What’s the difference between zero, double and triple offset butterfly valves?
Butterfly valve offset is a term used to describe the distance between the centerline of the valve stem and the centerline of the valve disc. This offset creates a camming action when the valve is opened or closed, which can affect the torque required to operate the valve and the flow characteristics through the valve.
There are two types of butterfly valve offsets: zero offset and eccentric offset. In a zero offset valve, the stem and disc are centered in the valve body, and the valve rotates around its central axis. In an eccentric offset valve, the stem is positioned off-center, which creates an offset between the disc and the stem.
Eccentric offset butterfly valves can be further classified into two types: double offset or high performance and triple offset. In a double offset valve, both the stem and disc are offset from the center. This creates a camming action that causes the disc to move away from the seat as it is opened, which helps to reduce wear on the seat and disc. In a triple offset valve, the stem is positioned off-center for the first offset, the second offset is the disc being positioned off-center and at an angle to the stem and the third offset is the geometry of the seating surface, which creates a cone shape of the disc and seat. This creates a camming action that causes the disc to move away from the seat and to pivot away from the flow path as it is opened, which helps to reduce wear on the seat and disc and minimizes flow turbulence and pressure drop across the valve. Triple offset valves are classified as zero leak valves while double offset or high performance valves are classified as Class VI shut off valves.
The type of offset used in a butterfly valve depends on the specific application and the desired performance characteristics of the valve. Zero offset valves are commonly used in HVAC applications where pressure & temperature requirements are low, while eccentric offset valves are often used in applications where flow control is important, such as in the chemical, food and beverage, and pharmaceutical industries.
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