NFPA 652 Combustible Dust Hazard Risk and Requirements For Enclosureless Dust Collectors

by Ronald L. Allen, MS, PE, CSP, Senior Consulting Engineer, Fauske & Associates, LLC

Requirements, Risks and Incidents Associated With NFPA 652 Direction to Control Combustiblle Dust Hazards. Not all Enclosureless Dust Collectors (EDCs) are intended for combustible dust.

NFPA 652 – 2016, Standard on the Fundamentals of Combustible Dust, defines an enclosureless dust collector (EDC) as, “An air-material separator (AMS) designed to separate the conveying air from the material being conveyed where the filter medium is not enclosed or in a container”. Commodity-specific NFPA standards offer more detailed definitions. EDCs may also be known as Positive Pressure Open Dust Collectors.

Enclosureless dust collectors (EDCs) are commonly used in the woodworking industry and home woodworking shops. They are also present in other varied industries and applications – whether or not such applications are permitted by NFPA standards.

EDCs are available from a breadth of manufacturers. Designs vary considerably – from very simple to complex. Units are available with single or multiple filters (“bags”). Typically, EDCs are less expensive to purchase than traditional cartridge or baghouse style dust collectors. Not all EDCs are intended for combustible dust service. The sketch depicts a multiple filter EDC.

Deflagration Risks Associated with Enclosureless Dust Collectors

Some risks from traditional explosions are reduced with compliant EDCs since creation of projectiles would be limited given the fabric filter bags used in construction.  Yet, deflagration and flash fires risks remain as discussed below.

1. Low strength enclosures (i.e., filter bags) may be ruptured or burned by a deflagration. In such case, the resultant fireball could spread in an uncontrolled fashion in all directions.
2. Filter bags may be combustible
3. Improper application of EDCs (e.g., collection of combustible metal dust; utilization of units intended for non-combustible dust)
4. Assumed presence of oxygen, fuel, confinement, and dispersion – only ignition is missing to potentially create a deflagration
5. Inability to provide explosion venting
6. Infeasible to locate outdoors without shelter
7. Limited ability to provide suppression or isolation
8. Filter bag breakage could release a dust cloud exceeding MEC. Such releases could find a viable ignition source.
9. Recirculation of air from EDCs into the workplace can add fugitive dust to the environment
10. Fans may be located in the dirty air stream where ignition could be created from frictional heating
11. Removal of dust from filter bags can create uncontrolled dust clouds

NFPA Requirements for Indoor Use of Enclosureless Dust Collectors

Because of this blog’s space constraints, discussion is limited to indoor, prescriptive requirements for utilization of EDCs^[1]. NFPA 652 does not permit EDCs to be located indoors unless the appropriate industry or commodity-specific standards allow such installations. Only NFPA 664 - 2012, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, and NFPA 654 - 2013, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, allow indoor use of the EDCs. Indoor use of EDCs is not permitted by inference when collecting combustible agricultural dust since they are not referenced in NFPA 61 - 2013, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities (Note: performance based design option could be used to justify use of EDCs for some agricultural dusts). NFPA 484-2015, Standard for Combustible Metals) specifically bans the use of EDCs.

NFPA 664 states that explosion protection and isolation requirements do not apply to compliant EDCs and that compliant EDCs are permitted to exhaust into the buildings.

NFPA 654 warns that EDCs are not meant for use with most dusts created during the venting of process equipment or other aerated dust sources. The Standard indicates that fine dust will rapidly blind the filter, which results in reduced performance and a significant increase in deflagration hazards associated with the system operation and performance.

Requirements for indoor usage of EDCs vary somewhat between NFPA 654 and NFPA 664. Common and unique requirements appear below:

1. Common Requirements
   
   
   1. The filter medium is not shaken or pressure-pulsed to dislodge dust during operation^[2]
   2. The collector is located at least 6.1 m (20 ft.) from any means of egress or area routinely occupied by personnel
   3. The collected dust is removed daily or more frequently if necessary to ensure efficient operation^[3]
   4. Multiple collectors in the same room are separated from each other by at least 6.1 m (20 ft.)^[4]
      
      
2. NFPA 664 Requirements
   
   
   1. The collector is used only for dust pickup from wood processing machinery (i.e., no metal grinders and so forth)
   2. The collector is not used on sanders, molders, or abrasive planers having mechanical material feeds through the machine
   3. Each collector has a maximum air-handling capacity of 2.4 m³/sec (5000 cfm)
   4. The fan motor is of a totally enclosed, fan-cooled design
   5. The collected dust is removed daily or more frequently if necessary to ensure efficient operation
   6. The filter media are not enclosed or in a container
   7. The filter media are not under positive pressure
   8. Removal of the collected dust is not continuous or mechanical (intended to effectively limit the size of the collector because, without continuous or mechanical removal of collected dust, it is not practical to manually remove the dust on the larger systems).
   9. Fans and blowers be located upstream of EDCs regardless of the moisture content or particle size of the material conveyed

While automatic sprinkler protection is not required in EDCs, NFPA 664 recommends protecting EDCs with either an automatic sprinkler located above the unit or a spark detection and extinguishing system in the main duct, upstream of the unit.

        C. NFPA 654 Requirements

1. The AMS is not used to vent or serve metal grinders, hot work processes, or machinery that can produce sparks
2. The AMS is not used to vent or serve sanders, abrasive planers, or similar sanding process equipment
3. Each collector system has a maximum airflow– handling capacity of 3000 cfm (1.4 m³/sec). (Interpretation: Many of the enclosureless dust collectors are manifolded into multiple bags with containers. The 3000 cfm limit refers to the overall airflow through the assembly and not just to a single bag with collected material container).
4. The fan motor is suitable for Class II, Division 2, or Class III, as appropriate
5. The minimum ignition energy (MIE) of the collected materials is greater than 500 mJ. (Interpretation: MIE is determined by testing the material as received with respect to particle size).
6. The fan construction is spark resistant and meets the criteria in 7.12.2.5
7. The filter medium is not located within 35 ft. (10.7 m) of any open flame or hot surface capable of igniting a dust cloud of the material it contains

Incidents Involving Enclosureless Dust Collectors

Deflagrations involving EDCs have been reported, but broad-based data is not readily available.  Readers of this blog are encouraged to share their experiences. For more information on this and other Dust Hazard Analysis (DHA), contact Ron Allen at Allen@fauske.com,  608-698-6105 or 630-323-8750

^[1] NFPA 664 allows for indoor locations of EDCs but highly recommends outdoor locations. NFPA 664 advises against locating dust collectors on the roofs of buildings.

^[2] NFPA 664 permits shaking or pressure-pulsing if the fan is off.

^[3] NFPA 654 interpretation: Dust must be removed daily and is limited a maximum of 22 lbs. (10 kg) per day.

[4] NFPA 654 interpretation: Enclosureless dust collectors are often manifolded into multiple bags (with collected material containers). Each such manifolded assembly must be separated by the required 20 ft. or 6.1 m.)