The Introduction of Eccentric Butterfly Valve Structure

Various of Butterfly valves, structurally classified, include center-mounted and eccentric (single eccentric, double eccentric, and triple eccentric) types.

Based on their end connections, they include flanged, weld-on, and wafer/lug types. This section primarily discusses their structural classifications and characteristics.



The following diagram illustrates center-mounted, single-eccentric, double-eccentric, and triple-eccentric butterfly valves.

Center-mounted: The butterfly plate's central axis aligns with the valve body's vertical centerline. A non-metallic pressure valve body or seat (such as rubber) is used. The outer ring of the butterfly plate is pressed against the soft-seal seat, causing elastic deformation and sealing the valve. Advantages include simple structure, easy operation, and light weight. Disadvantages include the worst sealing performance, and adaptability to temperature, pressure, and media is limited by the material of the soft sealing surface.



Single eccentric: The butterfly plate's central axis (the axis of medium flow) is aligned with the valve body's vertical centerline, but the butterfly plate is offset by OFFSET1 along the medium flow axis. This means the butterfly plate sealing surface and the valve seat sealing surface are offset from the butterfly plate's central axis (the valve body's vertical centerline). The single eccentricity OFFSET1 setting ensures the butterfly plate and valve seat sealing surfaces form a complete circle, reducing mechanical wear and compression between the sealing surfaces during opening and closing. This improves sealing performance, applicable working pressure, and service life compared to the centerline type. Disadvantages include: (a) mechanical friction between the butterfly plate and valve seat sealing surfaces, leading to scratches; (b) butterfly plate compression of the valve seat, causing deformation and cold flow failures in the elastic layer; (c) a more complex structure and higher cost compared to the centerline type; and (d) cavitation and erosion of the sealing surface by the medium.

Double eccentric type. Based on the single eccentric design, the butterfly plate shaft is offset radially by OFFSET2. This reduces mechanical wear and compression when the butterfly valve is open, and when closed, the outer sealing surface of the butterfly plate presses against the valve seat, resulting in a larger and better sealing specific pressure. This structure has better sealing performance than the single eccentric design, higher operating pressure, and longer service life. The disadvantages are: (a) mechanical wear and scratches occur between the butterfly plate and the valve seat sealing surface; (b) the butterfly plate pressing against the valve seat can cause cold flow failure due to the elastic deformation of the soft-seal valve seat; (c) the structure is more complex and slightly more expensive than the single eccentric design; and (d) the medium can cause cavitation and erosion of the sealing surface.

Triple eccentric design. Based on the double eccentric design, the valve seat centerline is offset by OFFSET3 from the horizontal centerline of the valve body. When opening, the butterfly plate immediately leaves the valve seat sealing surface; when closing, the butterfly plate sealing surface only contacts and presses against the valve seat sealing surface at the moment of closing. The sealing specific pressure between the two sealing surfaces is determined by the eccentricity and the torque of the valve stem. The advantages are significantly improved sealing performance and service life, eliminating factors such as cold flow elasticity failure of soft-seal valve seats in other butterfly valve structures. The disadvantages are complex structure, high cost, and cavitation of the sealing surface by the medium.