Transient phenomena during the emptying process of water in pressurized pipelines

Tesis por compendio[ES] El análisis de los fenómenos transitorios durante las operaciones de llenado en conducciones de agua ha sido estudiado de manera detallada comparado con las maniobras de vaciado. En este último se encontró que no existen modelos matemáticos capaces de predecir el fenómeno. Esta investigación inicia estudiando el fenómeno transitorio generado durante el vaciado en una tubería simple, como paso previo para entender el comportamiento de las variables hidráulicas y termodinámicas durante el vaciado de agua en conducciones presurizadas de perfil irregular. Los análisis son realizados considerando dos situaciones: (i) la situación No. 1 corresponde al caso donde no hay válvulas de aire instaladas o cuando éstas han fallado por problemas operacionales o de mantenimiento, que representa la condición más desfavorable con respecto a la depresión máxima alcanzada; y (ii) la situación No. 2 corresponde al caso en donde se han instalado válvulas de aire en los puntos más elevados de la conducción para dar fiabilidad mediante el aire introducido al sistema previniendo de esta manera la depresión máxima. En esta tesis doctoral se ha desarrollado un modelo matemático para predecir el comportamiento de las operaciones de vaciado. El modelo matemático es propuesto para las dos situaciones mencionadas anteriormente. La fase líquida (agua) es simulada con un modelo de columna rígida, en el cual se desprecia la elasticidad del agua y de la tubería debido a que la elasticidad del aire es mucho mayor que estas; y la interfaz aire-agua es modelada con un modelo de flujo pistón, el cual asume que la columna de agua es perpendicular a la dirección principal del flujo. La fase de aire es modelada usando tres ecuaciones: (a) un modelo politrópico basado en el comportamiento energético, que considera la expansión de las bolsas de aire; (b) la formulación de las válvulas de aire para cuantificar la magnitud del caudal de aire admitido; y (c) la ecuación de continuidad de la bolsa de aire. Un sistema ordinario de ecuaciones diferenciales es solucionado utilizando la herramienta de Simulink de Matlab. El modelo matemático es validado empleando bancos experimentales localizados en los laboratorios de hidráulica de la Universitat Politècnica de València (Valencia, España) y en el Instituto Superior Técnico de la Universidad de Lisboa (Lisboa, Portugal). Los resultados muestran que el modelo matemático predice adecuadamente los datos experimentales de las presiones de las bolsas de aire, las velocidades del agua y las longitudes de las columnas de agua. Finalmente, el modelo matemático es aplicado a un caso de estudio para mostrar su aplicabilidad a situaciones prácticas, con el fin de poder ser empleado por ingenieros para estudiar el fenómeno en conducciones reales y así tomar decisiones acerca de la planificación de esta operación.[CA] L'anàlisi dels fenòmens transitoris durant les operacions d'ompliment en conduccions d'aigua ha sigut estudiat de manera detallada comparat amb les maniobres de buidatge. En este últim es va trobar que no hi ha models matemàtics capaços de predir el fenomen. Esta investigació inicia estudiant el fenomen transitori generat durant el buidatge en una canonada simple, com a pas previ per a entendre el comportament de les variables hidràuliques i termodinàmiques durant el buidatge d'aigua en conduccions pressuritzades de perfil irregular. Les anàlisis són realitzats considerant dos situacions: (i) la situació No. 1 correspon al cas on no hi ha vàlvules d'aire instal·lades o quan estes han fallat per problemes operacionals o de manteniment, que representa la condició més desfavorable respecte a la depressió màxima aconseguida; i (ii) la situació No. 2 correspon al cas on s'han instal·lat vàlvules d'aire en els punts més elevats de la conducció per a donar fiabilitat per mitjà de l'aire introduït al sistema prevenint d'esta manera la depressió màxima. En esta tesi doctoral s'ha desenrotllat un model matemàtic per a predir el comportament de les operacions de buidatge. El model matemàtic és proposat per a les dos situacions mencionades anteriorment. La fase líquida (aigua) és simulada amb un model de columna rígida, en el qual es desprecia l'elasticitat de l'aigua i de la canonada pel fet que l'elasticitat de l'aire és molt major que estes; i la interfície aire-aigua és modelada amb un model de flux pistó, el qual assumix que la columna d'aigua és perpendicular a la direcció principal del flux. La fase d'aire és modelada usant tres equacions: (a) un model politròpic basat en el comportament energètic, que considera l'expansió de les bosses d'aire; (b) la formulació de les vàlvules d'aire per a quantificar la magnitud del cabal d'aire admés; i (c) l'equació de continuïtat de la bossa d'aire. Un sistema ordinari d'equacions diferencials és solucionat utilitzant la ferramenta de Simulink de Matlab. El model matemàtic és validat emprant bancs experimentals localitzats en els laboratoris d'hidràulica de la Universitat Politècnica de València (València, Espanya) i en l'Institut Superior Tècnic de la Universitat de Lisboa (Lisboa, Portugal). Els resultats mostren que el model matemàtic prediu adequadament les dades experimentals de les pressions de les bosses d'aire, les velocitats de l'aigua i les longituds de les columnes d'aigua. Finalment, el model matemàtic és aplicat a un cas d'estudi per a mostrar la seua aplicabilitat a situacions pràctiques, a fi de poder ser empleat per enginyers per a estudiar el fenomen en conduccions reals i així prendre decisions sobre la planificació d'esta operació.[EN] The analysis of transient phenomena during water filling operations in pipelines of irregular profiles has been studied much more compared to emptying maneuvers. In the literature, there is a lack of knowledge about mathematical models of emptying operations. This research starts with the analysis of a transient phenomenon during emptying maneuvers in single pipelines, which is a previous stage to understand the emptying operation in pipelines of irregular profiles. Analysis are conducted under two typical situations: (i) one corresponding to either the situation where there are no air valves installed or when they have failed due to operational and maintenance problems which represents the worse condition due to causing the lowest troughs of subatmospheric pressure, and (ii) the other one corresponding to the situation where air valves have been installed at the highest point of hydraulic installations to give reliability by admitting air into the pipelines for preventing troughs of subatmospheric pressure. Particularly, this research developed a mathematical model to predict the behavior of the emptying operations. The mathematical model is proposed for the two aforementioned situations. The liquid phase (water) is simulated using a rigid water column model (RWCM), which neglects the pipe and water elasticity given that the elasticity of the entrapped air pockets is much higher than the one from the pipe and the water. The air-water interface is simulated with a piston flow model assuming that the water column is perpendicular with the main direction of the flow. Gas phase is modeled using three formulations: (a) a polytropic model based on its energetic behavior, which considers an expansion of air pockets; (b) an air valve characterization to quantify the magnitude of admitted air flow; and (c) a continuity equation of the air. An ordinary differential equations system is solved using the Simulink tool of Matlab. The proposed model has been validated using experimental facilities at the hydraulic laboratories of the Universitat Politècnica de València, Valencia, Spain, and the Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal. The results show how the mathematical model adequately predicts the experimental data, including the pressure oscillation patterns, the water velocities, and the lengths of the water columns. Finally, the mathematical model is applied to a case study to show a practical application, which can be used for engineers to study the phenomenon in real pipelines to make decisions about performing of the emptying operation.Coronado Hernández, ÓE. (2019). Transient phenomena during the emptying process of water in pressurized pipelines [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/120024TESISCompendi

