Fog, Straight, and Solid Streams
These are the extinguishment tools of the trade.
NFPA Journal®, July/August 2008
A discussion among firefighters as to when and how to use various fire streams is sure to evoke a lively debate. Before the late 1800s, the only fire stream available was a solid stream produced by a smooth-bore nozzle. Then variable stream nozzles that allowed the operator to select a straight or fog stream that can be varied from a narrow to a wide angle were introduced in 1863. Many departments developed extinguishment tactics using these streams.
Variable stream nozzles available today are much improved and used by virtually all fire departments, although they have not made the smooth-bore nozzle obsolete. While smooth-bore nozzles lack the versatility of variable stream nozzles, the solid stream maintains better stream continuity, especially during windy conditions, provides greater reach, and penetrates the fire plume better than straight streams produced by variable stream nozzles. Under conditions typically encountered during an interior offensive attack, however, the solid stream and straight stream are equally effective.
When deciding on a nozzle type, it is important to consider tactical objectives. In most cases, the objective of the nozzle team is to extinguish the fire quickly, and understanding the physics of heat absorption is central to achieving this goal. Maximum heat absorption is achieved when water is converted to steam. Yet a misunderstanding of the water-to-steam phenomenon has resulted in a commonly held fallacy that cooling the atmosphere is as effective as cooling the fuel. This fallacy led to a preference for indirect application, where high-pressure fog streams are operated into unventilated enclosures.
From a tactical perspective, there are two major problems with using this indirect attack during offensive operations. First, indirect application disrupts the natural heat layering effect in fire compartments where ceiling temperatures are extremely high and floor-level temperatures are much lower and more likely to be survivable. With indirect application, the entire compartment will be nearly the same temperature, usually in a range that is not survivable. Adding to these lethal conditions is the high humidity caused by the steam. Second, cooling the atmosphere is not as effective as cooling the fuel because the entire atmosphere must be below the ignition temperature of the burning fuel.
In most structural firefighting applications, applying water directly to the burning fuel is the most effective tactic. Water cools the fuel below the temperature necessary for pyrolysis, halting the production of the vapor fuel necessary for continued combustion. Applying water at or above the required rate of flow, combined with proper ventilation, makes the building much safer for occupants and firefighters.
Fog streams are most effective for fighting fires in closed, unoccupied compartments. Fog streams also have a tendency to “push” the fire into previously uninvolved areas. For these reasons, they have limited use in offensive structural firefighting. However, the fog attack may be preferred when fighting a fire in an unoccupied, closed compartment that cannot be safely entered. Further, the “pushing” effect of the fog stream can be used to push the fire away from occupied areas of a building or to create a barrier between the fire compartment and uninvolved areas. However, these scenarios are the exceptions rather than the rules of offensive firefighting.
The fog stream has greater application in defensive operations, where it can be effectively used to push the fire away from victims threatened by the fire or from threatened exposures.
A narrow-angle fog stream is preferred when wetting exposure walls facing the fire building because a solid or straight stream can damage exposed buildings and break windows. Broken windows provide an additional pathway for the fire to spread to exposures by way of convection heat and flying brands.
The reasons for choosing the type of fire stream and how that choice is used is critically important knowledge for firefighters. Their decision should be based on an understanding of the advantages and disadvantages of each type of stream and when each should be used.
This column is adapted from the authors’ book Stuctural Fire Fighting, available at www.nfpa.org or (800) 344-3555.