Mistakes in the balance flowsheet - topology errors

Another suspicious case is so-called loop as represented by stream 2 in Fig. 13-5b. Although theoretically admissible, the existence of a loop is of no use in balancing. If the flow is measured, the value does not influence other values if not, it will be unobservable. 

Whereas the two above mentioned cases do not hinder carrying out the balance calculations, an excessive use of fixed quantities (constants not affected by reconciliation) can make the balance problem unsolvable. Fig. 13-5c shows a simple case where all streams connected with a node are fixed. If the values of the flows are chosen independent one from another, the balance equation around this node will probably not be satisfied. The following Fig. 13-5d illustrates the same problem in a slightly more complex form (fixed streams around subsystem formed by two nodes). 

Not all erroneous schemes can be detected in the manner shown above (for example omitting one stream need not result in an absurd scheme). Further mistakes can be revealed by the analysis of disproportions between balance relations and measured data see the following subsection. 

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