What Are The Differences between Single, Double And Triple Eccentric Butterfly Valves

When to use a concentric, double-offset or tripleoffset butterfly valve?

Selecting the wrong valve type can harm your operation. A few butterfly valves in the market are the zero offset, double offset and triple offset designs.The concentric butterfly valve, single eccentric butterfly valve, double eccentric butterfly valve and triple eccentric butterfly valve signify increasing levels of labor-saving in opening and closing as well as reducing wear on the sealing surface. Changes in the position of the shaft of the plate of a butterfly valve set change to seal and open conditions for a butterfly valve.

Center line-type butterfly valve

Center butterfly valve, in terms of structure, has the valve stem axis, butterfly plate center, and body center at one place. The design is simple and easy to produce. This includes most rubber-lined butterfly valves. This is because of the squeezing that always exists between the butterfly plate and the seat of the valve. The use is limited to temperature conditions as it must withstand heat due to metal seat sealing for high temperatures; it is necessary not only for overcoming valve tightness during squeezing or scratching but also for limiting temperature which should be based on these factors. That is why there has been this notion that Butterfly Valves are not applicable for high-temperature services because they cannot withstand them.

Single eccentric butterfly valves

To address this extrusion problem between the butterfly plate and the valve seat of the concentric butterfly valve, the single eccentric butterfly valve was developed. Its structure is such that the axis deviates from the center of the butterfly plate so that the upper and lower ends of plates do not revolve around becoming the rotation axis anymore. It scatters and diminishes excessive extrusion between upper and lower ends with a valve seat. But because it is a single eccentric structure, during operation of these valves, never does rubbing between plates on seats disappear entirely. The range of applications is identical to that of concentric butterfly valves; therefore, it is not widely used.

Double eccentric butterfly valves

Compared to the single eccentric butterfly valve, the double eccentric is the most widely used today. Its structure has the feature that the axis of the valve stem is both off center from the center of the disc and also from the center of body. In effecting a double eccentric action, this allows the disc to pull away quickly from its seat once opened, therefore very much reducing damage caused by unnecessary friction between disc and seat during their periods of contact, diminishing impedance to flow, reducing wear, and increasing seat life.

The anti-scratching is much decreased, and the double-eccentric butterfly valve can also adopt a metal valve seat which enhances high-temperature applications for butterfly valves. But because its sealing principle is a positional seal structure, that is, the sealing surface of the butterfly plate and the valve seat is linear contact and elastic deformation generated by the butterfly plate pressure to squeeze the valve seat that plays the role of a seal so closing positioning and bearing pressure conditions are very high. Low, this is why traditionally people think that butterfly valves are not resistant to high pressure and have large leakage.

Triple eccentric butterfly valves

At very high temperatures, hard seals are used, but these result in big leakages; for zero leakage, use of soft seals is to be applied, but they are not resistant to high temperatures. The way the contradiction of the double-eccentric butterfly valve was solved was by making the butterfly valve eccentric for the third time. Its uniqueness lies in that while the double-eccentric valve stem's axial position is eccentric, the conical axis of the sealing face of the butterfly disc is also offset from the axis of the valve body cylinder. This means that after being triple offset, section on which it seals as a disc does not form a perfect circle. This circle has a larger diameter than normal (i.e., greater than 360 degrees), and its shape is called an ellipse; consequently, its sealing face shape becomes asymmetrical. One side slants towards (or converges to) the body's centerline while another runs parallel to this centerline.

The triple eccentricity is biggest in that the sealing structure is fundamentally changed. It is a torsion seal and no longer a position seal. That is, it does not rely on the elastic deformation of the valve seat but entirely relies on contact surface pressure of the valve seat to achieve sealing. Therefore, metal valve seats' zero leakage problem is solved at one stroke. Since contact surface pressure is proportional to medium pressure, it also simply solves high-pressure and high-temperature resistance.

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