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Fire and Explosion Hazards in Enclosed Powder Conveyors

March 25, 2016
Figure 1: Still shots from the CSB video “Inferno: Dust Explosion at Imperial Sugar” showing the belt conveyor under the silos before being enclosed (left) and after being enclosed (right).
Figure 1: Still shots from the CSB video “Inferno: Dust Explosion at Imperial Sugar” showing the belt conveyor under the silos before being enclosed (left) and after being enclosed (right).

Catastrophic dust explosions during the last 15 to 20 years have placed an increased emphasis on the prevention and mitigation of dust explosions. In response to these incidents and subsequent Chemical Safety Board (CSB) investigations and recommendations, OSHA introduced a Combustible Dust National Emphasis Program (NEP) and has greatly increased enforcement efforts. One area, where prescriptive guidance often seems unclear to facilities, is protection methods for fire and explosion hazards in conveying systems. Those systems are discussed in more detail in this article.

Focus on Enclosed Conveyor Hazards
Conveyor sections are widely used in bulk powder processing and are often enclosed, in order to reduce the amount of fugitive dust emissions and prevent outside contamination. Although enclosing a combustible powder conveyor may reduce the combustible dust hazard in the surrounding area, it can introduce a new dust explosion hazard potential inside the enclosure.
    
The February 7, 2008 dust explosion at a sugar facility in Georgia brought increased attention to explosion hazards in conveyors. The incident led to the death of 14 workers at the facility. One theory, put forth by the CSB, is that the explosion initiated in an enclosed conveyor below silos. The CSB investigation found that a belt conveyor running in a tunnel under the silos had been enclosed in the year prior to the incident in order to minimize the possibility of contamination of sugar for food safety issues. Figure 1 shows still shots from a video produced by the CSB that highlight the installation of the belt conveyor enclosure. The CSB theorized that this increased enclosure led to the creation of dust clouds above the minimum explosible concentration that were ignited, and initiated an explosion that propagated first through the enclosed conveyor, and subsequently throughout the facility.

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The CSB concluded that the enclosed conveyor under the silos played a major role in the incident by creating a confined and unventilated space where dispersed dust could reach an explosible concentration [1]. According to the CSB report: “[a]lthough sugar dust was generated as the sugar flowed onto the belt and when flow blockages caused sugar to spill off the belt, these new enclosures were not equipped with a dust removal system and were not equipped with explosion vents.” While the CSB pointed out the lack of explosion venting in the enclosed conveyor, it added that “designing a deflagration vent system for the belt conveyor in the silo tunnel was impractical [since that] conveyor was located on the ground floor in the center of the building […] far from any building exterior wall or roof” [2]. Thus, while the CSB identified the lack of explosion protection for the conveyor created a hazard, it did not identify a practicable method to protect an enclosed conveyor system located within a building.

In response to several large dust explosion incidents and subsequent CSB recommendations, OSHA introduced a Combustible Dust National Emphasis Program (NEP) in 2007-2008 and greatly increased enforcement efforts. OSHA does not currently have a comprehensive general industry regulation for the prevention of dust explosions. Where OSHA believes a dust explosion hazard exists, but there is not an applicable OSHA regulation, OSHA relies on the General Duty Clause [3].

The frequency and types of OSHA citations listed under the general duty clause were examined by searching for citations containing the words “dust explosion”, “combustible dust”, and “conveyor” in an OSHA general duty clause citation database for the past 27 years [4]. Figure 2 shows the number of OSHA general duty clause related to dust explosion hazards, as well as those including reference to conveyors, contained in this database. The database and this search criterion likely do not identify all citations that have been issued by state or federal OSHA related to combustible dust hazards, but does provide a general trend of citation history. The data show how in 2007/2008 there is an uptick in general duty clause citations associated with combustible dust hazards. A similar increase in citations involving conveyors is also seen.
 
