See Introduction to Inherently Safer Chemical Process for more information relating to The Role of Inherently Safer Design Concepts in Process Risk Management.
How does inherently safer design fit into an overall process risk management program? To answer this question, it is first necessary to understand the definition of risk. Risk is defined as a measure of economic loss, human injury, or environmental damage in terms of both the incident likelihood and the magnitude of the loss, injury, or damage. Any effort to reduce the risk arising from the operation of a chemical processing facility can be directed toward reducing the likelihood of incidents (incident frequency); reducing the magnitude of the loss, injury or damage should an incident occur (incident consequences), or some combination of both. In general, the strategy for reducing risk, whether directed toward reducing frequency or consequence of potential accidents, can be classified into four categories. These categories, in decreasing order of reliability, are:
- Inherent: Eliminating the hazard by using materials and process conditions which are nonhazardous; e.g., substituting water for a flammable solvent.
- Passive: Minimizing the hazard by process and equipment design features which reduce either the frequency or consequence of the hazard without the active functioning of any device; e.g., the use of equipment rated for higher pressure.
- Active: Using controls, safety interlocks, and emergency shutdown systems to detect and correct process deviations; e.g., a pump that is shut off by a high level switch in the downstream tank when the tank is 90% full. These systems are commonly referred to as engineering controls.
- Procedural: Using operating procedures, administrative checks, emergency response, and other management approaches to prevent incidents, or to minimize the effects of an incident; e.g., hot-work procedures and permits. These approaches are commonly referred to as administrative controls.
Risk control strategies in the first two categories, inherent and passive, are more reliable because they depend on the physical and chemical properties of the system rather than the successful operation of instruments, devices, procedures, and people. Inherent and passive strategies differ, but are often confused. A truly inherently safer process will reduce or completely eliminate the hazard, rather than simply reducing its impact. Table below gives examples of the four risk management strategy categories. These categories are not rigidly defined, and some strategies may include aspects of more than one category.