Using Hierarchy of Controls to Reduce the Likelihood of High Severity Scenarios

The Center for Chemical Process Safety (CCPS) Guidelines on Inherently Safer Chemical Process A Life Cycle Approach(2009) defines a hierarchy that can be used to decrease the risk of a system.  These guidelines include a hierarchy of layers of protection that can be used in Process Hazard Analyses (PHAs) to determine adequacy of the existing safeguards.  This approach can be particularly useful for small businesses that do not elect to apply Layer of Protection Analysis (LOPA) to semi-quantitatively assess risk.

Releases of highly hazardous materials have occurred that could have been prevented by implementation of engineered safeguards.  From a review of these incidents, it had become evident that the owners and operators of the affected facilities relied on procedural safeguards to prevent occurrence of those events.  Examples of such cases have involved chlorine and anhydrous ammonia, which are two commonly used highly hazardous chemicals at facilities in the United States.    

Process Hazard Analysis (PHA) provides a thorough and efficient method for systemically reviewing complex systems for safety concerns.  A commonly applied PHA method is the Guide-Word Style Hazard and Operability (HAZOP) approach, which breaks complex systems into focused sections called nodes.  HAZOPs are a team effort and risk determination varies based on the experiences and safety culture of team members.  Some teams may determine that procedural safeguards are adequate to prevent against a scenario that could lead to a severe event (e.g., liquid release of a volatile toxic material such as chlorine.)  However, accidental releases have been experienced demonstrating inadequacy of procedural safeguards to minimize the risk of a severe event.

The above noted CCPS guidelines considers procedures as the least effective protection layer.  An active engineered safeguard is considered a more reliable protection layer.  A method is demonstrated in this paper for establishing adequacy of available safeguards and the use of CCPS hierarchical definitions to establish the risk gap.  The method also provides the bases for recommending implementation of additional safeguards that can be effective in reducing the risk.  Application of the proposed method will be demonstrated through at least two specific events: release of liquid anhydrous ammonia during oil draining operation and release of liquid chlorine due to overfilling a storage tank.