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Positioner controls large valves

20 September 2007

Samson Controls, a valve specialist, has introduced a new positioner package which facilitates exact positioning with fast response times over the entire operating range, including large control valves which can be problematic.

For example, in most process control applications, larger valves are fitted with pneumatic actuators, and these provide significant positioning forces. In these situations, the positioner is used to precisely position the valve by filling the actuator with the corresponding amount of air, or by venting air from the actuator.

The air output capacity required to achieve fast positioning in larger actuators (with diaphragm areas of 1400 cm² up to 2800 cm²) is simply too much for normal positioners, and so positioning accuracy can be compromised. In these situations, the positioner controls a volume booster which achieves a much higher air output capacity than the positioner. Consequently, this results in an overshoot on responding to small signal changes.

The new Samson positioner package, which consists of a positioner and a special hook-up of several volume boosters with different air output capacities, accomplishes this task. Closing and opening times of less than two seconds can be achieved over the entire positioning range. This package has already been used successfully for compressor bypass valves (anti-surge valves), as well as in petrochemical applications involving adsorption processes in moving-bed adsorbers.

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