Flownex is an excellent tool to model water hammer and can be used to determine pressure pulse peaks, frequency, damping and helps to give an indication of when flashing can occur. This article lists important aspects to consider when modelling water hammer. |
When modelling water hammer in Flownex, the following key aspects should be considered during the modelling approach:
Increment Pipe Components
It is important that the pipes should be incremented as pressure pulse propagation is grid dependent. Several degrees of pipe incrementation should be tested in order to obtain grid independence to ensure that the calculated results are accurate.
Specify Pipe Wall Material Properties
Always ensure that the pipe material, Young’s modulus and pipe wall thickness is specified. This means that the elasticity effect of the pipe will be taken into account. If these properties are not specified, Flownex simply assumes a completely rigid pipe wall.
Fluid Properties
In Flownex, it is recommended to use a pure incompressible fluid to model water hammer. When specifying the fluid, always ensure that the Bulk modulus of the fluid is correctly specified, as it is a critical parameter for the Flownex pressure wave calculation.
Time Step Size
It is important to specify a time step size small enough for the simulation to ensure accurate results and that the simulation is time step independant. The time step size can be specified in the Scheduler.
Flow Solver Settings
When a pure fluid is used, Flownex will indicate when a phase change occurs and will give an error if the option is specified to treat phase change warnings as errors. This option can be disabled and then the network will continue to solve and provide the user with the necessary warnings that phase changes occur.
Note that Flownex does not use a dedicated water hammer solver like the “Method of Characteristics”. Rather, it uses the same implicit pressure correction method for fluid pressure pulses that is used for steady-state solution or any other transient simulation. This simplifies the approach considerably, as the simulation can go seamlessly from steady-state into transient events of any timescale, even water hammer. As such, the wave propagation is not explicitly calculated, but it follows from the combined effect of pipe increments, fluid and pipe wall properties.