As process plants become more highly automated, information is gathered at the device level to provide feedback for control loops. Yet, increasingly intelligent devices are now talking to each other and working independently. Paul Trevitt, general manager, Bürkert Fluid Control Systems, explains the different areas where centralised control and de-centralised control fit
Amazingly the modern process valve has been with us for over 300 years, and has remained the same in principle. However, with the growing automation of drinks, food and confectionery production, the control of process valves has inevitably become more complex.
Factory automation trends are heading towards more precise control and more information being gathered at device level, in order to provide feedback for control loops and better information for plant visualisation, management and reporting software.
Better control has also paved the way for continuous processes, reduced waste, improved hygiene and greater efficiency. However, in some applications the focus is turning back towards the valve itself; increasingly intelligent and robust field control devices are allowing valves to talk to each other and work independently to a complex set of rules and parameters.
So, when is centralised control still desirable and how can de-centralised control be employed to improve process systems?
Automated process valves, depending on their size and design can either be ‘piloted’ i.e. a pressurised air supply or pressure from the medium being controlled is used to actuate the valve, or, an external actuator is used (typically an electric ball screw or pneumatic/hydraulic cylinder on larger valves), or, an electric solenoid is used.
These items are usually connected to a PLC or other electronic controller, one which uses a software program or routine in order to keep the sequence and operation of the process valves in synchronisation with the pressure, flow and mixing required of the fluid being controlled.
If a compressed air supply is available, then piloting the valves may be the right option. There are solutions available to connect large numbers of pneumatic control air lines to a control valve manifold block in a hygienic environment.
One example is Bürkert’s AirLINE Quick interface, a solution for the direct mounting of valve islands and automation equipment into a stainless steel control cabinet, suitable for food grade wash down environments. Eliminating the need for individual bulkhead connections, and internal piping, it has rows of push-fit connectors and exhaust valves integrated into one stainless steel interface plate. It allows for smaller designs of control cabinets for hygienic process actuation.
When connecting larger numbers of process valves, which are often required in continuous process applications such as supplying fluid ingredients or for CIP processes in confectionery production - then a distributed control approach may be more suitable. Using fieldbus protocols such as ASi, Profibus and DeviceNet allows individual valves to be connected using just one or two cables.
Distributed control solution
Again, Bürkert offers a solution that allows process valves from a variety of manufacturers to be controlled centrally, or given a level of independence; Bürkert’s new Type 8681 control head offers the advantage of universal fitment onto third-party hygienic valves: single seat valves, double seat, mix proof, and also quarter turn ball and butterfly valves that have linear stainless steel actuators. The 8681 is compatible with various fieldbus networks, and is enclosed in a IP67 rated housing.
For the final layer of distributed control, many process valve automation applications can now dispense entirely with the control cabinet, bringing process automation down to valve level. Bürkert’s type 8793 controllers can be used to replace an entire control cabinet. Therefore, the relative costs of a separate enclosure, rack mounted PLC, I/O, cabling, power supply, HMI etc. can effectively be replaced with just one small control unit that can be mounted either on, or very near to, the process valve.
The key to finding the best possible automation solution is ultimately through an analysis of each individual part of the plant or installation. In this way, the question of where plant intelligence should rest can be answered.
Burkert Fluid Control Systems
T: 01453 731353