The opening & closing part of gate valve is gate disc, its movement direction is vertical to vertical to that of liquid, and the gate valve is only used to open and close fully, but not to adjust the flux. The disc has two seal surfaces, for the most general wedge gate valve ,its two seal surfaces form the wedge each other, and the wedge angle is different for different valve parameters, it is 5°generally, and 2°52＇when the medium temperature is not high . The disc of wedge gate valve may be made into a whole, i.e. the rigid gate disc, or may be made into a kind of little distortion disc, which is available to improve its craft work and makes up the deviation of seal surface angle caused in the processing course, I . e .the flexible gate disc.
　　When the gate valve is closed, the seal surface iy available to seal by the pressure of medium, i.e. relies on the medium pressure to push the seal surface of gate disc down to the seat on the other side, which is just called self-sealing. Most of gate valves adopt the compulsive seal , i.e. when the valve is closed, it relies on external force to compulsively push gate disc down to valve seat to ensure the seal of seal surface.
　　Which its gate plate moves in straight line together with valve stem, is called rising stem type gate valve(also outside screw rising stem gate valve). Genrally, the ladder-shape screw on rising stem changes the rotary movement into a straight-line movement through the nut to top of valve and the guidance on valve body. i.e.changes the operation torsion into operation thrust.
　　When open the valve , and the gate disc rises to the height as much as the valve diameter, the liquid passage is through fully,but it can’t be monitored on this position. In the practical operation, it uses the peak of valve stem as the mark, i. e. the non-lifted position as its full-open position. Considering that the change of temperature probably results in locked status, the stem is usually returned 1/2~1 round when the gate disc rises to the top position, i. e. as the full-open position. Therefore, the full-open position is decided by the position of gate disc(i.e. the stroke of gate disc).