What's the Difference Between Zero, Double, and Triple Offset Butterfly Valves?

Butterfly valve offset refers to 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, impacting the torque required for operation and the flow characteristics through the valve.

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Types of Butterfly Valve Offsets

There are two primary types of butterfly valve offsets: zero offset and eccentric offset. In a zero offset valve, the stem and disc are aligned with the center of the valve body, allowing the valve to rotate around its central axis. In contrast, an eccentric offset valve features an off-center stem, creating an offset between the disc and the stem.

Eccentric Offset Valves

Eccentric offset butterfly valves can be further classified into two types: double offset (high performance) and triple offset valves. A double offset valve has both the stem and disc displaced from the center, generating a camming action that moves the disc away from the seat during opening. This design minimizes wear on both the seat and the disc.

On the other hand, a triple offset valve includes three offsets: the first is the off-center position of the stem, the second is the positioning of the disc, which is off-center and angled to the stem, and the third is the conical geometry of the seating surface. This unique design allows the disc to pivot away from the flow path as it opens, significantly reducing wear and minimizing flow turbulence and pressure drop across the valve. As a result, triple offset valves are categorized as zero-leak valves, while double offset valves are classified as Class VI shut-off valves.

Choosing the Right Offset

The type of offset used in a butterfly valve is largely determined by the specific application and desired performance characteristics. For instance, zero offset valves are ideal for HVAC applications with low pressure and temperature requirements. In contrast, eccentric offset valves are commonly employed in scenarios where precise flow control is crucial, such as in the chemical, food and beverage, and pharmaceutical industries.

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