In today’s episode of the Dust Safety Science podcast, Kevin Cardwell, combustible dust consultant with Airdusco Engineering and Design Services, returns to the podcast. Last week, he addressed common difficulties encountered while investigating combustible dust fires and explosions. Today, he talks about spark detection, arresting, aborting, and extinguishing systems.
The subject was inspired by a question submitted to the Dust Safety Science Helpdesk. It was a question about a very specific system, and some prevention aspects to it, and some protection aspects to it. Not only was Kevin able to help, but he agreed to come on the podcast and clear up a lot of the confusion surrounding spark detection, arresting, aborting and extinguishing systems.
What Are the Differences Between Spark Detection, Arresting, Aborting and Extinguishing Systems?
Kevin explained that these systems protect process and dust collection systems and equipment.
Spark arrestors, also known as spark traps, are usually installed in dust collection lines and pickup hoods where spark or ember-producing equipment and processes are present. Examples include: grinders, sanders, polishers, saws, planers, welding tables, laser, plasma-cutting tables, that type of thing.
While there are different types of spark arrestors, most work by either providing a convoluted path for the spark to travel through, giving it time to cool down before continuing on into the rest of the dust collection system. Another type slows down the spark or ember, allowing it time to cool before continuing into the ducting and on to the collector.
Spark detectors are installed in the same dust collection systems as arrestors, but further downstream. As a rule, they are either infrared sensors or photosensors that see a spark moving in the air stream and then activate a mitigation device downstream.
“[You can have] a detect and spray extinguishing system or a detect and abort gate system,” Kevin explains. “The extinguishing systems use water or another type of extinguishing agent to spray the ember, cool it down, and put it out. Abort gates redirect embers or sparks around dust collectors or cyclones. They direct the dust-laden air stream right into the air material separator. If you’re recycling your air system back into the facility, there’s usually another abort gate or some type of diverter valve and the clean air return line that actuates along with the abort gate to send a spark to a safe location outside.”
What Are the Differences Between Explosion Suppression and Mechanical Isolation?
According to Kevin, the main difference is that ignition source control devices help to prevent a fire and explosion. Fire protection or explosion protection devices come into effect when you have an explosion. If you can prevent that explosion from occurring, they are not used.
“The systems that we’re talking about today prevent credible ignition sources from getting to the air material separator where the other systems only activate once you have an incident, be it a fire or explosion,” he says.
What Are Common Design Considerations?
Asked about design considerations for ignition control with a laser-cutting table, Kevin said, “Any one of the systems that we mentioned, whether it is a spark trap or spark arrestor to detect and spray, detect and abort, any of those can be used. However, if you have a short duct run where you don’t actually have enough room to install a spark detector and then a downstream mitigation, you normally see spark traps close to the hood. If you have a slightly larger system with multiple tables – all being manifold together into the main duct and then a decent distance of duct, you might see a combination of those. You might see a spark trap by the hoods. You might see detect and spray and/or detect and abort in the main duck before you get to the dust collector. But what you have to look at is how much room that you have in the duct and how many devices you’re protecting.”
He explained that when he is designing a mitigation solution for a given system, he will look at how frequently sparks are created. If it is grinding on equipment that sparks frequently, he will put a spark trap in as a first step and possibly something else downstream.
“A sanding system will occasionally create a spark or an ember. Let’s say a given production sander might have six or eight pickups on one unit – is it worth putting a spark trap on each of those to collect the occasional spark? Or is it better to just provide a detect and spray or multiple zones of that? Maybe one or two zones of detect and spray, and then a zone of detect and abort to protect it. As a rule, it’s usually better to not put the spark arrestors on each of those individual ducts, and just protect the system as a whole. Those are the types of things that I look at when I’m determining what the appropriate equipment is to protect in the given scenario.”
He reminded listeners that they need to pay attention to the given velocity range of a spark detector. If they have a spark arrestor designed to work up to 5,000 or 5,500 feet a minute and their dust collection system is running 6,000 feet a minute in that particular duct, that arrestor is not going to catch very much because they’re over-running that system.
Another consideration with detect and spray extinguishing systems is that you have to look at a combination of your system activation time, duct velocity, and the distance between the detector and the mitigation downstream. If you have a system where your activation time from the moment the sensor sees a spark until the extinguishment or the abort gate activates, is 500 milliseconds and you have a duct velocity of 5,280 feet per minute, you’re going to overrun your protection when you’re running at your maximum airflow. This is because at 5,280 feet per minute, the spark can travel 44 feet in 500 milliseconds or half a second.
“Your solution for that is either to slow down your velocity slightly or extend the distance between detection and activation,” Kevin says.
The last thing to think about is the location of your detection and your extinguisher abort equipment. You don’t want to mount either of those in a turbulent area in your ducting, as turbulence can change the velocity in that little section of the duct and cause you to over-run your protection, resulting in less than optimal performance.
Conclusion
“The one thing I want people to understand is that you need to have a qualified person or a qualified company determine your mitigation plan and what equipment that involves. I have seen equipment that was over-protected,” Kevin says. “You can spend a lot of money doing absolutely everything. Sometimes, you can have false activations or other things that actually cost you downtime just by saying, “Well, let’s just do everything.” Other times, I’ve seen people not consider certain scenarios that are very valid, and therefore they are under-protected. If you have a qualified person or a qualified company to help you determine your mitigation strategy, what equipment you need and how it’s installed, that will go a long way in both ensuring safety and making sure that you’re not spending money that you don’t necessarily have to spend.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Kevin Cardwell directly:
Email: [email protected]
Cell: 901-378-8038
LinkedIn: https://www.linkedin.com/in/kevin-cardwell-cfei-cfii-cfps-5935a182
Website: http://www.airdusco.com/
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
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Dust Safety Share
Companies
Airdusco Engineering and Design Services
Previous Episodes
DSS079: Three More Challenges in Baghouse Maintenance with Kevin Cardwell
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