April 2017

Environment and Safety

Process safety incidents, cognitive biases and critical thinking

Process hazard analysis (PHA) is performed to identify possible hazard scenarios that may occur in a process.

Baybutt, P., Primatech, Inc.

Process hazard analysis (PHA) is performed to identify possible hazard scenarios that may occur in a process. This type of analysis enables companies and operations personnel to take suitable measures to protect against process safety incidents resulting from toxic, flammable, explosive or reactive hazards that may adversely impact people, property or the environment.1,2

Various human factors can influence the performance of studies.3 Psychological processes involved in hazard and operability (HAZOP) studies—specifically interactions between team members, and how team members perceive, remember, judge and reason—have been addressed.4 In particular, decisions made during the performance of PHA studies are subject to various cognitive biases.5

Cognitive psychologists have studied how people make decisions and the conditions under which decisions may be unreliable. It has been shown that various cognitive biases influence decisions and can hinder rationality.6,7 Cognitive biases are unconscious, automatic influences on human judgment and decision-making that can cause reasoning errors; distort perceptions, interpretations and judgments; and produce irrational decisions. They arise from various mental processes that can be difficult to distinguish, including information-processing shortcuts and motivational and social factors. Many cognitive biases have been documented in literature, and their nature and causes described. Cognitive biases occur commonly.

Unfortunately, cognitive biases are difficult to detect and override because they are used unconsciously and automatically. Even those aware of their existence are unable to detect bias in their decisions when it occurs. Consequently, mitigation of cognitive biases poses challenges. Described here is the possible role of cognitive biases in a hazard analysis, one that missed the identification of a hazard scenario that subsequently occurred as a process safety incident.

Cognitive bias and a process safety incident

On June 3, 2014, an explosion and fire occurred in a styrene monomer and propylene oxide (SMPO) unit at the Royal Dutch Shell petrochemical plant in Moerdijk, south of Rotterdam in the Netherlands.8,9 Two contract workers were injured, and the explosion caused extensive process damage requiring over a year to repair.

The incident occurred during the pre-startup phase, after a scheduled shutdown and replacement of catalyst in the hydrogenation reactors. The hydrogenation unit was being heated in preparation for a catalyst reduction step when one reactor and a separation vessel suddenly exploded. The incident investigation concluded that both the reactor and separation vessel had failed due to overpressure resulting from unexpected exothermic reactions between ethylbenzene and the copper chromite oxide catalyst. Various factors combined to cause this reaction.8,9 The key factor was a lack of full understanding of the reaction chemistry.

A reactive hazard assessment (RHA) had been conducted in 2011 for this section of the SMPO unit. However, the analysts did not identify the reaction of ethylbenzene that caused the explosion. Certainly, there are no guarantees that a hazard analysis will identify all possible hazard scenarios. Present PHA methods exhibit a variety of weaknesses,10,11 and PHA for reactive chemical hazards can be challenging.12 Of interest in the Moerdijk incident is how the RHA team missed identifying the scenario that occurred.

The incident description states, “Prior to the incident, it was generally believed that under inert nitrogen conditions, ethylbenzene would not react with the passivated catalyst.”8 It was subsequently determined, and proven in the laboratory, that two exothermic reactions actually occurred during the heat-up. The RHA relied on the belief that ethylbenzene was an inert medium in combination with the catalyst, although this assumption had not been verified empirically.9

As is usual in such cases, the incident investigation focused on the technical issues involved. However, fundamentally, the incident may be attributed to the decision made during the RHA regarding the lack of reactivity of ethylbenzene with catalyst. A key question is why the assumption of lack of reactivity of ethylbenzene with catalyst was accepted, even though this was not the case.

Based on the description of the RHA,8,9 it is likely that one or more cognitive biases played a role in the process safety incident. Hazard analysis studies are subject to various types of cognitive biases.13 For example, groupthink is a phenomenon in which a group of people share common but possibly false beliefs, and think and make decisions in the same way. Mindsets are assumptions held by an individual that are so established that the individual does not recognize that they exist, and continues to accept prior choices as valid. Sometimes, people defer to the opinions of perceived authorities without substantiation of the views expressed.

Another cognitive bias, group polarization, refers to the tendency of a group to make decisions that are more extreme than the initial inclinations of its members. This type of cognitive bias also could have played a role in the RHA. Decisions subject to group polarization can be more risky or more cautious. The phenomenon is of most concern when a group tends to be more accepting of risk than an individual; in such cases, it is called a risky shift.

