In subsurface injection, foam is not delivered from the top of the tank. It is pumped straight into the product at a point near the tank bottom. The foam rises through the liquid and forms a blanket on the surface. Pumping foam into a flammable liquid sounds backwards, but in the right tank and within clear limits it works well. NFPA 11-2024 covers the method in 5.2.6, and its relative, semi-subsurface injection, in 5.2.7. This article explains which tanks qualify, how many outlets are needed, how foam velocity is checked and where the method is not allowed.
In short: Subsurface injection is only used on vertical fixed-roof atmospheric tanks holding liquid hydrocarbons (5.2.6.1). It is not used for Class IA liquids, alcohols or other products that need alcohol-resistant foam (5.2.6.1.1). Nor is it used on floating roof tanks (5.3.3.1, 5.4.2.2.4). The rate is 4.1 mm/min, and the time is 30 or 55 minutes (Table 5.2.6.5.1). Foam velocity at the outlet must not exceed 3 m/s for Class IB liquids or 6 m/s for other liquids (5.2.6.2.3).
Why inject from the bottom?
Annex A notes that experience with storage tank fires shows the main problem is operational. It is hard to deliver foam gently to the fuel surface at a rate high enough to put the fire out (A.5.2.6.1). Roof-mounted foam chambers are often damaged by the explosion that starts the fire. In a subsurface system the foam line enters low on the shell. The foam maker and the crew can work from a safe distance outside the dike. Annex A also notes that the rising foam carries cooler product from the bottom to the surface. This circulation helps extinguishment, on top of the smothering effect of the foam and the cooling effect of its water (A.5.2.6.2.8).
Because the foam passes through the fuel, the concentrate and equipment must be listed for subsurface use (5.2.6.1.2). Expansion ratios for subsurface injection are typically between 2:1 and 4:1 (A.5.2.6.1.2).
Where it is not used
- Class IA liquids: flash point below 23 °C and boiling point below 38 °C (3.3.8.1, 5.2.6.1.1).
- Products needing alcohol-resistant foam: alcohols, esters, ketones, aldehydes, anhydrides and similar products (5.2.6.1.1).
- Open-top floating roof tanks: the roof, even partly sunk, can stop foam spreading properly over the fuel (5.3.3.1).
- Internal floating roof tanks: excluded for the same reason (5.4.2.2.4).
- Very viscous products: according to Annex A, the most viscous fuel extinguished by subsurface injection when stored at ambient conditions had a viscosity of 440 cSt (2000 SSU). The method is generally not recommended for fuels above that viscosity at their lowest expected storage temperature (A.5.2.6.2.8).
Number of outlets: Table 5.2.6.2.8
The subsurface outlet table is not the same as the surface application table. The diameter bands are the same, but liquids with a flash point of 38 °C or higher need fewer outlets.
| Tank diameter (m) | Tank diameter (ft) | Flash point below 38 °C | Flash point 38 °C or higher |
|---|---|---|---|
| Up to 24 | Up to 80 | 1 | 1 |
| Over 24 to 37 | Over 80 to 120 | 2 | 1 |
| Over 37 to 43 | Over 120 to 140 | 3 | 2 |
| Over 43 to 49 | Over 140 to 160 | 4 | 2 |
| Over 49 to 55 | Over 160 to 180 | 5 | 2 |
| Over 55 to 61 | Over 180 to 200 | 6 | 3 |
| Over 61 | Over 200 | 6 + 1 for each additional 465 m² (5000 ft²) | 3 + 1 for each additional 700 m² (7500 ft²) |
According to Annex A, the table is extrapolated from fire tests on tanks of 7.6 m, 28 m and 35 m diameter holding petrol, crude oil and hexane (A.5.2.6.2.8).
Outlet arrangement (5.2.6.2 and 5.2.6.3)
- The outlet into the tank may be the open end of a foam line or a product line (5.2.6.2.1).
- Where more than one outlet is needed, they are placed so that foam travel across the surface does not exceed 30 m (100 ft) (5.2.6.2.4).
- Each outlet is sized to deliver at least the minimum rate on its own (5.2.6.2.5).
- For even distribution, outlets may be separate shell connections. They may also be fed through a manifold inside the tank from one shell connection (5.2.6.2.6). Manway covers may be used instead of new tank nozzles (5.2.6.2.7).
- Outlets must not discharge into a water bottom. To ensure this, they sit at least 300 mm (12 in) above the highest water level (5.2.6.3).
