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PT Notes

The ALARP Principle in Process Safety

PTNotes is a series of topical technical notes provided periodically by Primatech for your benefit. Please feel free to provide feedback.

The As Low As Reasonably Practicable (ALARP) principle provides a sensible basis for managing process risks. It embodies several concepts.

First, risks for processes can be partitioned into three regions:

  • Unacceptable risks that cannot be justified except under extraordinary circumstances.
  • Tolerable risks that are considered acceptable if further risk reduction is impractical, i.e. the benefit does not outweigh the impact.
  • Broadly acceptable risks that are so low as not to be of concern.

Second, as risk decreases, the proportional benefit of risk reduction diminishes.

Third, efforts to reduce risk should be continued until the incremental sacrifice is grossly disproportionate to the value of the incremental risk reduction achieved. Incremental sacrifice is defined in terms of cost, time, effort, or other expenditures of resources.

Application of the ALARP principle involves specifying two sets of risk tolerance criteria. The first set of criteria, corresponding to the dividing line between the unacceptable and tolerable regions, usually called the maximum tolerable risk, is a minimum requirement that must be met. The second set of criteria, corresponding to the dividing line between the tolerable and broadly acceptable regions, usually called the broadly acceptable risk, is a goal which may not be reached but towards which progress must be made until risk reduction measures involve grossly disproportionate sacrifices. The residual risk, that is the risk remaining after controls have been implemented, should fall either in the broadly acceptable region, or near the bottom of the tolerable region. The ALARP principle can be incorporated into both qualitative and quantitative risk analysis.

The key concept in using the ALARP principle is the tradeoff between the benefits and costs of risk reduction measures. Eventually a point of diminishing returns is reached wherein expenditure increases markedly as risk reduction diminishes rapidly. Cost-benefit analysis (CBA) can be used with risk analysis and entails a comparison of the sacrifice (costs) and the risk reduction (benefits) achieved for a risk reduction measure.

CBA helps in making judgements on whether risk reduction measures are reasonably practicable. In standard CBA, usually a measure is adopted only if benefits outweigh costs. However, in ALARP determinations, a measure is adopted unless the sacrifice is grossly disproportionate to the risk. A Disproportion Factor (DF) is used so that a risk reduction measure would not be considered worth the risk reduction achieved if: Costs / Benefits > DF. The greater the risk, the greater should be the DF to achieve a given benefit.

Cost-benefit analysis is useful to assist in deciding what risk reduction measures are warranted for the costs involved. The use of a disproportion factor in the analysis formalizes the concept of reasonably practicable and provides justification and transparency for decisions made.

For more information, you can contact Primatech or consult the article:

The ALARP Principle in Process Safety, by Paul Baybutt, Process Safety Progress, Vol. 33, Issue 1, Pages: 36–40, March 2014.

The article is available at: http://onlinelibrary.wiley.com/doi/10.1002/prs.11599/abstract

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