When a gate valve is closed, the sealing surface can be sealed solely by the pressure of the medium. This means the medium pressure forces the gate's sealing surface against the valve seat to ensure a seal; this is called self-sealing. Most gate valves use forced sealing, meaning that when the valve is closed, external force is required to press the gate against the valve seat to ensure a tight seal.
Motion: The gate of a gate valve moves linearly along with the valve stem; this is also called a rising stem gate valve. Typically, there is a trapezoidal thread on the rising stem. Through the nut at the top of the valve and the guide groove on the valve body, the rotational motion is converted into linear motion, that is, the operating torque is converted into operating thrust. When opening the valve, when the gate's height is equal to 1:1 times the valve's nominal diameter, the fluid passage is completely unobstructed. However, this position cannot be monitored during operation. In actual use, the top of the valve stem, the position where it cannot be opened further, is considered the fully open position. To account for potential lock-up due to temperature changes, the valve is typically opened to its maximum position and then reversed 1/2 to 1 turn to achieve the fully open position. Therefore, the fully open position of the valve is determined by the position of the gate (i.e., the stroke). Some gate valves have the stem nut located on the gate; rotating the handwheel drives the valve stem to rotate, thus raising the gate. This type of valve is called a rotary stem gate valve or a non-rising stem gate valve.
