What are the three types of butterfly valves?

Midline butterfly valve stem axis line and butterfly plate center plane in the same plane and with the valve body pipeline center line vertical intersection, and the butterfly plate two side area for the stem axis symmetry. Midline butterfly valve is generally made in the form of rubber lining, which is widely used in medium and small butterfly valves because of its simple structure, center symmetry (type I) bi-directional sealing effect, and smaller flow resistance and switching torque. But the shaft head is often in the friction state, wear faster than other parts, easy to leak here, so the rubber-lined butterfly valve is sometimes lined with teflon film in the shaft head to reduce friction or increase the spring to compensate for wear and so on. Obviously, the centerline type such as made metal to metal, to seal some difficulties, tilt plate and offset plate butterfly valve shaft head without friction, but their flow resistance and sealing torque are greater than the center symmetric butterfly plate.

(2)Double eccentric butterfly valve

Double eccentric butterfly valve, in order to improve the situation of single eccentric, the valve shaft is offset by a distance e, so that the line between the shaft center and the sealing point and the sealing surface is at an obtuse angle (more than 90°), so that there is no interference when sealing, and the more it is closed, the tighter it is, producing a greater sealing surface compression force. The feature of double eccentric is that when the valve is opened, the sealing surface of the butterfly plate can be quickly detached from the valve seat, and the contact scraping effect is greatly reduced, and it can be made into a metal sealed valve.

Double eccentric butterfly valves can be designed in the form of rubber, PTFE and metal hard seal. For example, C-shaped, O-shaped or U-shaped genus seal ring and other forms.

(3) Single eccentric butterfly valve 

Single eccentric butterfly valve valve shaft deviates from the center plane of the butterfly plate to produce an eccentric h, so that the upper and lower ends of the butterfly plate against the valve shaft no longer become a rotary axis, eliminating the friction and wear of the upper and lower axis ends to produce leakage. The sealing effect is better, but the sealing effect is not consistent in both directions. Generally, the forward direction (flowing from the valve shaft to the sealing surface) is easy to seal, while the reverse direction is easy to leak because there is no supporting effect of the sealing surface. But the use of rubber soft seal, the use of its elasticity, can be very good to achieve a two-way seal. Since the sealing contact mechanism belongs to "ball in cone" (ball in cone), the butterfly plate always has two points that cannot be out of contact under each opening degree, and when used for adjustment, these two points will wear more than other parts, which will have a negative impact on the seal. If it is made into a metal hard seal, two positive cone fits, when turning the butterfly plate, the sealing surface will interfere and there will be difficulties in switching.