A high-expansion foam total flooding system fills an enclosed volume with foam to above the hazard and holds that level until the fire is under control. The design comes down to three questions: how deep the foam must be, how quickly that depth must be reached, and how much foam the generators must produce once leakage, shrinkage and sprinkler water are allowed for. NFPA 11-2024 answers them in 7.12. This page goes through the clauses in order and finishes with a worked example using the formula.
Clauses, table and formula are from the 2024 edition, read from the standard's PDF pages. Metric values are those the standard gives in brackets.
What counts as high expansion, and where is it used?
The explanatory annex divides foams into three ranges by expansion: low expansion up to 20, medium expansion from 20 to 200 and high expansion from 200 to about 1000 (A.3.3.9). Medium- and high-expansion systems may protect ordinary combustibles, ignitable liquids and combinations of the two; liquefied natural gas is protected by high expansion only (7.3). They are not used on oxygen-releasing chemicals such as cellulose nitrate, unenclosed energised electrical equipment, water-reactive metals and materials, or liquefied flammable gas, unless an evaluation including tests shows them to be acceptable (7.3.2).
A total flooding system consists of fixed foam generators with a piped supply of concentrate and water, discharging into an enclosed space or an enclosure around the hazard (7.12.1). It needs a permanent enclosure so that the foam can build up to the required depth and stay there long enough (7.12.2).
The enclosure: leakage and venting (7.12.4)
- Openings: doors and windows below the design filling depth close automatically before or as foam discharge starts, with due regard for evacuation (7.12.4.1.1). Closures must stay shut during the fire and withstand foam and sprinkler water pressure.
- Openings that cannot close: the system is designed to make up the probable foam loss and the design is verified by test; if discharge may start before closure is complete, extra output is provided for the losses (7.12.4.1.1.2).
- Venting: where outside air feeds the generators, high-level venting is provided for the air displaced by the foam (7.12.4.1.2.1), with a velocity not above 305 m/min (1000 ft/min) in free air (7.12.4.1.2.2).
- Fans: forced ventilation that disturbs foam build-up is shut down or closed automatically (7.12.4.1.2.5). Exhaust fans, where the design needs them, are rated for high temperature and must not upset generator operation.
Foam depth (7.12.5.2.1)
For high-expansion foam the total depth is at least 1.1 times the height of the highest hazard and never less than 600 mm (24 in.) above it (7.12.5.2.1.1). Both conditions apply together. For a 4 m stack, 1.1 × 4 = 4.4 m leaves only 0.4 m of cover, so the depth becomes 4 + 0.6 = 4.6 m. Below roughly 6 m of hazard height the 600 mm rule usually governs; above it, the 1.1 factor does.
For ignitable liquids the depth may need to be considerably greater and may not be less than the depth found by tests that reproduce the expected fire (7.12.5.2.1.2). For medium-expansion foam the depth varies with expansion and is set by tests (7.12.5.2.2).
Submergence volume (7.12.6)
- In sprinklered areas the submergence volume is the required depth multiplied by the floor area protected (7.12.6.1).
- In unsprinklered rooms of combustible construction or finish, the whole volume including concealed spaces is used (7.12.6.2).
- The volume of vessels, machinery and other permanent equipment may be deducted (7.12.6.3); the volume of stored material may not (7.12.6.4).
Submergence time: Table 7.12.7.1
The table gives the maximum time, in minutes from the start of discharge, to reach the submergence volume (7.12.7.1). The columns are two groups of NFPA 220 construction types. The first is Type II (000), Type III (200) and Type V (000): buildings with unprotected structural members. The second is Type I (all), Type II (222, 111), Type IV (2HH) and Type V (111): fire-resistive, protected or heavy-timber buildings.
