Hydraulic Transients in Water Distribution Systems

1. Introduction

Hydraulic transients in water distribution systems (WDS) is a field of research that has advanced greatly in the last few decades. However, there are still many aspects of hydraulic transients that require further research. Hydraulic transients result from sudden changes in flow conditions in pipeline systems because of planned or accidental maneuvers. Failures related to the effects of hydraulic transients can lead to major accidents and significant damage to pipeline systems. Nowadays, hydraulic transient analysis is a fundamental part of the design of water distribution systems. Of particular importance are the maximum and minimum pressures which are reached during hydraulic transients.

Hydraulic transient analysis is a complicated research topic. In recent years, considerable progress has been made due to developments in computer science, numerical models, and novel analysis techniques. This Special Issue focuses on all advancements related to hydraulic transients in water distribution systems, mathematical simulations, new analysis techniques, laboratory tests, protection elements and systems against water hammer, innovative strategies for controlling water hammer, hydraulic transients with trapped air, hydraulic transients with water column separation, the consequences and risks of hydraulic transients, etc. This Special Issue aims to collect novel research related to hydraulic transients in any subject.

Hydraulic transients with entrapped air are a particular case which are even more complex because there are two fluids (water and air) in two phases (liquid and gas). Many causes lead to the presence of air inside pipelines. They include filling and emptying the hydraulic system, transient interruptions in the supply, deliverance of dissolved air, vortex creation in the pump groups, depressurization occurring in the air valves due to transient operations, etc. High points along pipelines are likely locations for the accumulation of air pockets, which can experience pressure surges during a filling process.

The effects of entrapped air in water pipelines are generated with regard to two features: (i) air density is much lower than water density by a ratio of approximately 1:800, considering atmospheric conditions and a temperature of 20 °C; and (ii) the elasticity of air is much higher than the elasticity of water.

Whatever the origin of air in the pipes, its presence creates unwanted problems in the majority of cases [1]. One of the most important problems is overpressure generation. Other problems include section reduction inside the pipe, which may cause collapse; the additional generation of head losses, which cause an increase in the electricity consumption; reduction in the efficiency of the pumps; problems with noise and vibrations; failures in the measurement instrumentation; and interior corrosion due to the oxygen transported by the air.

In water distribution networks under pressure, the installation of air valves is necessary. Nevertheless, air valves are not always a guarantee of good system performance. They can cause some problematic circumstances. An inappropriate valve selection, an inaccurate performance measurement, or inadequate maintenance may cause significant system problems [2].

Filling and emptying processes are common maneuvers while operating, controlling, and managing water pipeline systems. Currently, these operations are executed following recommendations from technical manuals and pipe manufacturers. However, these recommendations demonstrate a lack of understanding about the behavior of these processes. The application of mathematical models considering transient flows with entrapped air pockets is necessary because a rapid filling operation can cause pressure surges due to air pocket compressions [3], whereas an uncontrolled emptying operation can generate troughs of sub-atmospheric pressure caused by air pocket expansion [4].

Hydraulic transients (with and without air) is a subject on which much remains to be investigated. This Special Issue includes various articles dealing with hydraulic transients, filling and emptying processes, air valves, etc.

2. Articles

In total, six papers were published in this Special Issue. The article titles, authors and keywords are summarized in Table 1.

Table 1. Summary of the papers published in the Special Issue entitled “Hydraulic Transients in Water Distribution Systems” for the journal Water.

Title	Authors	Keywords
Assessment of Steady and Unsteady Friction Models in the Draining Processes of Hydraulic Installations	Óscar E. Coronado-Hernández Ivan Derpich Vicente S. Fuertes-Miquel Jairo R. Coronado-Hernández	air pocket draining process friction factor transient flow unsteady
Analysis of Sub-Atmospheric Pressures during Emptying of an Irregular Pipeline without an Air Valve Using a 2D CFD Model	Aris D. Hurtado-Misal Daniela Hernández- Sanjuan Óscar E. Coronado-Hernández Héctor Espinoza-Román Vicente S. Fuertes-Miquel	sub-atmospheric pressure OpenFOAM emptying process air pocket irregular pipeline CFD
2D CFD Modeling of Rapid Water Filling with Air Valves Using OpenFOAM	Andres M. Aguirre-Mendoza Sebastián Oyuela Héctor G. Espinoza-Román Óscar E. Coronado-Hernández Vicente S. Fuertes-Miquel Duban A. Paternina-Verona	computational fluid dynamics pipeline filling transient flow OpenFOAM air valve
Effects of Orifice Sizes for Uncontrolled Filling Processes in Water Pipelines	Andres M. Aguirre-Mendoza Duban A. Paternina-Verona Sebastián Oyuela Óscar E. Coronado-Hernández Mohsen Besharat Vicente S. Fuertes-Miquel Pedro L. Iglesias-Rey Helena M. Ramos	air valves computational fluid dynamics pipeline filling hydraulic transients
Concerning Dynamic Effects in Pipe Systems with Two-Phase Flows: Pressure Surges, Cavitation, and Ventilation	Helena M. Ramos Vicente S. Fuertes-Miquel Elias Tasca Óscar E. Coronado-Hernández Mohsen Besharat Ling Zhou Bryan Karney	pipelines entrapped air two-phase flow air valves hydraulic transients cavitation
The Crucial Importance of Air Valve Characterization to the Transient Response of Pipeline Systems	Elias Tasca Bryan Karney Vicente S. Fuertes-Miquel José Gilberto Dalfré Filho Edevar Luzivotto, Jr.	air valve characteristic curve air pocket water hammer pipeline water supply

