Precise, fast response when controlling large valves
Larger valves (200 mm and above, with flowrates in excess of 200 m3/h) tend to be fitted with pneumatic actuators. However, the air-output capacity required to achieve fast positioning in larger actuators is simply too high for normal positioners and accuracy can be compromised. An added complication in these situations is that the positioner controls a volume booster, which results in a much higher air capacity than the positioner and this can lead to an overshoot on responding to small signal changes. To overcome these problems, this company developed a control-valve positioner package (photo) that ensures precise positioning with fast response times, over the entire operating range, for example strokes of 60 mm and greater. The package consists of a positioner and a special hookup of several volume boosters with different air-output capacities that perform this task. Closing and opening times of less than 2 s can be achieved over the entire positioning range. — Samson Controls, Redhill, Surrey, U.K.