In today’s episode of the Dust Safety Science podcast, we’re talking about non-destructive evaluation to verify vessel pred values. We’re doing that with Bryn Johnson, President of JBW Consulting Engineers based out of Duluth, Minnesota. In addition to talking about his role in the industry, we answer questions like:
- Why do we care about the reduced pressure value?
- Why do we care if it’s the same as one thing was put in there?
- How do we do this type of evaluation to understand if it’s still sufficient today?
Bryn explained that JBW Consulting Engineers provides design and evaluation solutions for industrial facilities.
“I’m the President of the company and we’re about 22 people currently and we do offer all the disciplines, but our largest is mechanical and process. And so there’s about three of us here that currently will perform combustible dust related consulting services, whether it’s DHA (Dust Hazard Analysis) or whether it’s coming in after DHA has already been done and helping implement some of the solutions or both. Also, we routinely get called for, you know, combustible dust collection system design and just to make sure that from the get go, the system is designed correctly. So kind of the whole gamut there. But my specialty is combustible dust. And so that’s one of my bigger focuses.”
Why Do We Need To Know Pred For Vessels And Industries Handling Combustible Dust?
Pred is a value used in the computation of explosions, specifically for venting or explosion mitigation tools in devices like dust collectors or any vessel that might undergo a deflagration. Pred represents the pressure required for designing the venting system, rather than the actual pressure capacity of the vessel itself. This is detailed in NFPA 68.
Bryn encountered this factor when assisting a client in the ethanol industry. He performed a facility DHA on the dry part of their process, identifying a particular dust collector that had its vents directed close to another equipment. Bryn flagged this as a potential risk. Furthermore, he noticed that the vents were not up to the current standards in terms of size. While not every professional checks for this during their DHA, Bryn does and ensures clients are informed. He always advises adhering to the latest NFPA 68 standards. A significant recent change in these standards, specifically related to explosion venting size calculations, concerned the L/D calculation that affected the bag area dimensions.
In the last eight years, there have been three or four adjustments, likely causing these panels to increase in size for safety reasons. When Bryn addressed these issues with the client, they were eager to remedy the situation. Bryn reached out to the equipment’s manufacturer for advice on a new vent. Their response was non-committal as they couldn’t verify if their Pred value remained valid without knowing the current condition of the dust collector. This sort of reply is not uncommon from equipment manufacturers and is understandable. They can’t ascertain the wear and tear on the vessel over time, especially not knowing whether it had been subjected to abrasive conditions that might wear down the walls.
Bryn felt their response was logical but believed there must be another solution besides replacing the dust collector. Visually, the equipment appeared relatively new, possibly around ten years old, and seemed well-maintained. As it turned out, Bryn’s lead mechanical designer had a background as a nondestructive testing technician and held certifications in API 510 and API 570, with the former relating to American Petroleum Institute vessels.
Bryn believed that if he could ascertain the vessel’s condition, ensuring there wasn’t any substantial thinning or observable damage or imperfections that could affect the pressure capacity, it would be justifiable, based on engineering judgment, to either use the original Pred or adjust it based on the findings. Perhaps even suggest repairs to restore its original condition. Bryn presented this approach to the client, and they were keen to proceed.
From Bryn’s perspective, such a decision seemed like standard engineering judgment, akin to practices in the oil and gas industry or with API vessels. Regularly, these vessels undergo evaluations to determine their remaining lifespan and to decide if replacement is necessary.
The ultrasonic thickness tester was used to examine various vertical lines on the vessel’s exterior, paying special attention to high erosion areas like the inlet and outlet. During the assessment, a few dents were noticed in the cone area. However, the vessel largely retained its original thickness, indicating it hadn’t been subjected to extremely abrasive conditions. Based on this information, Bryn concluded that retaining the original Pred was a sensible decision, believing the vessel still had that capacity. He proceeded to specify the new vents and assisted the client in installing them in a more optimal location. Bryn had employed this approach for other clients as well.
The critical point Bryn wanted to emphasize was that most clients he worked with genuinely aimed to do the right thing. Yet, when they’re informed, whether by him or someone else, that their long-serving dust collector needs replacement, especially one that has possibly even survived an explosion, it’s not always an easy pill to swallow.
“They hear they need to replace this vessel and they think, well, it’s perfectly good and a lot of them would be happy to upgrade. It’s much cheaper, obviously, to cut in larger explosion vents if that’s what you need to do or relocate them or whatever. So it allows them to, it gives them an easier path to compliance than just replacing the vessel,” he said.
