Protecting a fixed-roof tank with foam involves two separate calculations: the primary protection that extinguishes the tank itself, and supplementary foam hose streams for small spill fires around it. NFPA 11-2024 covers the first in 5.2 (fixed outlets, subsurface injection, or monitors and handlines) and the second in 5.9. This page sets out, clause by clause, when monitors and handlines may serve as primary protection (5.2.4, Table 5.2.4.2.2) and how supplementary hose streams are sized (5.9, Tables 5.9.2.2 and 5.9.2.4).
Clause and table numbers are from the 2024 edition, with values read from the standard's PDF pages. Metric figures are the ones the standard itself gives in brackets.
Primary versus supplementary protection
For outdoor fixed-roof (cone) tanks NFPA 11 accepts four methods, listed in no order of preference: foam monitors and handlines, surface application through fixed discharge outlets on the shell, subsurface application, and semi-subsurface injection (5.2). Whichever is chosen, supplementary protection to 5.9 is provided in addition (5.2.1). The system is sized on the tank that needs the largest foam solution flow, and that flow includes the supplementary hose streams (5.2.2).
Keep the two ideas apart. Primary protection is there to put out a fire on the liquid surface. Supplementary hose streams, according to the explanatory annex, are not meant for major spill fires; they are first-aid protection for small spills (A.5.9).
Can monitors and handlines be the primary protection?
Yes, with conditions. Monitor nozzles may be the primary protection for a cone-roof tank only as part of a portable, mobile or semi-fixed system, meaning adequate fire water, foam concentrate supply and portable or fixed monitors, that can be put into service within 4 hours. Three further conditions apply: a risk assessment has been completed, a pre-incident contingency plan has been completed, and the authority having jurisdiction has approved it (5.2.4.1.1).
Handlines are more restricted. They may not be the primary protection for fixed-roof tanks over 9 m (30 ft) in diameter or over 6.1 m (20 ft) in height (5.2.4.1.2). Exceeding either limit is enough to rule them out.
The annex (A.5.2.4.1.1) points out that most cone-roof tank fires start with an overfill or with an internal explosion that partly or completely separates the roof. Where mobile monitors or handlines are the chosen method, response tactics for each credible scenario should be worked out in advance.
Table 5.2.4.2.2: rates and times for hydrocarbon tanks
| Hydrocarbon type | Minimum application rate | Minimum discharge time |
|---|---|---|
| Flash point between 38 °C and 60 °C (100–140 °F) | 6.5 L/min·m² (0.16 gpm/ft²) | 50 min |
| Flash point below 38 °C (100 °F), or liquid heated above its flash point | 6.5 L/min·m² (0.16 gpm/ft²) | 65 min |
| Crude petroleum | 6.5 L/min·m² (0.16 gpm/ft²) | 65 min |
The table note equates L/min·m² with mm/min. The annex recommends a minimum discharge time of 35 minutes for hydrocarbons with a flash point above 93.3 °C (200 °F) (A.5.2.4.2.2). It also notes that liquids boiling below 38 °C may need higher rates, and that liquids with a wide boiling range can build a heat layer after prolonged burning and then need 8.2 L/min·m² (0.2 gpm/ft²) or more.
Wind and ambient losses: the 1.5 factor
Part of the foam thrown by a monitor or hose never reaches the target because of wind, temperature or rain. The standard requires these losses to be considered when the actual solution flow is calculated (5.2.4.2.1.1) and sets a factor of at least 1.5 on the application rate to cover them (5.2.4.2.1.2). In practice the table value of 6.5 L/min·m² becomes at least 9.75 L/min·m² for primary protection by monitor. The annex explains that the minimum rate in the table assumes the extra solution added by the 1.5 factor carries the required foam onto the target area (A.5.2.4.2.1).
Liquids that need alcohol-resistant foam
Water-soluble liquids and polar solvents that break down ordinary foam are protected with alcohol-resistant (AR) foam (5.2.4.3.1). For liquid depths greater than 25 mm (1 in.), monitors and foam hose streams are limited to special alcohol-resistant foams listed or approved for that purpose (5.2.4.3.2). In every case, the manufacturers of the concentrate and the foam-making equipment are consulted on limitations based on listings or fire tests (5.2.4.3.3).
For these tanks the operating time of monitors and handlines is 65 minutes at listed application rates, unless the foam manufacturer has shown by fire test that a shorter time is enough (5.2.4.4). The annex adds that foam should not be plunged into polar solvents and should be applied to the surface as gently as possible (A.5.2.4.4).
Supplementary foam hose streams (5.9)
Approved foam hose equipment is provided in addition to the tank foam installation, as supplementary protection for small spill fires (5.9.2.1). The number of streams depends on the diameter of the largest tank on site (5.9.2.2), and the equipment producing each stream must deliver at least 190 L/min (50 gpm) of solution (5.9.2.3).
