Process valves designed to eliminate energy losses

The 8690, 8691 and 8695 valve 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. Adopting digital positioners with integral solenoid valve control heads for regulating modulating process control valves will automatically lead to air savings, says Bürkert, 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 twenty 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, is designed to provide decentralised control loop in combination with the associated process sensor. According to Bürkert, this arrangement also reduces complexity and saves unnecessary components and wiring.