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EPANET Program Methodology

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EPANET Program Methodology CHAPTER 6 EPANET Program Methodology Continued global growth has placed increasing demand upon existing water distribution systems. This growth has fueled the increasing need to analyze existing and design new water distribution systems. In addition, recent concern and awareness about the safety of our drinking water has raised other questions about water quality of existing and proposed municipal water distribution systems. MIKE NET can be used to simulate existing and proposed water distribution systems. It can analyze the performance of the system and can be used to design system components to meet distribution requirements. In addition, it can perform water quality modeling, determining the age of water, performing source tracking, finding the fate of a dissolved substance, or determining substance growth or decay. This chapter describes the EPANET analysis program that MIKE NET uses to perform its water distribution network analysis. The program’s theoretical basis, its capabilities, and ways of utilizing the program for analyzing water distribution networks is discussed. 6.1 Overview The EPANET computer model used for water distribution network analysis is composed of two parts: (1) the input data file and (2) the EPANET computer program. The data file defines the characteristics of the pipes, the nodes (ends of the pipe), and the control components (such as pumps and valves) in the pipe network. The computer program solves the nonlinear energy equations and linear mass equations for pressures at nodes and flowrates in pipes. EPANET Input Data File The EPANET input data file, created automatically by MIKE NET, includes descriptions of the physical characteristics of pipes and nodes, and the connectivity of the pipes in a pipe network system. The user can graphically layout the water distribution network, if desired. Values for the pipe network parameters are entered through easy-to-use dialog boxes. MIKE NET then creates the EPANET input data file in the format required to run the analysis. The pipe parameters include the length, inside diameter, minor loss coefficient, and roughness coefficient of the pipe. Each pipe has a defined positive flow direction and two nodes. The parameters of nodes consist of the water demand or supply, elevation, and pressure or hydraulic grade line. The hydraulic grade line (HGL) is the summation of node elevation and pressure head at the node. The control components, which usually are installed on pipes, include control valves and booster pumps. They are also part of the input data file. 6-1 MIKE NET EPANET Computer Program The EPANET computer program was developed by the U.S. EPA (Environmental Protection Agency). The program computes the flowrates in the pipes and then HGL at the nodes. The calculation of flowrates involves several iterations because the mass and energy equations are nonlinear. The number of iterations depends on the system of network equations and the user-specified accuracy. A satisfactory solution of the flowrates must meet the specified accuracy, the law of conservation of mass and energy in the water distribution system, and any other requirements imposed by the user. The calculation of HGL requires no iteration because the network equations are linear. Once the flowrate analysis is complete, the water quality computations are then performed. 6.1.1 History Pipe network analysis of water distribution systems has evolved from a time- consuming process done infrequently to a quick and easy process done regularly on systems of all sizes. Pipe network analysis initially started early in 1940. Years later, two network analysis programs were introduced by Shamir and Howard (1968) and Epp and Fowler (1970). Both programs used the Newton-Raphson method to linearize the nonlinear mass and energy equations. The major differences between these two programs are: 1. The Shamir-Howard program is based on node-oriented equations, while the Epp-Fowler program is based on loop-oriented equations. 2. The Shamir-Howard program solves for pressure, demand, and the parameters of pipes and nodes, while the Epp-Fowler program solves only for pressures and flowrates. Since then, several programs have been developed, based on improved computing techniques as well as advances in computer hardware. Recently, several computer programs running on personal computers, such as EPANET, UNWB-LOOP, WADISO, U of K KYPIPE, and WATER have been created and made available. Of the four programs, only EPANET and U of K KYPIPE can perform dynamic simulation over a extended period of time and only EPANET can perform water quality analysis. In addition, WADISO can perform optimization analysis. 6.1.2 Analysis Methods Three types of analysis may be conducted using MIKE NET: steady state (static) analysis, extended period (dynamic) analysis, and water quality analysis. Steady state analysis is used to compute the pipe flowrates and the node HGL in a steady state pipe network system. Extended period analysis simulates the continuous flowrate and pressure changes over a period of time. Water quality analysis is used to compute the age of water, perform source tracking, calculate the fate of a dissolved substance, or determine the growth or decay of a substance. Each of these analysis types are discussed in detail in the following sections. 