Use of Foam for Fire Fighting in Tank Farms of the Oil- and Petrochemical Industry

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 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.

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.

Diverse regional and national laws exist worldwide concerning fire protection. This article will be addressing 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.

Foam has proven to be the best medium to extinguish fluid fires. Foam consists of the components; 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 to 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 which is unable to develop its full extinguishing power.

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 which 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.

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 case of a crude oil tank with 60m diameter, NFPA 11 requires an application of 6.5l/(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 liters of foam concentrate and a required extinguishing water flow rate of approx. 18 000 l/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, e.g., wind and other factors. This results in a stock of 144 000 l of 3% foam concentrate.
• If alternatively a 1% foam concentrate is used the total storage quantity would amount to only 48 000 l requiring smaller storage tanks and set up space.
• The choice of foam concentrate and proportioning rate is dictated by the fluid to be extinguished.

NFPA 11 describes various types of foam concentrate proportioning equipment. In the following, three systems are looked at which are most common. Tight limits for the proportioning of foam concentrate apply to all of them.

The proportioning rate must not be less than the permitted values – i.e. 3% for a 3% foam concentrate or 6% for a 6% foam concentrare.
The proportioning rate must not exceed 30% above the permitted value i.e. 3.9% for a 3% foam concentrate or 7.8% for a 6% foam concentrate; respectively, the proportioning rate is allowed to be an absolute maximum of 1% above the permitted value – i.e. 4% for a 3% foam concentrate or 7% for a 6% foam concentrate (the smaller value must be used respectively).

To guarantee correct proportioning, the proportioner, including the proportioning rate must be tested at least once per year and its correct functioning must be checked.

The bladder tank with a proportioner is a proven and cost-effective technology. The bladder tank is a pressurized vessel with a bladder inside which is filled with foam concentrate. The tank is pressurized with water from the fire extinguishing line and discharges the foam concentrate from the bladder as required. The bladder is connected to a proportioner which operates using the venturi principle. When the fire pumps are activated, pressure is generated by the pump, causing delivery of foam concentrate to the proportioner. The extinguishing water flows through the venturi proportioner. The resulting vacuum induces the foam concentrate into the extinguishing water flow. The advantages of this system are its simple design without moving parts and its easy operation. No external energy is required and the system is relatively inexpensive.

A disadvantage is that the system is a pressurized vessel subject to corresponding regulations such as ASME Boiler & Pressure Vessel Codes. In order to refill foam concentrate, the system must be shut down and drained. The rubber bladder is sensitive; when damaged, water will contaminate the foam concentrate. At a given proportioning rate, the system is only suitable for low variations in the extinguishing water flow pressure and volume. Adding or changing individual foam discharge devices is possible only to a very limited extent. The system is also unsuitable for proportioning highly viscous foam concentrates.

To conduct any mandatory required annual testing, the system must be activated and premix generated at the venturi proportioner within in the extinguishing water line. The correct proportioning rate must be measured in the premix by laboratory analysis. The generated premix must be disposed of. The consumed foam concentrate in the bladder tank needs to be replaced.

The system consists of an atmospheric tank for the foam concentrate, an electric or diesel-powered foam concentrate pump with an electronically controlled valve and a flow meter in the extinguishing water flow line. When the fire pumps are activated, the foam concentrate pump drive and electronic control system must be activated. The extinguishing water flow rate is measured by the flow meter and the control system adjusts the correct foam concentrate quantity via the control valve. The foam concentrate is injected into the extinguishing water flow by the foam concentrate pump. If there is a change in the flow rate, the amount of injected foam concentrate is regulated by the control valve.

The system's advantage lies in the precise proportioning of the foam concentrate, independent of the extinguishing water pressure or flow rate. Foam concentrate can be topped up during the extinguishing operation. The system is capable of proportioning highly viscous foam concentrates. For the purpose of annual testing, the system must be activated; however, the delivered foam concentrate can be measured via a return line.The proportioning rate is calculated from the extinguishing water / foam concentrate flow rate. No premix is produced; and as the foam concentrate is passed back into the tank, no foam concentrate needs to be refilled.

Disadvantages are the requirement for an external interruption-free energy supply for the foam concentrate pump and the control system, as well as the need for a sophisticated control system and the comparatively higher purchasing costs. Furthermore, it must be accepted that a delay occurs between the change of the extinguishing water flow rate and the newly adjusted foam concentrate amount. The foam quality may be compromised when constantly changing operating conditions as foam discharge devices are turned on or off or changed.

The system consists of an atmospheric tank for the foam concentrate, a water motor installed in the extinguishing water flow line and a foam concentrate pump which is connected directly to the water motor. Water motor and pump form one compact unit. Upon activation of the fire pumps, rotation in the water motor starts. The direct coupling to the foam concentrate pump affects immediate foam concentrate injection into the extinguishing water. If the flow rate changes, the amount of foam concentrate is adapted immediately.

