Using Water as an Extinguishing Agent

Using Water as an Extinguishing Agent

Part 3 – Special Circumstances & Overhaul

What size line? If this area were 80 by 20 by 10, the ideal rate of flow would be 160 gpm. Thus could be provided toith two 1 1/2-inch lines or one 2 1/2-inch line. If openings at B and C were usable, the two 1 1/2-inch lines would give the best distribution. If the only us able opening was at A, the l 1/2-inch lines would not have sufficient reach and a 2 1/2-inch line would be needed. Wind direction might make either one of these two choices the proper one on a given dayThermal balance—cross section end view of fire area. During buildup a fire heats the ceiling areas of a structure evenly and establishes a thermal balance with smoke escaping from the top part of openings and fresh air entering at the lower levels. By careful distribution of water this balance can be maintained during knockdownThermal imbalance. If water distribution is poor during knockdown, the thermal balance will be upset. Hot air goes up, cool air goes down, the greater the heat differential, the nwre violent the rolling action will be

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.

Clockwise rotation

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.

Overhaul

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.

Thermal balance

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.

To get to this fire for effective application of water, the fire fighter must approach through Room I where the products of combustion are traveling across the ceiling and spilling to the outsideIf a wide fog pattern is used in Room 1, the thermal balance will be upset and a wall of steam and smoke will drop in front of the fire fighterIf the fire fighter approaches with a narrowed fog pattern directing his water into Room 2, he can get into position for effective use of his stream and at the same time maintain a flow of fresh air behind him for visibility

Continued on page 1147

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.

Thermal imbalance

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.

Total cooling

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.

Correcting distribution

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.

  1. 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.
  2. Spectators or firemen may block openings where streams of fresh air are flowing into the building.
  3. 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.
  4. 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.

Using Water as an Extinguishing Agent

20

Using Water as an Extinguishing Agent

Part 2 – Utilizing Heat

PART I of this series, in the September issue dealt with some of the characteristics of heat and its production in fires. One of the main premises advanced in Part I was that in a 6-foot-by-6-foot-by-6-foot enclosure (200 cubic ft.) there would be a relatively constant amount of heat, 7,400 Btu present in the enclosure at any given moment during the early stages of a fire. The article also pointed out how the heat of vaporization of water, 971 Btu per pound works to carry the heat of combustion throughout the structure and to rapidly level off the temperatures in a given confined area.

Of major interest to both the officer and the nozzleman in fire situations is the amount of water needed to control a given fire. The officer is concerned with the total number of lines that must be laid and supplied with water, while the nozzleman is concerned with the size of line or rate of flow needed to control the fire in the area that has been assigned to him.

Heat absorption

This article is concerned with the rate of flow necessary for the nozzleman or nozzlemen in approaching a given confined area. To aid in determining the necessary rate of flow we can review the amount of heat a gallon of water is capable of absorbing.

The figures in the following table and some others in these articles are approximate but sufficiently accurate for the fire fighter.

Heat to raise 1 pound of water 1°F = 1 Btu

Heat to vaporize 1 pound of water at 212°F=971 Btu

Heat to vaporize 1 gallon of water at 212° F 8,080 Btu

From the table above it can be seen that 1 gallon of water (8.336 pounds) at 62°F, applied to a fire and changed to steam, will absorb 9,330 Btu of heat.

To apply water to a fire so that 100 per cent of it is converted to steam would require very skillful application. If, however, we take 80 percent of 9,330 we find that 1 gallon of water will still absorb 7,460 Btu of heat when 20 per cent of it is wasted. We assume this figure as reasonable efficiency.

Referring back now to our 200-cubic-foot model room, we find that 1 gallon of water will absorb the heat present in 200 cubic feet of fire area at 80 per cent efficiency.

From the above we can arrive at a suggested formula for the required amount of water in a given confined area.

cubic feet involved / 200 = required gallons.

If there was no air flow into the model area and therfore no continuing heat production we could spend perhaps 5 to 10 minutes getting the gallon of water into the 200 cubic feet area and still absorb all of the heat. In practical situations, however, there is usually some air flow into the fire area and heat production during application becomes a factor.

Analysis of fire test data (See FIRE ENGINEERING, August 1959) has shown that results are best when the required amount of water is distributed in the fire area in 30 seconds time. Therefore the rate of flow in gpm will be twice the required gallonage or:

cubic feet involved /100 = gpm flow

Action of steam

In addition to having the ability to absorb the heat produced in 200 cubic feet of space, 1 gallon of water converted to steam will occupy 200 cubic feet of space at 212°F to inert the atmosphere in that space. When temperatures are above 212° steam superheats and expands to occupy more space. At atmospheric pressure (14.7 psi) and 1000°F, the steam from 1 gallon of water will occupy 400 cubic feet.

The following table gives the approximate volume which steam from 1 gallon of water will occupy at different temperatures under normal atmospheric pressure:

The additional space occupied by steam under high heat conditions is a very important factor in blacking out a fire quickly when the rate of water flow and distribution are proper.

The formula cubic feet / 100 = gpm when applied to fires in small structures seems ridiculously small and would in most cases call for only 1-inch lines in attacking a residence fire. However, when applied to even modest-size supermarkets or churches it will be found that the required flows inmany cases exceed the available water supplies or the fire departments ability to pump and distribute the water. This is evidenced by the continued loss of such structures when fully involved in fire.

