The simulation has a large number of time steps

A model with a large number of time steps will require a large number of calculations. The more calculations that need to be performed, the longer the run time.

How are the time steps contributing to long run times?

Impulse sections pipe to apply the Method of Characteristics solution method. The Method of Characteristics requires a uniform time step to be used for all pipes in the model, which is determined during the sectioning process using the section length and adjusted wavespeed. On average the wavespeed is typically on the magnitude of 10³, producing a time step on the magnitude of 10^-3.

A time step of 0.001 seconds means that a transient simulation of 1 minute will have 60,000 total time steps. Similarly, a transient simulation of 1 hour will have 3.6 million time steps. Run time is roughly proportional to the number of time steps, and models with large numbers of time steps will generally have long run times. In addition, models with higher wavespeed may require shorter time steps, increasing the number of time steps required.

What can the user do?

There are several different approaches to reduce run time when the number of time steps is larger than expected.

Consider reducing the simulation duration. The simulation duration is defined on the Simulation Mode/Duration panel. For a simple system, the full effect of the waterhammer cycle occurs over twice the communication time, where the communication time is the time it takes a pressure wave to travel the length of the system and back (length of system / wavespeed). Determining the communication time can become more complex for systems with branching paths or major reflection points in the path. However, limiting the simulation duration to 2 times the communication time may be a good starting point. If multiple transient events occur the simulation duration should be extended to end after the waterhammer cycle has completed for the last transient event.

In addition, for a piping network attached to a very long pipe it may not be necessary to include the length of the very long pipe in the communication time calculation. Use your engineering judgement on whether the reflections from the pipeline will be important to consider.

Consider reducing the number of unique pipe lengths. Models with numerous unique pipe lengths will often require the controlling pipe to be broken into multiple sections to satisfy the sectioning requirements for the Method of Characteristics. Artificially adjusting those pipe lengths such that all are divisible by the controlling pipe length can reduce the number of sections in the controlling pipe, increase the length of each time step, and reduce the total number of time steps required for the simulation. For more information, see the topic discussing Models with many unique pipe lengths.

Consider lengthening the controlling pipe. The controlling pipe length is used to determine the time step for the simulation, setting the time step equal to the section length divided by the wavespeed. Artificially lengthening the controlling pipe means the time step for the simulation can be longer, and fewer time steps are required for the simulation.