What is a DHA?

DHA is an acronym from NFPA’s 652, Standard on the Fundamentals of Combustible Dust, 2016 Edition. This NFPA standard came out in 2015 and is VERY MUCH in line with OSHA’s process safety principles, in that it outlines how combustible dust hazards are to be identified and managed. One of these management tools is a Dust Hazards Analysis (DHA). In this article I will breakdown the DHA element of NFPA 652 so that we can see how similar it is to a PHA and the specifics that must be addressed. The dust hazard analysis methodology is not necessarily the same as that which is found in OSHA’s PSM regulation but as we will see, those who are already doing PHA(s) will recognize the efforts to conduct a DHA.

NFPA states

“A DHA is a careful review of the fire and explosion hazards to determine the consequences of what could go wrong and to determine what safeguards could be implemented to prevent, protect, and/or mitigate those consequences.”

The requirements for the DHA “shall apply retroactively”, MEANING that the NFPA intends facilities to do a DHA even though the standard was published in 2015, meaning the DHA requirement is NOT intended ONLY for new processes built after 2015!  NFPA 652 even provides some guidance on WHEN the DHA is to be done:

1. For existing processes and/or facility compartments that are undergoing material modification (e.g modifications or maintenance/repair activities that exceed 25% of the original cost), do the DHA as part of the project.
2. For existing processes and facility compartments that are NOT undergoing material modification, schedule and complete DHAs of existing processes and facility compartments within a 3-year period from the effective date of the standard (e.g. September 7, 2015). We should be able to demonstrate reasonable progress in each of the 3 years.

NOTE: “It is not the intent of this requirement to permit a delay in the completion of all DHA until the third year”.

The DHA is intended to evaluate the fire, deflagration, and explosion hazards and provide recommendations to manage combustible dust hazards. The DHA should consider the four conditions that are required for a deflagration:

1. A combustible particulate solid of sufficiently small particle size to deflagrate
2. A combustible particulate solid suspended in air to deflagrate (or other oxidizing medium)
3. A combustion particulate solid suspension of sufficiently high concentration to deflagrate
4. A competent igniter applied to the suspension of combustible particulate solids where the concentration is sufficient for flame propagation.

If these four (4) conditions can be obtained within handling/processing equipment OR outside this equipment should the dust find its way outside its primary containment OR it is allowed to accumulate to hazardous levels the DHA should address ALL three (3) situations.

DHA Team and Leader

The DHA should be performed or led by a “qualified person”. The qualified person must be familiar with conducting a hazard analysis, such as a PHA. It is my professional option that a person who has been trained in PHA methodologies would be suited to lead a DHA. But being trained in PHA methodologies is NOT enough to lead a DHA, as NFPA states the qualified person should also be familiar with the hazards of combustible dusts.

A team should perform the DHA. For some processes this team may be a little as two to three people and for larger or more complex processes, the team might require a more persons. This team is made up of a variety of persons whose background and expertise should include the following:

1. Familiarity with the process
2. Operations and maintenance
3. Process equipment
4. Safety systems
5. History of operation
6. The properties of the material
7. Emergency procedures

DHA Scope and Methodology

The DHA shall include the following:

1. Identification and evaluation of the process or facility areas where fire, flash fire, and explosion hazards exist
2. Where such a hazard exists, identification and evaluation of specific fire and deflagration scenarios shall include the following: 
   1. Identification of safe operating ranges
   2. Identification of the safeguards that are in place to manage fire, deflagration, and explosion events
   3. Recommendation of additional safeguards where warranted, including a plan for implementation

The process safety data for the process or facility compartment determined to be combustible dust hazards should include, but not be limited to, the following:

1. Test reports
2. Drawings
3. Sizing calculations

This includes the process systems and ancillary equipment such as dust collection systems.

Each part of the process system where combustible dust is present or where combustible particulate solids could cause combustible dust to be present should be included in the scope of the DHA. This evaluation should address the following:
(1)Potential intended and unintended combustible dust transport between parts of the process system
(2)Potential fugitive combustible dust emissions into a building or building compartments
(3)Potential deflagration propagation between parts of the process system

Each part of the process that contains a combustible particulate solid and that can potentially include both of the following conditions shall be considered a fire hazard and shall be documented as such:
(1)Oxidizing atmosphere
(2)Credible ignition source

Each part of the process that contains a sufficient quantity of combustible dust to propagate a deflagration and that can potentially include all the following conditions shall be considered a dust deflagration hazard and shall be documented as such:
(1)Oxidizing atmosphere
(2)Credible ignition source
(3)Credible suspension mechanism

Each and every process component should be evaluated, including ducts, conveyors, silos, bunkers, vessels, fans, and other pieces of process equipment. Each point along the process should be described, and hazards at each point should be identified. Remedial measures for each hazard should be identified and documented.

Each building or building compartment where combustible dust is present shall be evaluated. Where multiple buildings or building compartments present essentially the same hazard, a single evaluation shall be permitted to be conducted as representative of all similar buildings or building compartments.

The evaluation shall address potential combustible dust migration between buildings or building compartments.
The evaluation shall address potential deflagration propagation between buildings or building compartments.

Each building or building compartment that contains a combustible particulate solid and that can potentially include both of the following conditions shall be considered a fire hazard and shall be documented as such:
(1)Oxidizing atmosphere
(2)Credible ignition source

Each and every facility compartment containing combustible particulate solids should be evaluated. The complete contents of the compartment should be considered, including hidden areas. Each area in the compartment should be described, and hazards at each point should be identified. Remedial measures for each hazard should be identified and documented.

The evaluation of dust deflagration hazard in a building or building compartment shall include a comparison of actual or intended dust accumulation to the threshold housekeeping dust accumulation that would present a potential for flash-fire exposure to personnel or compartment failure due to explosive overpressure.

Each building or building compartment that contains a sufficient quantity of combustible dust to propagate a deflagration and that can potentially include all of the following conditions shall be considered a dust deflagration hazard and shall be documented as such:
(1)Oxidizing atmosphere
(2)Credible ignition source
(3)Credible suspension mechanism

The results of the DHA review shall be documented, including any necessary action items requiring change to the process materials, physical process, process operations, or facilities associated with the process.