reducing the pressure, which can achieve the purpose of use. It is more suitable for pipelines where the flow direction of the medium may change. (4) The structural length is shorter because the gate of the gate valve is placed vertically in the valve body, while the disc of the globe valve is placed in the valve body horizontally, so the structural length is shorter than that of the globe valve. (5) The sealing performance is good and the sealing surface is less eroded when fully open. (6) When fully opened, the erosion of the sealing surface by the working medium is smaller than that of the globe valve. (7) The shape is relatively simple, the casting process is good, and the application range is wide. Disadvantages of gate valve: (1) The sealing surface is easy to damage the two seals that contact the gate and the valve seat when opening and closing, and there is relative friction between them, which is easy to damage, affects the performance and service life of the seal, and is difficult to maintain. (2) The opening and closing time is long and the height is large. Because the gate valve must be fully opened or fully closed when opening and closing, the gate travel is large, and a certain space is required for opening, the external dimensions are high, and the installation space is large. (3) Gate valves with complex structures generally have two sealing surfaces, which increase processing, grinding and maintenance. There are many difficult parts, manufacturing and maintenance are more difficult, and the cost is higher than that of globe valves. Diameter contraction of gate valve: If the diameter of the channel in a valve body is different (often the diameter at the valve seat is smaller than the diameter at the flange connection), it is called diameter contraction. The diameter shrinkage can reduce the size of the parts, the force required for opening and closing is correspondingly reduced, and the application scope of the parts can be expanded. But after the diameter shrinks. Fluid resistance loss increases. Under certain working conditions in certain departments (such as oil pipelines in the petroleum sector), valves with reduced diameters are not allowed. On the one hand, it is to reduce the resistance loss of the pipeline, and on the other hand, it is to avoid obstacles to the mechanical cleaning pipeline after the diameter shrinkage. The installation and maintenance of the gate valve should pay attention to the following matters: 1. The handwheel, handle and transmission mechanism are not allowed to be used for lifting, and collision is strictly prohibited. 2. The double gate gate valve should be installed vertically (that is, the valve stem is in a vertical position, and the handwheel is at the top). 3. The gate valve with the bypass valve should open the bypass valve before opening (to balance the pressure difference between the inlet and outlet and reduce the opening force). 4. The gate valve with transmission mechanism shall be installed according to the provisions of the product instruction manual. 5. If the valve is frequently switched on and off, lubricate it at least once a month.
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