Published on 25 Apr 2019 by Arun Patel
See Inherent Safety Trade-offs for related information to Inherent Safety Issues.
In considering inherently safer design alternatives, it is essential to remember that there are often, perhaps always, conflicting benefits and deficiencies associated with the different options.
Deciding among a number of process options having inherent safety advantages and disadvantages with respect to different hazards can be quite difficult. The first step is to understand thoroughly all hazards associated with the process options. Process hazard analysis (PHA) and evaluation techniques are appropriate tools including:
- past history and experience
- interaction matrices
- what if
- what if/checklists
- hazard and operability (HAZOP) studies
The hazard identification step is perhaps the most important, because any hazard not identified will not be considered in the decision process. For example, the impact of chlorofluorocarbons on atmospheric ozone was unknown for much of the period of their use, and this potential hazard was not considered until later.
Once the hazards have been identified, the process options can be ranked in terms of inherent safety with respect to all identified hazards. This ranking can be qualitative, placing hazards into consequence and likelihood categories based on experience and engineering judgment. More quantitative systems can also be used to rank certain specific types of hazard, for example, the Dow Fire and Explosion Index and the Dow Chemical Exposure Index. Unfortunately, none of these indices consider the full range of hazards. To get an overall assessment of the process options, it is necessary to use a variety of indices and qualitative techniques and then combine the results.
Sometimes the consequences of all hazardous incidents can be expressed by a single common measure; for example, dollar value of property damage, total economic loss, risk of immediate fatality due to fire, explosion or toxic material exposure. If all consequences can be measured on a common scale, the techniques of quantitative risk analysis may be useful in assessing the relative magnitude of various hazards, and in understanding and ranking total risk of process options.
In many cases, it is not readily apparent how the potential impacts from different hazards can be translated into some common scale or measure. For example, how do you compare long term environmental damage and health risks from the use of CFC refrigerants to the immediate risk of fatality from the fire, explosion, and toxicity hazards associated with many alternative refrigerants? This question does not have a "right" answer. It is not really a scientific question, but instead it is a question of values. Individuals, companies, and society must determine how to value different kinds of risks relative to each other, and base decisions on this evaluation.
Formal tools for decision making can be useful particularly if the hazards vary greatly in type of consequence or impact. Many of these tools introduce additional rigor, consistency, and logic into the decision process. Some available methods include:
- Weighted scoring methods, such as Kepner Tregoe method (KT-method) and the Analytical Hierarchy Process (AHP)
- Cost-benefit analysis
- Payoff matrix analysis
- Decision analysis
- Multi-attribute utility analysis