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I have a fixed head variable speed pump that is not able to apply the fixed hydraulic grade at the junction downstream that is specified for it in a steady state scenario. The model is running fine though with no red user notifications. How can I fix this issue?
One reason this has been found to occur is when there are two VSP's serving the same presssure zone. Both pumps are probably set to an inital status of 'On'. As our help documents indicate there should only be a single VSP serving a given pressure zone. What happens is the two different nodes in the model are usually very close hydraulically which results in each VSP fighting with the other one as to which has control.
In many cases this causes an error to result, but in other cases the model converges, but has to settle on a solution where there is an energy balance for the network and the demands are fully satisfied. Basically, behind the scenes what is happening is one pump is trying to meet the fixed head you assigned to it. Then as that happens the other pump is also trying to meet the fixed head you assigned to it. At this time the calculation engine is trying to balance the energy in the model and meet all the demands, so it goes through a few iterations until it stops at some number of trials, where it has converged on a solution that requires the VSP's to run in variable speed override in order to have an energy balance. (Variable speed override means the pumps run at a relative speed factor of 1.00 or 100% instead of something like .80 or .78.) This fighting of the two VSP's in the model to try to maintain their fixed head setting creates hydraulic conditions that are impossible to maintain.
The issue can be resolved by setting the inital conditions for one of the pumps to 'Off'.