NFPA 11 Dikes, Spill Areas and Loading Racks

A tank farm fire does not always start inside a tank. An overfilled tank, a leaking flange or a dropped hose during loading can spread liquid outside the tank, into the dike or across open ground. NFPA 11-2024 Chapter 5 treats these fires in three separate areas. These are loading racks (5.6), outdoor diked areas (5.7) and nondiked spill areas (5.8). In all three, foam goes onto a layer of fuel spread over the ground rather than onto a tank surface. What differs is the fuel depth, how the foam is applied and how long it must run. This article sets out the tables and layout rules for the three areas side by side.

Three areas, three definitions

The standard states that the list of methods for diked areas is not in order of preference (5.7.3.1).

Loading racks (5.6)

Many factors go into the design of a loading rack foam system. These include total rack size, the products handled, nearby hazards and exposures, drainage, wind, ambient temperature and available staff (5.6.2). A rack protected by foam-water sprinklers is designed to Chapter 6 (5.6.4). That method is covered in foam-water sprinkler systems.

For monitor protection, the design area is the total ground area within the curb (5.6.5.1). The aim is to protect the canopy, pumps, meters, vehicles and loading equipment in a spill fire (5.6.5.2).

Foam types not in the table are accepted when used in line with their listing (5.6.5.3.1.1). If fuel deeper than 25 mm can collect in the protected area, the rate rises to 6.5 mm/min whatever the foam type (5.6.5.3.2). At that depth the fuel behaves more like a pool than a thin spill.

Annex A makes two more recommendations. The first concerns monitor locations. Traffic, obstructions, wind and effective nozzle range decide whether foam reaches the whole area at the required rate. The monitor manufacturer's data on range and pattern should be used (A.5.6.5.1). The second concerns actuation. Automating the system on a truck loading rack should be considered, to reduce loss of life and property. This means either fully automatic detection and actuation or push-button stations. Manual operation alone is only acceptable where the AHJ agrees (A.5.6).

Outdoor diked areas (5.7)

A diked area is protected by low-level fixed outlets, fixed or portable monitors, or foam hoselines (5.7.2). For hydrocarbon dikes, the figures for fixed application are in Table 5.7.3.2:

Class I is a liquid with a flash point below 38 °C (3.3.8). Annex A, drawing on NFPA 30, describes Class II as a liquid with a flash point from 38 °C up to 60 °C (A.3.3.20.1).

Low-level fixed outlets

Low-level outlets can be open pipe fittings or directional nozzles. They deliver a compact, low-velocity foam stream onto the inside of the dike wall, or straight onto the dike floor where needed (5.7.3.5.1). They are placed around the dike wall, and inside the dike if necessary, so that foam covers the whole area evenly (5.7.3.5.2, 5.7.3.3.1). The spacing limit depends on flow per outlet:

• With 230 L/min (60 gpm) or less per outlet, no point in the dike may be more than 9 m (30 ft) from an outlet (5.7.3.5.3.1).
• With more than 230 L/min per outlet, the distance between outlets is at most 18 m (60 ft) (5.7.3.5.3.2).

Large dikes may be subdivided to keep the total design flow within practical limits (5.7.3.3.2). Annex A notes that low-level outlets cannot reach overhead structures or tank surfaces, so supplementary overhead foam spray may be needed (A.5.7.3.5.3).

Monitors and foam-water sprinklers

Monitors discharging into a dike are located outside it (5.7.3.5.4). Where monitors are the primary protection, the design allows for some foam being blown beyond the spill area (5.7.3.5.4.1). The same wind allowance applies to foam-water sprinklers or nozzles (5.7.3.4.2). A compact, high-velocity foam stream is not plunged straight into the burning liquid. It is played against dike walls, tank surfaces or other structures (5.7.3.5.4.2). Sprinkler and nozzle systems are designed to Chapter 6 (5.7.3.4.1). Annex A notes that overhead application may need low-level outlets to cover the area under large obstructions. It adds that overhead pipework can be damaged by an explosion (A.5.7.3.4.2). Annex A also says portable monitors and foam hoselines have generally been adequate for diked fires. The recommended tactic is to put out and secure one section of the dike before moving on to the next (A.5.7).

