Question:

Which one of these layers of protection (operator response to alarm, relief valves, dikes, and safety instrumented systems) is not like the other? 

Answer:

Operator response to alarm (Operator Intervention), because of the “Human” factor.

It is very difficult to calculate the probability of failure on demand (or the risk reduction factor) for an operator’s response to alarm because it is dependent on many different things. The same operator can perform differently from one day to the next depending on their physical condition, amount of sleep, number of days worked in a row, and the level of stress in their personal life.  Performance is also driven by issues such as nuisance alarms, alarm floods, poor human-machine interface (HMI) design, and insufficient operator training. A recent article published in Chemical Engineering Progress (CEP) discusses how human factors best practices can be applied to alarm management in order to change operator behavior and improve operator response.

There are many great takeaways from this paper. One that “sticks” with me concerns operators ignoring nuisance alarms, which is commonplace in control rooms today. When nuisance alarms contribute to an incident, a typical corrective response is to add more alarms (to back up the ones the operator is ignoring) or discipline the operator. These actions only make the situation worse.

When one looks at the situation through the lens of human factors, we gain a different understanding of how to correct the situation.  Nuisance alarms represent conditions where the alarm is not true, or when no action is needed on the part of the operator.  Since operators are busy people and have a lot of data to process through their mental models, the ability to discount data that is not important in order to focus on critical data, is a useful and desirable skill. Consequently, the ignoring of nuisance alarms is NORMAL human behavior and the best way to stop operators from ignoring nuisance alarms is to eliminate the nuisance alarms…

As stated by Mica Endsley, a leading human factors expert,

“A person’s reluctance to respond immediately to a system that is known to have many false alarms is actually quite rational. Responding takes time and attention away from other ongoing tasks perceived as important” (Ref: “Designing for Situation Awareness: An approach to User Centered Design”)

To learn more, please download the article “Plug the Holes in the Swiss Cheese Model"