(18a) A Conceptual Approach to the Evaluation of Layers of Protection for Inherently Safer System Design

First order inherently safer technology (IST) involves the elimination of a hazard through one of four traditional approaches. While these methods are recognized as preferred means of addressing chemical facility hazards, all hazards cannot be removed through the use of these methods due to technical, financial or end-use constraints. When a hazard still exists after the application of first order inherently safer approaches, second order layers of protection can be applied to appropriately reduce the risks of a process to a tolerable level. For any given application, judgment can be applied to order the possible layers of protection in terms of their quality as second order inherently safer solutions.

In this paper, two unit operations, distillation and chemical storage, have been analyzed using Layers of Protection Analysis (LOPA) to determine the appropriate reliability of risk reduction measures. Once the required reliability was established, additional qualitative and quantitative analysis has been conducted to evaluate, as inherently safer solutions, different layers of protection for efficacy, cost effectiveness, and quality. In each case, estimates have been made for capital cost as well as ongoing operating costs for maintaining the layers of protection to determine an appropriate life cycle cost for each implementation. While the most cost effective approach for a given implementation will vary based on the specific situation to which the method is applied, this paper presents a conceptual approach to analysis that can be applied to most unit operations to indicate the cost and benefit of using layers of protection with a higher degree of second order inherent safety which meet the reliability criteria established by a LOPA.