Page 40 - Occupational Health & Safety, May 2019

COMBUSTIBLE DUST
shall be completed as part of the project. Finally, the DHA must be reviewed and updated at least every five years.
■ Managing identified fire, flash fire, and explosion hazards.
■ Establishing written safety management systems. NFPA 652 requires the owner/operator to establish written management sys- tems for operating their facility and equipment to prevent or miti- gate combustible dust fires, deflagrations, and explosions. These written management systems include operating procedures and practices, training, incident investigation, and employee participa- tion. Written management system requirements shall apply to new and existing facilities and processes.
Towards Compliance
Managing dust cloud flash fire and explosion hazards could include:
■ Data collection on your powders is a good first step. Specific testing on representative samples from your facility provides the highest confidence in data compared to those obtained from litera- ture sources. Typical tests/data to consider include:
3 ExplosibilityScreening(Go/NoGo)—ASTME1226 If Explosible (Go), then:
3 Ease of Ignition?
■ Minimum Ignition Energy (Dust Clouds)—ASTM E2019
■ Minimum Ignition Temperature (Dust Cloud)—ASTM E
1491
■ Minimum Ignition Temperature (Dust Layer)—ASTM
E2021
■ Thermal Instability
3 Explosion Effects?
■ Maximum Explosion Pressure—ASTM E1226 ■ Maximum Rate of Pressure Rise—ASTM E1226
3 Controlling Flammable Concentrations?
■ Minimum Explosible Concentration—ASTM E1515
■ Limiting Oxygen Concentration—ASTM E2931
■ Avoidance/Control of explosible dust cloud (Fuel). Dust
accumulations on surfaces outside of processing equipment could result in devastating secondary dust explosions. To the extent pos- sible, one should prevent dust escape from the processing equip- ment through appropriate equipment design for dust containment and release source reduction. Additionally:
3 Use correctly designed, operated, and maintained local exhaust ventilation in those locations where any dust release is anticipated.
3 Establish an effective housekeeping program to prevent hazardous dust accumulations on all surfaces in the facility.
■ Avoidance/Control of Oxidant. A dust explosion within process equipment can be prevented if the oxidant (oxygen) con- centration can be maintained below the Limiting Oxidant Concen- tration (LOC), typically by replacing the oxidant with an inert gas. This is commonly referred to as inert gas blanketing, inert gas purg- ing, or simply inerting. Inerting can be considered if the equipment containing the explosible atmosphere can be totally contained. It should be noted that LOC for combustion depends on the nature of the dust (fuel) and the inert gas that is used. Commonly used inert gases are nitrogen, carbon dioxide, and argon. NFPA 69 provides requirements for installing systems for the prevention and control of explosions in enclosures that contain flammable mixtures, in- cluding inert gas blanketing.
■ Elimination/Control of Ignition Sources. Ignition sources that could be present during normal and foreseeable upset condi- tions must be identified and controlled. Common ignition sources include heat sources, friction and impact sparks, electrical arcs and sparks, and electrostatic discharges. This can be undertaken by:
3 Performingregularinspectionandmaintenanceofplant to prevent mechanical failure.
3 Performingwelding,cutting,andotherhotworkopera- tions under a hot work permit system according to NFPA 51B.
3 Electricallygrounding(earthing)allmetal(conductive) items of the plant. Where bonding/grounding system is all metal, resistance in continuous ground paths should be less than 10 ohms.
3 For locations where an explosible dust cloud or indeed any flammable atmosphere with a Minimum Ignition Energy (MIE) less than 30mJ might exist, operators must be adequately grounded. Static dissipative footwear used together with conduc- tive or static dissipative flooring with a total resistance between 106 ohms and 108 ohms provides a means to control and dissipate static charges from the human body.
3 Using appropriately rated electrical equipment and de- vices in locations where explosible atmospheres are/may be pres- ent. This is achieved by conducting a Hazardous Area Classifica- tion according to the requirements of Article 500 of the National Electrical Code (NFPA 70). The intent of Article 500 is to prevent electrical equipment from providing a means of ignition for an ig- nitable atmosphere.
3 Determining powder’s exotherm onset temperature for www.ohsonline.com
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