In the previous installment we talked about the types of petroleum storage tanks, their locations, and common fire hazards. In this article we will discuss the various types of fires and the fire protection methods available.
In the last article it was mentioned that different types of tanks have various common types of fires. Delving into these fires they can be further identified as follows:
Types of Fires
Overfill Ground Fires
Overfill ground, dike, or bund fires result from piping or tank leakage. This may be caused by operator error, equipment or instrumentation failure. These incidents are considered the least severe.1 When a leak occurs without ignition, all ignition sources should be eliminated and vapors suppressed. Remember that our fire appliances (apparatus) can be the source of ignition. If ignition does occur, the fire is treated as a large pool fire. If valves or piping are leaking, treat these areas as three dimensional fires.
Vent fires are typically associated with fixed roof tanks such as cone roof, covered floating roof tanks, dome roof, vertical low pressure fixed roof, and horizontal tanks. These are less severe fires and can usually be extinguished using a dry chemical extinguisher or reducing the pressure in the tank. Vent fires can be caused by lightning or sometimes hot work in the vicinity.
Rim Seal Fires
Rim seal fires comprise the large majority of fires in open top floating roof tanks. As with vent fires, lightning is usually the primary source of ignition. If the seals are not maintained, deterioration may cause fugitive vapor emissions to escape. When these vapors are in the flammable range they can ignite when exposed to an ignition source. There is usually a high rate of extinguishment success if there is no associated damage such as a pontoon explosion or if the suppression efforts do not sink the roof by using excessive water during firefighting.
In internal floating roof tanks, rim seal fires may be extremely challenging, especially if there is no permanently attached foam system. With no foam system in place, the only openings to apply foam into the tank would be the small vent openings at the top of the tank. These openings typically have protective screens in them, hampering foam application further. With open top and internal floating roof tanks, fixed or semi-fixed foam systems are the preferred method of extinguishment for rim seal fires.
Unobstructed Full Surface Fires
With sufficient resources and trained personnel, unobstructed full surface fires are relatively easy to extinguish on relatively small tanks (less than 45.72 meters [150’]). Larger tanks greater than this size will be much more challenging because of the size of the tanks, application and flow rates required, resources required, and the logistics involved with the firefighting efforts. Unobstructed full surface fires can occur in fixed roof tanks without internal roofs when the weak seam at the roof-shell joint separates from an explosion or other over-pressure event, leaving an exposed full liquid surface in the tank. External floating roof tanks can have unobstructed full surface fires when heavy loads (e.g., rains) exceed the weight capacity of the roof, causing it to sink below the surface of the product. This may also occur if the roof drains have been left closed during periods of heavy rains.
Obstructed Full Surface Fires
Full surface fires with a fully or partially sunken roof can occur where tanks have fixed roofs, internally floating roofs, or external floating roofs. These fires are challenging because access to the burning surface is blocked by the roof or floating roof (pan). An increase in vapor pressure between the liquid and the roof can cause the pan to tilt, partially exposing product above the pan. Pontoon failure may also cause a pan to sink. Heavy rains may also tilt or sink a pan.
Fire Protection Systems
Fixed and semi-fixed fire protection systems are used in industry to protect various equipment and processes. They are used in many cases to protect petroleum storage tank facilities. In differentiating between a fixed and semi- fixed fire protection system we find that a fixed fire protection system has all of the components to produce water or foam discharges without the aid of a fire vehicle. Water and foam based systems may be activated manually or activated by the use of devices such as smoke, heat, or flame detectors. A semi-fixed system is required to be supported by a fire vehicle.
The following are some of the systems that may be found at petroleum storage tank facilities:
Rim Seal Protection
This is the most common type of system found on atmospheric storage tanks. They are installed on either an open top or covered floating roof tank. The rim seal will have a 30 cm (12 inch) to 61 cm (24 inch) foam dam designed to hold the foam in the rim seal area so that it can flow and extinguish fire along the seal. These systems can either be fixed or semi-fixed. Mostly, these systems will be manually activated. The manual procedures could include starting the fire and foam pumps as well as opening and/or closing valves. A semi-fixed system will require a fire vehicle to connect to a lateral terminal connection and supply the foam chambers with foam solution at a specific pressure. These pressures should be identified during pre- incident planning and signs posted at the manifolds indicating these pressures.
