Basic Knowledge and Working Principle of Gate Valves
Posted: 10/27/2021 23:19:15 Hits: 9
The opening and closing parts of the gate valve are the gate plate. The movement direction of the gate plate is perpendicular to the direction of the fluid. The gate valve can only be fully opened and closed, and it is not adjusted and throttled. The gate plate has two sealing faces. The most commonly used mode of gate valves is that the two sealing faces form a wedge. The wedge angle varies with valve parameters, usually 50 degrees. The degree of the wedge angle is 2°52’’ when the medium temperature is not high. The wedge gate valve disc can be made into a whole, which can be called the rigid gate. It can also be made into a gate in order to produce slight deformation, improve its technology and compensate for the deviation of sealing face angle in the process of processing. This gate plate is called an elastic gate.
When the gate valve is closed, the sealing surface can only rely on the medium pressure to seal, that is, rely on the medium pressure to press the sealing surface of the gate to the other side of the seat to ensure the sealing surface itself. We called it self-sealing. Most gate valves are force-sealed. When the valve is closed, the gate is forced against the seat by external forces to ensure the sealing face. The disc of the gate valve moves in a straight line with the valve stem.
Usually, trapezoidal threads on the lifting rod convert the rotary motion into linear motion. The operating torque into operating thrust by means of the nut at the top of the valve and the guide slot in the valve body. When the valve opens and the lift height of the gate is equal to 1:1 valve size, the flow path is clear. However, this position cannot be monitored during an operation. In actual use, the vertex of the stem will be used as a mark. The position does not be moved as its full open position. In order to consider the occurrence of locking phenomenon because of temperature changes, we usually open the top position and then rewind 1/2-1 turn as the position of the fully open valve. Therefore, the full open position of the valve is determined by the position of the gate, i.e. the stroke. Some gate valves, stem nuts in the gate are on the valve plate. The hand wheel rotation can drive the stem rotation to lift the gate. The valves are called rotary rod gate valves or non-rising stem gates.
The gate valve has the following advantages:
1. The fluid resistance of the gate valve is small, and the sealing surface is subjected to small media erosion and strong brush.
2. More energy can be saved when we open and close the valve.
3. The flow direction of the gate valve medium is not restricted. No turbulence and no pressure reduction.
4. The gate valve has a simple shape, short structure length, good manufacturing technology and a wide application range.
The disadvantages of gate valves are as follows:
1. It is easy to cause erosion and abrasion between the sealing surfaces of gate valves, and the maintenance is difficult.
2. The gate valve has a large size, requires a certain space to open, and takes a long time to open and close.
3. The gate valve structure is complex. The classification of gate valves can be divided into wedge type gate valves and parallel gate valves according to the sealing surface configuration. Wedge type gate valves can be divided into single gate types, double gate types and elastic gate types. Parallel gate valves can be divided into single gates and double gates. According to the thread position of the valve stem, it can be divided into rising stem gate valves and non-rising stem gates.
Notes for installation and maintenance of gate valves:
1. The hand wheel, handle and transmission mechanism of gate valves are not allowed to be used for lifting, and the collision is prohibited.
2. The double disc of the gate valve should be vertically installed. That is, the stem should be in a vertical position with the hand wheel at the top.
3. Gate valves with bypass valves should be opened before opening to balance the inlet and outlet pressure difference and reduce the opening force.
4. Gate valves with belt transmission mechanism should be installed according to the provisions of the product manual installation.
5. Lubricate the gate valves at least once a month if it is used frequently.