The advantage of the system is its independence from external energy sources as well as a precise and immediate foam concentrate proportioning regardless of the extinguishing water pressure or flow rate. If a piston or plunger pump is used, adjustment or calibration after installation is not necessary since the water motor and the pump are volumetric devices firmly connected to each other. Foam concentrate refilling during operation is possible. The system is also capable of proportioning highly viscous foam concentrates. The system must be activated for annual testing; however, the delivered foam concentrate can be measured via a return line. The proportioning rate is calculated from the extinguishing water / foam concentrate flow rate. No premix is generated; and if the foam concentrate is passed back into the tank, no foam concentrate needs to be topped up. The larger design and the comparatively higher purchasing costs are a disadvantage of the system.

With any system, consideration should be taken into account for the annual testing costs, which can be considerable in terms of replacement foam concentrate, disposal of premix and manpower.

As the stationary foam discharge equipment can be damaged in extensive fires in the tank or in the dyke area and thus lose effectiveness, mobile fire monitors and foam pipes may be used.

Foam pipes are normally held by firefighters, making them very flexible. Yet they have only limited extinguishing agent flow rates and reaches.

Firefighting monitors are discharge devices mounted on vehicles or trailers and available in many sizes. The extinguishing agent flow rate can be up to 60 000 l/min and the reach can be up to 180m if the pressure of the fire pumps is sufficient. They are suitable to discharge foam, e.g., to extinguish a surface fire in a tank; or water, to cool down a neighboring tank or the tank wall of a burning tank in order to prevent reaching the critical temperature for a boilover or to keep the flames from spreading. The accumulation of water inside the dyke area should always be observed to avoid an overflow of the dyke.

Mobile fire monitors can be supplied either by the extinguishing water of the stationary fire pumps or by mobile pumps. The injection of the foam concentrate usually takes place via mobile proportioners. This clearly points towards the advantage of energy independence for water motor-driven proportioning pumps.

The strategic decision for the sizes of mobile units available as back-ups is shown by the following example for the placement of monitors for fire extinguishing at tanks which are 45m in diameter and 15m high. According to NFPA, 32 000 l of premix per minute are required. This results in several alternatives for the monitors. Generally, foam concentrate proportioners for at least 32 000 l/min are required, which should be able to handle varying flow rates to guarantee flexibility during extinguishing operations. Depending on the local conditions, the monitors will need to keep a minimum distance to the burning tank or may not be able to be positioned near to the tanks due to debris. In addition, it will not always be possible to position several monitors around the tank. It must be ensured the monitor has sufficient throwing height in relation to the tank height, to deliver foam into the inside of the tank. The dimensioning of the entire fire fighting system is made following legal regulations as well as recommendations by associations like NFPA and ist not looked at more closely in the present article.

* Distance between monitor and tank dependent on low rate

As mentioned in the introduction, it seems that many authorities and companies have not learned the necessary lessons from disastrous fire incidents of past years. Tank farm fires in the oil and petrochemical industry do not happen frequently. When they do, they usually have devastating consequences. Let us remember the tank farm fire at Deer Park, Texas in March 2019 mentioned in the introduction.

The fire developed after over 30 000 l butane-enriched naphtha had been leaking from a faulty valve for 30 minutes and caught fire for yet unknown reasons. The plant had no gas warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas did not have fixed extinguishing systems installed. All 15 tanks were surrounded by one single dyke. The owner had relied on the local fire service which was on the spot very quickly but could not take control over the fire with the equipment available, partially because flammable substance was continuously leaking from the tank. 36 hours after the fire had broken out, a contract was made with an external fire fighting company to do the extinguishing work. Mobilization of equipment and foam concentrate as well as the preparation of a plan of action took approx. 13 hours. The fire was finally extinguished after 14 hours of fire fighting, 63 hours after it initially broke out. Extinguishing attempts were made over three days, with foam concentrate and water shortages occurring in the meantime. In total, over 500 000 l of foam concentrate were used. Instead of a fire in the dyke area, 11 of the 15 tanks burnt down.

It is very probable that the fire would have been extinguished quickly if the warning systems and valves had worked and a fixed fire extinguishing system had existed. It is also probable that the fire would have been extinguished quickly if the extinguishing operation, which succeded eventually, had been started earlier. Both cases would have resulted in notably less damage.

[1] National Fire Protection Association, NFPA 11 Standard for Low, Medium, and HighExpansion Foam 2005 Edition.

[2] U.S. Chemical Safety and Hazard Investigation Board (CSB), Factual Update Storage Tank Fire at Intercontinental TerminalsCompany, LLC (ITC) Terminal, Published: October 30, 2019.

[3] Nico Zorzetto PhD, Modern Safety Technologies on highimpact mega-fires, GULF FIRE, January 2020

This article is reproduced by kind permission of MDM Publishing Ltd - this article first appeared in the July 2020 issue of Gulf Fire magazine - www.gulffire.com