John,

You have presented an interesting finding about large surcharged models in Extran. What you have done by increasing the SURTOL parameter is to minimize the usage of the Surcharge Algorithm in Extran. Extran for the ISOL=0 and ISOL=1 solution tries to balance the flows at the surcharged nodes by iterating until the sum of the flows at the surcharged nodes is within the allowed tolerance.

The allowed tolerance is the value of SURTOL times the average flow in the WHOLE network (surcharged and non-surcharged links). If the sum of the flows is not lower than the tolerance then Extran will continue iterating until the maximum number of iterations is reached. The equation that Extran uses for calculating the surcharged node depth is different than the node continuity equation for non-surcharged nodes.

It seems in your network the surcharge algorithm causes a significant continuity error if the tolerance is too small. As you increased the value of SURTOL the number of surcharge iterations decreased and the continuity error decreased because in this case the iterations either were not converging or were converging to a solution that caused the continuity error.

Usually in most networks you decrease the continuity error by lowering the value of SURTOL but it is interesting to know that sometimes an increase in the value of SURTOL improves the overall model. You could improve your table by adding the number of iterations alongside the percent continuity error.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM