Unwanted Accelerated Burning After Suppressant Delivery
Anthony P. Hamins, Kevin B. McGrattan, Glenn P. Forney
In a number of real-scale fire tests, accelerated or enhanced unwanted burning has been observed immediately following the delivery of a fire suppressant. The conditions under which this phenomena occurs, its potential for harm, and its effect on the ability of an agent to extinguish a fire have not been comprehensively documented. The objective of this work is to identify the causes of enhanced burning with an emphasis on aircraft dry bay and engine nacelle applications and to provide recommendations on how it can be avoided when it may become a significant safety risk.A survey of the fire and combustion literature was undertaken to document cases of enhanced burning after suppressant delivery. Informal interviews were conducted with a sample of people involved with full and reduced-scale fire suppression experiments. Analysis of the information suggests that there is no common terminology or unique definition used to describe unwanted enhanced burning. Several instances of enhanced burning have been documented and it is apparent that unwanted enhanced burning is not uncommon. Manifestation of the phenomena has been documented through measurements of increased pressure, temperature, and heat release rate as well as changes in the appearance (intensity, color, volume) of a fire for a number of experimental configurations. Although many events could be described as enhanced unwanted burning after suppressant delivery, the main hazard associated with this phenomena appears to be system over-pressurization due to enhanced combustion from a vaporizing liquid fuel that mixes with air after suppressant delivery. In many instances, the phenomena do not represent a significant safety risk.To further understand the phenomena, three experiments in which system over-pressurization was suspected were analyzed in some detail. The cases involved over-pressurization of full-scale suppression experiments for an aircraft dry bay and two cases of over-pressurization of full-scale enclosure fire experiments. The three cases had many differences in terms of experimental conditions, timescales of the suppression events, and the geometric configuration of the experiments, yet in all three cases volatile liquid fuels were present.In summary, the main hazard associated with unwanted burning after suppressant delivery appears to be over-pressurization likely due to enhanced mixing of vapor with air. From the investigation of three key case studies, the most important overall finding is that suppressant delivery does not appear to pose undue safety risk unless insufficient amounts of suppressant are applied to a fire. It is concluded that these effects are already embedded in full-scale test results and associated design equations. This report concludes with a discussion of safety issues associated with unwanted enhanced burning after suppressant delivery.
, McGrattan, K.
and Forney, G.
Unwanted Accelerated Burning After Suppressant Delivery, Special Publication (NIST SP), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.sp.1004
(Accessed September 26, 2022)