Effect of Inert Gas Discharge Time on Wood Crib Fires in Reduced-Scale and Full-Scale Experiments Public
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Currently, the UL 2127, Inert Gas Clean Agent Extinguishing System Units, and FM 5600, Clean Agent Extinguishing Systems, standards are used to test inert gas suppression systems and to determine the minimum extinguishing concentration (MEC) for total-flooding suppression system applications. For Class A ordinary solid combustible test articles, both UL and FM standards follow equivalent methodologies; only UL will be referred to for brevity. In this standardized test, the article is placed within a 100 m3 enclosure, burned, subjected to a suppression event over a certain discharge period, and then allowed to soak for 10 minutes before exposure to fresh air, checking for reignition, to determine the required MEC. The UL wood crib was selected as the sole test article to study over the UL plastic sheet arrays to simplify testing requirements and facilitate the study of re-ignition potential at the longer discharge period. In 2012, the NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, was revised to allow a maximum 120 s discharge period for systems that protect Class A fire hazards with inert gases. Preliminary tests conducted using Class A articles indicate that this change in discharge period can potentially change the suppression dynamics, resulting in different listed MECs, and created confusion in the inert gas industry. Confusion around these changes and results arises from two primary sources. The first being the requirements of the UL 2127 only to achieve crib extinguishment and reignition prevention. The second is the issue where the selected discharge period is not reported on test results. By not publicizing the discharge period, it is impossible to understand if a lower listed value for MEC is due to a design advantage or from a change in extinction dynamics between the two discharge periods. To answer these questions and provide more clarity to the inert gas industry, the underlying physics of the extinction dynamics within the test enclosure and near the burning test article must be better understood. In this work, a series of full-scale experiments were conducted to study the effect of discharge time over the extinction of a Class A test article following the methodology presented in the UL 2127 standard. During these experiments, the instrumentation was increased above UL requirements to capture the evolution of the article’s mass and changes in the oxygen content, gas velocity, and temperature in the atmosphere that surrounds it. Additionally, a novel ¼ scale enclosure and crib were designed to replicate the full-scale suppression dynamics with significantly reduced resource demands for testing. The scaling of the crib was performed following the method proposed by Heskestad and Croce et al. The reduced-scale enclosure was designed such that the only change in burning rate would be from the discharge event itself. The results obtained support the hypothesis that the fundamental changes in the suppression dynamics are caused by recirculating flows of agent and entrained air created during the high-pressure discharge. The impact of these recirculating flows, designated as bulk flows, alters the dynamics of the suppression event. The bulk flow effects, quantified by gas-phase velocity through the crib, were dependent on the agent discharge flow rate and were most prevalent at the full-scale test and 60 s discharge periods. Furthermore, the change in discharge time from 60 to 120 s had a negligible effect on the average agent extinguishment concentration at both scales. No crib at either scale or discharge period reignited after suppression or when exposed to fresh air after the prescribed 10-minute soak period. The constant agent extinguishing concentration results indicate that any differences between approved and listed MEC are likely not related to the discharge period selection.
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