Ball valves are a large category, and fixed ball valves are a type of ball valve. In addition, there are floating ball valves, etc. There are obvious differences between fixed ball valves and floating ball valves in terms of structure, working principle, sealing performance, and applicable working conditions. The following is an introduction for you:
Structure
Fixed ball valve: The ball is fixedly connected to the valve body through the upper and lower valve stems. There is a fixed shaft at the bottom of the ball, which can only rotate around the axis of the valve stem and will not move after being compressed. Thrust or sliding bearings are usually installed on the upper and lower valve stems connected to the ball.
Floating ball valve: The ball is in a floating state in the valve body, there is no lower fixed shaft, and usually only one upper valve stem is connected to the ball.
Working principle
Fixed ball valve: Usually with a floating valve seat, under the action of the medium pressure, the valve seat moves, so that the sealing ring is pressed tightly against the ball to achieve sealing. Even when there is no fluid pressure, the preload between the ball and the valve seat and the transmission of the valve stem can achieve a tight fit and seal between the ball and the valve seat. When opening, the valve stem moves in the opposite direction, causing the ball to detach from the valve seat and rotate 90 degrees around the center of the valve stem to the fully open position; the closing process is the opposite, the valve stem descends and mechanically wedges the ball to press it tightly against the valve seat to achieve sealing.
Floating ball valve: The ball is pushed by the medium pressure, causing the ball to have a certain displacement and press against the sealing surface at the outlet end, thereby achieving the outlet end sealing. When opening, the valve stem drives the ball to rotate 90 degrees around the center of the valve stem to open the channel; when closing, it rotates 90 degrees in the opposite direction, and the ball is again tightly attached to the valve seat under the medium pressure.
Sealing performance
Fixed ball valve: Bidirectional sealing is achieved through mechanical preload (spring or valve seat floating design), which is more reliable. The valve seat can automatically compensate for wear under the action of the medium pressure and maintain good sealing performance for a long time. At the same time, the sealing surface of the fixed ball valve is usually made of high-performance materials, such as PTFE, which further improves the sealing performance.
Floating ball valve: Relying on medium pressure sealing, it may leak at low pressure, and is mainly one-way sealing. When subjected to high pressure shock, the ball may shift, affecting the sealing effect.
Applicable working conditions
Fixed ball valve: can withstand high pressure (up to Class 2500), suitable for large diameter (up to NPS 60), can adapt to media containing particles, and has good wear resistance. Widely used in high-pressure and large-diameter occasions such as petrochemicals, long-distance pipelines, and electricity, such as pipeline ball valves above Class 1500.
Floating ball valve: suitable for medium and low pressure (such as ≤Class 600), usually used for small and medium diameters (≤NPS 8), suitable for clean media, to avoid particles accelerating seal wear. Commonly used in medium and low pressure small-diameter pipelines such as water and gas, such as cut-off valves in general industrial systems.
Operating performance
Fixed ball valve: The ball is fixed to reduce friction, opening and closing is easier (especially large diameter and high pressure conditions), and the operating torque is small, so the operation is more labor-saving.
Floating ball valve: The operating torque is large under high pressure, requiring a larger actuator or gear device.
Maintenance cost
Fixed ball valve: The valve seat is adjustable or self-compensating, and the maintenance cycle is long, but the maintenance cost is high.
Floating ball valve: The structure is simple, but the valve seat needs to be replaced frequently after the seal is worn.
