Here are the sealing principle and structural characteristics of fixed ball valves.
1. The sealing principle of fixed ball valves
The opening and closing component of the fixed
ball valve is a ball with two supporting shafts, with a channel in the middle that is equal to the diameter of the pipeline, and two floating valve seats on both sides. When the ball rotates, it does not deviate from the axis of the supporting shaft on the ball. The supporting shaft is supported by two sliding bearings on top and bottom, and the valve seat is floating. Under the action of medium force, it pushes the ball to generate a contact stress to obtain reliable sealing. Like floating ball valves,
valves have a 90 degree angle travel from open to closed or from closed to open.
2. Structural characteristics of fixed ball valves
The body of fixed ball valves is composed of the main valve body and two auxiliary valve bodies. The ball is a ball with a shaft neck at the upper end and a shaft hole at the lower end. The upper part of the valve body has a packing box, and there is a sliding bearing at the junction with the shaft neck of the ball. The lower end of the ball has a shaft hole, which is equipped with a sliding shaft bearing sleeve. At the bottom of the valve body, there is a bearing inserted into the shaft hole at the lower end of the ball. In this way, the sphere is kept on the centerline of rotation, and there will be no deviation during rotation. There are two floating valve seat components symmetrically on both sides of the sphere. This valve seat component consists of a metal seat ring, a PTFE sealing ring embedded in the seat ring, a rubber O-ring on the outer edge, and a back spiral spring. When the valve is closed, the medium force acts on the components of the sealing seat, causing the valve seat to come into contact with the ball, generating contact stress and achieving effective sealing. The valve stem and packing box are sealed with O-ring rubber rings.
The structure of a fixed ball valve is much more complex than that of a floating ball valve. Its principle is that the medium pressure pushes the valve seat component to act on the ball to generate contact stress to obtain sealing. The axis of the rotation center of the ball does not deviate, and the valve seat component is floating, so it is called a fixed ball floating valve seat structure. Due to the medium force acting on a circular area, the medium thrust is much smaller than that of the medium of the floating ball valve. Therefore, the structure of fixed ball valves is suitable for high pressure and large diameter, with a pressure range of Class 150 to 2500 and a valve diameter of NPS 2 to 60. For structures that use polymer materials such as PTFE, reinforced PTFE, nylon, PEEK, etc. as sealing rings, the design still needs to meet the requirement that the sealing specific pressure q is greater than the necessary sealing specific pressure qb and less than the allowable specific pressure of the material [q], that is, qb ≤ q ≤ [q].
From this, it can be seen that compared to floating ball valves, the characteristics of fixed ball valves are as follows.
① The size of the combined ball and valve seat is a free size, unlike the floating ball valve structure that forms a closed size chain. Therefore, its sealing performance is not affected by temperature and pressure fluctuations structurally.
② The same principle applies to the self-sealing of the medium, but the medium force of the fixed ball valve acts on the annular area of the valve seat, which is much smaller than that of the floating ball valve. Moreover, it can be controlled through reasonable design. Therefore, this structure is suitable for all ball valve specifications below NPS 60 and pressure rating below Class2500, becoming an important structural form in the development of ball valves.
③ Based on the same reasons mentioned above, the operating torque of ball valves is also much smaller than that of floating ball valves.
④ Its structure is more complex than floating ball valves, it is heavy, and its price is also more expensive than floating ball valves of the same specifications.
⑤ It can be designed as full diameter, standard diameter, and reduced diameter, with larger flow capacity and lower flow resistance coefficient.