| Hazard | II (000), III (200), V (000) | I, II (222, 111), IV (2HH), V (111) | ||
|---|---|---|---|---|
| Sprinklered | Not sprinklered | Sprinklered | Not sprinklered | |
| Ignitable liquids, flash point below 38 °C, vapour pressure not above 276 kPa (40 psia) | 3 | 2 | 5 | 3 |
| Ignitable liquids, flash point 38 °C and above* | 4 | 3 | 5 | 3 |
| Low-density combustibles (foam rubber, foam plastics, rolled tissue, crepe paper) | 4 | 3† | 6 | 4† |
| High-density combustibles (rolled kraft or coated paper, banded) | 7 | 5† | 8 | 6† |
| High-density combustibles (rolled kraft or coated paper, unbanded) | 5 | 4† | 6 | 5† |
| Rubber tyres | 7 | 5† | 8 | 6† |
| Combustibles in cartons, bags or fibre drums | 7 | 5† | 8 | 6† |
* Polar solvents are not covered; liquids boiling below 38 °C may need higher application rates. † These times may not apply directly to storage piled above 4.6 m (15 ft) or where fire spreads very rapidly through the contents.
The times assume no more than 30 seconds between fire detection and the start of discharge; any delay beyond 30 seconds is deducted from the table time (7.12.7.3, 7.12.7.4). The factors in 7.12.8 may call for shorter times (7.12.7.2). Where high-expansion foam is proposed for polar solvents, the equipment supplier must substantiate its suitability (7.12.7.5). The shorter times in the unprotected-construction column reflect the annex point that light, unprotected structural members are especially vulnerable to fast-developing fires (A.7.12.7.1).
The discharge-rate formula (7.12.8)
For medium expansion the rate is set by tests (7.12.8.1). For high expansion the rate must meet the depth and time of Table 7.12.7.1, compensating for normal shrinkage, leakage and breakdown by sprinkler water (7.12.8.2.2). The minimum discharge rate, or total generator capacity, is (7.12.8.2.3.1):
R = (V / T + RS) × CN × CL
R: discharge rate, m³/min · V: submergence volume, m³ · T: submergence time, min · RS: rate of foam breakdown by sprinklers, m³/min · CN: normal shrinkage factor · CL: leakage factor
- RS = S × Q (7.12.8.2.3.2), where Q is the total flow of the maximum number of sprinklers expected to operate, in L/min. Without specific test data, S = 0.0748 m³/min per L/min (10 ft³/min per gpm) (7.12.8.2.3.3).
- CN = 1.15, an empirical allowance for losses from drainage, fire, wetting of surfaces and absorbency of stock (7.12.8.2.3.4).
- CL covers losses around doors and windows and through openings that cannot be closed, and is set by the designer after assessing the building. It is never below 1.0, even for a building fully tight below the filling depth, and may be as high as 1.2 for a building with all openings normally closed, depending on expansion ratio, sprinkler operation and foam depth (7.12.8.2.3.5).
The annex reports that with expansions above 400:1, extinguishment times for ignitable liquid fires rose markedly when the foam rose more slowly than 0.9 m/min (3 ft/min) (A.7.12.8).
Concentrate and water quantity (7.12.9)
High-expansion concentrate and water must allow the whole system to run for 25 minutes or to generate four times the submergence volume, whichever is less, but never less than 15 minutes at full output (7.12.9.1). For medium expansion the quantity comes from tests by an independent laboratory (7.12.9.2). A reserve supply is also required (7.12.9.3).
Holding the submergence volume (7.12.10)
To secure control or extinguishment, the submergence volume is held for at least 60 minutes in unsprinklered locations and 30 minutes in sprinklered ones (7.12.10.1). Where the hazard is ignitable liquid in noncombustible containers, the time may be reduced (7.12.10.2). The volume may be held by running some or all generators continuously or intermittently, with arrangements and procedures in place to do so without wasting concentrate (7.12.10.3). Overhaul is pre-planned so that control is not lost (7.12.10.4). The annex adds that a soaking period of up to an hour may be needed before the foam is removed (A.7.12.10).
At acceptance, total flooding systems are also tested to show that automatic closers on doors, windows and conveyor openings, equipment interlocks and automatic vent opening all work when the system operates (12.5.2).
