Run duration is measured either in seconds or as a number of time steps. HAMMER determines the length of each time step automatically. Time steps typically range from a few hundredths of a second to a few seconds, depending on the system and the pressure wave speeds. The run duration has a direct effect on the modeling computation time, along with the time step selected for the simulation.

For simple systems or if the time required to compute the HAMMER model is not a concern, it is ideal (but not always necessary) to set run durations long enough to allow a final steady state to be achieved once all transient energy attenuates. This is quite manageable in many cases, such as for the sample file sample02.wtg, which requires about 30 to 40 seconds to reach a final steady state. Each system requires a different amount of time to reach a final steady state.

Transient Tip: Every pipe system has a characteristic time period, T = 2 L/a, where L is the longest possible path through the system and a is the pressure wave speed. This period is the time it takes for a pressure wave to travel the pipe system's greatest length two times. It is recommended that the run duration equal or exceed T. Another factor to consider when determining run duration is to allow enough time for friction to significantly dampen the transient energy. If in doubt, run HAMMER for a longer duration and examine the resulting graphs and time histories.

For larger systems, you can use the following guidelines to decide on the most appropriate run duration:

First run HAMMER for only a few time steps to identify the sources of transients (remember to output every time step using the Report Times attribute of the Calculation Options). You can also check for input errors by clicking the Validate button. Finally, click Compute to run the model, and then look for errors in the steady-state model or other initial transients in the comments at the end of the output file (.out).

Run HAMMER again for a duration of T=4 L/a (or greater) to verify that your simulation includes the maximum and minimum transient heads (Change the duration in the Calculation Options). These normally occur within this time frame. A longer run duration may be required if air pockets form or if a gas vessel or surge tank is installed, due to the persistence of oscillations in the system.

Run HAMMER again for a duration of T=20 L/a or greater, whatever is enough to allow friction to attenuate the transient energy and, consequently, to let the system approach or achieve a final steady state. See Selecting the Transient Friction Method.

The preceding procedure increases the likelihood that you will correctly simulate the key aspects of the hydraulic transient event for your system. However, remember that L is only a characteristic length which may not be directly applicable to branched or looped networks or plants. Always use sound engineering judgment in reviewing HAMMER results and interpreting the output.