Selection of hydrological probability distributions for extreme rainfall events in the regions of Colombia

Frequency analysis of extreme events is used to estimate the maximum rainfall associated with different return periods and is used in planning hydraulic structures. When carrying out this type of analysis in engineering projects, the hydrological distributions that best fit the trend of maximum 24 h rainfall data are unknown. This study collected maximum 24 h rainfall records from 362 stations distributed throughout Colombia, with the goal of guiding hydraulic planners by suggesting the probability distributions they should use before beginning their analysis. The generalized extreme value (GEV) probability distribution, using the weighted moments method, presented the best fits of frequency analysis of maximum daily precipitation for various return periods for selected rainfall stations in Colombia

Emptying operation of water supply networks

Recently, emptying processes have been studied in experimental facilities in pipelines, but there is a lack regarding applications in actual pipelines, which permits establishing the risk of collapse because of sub-atmospheric pressure occurrence. This research presents a mathematical model to simulate the emptying process of water supply networks, and the application to a water emptying pipeline with nominal diameter of 1000 mm and 578 m long which is located on the southern of Cartagena, Bolívar Deparment, Colombia. In the application, both pipes and the air valve data manufacturer were considered. The behavior of all hydraulic and thermodynamic variables is considered. Results show that is crucial to know sub-atmospheric pressure values to prevent the collapse of the pipeline. The application of the mathematical model confirms that the hydraulic system is well designed depending on air valve sizes and maneuvering of drain valve. © 2018 by the authors

Assessment of Steady and Unsteady Friction Models in the Draining Processes of Hydraulic Installations

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, this research improves the current mathematical model considering unsteady friction models. An experimental facility composed by a 4.36 m long methacrylate pipe 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 improve the results compared to steady friction models

Isohyetal maps of daily maximum rainfall for different return periods for the Colombian Caribbean Region

In Colombia, daily maximum multiannual series are one of the main inputs for design streamflow calculation, which requires performing a rainfall frequency analysis that involves several prior steps: (a) requesting the datasets, (b) waiting for the information, (c) reviewing the datasets received for missing or data different from the requested variable, and (d) requesting the information once again if it is not correct. To tackle these setbacks, 318 rain gauges located in the Colombian Caribbean region were used to first evaluate whether or not the Gumbel distribution was indeed the most suitable by performing frequency analyses using three different distributions (Gumbel, Generalized Extreme Value (GEV), and Log-Pearson 3 (LP3)); secondly, to generate daily maximum isohyetal maps for return periods of 2, 5, 10, 20, 25, 50, and 100 years; and, lastly, to evaluate which interpolation method (IDW, spline, and ordinary kriging) works best in areas with a varying density of data points. GEV was most suitable in 47.2% of the rain gauges, while Gumbel, in spite of being widely used in Colombia, was only suitable in 34.3% of the cases. Regarding the interpolation method, better isohyetals were obtained with the IDW method. In general, the areal maximum daily rainfall estimated showed good agreement when compared to the true values. © 2019 by the authors

Experimental and numerical analysis of a water emptying pipeline using different air valves

The emptying procedure is a common operation that engineers have to face in pipelines. This generates subatmospheric pressure caused by the expansion of air pockets, which can produce the collapse of the system depending on the conditions of the installation. To avoid this problem, engineers have to install air valves in pipelines. However, if air valves are not adequately designed, then the risk in pipelines continues. In this research, a mathematical model is developed to simulate an emptying process in pipelines that can be used for planning this type of operation. The one-dimensional proposed model analyzes the water phase propagation by a new rigid model and the air pockets effect using thermodynamic formulations. The proposed model is validated through measurements of the air pocket absolute pressure, the water velocity and the length of the emptying columns in an experimental facility. Results show that the proposed model can accurately predict the hydraulic characteristic variables. © 2017 by the authors

Effect of a commercial air valve on the rapid filling of a single pipeline: A numerical and experimental analysis

The filling process in water pipelines produces pressure surges caused by the compression of air pockets. In this sense, air valves should be appropriately designed to expel sufficient air to avoid pipeline failure. Recent studies concerning filling maneuvers have been addressed without considering the behavior of air valves. This work shows a mathematical model developed by the authors which is capable of simulating the main hydraulic and thermodynamic variables during filling operations under the effect of the air valve in a single pipeline, which is based on the mass oscillation equation, the air-water interface, the polytropic equation of the air phase, the air mass equation, and the air valve characterization. The mathematical model is validated in a 7.3-m-long pipeline with a 63-mm nominal diameter. A commercial air valve is positioned in the highest point of the hydraulic installation. Measurements indicate that the mathematical model can be used to simulate this phenomenon by providing good accuracy. © 2019 by the authors.This work is supported by Fundacao para a Ciencia e Tecnologia (FCT), Portugal (grant number PD/BD/114459/2016)

Effect of the non-stationarity of rainfall events on the design of hydraulic structures for runoff management and its applications to a case study at Gordo Creek watershed in Cartagena de Indias, Colombia

