The vertical fixed-roof (cone-roof) atmospheric tank is the oldest and best understood application of low-expansion foam. A fire usually starts when part or all of the roof is lost, and the whole liquid surface then burns. NFPA 11-2024 recognises four methods for this tank: foam monitors and handlines, surface application with fixed foam discharge outlets attached to the tank, subsurface injection and semi-subsurface injection (5.2). The standard says that the list is not an order of preference. This article covers the second method, surface application through fixed outlets (5.2.5). It shows where each number comes from, how many outlets are needed and when the discharge time may be shortened.
In short: For a cone-roof tank holding hydrocarbons, the minimum rate for fixed-outlet surface application is 4.1 mm/min (0.10 gpm/ft²). The minimum discharge time is 30 minutes for liquids with a flash point between 38 °C and 60 °C, and 55 minutes for liquids with a flash point below 38 °C, liquids heated above their flash point and crude oil (Table 5.2.5.2.2). The number of outlets is read from tank diameter (Table 5.2.5.2.1). For liquids that need alcohol-resistant foam, the rate comes from the product listing and the time is 55 minutes (Table 5.2.5.3.4).
What a fixed outlet is and where it goes
A fixed foam discharge outlet is a device permanently attached to the tank that delivers foam into it. On site it is usually called a foam chamber (A.5.2.5.1). NFPA 11 now treats the Type II outlet as the norm. A Type II outlet delivers foam onto the burning surface, partly submerging the foam while keeping agitation of the fuel limited (3.3.3.3). Type I outlets are considered obsolete. A damaged Type I outlet is in effect treated as a Type II outlet (A.5.2.5.2.2).
Clause 5.2.5.1 sets these rules for the outlets themselves:
- Outlets are attached to the tank (5.2.5.1.1) at the top of the shell, and are located or connected so that the tank contents cannot overflow into the foam lines (5.2.5.1.3).
- Where two or more outlets are needed, they are spaced equally around the tank periphery (5.2.5.1.2).
- Each outlet is piped on its own and can be isolated by its own valve outside the dike (5.2.5.1.2.1). A line damaged by the initial explosion then does not take the whole system out.
- Each outlet is sized to deliver at least the minimum application rate on its own (5.2.5.1.2.2).
- The outlet is attached so that movement of the roof will not damage it (5.2.5.1.4).
- The outlet has a seal that breaks at low pressure and keeps tank vapours out of the foam piping (5.2.5.1.5). There must also be a way to inspect the outlet and replace the vapour seal during maintenance (5.2.5.1.6).
Fixed outlets are not used to protect horizontal or pressure tanks (5.2.3).
Number of outlets: Table 5.2.5.2.1
The minimum number of outlets depends on tank diameter (or equivalent area). The same table applies to hydrocarbon tanks and to tanks of liquids that need alcohol-resistant foam (5.2.5.2.1, 5.2.5.3.3).
| Tank diameter (m) | Tank diameter (ft) | Minimum number of outlets |
|---|---|---|
| Up to 24 | Up to 80 | 1 |
| Over 24 to 37 | Over 80 to 120 | 2 |
| Over 37 to 43 | Over 120 to 140 | 3 |
| Over 43 to 49 | Over 140 to 160 | 4 |
| Over 49 to 55 | Over 160 to 180 | 5 |
| Over 55 to 61 | Over 180 to 200 | 6 |
| Over 61 | Over 200 | 6 + 1 for each additional 465 m² (5000 ft²) |
For tanks over 61 m the table does not say where the additional area is counted from. Annex A recommends at least one more outlet for each additional 5000 ft² of liquid surface or fractional part thereof (A.5.2.5.2.1). In the example below we count the area above that of a 61 m tank and round any fraction up. Confirm that reading with the authority having jurisdiction on a large tank. The same annex note says experience with fires in fixed-roof tanks larger than 43 m (140 ft) is limited, so the requirements for them are extrapolated from successful extinguishments in smaller tanks. Tests have shown that foam can travel effectively across at least 30 m (100 ft) of burning surface.
Rate and time: Table 5.2.5.2.2
For a cone-roof tank holding hydrocarbons and fitted with Type II fixed outlets, the minimum values are (5.2.5.2.2):
| Hydrocarbon type | Minimum rate | Minimum time |
|---|---|---|
| Flash point between 38 °C and 60 °C (100–140 °F) | 4.1 mm/min (0.10 gpm/ft²) | 30 min |
| Flash point below 38 °C (100 °F), or heated above its flash point | 4.1 mm/min (0.10 gpm/ft²) | 55 min |
| Crude oil | 4.1 mm/min (0.10 gpm/ft²) | 55 min |
The split follows the NFPA 11 liquid classes. A liquid with a flash point below 38 °C is a Class I (flammable) liquid (3.3.8). Annex A, drawing on NFPA 30, describes a liquid with a flash point from 38 °C up to 60 °C as Class II (A.3.3.20.1). The table notes in Annex A add three warnings (A.5.2.5.2.2):
- Gasolines and gasohols with no more than 10 % oxygenated additives by volume fall under this table. Above 10 %, protection normally follows the alcohol-resistant foam rules of 5.2.5.3.
- Liquids boiling below 38 °C may need higher rates, which should be found by test.
- For high-viscosity liquids heated above 93 °C, a lower initial rate may be preferable to limit frothing and expulsion of the contents.
Does a higher rate shorten the time?
Yes, down to a floor. If the equipment delivers more than 4.1 mm/min, the time may be reduced in proportion, but not below 70 % of the minimum time in the table (5.2.5.2.3). For a 55-minute tank the floor is 38.5 minutes. At 5.0 mm/min the proportional time is 55 × 4.1 / 5.0 = 45.1 minutes, which is above the floor. At 6.5 mm/min the proportional figure would be 34.7 minutes, but the time cannot go below 38.5 minutes.
Liquids that need alcohol-resistant foam (5.2.5.3)
Water-soluble liquids, polar solvents and certain other ignitable liquids that break down non-alcohol-resistant foam need alcohol-resistant foam (5.2.5.3.1). The outlet count still comes from Table 5.2.5.2.1. The standard does not give a fixed rate for these tanks: the rate comes from the manufacturer's listing for the product stored. With a Type II outlet the minimum time is 55 minutes (Table 5.2.5.3.4). The foam concentrate and foam-making equipment manufacturers must be consulted in every case (5.2.5.3.2). Annex A recommends designing for a fire in one tank at a time. The rate for each tank is worked out from its liquid surface area and the type of liquid it holds (A.5.2.5.3). A small tank of a more volatile product can therefore need more foam-generating capacity than a larger tank.
Design basis and supplementary hose streams
The system is sized for the tank that needs the largest foam solution flow, including supplementary hose streams (5.2.2). Supplementary protection is mandatory and follows Section 5.9 (5.2.1). The number of hose streams depends on the diameter of the largest tank (Table 5.9.2.2), and so does their operating time (Table 5.9.2.4). Each stream delivers at least 190 L/min (50 gpm) of solution (5.9.2.3). Those tables are covered in NFPA 11 supplementary foam hose streams and monitors.
Worked example: 30 m petrol tank
A 30 m diameter cone-roof tank holds Class I petrol, and a 3 % concentrate is used.
| Step | Calculation | Result |
|---|---|---|
| Liquid surface | π × 15² | 706.9 m² |
| Solution flow | 706.9 × 4.1 | 2,898 L/min |
| Outlets | Table 5.2.5.2.1, over 24 to 37 m | 2 outlets, at least 1,449 L/min each |
| Time | Table 5.2.5.2.2, flash point below 38 °C | 55 min |
| Tank solution | 2,898 × 55 | ≈ 159.4 m³ |
| Hose streams | Table 5.9.2.2 (20–36 m): 2 × 190 L/min; Table 5.9.2.4 (over 29 m): 30 min | 380 L/min; 11.4 m³ solution |
| Design flow | 2,898 + 380 | 3,278 L/min |
| Concentrate (3 %) | (159.4 + 11.4) × 0.03 | ≈ 5.1 m³ |
If the same tank held a product with a flash point between 38 °C and 60 °C, such as diesel, the outlet count and flow would not change. The time would drop to 30 minutes, and the tank solution to about 87 m³. Pipe filling and the reserve supply the standard requires are added to the concentrate figure. These are covered in foam concentrate supply and proportioning. To try the numbers for your own tank, use the foam concentrate calculator.
For a larger example, take a 75 m tank. Its surface is about 4,418 m². A 61 m tank has a surface of about 2,922 m², so the difference is about 1,495 m². That is 3.2 times 465 m². Rounding the fraction up adds 4 outlets, for a total of 10.
