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NFPA 664: preventing fires and explosions among conveyance equipment

June 23, 2021  By Joel Dulin

Photo: Annex Business Media

It’s no secret that risk accompanies wood processing. Wood burns. Dry wood burns easily. Wood dust can explode. While history has afforded many incidents that highlight the seriousness of these realities, the danger of these risks – the latter in particular – gained public attention in 2012 when Lakeland Mills and Babine Forest Products in British Columbia experienced catastrophic explosions and subsequent fires that killed several workers. These events, among others, underlined inadequate safety measures surrounding wood dust in wood-processing industries. The Canadian government quickly implemented new safety regulations, and within a few years the National Fire Protection Agency (NFPA) published NFPA 652, Standard on the Fundamentals of Combustible Dust – a resource the agency had identified in the early 2000’s as something it needed to develop.

After publishing the standard, the NFPA began updating other standards to align with it. Its Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities (NFPA 664) was revised with these updates in 2017 and again in 2020. The 2020 edition represents a major overhaul of the standard with changes to its layout and material additions related to, among other subjects, hazard management. In the chapter dedicated to this subject, NFPA 664 details what the NFPA considers inherently safe designs for equipment that handles cellulosic dust.

[Editor’s note: This article is part of Dust Safety Week 2021. Find more articles here.]

While the NFPA doesn’t require owners get their operations in compliance with the revised 664 standard overnight, it does require them to get in compliance within a structured timeline. Existing, uncompliant systems are not “grandfathered in.” NFPA 652 8.7.4 states:


Where equipment deficiencies that affect the prevention, control, and mitigation of dust fires, deflagrations, and explosions are identified or become known, the owner/operator shall establish and implement a corrective action plan with an explicit deadline.

This code applies retroactively to both new and existing facilities (8.1). This means that as wood-processing facilities conduct their dust hazards analysis (DHA) every five years (664 7.1.6, ref. 652 chapter 7, also applied retroactively) and identify equipment or processes that are not inherently safe, they must create and implement a plan to address problem issues. The codes we shall discuss, therefore, apply not only to new installations but also to those already in place.

When is there a risk for an explosion?

When it comes to NFPA 664, you should first understand that if an operation hasn’t completed a DHA, a deflagration risk exists by default ( It’s thus necessary for wood-product manufacturers to complete their DHAs as required. Otherwise, they risk having the fire marshal shut them down or their insurance company drop their coverage. An assumption of risk flows throughout the standard, with one portion stating explicitly that owners should, “Assume that all wood waste in an enclosed dust collector is potentially deflagrable, unless a dust deflagration test demonstrates it is not (A.” The same section of code identifies other assumptions. Unless otherwise proven, the NFPA considers a deflagration risk to exist when:

  • The mean dust size is 500 microns or less.
  • At least 10 per cent of the dust mixture contains dust 80 microns or less in size.
  • The dust is produced from fine cutting, such as sanding (1).
  • The dust is created from sawing or machining hardwood (3).
  • The dust is created from sawing or machining fiberboard (4).

The NFPA does not usually consider dust created from sawing or machining softwood deflagrable unless it accumulates (2). However, NFPA 664 states that any cellulosic dust poses a risk when it accumulates to an average of 1/8-inch on all upward-facing surfaces in a building compartment ( These concerns apply only to materials that haven’t been evaluated.

But, according to the standard, the NFPA does not deem fire and deflagration hazards to exist in systems that handle only green material ( (NFPA 664 defines green material as wood with a moisture content of 25 per cent or higher (3.3.17).). The NFPA does, on the other hand, consider dry material a risk ( It also deems risk existent if part of a dust handling system is combustible, whether or not the material is dry or green (, examples being a rubber conveyor belt or a baghouse filter that can burn. The agency, likewise, always considers a suspension a risk when the material is concentrated at 25 per cent above its minimum explosible concentration (

Standards for conveying systems

Understanding where risks for fire, flash fires, and explosions exist are necessary because it is in these locations that 664’s standards for inherently safe material handling equipment apply. Conversely, these standards do not apply where there are no identifiable risks. An owner needn’t install a conveyor that’s compliant with 664’s standards for dust control if it’s outdoors and handling green material, though there are other reasons an owner may wish to do so, of course (reducing clean-up, for example).

As for conveyance systems in at-risk areas, NFPA 664 lists a slew of requirements. Among these, the standard states all ducts and enclosed mechanical conveyors must be designed with at least one of the following methods to mitigate explosions (

  • Strength to withstand the maximum, unvented deflagration pressure (1).
  • A listed deflagration-suppression system (2).
  • Relief vents and ducts (3, 5).

Indoor systems may vent through flame-quenching devices (4), and all systems must be able to withstand the maximum, unvented deflagration pressure of its relief or suppression system if equipped with these devices (2-5).

The code does allow some flexibility around the strength of the conveyor if the conveyor is outdoors. Owners may conduct a risk analysis in such scenarios and install a conveyor with a weaker box, but the conclusions of the analysis must be acceptable to the local regulatory official (6).

