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Introduction to Basic Knowledge of Gate Valves

Gate valve refers to a valve whose closing element (gate) moves vertically along the centerline of the passage. Gate valves are mainly used for cutting off in pipelines.

Gate valve is a widely used type of valve, generally used for cutting devices with a diameter of DN ≥ 50mm. Sometimes, gate valves are also used for cutting devices with very small diameters.

1、 Advantages of gate valves

1. Low fluid resistance.

2. The external force required for opening and closing is relatively small.

3. The flow direction of the medium is not restricted.

4. When fully open, the sealing surface is less susceptible to erosion by the working medium than the globe valve.

5. The body shape is relatively simple and the casting process is good.

2、 Disadvantages of gate valves

1. The external dimensions and opening height are both relatively large. The installation requires a large amount of space.

2. During the opening and closing process, there is relative friction between the sealing surfaces, which can easily cause abrasion.

3. Gate valves generally have two sealing surfaces, which add some difficulties to processing, grinding, and maintenance.

3、 Types of gate valves

1. According to the structure of the gate, it can be divided into

1) Parallel gate valve: a gate valve whose sealing surface is parallel to the vertical centerline, meaning that the two sealing surfaces are parallel to each other.

In parallel gate valves, the structure with thrust wedges is the most common, with double-sided thrust wedges between the two gate plates. This type of gate valve is suitable for low-pressure small and medium-sized (DN40-300mm) gate valves. There are also springs between the two gate plates, which can generate preload force and facilitate the sealing of the gate plates.

2) Wedge gate valve: A gate valve with a sealing surface at a certain angle to the vertical centerline, where the two sealing surfaces form a wedge shape.

The inclination angle of the sealing surface generally includes 2 ° 52 ′, 3 ° 30 ′, 5 °, 8 °, 10 °, etc. The size of the angle mainly depends on the temperature of the medium. The higher the working temperature, the larger the angle should be taken to reduce the possibility of wedging during temperature changes.

In wedge gate valves, there are single gate valves, double gate valves, and elastic gate valves. Single gate wedge gate valve has a simple structure and reliable use, but it requires high accuracy in the sealing surface angle, making it difficult to process and maintain. There is a high possibility of wedging when the temperature changes. Double gate wedge gate valves are commonly used in water and steam medium pipelines. Its advantages are: lower precision requirements for the sealing surface angle, less risk of wedging caused by temperature changes, and the ability to compensate with gaskets when the sealing surface wears out. But this type of structural component has many parts and is prone to sticking in viscous media, affecting sealing. More importantly, the upper and lower baffles are prone to rust and the gate is prone to detachment after long-term use. Elastic gate wedge gate valve has the advantages of simple structure and easy use of single gate wedge gate valve. It can also generate trace elastic deformation to compensate for the deviation of sealing surface angle during processing, improve processability, and has been widely adopted.

2. According to the structure of the valve stem, gate valves can be divided into

1) Bright stem gate valve: The valve stem nut is attached to the valve cover or bracket. When opening and closing the gate, the valve stem nut is rotated to achieve the lifting and lowering of the valve stem. This structure is beneficial for the lubrication of the valve stem and has a clear degree of opening and closing, so it is widely used.

2) Hidden stem gate valve: The valve stem nut is located inside the valve body and comes into direct contact with the medium. When opening and closing the gate, use a rotating valve stem to achieve this. The advantage of this structure is that the height of the gate valve remains constant, so the installation space is small, making it suitable for gate valves with large diameters or limited installation space. This structure should be equipped with an opening and closing indicator to indicate the degree of opening and closing. The disadvantage of this structure is that the valve stem thread not only cannot be lubricated, but also directly accepts the erosion of the medium, which is easy to damage.

4、 Diameter shrinkage of gate valve

If the diameter of the channels inside 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 shrinkage.

Shrinkage of the diameter can reduce the size of the parts, correspondingly decrease the force required for opening and closing, and expand the application range of the parts. But after the diameter shrinks. The fluid resistance loss increases.

Under certain working conditions in certain departments (such as oil pipelines in the petroleum sector), it is not allowed to use valves with reduced diameter. On the one hand, this is to reduce the resistance loss of the pipeline, and on the other hand, it is to avoid obstacles to mechanical cleaning of the pipeline caused by diameter shrinkage.