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1. Fixed ball valve sealing principle

The opening and closing parts of the fixed ball valve is a sphere with two support shafts, with a channel in the middle equal to the diameter of the pipe, with two floating seats on both sides, the sphere does not deviate from the axis of the support shaft on the sphere when the sphere is turned, the support shaft is supported by the upper and lower two sliding bearings, the seat is floating, under the action of medium force, pushing the sphere to produce a contact stress to obtain a reliable seal. Like the floating ball valve, the valve swings 90° from open to close, or from close to open.

2. Fixed ball valve structural features

Fixed ball valve body consists of the main body, left and right two sub-body, the ball is a journal at the upper end, the lower end has a shaft hole of the ball, the upper part of the body has a stuffing box, and the ball journal is connected to a sliding bearing, the lower end of the ball has a shaft hole, equipped with a sliding shaft bearing sleeve, the bottom of the valve body has a bearing inserted into the shaft hole at the lower end of the ball. This way the ball is kept in the center line of rotation, rotation will not produce deflection. There are two floating seat assemblies on both sides of the ball symmetrically, this seat assembly consists of a metal seat ring, PTFE seal ring embedded in the seat ring, rubber O-ring on the outer edge, and back coil spring. When the valve is closed, the medium force acts on the assembly of the seal seat, making contact between the seat and the ball, generating contact stress, and obtaining an effective seal. The valve stem and stuffing box body are sealed by an O-shaped rubber ring.

The structure of a fixed ball valve is much more complex than a floating ball valve, the principle is that the medium pressure pushes the seat assembly to act on the ball to produce contact stress to obtain a seal, and the ball rotation center axis does not shift, the seat assembly is floating, so it is called fixed ball floating seat structure. Because the medium force is acting on a ring area, the medium thrust is much smaller than the floating ball valve medium force, so the fixed ball valve structure is suitable for high-pressure large diameter, the applicable range of pressure class 150 ~ 2500, valve diameter NPS 2 ~ 60. For the use of PTFE, enhanced PTFE, nylon, PEEK and other polymer materials for the structure of the sealing ring, design For the structure using PTFE, reinforced PTFE, nylon, PEEK and other polymer materials as sealing ring, the design should still meet the sealing pressure q is greater than the necessary sealing pressure qb, less than the allowable specific pressure [q], i.e. qb ≤ q ≤ [q].

It can be seen that, compared with the floating ball ball valve, the fixed ball ball valve has the following characteristics:

① The size of the ball and seat after combination is a free size, and does not form a closed size chain like the floating ball ball valve structure. Therefore, the sealing performance is structurally independent of temperature and pressure fluctuations;

②The same principle of media self-sealing, but the fixed ball valve media force is acting on the annular area of the seat, than the floating ball valve media force on the entire channel cross-section is obviously much smaller, and can be controlled by a reasonable design, so this structure is suitable for NPS60 or less, the pressure level is applicable to all ball valve specifications below Class2500, become an important ball valve development It has become an important structural form in the development of ball valves;

③For the same reason, the operating torque of the ball valve is much smaller than that of the floating ball valve;

④The structure is more complex and heavier than floating ball valves, and more expensive than floating ball valves of the same size;

⑤It can be designed as full bore, standard bore and reduced bore, with larger flow capacity and lower flow resistance coefficient.