Trouble representing a problem
I would definitely use a graph (Though, this clearly depends on the final application, maybe you could add more information) 

For the nodes, you should consider as nodes not only tanks but also points were pipelines change name or bifurcate. For instance, following your example:

    e1     e2        e3          e4        e5          e7           
    +---+----------.---------+----------.----------+-----------      
    |          |         |                     |          |      
[R tank 1]  [S tank 1] [S tank 2]              |e6     [S tank 3]
                                               |                 
                                           [S tank 4]            


Now adding the nodes:  

    e1 n2  e2     n4   e3   n6   e4    n8  e5     n9   e7           
n1 +---+----------.---------+----------.----------+----------+      
   |          |         |                     |              |      
  n3         n5        n7                     |e6           n11     
                                              |                 
                                             n10                


Lastly, you need some kind of mapping. 
Some of the nodes will map to tanks:

[R tank 1]     n3                                                   
[S tank 1]     n5                                                   
[S tank 2]     n7                                                   
[S tank 4]     n10                                                  
[S tank 3]     n11                                                  


And the pipelines will be represented by paths in the graph

Pipeline 1     e1 e2                                                
Pipeline 2     e3 e4                                                
Pipeline 3     e5 e7                                                
Pipeline 4     e6