Double Eccentric Butterfly Valves
Double eccentric butterfly valves, also known as high-performance butterfly valves, have the shaft mounted slightly offset from the pipe centerline, and the shaft is mounted behind the sealing band of the seat. The seat is a narrow resilient band, clamped in the body and must be resilient. The double eccentric butterfly valve is resilient enough to seal, and the disc only wipes the seat for a few degrees at seating. This results in a lot less friction, meaning the seats can last longer. Metal seats are available for HP Butterfly valves allowing operation to around 1200F. Since the disc is offset from the shaft, there is a lot of metal in the flow stream in the wide-open position, and double-offset valves have somewhat less maximum capacity than flat-disc, non-offset valves. These valves are also position-seated. Unlike the lined valves, these valves can actually damage the seats if they are overstroked so they almost all have a travel-stop pin in the body.
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