What are Your Safety Layers and How Do They Compare to the Safety Layers at Bhopal before the Accident?

As we remember the 30th anniversary of the December 1984 accident in Bhopal, India, sadly articles such as this one are still warranted. Despite tremendous efforts across our industry to prevent events like this one via self-improvement and government regulations, key lessons learned from this event are occasionally neglected by industrial management. Furthermore, a generation of new engineers has evolved without direct knowledge of this incident. We must not forget the poor decisions that were made to override safety layers.
 Chemical process industries (CPI) plants use various means, referred to as layers of protection, to protect and mitigate a process safety incident, such as a major fire, release, or explosion. These layers include: process design; basic controls, process alarms, and operator supervision; critical alarms, operator supervision, and manual intervention; automatic action via a safety instrumented system (SIS) or emergency shutdown device (ESD); physical protection via relief devices; physical protection via containment dikes; and finally plant emergency.
 This article reviews the layers of protection that failed, and those that worked, at the Bhopal pesticide plant in December 1984.