Using Water as an Extinguishing Agent
Part 3 – Special Circumstances & Overhaul
PART 2 of this series in the November issue set forth a method of determining the required rate of nozzle flow for a given area and stressed the importance of careful use of the heat present for steam production and ventilation. It also defined the indirect and combination methods of attack and gave some of the factors which an officer or nozzleman should consider in choosing between them.
In an article in FIRE ENGINEERING, August 1959, it was pointed out that the direction of rotation of the fire stream should be clockwise when viewed from the base of the nozzle. This rotation helps to push flame, smoke, and steam away from the nozzleman when this is a problem. A counterclockwise rotation will not have the same effect and in many cases seems to draw flame, smoke and steam toward the nozzleman. To date, this effect has been observed many times on the fire ground. It has not been the subject of sufficient research to specify the exact scientific cause for the observed results but it is surmised that the effect is created by electrical forces.
It is known that in flame production areas there are free electrons and positive and negative ions. Since there are many fire fighters who are not satisfied to know that something happens, but want to know “why,” this effect of clockwise rotation needs to be studied. In many cases, determining exactly what causes certain phenomena leads to fuller utilization of the natural laws in force by the service involved.
Choosing the size of lines
No mention was made in the previous article of factors influencing the size of line to be used in distributing the required rate of flow. No hard and fast rules can be laid down for making this choice under varying conditions. The flow needs to be at least that required by the formula but it does not need to be exact. For example, a required flow of 50 gpm could be supplied by a 100-gpm nozzle. The knockdown and point of shut-off will simply be quicker.
In another case, we may have a flow which can be supplied by either a 2 1/2-inch line or two l 1/2-inch lines.
In making this choice several factors have to be considered such as the available openings—doors, windows, etc.—the size of and accessibility of these openings, the shape of the fire area in relation to these openings and the wind direction. The aim of the attack is to get the required flow distributed as evenly as possible over the entire area. It does little good to know that a line will control a given fire if the wind direction is wrong and the line cannot be advanced to the vantage point. It is just as futile to try to control a given fire requiring the flow that can be furnished by a 2 1/2-inch line if the line has to be operated at a distance from a ladder. Where the vantage point is such that the 2 1/2-inch line can be worked, it has about twice the capability of the 1 1/2-inch, especially where the shape of the area is such that the extra reach of the 2 1/2-inch line is needed.
Knowing the required rate of flow is helpful in many fire situations. However, this knowledge will be of little value if the department has not prepared to overcome the many difficulties it will encounter in distributing the water. Buildings with no windows or available openings, or fires above the second floor present special problems in distribution of the required rate of flow. An officer who knows the flow required is still behind the eight ball if he does not have the manpower, equipment or water supplies in the area to meet that requirement.
There are many definitions for the word “overhaul” as used in the fire service. It is often hard to tell where the knockdown or control phase ends and overhaul begins. In many cases, the changeover from attack to overhaul is a feeling of intuition on the part of the fire fighter rather than a point that can be exactly determined. For the purpose of this article, overhaul will be considered as the fire fighting that takes place on the inside of the fire area after the main body of fire has been controlled. In some cases, overhaul may involve extinguishment of pockets of fire involving entire rooms within the structure.
In many cases considerable fire or water damage takes place after a fire has been controlled because of the inability of the department to quickly carry out the overhaul function. Fires have burned in false ceilings, through partitions, or up walls into new fuel areas and gotten completely out of control before the overhaul could be? completed.
The use of masks
In many cases, fires that get out of hand during the overhaul phase could have been controlled if the department had been equipped with a sufficient number of self-contained masks. However, the mere fact that a department is equipped with masks will not assure efficient overhaid. Masks provide a means for the fire fighter to breath in a smoke-filled building but they do not assure that the man will be able to see what he must do if overhaul is to be efficient. Neither do masks give full protection against heat and flames driven toward a fire fighter by other crews operating in the building.
The time to start the planning for efficient overhaul is during pre-fire drill sessions, but unless careful thought is given to overhaul during the knockdown, conditions will be created which will make this function difficult, if not impossible.
Nozzlemen making die knockdown should strive for the ideal. If, at the conclusion of the knockdown, the fire area is left with an even ceiling temperature of 300°F., conditions will be ideal for natural ventilation and easy and efficient overhaul.
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Continued from fMHU’ 1119
The lifting forces of the warm air (thermals) will be in balance throughout the area and we can say that we have left the area with the same thermal balance that was developed as the fire built up at a somewhat lower temperature. This will permit overhaul crews to move in rapidly as the fresh air enters the building at the lower levels and the remaining steam and smoke escape from upper levels.
