Use of foam for firefighting in tank farms of the oil and petrochemical industry – Part 1

Tank farm fires in the oil and petrochemical industry do not occur often. When they do occur, it is with devastating consequences and negative publicity. This article will describe the typical incident scenarios and present foam as the most suitable extinguishing agent together with the firefighting equipment most commonly used. Finally, it is questioned whether the lessons learned from every incident are being taken into consideration.

Most recently, a number of fires have caused massive damage to tank farms, oil refineries and loading terminals. Prominent examples include the ITC tank farm at Deer Park, Texas, USA in March 2019 and the refinery fire at Baton Rouge, Louisiana, USA in February 2020.

Sadly, only news of this kind makes its way into the news, unlike near-miss accidents where improved fire-protection technology has prevented worse outcomes. This means that both fire protection and extinguishing methods must be improved to minimize the effects of future fires. Fire-protection systems in hazardous areas of the oil and gas industry (fire class B) require unique extinguishing systems as foam is considered the best extinguishing agent for liquid fires. The use of water can be important as well, however, for cooling the tank wall and adjacent tanks.

Luckily, such large-scale incidents do not happen frequently, but when they do, they may have a severe impact on humans, local eco-systems and the general environment.

Extinguishing full-surface oil-tank fires is very challenging and the rate of success is low. Many authorities and companies have not learned from such incidents, some of which can be disastrous.

Common tank farm configurations

The tanks in use are typically classified by roof type and diameter. They are distinguished between fixed roof, floating roof and fixed roof tanks with a floating cover. Flammable liquids are commonly stored in tanks with a floating cover or roof; explosive fluids, in turn, are stored in tanks with a fixed roof. The tank diameters are normally between 20m and 80m, although larger tanks up to 120m tanks are also used. Their heights usually vary between 10m and 25m. This article is focusing on firefighting techniques and will not deal with tank types in more detail.

Laws and regulations

Diverse regional and national laws exist worldwide concerning fire protection. This article will address the basics of the globally applied NFPA11 issued by the National Fire Protection Association, the standard for firefighting foam.

In place of various national and international certifying bodies, the following should be mentioned: the VdS approval in Germany (Verband der Sachversicherer translated to Association of Property Insurers) and the FM approval (Factory Mutual) on an international stage. Specifically, VdS and FM have prepared particular testing procedures for firefighting systems and have the necessary facilities and specialist personnel for testing.

Typical incidents

Fire incidents in tank farms of the oil and petrochemical industry occur in different scenarios, with consequences ranging from minor to disastrous. The most typical scenarios are listed as the following:

Rim-seal fire

Rim-seal fires occur on tanks with a floating cover. They can be extinguished quickly by the use of stationary systems as long as they are detected early enough. A longer fire, however, may damage the seal and, hence, cause an excess release of oil. This may develop into an extensive fire. A seal failure releasing excess fluid, or excessive use of extinguishing fluid, may lead to sinking of the floating cover and eventually to a full surface fire.

Vapor ignition

When storing petrochemical fluids, vapors may leak and be ignited by external sources, such as a lightning strike.

Dyke area fires

Tank farms are normally safeguarded by a dyke or located inside a basin to contain any escaping liquids. If oil or petrochemical fluids leak from a tank unintendedly, they may catch fire. If the fire is detected early enough, such fires can normally be extinguished easily and quickly by the use of fixed or mobile extinguishing equipment. If this is not the case, it can lead to a large surface fire which can also spread on the tank.

Explosion

Transformation of explosive gases by an ignition source. This normally causes damage to stationary extinguishing systems and mobile systems must be used. In addition, an explosion usually results in spreading the flammable liquids and devastation of the adjacent areas and access roads.

Boilover

A boilover may happen in the case of a long-lasting oil-tank fire where water has gathered at the bottom of the tanks. Due to the oil heating up to several hundred degrees Celsius, the water may evaporate abruptly. The water steam rises to the surface in bubbles which burst at the surface and spread the oil above the tank. The enlarged surface area causes an abrupt fireball.

Full-surface tank fire

If oil or petrochemical liquids escape, they gather on the floating roof. In the worst case, even a sinking of the floating roof can occur. If this surface catches fire, a full-surface tank fire develops rapidly. Fires on tanks with a floating roof can be extinguished either by stationary extinguishing systems from inside or by mobile extinguishing systems from outside the dyke area. In the case of fixed-roof tanks, only the internal stationary extinguishing systems can be used until the roof collapses.

Foam as an extinguishing agent

Foam has proven to be the best medium to extinguish fluid fires. Foam consists of water, foam concentrate and air. The foam concentrate is mixed with the extinguishing water at a precisely defined rate. Air is then added to this premix to generate the foam. Depending on the foam concentrate and the quantity of air, different types of foam are produced to extinguish different types of fire. Foam forms a homogenous layer of air bubbles, increasing the extinguishing agent’s volume and, hence, reducing its density. The foam floats on top of the flammable liquid and spreads across its surface. Due to this and its chemical properties, the foam blanket suppresses the release of flammable vapors, cuts off the supply of air and cools down the substance on fire. Consequent application of foam until fully covering the entire surface of the burning liquid will finally smother the fire.

