The butterfly valves, model G.S.40, are worked by a single-action hydropneumatic actuator with return spring, hand-wheel for turning it off and bushing for regulation of the piston stroke. The use of large diameter cylindres ensures that the valves can function even when drive pressures are relatively low. At customers' request the valves can be supplied set either "normally open" or "normally closed". These valves are made in n. diams. from 50 to 200 mm, and differ from the G.S. model in that they have a "Wafer" valve body for insertion between the flat flanges.