How is high-expansion foam total flooding calculated?
In a total flooding system, foam generators fill an enclosed volume until the foam rises above the protected hazard, and that level is held until the fire is under control. NFPA 11-2024 splits the design in 7.12 into three questions: how deep the foam must be, how quickly that depth must be reached, and how much foam the generators must produce per minute once losses are counted. The tool answers them in turn, then gives the number of generators and the quantity of concentrate and water.
1. Submergence depth and volume
The total foam depth must be at least 1.1 times the height of the highest hazard and at least 24 in. (600 mm) above it (7.12.5.2.1.1). Both conditions apply together: the 600 mm margin governs for stacks up to about 6 m, the 1.1 factor above that. For ignitible liquids the depth may not be less than the depth found by tests that duplicate the anticipated fire (7.12.5.2.1.2); the tool has a separate input for it.
In a sprinklered area the submergence volume is the depth times the floor area (7.12.6.1). In an unsprinklered room of combustible construction or finish the entire volume, including concealed spaces, is used (7.12.6.2). Vessels, machinery and permanent equipment may be deducted (7.12.6.3); stored material may not (7.12.6.4).
2. Submergence time: Table 7.12.7.1
The table gives the maximum time from the start of foam discharge to reach the submergence volume. Rows are the hazard (ignitible liquids, low- and high-density combustibles, rubber tyres, combustibles in cartons); columns are the two NFPA 220 construction groups and whether the area is sprinklered. Times run from 2 to 8 minutes. The table assumes no more than 30 seconds from detection to discharge; any excess is deducted (7.12.7.3, 7.12.7.4). For the rows marked with a dagger, the times might not apply to storage piled above 15 ft (4.6 m); the tool flags this.
3. Foam discharge rate: 7.12.8.2.3
- RS = S × Q: foam breakdown by sprinkler water. Q is the total flow of the maximum number of sprinklers expected to operate; without test data S = 10 ft³/min per gpm (0.0748 m³/min per L/min).
- CN = 1.15: normal foam shrinkage from drainage, fire, wetting of surfaces and absorbency of stock.
- CL: loss around doors and windows and through unclosable openings, set by the designer. Never below 1.0, even for a tight structure, and up to 1.2 for a building with all openings normally closed.
Under the formula the standard prints the unit of V as “ft³/min (m³/min)”; V is a volume (ft³, m³), and the tool treats it as such. The reference in 7.12.6.1 to the medium-expansion depth clause (7.12.5.2.2.2) is also a misprint; for high expansion the depth comes from 7.12.5.2.1.1.
4. Generators, concentrate and water
The number of generators is R divided by the capacity of one generator, rounded up. Concentrate and water must allow the entire system to run for 25 minutes or to generate four times the submergence volume, whichever is less, but never less than 15 minutes of full operation (7.12.9.1). The tool works out that time from the installed capacity (number × generator capacity). Solution flow is the capacity divided by the expansion ratio; expansion ratio, generator capacity and proportioning rate are product values taken from the listing. A reserve supply is also required (7.12.9.3).
5. Holding the level and the enclosure
The submergence volume is maintained for at least 60 minutes in unsprinklered and 30 minutes in sprinklered locations (7.12.10.1). Openings below the filling depth close automatically with the discharge (7.12.4.1.1). Where outside air is used, high-level venting is needed for the displaced air at no more than 1000 ft/min (305 m/min) (7.12.4.1.2); taking the displaced air as equal to the foam volume, the tool shows the minimum vent area.
Example
A 40 m × 25 m sprinklered warehouse with unprotected structural members; combustibles in cartons stacked 4 m high; 4,320 L/min sprinkler flow; CL = 1.1. Depth 4.6 m, volume 4,600 m³, time 7 min; RS = 323.1 m³/min and R = (657.1 + 323.1) × 1.15 × 1.1 ≈ 1,240 m³/min. With 4V/R ≈ 14.8 min the 15-minute minimum applies; at 500:1 and 3% concentrate that is about 1,116 L of concentrate plus the reserve. The same example is worked step by step in NFPA 11 high-expansion foam.
Scope
For medium-expansion foam, depth, rate and quantity are set by tests (7.12.5.2.2, 7.12.8.1, 7.12.9.2); the tool covers high expansion only. For low-expansion tanks, dikes, loading racks and foam-water sprinklers use the foam concentrate calculator.
Frequently Asked Questions
How is the submergence depth for high-expansion foam found?
At least 1.1 times the height of the highest hazard, and at least 24 in. (600 mm) above it; the larger governs (NFPA 11-2024 7.12.5.2.1.1). For ignitible liquids it may not be less than the depth found by tests.
What are the factors in the NFPA 11 foam rate formula?
R = (V/T + R_S) × C_N × C_L. R_S = S × Q with S = 10 ft³/min per gpm (0.0748 m³/min per L/min) without test data; C_N = 1.15; C_L at least 1.0 and up to 1.2 for a building with all openings normally closed (7.12.8.2.3).
How many minutes of concentrate are needed?
Enough for the whole system to run 25 minutes or to generate four times the submergence volume, whichever is less, but never less than 15 minutes of full operation (7.12.9.1). A reserve is also required (7.12.9.3).
Why are the expansion ratio and generator capacity inputs?
The standard does not give them; they depend on the listing of the foam generator and concentrate. The tool takes the expansion ratio, generator capacity and proportioning rate from the data sheet and marks them as product values in the result.
MEP Calc — 110+ Engineering Calculators
MEP Calc bundles 110+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App Store
SprinkCalc — Fire Sprinkler Design in Three Standards
Run these calculations on site, offline and with PDF reports: NFPA 13, FM Global and BS EN 12845 in one iOS app.
Get SprinkCalc on the App Store