Rate and time: Table 5.2.6.5.1
| Hydrocarbon type | Minimum rate | Minimum time |
|---|---|---|
| Flash point between 38 °C and 60 °C (100–140 °F) | 4.1 mm/min (0.1 gpm/ft²) | 30 min |
| Flash point below 38 °C (100 °F), or heated above its flash point | 4.1 mm/min (0.1 gpm/ft²) | 55 min |
| Crude oil | 4.1 mm/min (0.1 gpm/ft²) | 55 min |
A note to the table also sets a ceiling: the maximum rate is 8.1 mm/min (0.20 gpm/ft²). Foam pumped in too fast agitates the surface and breaks up the blanket. A higher rate allows a proportional cut in time. The reduced time may not fall below 70 % of the tabulated time, and the velocity limit still applies (5.2.6.5.3). Where hydrocarbons contain foam-destructive products, the concentrate manufacturer's listing-based advice is needed (5.2.6.5.2). Annex A warns that some foams may fail to extinguish fires in petrol containing oxygenates at the usual rate when applied subsurface (A.5.2.6.5.2).
Foam velocity and back pressure
The velocity limit applies at the outlet, where the foam enters the product. The limits are 3 m/s (10 ft/s) for Class IB liquids and 6 m/s (20 ft/s) for other liquids. They may only be exceeded where actual tests prove a higher velocity is satisfactory (5.2.6.2.3). The check uses expanded foam flow, not solution flow. Annex A takes the solution flow, adjusted to the supply curve, and multiplies it by the maximum expansion ratio of 4. In metric form the velocity is (A.5.2.6.2):
v (m/s) = foam flow (L/min) / d² × 21.22 (d: pipe internal diameter, mm)
The pipe sizes and lengths downstream of the foam maker, together with the maximum expected fuel depth, create a back pressure. That back pressure must stay within the range for which the device was tested and listed (5.2.6.4). It is the static head plus pipe friction between the foam maker and the tank inlet. The friction curves in Annex A also assume an expansion ratio of 4 (A.5.2.6.4). This is why subsurface systems use high back-pressure foam makers.
Worked example: 40 m petrol tank
| Step | Calculation | Result |
|---|---|---|
| Liquid surface | π × 20² | 1,256.6 m² |
| Solution flow | 1,256.6 × 4.1 | 5,152 L/min |
| Outlets | Table 5.2.6.2.8, over 37 to 43 m, flash point below 38 °C | 3 outlets, about 1,717 L/min solution each |
| Foam per outlet | 1,717 × 4 | about 6,869 L/min |
| Velocity limit (Class IB) | d² ≥ 6,869 × 21.22 / 3 | Internal diameter ≥ about 220 mm |
| Time and solution | 5,152 × 55 | about 283.4 m³ |
If the same tank held a product with a flash point above 38 °C, the table would give 2 outlets. Foam per outlet would be about 10,305 L/min. With the 6 m/s limit the required internal diameter falls to about 191 mm, and the time drops to 30 minutes. Two more checks follow. The first is the 30 m surface travel limit. The second is that outlets sit at least 300 mm above the highest water level. Supplementary hose streams (5.2.1, Section 5.9) and the concentrate supply are added separately. You can try the flow, time and concentrate figures in the foam concentrate calculator.
During the fire: Annex A guidance
Annex A suggests an operating sequence for subsurface systems (A.5.2.6.1). Once the water and foam connections are made, foam pumping starts at the same moment as the block valves to the tank are opened. Solution pressure is brought up to the design value and held there. When foam first reaches the burning surface, the flames may briefly grow because of the steam it produces. They then die down quickly. The other Annex A points are:
- If there is enough water, cool the shell at and above the liquid level. Do not direct water into the tank where it can break up the foam blanket.
- With liquids flashing below 38 °C (Class IB and IC), fire can persist over the turbulent area above the injection point. Intermittent injection then lets foam cover that area while injection is paused.
- With products that can form a heat wave, slop-over can occur whether foam goes in from the top or the bottom. The risk is greatest if the tank has burned for 10 minutes or more. Slop-over is controlled by intermittent injection or by lowering foam-maker inlet pressure.
