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Ball valve, is one of the more widely used valves, it appeared very early to promote the use of late. Its working principle is the same as the plug valve, which is to open and close the valve by rotating the valve plug, but it is lighter and lighter than the plug valve switch, so it can be made into a large-caliber valve. Ball valve seal is reliable, simple structure, easy maintenance, the sealing surface and the ball are often closed, not easy to be media erosion. Currently, the diameter of foreign ball valves has reached 3050cm, widely used in petroleum, chemical, metallurgy, textile, food, atomic energy, aviation and other departments. 1, the classification of ball valves Ball valves are divided into two categories: floating, fixed (see Figure 1) a, floating ball valve b, fixed ball valve Figure 1 Floating, fixed ball valve structure 1.1, floating Ball Valve Ball valve ball Is floating, under medium pressure, the ball can produce a certain displacement and squeeze the sealing surface at the outlet end, to ensure that the ball valve seal. Floating ball valve structure is simple, good sealing, but the ball to withstand the load of the working medium all to the export seal, so to consider whether the seal material can withstand the working load of the ball medium. This structure is widely used in low pressure ball valve. 1.2, fixed ball valve Ball valve ball is fixed, does not produce pressure after the move. Fixed ball valve with a floating valve seat, subject to media pressure, the valve seat to move so that the ring is pressed on the ball in order to ensure the seal. Usually with the ball on the lower shaft equipped with bearings, operating torque is small, suitable for high pressure and large diameter of the valve. In order to reduce the operating torque of the ball valve and increase the reliability of the seal, in recent years there have been oil seal ball valves, both in the sealing surface injection of special lubricants to form a layer of oil film, which enhanced the tightness and reduced operating torque , More suitable for high pressure large diameter ball valve. This paper mainly on the floating large-diameter ball valve seal opening and closing torque for a superficial discussion, hoping to help the application of the ball valve. 2, floating ball valve seal Floating ball valve structure is simple, good sealing, but the ball to withstand the load of the working medium are all passed to the export seal, the seal material can withstand the ball bearing media load is the key. In general, seals are made of polytetrafluoroethylene and other elastic materials, metal and non-metallic materials to form a sealing pair, which is commonly known as the soft seal. Sealing the soft seal easy to ensure, but also on the sealing surface of the machining accuracy and surface roughness requirements are not high. Under the pressure of the medium, the ball displaces to the outlet end and compresses the valve seat. At this moment, the force QMJ of the ball body pressing on the valve seat is: Formula 1 Where: DMW- outer diameter of valve seat sealing surface DMN- valve seat sealing surface Internal diameter P-medium working pressure In order to ensure low-pressure sealing, the pre-tightening pressure of the ball valve and the valve seat should not be less than 1200N · M, the pre-tightening seal mainly depends on the elasticity of the valve seat itself, so the width of the sealing surface is constant Restricted by the formula: Type 2 Where: [q] - allowable stress (Teflon allowable stress 1530N · M) For the valve seat with elastic components, its reliability and useful life is very large To a degree, it depends on the correct choice of the pre-compression force of the sealed seat. Under low pressure, under the condition of insufficient pre-tightening force, the sealing of the ball valve can not be guaranteed. The transitional compression will cause an increase of the friction torque between the ball and the valve seat and may cause the plastic deformation of the valve seat material. The gap δ1 between the valve seat and the valve body groove is the basic condition to ensure the normal operation of the ball valve. When selecting the gap δ1, the uniform elongation of the pre-compressed valve seat must be considered within the range of the total elastic deformation. For this purpose, Formula 3 Where: DMP - average diameter of the valve seat; σMJ - Allowable stress of the valve seat cross section in the elastic deformation range, σMJ = 820 (NM) for seat made of polytetrafluoroethylene; Elasticity of seat material Modulus, on the seat made of Teflon, E = 48000 ~ 81600 (NM). Seat sealing materials are generally used PTFE, in the existing plastics, chemical stability is the strongest polytetrafluoroethylene, its stability and chemical stability even more than the precious metal gold and platinum, glass, ceramics, Enamel, special steel and alloy. Chemically corrosive substances - concentrated acid, dilute acid, concentrated lye, the strongest oxidant can not play any role in the PTFE, a "plastic king" said. In addition to some amine halide or aromatic hydrocarbon polytetrafluoroethylene plastic swell a slight phenomenon, the ketones, acids, esters and other organic solvents have no effect on the PTFE, will not occur weight changes and swelling . It can only play a role in the molten alkali metal, chlorine trifluoride and elemental fluorine, but only under high temperature and high pressure was significant. In addition, PTFE is not affected by oxygen or ultraviolet light, non-absorbent, non-flammable. It has a high heat resistance and cold resistance, the use of temperature range -180 ~ 250 ℃, 250 ℃ 240h treatment, no reduction in mechanical properties, 390 ℃ 0.006% weight loss per hour. Its excellent dielectric properties, and electrical properties are not affected by the frequency and temperature, long-term work at 250 ℃. It can be seen from the above, with PTFE as a ball valve seal is more ideal. Teflon elastic modulus value is very large, therefore, the material properties determine its elastic deformation is small, but the long-term use, the material's elasticity is not enough to make up for sealing surface wear, then the ball valve seal will be destroyed Therefore, considering how to structurally increase its elasticity when designing the valve seat, we chose a flexible structure (see Figure 2). Figure 2 seat ring This seat has the following characteristics: (1) the sealing surface has an angle of 40 ° ± 1 '; (2) seal the bottom of the valve seat has a semi-circular groove R2; (3) and the valve body With the gap left δ1 can increase the flexibility. The principle of the structure is as follows: During the assembly without pretightening force, the ball is tangent to point A, the pre-tightening force is adopted to displace the ball and the displacement is generally between 0.2 and 0.4 (mm). When the ball is under pressure, Close to the sealing surface, then the ring outward expansion, so that the sealing surface and the ball will not be too tight and cause torque increases, after the pressure relief, sealing the valve seat is basically restored to the original installation position. Since the tight seal, is a more special form of seal, I plant the production of large-diameter ball valve, stem seal all adopt the structure. Working principle is: the use of media pressure self-tight seal, and its sealing ring mounted inside the cone, with the media side of an angle, the media pressure transmitted to the cone, to pass to the ring, cone at a certain angle There are two components of the surface, one parallel to the centerline of the body outward, the other pressure to the body wall,