Canadian Biomass • Fall 2022 • Need to vent?

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 toward compliance. And navigate they must because the standards for conveyors alone are extensive and can be expensive to implement, especially where mitigation devices are involved. Plus, as any non-expert who has delved into the subject knows, the forest of technical information about it isn’t easy to navigate. Neither does the information usually help with practical issues, such as whether to choose active or passive mitigation methods or whether you can mitigate explosion risks without expert assistance.

This article will thus address these issues and more as they relate to explosion venting in conveyors.

Where an explosion hazard exists, the facility owner has the choice to address it with active or passive mitigation devices. Active systems are more complex. These rely on a sensor to trigger a suppressant-containing device and are set up with electrical controls that can include fault modes to prevent unwanted release of the suppressant.

However, the complexity of these systems is a downside, as more can go wrong. Also, technicians may need to clean their conveyors after the suppressant has been 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, engineering manager at CV Technology, advocates for passive devices for this reason. In an interview with Dr. Chris Cloney on the Dust Safety Science podcast, Krbec insisted passive systems are “readily available” and “failsafe,” which gives them an advantage over tuned, active systems. Passive devices, he said, are “designed to open at a pre-set pressure. … And once that pressure is exceeded, they open, whether it’s for a deflagration event, explosion event, or a process reason.” In other words, there is no off mode for a passive system. The system is always ready to perform. Its simplicity makes it reliable.

Cloney followed up on 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 failsafe, it’s even better. It has less chance of things going wrong.”

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

But those tolerances are only as good as the data a conveyor manufacturer provides about the pressure capabilities of their system. Getting that data takes effort, and not all conveyor manufacturers go through the rigorous testing required to obtain it, which can include computer analysis, field testing, and third-party evaluation.

Engineers who design blast vents for conveyors need accurate information because the pressure characteristics affect the mitigation system’s design. 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 strength of their equipment, the vents designed for it may fail to prevent an explosion.

These concerns underscore the fact that mitigation is too specialized to attempt without consulting an expert. Yet we know that wood-industry professionals prefer to do things themselves. If they can strap a solution together, it’s what they do. Large companies are no exception. Plus, they have engineers on staff to handle complex issues.

But the knowledge required to design a reliable mitigation system that conforms to NFPA standards is highly specialized. NFPA 68 alone has some 84 pages of codes, tables, calculations, and exceptions for explosion mitigation devices, and missing one detail can put a facility out of compliance. Worse, it may nullify the system’s effectiveness.

Bernardo Sanson, a sales engineer with CV Tech, spoke to this point on a recent call, saying, “Ventilation requires expertise in the sense you’re required to know and be able to determine the burst pressure of the explosion panels. In the past, they were manufactured without much control for bust pressure. So, without knowing that, you don’t know the side effect a deflagration would have on your conveyor or the atmosphere. That’s only determined with testing. Plus, you have to be compliant with ATEC’s approvals in quality and protocol [as they relate to testing and manufacturing controls].”

Army Test and Evaluation Command approval isn’t likely something a wood processor will get from a panel designed by a staff engineer and manufactured in a company fab shop. Manufacturing intricacies are yet another reason to rely on professionals for this service and not attempt a do-it-yourself solution.

DIY efforts do come into play post instalment, of course. While passive systems require less care than active systems, they still need attention. As with other systems, plant personnel must know how post-instalment work may affect them and how they wear over time.

According to Krbec, it’s not uncommon for technicians to add insulation to blast panels on their equipment. This is a problem, as insulation adds inertia to the panel and affects how it will perform in the event of a deflagration. The same idea holds for changes to the conveyor the vents protect. For example, replacing a top or bottom panel with material thinner than original equipment manufacturer specifications makes the conveyor weaker. Because the system’s parameters have changed, the vents may no longer adequately protect it.

Adding components around a blast panel likewise can affect how the system performs. Objects placed to the side of a panel may deflect energy up and increase the distance the fireball travels. Changes to the material inside the vessel may also affect the system, as can process changes that add vibrations or alter the air pressure. Due to the complexities associated with mitigation it’s best to consult the blast panel’s manufacturer before making changes.

Plant personnel must also maintain blast vents to ensure they remain functional. Panels must be kept free of debris, snow, ice, and large amounts of dust. They may also need protection from pests and precipitation. Furthermore, panels are not rust-proof, and vibrations will weaken them over time. A panels manufacturer can provide the best estimate for a panel’s expected lifespan.

Because mitigation systems are so nuanced, it’s best to talk to an expert before altering anything that may affect them. Professionals understand the ins and outs of these systems – what’s required, what to avoid, and how to manufacture devices to code. The forest of information on mitigation and dust safety may be thick, but such experts can help you navigate it. •