CRITICAL FLOW RATE
In spite of horrendous loss of life, property, and means of production from structural fires, conventional manual fire suppression technology is still essentially at the World War II level. The old ‘’Save-yourwater”/ “Catch-up” approach is still widely used.
Although there has been fire behavior, suppressant, and building material research, practical suppression research involving both the fire service a fid the scientific community has been neglected, largely because of its complexity.
This now is changing. After exploratory research burns to evaluate modem Class A foams and compressed-air foam systems (CARS), Ron Rochna of the Boise Interagency Fire Center suggested that the author initiated joint US/Canadian research project: “Quantitative Evaluation of Enhanced Water Fire Suppression.”
Experience has indicated that advanced Class A fire suppression technology allows a nozzleman to darken between three and 20 times as much fire as the conventional plain water system. The research project will answer the important questions: How much more effective is advanced technology than plain water in practical fire suppression? How can its use be optimized? It will feature 50 research fires this month near Vernon B.C., Canada, to compare Class A foam solution, aspirated foam, atid CARS with plain water.
This article and others that follow will give the foundation of new suppression technologies and explore exciting advances in fire suppression. The results of the research tests will be published in future issues of Fire Engineering.
Traditional “save-your-water” firefighters, faced with limited water, tend to use their many years of experience to estimate how long the fire will take to burn itself out. Then the available water is cunningly rationed to last until after the rubble has cooled.
The objective seems to be to makesure that water doesn’t run out. What about saving the building? Skeptics could suggest that traditional ‘saveyour-water” fire departments might as well stay in the firehouse since that saves all their water and they don’t save buildings anyway.
The traditional cry “Save your water!” is correct, but not in the way it usually is interpreted. In fact, the higher the initial attack fire flow in gpm, the less water it takes to darken a fire —believe it or not.
Think of a large, fully involved, 2,000-square-foot fire. Suppose only 500 gallons of water are available. The fire won’t even notice being “piddled on” with a booster line or even a 2⅛inch handline. Water certainly will be saved, but the building, just as certainly, will be lost —and buildings arcmore expensive than water.
Attack at 1,000 gpm for 1 5 seconds, though, and you will darken an inferno. You can save a large building with 250 gallons of water and still have 250 gallons left to prevent rekindles whilewater supply is established. That is effective firefighting. Applying “common sense” to fire suppression saves water and prevents water damage.
CRITICAL FLOW RATE
There arcseveral mechanisms by which water suppresses fire. A major mechanism is applying water so it boils, thereby taking away enough heat from the fire to lower the fire compartment temperature below the boiling point of water (212°F), which is below the ignition temperature of most Class A fuels. T his is what causes the immediate darkening of the fire volume being hit by a fire stream.
Every gallon of water effectively applied to a fire takes about 8,000 Btus (British thermal units; one Btu is the amount of heat energy needed to raise the temperature of one pound of water by one Fahrenheit degree) of heat away from it. This means that one gallon per minute takes away about 500,000 Btus of heat from the fire per hour. To put this into perspective, one gpm would take away the heat produced by five gas-fired, househeating furnaces.
Water effectively applied from a 200-gpm handline can remove the heat produced by 1,000 gas-fired, house-heating furnaces all operating full-blast.
When the gpm is high enough to use up heat energy at the same rate at which it is being produced by the fire, there is an equilibrium situation: The fire can’t grow or get hotter. This gpm is called the critical flow rate (CFR).1 If water is applied below the CFR, the fire won’t be darkened.
Consider fire in a fully involved, single-story office or living room that’s eight feet high by 20 feet by 25 feet—an area of 500 square feet. The amount of water needed to darken the fire as a function of flow rate (gpm) is shown in the graph below. The CFR is 160 gpm for that room volume, which means that if less than 160 gpm is applied, the fire cannot be darkened. The fire will barely even notice a conventional 1 ½-inch fire stream. If 160 gpm is applied for many minutes, the fire eventually may be darkened— say in five minutes after using 800 gallons of water. But note what happens if 250 gpm is applied effectively. The fire is knocked down in four seconds flat, using only 17 gallons of water!
