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In this file we will discuss about how to estimate in a first order basis the pressure pulse in a pipe when the flow is suddenly changed (known as 'water hammer'). Before starting the unsteady considerations, the system has to be solve working in steady conditiones. What is important to know is the Piezometric Head (z + pressure/liquid's specific gravity). Hence, knowing the heigth (z) of the pipeline pressure is also known.
The next step is to evaluate the maximum pressure pulse than can develops in the pipeline. This pressure (or piezometric head) pulse is also known in bibliography as Jouckowski's pulse. This is the maximum pressure pulse that can develops in the pipeline in most cases, exceptions are rarely seen as for instance when resonance phenomenoes occurs. (How to calculate Jouckowsky's pulse can be found in other file in the site)
When steady pressure and Jouckowsky's pulse have been estimated, maximum and minimum pressure in each point of the pipeline system must be evaluated by adding and decreasing the pressure's pulse to the steady pressure.
Unsteady pressure fluctuations must be considerer for designing the pipeline if:
Maximum pressure is greater that pipeline's maximum supported pressure in any point, or
Minimum absolute pressure is closer to the steams's (vapour) fluid pressure (for water at 20 degrees celcius: absolute pressure about 0.4 meters or manometric pressure around -10). If this happens the liquid will vaporice and cavitations occurrs.
Other phenomenos as resonance can devolops in the pipeline
If any of the previous factors ocurrs, a more detailed study regarding unsteady transient hydraulics is recomended. Numerical modelling with model's as HiTrans is higly recommended.
Prior to finish, is important to note that this way to calculate pressure's pulse can lead to very big safety margins, usually greater than the permissible ones, reason why it is recommended to continue evaluating the case if the importance of the project deserves it.
On the other hand the vaporization of the water contained in the line when the pressure falls not necessarily below the steam pressure is a risky event for the structural integrity of the line.
Some others situation that can turn out from interest to analyze relative to unsteady flow, is to estimate the reversal turn of the pumping equipment if check valve is not present or it fails. This reversal turn speeds can even become greater than the steady regime one (but in opposite direction) and then it can lead to broke pump's integrity.
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