The Model Incident Command System Series

The Model Incident Command System Series

STRATEGY AND TACTICS

Water and Flow

This is the second in a series of articles on incident command.

In this issue, the authors will focus on water and fire flow calculations.

BY AND

RESOURCE DETERMINATION: Water

As a review, what are some of the qualities that make water the most commonly employed extinguishing agent? Water is:

• efficient
• relatively inexpensive
• abundant
• easily moved
• readily stored
• relatively inert (non-reactive)

Of all these features, the efficiency of water in absorbing heat makes it our most valuable substance in controlling and extinguishing fire. In order to develop our understanding of the capabilities of water, we must first define the terms specific heat and latent heat of vaporization.

Specific heat. Specific heat or thermal capacity of a substance is: The number of Btu required to raise the temperature of one pound of a substance 1°F, or the number of calories needed to raise one gram of a material 1°C.

Water absorbs more heat when its temperature is increased than most substances and has the highest specific heat value of all common materials. It requires 1 Btu to raise the temperature of water 1°F. When raising the temperature of one pound of water from 70°F to 212°F, 142 Btu of heat are absorbed by the water.

Latent heat of vaporization. Once a liquid, such as water, has been heated to its boiling point, an additional quantity of heat is required to convert it from a liquid to a vapor (gaseous) form. The latent heat for water is accepted as 970 Btu per pound. This means that water will absorb an additional 970 Btu per pound in the process of changing from a boiling liquid to steam. It is apparent that the real efficiency of the heat-absorbing capability of water is as it converts from liquid to steam.

The following calculation shows the amount of heat absorbed by a pound of water that is heated to the boiling point and converted to steam:

Practical conclusions

One gallon of water being raised from 70°F and converted to steam will theoretically absorb the heat from the complete combustion of one pound of wood. At a flow rate of 100 gpm, it is, in theory, possible to absorb 926,300 Btu every minute. However, in actual firefighting, there are factors that reduce the theoretical capability.

Practical considerations

In order to achieve maximum cooling, 100% of the water applied must be converted to steam. Even in an extremely hot fire, not all the water directed onto the fire is converted. In fact, depending on fire conditions, application, and other factors, the conversion range of the water to steam is between 10% and 90%.

The conversion of water to steam and its expansion ratio are important factors. The expansion ratio at 212°F is approximately 1,700 to 1. A gallon of water occupies a space of 231 cubic inches. If the same gallon of water is converted to steam at 212°F with a 100% efficiency rate, it would fill an area of 227 cubic feet with steam.

The expansion ratio is a function of the temperature of the fire area. The following table will illustrate the volume that 50 gallons of water converted to steam at a 90% efficiency rate will fill.

The efficiency rate on the fireground is much lower than 90%. It is probably closer to 50% and the expansion table should be adjusted accordingly.

The value of steam expansion is important; however, fires in ordinary combustibles are normally extinguished by the absorption of the heat (Btu). The cooling effect extinguishes the fire while the smothering effect of the steam will tend to suppress the active flaming. The steam expansion will also force smoke and other products of combustion from the involved area while reducing the amount of oxygen available for combustion.

Having refreshed our understanding of water as an extinguishing agent, we must consider other factors. Every member of the department must be thoroughly familiar with the water supply and the distribution system in the community. Officers should become familiar with the available fire flows in their response area. There will be times when there will be sufficient personnel and apparatus, but a less than adequate water supply.

Many municipalities enjoy satisfactory water supply. Large size mains and well-spaced hydrants supply sufficient water at adequate pressure. Still, many areas of the country are not as fortunate. They must rely on a more limited supply from smaller mains with poorly spaced hydrants. Often, departments are compelled to use ponds, creeks, or tankers to supply firefighting water. It is critical to have this knowledge because it will have an impact on strategic decisions and tactics available to the incident commander.

Whatever the area of the country, maps showing the size and location of hydrants or static water supplies should be carried on all apparatus. The maps could possibly be mounted on the inside of the cab above the officer and protected by a plastic cover. They can be used by initial units enroute to an incident, and later by a water officer (should one be designated by the commanding officer) at an escalating incident. As the need for a greater water supply becomes evident, incoming companies could be directed to the proper locations. Also, large caliber streams will probably be used, and it is a good rule of thumb to have one engine company and sufficient water supply available to supply each large caliber stream.

The’ presence of a hydrant system does not guarantee that an adequate water supply will be available. Many areas are experiencing rapid growth, and, unfortunately, some departments do not learn that their water supplies are insufficient to contain a major fire until after the fact (see “Rapid Strategy Changes Save Furniture Factory,” FIRE ENGINEERING, July 1984). Flow tests must be conducted to determine how much water is available in a specific area to control a major incident. Departments should consider:

• Fringe areas of the community.
• New developments in the outlying sections of the community.
• Expansion of existing industrial complexes.
• Areas with small-sized or deadend mains.

The results of the tests will point out the problem areas. The options and solutions to overcome these problems can then be worked out before the fire-not as one of the strategic factors to be overcome while the building is burning. Determining the available fire flow should be part of every pre-fire plan.

The fire flow information can be used to advise the builder of the need to ensure the availability of an adequate water supply. Several committees have refused to issue building permits due to an inadequate fire flow in the area of the new construction. Understanding the available fire flow and the needed fire flow can avoid serious problems.

