"Functional Safety" refers to instrumented systems intended to perform a safety function. Instrumented systems consist of sensors (signaling devices), logic (decisive components) and actuators intended to perform a control or safety function for processes in the chemical, petrochemical or other process industries. Instrumented systems intended to perform a safety function are generally referred to as Safety Instrumented Systems(SIS). These systems must comply with IEC 61508, the international standard for Safety Instrumented Systems.
A safety function is substantially different in concept from a control function:
- the purpose of a control function is to bring a process to its optimum state (the most effective or efficient combination of process conditions);
- a safety function monitors the limits or range of permissible process conditions;
- a safety function generally operates in "on/off" mode, in other words it intervenes forcefully or not at all, but it will not intervene gradually;
- the settings of a safety function are normally fixed and follow from the process design while the settings of a control function vary depending on current process conditions.
A SIS performs an important role in containing chemicals and high pressures and as such, it serves as an important protection in preventing unsafe conditions leading to an incident.
Example "layer of protection analysis (LOPA)"
In the above scheme of different layers of protection, the SIS responds when both the control system (Process Control) and the operator intervention fail to resolve a problem. The failure of the SIS will in turn likely lead to an uncontrolled discharge to the flare or to the vent and perhaps even to an even more dangerous situation. For this reason, the SIS must meet very strict requirements and criteria regarding availability, reliability and probability of failureon demand.