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Sunday
Feb272011

TFE-2

 

Low Pressure Vertical Dome and Horizontal Bulk Storage Tanks

     Section 12-2.1.3 of the FLBSS deals with smaller stationary bulk storage tanks, both vertical dome and horizontal in design. Horizontal tanks as well as cryogenic vertical tanks can be identified from a distance by the inclusion of legs, beneath their base. These are primarily designed for insulation properties depending on the hazard type. Newer tanks {that are vertical} also incorporate a space saving feature because of this configuration. Horizontal and Dome tanks can store, flammable and combustible liquids, corrosives, poisons, fertilizers, chemical solvents and more depending on their rating. Usually, these are found with specialty products in smaller volumes than tank farms. Either the business uses smaller quantities for production, or larger storage volumes are not permitted due to the hazards municipal risk. Underground horizontals facilitate easier fire mitigation, but are more costly for installation and environmental monitoring.

     Regardless of the hazard, horizontal tanks must be approached from the middle section of the tank. End welds, used in construction have been found to fail under chemical, heat, or pressure stress, much sooner than mid-sections. Depending on the nature of the released hazard, the supporting legs become an immediate and constant concern. Corrosive spills can weaken un-protected steel while fire can melt un-protected metal legs. Fire Department tactics for these “legs” should immediately push away or extinguish flames before support legs fail. If the legs fail, and the tank topples, the entire contents of the tank could be released all at once. This volume could over-run firefighter positions and be catastrophic. Underground storage horizontals, primarily seen at fuel stations, will self extinguish after the primary firefighting or spill leak mitigation procedure is performed. Always shut off the FUEL first. Confinement of a lesser volume of fire or hazard spill is the immediate goal.

     Verticals and horizontals will have valves, while fuel stations all have emergency pump stop panic buttons near the cashier for these underground storage tanks. The first step for an underground spill {or fire} is for the attendant to shut off the pumps inside the station. Once this is accomplished, fire suppression is best performed by the application of class “B” firefighting Foam. For above ground tanks protect the tank remaining tanks content, by flushing fire and spilled product away from the un-protected legs of both verticals and horizontals. Once this is done, Haz-Mat “Technicians” can make entry to close the product lines. Depending on the product, the damaged tank can then be off-loaded and repaired by site personnel or specific contractors.

     In the advent of fire, two {2} procedures must be done immediately. 1] Using hose streams, flush fire away from tank legs. This will increase tank stability while eliminating heat contact on the lower portion of the tank. 2] Extinguish all fire contact above the level of the product on the tanks exterior. If venting devices are functioning, even if they are on fire, this is good. This means that the pressure inside the tank is either falling or failing to increase, both good. Increased pressurization can lead to tank failure with mass detonation. 18,000 gallons of LPG {Liquefied Petroleum Gas} can produce a fireball 300 meters wide by 90 meters high. History has demonstrated that detonations have occurred in as little as 6 minutes of over pressurization. An increasing pitch in relief valve sound may indicate impending detonation. An interesting phenomenon occurs when LNG {Liquefied Natural Gas} is spilled on water. The chemical process is complex and the physical action is dramatic. Loud cracking and popping noises, some quite startling occur. This is normal, and should not be regarded as an indication of increasing danger.  Once flame impingement is eliminated from the upper sections of the tank, care should be taken to attempt cooling relief valves without extinguishing them. Extinguishment of the vent flame should only be done by fuel shut-off. In this case, if you cool the tank you lower the internal pressure of the tank, causing the relief valve to shut, stopping the release of fuel. If the relief valve has been damaged, it may remain open and continue to burn. At this point, tank personnel should be able to gauge the remaining pressure inside the tank and determine if the valve has failed in the open position. The end result in this scenario will be either a “controlled burn” until the tank has been “flared off” or tank technicians may attempt a product transfer while the relief valve is discharging. Both choices can be done safely as long as you “respect” the dangers of the hazard.

 

     Notice in the drawings how the fire suppression system spray is directed to cover the bottom sections of the horizontal tank. Depending on the product, sprinkler protection for horizontals could also be applicable.

                                     Haz Mat Mike

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