Foam chambers are typically installed for rim seal protection, but in some instances they may be installed for extinguishment of full surface fires in both floating roof and covered storage tanks. Once again, pre-incident planning will be crucial to allow the fire service to know how much foam solution should be flowed and at what pressures.
Subsurface injection may be found on some covered roof storage tanks. When a floating roof is present, this system is not practical and cannot be used. The floating roof will keep the foam from reaching the surface of the fuel. These systems can either be fixed or semi-fixed. Aqueous film forming foams have low fuel tolerance and will not effectively rise to the surface. Subsurface injection also can not be used with polar solvent products. Once again, during pre-incident planning consult the plant operators to ensure that your personnel know how these systems operate and ensure that foam supplies, both fire service and plant, are compatible with the fuel in the tanks. The foam solution must be injected above the water level in the tank. Foam injected below this level will dissolve into the water and not rise to the surface.
Another system that may be used is a cooling system. Cooling rings on the exterior of the tank are placed so that they discharge water onto the tank’s exterior, protecting it from radiant heat from fire in close proximity. In many instances, these systems will have to be manually activated.
Looking back at storage tank fire protection we see that while fixed and semi-fixed systems have basically remained unchanged for 60 years but the size of the tanks has grown larger. In recent years, new technologies have been developed to make fixed and semi-fixed systems more efficient. As tank diameters increase, the degree of difficulty for extinguishment also increases. As with the rest of the fire service, new technologies help improve efficiencies and performance. Some of the new technologies for storage tank fire protection are:
Dual Agent Chambers
Traditional foam chambers would aerate the foam and distribute it evenly on rim seal or full surface fires. A dual agent chamber is a two-phase application through the same chamber. The first phase cascades a rich blanket of foam to extinguish and seal the surface of the contained product, while the second phase injects Purple K dry chemical to evacuate the internal vapor space of any lingering flammable vapors, rendering the space inert. These systems would be used on covered tanks.
Point and Shoot System
The Point and Shoot system provides a more aggressive and more efficient foam application as compared to traditional foam chambers. In actual tests, the Point and Shoot system provides 73 meters (240 feet) of coverage from one wand, a 200% increase in performance over a foam chamber. The foam wand will distribute foam from three orifices, two lateral and one discharging down the wall of the tank immediately below the wand. An optional feature is the addition of a separate riser to allow a specialized monitor to be quickly deployed and mounted to the tanks rim for additional extinguishment and/or vapor suppression.
The Hollow Point system was developed to overcome issues faced by foam chambers when they are installed on a fixed or cone roof tank for the purpose of full surface protection. When foam chambers are installed, they rely on gravitational forces and the static head pressure of the foam to provide the propulsion for the foam to cover the surface area of the product. The Hollow Point system projects the foam toward the center of the tank as well as to the left and right along the tank wall.
The Ambush system is designed for use on large diameter jumbo storage tanks. Type II “Over the top” applications on these tanks require huge amounts of resources and firefighting equipment. In some locations, 122 meter (400+ feet) diameter tanks are present. The Ambush is designed to provide enhanced fixed system response for external floating roof storage tanks. The system reduces foam fallout from the stream to provide more efficient foam/water application densities. It addresses four distinct areas of the tank simultaneously; the area to the left of the wand, the area to the right of the wand, the center of the tank, and the area directly below the wand. Multiple units may be required for complete coverage depending on the tank diameter.
Additional information on these new technologies can be obtained direct from the manufacturer of this equipment. An internet search will assist with locating the manufacturer.
This article and our previous one have discussed the types of atmospheric petroleum storage tanks, the types of fires, and fire protection systems. Our next installment will discuss the strategy and tactics involved with fighting atmospheric petroleum storage tanks as well as pre-incident planning techniques.
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