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06/07/2010
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Traditionally, the majority of pneumatically piloted valves on production and process lines centralise pneumatic control around valve islands in a control cabinet or enclosure.
 This arrangement means that the pipework carrying the pilot pressure to the actual valve from the valve island can travel for many metres before it reaches the valve head. As a result, the venting cycle of the valves operation will exhaust proportionately more air than is necessary.
Burkert believes a solution to this problem is its type 8690, 8691 and 8695 valve control heads. Developed for use in the food, beverage and pharmaceutical industries, theunits provide a decentralised method of opening and closing of process valves automatically, eliminating the need for the wasteful venting of control air normally associated with pneumatic tubing between the process valve and its related control solenoid valve.
The control heads integrate electrical and pneumatic control components as well as position feedback units and, optionally, field bus interfaces for AS-Interface or DeviceNet. They are mounted directly above the valve body, and, because there is little or no distance between the actuator and the valve that it is piloting, there is no air bleed: it is sealed. With this system the pressure feed goes directly into the valve head and the control signal is supplied either from a local closed loop control sensor or switch; or from a PLC / machine controller via a control bus - AS-Interface or DeviceNet – or multipole (parallel) directly into the valve.
Similarly, adopting digital positioners with integral solenoid valve control heads for regulating modulating process control valves will automatically lead to air savings, as these will normally ensure zero-air use in their stable state. The traditional technique of process valve positioners incorporating pneumatic flapper-nozzle systems means that air is being bled constantly, even when the valve is at rest. This can average the equivalent of a 0.75kW (or one-horsepower) in compressed air for every 20 valves in operation; a large process site can therefore be using a vast amount of energy – unnecessarily.
Choosing the option of an embedded PID process controller, working in conjunction with the positioner, provides a fast-acting, decentralised control loop in combination with the associated process sensor. This arrangement also reduces complexity and saves unnecessary components and wiring.
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Author
Chris Shaw
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