Availability versus Safety
This is an important point to be considered when designing a control system with a programmable logic controller. The analysis must be performed with all the elements involved: the PLC, field instruments, valves, motors, dampers, etc.
In addition, it is essential to analyze the process or machine. The design will be very different for a Water Treatment Plant than for an Emergency Shut down System in a Refinery or for a Boiler Protection System.
Let’s see these concepts and ideas about design with a simple part of a process: a line with a fluid, a shutoff valve that opens when PLC output is energized, and a high-pressure sensor which should give closure order to valve when the pressure exceeds a value. Depending on the fluid (water, high pressure steam, gas, nitrogen, etc.) design criteria should be very different.
For safety reasons, the pressure switch contact is closed when the pressure is low (contact N.O.) which is usual in the case of any safety instrument. All elements that can fail are shown in red color. Let’s see how we could make the design of the control cabinet (PLC and external intermediate relays).
If the priority is for “Availability”, i.e., we want to minimize the probability to close the shutoff valve by any failure, we can design as follows:
In this way we ensure the following:
1-Valve will not close due to failure of a PLC input, since we use 2 inputs for the same pressure switch.
2-Valve will not close due to failure of a PLC output, since we use 2 outputs and contacts in parallel.
If the priority is the “Safety”, i.e., we do not want any dangerous situation occurs by the failure of some element, even if valve close, we could design as follows:
Thus we get that the failure of an input or output produces the immediate closure of the valve.
If we want both, “Availability and Safety”, a possible design would be:
With this configuration the valve failure is the weakest point. If we want “Availability” we can add another valve in parallel, for “Safety” the second valve should be in series.
These examples of design with a general purpose PLC are illustrative of the advantages of the “safety PLC”, especially when we have many I/Os and many critical elements that must be designed with the criterion of “Priority to Safety”. With the safety PLC we save a lot of external wiring.
Safety PLC also has a very clear plus: meets the safety standards and incorporates diagnostic functions in all internal components, including CPU, memory, etc.