27 September

It’s Time to Review and Update your DHA (Dust Hazards Analysis)

Many companies are now reviewing their initial DHAs, performed as a requirement of NFPA 652 (Standard on the Fundamentals of Combustible Dusts). Why are they doing this now and should you?

Following a series of severe industrial dust explosions in North America, the National Fire Protection Association released their NFPA 652 Standard on the Fundamentals of Combustible Dusts back in 2015. This was seen as an important step in helping industry reduce its risk of dust explosion and fire. That standard introduced the concept of the DHA (Dust Hazards Analysis) with the following text:

“The owner/operator of a facility where powders that have been determined to be combustible or explosible [….] are present in an enclosure, shall be responsible to ensure a DHA is completed. “

Furthermore, in the 2019 edition of the same standard is stated

“The DHA shall be reviewed and updated every 5 years.”

In fact, the 5-year expiry date of current DHA’s is only one reason why companies are asking us to help with review and update of their DHA. In this article we look at the reasons why you may now want to check and revise YOUR INITIAL DHA. Some reasons are quite surprising; so, read on….

Why now is the time to review and update your DHA:

Reason 1: Findings from last DHA not fully implemented or wrongly implemented

What we have found is that not all companies that have previously commissioned a DHA have properly implemented its recommendations. Sometimes there has been inertia in the company, but other times the DHA recommendations have been impractical – or in some cases simply too expensive to justify. And occasionally we have found that recommendations have been incorrectly interpreted and therefor are ineffective. Companies struggling for these reasons are contacting us with a request for help with understanding and interpretation, and sometimes with a request to find alternative ways of achieving safety that are more practical or more cost effective.

And sometimes we are simply being asked to check that the DHA’s recommendations have been correctly implemented.

One of our Process Safety Consultants says “There is usually more than one way to skin a cat! May be a basis of safe operation can be found that relies on control of ignition sources rather than plant redesign and installation of involved explosion protection equipment. Or perhaps it’s possible to limit/ control flammable atmospheres more effectively, rather than upgrading electrical equipment to higher IP ratings or to explosion proof design, for example. Even electrostatic sparks can sometimes be eliminated by design rather than adopting expensive counter measures”.

Another of our Process Safety Specialists goes on to say that “if a DHA report contains recommendations that are impractical, then the DHA assessor has failed. Companies should be demanding of their assessor, if they have one, to ensure DHA projects are only signed off once the client understands the DHA report and recommendations and is sure everything is practical.”

Yet another of our consultants revisited a site and quickly realized that almost none of his recommendations had been implemented. Then he understood why he was re-visiting; the company had just been indicted by OSHA!

Reason 2: Product or powders have changed

Perhaps your powders supplier has changed or may be the powders being processed have ben substituted. In either case, changes in powders specification can change the combustibility properties of associated powder accumulations and dust clouds. Obviously, a powder that has been switched to one with different chemical composition will have different combustibility properties, but physical properties of powders will also affect combustibility. A powder that has become finer and drier will likely be easier to ignite as a dust cloud (lower minimum ignition energy) and will explode with greater severity (increase in maximum explosion pressure and Kst value); and its propensity to pick up and retain electrostatic charge may also have changed if moisture content has changed.

Such physical property changes and chemical property changes may therefore affect the basis of safety established in the DHA. Explosion protection may not be adequate, protection against static electricity may be invalidated, dust collection and housekeeping arrangements may not be sufficient…. and so on. All good justification to review your DHA.

Reason 3: Process or equipment has changed

Process equipment can be subject to regular modification. If a hopper or conveyor is replaced, has the new unit been fitted with the correct explosion protection, for example? An old storage hopper fitted with an explosion relief vent can be replaced with a new one with an explosion relief vent. But if the new hoper is of a different strength, different volume or simply a different shape, the required explosion relief area may be different.

Same thing for most other equipment. Changing from metal to plastic equipment can introduce electrostatic hazards – and even a change to issued footwear can affect precautions against static electricity.

Drift/ changes over time; yet another good reason to revalidate your DHA now.

Reason 4: Training and management systems have not been reviewed, updated or fully implemented


NFPA 652 is specific on the need for training on dust combustibility hazards for employees (and contractors). Specifically:

“General safety training and hazard awareness training for combustible dusts and solids shall be provided to all affected employees“, and

“Job-specific training shall ensure that employees are knowledgeable about fire and explosion hazards of combustible dusts and particulate solids in their work environment”

We have found that job-specific and general dust training can easily lapse in an organization as other training and competing priorities come along. And, of course, new employees must be trained as they join the organization.

At Stonehouse we have helped many companies with both general and specific training courses, both live and on-line virtual training.

Management Systems

Management systems are essential and effective if they are properly implemented and maintained. But even here there can be a tendency for drift in application. NFPA 652 says:

“The owner/operator shall evaluate the effectiveness of the management systems presented [….] by conducting a periodic review“ and

“The owner/operator shall be responsible for maintaining and evaluating the ongoing effectiveness of the management systems”

How has your management systems changed since your DHA was completed?

Reason 5: The 5-year review point has arrived

We started this article by referencing 2016 when the first version of NFPA 652 was issued. Some companies were ahead of the game, but for many the 5-year review point is fast approaching.

NFPA 652 states:

“The DHA shall be reviewed and updated at least every 5 years”

What to do

There are a number of reasons why companies in North America are now looking to review and update their Dust Hazards Analyses (DHAs). Once a review is completed, the DHA can sometimes be revalidated by a side letter, if all is in order. In other cases, an update by addendum is possible. Of course, in cases where there have been significant changes to plant, equipment, or powders, a new DHA may be the best way of proceeding.

If you would like more information on how Stonehouse Process Safety consultants can help you with DHA review and update, please contact us by emailing us at info@stonehousesafety.com or call us on 609-455-0001.

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