Unconfined Deflagration Testing for the Assessment of Combustible Dust Flash Fire Hazards

The presence of combustible dust can result in associated flash fire and dust explosion hazards at manufacturing facilities.  The hazard and mitigation strategies for combustible dust explosions are relatively well understood, but flash fire hazard assessment methods are currently lacking.   Improved flash fire hazard assessment methods are of significant industrial interest because of the lack of a standard test method for assessing the hazard of unconfined deflagrations.  Dust explosion hazards are currently assessed by OSHA and NFPA regulations using standardized tests that measure the maximum pressure rise (P_max) and the deflagration index (K_st) in a confined system, but no similar hazard classification exist for flash fire hazard potential.  Simulated flash fire testing performed by Exponent has demonstrated that dusts with similar deflagration indexes (K_St) and maximum pressures (P_max) as measured in a standard 20 L combustion chamber can exhibit radically different flash fire behaviors. 

This paper addresses the need for developing flash fire hazard evaluation methods.  Using an unconfined dust deflagration test setup, we will present data for a number of typically-encountered combustible dusts, and demonstrate how the flash fire hazard of a combustible dust is not effectively characterized by the current standard test methodologies.  We will consider whether testing in a confined 20 L combustion chamber can accurately predict the unconfined flash fire hazard using flame propagation models. Recommendations will be provided for a standardized test which could accurately evaluate an unconfined deflagration hazard.