The risks associated with unsteady two-phase flows in pressurized pipe systems must be considered in both system design and operation. To this end, Ramos et al. [5] present experimental tests and numerical analyses that highlight key aspects of unsteady two-phase flows in water pipelines. The essential dynamics of air–water interactions in unvented lines are first considered, followed by a summary of how system dynamics change when air venting is provided. System behavior during unsteady two-phase flows is shown to be complex, counter-intuitive, and surprising. The role of air valves as protection devices is considered, as is the reasonableness of the usual assumptions regarding air valve behavior. The paper then numerically clarifies the relevance of cavitation and air valve performance to both the predicted air exchanges through any installed air valves and their role in modifying system behavior during unsteady flows.

The study of draining processes without admitting air has been conducted using only steady friction formulations in the implementation of governing equations. However, this hydraulic event involves transitions from laminar to turbulent flow, and vice versa, because of the changes in water velocity. In this sense, Coronado-Hernández et al. [6] improved the current mathematical model considering unsteady friction models. An experimental facility was configured, and measurements of air pocket pressure oscillations were recorded. The mathematical model was performed using steady and unsteady friction models. Comparisons between measured and computed air pocket pressure patterns indicated that unsteady friction models slightly improved the results compared with steady friction models.

The rapid filling process in pressurized pipelines has extensively been studied using mathematical models. On the other hand, the application of computational fluid dynamics models has emerged during the last decade, which consider the development of CFD models that simulate the filling of pipes with entrapped air, and without air expulsion. Currently, studies of CFD models representing rapid filling in pipes with entrapped air and with air expulsion are scarce in the literature. Aguirre-Mendoza et al. [7] developed a two-dimensional model using OpenFOAM software to evaluate the hydraulic performance of the rapid filling process in a hydraulic installation with an air valve, considering different air pocket sizes and pressure impulsion by means of a hydro-pneumatic tank. The two-dimensional CFD model captures the pressure evolution in the air pocket very well with respect to experimental and mathematical model results, and produces improved results with respect to existing mathematical models.

Studying sub-atmospheric pressure patterns in emptying pipeline systems is crucial because these processes could cause collapses depending on the installation conditions. Pipeline studies have focused more on filling than on emptying processes. Hurtado-Misal et al. [8] present an analysis of the hydraulic transient during the emptying of an irregular pipeline without an air valve by two-dimensional computational fluid dynamics model simulation using OpenFOAM software. The mathematical model predicts the experimental results. Water velocity vectors are also analyzed within the experimental facility, assessing the sensitivity of the drain valve to different openings and changes in water column length during the hydraulic phenomenon.

The sizing of air valves during the air expulsion phase in rapid filling processes is crucial for design purposes. Mathematical models have been developed to simulate the behavior of air valves during filling processes for air expulsion. The effects of air valves under scenarios of controlled filling processes have been studied by various authors. However, the analysis of uncontrolled filling processes using air valves has not yet been considered. In this scenario, water columns reach high velocities, causing part of them to close air valves, which generates an additional peak in air pocket pressure patterns. Aguirre-Mendoza et al. [9] developed a two-dimensional computational fluid dynamics model in OpenFOAM software to simulate this phenomenon.

Obviously, air valves are often crucial components in an air management strategy for pressurized water systems. However, the reliability of characteristic curves of air valves found in product catalogs is quite variable. Tasca et al. [10] evaluated the consistency of a selection of product curves to basic airflow principles. Several recurring issues are identified: catalogs that present identical curves for admission and expulsion (they are, in fact, quite distinct); admission curves that are inconsistent with the isentropic inflow model; inflow (admission) curves which are actually consistent with the shape of the isentropic outflow model; limited validity curves that encompass only part of the subsonic flow regimen; and unclear or unstated specifications regarding the conditions under which the characterization tests are performed or their results displayed. To examine the significance of these representational issues related to air valve capacity on system behavior, this paper presents a case study involving the simulated transient response arising from a pump trip at the upstream end of a rising water line having a distinct high point fitted with an air valve. It was found that employing inaccurate air valve characteristics in a transient simulation may potentially result in appreciable or even dangerous simulation errors.

3. Conclusions

This Special Issue highlights and discusses topics related to hydraulic transients with entrapped air. One paper deals with dynamics effects in pipe systems with two-phase flows (pressure surges, cavitation, and ventilation). Another article evaluates steady and unsteady friction models in the draining processes of hydraulic installations. Three papers focus on the two-dimensional computational fluid dynamics models to simulate hydraulic transients during filling and emptying processes in water pipelines. The last article deals with the crucial importance of air valve characterization to the transient response of pipeline systems.

In any case, it is important to point out that there are still many aspects of hydraulic transients that require further research.