Required water flow

For example a 100-foot by 100-foot by 10-foot open area would require a flow of 1,000 gpm properly distributed to control a fire when the area was fully involved. The same area with a partition down the center could be controlled with a 500 gpm flow by making application fust in one area, then in the other.

It should be stressed here that this is the flow required for the actual attack on the fire area to achieve control and in many cases does not constitute the entire water requirements for a given fire. Sound strategy will lead the officer to have additional lines to cover exposures and back up the lines actually working on the fire.

In a fire of any consequence stretching in additional lines to furnish master streams will be carried out while the attack is being made on the area of major involvement. Many factors, such as: Height, lack of openings, smoke, or unsafe structure may prevent close approach to an involved area for proper distribution of water and master streams will be needed to hold the situation.

If conditions are such that streams providing necessary rate of flow can be advanced to the involved area, skillful handling of the lines will control the fire, and make overhauling relatively easy. Inefficient handling of the required flow, or using flows greatly in excess of the required gallonage can allow the fire to get out of hand, or at best, make overhaul difficult.

Where large areas require several lines for the required flow the coordination of these lines present a problem. Unless they are very carefully coordinated, results will be less than ideal, because results are best when the ideal flow is introduced into an area, distributed throughout it, and cut off when the fire blacks out, usually in about 30 seconds. This leaves the involved area full of steam to hold fires in remote areas in check, while overhaul is carried out. If flows are continued past the blackout point, steam in the area will be condensed, allowing air to flow in to feed flames that were shielded from the water in the initial attack. This may not be too critical if only one floor is involved, but if there are areas overhead such as false ceilings or other stories above the one where application has been made, fresh air instead of steam flowing into such areas can lead to the loss of the structure.

Heat useful for control

Firemen approaching a fully involved area, where a large amount of heat is accumulated and being produced, should regard the heat present as a most valuable weapon for controlling the fire. Skillful application of water can produce large volumes of steam to flow upward through a structure and control a fire in a remote area.

There are usually heated areas above a fire that cannot easily be reached with water streams If a flow of steam can be continued into these areas, to suspend flame production, natural loss of heat will lower the temperatures in those areas. Even if some water can be applied in these remote areas, steam flowing into these areas can be helpful.

It is not possible in all cases to say which is most important—the cooling action of the water or the smothering action of the steam. The two effects are complementary, that is, either may accomplish extinguishment results impossible for the other. The smothering effect of steam is especially important in remote areas.

The wasting of heat by overapplication of water can make overhaul of a fire very difficult. The natural ventilation offered by ceiling temperatures between 212 and 300° cannot be replaced by forced ventilation. Smoke-laden air excessively cooled by overapplication of water often refuses to leave the structure under any amount of forced ventilation. In some cases it may be forced out of windows on an upper story and cascade down the side of the building to again enter the building at a lower level.

While close coordination of all lines operating on a given situation is ideal, it is not easily accomplished on the fire ground. In an actual situation some lines may be stretched in and ready to go several minutes before enough lines are ready to furnish the required flows. To have these early lines stand by and wait would not be practical, yet if they make a partial attack and cool one end of the involved area prematurely, over-all results will be questionable.

However, if early lines are operated carefully they can carry out an effective holding operation until a sufficient number of lines are in position for a killing attack. This holding operation is possible by having the early lines make a careful indirect attack as defined in the following explanation.

Methods of attack

  1. Direct method, applying the water directly to the materials involved in fire.
  2. Indirect method, applying water to the heated atmosphere, striking as little of the fuel or the walls of the structure involved, as possible.
  3. Combination method, rolling the hose stream around the perimiter of the area striking the walls of the structure and the fuel with the outside edge of the stream and letting the inner part of the spray stream cool the hot gases.

The direct method of attack is usually employed where heat accumulation has not yet become critical or the arrangement of the fuel is such that all that is involved in fire can be readily hit from the point of stream application.

Continued on page 1083

WATER—EXTINGUISHING AGENT

Continued from page 1037

The indirect method involves using spray streams to cool the heated overhead. The spray stream cools the hot gases and steam is produced. This steam has some smothering effect in the base fire area but it is of most benefit because of the smothering effect in remote areas where water cannot be readily or immediately applied by fire fighters.

If the volume of water is sufficient to cool the fire gases, this will be accomplished in 15 to 20 seconds. If the volume of the spray is excessive, this is not too critical as long as the stream does not hit the walls or ceiling of the structure. With the gases cooled, the spray will merely fall to the floor.

With the gases cooled, heat will radiate from the hot walls and ceiling or heat-production areas to the cooled gases in the vicinity of the spray stream and steam production will take place at the perimeter of the stream to continue the flow of steam into the remote areas. In this manner it may be possible to push steam into the remote areas for four to five minutes.

When an indirect attack is being used as a holding operation, nozzlemen should be particularly careful not to strike the heated walls or fuel involved, until all lines are in position. Then all lines can be coordinated in making a combination attack afterwards shutting down and going in for overhaul if the structure is safe.

When a successful knockdown has been accomplished and sufficient heat left in the structure to provide natural ventilation, the building must be entered Immediately and all overhaul lines operated carefully if the benefits of natural ventilation are to be preserved while overhaul is carried out. The maintenance of thermal balance in an area during overhaul and certain other types of attack will be the subject of the concluding article in this series.