Does HAMMER support initially negative pressures for a transient simulation?
Yes, it does, but there are two things to consider:
1) Make sure the initial conditions pressure is not negative at demand nodes. (because that demand would not be physically possible)
2) Make sure the initial conditions pressure is not below the vapor pressure limit. (because a vapor pocket would have formed)
These conditions are not supported with the transient solver.
In general, you would normally want to avoid any negative pressures in your system, both in the initial conditions and the transient simulation. Negative pressures can cause contaminant intrusion or joint failure.
Additionally, the model should be prepared for a transient analysis. Issues with the initial conditions should be resolved before moving to a transient simulation. See link below for more advice.
Troubleshooting negative pressures at pumps, junctions, & other node elements
User Notification "Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect"
Preparing an existing model for a transient analysis in HAMMER