Although the graph shows laboratory data, the model applied well to structural fires I have fought under normal ventilation conditions.
A comparable figure to CFR is promulgated by the National Fire Academy in Emmitsburg, Maryland, which recommends the following minimum fire flow for a single-story, fully-involved fire compartment:
Minimum gpm required = [Length x Width (in feet)]/3.
In the above example, the fire compartment length and width are 25 feet by 20 feet, so the minimum fire flow’ needed to darken the fire is 500/ 3=167 gpm, according to the NFA formula.
Note, though, that this is just above the CFR, so the fire will be darkened only after a struggle and after using a lot of w ater needlessly. A short-duration attack with the largest possible handline flow rate—about 300 gpm — is far more effective. It will reduce the probability of losing the building, save water, and minimize water damage. At 300 gpm, from the graph, knockdow n would take just three seconds, using under 15 gallons of water.
A house fire illustrates the difference between the traditional longduration l‘/2-inch attack and the short-duration high-gpm attack. The Wabasca (B.C., Canada) Fire Department, after we upgraded it, was called on mutual aid to a “working” house fire. It started in the back porch and spread to the kitchen through the open doorway. The first-due engine company, from a traditional department, initially attacked from outside the back porch with a so-called “95gpm” l 1/2-inch line, which drove fire throughout the wood-frame house.
Why did the attack fail? During the initial attack, the fire area was about 300 square feet, for which the CFR is about 100 gpm. The engine pressure was probably about 120 psi, so the actual flow was about 80 gpm. The fire, therefore, could not quite be darkened. Air entrained by the fire stream, however, blew fire into the living and dining rooms, which flashed over, increasing the fire area to about 750 square feet, for which the CFR is about 240 gpm.
Then a second 1 ½-inch line was applied from a defensive position through the living room windows. The total actual fire flow of 160 gpm was still below the CFR, so the fire continued to burn and extend.
As the 1,000-gallon tank ran dry, the Wabasca mutual-aid company arrived. ‘Hie nozzleman’s 250-gpm interior attack through the front door knocked down the fire, blowing it out the dining room window and back out the kitchen, one minute after the pumper’s brakes were set. He estimates that he opened his automatic flow-controllable nozzle to about 250 gpm, the maximum flow rate he could safely control. The fire darkened in less than 20 seconds, its predicted by the graph. (The graph is for a 500squarc-foot room. For a 750-squarefoot room, multiply graph gpm by 750/500 or 1.5. So CFR is 160 gpm X 1.5 = 240 gpm, and flow rate for 20second knockdown is 168 gpm X 1.5 = 250 gpm.) He applied about 80 gallons of water. Had he been able to use his full 350 gpm, the fire would have darkened in about four seconds, using only 22 gallons. That is how to save water and save the building.
When a department lacks the attack system and know-how to knock down a fire, it is called a “working fire.” We burn our mistakes.
The rule to remember is this: “The higher the initial gpm, the bigger the fire you can knock down, the faster it will be darkened, and the more water you will save.” Fires involving modern materials need a modern high-gpm attack. It sounds almost biblical: “Those who save their water will lose it (and the building, too).
HEAT-ABSORBING PROPERTIES OF WATER
A fire stream of water works by using up the heat of the fire to boil the applied water. This cools the fire below the fuel’s ignition temperature.
To boil one pound of water at 212°F takes 970 Btus of heat. Since one gallon of water weighs roughly eight pounds, to boil one gallon of water takes 7,760 Btus. This means that a one-gpm fire stream absorbs heat at the rate of 7,760 Btus per minute or 466,000 Btus per hour. This is more than 10 times the heat given off by the flame of a typical 40,000 Btu/hr. domestic gas hot water heater.
References
W.E. Clark. Firefighting Principles & Practices. Second Edition. Fire Engineering, 1991; p. 34.