All or some of these cognitive biases would help explain the decision made during the RHA regarding the lack of reactivity of ethylbenzene with catalyst. The telltale indicator that reveals cognitive bias is the use of the words “generally believed” in the description of the incident when referring to the lack of reactivity. Belief is not knowing. Qualification by “generally” implies broad agreement with the belief, although unproven.


Cognitive biases can affect all decisions made by people. Decisions made during PHA studies are particularly important, and significant efforts to address the impact of cognitive biases are warranted. The impact of cognitive biases on PHA studies can be minimized by using as much tangible data and information as possible to avoid the need to rely on opinions, which may be tainted by cognitive biases.

PHA practitioners must recognize when reliance is being placed on opinions and assumptions. The discipline of critical thinking is essential in this regard.14 Critical thinking can be applied in process safety to help ensure that cognitive biases, fallacious reasoning and rhetorical devices do not produce poor decisions.15

Also, the use of a “devil’s advocate” as a PHA team member can address many cognitive biases.16 A devil’s advocate challenges and debates views offered by others to help determine their validity. Devil’s advocates actually may agree with the views offered, but their role is to challenge them and possibly even take an opposing position.


PHA studies require practitioners to make many decisions. Human decisions are influenced by cognitive biases. Key process safety decisions can benefit from the application of the discipline of critical thinking, which can help identify flaws in reasoning and help prevent process safety incidents. HP


  1. American Institute of Chemical Engineers, Guidelines for Hazard Evaluation Procedures, 3rd Ed., Center for Chemical Process Safety, John Wiley & Sons, Hoboken, New Jersey, April 2008.
  2. Baybutt, P., “Analytical methods in process safety management and system safety engineering—Process hazards analysis,” Handbook of Loss Prevention Engineering, J. M. Haight, Ed., Wiley-VCH, Weinheim, Germany, 2013.
  3. Baybutt, P., “The role of people and human factors in performing process hazard analysis and layers of protection analysis,” Journal of Loss Prevention in the Process Industries, Vol. 26, 2013.
  4. Leathey, B. and D. Nicholls, “Improving the effectiveness of HAZOP: A psychological approach,” Loss Prevention Bulletin, No. 139, 1998.
  5. Baybutt, P., “The psychology of decision-making in process hazard analysis,” Spring Meeting & 11th Global Congress on Process Safety, Austin, Texas, April 2015.
  6. Pohl, R. F., Cognitive Illusions: A Handbook on Fallacies and Biases in Thinking, Judgement and Memory, Psychology Press, Hove, UK, 2004.
  7. Kahneman, D., Thinking, Fast and Slow, Farrar, Straus and Giroux, New York, New York, 2011.
  8. Nederlandse Vereniging voor Arbeidshygiëne, “Learning from incidents—Awareness alert: Explosions in SMPO hydrogenation unit,” September 2015, online: http://www.arbeidshygiene.nl/-uploads/files/insite/cgc-20160121-explosions-in-smpo-hydrogenation-unit.pdf
  9. Gonzales, N., “Explosions in SMPO hydrogenation unit,” 12th Global Congress on Process Safety, Houston, Texas, April 10–14, 2016.
  10. Baybutt, P., “Requirements for improved PHA methods: Addressing weaknesses in HAZOP and other traditional PHA methods,” 2014 Spring Meeting & 10th Global Congress on Process Safety, New Orleans, Louisiana, April 2014.
  11. Baybutt, P., “Requirements for improved process hazard analysis (PHA) methods,” Journal of Loss Prevention in the Process Industries, Vol. 32, November 2014.
  12. Baybutt, P., “Consider chemical reactivity in process hazard analysis,” Chemical Engineering Progress, Vol. 111, Iss. 1, January 2015.
  13. Baybutt, P., “Cognitive biases in process hazard analysis,” Journal of Loss Prevention in the Process Industries, Vol. 43, September 2016.
  14. Moore, B. N. and R. Parker, Critical Thinking, McGraw-Hill Education, New York, New York, 2015.
  15. Baybutt, P., “A framework for critical thinking in process safety management,” Process Safety Progress, Vol. 35, Iss. 4, December 2016.
  16. Baybutt, P., “Process hazard analysis (PHA) team member roles that may be overlooked,” Loss Prevention Bulletin, Iss. 247, February 2016.

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