In another instance, Bryn managed to relocate an existing dust collector. He raised concerns regarding the vents, suspecting they might be out of compliance. Given the dust collector’s age – even though it was in good condition – it was evident that the standards had evolved over time. Bryn employed the same evaluation process, which enabled the client to utilize the dust collector for a different service. He envisaged that if one had an old dust collector stored away and wanted to reuse it, this assessment method would be essential. Before re-commissioning such equipment, it’s crucial to align it with the current NFPA standards. This approach offers several benefits, including significant savings in both money and time, considering the costs and delays associated with procuring and installing a brand-new device, not to mention potential downtimes.
Can This Process Establish a Reduced Value for Your Vessel?
Bryn acknowledged that this issue arises frequently. When examining the NFPA standards, they suggest 0.2 bar as a viable assumption in the absence of more specific details. Whether this assumption is universally suitable remains debatable. However, merely knowing the vessel’s condition doesn’t necessarily provide the Pred value. Instead, it indicates if the original Pred value attributed to the vessel remains a safe choice.
If Bryn detects issues, such as thinning or visible damage, he’s faced with a couple of decisions. He could agree that the reduced pressure of the vessel is uncertain, and based on the observed wear or damage, conclude that the vessel isn’t safe for use. Alternatively, if the vessel’s defects are localized, Bryn might consider designing a solution to address these shortcomings. This could involve adding extra plating or restoring thickness to the vessel to ensure it can endure the necessary pressure. Implementing such fixes would likely necessitate finite element analysis and detailed calculations, but it’s a feasible route if significant deficiencies are discovered.
Visual Inspection vs. Ultrasonic Thickness Testing
From Bryn’s perspective, any API certified inspector would advise performing both evaluations to genuinely understand the vessel’s condition. In the API domain, there are more tools available, like phased array, which Bryn doesn’t get into. Given the limited thickness, he isn’t particularly searching for impurities in the metallurgy. When he conducts what he terms a “vessel inspection”, he visually examines it. He checks for dents, any damage to the tube sheet, misalignments, missing or sheared bolts, and other anomalies. It’s essential to remain vigilant. Even if a few ultrasonic thickness tests indicate the high-wear areas are fine, other potential issues with the vessel still need consideration.
One significant concern is corrosion, especially when visible from the outside. Even if the thickness readings appear satisfactory, visible corrosion in critical areas can be problematic. The trick lies in combining both evaluation methods – leveraging his experience with vessels and his understanding of them, while also employing ultrasonic thickness testing to pinpoint wear. The challenging part to assess visually is the metal’s thickness. While Bryn could potentially inspect it qualitatively and identify areas of thinning or wear, using the ultrasonic method provides precise data, thereby ensuring a more accurate assessment and validation of the original Pred.
One of the things that people might be able to do then to avoid needing to replace the vessel altogether, what kind of routes can they take to improve the strength or come up with a different design?
How Can People Avoid the Need to Replace the Vessel?
Bryn replied that if he observed thinning in a specific area, he could excise that section and replace it. It’s crucial to ensure the welds are designed to handle the pressure they were initially intended to bear. He could also restore the vessel’s thickness, especially if wear is the main concern.
When it comes to mechanical damage, the solution might be similar. For instance, if the damage is on the cone section, Bryn might affix a curved plate around it, essentially replacing the damaged part. He would need to demonstrate mathematically that he’s effectively reinstating the original strength. These are primary considerations he’d keep in mind. Regarding corrosion, the principle remains the same, but corrosion can be more complicated to navigate, potentially leading to more intricate challenges.
“We have done finite element analysis on the square to round transitions,” Bryan said. “So between the vessel and the backdraft damper on an application, your duct in between is supposed to be two times the route of the vessel or the backdraft damper, whichever is higher. And for round duct, that’s usually not a problem. The pressure capacity of a round duct is pretty high, but where you have a squared around transition, particularly on the flat side of those, there’s very little pressure capacity. The same concept could apply to really any vessel, right? You perform the finite element analysis to feel confident that the design will withstand the pressure, the two times the Pred in that case, or maybe the vessel itself. Maybe it’s an oblong, funny shape, whatever you can perform finite element analysis to to prove that you need to, I should say.”
Conclusion
At the end of the interview, Bryn said he has observed numerous facilities taking more proactive measures concerning combustible dust concerns. Over the past five years, he has noticed a stronger commitment to addressing these concerns, even from smaller establishments he’s collaborated with. This trend might be due to a rise in awareness, with incidents being more frequently reported.
“It’s encouraging to see such dedication,” he said.
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Bryn Johnston directly:
Website: https://www.jbwconsult.com/
LinkedIn: https://www.linkedin.com/in/bryn-johnston-pe-27b69376
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.
Resources mentioned
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Companies
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DSS243: Using Non-Destructive Evaluation to Verify Vessel Pred Values with Bryn Johnston