Figure 2: OSHA general duty clause inspections resulting in citations related to dust hazards since 1989. The left plot shows cited facilities were “combustible dust” or “dust explosion” was mentioned. The right plot shows the facilities cited for dust explosion hazards where the conveyors were mentioned.  
 
OSHA citations related to combustible dust conveying often identified missing systems that are intended to minimize ignition of dust or mitigate the consequences of fires and explosions. These missing systems include:
 
•    interlocks to shutdown plugged equipment
•    spark detection
•    belt alignment sensors
•    battery temperature monitoring
•    bonding and grounding
•    fire protection systems
•    explosion protection
•    explosion isolation

Guidance on Conveyor Hazard Mitigation
Most U.S. building codes provide at least minimal guidance related to facilities with dust explosion hazards. Other references are often used to address potential hazards. The National Fire Protection Association (NFPA) publishes several combustible dust standards that apply to different types of industries. In addition, a new combustible dust standard NFPA 652 Standard on the Fundamentals of Combustible Dust was published in 2015, and provides the basic principles of and requirements for identifying and managing the fire and explosion hazards of combustible dusts.
      
NFPA 652 requires facilities to perform a Dust Hazard Analysis (DHA), in which explosion scenarios should be considered for any enclosed conveyor where “combustible dust in sufficient quantity or conditions to support the propagation of a flame front during startup, normal operating conditions, or shutdown” or which contain “a combustible dust in sufficient quantity or conditions to support the propagation of a flame front under conditions of production upset or single equipment failure” [5,6]. This standard, however, does not offer any specific prescriptive requirements for enclosed conveyors. Several commodity-specific NFPA standards (NFPA 61, NFPA 484, NFPA 654, etc.) provide some additional guidance to mitigate the hazard.
      
NFPA 654 (2013) Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids contains some requirements for the protection of combustible dust handling conveying systems. When considering explosion protection in dusty areas within the conveyor, NFPA 654 (2013) provides the following requirements for “enclosed conveyors”.

7.11* Enclosed Conveyors
A.7.11 Explosion protection should be provided when the risk is significant. Where coverings are provided on cleanout, inspection, or other openings, they should be designed to withstand the expected deflagration pressure. (See 7.1.4.)
7.11.1 Housing and Coverings
7.11.1.1 Where an explosion hazard exists within enclosed conveyors, they shall be protected in accordance with 7.1.4.
     
As shown above, explanatory material in the annex states that explosion protection should be provided when the risk is “significant”. However, there is little guidance on what represents a “significant” amount of risk. Section 7.1.3 of NFPA 654 also allows for the use of a documented risk evaluation to determine the level of protection to be provided for such equipment.

Hazard Analysis of Dust Conveyors
In order to determine the level of explosion protection required for an enclosed conveyor, however, the potential risk of an explosion needs to be further evaluated using a dust hazard analysis (DHA). A DHA starts by identifying the combustible dust hazards in the process. For example, a potential dust explosion could occur in transfer areas of the conveyors, where dust in suspension is generated during normal operation. The risk posed by the transfer and processing of combustible dust will depend on a number of factors, including:
•    The properties of the combustible dust:
o Explosion characteristics (Pmax / KSt)
o Particle size (propensity for entrainment)
o Ease of ignition (Minimum Ignition Energy, Minimum Ignition Temperature)
o Propensity to self-heat
•    Process conditions:
o Is the process at elevated temperatures?
o Does the process involve heating, grinding or spark generation?
o Are any competent ignition sources present in or near the equipment?
o Is there a large amount of fugitive dust generated by the process?
o Does the equipment have dust collection?
o Does the equipment follow prescriptive requirements in NFPA standards?
•    Potential consequences of a dust explosion:
o Is the process located indoors?
o Is the process located on or near a critical structure?
o Is the area around the process normally occupied?
o Will an explosion/fire cause structural failure of the equipment?
o Is there a fire/explosion emergency action plan that covers such a scenario?

