In the wake of catastrophic incidents such as the Bhopal disaster in India, the BP explosion in Texas City, TX and the Buncefield Petrol Depot explosion in the UK, safety has become a top priority for industrial companies running critical processes that contain extreme temperatures and pressures, and flammable or toxic materials. To improve the safety of such processes, the ISA SP84 committee released IEC 61508 (and more specifically IEC 61511 – which is the Process Sector Implementation of IEC 61508). This standard is helping to increase the reliability of the systems that ensure the safety of these processes.
All safety standards exist to reduce risk, which is inherent wherever manufacturing or processing occurs. Although eliminating risk entirely and bringing about a state of absolute safety is not attainable, the goal of any modern safety system is to reduce risk to an acceptable level.
The formula for risk is:
Risk = Hazard Frequency x Hazard Consequence
Risk can be minimized first by incorporating inherently safe process and control system designs but, ultimately, a full safety shutdown system may be necessary.
No single safety measure can eliminate risk and protect a plant and its personnel against harm or mitigate the spread of harm if a hazardous incident occurs. For this reason, safety exists in protective layers: a sequence of mechanical devices, process controls, shutdown systems, and external response measures that prevent or mitigate a hazardous event. Detailed evaluation, including a hazard and risk assessment, is required to identify the overall risk reduction requirements and to properly allocate them into the independent protection layers.
If any protection layer fails, successive layers are available to take the process to a safe state. If one of the protection layers is a safety instrumented function (SIF), the risk reduction allocated to it determines its Safety Integrity Level (SIL). As the number of protection layers and their reliabilities increase, the safety of the process increases. Figure A shows the succession of typical safety layers in order of their activation.
To determine the levels of protective layers required, your company will need to conduct a Process Hazards Analysis to analyze hazards and risks within a process. Depending upon the complexity of the process operations and the severity of its inherent risks, such an analysis may range from a simplified screening to a rigorous Hazard and Operability engineering study, including reviewing process, electrical, mechanical, safety, and managerial factors.
Once the risks and hazards have been assessed, one can determine if they are below acceptable levels. If the study concludes that existing protection is insufficient, a Safety Instrumented System may be required.
Even when emergency shutdown systems are not mandatory, many process control industries today are using the reliability specifications defined in IEC 61508 to separate great products from good ones. In fact, many companies are using a key parameter – Safe Failure Fraction (SFF), which is an indication of all of the safe and dangerous detected failures within a device, to objectively compare the reliability of instruments from different suppliers.
Follow Our Safety Integrity Level Discussion
Next week, we will examine the role of a Safety Instrumented System in industrial process systems. For more information about this topic, you can also download the MAGNETROL® Understanding SIL Technology bulletin.