Mandatory seal the necessary condition is that the media Thanksgiving and operating force, the sealing surface can still maintain a certain pressure of the non-Genuine. It should be emphasized that in a forced seal, the media pressure tends to reduce the pre-tight seal pressure to lower the seal performance. The typical structure of forced seal is flat pad seal, wound pad seal and toothed pad seal. Usually used for low pressure, medium pressure and small diameter of the valve. (2) Prior to self-tightening of the valve, tighten the bolts to raise the valve cover so that the initial sealing condition between the bonnet and the wedge seal and between the valve body and the wedge seal - on the sealing surface The preload than the pressure. When the media pressure rises, the valve cover by the media pressure, moving up, the valve cover and the wedge seal, and the valve body and the wedge seal between the seal than the pressure, increasing with increasing pressure. In the self-tight seal, the sealing surface of the working pressure than the pressure from the synthesis of two parts: First, pre-tight than the pressure seal, the second is the media pressure formed by the pressure. It should be emphasized that: Self-tight seal, the medium pressure tends to increase the pre-tight seal than the pressure to increase the sealing performance. The higher the media pressure, the greater the work pressure ratio, the more sealing performance. According to this feature, since the tight seal as a high-pressure sealing technology, commonly used in high temperature, high pressure large diameter control valve. Self-tight seal in accordance with the media pressure acting on the gasket in the direction of force, but also can be divided into axial self-tight seal and radial self-tight seal. Self-sealing axial wedge pad seal (Wood Seal), wedge seal, flat pad self-tight seal, C-ring seal and O-ring seal. Radial self-tight seal with a double cone seal, B-ring seal, triangular pad seal, octagonal ring (oval ring) seal and lens pad seal.