The cut-off valve belongs to the forced seal type valve, so when the valve closes, must exert pressure on the disc to force the sealing surface not to leak. When the media is entered through the valve under the disc six o'clock, operating force need to overcome the resistance, is the stem and filler friction and pressure from the media generated by the force, the valve closed than open the force of the valve, so the stem diameter should be large, otherwise the valve stem top bending failure. In recent years, from the seal of the valve appears, the cut-off valve of the media flow over the disc into the valve cavity, at this time under the pressure of the media, close the valve force is small, and open the valve of the force, valve stem diameter can be correspondingly reduced. At the same time, in the medium action, this form of valve is also more stringent.
China's valve "three to" has stipulated that the cut-off valve flow, all use Top-down. When the cut-off valve is open, the opening height of the disc is the nominal diameter of the 25%~30%, the flow has reached the maximum, indicating that the valve has reached the full open position. Therefore, the full open position of the cut-off valve should be determined by the stroke of the disc.