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Haz Mat "Specialist Course"
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Saturday
Oct302010

Boil-over and Slop-overs

     These photographs give a vivid example of firefighting and Haz-Mat concerns involving Flammable Liquid Tank Farm operations. In a large scale fire or Haz-Mat spill involving these tanks, the initial tactic used to control the event is likely to be vapor suppression and or extinguishment by Foam. Therefore, the immediate goal will be to stop vaporization of these products whether they are on fire or not. There are many issues with an operation of this magnitude and many players requiring much pre-planning. When the event became involved, Sonny Crockett used to say, “There are so many players you need a program”. This is certainly the case when dealing with boil-over and froth-over situations. These topics will be addressed individually in my following series on “Tank Farm Engineering”. For now, lets’ find out how this hazard came to be.      Spills or Fires at tank farms generally fall into three categories. 1] Accidental or equipment failure, 2] sabotage from terrorist activities, and 3] ignition or spillage from natural causes such as “lightening strikes”. While maintenance and security can reduce the number of these events, natural weather conditions are more difficult to parry.      A “Boil-over” is an event involving a container of “medium grade” petroleum becoming super-heated by a free-burning fire of its own vapors on its’ surface. Back in the 1960’s, many “medium grade” petroleum products were housed in tank farm storage tanks. These can vary from between 1 to 6 million gallons of product. Today, there may be larger fixed site tanks in operation. As a result of these fires,“Medium grade” products are no longer stored in bulk quantities. Herein is where this problem came to be. The medium grade petroleum “phenomena” radiated heat from a surface fire “down” towards the bottom in the form of a “heat wave”. Since the bottom of the tank contained a few feet of water, it became super-heated. Water was used in the bottom portion of these tanks as a fail safe for leakage. If leaks occurred due to damage or corrosion, water, rather than the petroleum product would leak out. This was very beneficial to the environment as water has no environmental threat. Also, the non-flammable nature of water facilitated easier repairs.      The phenomenon that occurs with “medium grade” petroleum if allowed to continue for hours, {as most due, because of fire load} forces this “Heat Wave” towards the tank bottoms water layer. When these two entities connect, and the water rises above 212 degrees, the entire body of water is instantaneously turned into steam. This “steam explosion” forces millions of gallons of fuel up, out, and over the top edge of the tank wall. As it meets the atmosphere it ignites either from conduction, or contact with heated tank “courses”, and rains down as a river of fire. Remember, this phenomenon only occurs with “medium grade” petroleum. Since then the API {American Petroleum Institute} has declared no medium grade products shall be stored in this manner. Fortunately, no one was killed by this event. By watching the flow of this burning river of fuel, you can see that it did cross and destroy a large section of I-75 highway in Ohio.      A “Slop-Over” is another matter. These photographs were taken the evening of a tank farm fire and graphically demonstrate the slop-over principle. A slop-over may occur with all burning products stored in customarily large storage tanks. Here the same principle of heating occurs, except the products are not “medium grade” and do not radiate a “Heat Wave” downward towards the water barrier. As firefighting Foam drains {1/4 drain time}, back into water it is turned to steam close to the tanks surface and begins to slop around within the tank walls. If drainage is not monitored by site personnel, the tank volume accumulates and the burning products “slop-over” the tank wall, possibly onto responders. As you can see, this also increases the danger of the tank wall melting away or failing completely releasing all the tank contents. More about preparing for, and pre-planning this type of a response in the “Tank Farm Engineering” series.                                                                                       

                          Haz Mat Mike                                                                                            

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