DSS026: Different Types of Dust Hazard Analysis with Marc Hodapp

In this episode of the Dust Safety Science Podcast, we interview Marc Hodapp, Senior Fire Protection Engineer at Jensen Hughes in Baltimore, Maryland, about the different approaches to dust hazard analysis. In the process, he answers the following important questions:

What is the traditional approach to dust hazard analysis, including a prescriptive approach?
What can you do when the prescriptive approach can’t be implemented, which can happen when a facility is older and/or there are difficulties meeting these guidelines or regulations?
What’s the difference between performance-based design and risk analysis?
What are some of the advantages of using these different methods?
What success stories does he have to share?

Marc has over 10 years’ experience as a fire protection engineer and has done a lot of performance-based design in that area. He and Martin Clouthier, who was interviewed in Episode 21 of this podcast, are senior members of the combustible dust team at Jensen Hughes. He estimates that he’s done over 300 dust hazard analyses, so he has a highly informed perspective.

Prescriptive vs. Risk-Based DHAs

In May 2017, the Center for Chemical Process Safety (CCPS) published Guidelines for Combustible Dust Hazard Analysis, which defined traditional (prescriptive) vs. risk-based DHAs. There are core similarities in both methods, mainly:

Understanding the properties of the dust in the facility, including flammability and combustibility
Determining whether or not an ignition source exists

Traditional, or prescriptive, DHAs are guided by standards like NFPA652. Although commonly applied, there are cases where the traditional standards can’t feasibly be implemented, and a risk-based DHA is a viable alternative.

Marc said that he has been getting a lot of DHA requests from companies that are trying to improve or stay on top of combustible dust safety in their facilities. Some clients have up to 200 facilities, which is a monumental task if many of them were designed 50 years ago or the designers weren’t familiar with the standards. Sometimes the prescriptive requirements can’t be feasibly implemented or there is no prescriptive requirement that addresses a particular hazard.

“You’re going through a DHA, coming up with the list of 100 recommendations and you multiply that by 100 different sites,” he explained. “We’ve even had some clients say it would be cheaper for them to build a new facility than implement all these recommendations.”

Fortunately, there are alternative methods of achieving compliance, and they address some of these real-world constraints. The two outlined in the NFPA standards are performance-based design and risk analysis.

Performance-Based Design Explained

The performance-based framework essentially mirrors the fire codes and the different fire protection standards. It evaluates fire and explosion protection scenarios to confirm outcomes such as:

Facility occupants aren’t exposed to untenable conditions
They can get out if necessary
The building won’t be irreparably damaged
Ignition has been prevented

Marc explained that he could create a detailed fire model of a building, apply the worst credible fire scenario, and show that everyone can get out of the building with a factor of safety at five. It’s a lot harder to do this with combustible dust, but not impossible.

With combustible dust, performance-based design is commonly applied to specific applications. For example, a closed-clearance rotary valve can’t be used due to material jams, so can an alternative with wider body clearance be applied instead? This scenario can be tested, but it becomes more difficult for full-facility DHAs.

Risk-Based Analysis Explained

Marc illustrated the use of risk-based analysis with an example. If the fire code says that two stairwells are needed in a building but the owner only wants one, they have to go through a rigorous analysis and obtain AHJ (authority having jurisdiction) approval. A thorough risk analysis can prove that this alternative design achieves an acceptable level of risk.

There are two types of risk models: qualitative and quantitative.

Qualitative vs. Quantitative Risk Models

With the qualitative risk model, a bidding system is used to evaluate risk. Many people use a numerical scale, such as a 1 to 5 likelihood or level of severity. Although less time intensive, it is also more subjective.

“If you assemble a group of five people at one plant and sit down with five different people at another plant, (someone) may rank something as low-risk and somebody else may rank it a medium risk, so you don’t have a lot of resolution in your results,” Marc explained.

When applying safeguards like explosion venting and isolation, there may not be enough of a resolution between a 1 and a 2 to really show that the needle has been moved much. With quantitative risk models, which include fault tree or event tree analyses, you’re quantifying probabilities and frequencies associated with the events, resulting in a number. If that number is compared to a quantitative risk acceptance criteria, it allows you to see exactly where you are on the risk spectrum, resulting in more resolution.

Marc mentioned one client with a large number of sites that did DHAs in all of them and found that a number of systems didn’t meet prescriptive requirements, but they didn’t have a lot of loss history with this equipment and the cost to protect it would be disproportionately high. If possible, the client wanted to apply their resources to addressing greater hazards.

What Marc did was develop quantitative risk models based on the parameters of these systems. One of them was a typical pneumatic starch silo and pneumatic conveying system. Risk model factors included:

Pipe diameter
Pipe length
Airflow rate in the pipes
How often the area around the equipment was occupied
Whether the equipment was located indoors or outdoors
Whether there was existing protection on the equipment, such as explosion venting and isolation

Marc developed risk models that allowed him to quantitatively calculate risk based on these various inputs and worked with the client to establish risk acceptance criteria based on industry standards and loss status.

Although effective, the quantitative-based risk model should not be a default DHA approach. If a facility is knee-deep in dust, the quantitative methodology is not going to work unless it’s applied after other inherent safety measures, such as removing workers from unsafe areas or even shutting down the equipment while they’re there. Marc said that the first step should always be a traditional DHA that quickly identifies the biggest problems.

Calculating Risk in Combustible Dust Facilities

He pointed out that combustible dust handling operations don’t have the same loss data as the nuclear industry, where everything is tabulated. Probabilities and frequency are based on system parameters. For example, if a dense phase conveying system is under steady-state operation, the concentration will be too dense to be explosive. The only time this becomes a risk is during startup or shutdown, so you can quantify how many hours a day it’s running in a steady state versus starting up or shutting down and get a probability for.an explosible concentration.

Marc mentioned that one of the key factors he looks at is the secondary consequences of an event. The risk models he uses include the potential for a secondary explosion, which propagates back into the building. If outside equipment doesn’t have isolation protection and fugitive dust is everywhere in the facility, his risk model warns that disaster could result.

Conclusion

In many ways, dust hazard analysis is similar to writing a math or physics test in high school. You don’t just write down an answer like “The water was going 1000 meters per second.” You have to ask yourself, “Does it make sense?” You never want to plug and play equations when it comes to safety. Using a quantitative approach supports better decision making and more informed risk ranking and can make you feel safer in your own facility.

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Marc Hodapp directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/marc-hodapp-p-e-44773a94/

Website: https://www.jensenhughes.com/engineering-services/expert-consulting/combustible-dust/

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.