Fuel Storage Tanks Foam Fire Fighting Systems per NFPA 11

Ing. José Félix Acevedo B.

3/10/20256 min read

Introduction

Fires in fuel storage tanks pose a high risk to industrial safety and the environment. Extinguishing these fires with foam is one of the most effective methods, as it allows the flames to be suppressed, the surface of the flammable liquid to be cooled and re-ignition to be avoided. Firefighting foam works by forming a layer on top of the fuel, insulating it from oxygen and reducing the release of flammable vapours.

The foam extinguishing process involves the following steps:

Foam generation: A foam concentrate is mixed with water and air to form low, medium or high expansion bubbles.
Fire Application: Foam is evenly distributed over the surface of the fuel through nozzles, generators, or foam chambers.
Vapor Suppression: The foam seals the surface of the liquid, preventing the evaporation of flammable vapors.
Fuel cooling: The moisture contained in the foam helps reduce the temperature of the fuel and its surroundings.
Preventing re-ignition: The foam layer acts as a barrier that prevents oxygen from coming back into contact with the fuel.

NFPA 11 sets forth requirements for the design, installation, and maintenance of low, medium, and high-expansion foam systems for extinguishing fires in flammable liquids. In this document, the types of foam, tanks, application systems, injection methods, application rates, discharge times, and foam concentrate storage capacity required to fight fires in tanks are presented.

Types of Foam

NFPA 11 recognizes several types of firefighting foams in fuel storage tanks:

Protein Foam: Based on natural products, it offers good heat resistance and is economical.
Fluoroprotein (FP) Foam: Improves resistance to re-ignition and has better mechanical stability.
Aqueous Film Forming Foam (AFFF): Creates an aqueous film that suppresses flammable vapors.
Aqueous Film Fluoroprotein Foam Forming (FFFP): Combination of FP and AFFF with advantages of both.
Alcohol-Resistant Foam (AR-AFFF): Designed for fires from polar fuels and hydrocarbon products.

The percentage of foam indicated in NFPA varies between 3% and 6%. In liquid fuel storage tanks, a foam concentration of 3% is typically used for fire suppression.

Types of Applications

Surface Injection: Through foam chambers or foam makers.
Subsurface injection: Typically used in tanks that require protection of the entire surface of the liquid. In this type of application, the foam floats progressively and covers the entire surface of the liquid.

Types of Tanks and Foam Systems Used

Depending on the tank configuration, different foam systems are used:

Fixed Roof Tanks:

They use foam chambers with baffles, which caused the foam to run down off the tank shell.
They can be combined with Subsurface injection systems.

Superficial Injection

Subsurface Injection

Fixed Roof Tanks with Internal Floating Roof (IFR):

Tubular pontoons IFR: They require protection on the entire surface.
Full-Contact IFR: Foam is applied to the seal area. To do this, foam dams are installed that confine the foam in this area.
Pontoon or double deck steel IFR: Need protection of the seal area.

For these tanks, foam injection is carried out by means of foam chambers, with baffles, in these tanks the use of flat baffles is recommended to avoid damage of the floating roofs seal in case of filling above the normal operating level.

External Floating Roof Tanks:

The foam is applied to the seal area.
Foam dams are installed to confine the foam in the seal area.
They can be supplemented with fire monitoring systems to detect early ignitions.
Foam injection is done by foam chambers or foam makers mounted on a shield installed above the tank shell

Application Rate and Discharge Times

NFPA 11 establishes application rates and minimum foam discharge times for different scenarios:

Fixed roof tanks: 4.1 L/min/m², minimum 55 minutes for crude oil, class I combustible liquids or liquids stored above the flash point and minimum 30 minutes for class II and class III liquids.
Fixed roof tanks with internal tubular pontoons IFR: 4.1 L/min/m², minimum 55 minutes for crude oil, class I combustible liquids or liquids stored above the flash point and minimum 30 minutes for class II and class III liquids.
Fixed roof tanks with full contact IFR: 12.2 L/min/m², minimum 20 minutes.
Carbon steel external floating roof tanks: 12.2 L/min/m², minimum 20 minutes.

Foam Storage Capacity

When the minimum storage capacity of foam concentrate is to be known, it must be calculated based on:

Foam application rate (L/min/m²).
Tank coverage area.
Minimum recommended application time.

Example of calculation (surface injection):

Fixed roof tank with a diameter of 30 m → 706 m² of surface.
Application rate of 4,1 L/min/m² → 2.898 L/min.
Application time of 30 minutes → 86.943 L of foam solution.
With a mixture of 3% → 2.608 L of foam concentrate.
Additional percentage 10% → 261 L
Total foam concentrate required → 2,869 L

This is the minimum recommended amount of foam concentrate for firefighting in the tank. In this example, the requirements for additional protection with hose were not considered. The number of hoses required to supplement the protection of the storage tanks is indicated below:

Tanks up to 20 m in diameter: 1 hose.
Tanks with diameter over 20 m to 36 m: 2 hoses.
Tanks with diameter over 36 m: 3 hoses.

Foam Chambers and Pressure Required

Foam chambers are devices designed for the controlled application of foam in flammable liquid storage tanks. These chambers contain a baffle that allows the foam drain down the tank shell maximizing the effectiveness of the extinguishing agent.

For proper operation, foam chambers require a minimum operating pressure of 40 psi (2,76 bar) and an optimal 65-100 psi (4,5 to 6,9 bar), depending on the system design and manufacturer. Pressure is critical to ensure that the foam flows properly and reaches the necessary coverage inside the tank.

Foam Chamber Components

The number of foam chambers required depends on the type of tank, the system used, as well as the diameter of the tank and, in the case of floating roof tanks with foam dam, the height of the foam dam.

For fixed roof tank without IFR or with tubular pontoons IFR the quantity of foam chamber is as follow:

Tanks up to 24 m in diameter: 1 chamber.
Tanks with diameter over 24 m to 37 m: 2 chambers.
Tanks with diameter over 37 m to 43 m: 3 chambers
Tanks with diameter over 43 m to 49 m: 4 chambers
Tanks with diameter over 49 m to 55 m: 5 chambers
Tanks with diameter over 55 m to 61 m: 6 chambers
Tanks with diameter over 61 m: 6 chambers + 1 chamber per each 465 m2

For tanks with full contact IFR o external floating roof.

Floating roof with foam dam height of 305 mm: 1 chamber each 12 m, in perimeter
Floating roof with foam dam height of 610 mm: 1 chamber each 24 m, in perimeter

Conclusions

The design of a fire protection system in fuel storage tanks requires knowledge of the various types of foams, tanks, application systems, injection methods, as well as the application rates and times recommended by NFPA 11. The correct selection and design of the system guarantees safety and minimizes the risk of catastrophic fires in industrial facilities.