If, on the other hand, water distribution has been poor, we may have part of the area cooled to ambient temperature and other parts left with temperatures of 500°F. to 1000°F. The upward thermal forces in the hot areas will push steam and smoke out and downward in the cool areas and cause a circulation in the fire area which will limit visibility and hamper overhaul. The cool air will be heavily laden with carbon particles (smoke).
The greater the heat differential, the more violent this circulation will be. If cooling is continued on the perimeter, using a fog pattern too wide or a stream with insufficient reach, the problem is compounded and the heat in the hot spot will become more intense. This condition will keep the fire fighter from entering and may allow the fire to burn through to upper levels and get out of control. In a single story structure, burning through of the roof over the hot spot will relieve the situation.
When the area is quite large and several nozzle crews and more than one hot spot is involved, the interaction between the various thermal forces pose additional problems for the fire fighters.
If, on the other hand, rate of flows have been excessive or the right flow has been continued for too long a period, the entire fire area may be cooled to ambient temperature. In this case, as spot fires continue to give off smoke, we find a heavy, muggy, smoke-laden atmosphere which is very difficult to remove efficiently with any amount of forced ventilation. Overhaul will have to be carried out with limiter! visibility and gas masks will be an absolute necessity for entry.
A condition of thermal imbalance will be observed first by the nozzleman who has made the knockdown attack. If, for example, his pattern adjustment on attack has been too wide, immediately on shutdown he will observe a downward movement of steam and smoke in the opening through which he has made the attack. This can often be corrected by making another short attack with a narrowed fog pattern for longer reach unless a partition or stock is blocking his application into the hotter area. If the nozzleman at this point understands the effect of thermal forces, observation of smoke and steam circulation will give him a good indication of where his hot spots are, even when they may be off to one side.
Holding thermal balance during overhaul following knockdown
When a good thermal balance is established during knockdown, then it is up to the fire fighters who ventilate and overhaul to maintain that balance so that a rapid and complete overhaul can be carried out. Three main factors can operate to upset thermal balance and the orderly flow of clean clear air into a building during overhaul.
- Doors or windows broken or opened on the windward side of a building with a strong wind blowing or forced ventilation used in the wrong place can set up strong air currents that will hamper the orderly ventilation of the area being overhauled.
- Spectators or firemen may block openings where streams of fresh air are flowing into the building.
- Nozzlemen are the ones most often guilty of upsetting the thermal balance and hampering their own efforts or those of other overhaul crews operating in the building. Using a fog pattern during overhaul on the ground floor for example may upset the air supply and visibility for overhaul crews on the second and third floor. During overhaul all nozzles should be operated with relatively short bursts and on a quite narrow or straight-stream pattern.
- If master streams have been used to protect exposures or clear the way for attack at certain openings, these streams must be shut down or very carefully used during overhaul so that they do not hamper or, in some cases, actually prevent the effective overhaul of the fire. A heavy master stream used over a fire can in some cases so cool the products of combustion that they drop to the ground to flow back into the building and cut off visibility for the overhaul crews. A heavy stream operated from an aerial ladder downward through a roof opening may keep all overhaul crews out of a building.
Overhauling where large heat pockets exist
In some cases knockdown of a fire may leave some rooms in the structure still fully involved. We may also find on initial response that an interior room is fully involved and just beginning to push heat and products of combustion into the area through which the fire fighter must approach. In these cases a good understanding of the principles of thermal balance can aid the nozzleman in extinguishing the fire quickly and with a minimum of physical punishment.
The reaction of some nozzlemen in these cases is to use a relatively wide fog pattern on entering the structure. This action will cool the overhead and cause steam and products of combustion to drop to the floor thus cutting visibility and approach to the actual fire. If, on the other hand, a nozzleman crawls along the floor, perhaps he can reach the doorway of the fire room where proper application to the main body of the fire can be made. In cases where both rooms are involved, the fire in room one must be knocked down to the overhaul point before a crawling approach is made to the second room.
Application of principles
If all buildings were alike and the conditions which influence fire behavior were always the same, the fire fighter might be able to do a creditable job without an understanding of the factors which influence fire behavior. Since this is not the case and fires in the same building will behave different on different days, the fire fighter must have an understanding of the natural laws which influence fire behavior. The fire fighter who understands not only what, but why something happened the way it did in a previous fire, is in a position to apply basic principles and his skill to the job he finds at hand.