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Understanding Fire Flow – Part 1
In a previous article I wrote titled The Engine Company’s Primary Mission I stated that the engine company officer will have to make rapid and accurate decisions to quickly identify and initiate the appropriate course of action at the scene of a fire.
After deciding the “Mode of Attack” the officer will have to determine the needed “Fire Flow” to extinguish the fire. In a later article I went into detail about how to determine the necessary fire flow for the conditions you are confronted with, but after receiving some feedback it became clear that there is a lot of confusion and misunderstanding about what fire flow is and how important it is to fire suppression.
Many people believe that fire flow is the amount of water you will need to extinguish a given fire and although partially correct there is more to it than that. But before I go any deeper into defining fire flow we need to make sure that we have a thorough understanding of the theory of how to safely and efficiently extinguish a fire. We are taught in basic training that water extinguishes fire by absorbing heat. But in order to be effective water must be applied at a rate faster than the rate of heat being generated.
The goal is to cool the involved combustibles dropping them below the temperature at which they produce ignitable vapors and heat to support the fires growth. There is also an extinguishing affect when water is converted to steam which dilutes the oxygen supply. Although steam has no ability to cool or absorb heat, it can reduce the temperature of the flames by modifying the air which supplies the flames oxygen.
According to NFPA, fire suppression can be accomplished by:
1. Cooling the gaseous combustion zone by flowing water into the overhead smoke layer and therma
l column to disrupt the flow of heat and combustible gases.
l column to disrupt the flow of heat and combustible gases.
2. Cooling the solid or liquid combustible by flowing water directly onto the burning solid materials to prevent the production of combustible vapors and gases.
3. Application rate and type of stream are the key factors which will ultimately determine the speed of extinguishment.The rate at which water is applied (GPM) is the definition of fire flow. And it is this application rate (GPM) that will have the greatest impact on how quickly we extinguish a fire. As stated above the type of stream will also play a significant role but without applying water at a rate faster than heat is being generated the type of stream will have very little impact on the speed of extinguishment.
In Part 2 of this series we will discuss how the challenges of the modern fire ground have created a need for higher fire flows then typically used in the past.
(Photo Courtesy: Tom Rice)
Part 1 of 4
Steam may somewhat dilute the air although it is not a significant factor, humans have no problems breathing in saunas filled with hot steam…
The advantage of creating steam to suppress flames is that steam acts like a thermal ballast in the air and prevents the flame to reach the temperature to oxidise. So it is the cooling effect of steam (or anything else inert you put inside the flame) that will extinguish the flame.
A modern fire will not burn any hotter (fire is always the same temperature) or with any more heat being released unless you supply oxygen to it. Much oxygen, big fire, little oxygen, small fire. Ventilation of a ventilation controlled fire must be either combined with water application to the seat of the fire or preferably the seat of the fire and the smoke should be cooled before ventilating.