What is the purpose of the 'Accuracy' Calculation Option (listed under the Hydraulics section) in WaterCAD/WaterGEMS/HAMMER OR "Pressure Subnetwork Accuracy" in the Calculation Options for the GVF Convex solver in SewerCAD/SewerGEMS
The definition for 'Accuracy' that is provided with the program may be a bit difficult to understand, thus the definition/explanation has been expanded in this solution.
The 'Accuracy' is the convergence criterion used to signal that a solution has been found to the nonlinear equations that govern the network hydraulics. The trials end when the sum of all the flow changes divided by the sum of all link flows is less than this number (the default value for our programs is 0.001).
In other words, the program will go through multiple trials to attempt to converge on a solution that balances energy across the model, and compares the relative change in flow between trials until that change is less than the "accuracy" calculation option. In order to prevent an excessive number of trials, the default Accuracy is set to a reasonable, but non-zero value. Hence, you may see slight imperfections in results (such as flows that should be zero showing as something non-zero but very small such as 0.0005 gpm) However, a balanced timestep should be well within reasonable tolerances.
When the hydraulics are calculated, the solver starts with an assumed flow for all pipes in the network. At the beginning of the simulation, this is 1 cfs. Beyond that point, the flow is taken from the previous iteration and/or time step (as appropriate). The iterative solution method attempts to find a steady state condition that is consistent with the flows/demands in the system whilst meeting all system controls defined by the user (i.e. valve settings, simple controls, logic controls). At each iteration, an adjustment is made to the flows in all pipes, and this flow adjustment is the measure of convergence. The smaller the total adjustment needed,the closer the solution is to becoming balanced. But, that adjustment needs something to base the determination of "convergence" on, thus the change in flows becomes a relative (unit-less) value that in real terms means something different for each different model.
Engine Compatibility Mode and related Calculation options