Modeling of hydraulic transients in closed conduits
Date
2013
Authors
EL-Turki, Ali, author
Venayagamoorthy, Karan, advisor
Grigg, Neil, committee member
Wohl, Ellen, committee member
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Abstract
Hydraulic transients (often known as 'water hammer') occur as a direct result of rapid variations in the flow field in pressurized (closed-conduit) systems. For example, changes in velocity from valve closures or pump operations cause pressure surges that are propagated away from the source throughout the pipeline. The elasticity of the pipe boundaries and the compressibility of the fluid prevent these sudden changes in pressure from taking place instantaneously throughout the fluid. The associated pressure changes during a transient period are often very large and occur very rapidly (within a few seconds). If the maximum pressures exceed the bar ratings (mechanical strength) of the piping material, different types of failure such as pipe bursts can occur. Similarly, if the minimum pressure drops below the vapor pressure of the fluid, cavitation can occur and can be detrimental to the pipeline system. The purpose of this research is to model and simulate hydraulic transients in a closed conduit water system using different numerical methods. First, a numerical model was implemented to simulate the water level oscillations in a surge tank caused by the rapid closure of the outlet valve. The water surface oscillation results from the numerical model were compared with experimental results obtained from a surge tank experiment and found to be in good agreement. Furthermore, the stability and accuracy properties of the first-order explicit Euler time discretization scheme and the fourth-order Runge-Kutta (RK) time advancement scheme are highlighted using this example. It is found that using a higher-order scheme (such as the 4th order RK scheme) not only ensures a greater degree of numerical stability, but permits the use of larger time steps to achieve a similar degree of accuracy as the less stable first-order scheme. This is followed by a field test case study to investigate a pipe burst that occurred on a pipeline system in the Man-Made River in Libya. The Bentley HAMMER V8i software was employed to study this problem. A total of 28 scenarios were simulated using different combinations of the operating levels in the upstream Ajdabiya Reservoir and the downstream Gran Al-Gardabiya Reservoir and different time to closure of the valve. The simulation results show that the transient pressures in the pipeline exceeded the bar rating of the pipe where the burst occurred for most of the simulated scenarios. The range of results from the idealized simulations to the field test case study of hydraulic transients presented in this research highlights the importance of accurate prediction of the pressure fluctuations in order to ensure that a pipeline's integrity is not compromised.
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Subject
hydraulic transients