Semi-subsurface injection (5.2.7)
The body of the standard has a single rule for this method: all equipment must be listed or approved for the purpose (5.2.7). The detail is in Annex A. A flexible hose rising from the tank bottom is first held in a sealed housing. It is connected to an external foam generator that can work against the maximum product head. When the system operates, the hose is released and floats up on the buoyancy of the foam. Foam then discharges from its open end straight onto the surface (A.5.2.7). The foam does not pass through the product. Any concentrate suitable for gentle surface application can therefore be used. The method can also protect foam-destructive liquids, as long as the hose itself is not attacked by them. It is not suitable for floating roof tanks, with or without a fixed roof. The Annex A tables are:
| Product (Table A.5.2.7(a)) | Foam | Minimum time |
|---|---|---|
| Hydrocarbon, flash point below 38 °C | Protein, fluoroprotein, AFFF, FFFP, alcohol-resistant AFFF/FFFP | 55 min |
| Flash point 38 °C or higher | All foams | 30 min |
| Liquids needing alcohol-resistant foam | Alcohol-resistant foams | 55 min |
The number of units (Table A.5.2.7(b)) follows the same steps as the surface outlet table. That is 1 unit up to 24 m, 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 that, one more unit is added for each additional 465 m². Units are spaced equally. For hydrocarbons the rate is typically the same as for Type II topside application, 4.1 mm/min. For products needing alcohol-resistant foam, the manufacturer is consulted.
Common mistakes
- Checking velocity with solution flow instead of expanded foam flow, which is four times higher (A.5.2.6.2).
- Placing outlets in or just above the water bottom. They must be at least 300 mm above the highest water level (5.2.6.3.2).
- Keeping a subsurface system on a tank that has switched to an ethanol blend or a polar product (5.2.6.1.1).
- Raising the rate without respecting the 8.1 mm/min ceiling and the velocity limit (note to Table 5.2.6.5.1, 5.2.6.5.3).
- Choosing an unlisted foam maker without working out the back pressure (5.2.6.4).
For topside application, see NFPA 11 fixed-roof tank foam. For foam types and proportioning, see the foam system design guide.
Frequently Asked Questions
Which tanks can use subsurface foam injection?
Under NFPA 11-2024 5.2.6.1, only vertical fixed-roof atmospheric tanks holding liquid hydrocarbons. It is not used for Class IA liquids or for products needing alcohol-resistant foam, such as alcohols, esters and ketones (5.2.6.1.1). It is also excluded on open-top and internal floating roof tanks (5.3.3.1, 5.4.2.2.4).
What is the foam velocity limit for subsurface injection?
At the point where foam enters the product, velocity must not exceed 3 m/s (10 ft/s) for Class IB liquids or 6 m/s (20 ft/s) for other liquids. Higher values are allowed only if tests prove them satisfactory (5.2.6.2.3). Velocity is calculated with expanded foam flow, which is four times the solution flow (A.5.2.6.2).
How high above the tank bottom must a subsurface outlet be?
The outlet must not discharge into the water bottom. It is placed at least 300 mm (12 in) above the highest water level (5.2.6.3.1, 5.2.6.3.2).
Is there a maximum application rate for subsurface injection?
Yes. A note to Table 5.2.6.5.1 sets the maximum at 8.1 mm/min (0.20 gpm/ft²). The minimum is 4.1 mm/min. The time is 30 minutes for liquids with a flash point of 38–60 °C, and 55 minutes for the others and for crude oil.
How does semi-subsurface injection differ from subsurface injection?
In a semi-subsurface system the foam does not pass through the product. A flexible hose rises from the bottom to the surface and discharges foam directly onto it (A.5.2.7). Concentrates suitable for gentle surface application can therefore be used. Foam-destructive liquids can also be protected if the hose resists them. The equipment must be listed or approved for the purpose (5.2.7).
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Download MEP Calc on the App StoreNFPA 11, Standard for Low-, Medium-, and High-Expansion Foam, 2024 edition: 3.3.8.1 (p. 11-10); 5.2.6, 5.2.7, Table 5.2.6.2.8 and Table 5.2.6.5.1 (pp. 11-20, 11-21); 5.3.3.1 (p. 11-22); 5.4.2.2.4 (p. 11-27); Annex A: A.5.2.6.1, A.5.2.6.1.2 (p. 11-79), A.5.2.6.2, A.5.2.6.2.8 (p. 11-80), A.5.2.6.4, A.5.2.6.5.2, A.5.2.7 (pp. 11-82, 11-83), Tables A.5.2.7(a) and (b) (p. 11-85). Values were read from the standard's PDF pages. The binding text is the standard itself and the decision of the authority having jurisdiction.