The issue: Poor Incident Investigations and how to improve them

Often members of the Incident Investigation team miss some fairly obvious opportunities to improve their process safety. One trick is to use the Hierarchy of Controls as a brainstorming tool when coming up with causes and recommendations.


What is the Hierarchy of Controls and How can I use it as a tool during Incident Investigations?

The premise of the Hierarchy of Controls is that while hazards can be controlled in various ways, certain types of controls are inherently better than others. The hazard controls in the hierarchy are, in order of decreasing effectiveness:

Let’s take an example of an Incident Investigation concerning an unexpected employee NH3 exposure during an oil drain. While you will have to address any unique issues relating to the incident, here are some questions that the Hierarchy of Controls can provide for any oil drain incident:

Elimination: Physically removing the hazard. For example, when analyzing the risk of a valve packing leak in a process room, moving that valve to the roof would eliminate the hazard from the production room. Elimination is usually considered the most effective hazard control.

Substitution: Replacing the hazard with something that does not produce a hazard or something that produces a much smaller hazard. A common example of this is removing the hazard of NH3 in product chillers areas with the use of a secondary refrigerant such as CO2 or Glycol. Note that in some instances this results in simply relocating a hazard to another area with lesser consequences.

Note: We usually combine these two methods because if we don’t, we tend to spend more time arguing whether or not a control is an elimination or a substitution.

  • Can we avoid, or reduce the frequency of, the oil drains? Better coalescers, higher minimum head pressure to reduce oil blow-by, installation of an oil still to minimize oil draining from the system, etc.
  • Can we eliminate / reduce the NH3 involved in the oil drain? Pumpout of the oil pot and re-pressurization with shop air, conversion to a gravity drain oil pot, lower pressure suction during pumpout, etc.


Engineering Controls: These controls do not eliminate hazards but tend to attempt to control them or give notice when the process is approaching an unsafe state. Examples include NH3 sensors, Interlocks, High-Level Floats, Pressure and Temperature transducers, etc.

  • Is the equipment properly configured for a safe oil drain? Oil pot, “Dead-Man” valve, safe access, easy egress routes, etc.
  • Can we improve the ventilation in the area? Portable fans, local exhaust ventilation, manual use of existing Machine Room fans, etc.
  • Can we improve the hazard awareness? Local / Personal NH3 detector rather than relying on a fixed detector, pressure gauge installed during the pump-down, etc.


Administrative Controls: These controls are changes in the way the work is performed on or around the process. Training, Procedures, Signs and Warning labels are all administrative controls.

  • Can we improve the SOP? Better steps to address the hazards, mandating more oversight, required use of PPE, more effective use of ventilation, etc.
  • Can we improve the training? Better understanding of the hazards, procedures, PPE, tools, etc.


Personal Protective Equipment: PPE such as gloves, respirators, etc. is generally considered the last resort of hazard control.

  • Can we improve the PPE available? Can we make certain PPE mandatory? Improved gloves, smocks, respirators, etc.


Using the Hierarchy of Controls can be a great brainstorming tool to help you look at your possible causes, and your possible corrections from some new angles.