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Saturday
Aug212010

Frothovers

      “Frothovers” are seen at tank farms more often than expected. Unlike boil-overs and slop-overs, “frothovers” involve the rapid overfilling of a bulk tank from expansion when it is not on fire. This overflow of product is caused by the addition of product or water that is warmer than the existing tank product. This warmer product can sometimes boil underneath the surface of viscous oil. Often seen at the loading racks of a tank farm, this phenomenon more frequently involves tank cars. Although more rare, “frothovers” have been reported in the larger fixed site tanks under the same conditions. The great danger that exists during frothovers is the ignition of the resulting overage spilling onto the ground, around equipment, by contacting an ignition point. When this occurs, a resulting ground fire may break out and precipitate failure of a tank bottom. The worst result would be the subsequent launching of the entire tank and contents. Some of these “tank” launches have been reported to fly over 300 meters.

     One of the common procedures setting up this phenomenon is in the preparation of a tank car for loading. Commonly tank cars are washed at variable intervals to limit product contamination from previous product build-up or from carrying another product. After washing, because of the surface tension in water, a small amount of rinse water remains inside the tank car bottom. For a general service car, as much as 300 gallons can remain behind. As the tank car is inspected it may be re-drained before loading begins. Unfortunately, a substantial amount of the rinse water may still be present inside the tank bottom.

     As loading begins the “new” product starts to slightly cool from contact with the colder metal tank car while simultaneously vaporizing the rinse water. This continues due to the small amount of water and the large amount of “new” product being loaded into the tank car. As the car volume increases, the overall temperature rises from the new product being loaded. Unfortunately, by the time the tank car is near full, the water can become superheated and boil in the bottom. At this point the hot product overfills, spilling out of the loading port or “manway” and onto the ground towards an ignition source.

     The similar situation occurs on tank farms with larger bulk tanks containing “slops” or mixtures of residue collected to be “re-processed” at a later date. Here the slops are generally below 200 degrees F and contain either a water bottom or some type of wet emulsion to prevent product leaking into the environment. When new “slops” or product is loaded into this vessel, some times it may be above 212 degrees F. After a sufficient time has lapsed, the new product and temperature eventually reach the water or emulsion resulting in prolonged boiling. These events have been recorded to remove a tank roof and spread hot froth over wide areas.

                                     Haz Mat Mike

 

                                                              

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