When computing a transient simulation, the following error appears:"There is a data error affecting a diameter change or air valve. Change flow(s) in adjacent pipe(s) to preclude initial pocket formation."
"Invalid Data reached"
This may happen in some cases where extreme results are reached, such as very large air or vapor pocket formation, or extremely unstable conditions.
If using a surge tank, check the surge tank configuration for errors. If using the surge tank with a turbine, or in other cases where it should operate at "line" pressure, consider choosing setting the property field "Treat as junction?" to "True" to allow the initial equilibrium HGL to be computed for the tank.
If the notification refers to a pipe near an air valve, check the air valve parameters and adjacent pipes. If you have the "Extended CAV" transient calculation option set to "true", be sure to understand the limitations explained here:
Assumptions and limitations of tracking air or vapor pockets in HAMMER
In this case, if the pipes adjacent to the air valve are very short, instability can occur when the pipe is drained, which will trigger a transition in the air valve calculation method. Consider if the Extended CAV option is required (given the assumptions and limitations) or adjust the model such that the pipes adjacent to the air valve are longer.
If the transient timestep is set too small, it can also cause the issue in question if model conditions are also unstable. Generally speaking a timestep on the order of 0.01 seconds is typically, with down to 0.001 seconds for extreme cases. A timestep smaller than this can sometimes produce unpredictable results in certain situations and should be avoided. If you have selected a small timestep to avoid excessive wave speed or length adjustments (as indicated by a user notification), it would be better to eliminate short pipes instead. See: Understanding length/wave speed adjustments and their impact on results
Bad initial conditions
This could also occur based on the initial conditions in the model. Make sure that there are no error messages that need to be addressed, and that you are using a calibrated model. If you are running a pump shut down event, make sure that the pumps are on and flowing in the initial conditions. If the initial conditions are set up properly, make sure that the transient calculation option property "Specify Initial Conditions?" is set to False.