With a basic understanding of heat production, heat absorption, and available water supply, the contemporary fire officer is now ready to move on to calculating his district’s needed fire flow.

RESOURCE DETERMINATION: Fire Flow Calculations

Since we use water to extinguish fire, it is essential that we be able to approximate the fire flow requirements for a given structure. We do not, however, have time to work out any complex computations at a fast moving fire.

Cubic foot formula

Many of you are familiar with the cubic foot formula for determining the needed fire flow for a compartmentalized structure. The formula has been validated by years of use and is based on the theory that one gallon of water when converted to steam will inert 200 cubic feet of space. Being practical, and utilizing the accepted 50% efficiency factor, on the fireground we should expect no more than 100 cubic feet of space to be inerted after injection of one gallon of water. The important thing to remember is that these formulas give approximations from which the fire officer can estimate water flow requirements. Once the water is deployed, the incident commander must measure the effect and modifv the operation as necessary.

The cubic foot formula is:

length X width X height _

Example 1

Using the cubic foot formula:

100 X 50 X 10 100

500 gpm

Quick cubic foot formula

Since the objective is to obtain an estimate of fire flow from the formula, consider that all single-story buildings are 10 feet high. This way, we can eliminate estimating the height measurement in the calculation process, round the length and width measurements to the nearest tens or hundreds place and reduce the calculation to a mental process. Using the same building as previously computed, it would work this way:

1. Estimate the length and width to the nearest tens or hundreds place. Length = 100 Width = 50
2. Set aside the zeroes, drop one, and remember how many are left. 000 -*■ 00
3. Multiply the whole numbers together. 1X5 = 5
4. Put the zeroes on this product. 500 gpm

Example 2, Gable roof

Calculate gpm for one floor

1. Estimate the length and width to the nearest tens or hundreds place. Length = 50 Width = 30
2. Set aside the zeroes, drop one. 00 -*• 0
3. Multiply the whole numbers. 3X5= 15
4. Put the zero on this product. 150 gpm

for the first floor

5. Gable roof attic volume is one-half the first floor

150 gpm 2 75gpm

6. Add gpm for first floor and attic to get the total gpm needed for this structure. 150 + 75 = 225 gpm

Although the formula was worked with a building with a 10 foot height, it will be close enough for an 8or 12-foot height. We will discuss accuracy of measurement in more detail when we discuss pre-fire planning. For the present, the modified formula is accepted as reasonably accurate for fireground use. Remember, the purpose is to get an estimate of water needs. Other influences will change the estimate.

Fire flow calculations- modifying factors

There are three other factors that must be taken into account when doing fire flow calculations for specific buildings and occupancies:

• occupancy factor
• exposure charge
• percent of involvement

The formula that encompasses all the modifying factors is:

NFF = [(base flow X involvement%) X occupancy factor] + exposure charge Where:

NFF = the needed fire flow to extinguish the fire and protect exposures.

Base flow = the gpm fire flow determined by the formula for the given fire area (gpm = 1 wh/100), plus any modifications for a gable area. Involvement percent = for partially involved fire areas, multiply the 100% NFF by the percent of floor involvement. This effectively reduces the water required for fire extinguishment and exterior and interior exposure protection.

Occupancy factor = since buildings’ fire loads vary with the occupancy, resulting in different heat productions per square foot of fire area, a factor allowing for different fire loads must be applied (see factor table above). Exposure charge = a percentage of the base gpm modified by the occupancy factor. This percentage surcharge is added to the building extinguishment fire flow to provide water in exposure lines. The following table contains the parameters for determining the exposure charge (individual judgement is required in selecting these factors):

Interior Exposures (for non-fire-re-

sistive construction)

For each floor above the fire floor = 50%

It is usually only necessary to cal-

culate for five exposure floors as a maximum.

Interior Exposures (for fire-resistive

construction)

For each floor above the fire floor = 25%

Exterior Exposures (for each exposed

building)

One-, two-, or three-story exposure = 25%

Fouror more story exposure = 50%

The charge is a total charge for the entire building and not a charge for each floor.

NOTE: The completion of the formula without calculating in the percent of involvement renders the NFF for the structure at 100% involvement.

The entire mathematical operation for the fire flow formula can be reduced to a simple series of multiplications and additions.

An example of a fire flow calculation for a pre-fire planning exercise would go like this: The building is a two-story dress shop of non-fire-resistive construction, having a 100-foot length, 80-foot width, and 10-foot height (for each story). The exterior exposure on the left is two stories; the exterior exposure on the right is four stories. The second floor is an interior exposure. Fire is involving 50% of the first floor. Using the fire flow calculation form, it will take 800 gallons to extinguish the fire. Based on the amount of involvement, the figure is reduced to 50% or 400 gpm for base flow. Four-hundred gallons per minute is needed to provide extension protection to the second floor; 600 gpm is needed to protect the exterior exposures. Even though a number of parking lots are created as a result of serious fires, it is more practical to emphasize that most structural fires are much less than fully involved when fire departments arrive on the scene. It is reasonable to assume that 25% and 50% involvement as being within the ability of a fire department to extinguish before destruction of the building occurs. This is especially true when considering lightweight construction trends of the past 20 years.

By practicing with this formula and its factors, you will quickly become efficient at estimating the fire flow requirements for structures in your jurisdiction. Only after you know what your water flow requirements are can you possibly estimate the number and sizes of lines needed and the personnel required to place them in service.