Need to ventilate? Talk to an expert

While many wood processors have complied with the standards outlined in the National Fire Protection Association’s updated version of NFPA 664, others are still navigating the codes on their journey to compliance. And they have to navigate because the standards for conveyors alone are extensive and can be expensive to implement, especially when mitigation devices are involved. Also, as any non-expert who has looked into the subject knows, the forest of technical information on this subject is not easy to navigate. The information is also not useful for practical questions, such as the choice of active or passive mitigation methods or the possibility of mitigating the risk of explosion without the help of an expert. This article will therefore address these issues and more as they relate to the explosion vent in conveyors.

Passive or active attenuation?

When a risk of explosion exists, the owner of the installation has the choice to remedy it with active or passive mitigation devices. Active systems are more complex. These rely on a sensor to trigger a device containing a suppressor and are configured with electrical controls which may include fault modes to prevent unwanted release of the suppressor.

However, the complexity of these systems is a drawback, as others can go wrong. Additionally, technicians may need to clean their conveyors after the suppressor is released – a chore that can lengthen the downtime of an already disruptive event.

Passive mitigation is simpler and arguably safer than active systems. Jason Krbec, director of engineering at CV Technology, advocates passive devices for this reason. In an interview with Dr. Chris Cloney on the Dust Safety Science podcast, Krbec insisted that passive systems are “readily available” and “fail-safe”, giving them an edge over tuned active systems. Passive devices, he said, are “designed to open at a preset pressure. …And once that pressure is exceeded, they open, whether for a deflagration, an explosion or a process reason. In other words, there is no off mode for a passive system. The system is always ready to operate. Its simplicity makes it reliable.

Cloney followed Krbec’s point by comparing passive and active systems. “A passive system doesn’t need a controller,” he said. “No wiring. No redundant sensors. If it’s fail-safe, even better. There’s less chance of things going wrong.

But simplicity is also the downside of these systems. Since passive systems are designed to open whenever pressure reaches a certain threshold, process changes that affect airflow can cause vents to open when a deflagration has not occurred. . The vents are getting better in this regard, however. Krbec said vents are set to higher tolerances these days to prevent them from opening when they shouldn’t, although expert engineering is required to make a passive system a “set it and forget it” solution.

But these tolerances are only as good as the data provided by a conveyor manufacturer on the pressure capabilities of their system. Obtaining this data takes effort, and not all conveyor manufacturers go through the rigorous testing necessary to obtain it, which may include computer analysis, field testing, and third-party evaluation.

Engineers designing blast vents for conveyors need accurate information because pressure characteristics affect the design of the attenuation system. A conveyor with a strong frame, for example, needs fewer and smaller vents than a conveyor with a weaker frame. So if a conveyor manufacturer provides inaccurate information, such as overestimating the resistance of their equipment, vents designed for this purpose may not prevent an explosion.

Can you DIY a mitigation system?

These concerns underscore the fact that mitigation is too specialized to be attempted without consulting an expert. We know that professionals in the wood industry prefer to do things themselves. If they can find a solution together, that’s what they do. Large companies are no exception. Additionally, they have engineers on staff to handle complex issues.

But the knowledge required to design a reliable, NFPA-compliant mitigation system is highly specialized. NFPA 68 alone contains some 84 pages of codes, tables, calculations and exceptions for explosion mitigation devices, and the lack of one detail can put an installation out of compliance. Worse, it can negate the effectiveness of the system.

Bernardo Sanson, a sales engineer at CV Tech, spoke to this point on a recent call, saying, “Ventilation requires expertise in the sense that you need to know and be able to determine the burst pressure of panels of explosion. In the past, they were made without much control over chest pressure. So, without knowing it, you don’t know the secondary effect that a deflagration would have on your conveyor or on the atmosphere. This is only determined with tests. Additionally, you must comply with ATEC quality and protocol approvals (with respect to testing and manufacturing controls).

Army Test and Evaluation Command approval is probably not something a wood processor will get from a panel designed by a staff engineer and fabricated in an Army fabrication shop. company. The intricacies of fabrication are another reason to rely on professionals for this service and not attempt a do-it-yourself solution.

Maintenance after installation

DIY efforts come into play after the installment, of course. Although passive systems require less care than active systems, they still need attention. As with other systems, plant personnel should be aware of how post-installation work can affect them and how they wear over time.

According to Krbec, it’s not uncommon for technicians to add insulation to the blast panels of their equipment. This is a problem, because the insulation adds inertia to the panel and affects its behavior in the event of a deflagration. The same idea applies to conveyor modifications that vents protect. For example, replacing a top or bottom panel with material thinner than the original equipment manufacturer’s specifications will make the conveyor weaker. Because the system settings have changed, the vents may no longer protect it adequately.

Adding components around an explosion-proof panel can also affect system performance. Objects placed to the side of a panel can deflect energy upwards and increase the distance the fireball travels. Changes to the material inside the vessel can also affect the system, as can process changes that add vibration or change air pressure. Due to the complexities associated with attenuation, it is best to consult with the blast panel manufacturer before making any changes.

Plant personnel should also maintain vents to ensure they remain functional. Panels should be free of debris, snow, ice and large amounts of dust. They may also need protection from pests and precipitation. In addition, the panels are not stainless and vibrations will weaken them over time. A panel manufacturer can provide the best estimate of a panel’s expected life.
Because mitigation systems are so nuanced, it’s best to talk to an expert before changing anything that might affect them.

Professionals understand the ins and outs of these systems – what’s needed, what to avoid, and how to build devices to code. The forest of dust mitigation and safety information can be thick, but these experts can help you navigate it. Speak to one to ensure the safety of your facility. This is the most practical advice you can get.

Joel E. Dulin is Director of Marketing for Biomass engineering and equipment.

This article is part of Dust Safety Week 2022. To read more articles on dust safety, click here.

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