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Differences Between Central Line, Single Offset, Double Offset And Triple Offset Butterfly Valves

Central Line Butterfly Valve

Central Line Butterfly Valve Design And Its Downside

The central line butterfly valve has a simple design. Its main parts – the rod at the center, the middle of the butterfly-shaped disk, and the center of the main body – all line up in the same place. This makes it easy to build. Many butterfly valves that have a rubber lining fall into this category. However, there’s a downside: the butterfly-shaped disk and the part it sits against always rub against each other, causing a lot of wear and tear quickly. To deal with this rubbing and make sure the valve seals properly, the part the disk sits against is usually made of soft materials like rubber or PTFE (a type of plastic). But these materials don’t do well in high heat, which is why people usually think butterfly valves can’t handle high temperature.

Single Offset Butterfly Valve

Single Offset Butterfly Valve Design, Advantages And Disadvantages

To solve the problem of the butterfly-shaped disk squeezing against the seat in central line butterfly valves, the single offset butterfly valve was developed. In this design, the rod’s center is shifted away from the center of the butterfly-shaped disk. This change means that the top and bottom of the disk are no longer the pivot points, which reduces the excessive squeezing of the disk against the valve seat at these ends. However, even with this single offset design, the scraping of the disk against the seat during the opening and closing of the valve doesn’t completely go away. So, in terms of how it’s used, the single offset butterfly valve is quite similar to the central line butterfly valve and is not widely adopted.

Double Offset Butterfly Valve

Double Offset Butterfly Valve Design, Advantages And Disadvantages

The double offset butterfly valve is an improved version of the single offset butterfly valve and is quite popular. In this design, the rod’s center is shifted away from both the center of the butterfly-shaped disk and the center of the main body. Thanks to these two shifts (double offset), when the valve is opened, the disk quickly moves away from the seat. This greatly reduces unnecessary squeezing and scraping between the disk and the seat, making the valve easier to open, reducing wear, and increasing the life of the seat. With less scraping, double offset butterfly valves can also use metal seats, which makes them suitable for high-temperature situations. However, they seal by squeezing the disk against the seat to form a line contact, relying on the elastic deformation caused by this pressure for sealing. This means they need to be closed very precisely (especially with metal seats) and can’t handle very high pressure. That’s why people traditionally think butterfly valves aren’t good for high-pressure situations and tend to leak a bit.

Third Offset Butterfly Valves

Third Offset Butterfly Valves Design, Advantages And Disadvantages

To handle high temperatures, butterfly valves need hard seals, but these often leak a bit. For no leakage, soft seals are used, but they can’t withstand high temperatures. To solve this problem seen in double offset butterfly valves, a third offset was introduced, leading to the creation of the triple offset butterfly valve.

In this triple offset design, besides the offsets in the rod and axis found in the double offset, the sealing surface of the butterfly disk is also changed. Instead of being perfectly round, it’s shaped like a cone that’s tilted compared to the cylinder of the main body. This third offset makes the sealing section of the disk not a true circle but an ellipse, with its sealing surface asymmetric. One side of this surface tilts towards the center line of the main body, while the other side is parallel to it. This complex design helps in achieving high temperature resistance while minimizing leakage.

Conclusion

In conclusion, different butterfly valves have their unique advantages and applications. The choice depends on specific industrial needs, balancing cost, performance, and environmental considerations.

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