Features, Types and Structures of Gate Valves

1. Features of gate valve

Gate valves refer to the valve whose plate move in the vertical direction along the path centerline. A gate valve is mainly used for cutting off  the medium in pipelines.
A gate valve is a kind of widely used valve. Generally, it is selected as the cutting device with a DN greater than or equal to 50mm, and sometimes it is also selected as the cutting device with small calibers. 

Gate valve has the following advantages:
1. The fluid resistance is small.
2. The required external force of opening and closing is small.
3. The flow direction of media is unlimited.
4. When the gate valve fully opens, the erosion of sealing surface by working medium is smaller than that of globe valves.
5. The body shape is relatively simple and the casting technology is better.

Gate valves also have the following disadvantages:
1. The overall dimension and opening height are large. The required installation space is large.
2. In the process of opening and closing, there is relative friction between sealing surfaces of gate valves, which is easy to cause scratches.
3. Gate valves generally have two sealing surfaces, which make it difficult to be processed, ground and maintained.

2. Types of gate valves

A. According to the structure of ram, gate valves can be divided into the following types:

(1) Parallel gate valve
The sealing surface of this kind of gate valve is parallel to the vertical center line and two sealing surfaces are parallel to each other.
Among parallel gate valves, the most common structure is the valve with thrust wedges. There are double thrust wedges in the middle of two gate valves. This gate valve is suitable for low-pressure small and medium calibers (DN40 to 300mm) gate valves. There is also a spring between the two valves discs. The spring can produce a pre-tightening force, which is conducive to the sealing of the valve disc.

(2) Wedge gate valve
The sealing surface of this kind of gate valve has an angle with the vertical center line and the two sealing surfaces are wedge shaped.
Generally, the inclination angle of sealing surface is 2°52´, 3°30´, 5°, 8° and 10°. The degree of inclination mainly depends on the medium temperature. Generally, the higher the working temperature is, the larger the angle should be to reduce the possibility of wedging when the temperature changes.

There are single gates, double gates and elastic gates for wedge gate valve. Single gate wedge gate valves are simple in structure, but they require high accuracy of sealing surface angles. It is difficult to process and maintain the single wedge, and is very likely to be wedged when the temperature changes. The double gate wedge gate valve is widely used in the water and steam medium pipeline. ​Its advantages are  as the following: 

The accuracy of the sealing surface angle is low. The temperature change is not easy to cause wedging. When the sealing surface is worn, gaskets can be added to compensate. However, there are many parts of this structure, which are easy to bond in the viscous medium and affect the sealing. The main reason is that the upper and lower baffles are easy to be rusted after long-term use, and the valve disc is easy to fall off. The wedge gate valve with elastic gates has the advantages of simple structure and being easy to use. It can also produce a small amount of elastic deformation to make up for the deviation in the processing of sealing surface angle, and improve the processability. It has been widely used.

B. According to the structure of the valve stem, the gate valve can be divided into the following types:

(1) Rising stem gate valves
When the stem nut is on the bonnet or bracket and the gate is opened and closed, rotate the stem nut to realize the rising and falling of the stem. This kind of structure is advantageous to the lubrication of the valve stem, and the opening and closing degree is obvious. Therefore, it is widely used.

(2) Concealed stem gate valves
The stem nut is in the valve body and directly contact with the medium. When the plate is opened and closed, it is realized by rotating the valve stem. The advantage of this structure is that the height of the gate valve always remains the same, so the installation space is small, which is suitable for gate valves with large diameters or limited installation space. The structure shall be equipped with an opening and closing indicator to indicate the opening and closing degree. The disadvantages of this structure are that the stem thread is not only unable to be lubricated, but also directly eroded by the medium and easy to be damaged.