Working Principle of Forged Steel Flange Gate Valves

When a gate valve closes, the sealing surfaces can rely solely on the pressure of the medium to achieve a seal-meaning the medium's pressure forces the gate's sealing surface against the seat on the opposite side; this is known as self-sealing. However, most gate valves employ forced sealing, where external force is required to press the gate against the seat during closure to ensure a tight seal. Gate valves in which the gate moves linearly along with the stem are known as rising-stem (or "open-stem") gate valves. Typically, the stem features trapezoidal threads; a nut at the top of the valve and guide grooves on the valve body convert the rotational motion into linear motion-effectively transforming operating torque into operating thrust. When opening the valve, the fluid passage becomes fully unobstructed once the gate has been lifted to a height equal to the valve's bore diameter; however, this specific position cannot be visually monitored during operation. In practice, the fully open position is determined by the stem's topmost point-the limit of its upward travel. To prevent the valve from seizing due to thermal expansion, it is common practice to back the stem off by half a turn to a full turn from the maximum open position. Thus, the fully open position is defined by the gate's location (i.e., its travel). In some gate valves, the stem nut is located on the gate itself; turning the handwheel rotates the stem, thereby lifting the gate. These are known as rotating-stem or "non-rising stem" gate valves.