Table 5.9.2.2: minimum number of hose streams
| Diameter of largest tank | Minimum hose streams |
|---|---|
| Up to 20 m (up to 65 ft) | 1 |
| 20–36 m (65–120 ft) | 2 |
| Over 36 m (over 120 ft) | 3 |
Table 5.9.2.4: operating time alongside the tank installation
| Diameter of largest tank | Minimum operating time |
|---|---|
| Up to 11 m (up to 35 ft) | 10 min |
| 11–29 m (35–95 ft) | 20 min |
| Over 29 m (over 95 ft) | 30 min |
The footnote to the time table says the times assume the required minimum number of streams discharging simultaneously at 190 L/min (50 gpm) each. Enough additional foam-producing material is kept so the hose streams can run at the same time as the tank installation (5.9.2.4). Note that the two tables step at different diameters: the number of streams changes at 20 and 36 m, the time at 11 and 29 m. The text does not say which row applies to a diameter sitting exactly on a boundary, so confirm that case with the authority having jurisdiction.
A rough guide to coverage. The annex treats a supplementary hose stream as first-aid protection able to cover a small spill whose area in square feet is about six times the nozzle rating in gpm (A.5.9). For a 50 gpm stream that is roughly 300 ft², about 28 m². Where permanent foam hydrants are used, they should be near the hazard, in a safe and accessible place, and positioned so that long hose lays are not needed.
Worked example
Assumptions: a fixed-roof tank 8 m in diameter holding a hydrocarbon with a flash point below 38 °C, protected primarily by a mobile monitor (conditions of 5.2.4.1.1 met), 3 % concentrate. This is the largest tank on site.
- Surface area: π × 8² / 4 = 50.3 m².
- Table rate: 6.5 L/min·m² → 50.3 × 6.5 = 327 L/min.
- 1.5 factor: 327 × 1.5 = 490 L/min monitor flow.
- Time: Table 5.2.4.2.2 → 65 min. Monitor solution: 490 × 65 = 31,850 L.
- Supplementary hose: 8 m diameter → 1 stream (Table 5.9.2.2) for 10 min (Table 5.9.2.4). 190 × 10 = 1,900 L of solution.
- Total solution: 33,750 L. At 3 %, that is 1,012.5 L of concentrate, before adding the reserve supply (4.3.2.5.2) and the unusable volume below the tank suction (4.3.2.3.1.5).
Had the tank been 30 m in diameter, the supplementary side alone would need 2 streams for 30 minutes: 2 × 190 × 30 = 11,400 L of solution, and handlines could not be the primary protection at all (diameter above 9 m). You can repeat the concentrate arithmetic with the Foam Concentrate Calculator.
Common mistakes on site
- Leaving hose streams out of the concentrate figure. Clause 5.2.2 includes them in the design flow, so stock and pump capacity must cover them too.
- Forgetting the 1.5 factor. The 6.5 L/min·m² in Table 5.2.4.2.2 is not a loss-free value when monitors or handlines are the primary protection.
- Relying on handlines for a large tank. Above 9 m diameter or 6.1 m height they cannot be the primary protection.
- No paperwork behind the "4 hours". With mobile monitors as primary protection, the risk assessment, pre-incident plan and AHJ approval need to exist in writing.
- Ordinary foam on a polar liquid. Above 25 mm depth only special AR foams listed for the purpose may be used from monitors and hoses.
For an overview of tank foam systems see the Foam System Design Guide; for fixed outlets, NFPA 11 Fixed-Roof Tanks; for concentrate and proportioning, NFPA 11 Concentrate Supply and Proportioning.
Frequently Asked Questions
How many supplementary foam hose streams does NFPA 11 require?
It depends on the diameter of the largest tank: 1 stream up to 20 m, 2 streams for 20–36 m and 3 streams above 36 m (Table 5.9.2.2). Each stream must deliver at least 190 L/min (50 gpm) of solution (5.9.2.3).
How long must the supplementary hose streams run?
10 minutes where the largest tank is up to 11 m in diameter, 20 minutes for 11–29 m and 30 minutes above 29 m (Table 5.9.2.4), assuming the required streams run at 190 L/min each together with the tank installation.
Can handlines be the primary protection for a fixed-roof tank?
Only for small tanks. Handlines may not be the primary protection for fixed-roof tanks over 9 m (30 ft) in diameter or over 6.1 m (20 ft) in height (5.2.4.1.2).
What application rate applies to monitors used as primary protection?
For hydrocarbon tanks, 6.5 L/min·m² (0.16 gpm/ft²) from Table 5.2.4.2.2, for 50 or 65 minutes depending on flash point, with a factor of at least 1.5 to allow for wind and other ambient losses (5.2.4.2.1.2).
Are supplementary hose streams meant for large spill fires?
No. The annex describes them as first-aid protection for small spills, not for major fuel spills (A.5.9). Diked and non-diked spill areas are designed separately under 5.7 and 5.8.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App Store
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 StoreNFPA 11, Standard for Low-, Medium-, and High-Expansion Foam, 2024 edition: 5.2, 5.2.1, 5.2.2 (p. 11-18); 5.2.4 and Table 5.2.4.2.2 (p. 11-19); 5.9 and Tables 5.9.2.2, 5.9.2.4 (p. 11-29); Annex A: A.5.2.4.1.1–A.5.2.4.4 (pp. 11-77, 11-78), A.5.9 (p. 11-88). Table values were read from the standard's PDF pages. The binding text is the standard itself and the decision of the authority having jurisdiction.