NFPA 664 also stipulates that conveyance systems handling explosive dusts mustn’t leak dust. On this point, there is some ambiguity between standards 664 and 652. NFPA 664 states these systems must be designed to “minimize fugitive dust emissions” ( Meanwhile, 652 states, “Housings for enclosed conveyors (e.g., screw conveyors and drag conveyors) shall be of metal construction and designed to prevent the escape of combustible dusts (, emphasis added). NFPA 652 also states, “Where the equipment cannot be designed for dust containment,” it must have a dust collection system in place ( The words “minimize” in 664 imply conveyors do not have to be IP-65-rated dust tight, which would require a tightly enclosed system like motor housing. The only places where the standard mentions “dust tight” in relation to conveyors is for removable covers and hatches ( and for bearings and bushings ( Nowhere does the code state that conveyors as a whole be dust tight.

It’s also worth noting that NFPA 652 does not state the degree to which systems must prevent dust from escaping. The gist of the NFPA’s standards is that conveyors mustn’t leak any volume of dust that could constitute a hazard or build up to the point of creating a hazardous situation. Small openings in conveyors, such as rivet holes or gaps for plastic expansion, appear acceptable. Systems with any meaningful leakage will likely require dust collection, however (and those that leak large volumes will certainly require it).

Another area of concern with conveyance systems is their ability to isolate fires. NFPA 664 ( and 652 (, 3) require conveyance systems with identified risks be designed so fires not propagate to processing systems upstream. An example of such isolation devices is a flop gate, which enables a conveyor to quickly dump material.

Other requirements for mechanical conveyors include:

  • Design, installation, operation, and maintenance that prevents the buildup of excessive heat (
  • Unless impractical, bearings and bushings that are outside the equipment (
  • Shaft seals located where shafts penetrate equipment walls (

Standards for screening

Beyond setting standards for conveyance and processing equipment, NFPA 664 also stipulates manufacturers screen bulk, cellulosic material before processing it. “Wood stock must be inspected for foreign materials […] prior to being processed,” it states ( Also, “Foreign materials […] capable of igniting wood waste and wood dust shall be prevented from entering the wood and dust process equipment” ( These codes are targeted at metal contaminants because they can create sparks within processing equipment.

While the standard doesn’t specifically mention screening in Chapter 9, a note accompanies in the appendix that lists screeners, along with magnetic separators and grates, as a system capable of removing metals from the material stream. Even without the mention of screening in the appendix, all but requires it due to the nature of the task.

Safe systems

While any category of conveyance system can be designed to meet the standards outlined in NFPA 664, it’s worth noting that some systems are inherently safer than others.

Among the least safe are bucket elevators, according to Jack E. Osborn, member of the Correlating Committee on Combustible Dusts, which created standard 652 and the most recent version of 664. In a recent edition of Powder & Bulk Solids, Osborn wrote, “Fully enclosed bucket elevators represent one of the highest hazards and risk types of equipment when handling and conveying combustible materials. Deflagration event records confirm that bucket elevator bearings and belt-and-pully alignment represent consistent ignition source risks,” (“Is Your Facility Dust Compliant?” Vol. 39 (No. 5), 26-27). Osborn reached this conclusion after studying cross-industry data for years as part of his duties as a committee member at the NFPA.

According to Osborn, pneumatic conveyance systems constitute some of the safest systems when, he stressed, they are designed properly. But it is also true that mechanical drag conveyors are among the safest solutions for handling deflagrable dust – when designed properly.

Both pneumatic systems and mechanical conveyors have their downsides, as do bucket elevators (beyond their safety record). But all these systems will wear, and any effectiveness they have containing dust when first installed will diminish over time. Owners, as stated in 664, must therefore maintain their equipment to ensure it continues to operate safely.

The maintenance aspect of safety means that as owners and managers purchase new conveyance systems, they should consider how quickly a given system will wear and how easily crews can perform maintenance on it. The faster a system will wear and the more difficult the maintenance, the less likely it will receive the care it needs to meet NFPA 664’s criteria. Owners and managers do well to identify reliable systems that minimize downtime and which are designed to ease maintenance.

Commitment from management is key to safety. Yet while some wood processors have begun requiring NFPA-compliant conveyance systems, many mills prefer not to make such an investment, which can be significant. The alternative, though, is what happened recently to a shavings manufacturer in New York. In 2013, the mill experienced a catastrophic fire, which burnt down the operation. That fire came only five years after a previous fire incident. After rebuilding, a third fire occurred in April 2021. The business frequently had such incidents, the owner said (WWNY. “Firefighters battle blaze at Berry Brothers Lumber,” wwnytv.com, accessed April 27, 2021). Unkept machinery and a lack of safety systems will put any mill at risk of a similar event.

There are enough loopholes in the NFPA codes that a mill owner may get by with systems that don’t meet the standard (after all, it is the owner who conducts the DHA, and it’s a local-level fire marshal who conducts the inspections), but insurance companies aren’t going to turn a blind eye to repeated incidents – we can assume the insurance premium for Berry Brothers Lumber increased after the third major fire in less than fifteen years. Indeed, we’ve seen insurance companies already begin to make changes in what they will and will not insure among wood product industries. Many will no longer cover rotary dryers, for example.

Yet, the onus for change ultimately does not come from the insurance companies or even machinery suppliers. It rests with the operation owner (664 4.1.2). How quickly the NFPA’s requirements are adopted, and thereby how quickly risk is mitigated at wood processing facilities, depends on the priority management assigns compliance.

Joel Dulin is the director of digital marketing and sales co-ordinator for Biomass Engineering & Equipment.

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