Update of the Fire Flow Formula

Update of the Fire Flow Formula

STRATEGY AND TACTICS

The Model Incident Command System Series:

You pull up to a two-story taxpayer with stores on the first floor and apartments on the second. Fire is showing in one of the middle storefronts. In addition to the attack lines, you’ll need protection lines to cover at least the three adjacent primary exposures (the two stores on either side and the apartment directly above the fire store).

The firefighters are already pulling hose and getting into position.

You have to start water. How many gallons per minute should you be prepared to deliver on the attack line(s)? On the exposure streams? Remember the formula:

Needed Fire Flow (gpm) = {(LWH / 100 X Occupancy Factor) + Exposure Charge) X % Involvement

The questions start racing through your mind:

• What are the dimensions of the building? Thirty feet wide, 100 feet long, 20 feet high. That’s 60,000. Divide by 100, knock off two zeros; that’s 600.
• Add this to the exposure charge, which is what? Three exposures…. The occupancy factor! Multiply 600 by the occupancy factor!
• The percent of involvement seems to be increasing steadily!
• Back to the exposure charge. How is that figured again? Okay, okay, take it easy. Think.

When the pressure on the fireground builds, so do the chances of error.

For the first-arriving officer to properly develop his strategic goals and tactical objectives, he must know the quantity of water that will be required to contain the fire. And to determine the quantity of water, he must have a method of calculating this from selected building variables.

In this thirteenth article in the series on the National Fire Academy’s (NFA) model incident command system, we will discuss a new fire flow formula that was derived by the NFA’s course development team from a study of articles describing fire department actions taken at numerous working fires.

Using descriptions and drawings of the fire buildings in combination with line size and placement, an approximate gallon per minute flow rate was determined that is believed to more closely approximate actual fireground flow rates for free burning structural fires. By working with the formulas for volume (V = Length X Width X Height) and area (A = Length X Width), it was found that the actual fire flow that was applied to real incidents most closely approximated:

Needed Fire Flow (GPM) = x

Width/3

This calculation, which gives about a 31/3 times higher flow than that derived from the previous NFA formula, provides the fire flow required for 100% involvement of one story. It also considerably reduces the amount of calculations needed to determine the gallons per minute necessary to control a fire incident, as the officer simply estimates the building length and width. The numbers should be multiples of ten in order to make multiplication simple. When dividing by the constant of three, round quickly to the nearest hundred, e.g., 2,000 / 3 = 700. Do not waste precious time at an incident scene trying to solve to the closest tenth of a gallon.

Now, let’s go back to that taxpayer.

INCIDENT SCENE USE OF THE FORMULA

Using the new fire flow formula, let’s estimate the gallons per minute needed in a logical, step-bystep method that will allow the person under stress conditions to do as few calculations as possible.

1. Estimate the length and width of the structure (to the nearest multiple of 10). Again, the building length is 100 feet and its width is 30 feet.
2. Multiply the length times the width and divide by three. (Round the answer to the nearest 100.)

GPM = LXW/3 GPM = 100 x 30/3 GPM = 3.000/3

GPM = 1,000 for total floor involvement

3. 3. If there is less than 100% fire involvement on one floor, reduce the fire flow proportionally. About 25% of the store is involved, which would call for a fire flow of 250 gpm.
4. If more than one floor is well involved, increase the fire flow accordingly. Let’s say that the fire extends to Exposure 4, which has the same dimensions as the original fire store, and involves 50% of this building. That would mean an additional 500 gpm are needed for attack lines to this store.

Now, about the exposure streams. The gallons per minute required for both interior and exterior protection lines is estimated by taking 25% of the 100% involvement figure for each immediate exposure. That means that to protect each of the three adjacent primary exposures, we’ll need to flow 250 gpm (.25 X 1,000 gpm of total involvement of fire store = 250 gpm).

FORMULA FOR PRE-FIRE PLANNING

When unhampered by the urgency of the fireground, there is considerably more time to do additional calculations. We can expand on the basic fire flow formula by calculating the water required for exposure protection and the percent of involvement. The amended formula for the needed fire flow is:

GPM = (LengthXWith/3 + Exposure Charge) X % Involvement

Let’s take another example. A four-story 50 X 30-foot storage building.

1. 1.After measuring the structure being preplanned, round the length and width to the nearest 10.
2. Multiply the length times the width and divide by three. (Round the answer to the nearest 100.)

GPM = LXW/3 GPM = 50 X 30/3 GPM = 1,500/3

GPM = 500 gpm

3. Exposure charge:

For interior exposures, add 25% of the answer from Step 2 for each floor above the fire floor, not to exceed five floors. If we had a fire on the second floor of the storage building, we would add 125 gpm for the thirdand 125 gpm for the fourth-floor exposures, making a total of 250 gpm needed for interior exposure protection.

For exterior exposures, add 25% of the 100% involvement figure for each side of the fire building with an exposure facing it. If there was one exterior exposure, we would flow 125 gpm for protection.

Our total gallons per minute for exposure protection is 375 gpm.

4. to determine the full fire flow needed to extinguish the fire and to protect exposures, estimate the percent of involvement of the fire floor and multiply the total calculations from Steps 2 and 3 by the decimal value.

GPM = (500 for total involvement + 375 for total exposure

protection) X 50% involvement GPM = 875 X .50 GPM = 437 gpm

SUMMARY

The 100% involvement figure is purely theoretical, as are most of the percentage values exceeding 50% involvement. The formula indicates approximate fire flows required for an aggressive interior attack on small and moderate structures, or on large structures with limited involvement.

An interior attack is normally not made on structures with high involvement percentages. Exterior, defensive/offensive strategies require larger fire flows that are not addressed with any real accuracy with this formula.