How water darkens fire and how to use it most effectively
Fire suppression with water works by boiling enough of it to cool the fuel below its ignition temperature. Each litre of effectively applied water boiled by the fire takes approximately 2600 KJ of heat energy from the fire. If boiling the applied water takes all of the heat energy from the area instantaneously covered by the nozzle footprint, the fire within it goes out in a small fraction of a second. The flow rate to achieve that is the Critical Flow Rate (CFR) in litres per minute.
Of course unless proper procedure is used, the moment the nozzle is moved, fire behind it will boil off residual water, dry the fuel then rekindle the footprint area just darkened, wasting the water applied. To prevent immediate rekindling suppression of a wildfire line, which can be over 50′ from the “green” (the unburned area) into the “black” (the burned area), must begin at a secure anchor line (a safe incombustible area such as a road), then paint out the base of the burning fuel from the green back until only sparse heavy fuels are burning, then back to the green. The initial attack hose must be quickly followed by another to quell rekindles. Sprinklers should then be immediately set to cool hotspots that threaten unburned fuel in the green.
The CFR concept, known for over 40 years, but rarely emphasized during firefighter training, is almost always forgotten in practice, leading to needlessly horrendous fire losses.
Rapid rekindling, and how to prevent it, doesn’t seem to be mentioned so it’s usual for firestreams to be almost randomly directed into fire then moved away allowing it to rekindle. This has negligible impact on a fire and so wastes scarce water.
In practice, if water is applied effectively at 50% above the CFR, all fire reachable by the nozzle is darkened in a few seconds and rekindling is delayed. This is the Optimum Flow Rate (OFR). Table 1 shows the Optimum Flow Rate in litres per minute required to darken fires at the maximum of each Head Fire Intensity Class.
For those who haven’t been conditioned to suspend common sense in favour of following tradition, the CFR concept is intuitive. Ontario MNR, which has the advantage of many lakes and lack of mountainous terrain, has been routinely controlling wildfires up to Class 5 above the OFR with Mark 3 pumps and 1½” hose for many decades.
Often water is just randomly squirted onto a fire. If applied at, or above, the OFR, the fire will darken instantaneously but, as soon as the fire stream is moved away, the fire will be immediately rekindled by adjacent fire. To be effective, application of water must begin from a safe anchor point, methodically painting out fire along the flanks, preventing rekindles, to pinch off the head fire. The lead attack hose must be followed by other attack hoses to paint out the inevitable rekindles before they grow significantly. Sprinklers should then be set to suppress hot spots.
Just as water applied under the OFR wastes it without darkening the fire, applying water above the OFR uses more water than necessary for knockdown, but with the advantage that it darkens the fire and reduces rekindles. For decades, Ontario MNR IA crews have attacked with about 200 Lpm which is well above the OFR for Class 3 and many Class 4 fires thereby controlling them with one pass. Selectable flow nozzles permit setting the flow rate near optimum but if the flow rate can’t be decreased, sweeping the nozzle more quickly than usual will save water.
Because municipal firefighters have been arguing about the merits of a Smooth Bore nozzle stream vs a Straight Stream from a “fog nozzle” for over a hundred years without conducting any credible measurements, I set out, as a public service, to provide credible quantitative answers by conducting 50 reproducible 8 x 8 x 2’ crib burns based on the UL711 standard. This was funded by the Canadian Forces and the National Research Council, with generous in-kind contributions from many companies. Along with Sam Duncan of the US Army, Dan Madrzykowski of the National Institute of Science and Technology, and Wuppertal University, I was invited to present my results at the Fire Department Instructors Conference in Indianapolis ca 2000. Attempts by the others were inconclusive because human nozzle operators can’t apply water in the identical way each time. I overcame this obstacle by having built a robotic nozzle operator that attacked each fire in the same way. This permitted consistent quantitative determination of CFR for;
- A Straight Stream of plain water from a “fog nozzle” at 50 psi vs 100 psi nozzle pressure (50 psi is more effective)
- Smooth Bore vs Straight Stream at 50 psi nozzle pressure (essentially the same)
- A Straight Stream vs spray patterns of various diameters where it hit the fuel (12” diameter is most effective)
- Wet water vs plain water (wet water is much better)
- Aspirated Low Expansion Foam
- Compressed Air Foam at various water to air ratios
The optimum fire stream is a low-pressure spray of wet water, 12-inches in diameter where it hits the fuel. A Straight Stream darkens too small an area to permit rapid and uniform application of water to all of the burning fuel. Without sacrificing nozzle reach, the 12-inch diameter spray permits painting the area out quickly, like painting walls with a wide paint roller instead of with a narrow paint brush. Low nozzle pressure creates droplets that are large enough to reach the fuel by passing through flame without evaporating. The optimum nozzle pressure is the lowest possible pressure that permits adequate reach.
As per Table 1, the ubiquitous “Econoflo” flat garden hose and nozzle can only darken Class 2 surface fires, while 1½-inch hose with a Hansen nozzle can darken Class 3 fires. More intense wildfires cannot be darkened with the conventional Mark 3 pump and 1½-inch mainline hose, so the water applied is essentially wasted.
Wildfires at which conventional pump-and-hose attack fails, and fires that jump hand-guards or dozer-guards can “run”, slowed only by aerial attack, until they run out of fuel, extreme fire weather ends, or it rains at least 12 mm of rain in 24 hours. Fires that “run” can become enormous “project fires” which can take weeks to burn down to a level that can be controlled. Some project fires – called “holdover fires” last over the winter.
Class 3 fires defeat a Hansen nozzle if suppression requires over 1600 feet of hose ending 400 vertical feet above a Mark 3 pump (See Table 2 below), since there is not enough pressurized water to advance along the flanks, while still preventing rekindles along the fireline already darkened.
In BC, where water sources are often scarce, saving water is a priority. This is often attempted by using Econo garden hoses instead of Hansen nozzles. Paradoxically, the way to really save water is to apply it methodically at a high flow rate that is at or above the OFR in brief bursts, since that darkens fire reachable by the nozzle in a few seconds. This water-saving ability can be easily demonstrated on any Class 4 wildfire by comparing the water consumption of Econoflo hose with that of a 1½-inch attack hose feeding a plastic “fog nozzle”. The “fog nozzle” actually darkens fire in seconds which saves water, whereas applying water with Econoflo is futile and wastes the water applied.
Explain rekindle prevention