PT Notes
Safeguard Failures Can Be Initiating Events and Enablers
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In PHA studies, safeguard failures are considered when the PHA team identifies ways in which protection is provided against hazard scenarios. Safeguards may operate successfully or fail to do so when challenges occur as part of a hazard scenario. Safeguards may protect against initiating events, intermediate events, or the consequences of hazard scenarios. However, safeguards failures and spurious operation may also be initiating events for hazard scenarios. For example, a PSV is a safeguard against overpressure hazards but it may be a cause of less flow or low pressure hazards if it is stuck open or operates prematurely. Similarly, an isolation valve is a safeguard against unwanted flow but it may be a cause of unwanted flow if leaking or failed open, or a cause of no flow when flow is needed if it operates prematurely. The possible impacts of safeguard failures and spurious operation include the release of hazardous materials and shutting down equipment and processes.
Similarly, latent failures of safeguards may be enablers for scenarios. Enablers are events or conditions that must be present or active for a hazard scenario to proceed, for example, disabled alarms and safety systems. They do not by themselves initiate a hazard scenario but rather they make them possible. Enablers sometimes are called contributing causes. They may enable the initiating event or any other element of a scenario. Latent failures of prevention safeguards, such as the loss of grounding or corrosion protection, enable initiating events. Latent failures of detection safeguards allow scenarios to proceed while latent failures of mitigation safeguards enable adverse scenario consequences to occur.
The role of both active and latent safeguard failures should be addressed in PHA. Safeguards protect against hazard scenarios but safeguard failures may also be initiating events or enablers for hazard scenarios.
Further information on the treatment of initiating events, enablers, and safeguards is provided in:
Initiating events, levels of causality, and process hazard analysis, Process Safety Progress, Vol. 33, Issue 3, pages 217–220, September 2014.
Addressing enablers in layers of protection analysis, Process Safety Progress, Vol. 33, Issue 3, pages 221–226, September 2014.
On the validation of safeguards for process hazard analysis, Process Safety Progress, Volume 32, Issue 2, pages 165–169, June 2013.
Analytical Methods in Process Safety Management and System Safety Engineering – Process Hazards Analysis, in Handbook of Loss Prevention Engineering, J. M. Haight (ed), Wiley-VCH, Weinheim, Germany, 2013.
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