Transients can introduce large pressure forces and rapid fluid accelerations into a water distribution system. These disturbances may result in pump and device failures, system fatigue or pipe ruptures, and even the backflow/intrusion of dirty water. Many transient events can lead to water column separation, which can result in catastrophic pipeline failures. Thus, transient events cause health risks and can lead to increased leakage or decreased reliability. Transient flow simulation has become an essential requirement for ensuring safety and the safe operation of drinking water distribution systems. This article provides a basic understanding of the physical phenomena and context of transient conditions, presents practical guidelines for their suppression and control, and compares the formulation and computational performance of widely used hydraulic transient simulation schemes. Such capabilities greatly enhance the ability of water utilities to conceive and evaluate cost-effective and reliable water supply protection and management strategies and safeguard public health.A 112 MAY 2005 | JOURNAL AWWA • 97:5 | PEER-REVIEWED | BOULOS ET AL sient pressures are most important when the rate of flowis changed rapidly, such as resulting from rapid valve closures or pump stoppages. Such disturbances, whether caused by design or accident, may create traveling pressure and velocity waves of large magnitude. These transient pressures are superimposed on the steady-state conditions present in the line at the time the transient pressure occurs. The severity of transient pressures must be determined so that the water mains can be properly designed to withstand these additional loads. In fact, pipes are often characterized by their "pressure ratings" that define their mechanical strength and have a significant influence on their cost.

The authors compared the formulation and computational performance of two numerical methods for modeling hydraulic transients in water distribution systems. One method is Eulerian-based, and the other is Lagrangian-based. The Eulerian approach explicitly solves the hyperbolic partial differential equations of continuity and momentum and updates the hydraulic state of the system in fixed grid points as time is advanced in uniform increments.The Lagrangian approach tracks the movement and transformation of pressure waves and updates the hydraulic state of the system at fixed or variable time intervals at times when a change actually occurs. Each method was encoded into an existing hydraulic simulation model that gave initial pressure and flow distribution and was tested on networks of varying size and complexity under equal accuracy tolerance. Results indicated that the accuracy of the methods was comparable, but that the Lagrangian method was more computationally efficient for analysis of large water distribution systems.

Many surge analysis and design rules have evolved over time to help utilities cope with the complexity of transient phenomena. These rules have been widely applied to simplify analysis by restricting both the number and difficulty of the transient cases that need to be evaluated. On further reflection, however, the implicit assumption that elementary and conservative rules are a valid basis for design has often been shown to be questionable and sometimes dangerous. Indeed, many published guidelines are so misleading and so frequently false that they should only be used with extreme caution, if at all. This article specifically reviews a number of guidelines or suggestions found in various AWWA publications for water hammer analysis and provides a set of warnings about the misunderstandings and dangers that can arise from such simplifications. The authors conclude that only systematic and informed water hammer analysis can be expected to resolve complex transient characterizations and adequately protect distribution systems from the vagaries and challenges of rapid transients.

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