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Wednesday
Jun222011

TFE-11

Safety considerations for extinguishment using “Portables”

     Many flammable liquid storage tank fires are fought with portable application devices. Portable application devices have the ability to be quickly deployed, adjustable to a variety of situations, and can be moved to other sites assisting in multiple facilities tank fires. Because they are so mobile, they can be positioned to deal with not only full surface tank fires, but also low pressure vent fires, seal fires and dike fires. Each type of fire has its own unique set of safety hazards when fighting fire with portable devices.

     Dry chemical readily extinguishes flammable liquid vent fires. A good choice for most fuels is purple “K” agent. A tank vent will either burn with a yellow-orange flame emitting black smoke or a blue-red snapping flame with no smoke evident. If the fire is yellow-orange this indicates the air/vapor mixture inside the tank is above its flammable limits. This fire can be extinguished with dry chemical to the vent assembly.

     If the blue-red smokeless fire is observed, the air/vapor mixture inside the tank “IS” within its flammable or explosive limits. As long as the vent is “outbreathing” the fire cannot recede through the vent valve and ignite the interior tank space. However, if the fire has been burning long enough to damage the vent this could occur.       

     Therefore, proper extinguishment should be handled in one of two ways. 1] Pressure reduction, can be accomplished by either transferring the vapor gas or product, in which case the pressure reduction inside the tank slams the valve shut, extinguishing the fire. Sometimes this can be accomplished without transfer by applying cooling water to the exterior of the tank walls above the product level. This tactic cools the inner space, dropping the pressure, thus closing the vent relief-valve eliminating the fire. 2] Exceed the existing pressure by pumping in fuel gas. When the blue-red flame changes to yellow-orange, extinguish the vent fire with dry chemical.

     Seal fires in either an open or covered floating roof configuration can be extinguished with Foam extinguishers or dry chemical. The challenge in this situation is getting access to the seal area if the floating roof is covered. If this access is delayed, the entire seal may burn away positioning the roof for unpredicted sink age. Covered floating roofs usually have difficult access from the exterior. If there are rectangular vents near the top of the tank walls {seen in older tanks}, an extinguisher may be positioned at this point. If immediate improvement is not noticed, you may be missing contact with the flame and this technique should be abandoned before exposing firefighters to a full surface fire.

     If this happens, the full surface fire will require distance application from ground monitors. Access to the surface of the fire is the greatest challenge for portable monitors while the roof is still in place. When roof coverings are gone, proper application rates of Foam in conjunction with cooling water streams on exterior tank walls will successfully extinguish full surface tank fires. The key for portable monitors is being able to apply correct volumes of Foam, for an extended time period, at the proper rate. Cooling water streams used on the exterior tank walls above the product will cool the interior atmosphere minimizing Foam blanket destruction. This technique also cools the steel itself, preventing it from melting inward as fire contact intensifies the farther the tank level burns downward. The Foam can then smother the surface rather than become consumed by the fire.

     Dike and pipeline manifold fires may have something in common. If there has been enough spillage to form a pool of fuel around these areas, medium expansion, air aspirated Foam application from hose-lines will extinguish the pool fire. Since any type other than a pool fire is a 3 dimensional fire, the following adaptation should be made. First, extinguish all pooling with the aforementioned technique and secondly, approach the piping or uneven areas with dry chemical extinguisher for the final stop. If disturbance of the Foam pool blanket is made, have the Foam blanket re-applied while the firefighters advance through it. This will prevent re-ignition of the pool and provide firefighter security for the dry chemical activities.

 

                                            Haz Mat Mike   

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