When the gate valve is in the closed position, the sealing surface relies on the pressure of the medium to establish a seal. It means that the sealing surface of the gate is pressed against the valve seat on the other side only by the medium pressure, ensuring a self-sealing effect. However, in most cases, gate valves require external force to achieve a tight seal. This means that when the valve is closed, the gate needs to be forcefully pushed against the valve seat to ensure the tightness of the sealing surface. The external force is essential in achieving a secure seal.
A rising stem gate valve, also known as a straight-line gate valve, has a gate that moves in a straight line with the valve stem. The lift rod typically has trapezoidal threads, which, along with the nut at the top of the valve and the guide groove on the valve body, convert the rotary motion into linear motion. This conversion allows the operating torque to be transformed into operating thrust.
When the gate valve is fully opened, the lift height of the gate is equal to 1:1 times the diameter of the valve, ensuring a completely unobstructed fluid channel. However, this position cannot be monitored during operation. Instead, the apex of the valve stem is used as a reference point, indicating the position where the valve cannot be opened further. This position is considered the fully open position.
To account for potential locking caused by temperature changes, it is common practice to open the valve to the top position and then turn it back 1/2-1 turn. This position is regarded as the fully open position. Therefore, the determination of the fully open position is based on the gate's position or stroke.
Some gate valves have stem nuts attached to the gate. The rotation of the handwheel drives the valve stem to rotate, causing the gate to lift. These valves are referred to as rotary stem gate valves or dark stem gate valves.