11.3.2 Butterfly valve
The butterfly valve is shown in Fig. 11.7a and its flow versus travel characteristics
are shown in Fig. 11.7b. The relation between flow and lift is approximately
equal percentage up to about 50 percent open, after which it is linear. Butterfly
valves offer high capacity at low cost, are simple in design, easy to install, and
have tight closure. The torsion force on the shaft increases until open up to 70°
and then reverses.