Effectively Managing fire and explosion hazards posed by combustible dust can be a challenging task. After all, this requires not only a detailed understanding of the fuel/combustible material (dust) but also an understanding of the process equipment, operating conditions, maintenance practices, engineering and administrative controls currently in place, process design strategies, hazards analysis methods and the site’s safety culture. It’s not unusual to find facility management with a good understanding of a process but a limited understanding of the hazards posed by the combustible dust in the process.
Most organic solids are capable of burning when all three elements of the familiar fire triangle — fuel, heat and oxygen — are present at the same time. Any appropriate source of energy can supply the heat, and high enough concentrations of fuel and oxygen must exist to support combustion.
On the other hand, if a sufficiently large concentration of combustible dust is suspended and ignited in an enclosed space, the resulting combustion would develop pressure that can cause injuries and fatalities as well as damage or destroy equipment and buildings. The elements of dust suspension and combustion confinement commonly are added to the fire triangle to depict the “dust explosion pentagon” (Figure 1). If any element of the pentagon is missing, an explosion won’t occur. However, in the absence of confinement, suspended dust still can combust, creating a “flash fire” or fireball that can create a hazard to people and potential property damage.
This article outlines an effective approach to addressing combustible dust hazards by implementing a proactive and robust mechanical integrity (MI) program.
Generally speaking, an MI program aims to manage the maintenance of all processing equipment and control systems of a facility to ensure the process is operating safely and within its intended parameters. If equipment or systems are run outside their safe operating limits, the potential for equipment failure clearly is much higher.
At minimum, MI includes the inspection, testing and preventive maintenance of “safety critical” equipment, i.e., those units whose failure or malfunction could result in a combustible dust fire or explosion. A more-comprehensive approach to MI would cover all process equipment that could contain combustible dust during normal or abnormal conditions, along with instrumentation and alarm/interlock systems used to prevent combustion. Equipment in the scope of the MI program must remain “fit for service” for its entire lifecycle, from procurement and receiving to installation, maintenance and decommissioning.
Three brief examples demonstrate the importance of MI to the control of combustible dust hazards:
1. Overheated bearings are a well-known ignition source for combustible dust. Depending on the specific service, equipment may require an anti-friction bearing design. The bearings must be maintained per manufacturer recommendations, with proper lubrication and cleaning at a frequency that would prevent a hazardous buildup of dust. Alternatively, the design should provide for bearings that are outside the dusty environment.
An effective MI program would include temperature monitoring of the bearings, either by manual or automated means, to verify the bearing temperature remains at a safe margin below the layer minimum ignition temperature (LMIT) of the powder. LMIT typically is determined by a laboratory test according to ASTM E2021. For an automated monitoring system, maintenance must ensure a high degree of reliability.
2. Poorly maintained equipment in combustible dust service may leak or spill powder to the floor and onto equipment surfaces in the work area. Besides the obvious hazard of providing fuel in the form of a combustible dust layer, an additional hazard exists if a dust layer accumulates on equipment that can develop temperatures that could cause ignition of the powder.
Containment of dust within equipment will depend on frequent inspections or audits to detect incipient failures that could lead to leakage or spills. Also, equipment that could be exposed to combustible dust accumulation must operate with a surface temperature well below the LMIT of the powder according to ASTM E2021. In addition, it may make sense to determine the minimum ignition temperature of the dust cloud according to ASTM E1491 for the powder of interest.
3. To protect personnel, the facility and the community from the effects of an explosion should an explosive rupture of equipment occur, some processes call for special measures to minimize the consequences. The options are explosion relief venting, explosion suppression or explosion containment in a vessel that can withstand the maximum dust explosion pressure. The design of any explosion protection measure (venting, suppression or containment) requires appropriate data concerning the severity of the dust cloud explosion (maximum explosion pressure and Kst). These data are obtained by performing a laboratory test on a representative dust sample in accordance with ASTM E1226. The MI program should include regular inspection of explosion vents, quarterly tests of explosion suppression systems, and periodic checks of vessel integrity according to recognized and generally accepted good engineering practices (RAGAGEP) such as API 510 and API 570, and FM Global Data Sheet 7-43 [1,2,3]. Inspections and non-destructive tests should be performed by personnel with appropriate training and experience, and at a frequency that would ensure fit-for-service performance during the interval between the inspections and tests.