In today’s episode of the Dust Safety Science podcast, Robert Comer, author of the book Explosion Vented Equipment System Protection Guide, returns to talk about explosion and relief protecting systems. He goes into more detail on why you can’t simply add any vent or door to a dust collector: you need the design information for that protection system as well as the material properties.
“Dust collectors are not designed to handle high pressure very well,” he explained. “Even if there’s a vent on the collector, the pressure it experiences (referred to as Pred, a common standard) might exceed what the system or its parts can safely handle. It’s important to check if the collector can withstand this pressure.”
He emphasized the importance of making the NFPA 68 standard easier to understand. This standard covers how to protect against explosions through venting. Part two provides examples of different dust collector setups, where to place explosion vents, and the shape of dust collectors. NFPA 68 can be overwhelming at first glance, but part two aims to break down the key points and calculations in a simpler way. Any engineer with the right knowledge can figure out the needed size for explosion relief vents and the pressure from an explosion (Pred) that the system must resist without damage.
An Explosion Panel Alone Isn’t Enough for Safety
Simply having an explosion vent panel isn’t enough for safety. If your dust collection system lacks an explosion vent or the necessary details and analysis about explosion pressures, you must conduct a dust process analysis. This analysis will provide the information needed to ensure the system can handle the pressure without any part of it bending or breaking.
If a process changes, it’s crucial to reassess the system to account for those changes, specifically looking at the explosion potential index (KST) and the maximum pressure (Pmax). Bob owns a book with a table listing the characteristics of various materials’ dust, but he emphasized that these materials must be tested according to the ASTM standard E1226, which outlines the method for measuring pressure and the rate of pressure rise for combustible dust. The table’s values are for estimation only and must be verified with actual test results to ensure the system is designed correctly. Testing is necessary because Pmax can vary with the dust’s particle size, distribution, chemical composition, and moisture content. Actual dust characteristics can only be determined through testing.
The book analyzes square, rectangular, and cylindrical dust collectors, calculating their proper geometry and effective volumes. It also discusses the analysis for Pred (pressure) calculations, which are presented in the metric system, and includes a table of conversion factors to simplify this process.
The position of an explosion vent on a dust collector is critical, with the book covering potential locations such as near the top, near the bottom, or on top of the collector. Each location requires a different analysis. Part two of the book provides examples for existing dust handling systems that already have explosion relief elements sized and attached, as well as analyses for new systems. This ensures the explosion relief burst element is correctly sized from the start, allowing for a proactive approach to system safety for those who are in the process of ordering a unit.
Common Mistakes When Designing Explosion Protection Systems
Bob explained how duct back pressure affects the pressure (Pred) during flow. It looks into how the pressure and friction in the duct, especially if there’s a bend, can slow down the flow, meaning just looking at Pred isn’t enough. A common error is not using a filter bag cage at the entrance to the explosion relief vent. If the vent is too close to the filter bags, the bags can bend and block the vent, leading to dangerously high pressure. To prevent this, a special cage must be installed at the vent’s entrance to keep the bags from obstructing it.
Another oversight is not accounting for the reaction force on the duct and the dust collector when gas is expelled. This force can create a back pressure of several thousand pounds, potentially destabilizing the system or even tipping over the dust collector. To avoid this, the anchors securing the dust collector must be strong enough to handle such forces. This is considered one of the most critical mistakes in design.
“If it’s an outside unit, you have the wind loads. In addition, you could have an expelled expulsion of the hot gases, creating a back force. You could have the wind loads in the same direction. So everything has to be considered,” Bob said.
Other Dust Collection Safety Considerations
Grounding and keeping the area clean are just as important as having proper ventilation. Grounding helps prevent dust clouds from igniting accidentally, which can be caused by several things. For example, a metal grid inside the access door, used by technicians to change filter bags, wasn’t grounded and acted like a capacitor, sparking a dust explosion. Bob’s book outlines many grounding rules, including the need to ground dust collectors, filter bags, bag cages, and nozzles. It’s also recommended to use 304 stainless steel for all carbon steel bolts, nuts, and washers to prevent rust, which is non-conductive and could lead to similar issues.
The book contains various design guidelines for grounding, such as how to bond joints on pipes and which couplings need grounding. Keeping the workplace clean is crucial to avoid dust buildup, which poses a fire hazard. NFPA 68 sets the allowable dust limit to a 32nd of an inch over at least 5% of the floor area, including dust on beams, ducts, and equipment, which covers a large area. Vacuuming is the preferred cleaning method since it’s safe for dusty areas, while using compressed air, brushing, or sweeping is discouraged because they can make dust airborne and spread it around.
How Can You Avoid Building Up a Charge?
Bob explained that you don’t need to check the grounding constantly, but doing it once a year should be enough if it was set up correctly in the first place. It should stay fine for a year, unless some maintenance work is done and someone forgets to reconnect a grounding cable or something similar. Therefore, you should visually inspect the system annually to ensure all grounding straps are where they should be, bolts are tightened, and so on. Additionally, you can use an Ohmmeter to check that everything is still properly grounded. This can also be done once a year.
What Experience is Necessary for Evaluations?
A company engineer doesn’t need to be professionally licensed to perform analyses for their company. If they have the necessary education and experience and understand the concepts in the book, they can help their company comply with new dust collection system standards and reduce risks. They’re legally allowed to do this work for their company.
For example, I once worked with a company that had multiple dust collection systems. I analyzed one system and prepared a detailed report on venting analysis and design reinforcement. Then, I trained the company’s engineers for a couple of hours on the procedures. After that, they were able to analyze the rest of the systems to my satisfaction. This shows that engineers with the right guidance are fully capable of doing this kind of analysis.
If a company doesn’t have an engineer, or their engineer isn’t confident in doing this work, they should consider hiring a consultant with the appropriate experience. While it’s true that not many consultants have the specific expertise needed, resources like the book or information from the NFPA can provide significant support. However, I strongly recommend that in-house engineers, who are familiar with their company’s systems, take on this responsibility whenever possible.
Conclusion
At the end of the interview, Bob emphasized how important it was to thoroughly evaluate your system for safety. Here are some key safety measures:
- Connect the explosion vent burst sensor to the system’s power so it shuts down automatically during an explosion. This prevents the system from running after an incident.
- Place a magnet above the mill’s inlet to catch any metal. This helps avoid sparks that could ignite a dust explosion.
- Attach a vibration switch to mills (and similar equipment like conveyors or rotary valves) to automatically turn off the system if vibrations exceed normal levels, which could indicate a problem like friction or overheating.
- Install temperature sensors on bearings of rotating machinery to shut down the system if temperatures get too high or if there’s an explosion, preventing further damage.
- Use weather covers over ducts to prevent birds, animals, insects, and weather elements like rain or snow from getting in and causing blockages.
“I’ve also designed cost-effective and practical cover solutions that meet wind, ice, and snow standards,” Bob said. “If a duct discharges upward, you need to limit snow and ice accumulation to less than two and a half pounds per square foot to prevent bending. It’s best to angle ducts at 45 degrees to help snow and ice slide off. Upward-facing ductwork should only be used in areas where snow and ice are not a concern, like in southern regions.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Robert Comer directly:
Email: [email protected]
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
Standards
ASTM standard E1226,
Books
Explosion Vented Equipment System Protection Guide
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