Selection between fixed ball valves and floating ball valves
Thread Content
Ball valves are divided into fixed-ball and floating-ball types, which is a classification based on structure. A ball valve with a floating ball and a fixed seat is simply called a floating ball valve; a ball valve with a fixed ball and a floating seat is simply called a trunnion mounted ball valve. I. Sealing Principle 1. The operating element of a floating ball valve is a sphere with a hole in its center that has the same diameter as the pipeline. There are PTFE-sealed seats at both the inlet and outlet ends, and all these components are enclosed within a metal valve body. When the hole in the sphere aligns with the pipeline passage, the valve is in the open position; when it is perpendicular to that passage, the valve is in the closed position. From open to closed or from closed to open, the angle of rotation of the sphere is 90°. 2. The opening and closing element of a fixed ball valve is a sphere equipped with two support shafts; there is a passage in the center of this sphere whose diameter is equal to that of the pipeline. On either side, there are two floating valve seats. As the sphere rotates, it does not deviate from the axis of the support shafts. These support shafts are supported by upper and lower sliding shafts. The valve seats are floating, and under the action of the medium’s force, they press against the sphere, creating a contact stress that ensures a reliable seal. From open to closed or from closed to open, the angle of rotation of the sphere is 90°. II. Structural Features 1. The structure of the floating ball valve consists of a sealed dimension chain ； The size of the ball and seat assembly in a fixed ball valve is a free dimension. 2. The medium force acting on a fixed-ball ball valve is applied to the annular area of the valve seat, which is much smaller than the medium force that acts on the entire cross-section of the passage in a floating-ball ball valve. 3. The operating torque of a fixed ball valve is smaller than that of a floating ball valve. 4. The floating ball valve has a simple structure, while the fixed ball valve has a complex structure. III. Application Areas When selecting and using valves, attention should be paid to the pressure-temperature ratings of the valve body material; refer to ANSI B16.34. Floating ball valves are suitable for classes below Class 600 and pipe diameters below NPS8 (DN200). Fixed ball valves are suitable for classes below 2500 and pipe diameters below NPS60 (DN1500).
Additional notes: 1. Floating ball valve – The ball in a floating ball valve is free to float and does not have a fixed shaft. Under the pressure of the medium, the ball can move slightly and press against the seal at the outlet end, thereby ensuring sealing at that outlet; it operates on a single-sided forced-sealing principle. During design, special attention is paid to whether the material of the sealing ring can withstand the load exerted by the working medium acting on the ball. Since all the loads from the working medium acting on the ball are transmitted to the outlet sealing ring, this floating-ball structure is widely used in medium and low-pressure ball valves. Furthermore, the requirement for a high torque to operate large-diameter ball valves should also be taken into account during design. 2. Fixed ball valve: The sphere of a fixed ball valve is fixed and equipped with a fixing shaft; it does not move under pressure. Therefore, during design, the two sealing rings must have sufficient pre-tightening force to ensure sealing, resulting in a double-sided forced sealing mechanism. Typically, thrust or sliding bearings are installed on the upper and lower valve stems that are integrated with the ball. Compared to floating ball valves, in fixed ball valves the force exerted on the ball by the fluid pressure in the pipeline ahead of the valve is transmitted entirely to the upper and lower bearings; as a result, the ball does not move. Sealing is achieved by using springs and medium pressure to push against the valve seat, so the valve seat is not subjected to excessive pressure. Therefore, fixed ball valves exhibit minimal deformation of their valve seats, require less operating torque, and provide reliable sealing performance. They are suitable for long-distance pipelines as well as ordinary industrial pipelines. Their design takes into account safety and resistance to harsh environments, making them suitable for use with various corrosive and non-corrosive media. It has a long service life and is particularly suitable for high-pressure, large-diameter applications. To reduce the operating torque of ball valves and improve the reliability of sealing, oil-sealed ball valves should be used. Special lubricating oil is injected between the sealing surfaces to form an oil film, which not only enhances the sealing performance but also reduces the operating torque; such valves are more suitable for high-pressure, large-diameter applications. 3. Elastic ball valve: The ball of this valve is elastic. Both the sphere and the valve seat sealing ring are made of metallic materials, resulting in a very high sealing pressure. The pressure of the medium itself is not sufficient to ensure proper sealing; therefore, an external force must be applied. This type of valve is suitable for high-temperature and high-pressure media. An elastic sphere achieves elasticity by having an elastic groove at the lower end of its inner wall. When the channel is closed, the wedge-shaped end of the valve stem is used to expand the ball and press it against the valve seat to achieve sealing. Before rotating the sphere, release the wedge head; the sphere then returns to its original shape, creating a small gap between the sphere and the valve seat, which reduces friction on the sealing surfaces as well as the operating torque.
Ball valves can be divided into fixed ball valves and floating ball valves, which is a classification based on their structure. First of all, the sealing principle is different in comparison. Both the sealing seat and the ball of a floating ball valve can move, and they can make tight contact under pressure. In a fixed ball valve, the ball remains stationary, and the sealing effect is achieved through the movement of the valve seat. Secondly, their structural characteristics also differ. The floating ball valve has a simple structure; the ball moves freely within the central cavity, and sealing is dependent on the pressure of the medium. On the other hand, the fixed ball valve has a complex structure; high precision is required in the dimensions between the ball and the valve seat. The ball is held in place by upper and lower bearings, while the valve seat can move freely, resulting in a low operating torque. Finally, in terms of application areas, floating ball valves are typically suitable for applications with smaller diameters and lower pressures, such as those below Class 600 and with a nominal diameter of NPS8 (DN200) or less, as high pressures may cause the ball to move due to the applied forces, resulting in poor sealing. Fixed ball valves are used in applications with larger diameters and high pressures, such as those below Class 2500 and with a nominal diameter of NPS60 (DN1500) or less, as the ball does not move under stress, resulting in better sealing performance. .
Zero-friction sealed ball valves are now popular~
Although zero-friction ball valves can address the issue of component wear in ball valves, they also have some common problems (such as leakage, poor sealing when the valve is closed, loosening, and high noise), which increases the difficulty of maintenance and repair.
Are you sure you’re using a zero-friction ball valve? We have not encountered those problems with our valves that you mentioned.
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