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Operator Errors and Hindsight Bias
Posted on June 3rd, 2016 by Mike Schmidt in Chemical Manufacturing Excellence
“There is almost no human action or decision that cannot be made to look flawed and less sensible in the misleading light of hindsight. It is essential that the critic should keep himself constantly aware of that fact.” Lord Anthony Hidden
When Sidney Dekker quoted Lord Hidden in his book, Just Culture, he was warning us about the dangers of hindsight bias in our incident investigations. As he notes, “hindsight bias is one of the most consistent and well-documented biases in psychology, but incident reporting systems…have essentially no protections against it.” One of the insidious effects of hindsight bias is that it puts much too much emphasis on the impact of procedure violations in explaining an incident.
There is always—ALWAYS—a difference between what procedures say and what is normally done. One form of labor action that exploits this difference is called “working to the rule.” When a labor force cannot go on strike, it can still cause havoc by insisting on following every policy and procedure to the letter. This inevitably causes a significant slowdown. We don’t expect workers to “work to the rule,” under normal circumstances, but when an incident occurs, many investigators inevitably seize on a procedure violation as the cause, whether the violation is actually abnormal or not.
The Washington Utilities and Transportation Commission (WUTC) recently issued its report on the natural gas explosion that occurred on 31-Mar-2014 at the Williams Peak Shaving Plant in Plymouth, Washington. There is general agreement that the explosion was the result of a natural gas/air mixture remaining in the system following maintenance that subsequently auto-ignited upon startup, when the flammable mixture entered the inlet of a heater. It seems apparent that the system had not been successfully purged of air before starting up, and that is what the report states. However, the report makes much of something it called an operator error that involved a triple pressure purge of cycling between 100 psig and 5 psig, rather than cycling between 100 psig and 0 to 1 psig.
Auto-Ignition of Flammable Mixtures
For a natural gas/air mixture to auto-ignite, two conditions have to be met. First, the mixture has to be flammable. That is, above its lower explosion limit (LEL) and below its upper explosion limit (UEL). Secondly, the flammable mixture has to be above its auto-ignition temperature. The auto-ignition temperature of natural gas, which is essentially methane, is 999 F when it is at atmospheric pressure. However, the auto-ignition temperature decreases with increasing pressure, and a study conducted after the incident showed that at the normal operating pressure of 700 psig, the auto-ignition temperature of natural gas can be as low as 680 F, well above the normal operating temperature. Auto-ignition was inevitable if there was a flammable mixture.
Safe operation, then, depended entirely on assuring that there was no flammable mixture in the system when it started. A flammable mixture of natural gas and air at atmospheric pressure is one where the concentration of natural gas is above the LEL of 4.4% and below the UEL of 15%. Also, the mixture must contain at least a minimum amount of oxygen, regardless of the concentration of fuel. This is known as its limiting oxygen concentration (LOC). For natural gas, the LOC is around 12%. A mixture of natural gas and air that is 43% natural gas and 57% air cannot burn because it contains too much fuel (it is too rich to burn) and because it contains too little oxygen. Purging air from the natural gas system was key to handling the natural gas safely.
An Operator Error?
When the explosion at the Plymouth facility occurred, four workers sustained minor injuries and fifth worker was hospitalized with burns. The explosion caused $46 million in damages and lost product, including $71,000 in damages to public and other private property.
Obviously, there was a flammable mixture, or the explosion would not have occurred. It is reasonable to conclude that the procedures that were followed were insufficient to remove the flammable mixture. It is a mystery, though, how the investigator came to the conclusion that a triple purge cycle of 100 to 1 psig would have helped when a triple purge cycle of 100 to 5 psig was inadequate.
When a system is pressure purged, the pressure is increased from a low pressure to a high pressure by adding the purge gas. Increasing the pressure from atmospheric pressure (14.7 psia) to 100 psig (114.7 psia) has the effect of diluting the system by a factor of 7.8. The oxygen concentration drops from 20.9% to 2.3%, less than the LOC for methane. When the pressure is vented, the concentration stays the same, whether it is vented down to 1 psig (15.7 psia) or 5 psig (19.7 psia). Increasing the pressure to 100 psig a second time further dilutes the system: to 0.5% oxygen if starting from 5 psig, to 0.4% oxygen if starting from 1 psig. Venting and then increasing the pressure to 100 psig with purge gas a third time dilutes the system still more: to 0.08% oxygen if starting from 5 psig, to 0.05% oxygen if starting from 1 psig. Neither concentration is high enough to support ignition or auto-ignition.
Missing an Opportunity to Improve
To insist that the cause of the explosion at Plymouth was the result of the purge cycle is to miss the point and divert efforts away from the real cause. To seize on this deviation from “industry recognized” recommendations, as the press has done, is to fall into the trap of hindsight bias. That there was an unpurged fuel/air mixture in the system when it started up was not the result of an operator error during the purge cycle, but of the design of the system and the procedure to purge it.
No incident report is complete without recommendations to reduce the likelihood of a reoccurrence of the incident in the future. Any good recommendation addresses a cause of the incident, so correctly identifying causes is essential to improving safety. Defaulting to “procedure violations” as a cause when those violations have little or nothing to do with the incident is not helpful.
When you find yourself investigating an incident, be sure you are identifying the real causes so that your recommendations make a real difference in the future. Otherwise, safety is not advanced and blame, if it is necessary at all, is misplaced. It is not easy, but to do anything less is to miss out on an opportunity to improve.
 Dekker, Sidney, Just Culture: Balancing Safety and Accountability, 2nd Ed., Ashgate Publishing Company, Burlington, Vermont. 2012.
All opinions shared in this post are the author’s own.
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Principal, Bluefield Process Safety, LLC
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