Dikes with liquids that need alcohol-resistant foam (5.7.4)

These dikes need special engineering consideration (5.7.4.2). Fixed protection methods are the same as for hydrocarbon dikes (5.7.4.3.1). The rate comes from the manufacturer's listing- or approval-based advice for the product and the foam-making device (5.7.4.3.2). The minimum time is 30 minutes (5.7.4.3.3).

Nondiked spill areas (5.8)

First, the possible spill area is estimated (5.8.1.1). Table 5.8.1.2 then gives the design figures for portable nozzles or monitors (5.8.1.2). The table covers both hydrocarbons and liquids that need alcohol-resistant foam.

Foams not in the table are accepted when used in line with their listing (5.8.1.2.1). Annex A assumes that in a nondiked spill fire the average fuel depth does not exceed 25 mm and the fire is bounded only by the ground contour (A.5.8). If a spill collects in a pit or drainage channel and gets deeper than 25 mm, treat that part as a diked area.

Worked examples

In the dike example, choosing 230 L/min per outlet means at least 4,100 / 230 = 17.8 → 18 outlets. They are spread so that no point is more than 9 m from an outlet. With 600 L/min outlets, 7 outlets cover the flow. In that case the distance between outlets must not exceed 18 m. Section 5.7 does not say whether tank footprints inside the dike are deducted from the area. Agree the design area with the authority having jurisdiction. You can try flow, time and concentrate figures in the foam concentrate calculator. The number and duration of supplementary hose streams around tanks are covered in NFPA 11 supplementary foam hose streams and monitors.

Common mistakes

• Designing a poorly drained loading rack where fuel can pool at 4.1 mm/min (5.6.5.3.2).
• Placing dike monitors inside the dike. They belong outside it (5.7.3.5.4).
• Ignoring wind losses for monitors and sprinklers (5.7.3.4.2, 5.7.3.5.4.1).
• Plunging a compact, high-velocity stream into the burning liquid (5.7.3.5.4.2).
• Mixing up the 9 m rule for small low-level outlets with the 18 m rule for larger ones (5.7.3.5.3).
• Using the hydrocarbon rate in a polar liquid dike. There the rate comes from the product listing and the time is at least 30 minutes (5.7.4.3).

For protecting the tanks themselves, see NFPA 11 fixed-roof tank foam and floating roof seal protection.

Frequently Asked Questions

What foam rate and time apply to a tank dike?

Under NFPA 11-2024 Table 5.7.3.2, low-level fixed outlets apply 4.1 mm/min and monitors 6.5 mm/min. The time is 30 minutes for Class I and 20 minutes for Class II hydrocarbons. For liquids needing alcohol-resistant foam, the rate comes from the product listing and the time is at least 30 minutes (5.7.4.3).

What is the difference between a diked area and a nondiked spill?

A diked area is bounded by ground contours or barriers and can hold fuel deeper than 25 mm (1 in) (5.7.1). In a nondiked spill the average depth does not exceed 25 mm and the liquid is bounded only by the ground (A.5.8). Nondiked spills are protected with portable nozzles or monitors for 15 minutes (Table 5.8.1.2).

What monitor rate applies to a loading rack?

Table 5.6.5.3.1 gives 6.5 mm/min for protein, fluoroprotein and SFFF, and 4.1 mm/min for AFFF and FFFP. The time is 15 minutes over the total ground area within the curb (5.6.5.1). If fuel deeper than 25 mm can collect, the rate rises to 6.5 mm/min (5.6.5.3.2).

How closely are low-level outlets spaced in a dike?

With 230 L/min (60 gpm) or less per outlet, no point in the dike may be more than 9 m from an outlet (5.7.3.5.3.1). Above 230 L/min per outlet, the distance between outlets is at most 18 m (5.7.3.5.3.2).

Where do dike foam monitors go?

Monitors discharging into a dike are located outside it (5.7.3.5.4). The design allows for some foam being carried off by wind. A high-velocity stream is played against the dike wall or a tank surface, not plunged into the burning liquid (5.7.3.5.4.1, 5.7.3.5.4.2).

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