Retrofit safety with control valves
The food and beverage industry has embraced automation – partly due to the necessity of reducing production costs, but also to help meet food safety standards and personnel safety legislation. By taking a modular approach to automation, plant owners can gradually introduce new technologies and expand automated processes, in order to maintain productivity and keep up with safety legislation.
Increasing levels of automation in industrial processes has required additional levels of safety and reliability to be introduced. This has led to the adoption of the EU’s new Machinery Directive ISO 13849 and a number of new standards such as IEC 62061.
Within both of these pieces of legislation are details of increased safety functions. Safety circuits have very specific requirements, especially in terms of wiring and redundancy of components. As a simple example, an emergency stop circuit should have two sets of contacts in the pushbutton, which should be hard-wired to a safety relay or safety PLC which in turn is hard-wired to contacts that stop the process. This sort of arrangement would be classified as PLc or SIL1, which would be suitable for low level risks. If the plant risk assessment has identified a higher category for a particular process, then it will be necessary to improve the reliability of the safety circuit. This can be done by introducing additional redundancy, for example with the addition of a control valve that could shutdown the process in question in the event that the original contacts had failed.
Many automated processes use pneumatic valve islands, connected to PLCs, to maintain the production process and any safety related circuits are separate from the valve island. Fortunately, in many cases the new standards can be achieved without massive alterations to the existing control infrastructure – a simple retrofit can improve the safety levels and achieve compliance.
Plant managers can upgrade existing control structures to PLc or SIL1 with a compact solution and minimal changes to the circuit. The only addition to the control cabinet is the safety relay, which keeps both costs and space requirements to a minimum. In more demanding applications that require a higher level of safety, it is possible to introduce a redundancy valve block that will enable PLd or SIL2 to be achieved.
In processes where an inspection hatch can be opened for maintenance for example, the process must be stopped if a cover is removed at an inappropriate time. Under normal circumstances the safety valve on the pneumatic valve island will stop the process, but in situations where there is an increased risk, the redundancy block is positioned so that if the safety valve does not operate, then the process will be stopped by the backup valve.
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