Butterfly Valves, Technics : Gear Operated

A BUTTERFLY VALVE consists of a disc rotating in trunnion bearing. 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 25 mm to 6 (1 in to 25ft) diameter. They are particularly attractive because they take up little more room than the pipe flanges. In case some butterfly valves are designed specifically for insertion between pipe flanges, using through bolts. These are know as wafer butterfly valves. Other types are normally single-flanged or double-flanged . known also as lug-body valves.
Butterfly valve movement is simple and straightforward, requiring only 90o 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 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 a 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 particularly where percolation of gases through cast components is to be prevented.