DSS036: What is the Hierarchy of Controls?

In this episode of the DustSafetyScience Podcast, we talk about applying the hierarchy of controls to combustible dust safety. This concept was introduced during our conversation with Jeff Nichols in Episode #23, and today we explore the following:

The components of the hierarchy of controls
What this fundamental process looks like in practice

What is the meaning of ‘Hierarchy of Controls’?

The hierarchy of controls is a system of ranking different hazard control methods, from most effective to least effective.

We often see the hierarchy applied in reverse. For example, many facilities stress the importance of wearing personal protective equipment like safety glasses but this is actually the least effective way to safely engage in a task or remove a hazard.

The hierarchy of controls is like an inverted triangle, with the most effective parts on top and the least effective parts on the bottom. The National Institute for Occupational Safety and Health has a graphic that effectively illustrates the concept. (See link here)

There are five elements of the hierarchy of controls. From most to least effective, they are:

Elimination
Substitution
Engineering Controls
Administrative Controls
Personal Protective Equipment

These five elements can be subdivided into an inherently safer design (elimination and substitution) and applied or bolt-on safety, which includes the three remaining elements. These are procedures that are applied to equipment and worker attire.

Elimination

Elimination is the most effective hazard control method. As the late Trevor Kletz stated, “What you don’t have can’t explode.”

In his autobiography, Trevor provided the example of an engineer overlooking a large nitroglycerin container. Twenty years later, the same facility no longer stores nitroglycerin because its processes were improved. Now it handled only as much as it needed.

There are similar examples in the combustible dust world. Facilities have removed sawdust storage silos after improving their incoming raw material operations. These are all examples of dealing with hazards by eliminating them.

Substitution

The second element on the hierarchy is substitution, or replacing the hazard with a safer option. Can you use a less reactive or even inert material? Can you switch from bucket elevators to dense phase conveyance or some other conveying system? Can you move to a larger particle size if the particle size’s operating parameters can change?

Process Plants: a Handbook for Inherently Safer Design by Dr. Trevor Kletz [Affiliate] and Dr. Paul Amyotte addresses substitution. So does Dr. Amyotte’s book An Introduction to Dust Explosions [Affiliate].

Some aspects of inherently safer design fall under the substitution category, which can be broken down into three steps:

Minimization: Minimize the volume of hazardous substances being handled or generated. Examples can include running tests on smaller amounts or installing a planer that generates less dust.

Moderation: Change the operating conditions. Can you run at lower temperatures? Add an inert gas so that you can’t actually have an explosion?

Simplification: Avoid unnecessary complexity and opportunities for error. This could be simplifying our design, simplifying our plant layout, simplifying our safety solutions, or simplifying our engineering control safety solutions. These are all ways to have an inherently safer system at the end of the day.

Applied Safety / Bolt-On Safety

This category includes the three remaining elements of the hierarchy of controls.

Engineering Controls

These controls include ignition control, deflagration venting, suppression, and other technological approaches to increasing safety. They seek to prevent something that’s happening from getting worse or escalating and hurting people.

Engineering controls are less effective than elimination and substitution because something has to happen. If you allow a hazard to remain in place and put an engineering control on it, something has to happen (e.g., an explosion) and that something needs to trigger the response system.

Administrative Controls

Administrative controls consist of rules, regulations, policies, checklists, and other guidelines that everyone knows but doesn’t always follow. Their effectiveness depends on how they are implemented and whether the implementation involves a lot of time and expense.

If a control is time-consuming or expensive, it’s not likely to be applied, and when an explosion or other incident happens, the employees are usually blamed for not doing something that, in reality, no one at the facility does. If you compare your administrative controls to what people actually do in the facility every day, you get an idea of how effective they might be.

Personal Protective Equipment

This is the least effective hazard control method. It includes safety glasses and fireproof clothing. The reason why it’s the least effective is that has both engineered and administrative control elements. Protective equipment needs to be spec’d against the hazards that are potentially in the facility and it must be worn.

Although it’s at the bottom of the hierarchy, PPE is still important. In some cases, it can be your last line of defence: if there’s a fire or sudden backflash, fireproof gear can protect you from serious burns.

Conclusion

When does minimization, moderation, or simplification move into the realm of applied safety design instead of being inherently safer approaches? For example, cleaning up dust is minimization, but is it an administrative control? Is somebody responsible for cleaning it up? And are they? Is it possible they won’t?

These are some questions you need to ask yourself if you consider these measures to be part of an inherently safer design. If someone is being compelled to carry them out, it’s probably an administrative control. If there is a technology component, such as a failsafe, then it’s probably an engineering control. It’s important to be aware of these issues. Elimination and substitution are inherently safer but they need to be implemented correctly and not have an administrative or engineering aspect, or their effectiveness will be impacted.

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.