A 300MM Butterfly valve consists of a disc rotating in trunnion bearings. In the open position, the disc is edge-on, offering minimum resistance to flow. In the closed position, it is rotated against a seat. Butterfly valves are available in sizes from DN32-DN3000 (l 1/4" in to 120") diameter and include the popular 300MM butterfly valves. They are particularly attractive because they take up little more room than the pipe flanges. In fact some butterfly valves are designed specifically for insertion between pipe flanges, using through bolts. These are known as wafer butterfly valves. Other types are normally single-flanged or double-flanged, known also as lug-body valves.
300MM Butterfly valve movement is simple and straightforward, requiring only 90° rotation of the butterfly for full movement (or somewhat less in most designs). The main disadvantage is that a simple butterfly design is difficult to render completely 'tight' when closed, due to the absence of any wedging effect in this plane. This can be alleviated to a certain extent by seating the butterfly on a resilient ring on closure and/or offsetting the axis of rotation slightly so that, as well as rotating, the blade has a movement in a plane at right angles to the spindle axis.
One lesser drawback of the butterfly is that in the fully open position the butterfly, (now edge-on), offers some resistance to flow and could collect fibrous or stringy materials when used with contaminated fluid. It is thus more suited for use with clean fluids, where its basic advantages and low actuating power requirements can make it an attractive choice in all sizes.
Body construction is normally cast iron, although various other materials may be used depending on size and application. Welded materials (eg steel, stainless steel and titanium) may be used for certain valves for the chemical industry, and in particular where percolation of gases through cast components is to be prevented.
Discs are also usually of cast iron, although again alternative materials may be specified for particular services. Profile shapes vary, most having some form of convex streamline shape to minimize heat loss.
Seal design can vary considerably. A basic arrangement is a corrosion-resistant seat (eg bronze or stainless steel), into which a continuous rubber ring seal is fitted. Others include resilient plastic seal rings (for chemical duties) and flexible metal seal rings (for high-temperature services). Detailed designs may provide automatic adjustment to any eccentric motion of the disc and/or automatic compensation for seal wear. Drop-tight closure of any butterfly valve is normal and can be retained for a considerable time before seal replacement is necessary.
All types of control mechanisms can be used to operate butterfly valves - manual by lever or hand wheel with reduction gear, electrical by actuator or reduction gear, hydraulic actuator or pneumatic actuator. The choice largely depends on the size of the valve and the specific application. Special control systems can also be used for automatic closing.