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In the valve world, ball valves are known for their com […]
In the valve world, ball valves are known for their compact, durable designs and reliable, fast on/off control. Ball valves use a ball with a hole (called a port) drilled in the center to control the flow of media. The ball is rotated 90 degrees to align the ports with the valve body and allow media to flow through the valve. To close the valve, rotate the ball 90 degrees back to the closed position. All ball valves rely on the use of two seats, usually made of polytetrafluoroethylene (PTFE), to hold the ball in place and create a tight seal to prevent leakage of media through the valve. Floating and trunnion are the two basic types of ball valve designs. Floating ball valves are the most common type of valve used in industrial piping systems.
These valves are designed so that when the seat begins to wear, the pressure from the medium flowing through the valve will force the ball against the downstream seat to maintain a tight seal. In a trunnion ball valve, the ball is held in place by a bearing that prevents it from floating downstream. When the seat inevitably begins to wear, the spring-loaded seat mechanism ensures that the valve remains sealed. Floating ball valves are operated by a shaft or stem attached to the top of the ball, which rotates the ball 90 degrees (a quarter turn). As the ball rotates, the ports are covered or exposed by the walls of the valve body, releasing or stopping the flow of media.
The connection between the stem and the ball is loose enough that as the ball rotates about its axis, fluid pressure pushes the ball against the downstream seat, creating a tight seal. For this reason, floating ball valves may not seal effectively in very low pressure applications after a certain amount of seat wear has occurred. This is because there may not be enough media pressure to force the ball against the downstream seat to form a tight seal. In most applications, however, the downstream pressure is sufficient to maintain a tight seal long after the seat begins to wear.
A fixed ball valve works much the same way as a floating ball valve, but in this case the ball is anchored by a second rod opposite the operating rod. This second stem and bearing is called the trunnion. Unlike floating ball valve designs, the stem of a trunnion ball valve is rigidly attached to the ball, which prevents the ball from moving anywhere other than its axis. Since the ball does not "float" downstream with media pressure, a trunnion ball valve maintains a proper seal on both sides at the same time. This is especially important for multi-port valves and valves in very low pressure applications where there is insufficient media flow to push the ball against its downstream seat.