The 24-h maximum rainfall (P 24h-max ) observations recorded at the synoptic weather station of Rafael Núñez airport (Cartagena de Indias, Colombia) were analyzed, and a linear increasing trend over time was identified. It was also noticed that the occurrence of the rainfall value (over the years of record) for a return period of 10 years under stationary conditions (148.1 mm) increased, which evidences a change in rainfall patterns. In these cases, the typical stationary frequency analysis is unable to capture such a change. So, in order to further evaluate rainfall observations, frequency analyses of P 24h-max for stationary and non-stationary conditions were carried out (by using the generalized extreme value distribution). The goodness-of-fit test of Akaike Information Criterion (AIC), with values of 753.3721 and 747.5103 for stationary and non-stationary conditions respectively, showed that the latter best depicts the increasing rainfall pattern. Values of rainfall were later estimated for different return periods (2, 5, 10, 25, 50, and 100 years) to quantify the increase (non-stationary versus stationary condition), which ranged 6% to 12% for return periods from 5 years to 100 years, and 44% for a 2-year return period. The effect of these findings were tested in the Gordo creek watershed by first calculating the resulting direct surface runoff (DSR) for various return periods, and then modeling the hydraulic behavior of the downstream area (composed of a 178.5-m creek's reach and an existing box-culvert located at the watershed outlet) that undergoes flooding events every year. The resulting DSR increase oscillated between 8% and 19% for return periods from 5 to 100 years, and 77% for a 2-year return period when the non-stationary and stationary scenarios were compared. The results of this study shed light upon to the precautions that designers should take when selecting a design, based upon rainfall observed, as it may result in an underestimation of both the direct surface runoff and the size of the hydraulic structures for runoff and flood management throughout the city. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license

Analysis of sub-atmospheric pressures during emptying of an Irregular pipeline without an air valve using a 2D CFD model

Studying sub-atmospheric pressure patterns in emptying pipeline systems is crucial because these processes could cause collapses depending on the installation conditions (the underground pipe covering height, type, fill, and pipeline stiffness class). Pipeline studies have focused more on filling than on emptying processes. This study presents an analysis of the following variables: air pocket pressure, water velocity, and water column length during the emptying of an irregular pipeline without an air valve by two-dimensional computational fluid dynamics (2D CFD) model simulation using the software OpenFOAM. The mathematical model predicts the experimental values of the study variables. Water velocity vectors are also analysed within the experimental facility, assessing the sensitivity of the drain valve to different openings and changes in water column length during the hydraulic phenomenon

Filling and emptying manoeuvres in large pipes. Application to a cast iron pipeline DN400 located in Massamagrell, Valencia, Spain

Air pockets inside hydraulic installations during filling and emptying processes can generate pressure surges and negative pressure, respectively. Serious damages can be occurred in pipelines. In order to analyse hydraulic variables in filling and emptying operations, the selection of a mathematical model is chosen, which is suitable of simulating accurately the behaviour of both fluids (water and air) in pressurized water systems. The mathematical model proposed by the authors has been validated in small laboratory facilities. The aim of this work is to validate the mathematical model in current pipeline installations with large both nominal diameter and length. The pipeline is a nominal diameter DN400, and is located in Massamagrell, Valencia, Spain. The filling and emptying manoeuvres in the selected pipeline are performed by the Empresa Mixta Metropolitana S.A. (EMIMET). A good agreement is obtained when a comparison of absolute pressure and water flow is carried out between the mathematical model and the measurements.Debido a las bolsas de aire que hay en el interior de las tuberías durante los procesos de llenado y vaciado, se producen depresiones o sobrepresiones en el interior de las mismas, capaces de producir serios daños en las instalaciones. Para analizar todas las variables hidráulicas en las maniobras de llenado y vaciado, se opta por la aplicación de un modelo matemático, el cual es capaz de simular con exactitud el comportamiento de ambos fluidos, tanto la columna de agua como la bolsa de aire. El modelo propuesto por los autores ya ha sido validado en pequeñas instalaciones de laboratorio. En este trabajo, se pretende validar el modelo matemático en una instalación real de grandes dimensiones. Concretamente, se trata de una conducción de diámetro DN400, ubicada en Massamagrell (Valencia), donde se analizan las maniobras de llenado y de vaciado. Finalmente, se comparan los resultados que proporciona el modelo con las mediciones realizadas por la Empresa Mixta Metropolitana S.A. (EMIMET), obteniéndose una similitud muy aceptable