The level of risk associated with each hazard is compared to the allowable risk matrix for the facility, and mitigating measures must be applied where the risk is unacceptable. When deemed necessary by the DHA, the risk of an explosion inside an enclosed conveyor can be minimized by either (1) minimizing the likelihood of such an event, or (2) reducing the magnitude of the potential consequences.
      
The likelihood of an explosion event can be minimized by:
•    Removal of potential fuel sources. Dust collection can be used to reduce dust clouds where they are generated at transfer points.
•    Removal of potential electrical and static ignition sources:
o Ensuring there is no unclassified electrical equipment inside the conveyor enclosure, and that all electrical equipment in areas around the conveyors where dust may be present is classified for use in such areas.
o Bonding and grounding all conveyor components.
o Using a static dissipative belt, reducing the potential for static sparks as a viable ignition source.
•    Prevention of frictional heating of the material on the belt:
o Using a belt misalignment alarm, to prevent rubbing and frictional heating, as recommended in FM Global Datasheet 7-11 §2.5.3.
o Using a belt speed monitoring alarm and interlock to detect a stuck or broken conveyor belt, as recommended in FM Global Datasheet 7-11 §2.6.2 and required by NFPA 654 (2013) §7.11.2.1.
o Using ‘sealed for life’ bearings inside the idlers in the conveyor, preventing dust from entering the bearing
o Having adequate preventative maintenance programs to reduce the probability of bearing failure.  It should be noted that ‘sealed for life’ bearings can still fail and provide a potential source of ignition. An appropriate maintenance and housekeeping program is critical to reduce the probability of ignition caused by bearing failure.
•    Inerting of the atmosphere above the conveyor via oxygen reduction
•    Using a Hot Work Permit system when performing hot work near conveyors.

The consequences of a potential deflagration inside a conveyor can be minimized by:
•    Providing sufficient explosion vent area.
•    Locating conveyors outside
•    Limiting occupancy around conveyors during operation.
•    Having workers exposed to the combustible powder wear appropriate Personal Protective Equipment (PPE) appropriate for the location, including flame-resistant garments.
•    Using a structure supporting the conveyor that is non-combustible.
 
The use of some or all of the safeguards listed above can reduce the risk posed by an explosion inside a conveyor to below the level tolerable by the facility.

Conclusions
Enclosed conveyors, especially those located within a building or frequently occupied work area, can pose a significant hazard to a facility and personnel. In order to determine the level of explosion protection required for an enclosed conveyor, however, the potential risk of an explosion needs to be further evaluated. Explosion prevention and mitigation methods are available, but current industry standards do not address the specific requirements necessary for enclosed conveyors in detail. Therefore, facilities may need to rely on a hazard analysis, such as the dust hazard analysis described in NFPA 652, to determine the appropriate methods of prevention and protection.
 
Alfonso Ibarreta, Michael C. Stern, and Timothy J. Myers, Exponent Inc., Natick, MA. For more information email [email protected] or visit
www.exponent.com.

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References
[1] Chemical Safety Board, “Investigation Report No. 2008-05-I-GA” (2008), page 1. Available at: http://www.csb.gov/assets/1/19/CSB_CTA_Investigation_Report.pdf
[2] Chemical Safety Board, “Investigation Report No. 2008-05-I-GA” (2008), page 29. Available at: http://www.csb.gov/assets/1/19/CSB_CTA_Investigation_Report.pdf
[3] OSHA, “Status Report on Combustible Dust National Emphasis Program” October 2009. Available at: https://www.osha.gov/dep/combustible_dust/combustible_dust_nep_rpt_102009.html    
[4] OSHA General Duty Standard Search. Available at: http://www.osha.gov/pls/imis/generalsearch.html (Accessed 2016).
[5] NFPA 652, Standard on the Fundamentals of Combustible Dust (2016) Sec. 6.4.2.1
[6] NFPA 652, Standard on the Fundamentals of Combustible Dust (2016) Sec. 6.4.2.2
 

Exponent Inc.