179.400-7—Tank heads.
               		
               	 	
               	 	
               	 	               	 	               	 	               	 	               	 	
               	 	
    
        
        (a) 
         Tank heads of the inner tank and outer jacket must be flanged and dished, or ellipsoidal.
     
    
        
        (b) 
         Flanged and dished heads must have—
     
    
        
        (1) 
         A main inside dish radius not greater than the outside diameter of the straight flange;
     
    
        
        (2) 
         An inside knuckle radius of not less than 6 percent of the outside diameter of the straight flange; and
     
    
        
        (3) 
         An inside knuckle radius of at least three times the head thickness.