On the 22nd February 2022, HSE published a bulletin stating that HSE officers will be visiting woodworking businesses across the UK “to ensure duty holders know the risks associated with woodworking and have effective controls in place to keep workers safe…”. One of the ways in which the woodworking industry have a duty to keep their workers safe is by ensuring there is adequate dust extraction, or dust collection, systems in place. These dusts can be harmful to the lung health of employees, causing occupational asthma and, in the case of hardwoods, sinonasal cancer. That is just one of the reasons why dust collection systems are so important, and why HSE will be checking for them this year.
Dust collection systems are used to extract harmful dusts that can become present in the air during a manufacturing process, using centrifugal action. During everyday operation, dust mixed with air enter the unit at an angle. The circular air motion pushes the dust particles onto the cyclone’s wall, as they then fall down into a hopper below.
A dust extraction system and dust collection vessel can, in some cases, pose an optimal environment for a dust explosion to occur. Sufficient volumes of dust, dispersed into the air, confined within an enclosed vessel that is connected to other parts of machinery via ductwork, posing an intolerable risk if an ignition source were to enter the vessel. An ignition source could be a spark from faulty machinery, high temperatures from the process or even static electricity. It takes a matter of milliseconds for an explosion to occur, propagate and destroy a production facility, threatening the livelihood of surrounding personnel.
It is for this reason that HSE officers will most likely also be checking for appropriate explosion protection on all necessary equipment, including dust filters. Here’s why explosion protection is so important on dust handling systems:
Dust Collection Explosion Risks in the Woodworking Industry
When identifying explosion risks, the first question is – is the material being processed combustible? Wood and woodchip is of course combustible. That means an explosion is of high risk and this risk must be accounted for. During normal operation, the material being handled is dispersed into the air inside a vessel (confined space) with centrifugal force in the cyclone, thus creating an ideal environment for a dust explosion to occur. All that is missing so far in the equation is an ignition source.
An ignition source can come from any part of the process and can never be totally prevented. From spontaneous combustion to mechanical spark, electrostatic discharge, or already burning particles in the duct system, an ignition source can travel rapidly into the dust collector and before you know it you have a large scale explosion propagating from one part of the technology to another, with the results potentially catastrophic. Not only do you risk damages in equipment causing major downtime to production and funds to replace the technology, but you also put the livelihood of surrounding personnel at risk.
Protecting a Dust Filter from an Explosion
A variety of explosion protection solutions are available for use when it comes to dust collection systems. The best solution for explosion protection in whichever scenario is to use a combination. Explosion venting will relieve the explosion, whilst explosion isolation will stop the explosion from propagating into neighbouring machinery. If venting the explosion to a safe area, not occupied by surrounding equipment or personnel, is not an option, then flameless venting must be used. However, for the most part, suppression systems are the most flexible option if the application is difficult to protect (when it comes to installation parameters, specification of the vessel itself or location of the vessel for example).
B FLAP Isolation System
Featured in the example at point “A” you can see a B FLAP. A B FLAP is a form of passive explosion isolation. When an explosion occurs, it can travel into connecting equipment via the ductwork. A B FLAP has a heavy plate on the inside, that, to put it simply, will slam shut once hit by the initial pressure wave of the explosion. Using some of the best technology and research in the industry, the B FLAP has a locking mechanism that will lock the flap into place, so there is no risk of the flap opening again once the explosion has begun.
HRD Barrier System
At points “C” & “E” on the image above, you can see HRD suppression technology acting as an explosion isolation system. Much like the B FLAP, the HRD barrier will stop an explosion from propagating to connecting equipment. The system above uses the LumEx optical detector. As soon as an ignition source (source of light) is detected inside the vessel, a signal is sent to the CONEX control panel which will engage the HRD suppressant bottle, releasing suppressants into the ductwork at a high rate.
The HRD barrier suitably complements the HRD explosion suppression system used to protect devices. The appropriate combination of these two systems efficiently prevents property damage and saves human lives.
HRD Suppression System
In points “B” & “G” on the image above, the HRD system can be seen. The HRD system is a certified active explosion suppression system. Highly sensitive detectors can detect an explosion in a matter of milliseconds. The system then opens the valves of the HRD container unit and the pressure of the fire extinguisher pulls out special telescopic nozzles, which ensure efficient dispersion of the fire extinguisher into the entire protected area. Thanks to the HRD system, the increase in explosion pressure is under control – below the limit of the pressure resistance of the protected device, thus securing it against destruction. The HRD system is certified according to the EN 14373.
One of the most significant advantages of the HRD system is its flexibility. DetEx pressure detectors constantly monitor operating conditions and are able to detect an early explosion rapidly. The High Rate Discharge system is one of the most advanced extinguishing systems available; its quick response, high reliability, variability, and easy maintenance together create a perfect explosion protection solution for your facility.
VMP Explosion Venting
At marks “F” on the image above, you can see both options for venting – venting to a safe area using explosion vent panels and venting to an unsafe area using FLEX flameless vent panels.
Explosion relief panels are specially manufactured sheets of stainless steel which feature a carefully scored path around most sides of the panel edge. This scored path creates a weak outline which will burst at a nominal pressure.
Explosion vent panels need to be extensively tested and are commonly ATEX certified by a notified body according to EN 14797. There are different types of vent panels to suit a wide variety of applications, and it is important that a specialist is consulted to advise on each individual vessel/process. It is critical that the vent size is determined using the relevant calculation techniques, typically EN 14491 or NFPA 68. Using a panel which is undersized for the intended duty could prove catastrophic. There are several factors to take into account when selecting the correct vent panel, which is why it is so important that a professional, such as Coopatex, is involved in the design process.
FLEX Flameless Venting
At mark “F” on the image above, you can see a FLEX flameless venting device. The two basic functions of the FLEX flameless venting device are explosion venting and prevention of fire propagation into surrounding areas where personnel and neighbouring equipment may be located.
The explosion venting device, located inside the FLEX flameless housing, bursts opens due to fast increasing pressure from the start of an explosion and the FLEX housing absorbs flame, burning dust, and gases. As opposed to explosion venting, the FLEX device is capable to absorb these undesirable effects thanks to its construction. This means the FLEX can be used safely in spaces that are occupied by personnel and equipment.
The explosion venting can achieve temperatures up to 1 500 °C, with light metals such as aluminum and magnesium the temperature being around 3 000 °C. During explosion venting with the FLEX flameless equipment, the temperature is lowered to a safe level that is not dangerous for surrounding equipment and for work and movement of personnel.
If you would like to know more about how you can protect your process, please get in touch with us today.
