DSS028: Recent changes to NFPA 69 Standard on Explosion Prevention Systems with Timothy Heneks

In this episode of the Dust Safety Science Podcast, we interview Timothy Heneks, Director of Engineering Services with Dust Con Solutions at West Palm Beach in Florida, about recent changes to NFPA 69, which is the North American standard on explosion prevention systems. During our conversation, Tim discusses:

The new revisions and how they differ from the previous ones
Safety instrumented systems (SIS)
Limiting oxygen concentration, minimum explosible concentration, and other prevention methods in combustible dust safety
How attitudes toward safety in combustible dust handling industries differ from chemical processing industries

Tim’s role as Director of Engineering Services at Dust Con Solutions requires him to schedule and oversee dust hazard analyses (DHAs) as well as answer client questions on the subject. He deals with the NFPA 69 standard on a regular basis and has written an article about the three major changes that the revisions have created. He also hosted a combustible dust webinar on April 24 and will be presenting seminars at the Powder & Bulk Engineering Conference from September 18-19, 2019.

An Overview of the NFPA 69 Standard

Being the standard on on explosion prevention systems, NFPA 69 is used to determine how to prevent and mitigate the effects of an explosion from:

Combustible dust
Flammable vapors and gases

The NFPA 68 standard covers explosion protection by deflagration venting. In contrast, NFPA 69 prevents and mitigates explosions by:

Inerting systems
Oxygen concentration reduction
Combustible concentration reduction
Spark detection and extinguishment
Explosion suppression
Explosion isolation
Detonation arresters
Pressure containment

Tim describes NFPA 69 as a ‘how to’ standard because it describes the methods used to mitigate or prevent combustible dust hazards. In this respect, it’s different from NFPA 652, which is more of a prescriptive standard because it defines the hazards and points to how-to documents for the methods of control. For example, if you’re purchasing an explosion flap valve for explosion isolation in a dust collector, the NFPA does not have a testing protocol for it. You have to source the appropriate ATEX standard and use it to ensure that the valve is safe.

In his article, Tim identified three major differences between the old and new versions of NFPA 69. They are:

Safety instrumented systems (SIS)
Oxygen concentration data
Minimum explosible concentration level

Let’s take a closer look at all three.

Safety Instrumented Systems (SIS)

Safety instrumented systems, or SIS, is not a new concept, although it’s new to NFPA 69. It has been used in the chemical and petroleum industries for a long time. Tim’s first exposure to SIS was when he was working as a plant engineer in the chemical industry, where it was an integral part of the independent protection layers at the plant.

SIS prevents incidents that might cause loss or injury by monitoring processes and automatically intervening to bring the system back to a safe condition. It typically includes logic solvers, controllers, and final control elements. It’s important to note that an SIS is an independent layer of protection above and beyond the administrative controls or basic process control system.

To illustrate how an SIS might work, Tim presented a situation where the tank of a railcar has been overfilled, which could cause loss of containment. Perhaps the level sensor failed or someone forgot to turn off the pump. In either case, the situation is unacceptable, so an SIS might require you to install an independent high-level probe, a controller, and perhaps a shut-off valve on the line. These three elements will work together to alert you when the tank’s high level is reached and allow you to keep the system in a safe state.

An SIS is categorized based on the reliability of each component, or safety instrumented function (SIF). They are rated on safety integrity levels, or SILs, which range from 1 (the lowest level of reliability) to 4 (the highest). NFPA 69 now stated that all explosion prevention systems installed after November 5th, 2021 need to be installed as an SIS and that system needs to meet SIL 2 at the very least.

The best example of an SIS in the combustible dust world is the explosion suppression system. A system placed on a cyclone or filter above the packaging line is intended to prevent the propagation of flame within the vessel if ignition occurs. Without that suppression system in place, the pressure will build and possible rupture the vessel, which is an unacceptable outcome. By looking for pressure, the explosion suppression system is the first of your SIFs. The sensor collects information from the vessel and relays it to the controller before finally sending the signal to deploy the suppression canister.

One way to improve the SIL level in this situation is to use higher-quality or more advanced detection controls as well as apply redundancy. If there are two separate detectors looking for pressure build-up, you have a safeguard if one experiences an electrical failure or loses calibration.

Oxygen Concentration Data

The limiting oxygen concentration data found in the Annex C material of NFPA69 is changing, but the updates are specific to flammability to data issues and won’t have a major impact on dust handling operations.

Instead of subtracting 2% by volume, it’s providing a longer and more accurate formula for making that determination. The new text calls for subtracting 1.5% volume for LOC values of 10% or more, or multiplying a factor of .85 for LOC values less than 10%.

What’s important for combustible dust handling industries is that the NFPA69 document describes ways that you can design a system in which there’s not enough oxygen present to support a deflagration. This can be done by adding an inert gas such as nitrogen or CO2.

Minimum Explosible Concentration Level

Combustible dust needs to be in sufficient quantities to support combustion, so a minimum explosible concentration (MEC) is utilized. This value represents the lowest amount of fuel that can be present and still support deflagration or combustion.

The MEC value is used as a safety measure in certain situations. Tim provided the example of a dilute airstream coming into a dust collector. There isn’t enough combustible dust present to be a deflagration hazard. However, a filter may contain fine particles or there may be areas in the plant where there are higher concentrations of dust.

The NFPA 69 change ensures that in addition to the required amount of dust in the dust collector and the variations due to pressure, temperature, or operating controls, you look for concentration variation with time and location.

Tim mentioned that when a dust collector is new, it has a very dilute stream in it, so there may not be a deflagration hazard in the vessel for a long time. However, months and even years may pass before the filters are changed out. The same material is caked on the filters, pulsed off, and then returned, so even if you’re not bringing in high concentrations of dust, you’re going to end up with a combustible dust cloud surrounding those filter elements. Thanks to the NFPA 69 changes, all possible scenarios for concentration variation must be taken into account.

Loss of Containment

Tim addressed one hazard commonly seen in combustible dust environments: loss of containment, which refers to material that ends up where it doesn’t belong. This can happen for a number of reasons, including valve leakages or ruptured discs. You can pull up the bottom of a silo skirt and see piles of sugar dust, but the facility managers tend to worry more about food safety.

He pointed out that if a newly installed system was releasing fugitive dust, no one would see it as acceptable, but after it runs for a couple of years, the sight of dust becomes commonplace and you end up with a situation like the Imperial Sugar Refinery explosion.

Conclusion

Tim concluded the interview by posing a challenge to podcast listeners. “I’d like to see some success stories from our listeners,” he said. “I’d love to hear about when (an) explosion went off it was directed to a safe area and a disaster was averted.” Hopefully the NFPA 69 revisions will make those stories a frequent occurrence.

If you would like to share your success story, leave it in the comments section below. You can also reach Tim Heneks directly:
Email: [email protected]
LinkedIn: https://www.linkedin.com/in/timothyheneks/

Tim has also mentioned that for those seeking formal training in combustible dust safety, Kansas State University will be doing an online dust hazards course from May 14 – June 18, 2019.

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.