Where process control applications need larger valves it can sometimes be difficult to achieve the required levels of accuracy combined with fast positioning. Larger valves tend to be fitted with pneumatic actuators which provide significant positioning forces and in these situations the positioning of the valve is achieved by filling the actuator with the corresponding amount of air, or by venting from the actuator.
However, the air output capacity required to achieve fast positioning in larger actuators (with diaphragm areas of 1400cm2 up to 2800cm2) is simply too much for normal positioners and accuracy can be compromised. A further 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 control valve specialists Samson Controls have developed a control valve positioner package which ensures precise positioning with fast response times, over the entire operating range.
The package consists of a positioner and a special hook up of several volume boosters with different air output capacities which performs this task. Closing and opening times of less than two seconds can be achieved over the entire positioning range. Applications where the Samson package has been successful is for compress or bypass valves (anti-surge valves) as well as in petrochemical applications involving adsorption processes in moving-bed absorbers.