Where process control systems require larger valves it can sometimes be difficult to achieve the required levels of accuracy combined with fast positioning. Larger valves, for example where flow rates require valve sizes of 200mm and above enabling flows in excess of 200 m3/h, tend to be fitted with pneumatic actuators which provide significant positioning forces.
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.
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 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, for example strokes of 60mm and greater. The package consists of a positioner and a special hook up of several volume boosters with different air output capacities which perform this task.
Closing and opening times of less than two seconds can be achieved over the entire positioning range.The flow control 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 adsorbers.