Worked example
Assumptions (all for illustration): a warehouse with a 40 m × 25 m floor and unprotected structure (Type II (000)); wet-pipe sprinklers; goods in cartons stacked 4 m high; 30 sprinklers expected to open, 12 m² per sprinkler at 12 mm/min; all openings close automatically, with CL = 1.1 for losses around doors.
- Depth: 1.1 × 4 = 4.4 m; 4 + 0.6 = 4.6 m. The larger governs: 4.6 m.
- Volume: V = 40 × 25 × 4.6 = 4,600 m³ (the stock is not deducted, 7.12.6.4).
- Time: Table 7.12.7.1, combustibles in cartons, first construction group, sprinklered → T = 7 min.
- Sprinkler flow: Q = 30 × 12 × 12 = 4,320 L/min; RS = 0.0748 × 4,320 = 323.1 m³/min.
- Discharge rate: R = (4,600 / 7 + 323.1) × 1.15 × 1.1 = (657.1 + 323.1) × 1.265 = 1,240 m³/min of foam.
- Duration: 4V / R = 18,400 / 1,240 = 14.8 min. That is less than 25 minutes so it applies, but it falls below the 15-minute floor, so 15 min is used.
- Solution and concentrate: assume the generators deliver 500:1 and the concentrate is listed at 3 % (both are product values taken from the listing). Solution flow is 1,240 / 500 = 2.48 m³/min = 2,480 L/min; over 15 minutes that is 37,200 L of solution, of which 1,116 L is concentrate. The reserve comes on top (7.12.9.3).
Check: reaching 4.6 m in 7 minutes is an average rise of 0.66 m/min. Had the hazard been an ignitable liquid with expansion above 400:1, the annex observation about 0.9 m/min would call for a closer look.
Common mistakes: using only the 1.1 factor and missing the 600 mm cover; deducting stored goods from the volume; ignoring detection delay beyond 30 seconds; leaving out RS; forgetting high-level venting and its 305 m/min limit.
For the wider picture see the Foam System Design Guide; for concentrate stock and reserve, NFPA 11 Concentrate Supply and Proportioning; for acceptance testing, NFPA 11 Acceptance Tests and ITM.
Frequently Asked Questions
How is the submergence depth for high-expansion foam set?
At least 1.1 times the height of the highest hazard, and the foam must stand at least 600 mm (24 in.) above it; both conditions apply (7.12.5.2.1.1). For ignitable liquids the depth comes from tests and may be greater.
What are the factors in the NFPA 11 total flooding formula?
R = (V/T + R_S) × C_N × C_L. Sprinkler breakdown R_S = S × Q with S = 0.0748 m³/min per L/min (10 ft³/min per gpm); normal shrinkage C_N = 1.15; leakage C_L at least 1.0 and up to 1.2 for a building with openings normally closed (7.12.8.2.3).
How is the submergence time chosen?
From Table 7.12.7.1, between 2 and 8 minutes depending on hazard, construction group and sprinklers. The times assume at most 30 seconds from detection to discharge; any extra delay is deducted (7.12.7.3, 7.12.7.4).
How much concentrate does a high-expansion system need?
Enough to run the whole system for 25 minutes or to generate four times the submergence volume, whichever is less, but never less than 15 minutes at full output (7.12.9.1). A reserve is required in addition.
How long must the foam level be held?
At least 60 minutes in unsprinklered locations and 30 minutes in sprinklered ones (7.12.10.1). The time may be reduced where the hazard is ignitable liquid in noncombustible containers (7.12.10.2).
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Download MEP Calc on the App StoreNFPA 11, Standard for Low-, Medium-, and High-Expansion Foam, 2024 edition: 7.3 (p. 11-36); 7.12.1–7.12.7 (p. 11-39); Table 7.12.7.1, 7.12.8, 7.12.9 (p. 11-40); 7.12.10 (p. 11-41); 12.5.2 (p. 11-51); Annex A: A.3.3.9 (p. 11-59), A.7.12.7.1, A.7.12.8 (p. 11-96), A.7.12.10 (p. 11-97). Building, sprinkler, expansion and concentrate values in the worked example are assumptions. The binding text is the standard itself and the decision of the authority having jurisdiction.