This article originally appeared in the October 2019 edition of Water & Wastes Digest as "Water Hammer for Everyone: Part 2."
Part one of "Preventing & Controlling Transients" in the September issue of WWD discussed what water hammer is and provided information on wave speed as well as transient control measures. Part two of this series will review calculation options for transient analysis, skeletonization, calibration and training.
Hammer Calculation Options
The next considerations are the calculation options for the transient analysis. While extended period hydraulic model simulations might run with hourly time steps to simulate multiple days, transient events have a much shorter life span. The major effects of transient runs are usually complete within a minute and the nature of the equations that must be solved requires time steps that are on the order of fractions of a second. Large time steps can lead to inaccuracy while short time steps lead to long simulation run times. The duration of a transient run should be on the order of several times the “characteristic time” of the system to enable the modeler to view the decay of the transient wave.
Because it takes small time steps, a Hammer model can produce a great deal of output, much of which is not needed. The user has the option to not save all the results but only save results at specific time steps or at a period longer than a single time step. The user also does not usually need to see results at every point in the system. By choosing selected points, the user can focus on the locations that are most important. Hammer also can generate animations of transient waves moving through the system. The user has control concerning whether these results are saved.
Figure A on page 17 shows how the hydraulic grade line (HGL) can be animated to show fluctuations in pressure. The black line is the HGL before a sudden pump trip. The red line is the maximum HGL, the green line is the ground elevation, while the blue line is the minimum HGL.
The water hammer wave is dampened over time by control devices and friction. Hammer enables the user to use one of four friction methods, including steady, quasi-steady, unsteady and unsteady-Vitkovsky. The unsteady friction methods tend to be the most accurate. The Hammer model of this pipeline can be color-coded based on maximum pressure over the course of a transient event, with red being the highest and green the lowest.
Skeletonization. In general, transient models of full distribution systems can take much longer to run than hydraulic models, and skeletonization of the network usually can reduce run times without significantly affecting accuracy. However, it is important not to exclude locations that contain transient control devices or high points that are subject to column separation. The most important pipes with regard to transients are large pipes with high velocities.
Calibration. As with any model, calibration can detect shortcomings in the model and can greatly improve the confidence users have in the results. Collecting data for transient analysis is more difficult than normal hydraulic monitoring because of the speed at which transient events occur. Transient monitors with recorders that have fast sampling times are needed to do an adequate job providing calibration data.
Training. The real work does not lie in building the model but in understanding transients. It is highly recommended to take a class in transient analysis or read at least one of the publications on transients to understand the theory and application of transient modeling. Bentley provides the training and technical support for users to get up to speed using Hammer. The results of transient analysis may be non-intuitive, and it is important to understand the basic principles behind transient waves and not treat the models as a black box.
Transient analysis is an important tool for water and wastewater systems and their consultants. It is much less expensive to identify and correct problems before they occur than to deal with their aftermath.
“Gannett Fleming routinely conducts transient analysis in support of design and troubleshooting operational issues,” said Mike Brown, vice president of Gannett Fleming. “We rely on software such as Bentley’s Hammer, which can efficiently and effectively solve the complex computations required to analyze pressure transients, to identify potential pressure surge issues and recommend appropriate surge control measures.”
Most of the labor involved in constructing a transient model should have been performed developing and calibrating the hydraulic model of the system. However, transient models require a small but very crucial amount of information regarding the cause of transients and their control.