A gate valve refers to a valve, of which the switch device (the gate) is in a vertical motion trail along the central path. It is mainly used for cutting off the media in the pipeline.
The gate valve has a wide application range from the general cut-off devices with DN ≥50mm to the miniature ones sometimes.
Here is a list of its advantages.
1. Minor fluid resistance.
2. Less external efforts to switch off.
3. No limits to the direction of the flow.
4. Less erosion to the sealing surface by the medium in a full open than that of the block valve.
5. A simple structure with an excellent casting technique.
However, it has shortcomings.
1. Spacious room for installation due to its shape and the opening height.
2. Incidental abrasion between sealing surfaces by the switch on-and-off.
3. Two sealing surfaces embarrass the processing, grinding and maintenance.
1. Classification by the structure
The parallel gate valve: A gate valve with two parallel sealing surfaces, which are parallel to the vertical central line.
Among parallel gate valves, the thrust wedge valve is the most common one with a double-sided thrust wedge installed between the two gates. It is for low-pressure media and cut-off devices with DN 40-300mm. Some parallel gate valves have a spring between the two gates, hence a tightening force to a better sealing of the gate.
The wedge gate valve: A gate valve with two wedge-shaped sealing surfaces, which form into a certain angle with the vertical central line.
Mainly based on the medium temperature, the angle of the sealing surface generally is 2°52´，3°30´，5°， 8° and 10°. The higher the working temperature, the greater the angle to reduce the possibility of stuck.
The wedge gate valve can be further divided into single gated, double gated and elastic gated ones. The single-gate wedge gate valve is simple structured and reliable but its the high accurate angle embarrasses the processing and maintenance. Meanwhile, it has the possibility of being stuck in the case of the changeable temperature. The double-gate wedge gate valve is widely used in water and steam pipelines. Its advantages are low requirements of angle accuracy, the withstandability of changeable temperature and make-up gaskets for the worn sealing surface. However, it has plenty of parts and tends to be stuck in the viscous medium, which may result in leakage. Its key defect is that the upper and lower baffles are rust-prone consequently causing a fall-off gate. Advantaged with the similar simple structure of the single gated one, the elastic-gate wedge gate valve can be elastic deformed to make up for the deviation in the angle processing. Therefore, it is massively adopted.
2. Classification by the stem
The outside stemmed gate valve : By the rotating stem nut in the bonnet or bracket, the stem can be lifted to achieve the gate valve’s on-and-off. This structure is extensively applied because of its lubrication favourable to the stem and obvious on-and-off to the operation.
The inside stemmed gate valve : The stem nut inside the body is directly in contact with the media. By the rotating stem to achieve the gate valve’s on-and-off. Its advantages are that it suits the large calibre or limited installation space for the unchangeable total height depresses the installation space in need and that it provides an indicator to show its on-and-off. While the drawback is that the stem threads can neither be lubricated nor be erosion-proofed in the case of a direct medium contact. Therefore, it is easy to be damaged.
The diameter is different somewhere in the body channel (often the case is that the size of the seat is smaller than that of the flange connection) , which is known as the diameter shrinkage.
The shrinkage can shrink the size and enlarge the application range of the parts and reduce the effort required to switch on-and-off. On the other hand, the path shrinks leading to an increase of fluid resistance loss.
Under certain working conditions (such as the oil pipeline) , gate valves with shrinkage are rejected in order to reduce the loss of pipeline resistance and to facilitate the mechanical cleaning process.