Foam concentrates are developed for specific proportioning rates. The most common ones are 1% and 3%. As a general rule, a foam concentrate can form a stable and functioning foam only if it is mixed with the extinguishing water at no less than the correct proportioning rate. An increased proportioning rate will still form a stable and functioning foam; however, the foam concentrate stored will be used up faster. A proportioning rate falling short will produce a foam that is unable to develop its full extinguishing power.

The foam concentrates are divided into three groups:

Protein foam concentrates

Most prominently, there are film-forming fluoro-protein foam concentrates and film-forming alcohol-resistant* fluoro-protein foam concentrates.

Synthetic foam concentrates

The most important ones are film-forming and film-forming alcohol-resistant* foam concentrates. The majority of existing synthetic foam concentrates contain fluorine, which has good extinguishing properties. Mainly due to environmental reasons, fluorine-free synthetic foam concentrates, as the third group, have been and are being developed. Fluorine-free foam concentrates have yet to be fully proven for all foreseeable fire scenarios as suitable replacements for existing fluorine-based foam concentrates. In addition, due to their different physical properties (all existing fluorine-free foam concentrates have an increased viscosity and pseudoplasticity compared to the fluorine-containing foam concentrates), the necessary proportioning equipment has to be checked for its suitability.

After synthetic foam concentrates were introduced in the 1970s, various testing was conducted. One testing series by Shell in 1982 revealed that fires could be extinguished within 2 to 4 minutes if a good and suitable synthetic foam concentrate was used, both protein and synthetic. The extinguishing attempts were carried out in an open tank where the oil was on fire for 30 minutes and the surface temperature was about 800–900°C. Basically, the conditions for these low extinguishing times were as follows:

• suitable foam concentrate
• sufficient foam concentrate supply
• sufficient water supply
• functioning extinguishing equipment in sufficient quantity (well-maintained, quick and easy access, correct strategic placement)
• trained personnel
• quick implementation of a suitable plan of action

*Alcohols are water-miscible, which would destroy the foam. In alcohol-resistant foam concentrates, this is prevented by forming a polymer layer.

Extinguishing systems

Depending on the tank type and the size of the tank farm, the extinguishing systems must be designed differently. A fixed-roof tank must have a fixed extinguishing system that allows discharging foam under the roof. An application from mobile systems outside is possible only if the roof has been damaged or removed by a fire or an explosion. In case of a floating-roof tank, the foam can be applied by use of fixed or mobile systems from outside.

Fixed extinguishing systems typically consist of one or more stationary fire pumps, one proportioner and tank for the foam concentrate, discharge devices such as foam nozzles, sprinklers, foam pipes or fire monitors and the corresponding piping.

Mobile systems generally consist of the same components (fire pump, proportioner, supply tank); these must, however, be available in mobile form on vehicles or trailers. In addition, only fire monitors are usually used as discharge devices. The piping/lines consist of hoses and suction pipes.

Beside the tactical positioning of the foam discharge points, including mobile units, the foam concentrate and its proportioning into the extinguishing water are the most important factors for successful extinguishing. This will be looked at more closely referring to NFPA 11.

Foam concentrate

When storing the foam concentrate in suitable containers, attention must be paid that they are stored a sufficient distance away from the objects to be protected. The quantity must be sufficient to allow extinguishing of the largest protected object, or of the objects to be protected simultaneously as a minimum. The proportioning rate (1% or 3%) will dictate the quantity of the foam concentrate required. Generally, it is important not to mix different foam concentrates, or foam concentrates with different proportioning rates, as this can lead to unstable foam formation. Below is an example calculation for the foam demand according to NFPA 11:

Tank surface x specific extinguishing water quantity x proportioning rate of foam concentrate x requested minimum extinguishing time

In the case of a crude oil tank with 60m diameter, NFPA 11 requires an application of 6.5 litres/(min*m2) for an extinguishing time of 65 min. When using a 3% foam concentrate, this results in a minimum amount of approx. 36,000 litres of foam concentrate and a required extinguishing water flow rate of approx. 18,000 litres/min. NFPA 11 recommends stocking additional foam concentrate for the dyke area of about the same amount. In addition, a safety factor of 2 is recommended to compensate foam losses during extinguishing caused by, for example, wind and other factors. This results in a stock of 144,000 litres of 3% foam concentrate. If alternatively, a 1% foam concentrate is used the total storage quantity would amount to only 48,000 litres, requiring smaller storage tanks and set-up space. The choice of foam concentrate and proportioning rate is dictated by the fluid to be extinguished.

Part Two of this article will describe foam concentrate proportioning technologies, mobile extinguishing systems and conclude with lessons learned.

For more information, go to www.firedos.com

References

1. National Fire Protection Association, NFPA 11 Standard for Low, Medium, and High Expansion Foam 2005 Edition.
2. U.S. Chemical Safety and Hazard Investigation Board (CSB), Factual Update Storage Tank Fire at Intercontinental Terminals Company, LLC (ITC) Terminal, Published: October 30, 2019.
3. Nico Zorzetto PhD, Modern Safety Technologies on high impact mega-fires, GULF FIRE, January 2020.