When can monitors or handlines be the primary protection?
A cone-roof tank can be protected by monitors instead of fixed outlets. The portable, mobile or semi-fixed system must be able to go into service within 4 hours. A risk assessment and a pre-incident plan must also be complete, and the authority having jurisdiction must approve (5.2.4.1.1). Handlines cannot be the primary protection on a fixed-roof tank over 9 m (30 ft) in diameter or over 6.1 m (20 ft) high (5.2.4.1.2). For monitors, a factor of at least 1.5 is added to the application rate to allow for wind and other ambient losses (5.2.4.2.1.2). Table 5.2.4.2.2 gives different figures from the fixed-outlet table: 6.5 mm/min (0.16 gpm/ft²) for 50 or 65 minutes.
Indoor tanks (5.5)
Indoor storage tanks with a liquid surface of 37 m² (400 ft²) or more fall under Section 5.5 (5.5.1). A hydrocarbon tank gets Type II tank-mounted fixed outlets. For the number of outlets, 5.5.2 points to Table 5.2.6.2.8. The minimum rate is higher than outdoors: 6.5 mm/min (0.16 gpm/ft²) of liquid surface (5.5.3.1). The time comes from Table 5.2.5.2.2 (5.5.3.2). A higher rate again allows a proportional cut in time down to 70 % (5.5.3.3).
Common mistakes on site
- Feeding all outlets from one ring main with no separate isolation valves outside the dike (5.2.5.1.2.1).
- Getting the outlet count right but sizing each outlet for an equal share of the total flow. Each outlet must deliver at least the minimum rate on its own (5.2.5.1.2.2).
- Fitting vapour seals that cannot be inspected or replaced (5.2.5.1.6).
- Not reviewing the foam type when the product changes, for example when oxygenate content rises above 10 % (A.5.2.5.2.2).
- Cutting the discharge time without applying the 70 % floor (5.2.5.2.3).
- Leaving supplementary hose streams out of the design flow (5.2.2).
Frequently Asked Questions
How long must foam be applied to a cone-roof petrol tank?
Under NFPA 11-2024 Table 5.2.5.2.2, a liquid with a flash point below 38 °C needs at least 4.1 mm/min through Type II fixed outlets for 55 minutes. Crude oil also needs 55 minutes. Liquids with a flash point between 38 °C and 60 °C need 30 minutes.
How many foam chambers does a tank need?
Table 5.2.5.2.1 gives 1 outlet up to 24 m diameter, then 2 up to 37 m, 3 up to 43 m, 4 up to 49 m, 5 up to 55 m and 6 up to 61 m. Above 61 m, one outlet is added to the six for each additional 465 m² (5000 ft²). Outlets are spaced equally around the shell.
Can a higher flow shorten the discharge time?
Yes. If the applied rate is above 4.1 mm/min the time may be cut in proportion, but not below 70 % of the tabulated minimum (5.2.5.2.3). For a 55-minute tank the floor is 38.5 minutes.
What rate applies to an ethanol or polar solvent tank?
These liquids need alcohol-resistant foam (5.2.5.3.1). NFPA 11 does not give a fixed rate for them. The rate comes from the manufacturer's listing for the product, and the minimum time with a Type II outlet is 55 minutes (Table 5.2.5.3.4). The outlet count still comes from Table 5.2.5.2.1.
Can fixed foam outlets protect a horizontal tank?
No. NFPA 11-2024 5.2.3 states that fixed outlets are not used to protect horizontal or pressure tanks.
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Download MEP Calc on the App StoreNFPA 11, Standard for Low-, Medium-, and High-Expansion Foam, 2024 edition: 3.3.3.3, 3.3.8; 5.2–5.2.5; Tables 5.2.5.2.1 and 5.2.5.2.2 (pp. 11-19, 11-20), Table 5.2.5.3.4 (p. 11-20); 5.2.4 and Table 5.2.4.2.2 (p. 11-19); 5.5 (p. 11-27); 5.9.2 (p. 11-29); Annex A: A.3.3.20.1 (p. 11-62), A.5.2.5.1, A.5.2.5.2.1, A.5.2.5.2.2, A.5.2.5.3 (p. 11-78). Values were read from the standard's PDF pages. The binding text is the standard itself and the decision of the authority having jurisdiction.