When the
gate valve is closed, the sealing surface can only rely on the medium pressure to seal, that is, only 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 also called a rising stem 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 the 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 unblocked, 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 due to temperature changes, it is usually opened to the top position, and then back to 1/2-1 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). Some gate valve stem nuts are set on the gate, and the rotation of the handwheel drives the valve stem to rotate, which lifts the gate. This kind of valve is called Rotary stem gate valve or dark stem gate valve.
