3 concepts of great process safety minds that should be in your bowtie diagram

Note, that the barriers are ordered from left to right by their response time. So preferably, the operator proactively acts in time upon the tank level monitoring system and shuts down the flow before the high-level alarm is reached. If the operator fails to monitor the level, or if the monitoring system fails technically somehow, the high-level alarm will alert the operator to manually close the valve. Finally, the last line of defense is the high-level automatic trip, that only takes a split second to close the valve once its threshold is reached. Therefore, it sits on the far right and has the shortest response time. Of course, the response time of the high-level trip should be shorter than the time between reaching the trip’s threshold level and an actual overfill. If these are too close to each other, lower the threshold, or accelerate the response time of the trip function.

Barrier classification

Besides identifying performance attributes, Sklet also writes about barrier classification. Classifying barriers helps to understand them and identify if a company is reliant on too many barriers of the same kind. For instance, one can identify the different types of systems that can implement a barrier function. These systems usually consist of either some type of hardware, behavior or a combination of both.

Barrier system types

There are different ways to categorize these systems. In the default set-up of BowTieXP, you will find five different categories: Behavioral, Socio-Technical, Active hardware, Continuous hardware and Passive hardware.

Depending on the category, a barrier can contain three distinct parts. A detection mechanism, a decision based on what is detected, and an action that follows the decision. When analyzing a barrier, one needs to identify its parts, and whether those parts are behavioral or technical in nature. The different combinations determine the system type.

For instance, a double check is a purely behavioral barrier because the detection, decision and action are all behavioral. Another example could be a sprinkler system that is activated by pressing a fire alarm button. The detection and decision are behavioral, whereas the action is technical, which makes it a socio-technical barrier. A fence or dike does not detect, decide or act. Its existence alone is enough to have an effect (we are leaving maintenance out of the discussion, as that is on a different level and not part of the barrier itself), which makes it passive hardware.

1. Behavioral barrier: the detect decide act parts of the barrier are completely represented by people
2. Socio-technical: the detect decide acts parts of the barrier are a mix between people and hardware
3. Active hardware: the detect decide act parts of the barrier are completely hardware based
4. Continuous hardware: a barrier with no detection, but a continuous action (like for instance a ventilation system)
5. Passive hardware: is effective by just existing without any need for explicit action. Does not have detect, decide or act parts.

The added advantage of categorizing barrier systems lies in understanding the diversity of barriers. More diversity in the type of barriers you have is generally better. Having only behavioral barriers or only hardware barriers makes a system vulnerable, not only because barriers of one type can compensate for the weaknesses of other types, but also because barriers of the same type are more vulnerable to common mode failure.

Learn more about barrier classifications and performance attributes

There are multiple ways of classifying barriers and the default installation of BowTieXP can be adjusted to reflect the taxonomies of the framework that you would like to use. This blog covered only a few of them. For inspiration on more barrier classifications and performance attributes, read Snorre Sklet’s article “Safety barriers: Definition, classification, and performance” as it is a comprehensive summary of relevant sources. You can also use the References of Sklet’s article as a starting point to explore underlying sources of information and learn even more about safety barriers.