The transient calculation requires that there is not excessive friction in the pipelines. In some cases when the initial flow and headloss along a pipe are both very small, HAMMER will compute large friction factors for these pipes (generally because very low velocities result in small Reynolds number values, which results in high friction factors under laminar flow). This prompts an error message which prevents the model from running. To prevent this, it is possible to specify a Flow Tolerance value below which any flow is rounded down to zero. This prevents the friction factor error, because the friction factor for pipes with zero initial flow is based solely on the roughness parameter entered for the pipe. However, if the Flow Tolerance is adjusted, it is suggested that the 'Round Pipe Head Values?' parameter is set to 'True' and the pipe heads are rounded to a similar level of accuracy as the flows. This helps ensure that the head at either end of a pipe with zero initial flow is the same.

Note however, that in the majority of cases it is suggested that the default value is used for these parameters.