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Why foam is the most suitable extinguishing agent for tank farm fires

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Extinguishing monitor supplier, FireDos examines oil tank fire causes and incidents as well as how they can be extinguished efficiently and safely.

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 used.

Most recently, several 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 minimise 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.

Typical incidents

Rim Seal Fire: Rim seal fires occur on tanks with a floating cover. They can be extinguished quickly using stationary systems if 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.

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.

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 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%. Generally, 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.

Bladder tank with proportioner

The bladder tank with a proportioner is a proven and cost-effective technology. The bladder tank is a pressurised vessel with a bladder inside which is filled with foam concentrate. The tank is pressurised 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 pressurised vessel subject to corresponding regulations such as ASME Boiler & Pressure Vessel Codes. 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.

Firefighting monitors

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 neighbouring 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.

Overview

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.

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 succeeded eventually, had been started earlier. Both cases would have resulted in notably less damage.

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This article was published in the latest edition of International Fire and Safety Journal. Pick up your FREE digital copy here