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First, think about the most common faucet in your home, or the old-fashioned iron disc valve you have seen. The piece (disc) of that switch is threaded through a shaft (valve stem) in the center. When it's opened or closed, that disc rubs against the sealing ring, just like rubbing an eraser against a table. Over time, it will definitely leak.
The double eccentric butterfly valve is a solution devised by engineers to prevent them from wearing out the parts. They deliberately positioned the shaft slightly off-center twice: once away from the center of the valve plate, and another time away from the center of the entire pipeline. This is similar to a door that originally rotates around its center axis. Now, if you move the door axis to a slightly higher position, when you open the door, doesn't it become much easier to lift it off the ground without touching the floor? The double eccentric principle is the same. When the valve is opened, the valve plate quickly rises and leaves the sealing surface, resulting in much less friction. Therefore, it can use metal sealing, which is more durable, and can be used in situations with higher pressure and higher temperatures. However, when closing the valve, it still relies on forcefully pushing the valve plate into the sealing ring to achieve sealing, which can be considered as "pressing tightly" to prevent leakage.
The three-bias butterfly valve is even more ingenious. Based on the double-bias design, it has added a third trick: it makes the sealing surface of the valve seat inclined, giving it an angle. This is truly remarkable! This inclined angle ensures that when the valve plate closes, it doesn't push in straightly, but rather like a wedge, it "sticks" in at an angle.
Let me explain it this way: Double eccentric closing is like closing a book and pressing another book flat against it. While triple eccentric closing is like driving a triangular wedge into a wooden gap, the tighter you drive it, and there is almost no sliding friction between them. Due to almost no friction, the sealing performance of the triple eccentric butterfly valve is extremely good, achieving "zero leakage" (that is, not even a single drop leaks), and it is extremely durable because it doesn't wear at all. The materials it uses are all high-grade hard metals, and it can withstand high temperatures, high pressures, and strong corrosive environments.
So, to sum up briefly:
Double eccentricity is "lift and rotate, then squeeze tightly", which is already much better than the old method. It is a highly cost-effective powerhouse.
Triple eccentricity is "insert diagonally and there is no wear at all", it is the king of advanced technology, specializing in tackling those tough challenges that have strict requirements and cannot tolerate any leakage at all.
If you have plenty of money or the working conditions are very harsh, then choose the three-point eccentric design. It will definitely be the right choice. If the requirements are not that high, a double-point eccentric design will already be more than sufficient.
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