Protecting Tank Farms

Foam proportioning supplier FireDos expand on tank fire causes and incidents and how they can be extinguished efficiently and safely. Tank farm fires in the oil and petrochemical industry do not occur often. When they do, it is associated 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. 

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.

Full-surface tank fire: If hydrocarbons escape, they gather on the floating roof. In the worst case, the floating roof can even sink. 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. 

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 foam. 

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 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.

There are different foam proportioning concepts available in the market. The water-driven proportioning 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 and form one compact unit. Upon activation of the fire pumps water flow, rotation in the water motor starts. The direct coupling to the foam concentrate pump provides immediate foam concentrate injection into the extinguishing water. If the flow rate changes, the amount of foam concentrate is adapted immediately. 

One big 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 for the discharge of the foam concentrate, 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 like the fluorine-free ones. The proportioning rate is calculated from the extinguishing water / foam concentrate flow rate. No premix is produced; and as the foam concentrate is recirculated back into the tank, no foam concentrate needs to be refilled and no expensive disposal of foam / water solution is required.

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

Firefighting monitors are discharge devices mounted on vehicles or trailers and available in many sizes. The extinguishing agent flow rate can be up to 16,000 gpm and the reach can be up to 600ft 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 a critical temperature or to prevent the flames from spreading. 

Mobile fire monitors can be fed either by 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.

Tank farm fires in the oil and petrochemical industry do not happen frequently. When they do, they usually have devastating consequences. 

Having a leakage among others from a faulty valve or a failing measurement equipment, and if the plant has no gas warning system and no remote-controlled valves in place, fires can occur all of a sudden.  Without a fixed extinguishing system, the owner must rely on the local fire service and its immediate response as well as on their capability to fight these types of fires. 

With appropriate warning systems and at least monitoring devices for valves and other critical equipment, fires can be detected much earlier, extinguished much quicker if not even prevented from happening at all.