Rate-of-flow formulas for larger fires
Let’s return to the beginning, and consider again the question
How much water is needed to fight a given fire?
For one or two family detached dwellings with the fire still confined to the structure, the answer is fairly simple. For confined structure fires involving a room five to ten gallons of water converted to steam will control or extinguish that fire. This can be done with one pumper and three to four firefighters. In fact in the United States 75% of all structure fires are confined to the room of origin by using a single attack line.
However, occasionally fire departments (brigades) encounter a dwelling fire that is not confined. A fully involved house fire that is burning in the open requires more than one pumper and just a few firefighters. Besides this, every community has community or commercial buildings that are much larger than dwellings. Such fires need much more resources in the number of pumpers, the number of fire fighters, and the amount of water needed. So let’s shift from house fires to larger fires that occur in larger structures. Our question now becomes:
How much water is needed to fight a larger fire in a larger structure?
In any fire attack there are three operations that firefighters perform. For answers to this question we will turn to two books published by the National Fire Protection Association (NFPA) in 1966 and l968 authored by Warren Y. Kimball. He was then Manager of the NFPA Fire Service Department. The two books are titled: FIRE ATTACK, Command Decisions and Company Operations, and FIRE ATTACK 2, Planning, Assigning, operating. Our concern is primarily with the operations that provide the needed fire flow (NFF) that will control or extinguish the fire. The goal of any fire attack is to control or extinguish the fire with the minimum damage to life and property. In any fire attack there are three operations that firefighters perform.
The first is fire attack, that is, the operation of controlling or extinguishing the fire by the application of water from nozzles, or by using other extinguishing agents. The second is the supply of water with needed pressure and volume for the fire streams being used. The third are support operations needed to make a fire attack efficient and effective. Support includes forcible entry, ventilation, laddering, salvage, lighting, and rescue (1st priority).
Immediately, before fire attack even begins, the officer in command of the first pumper to arrive on the scene of a structure fire is faced with a dilemma. This dilemma is widely known in the fire service. It is whether to proceed directly to the fire and order a fast attack with small preconnected attack lines supplied by water in the tank on the pumper, or stop at the nearest hydrant before reaching the fire, and lay out a large supply line to be connected to the pump on the pumper.
The number of firefighters responding with the first pumper is critical here. The risk in the first operation is that the NFF may be greater than can be supplied using small lines with a limited supply of water. In other words, the officer runs the risk of running out of water before the second pumper arrives on the scene. The risk of the second operation is that the initial fire attack will be delayed long enough for a much larger fire to develop. The number of firefighters responding with the first pumper is critical here. One firefighter has to be left at the hydrant to make the connection and to turn on the hydrant. This firefighter may be needed to stretch the first attack line. So there is a real dilemma here. There are several things that can be done to get away from this dilemma., but further discussion on the dilemma will take us away from our priority of dealing with NFF.