6-2 EPANET Program Methodology Steady State Hydraulics The calculation of flowrates and pressures for a steady state pipe network system is called a steady state analysis. This analysis computes the pipe flowrates and the node hydraulic grade line elevations (HGL) so that the conservation of energy and mass are satisfied. The pipe network system can include pumps, check valves, and various types of control valves. Extended Period Hydraulics An extended period (dynamic) analysis is used to analyze a pipe network for an extended period of time. The total simulation time is usually divided into several time steps. At each time step an analysis is conducted for the pipe network based on the current network parameters and the pipe flowrates calculated from the previous time step. In an extended period simulation, storage tanks and hydraulic switches are often present as part of the water distribution system. The system operating parameters at each time step depend on external conditions and the pipe flowrates from the previous time step. External conditions are operating parameters controlled by factors outside the system, such as external demand or pump power. The previous time step flowrates are also used to predict the storage tank HGL for the current time step. Water Quality Analysis A water quality analysis can be used by operations, planning, and engineering departments to study the flow and distribution of water. Source tracking, travel time determination, water age, and concentration levels of chemical constituents and contaminants are the primary concerns addressed by water quality models. In addition, tracking paths of flow and distribution provide the engineer insight to the origin and amount of water supplied to a particular location, as well as concentration levels of chemical constituents and contaminants. Water quality models can also be used to study water retention time for reservoir operations, pipeline travel times, and the percentage of water supplied to a location from multiple sources (i.e., treatment plants, wells, and reservoirs). Additionally, water quality models can be used to develop a hydrant flushing program to reduce water stagnation at dead ends within the pipe network. And, site sampling locations, future rechlorination facility locations, cross-connection locations, and reservoir operating strategies can be designed and analyzed. 6.1.3 Applications of MIKE NET Water distribution analysis software, such as MIKE NET, is typically used for three broad areas of analysis. These areas of analysis are generally referred to as planning, design, and operation applications (AWWA Manual M32, 1989). Some examples of these applications include (1) analysis and design of booster pumps and storage tanks for municipal or rural water distribution systems, and (2) analysis and design of chlorination satellite stations. 6-3 MIKE NET Planning MIKE NET can be used in the planning of pipe network systems to meet forecasted demands of the next 10 years or 20 years. For example, the program can be used to develop long term capital-improvement plans for the existing pipe network system. These plans can include staging, sizing, and locating future pipe network and water chlorination facilities. The software can also be used in the development of a main rehabilitation plan or a system-improvement plan. And, a network analysis can provide suggestions and recommendations to prepare for the occurrence of any unusual events. Design MIKE NET can be used to design a new pipe network system or improve on the existing pipe network system. For example, the analysis conducted using MIKE NET could help users in selecting and sizing pipe network components, such as pipes, booster pumps, and pressure regulating valves. As a part of the analysis, the performance of the pipe network system can be analyzed to verify that the system satisfies fire-flow demand requirements. Operation The operating status of a water distribution system (e.g., the pipe flowrates and junction node pressures) can be determined by MIKE NET. The analysis can then be used to develop operational strategies based on the guidelines for maximum use of available water and efficient management of electrical energy. MIKE NET can also be used for system troubleshooting, such as finding the location of a pipe break. 6.1.4 Skeletonization In the past, water distribution models have not included all of the pipes contained in the network system due to the fact that the numerical modeling schemes used and the memory requirements required could handle only a limited number of pipes and nodes. These limitations required skeletonization of the pipe network system, where only a subset of all the pipes contained within the network system was defined. However, these limitations have eroded over the past years as enhanced programming methods and increased computer hardware capabilities have come into being.
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