PDV America, Inc. and Subsidiaries - Page 6

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          rafter support columns, the tanks contain no internal                       
          superstructure, and their external plating provides the sole                
          structural support.  The tank shell is thickest at the bottom and           
          gradually thins toward the top, which makes the center point of             
          the tank’s total weight lower than half the tank height.                    
          Due to the amount of steel in the tanks’ composition, the                   
          tanks have considerable weight.  For example, a 55,900-barrel               
          tank has a dry weight of 394,000 pounds, and a 151,000-barrel               
          tank exceeds 1 million pounds.  Consequently, tanks usually are             
          not tied down to their foundations, and none of CITGO’s tanks               
               The purpose of tank foundations is to spread the tank’s                
          weight load to help avoid tank settlement and to keep moisture              
          and other corrosive elements from deteriorating the tank’s steel            
          structure.  Tank foundations may consist of compacted sand or               
          soil, concrete ringwalls, crushed stone ringwalls, or concrete              
          slabs.  For sand pad foundations, CITGO replaces the top 3 to 6             
          inches of the soil with sand or granular backfill.  Concrete                
          ringwalls are circular concrete walls from 12 to 18 inches thick            
          that line up with the tank’s outer edge.  The ringwall is mostly            
          beneath grade and is filled with sand or other material to permit           
          sufficient drainage.  Similarly, crushed stone ringwalls are                
          circular gravel rings filled with sand or other drainage                    
          material.  Concrete slab foundations are more infrequently used             

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Last modified: May 25, 2011