Comparison of various phased approaches for the constrained minimum-cost design of water distribution networks

In most cases, water system design is based on a demand forecast at the end of some planning horizon based on the final configuration of the system at that time. This design approach (aimed at designing all the network at a time) is incompliant with its actual development, which instead takes place in phases. As a consequence, in order to follow the network demand and layout growth in time, practitioners prefer to sub-divide the whole construction life into various time phases thus including the different phases of construction in the network design. This work is aimed at analyzing and comparing three different phased approaches for constrained minimum-cost design of water distribution networks: the single-phase design with demand feedback, the multi-phase design without demand feedback and the multi-phase design with demand feedback. The difference between the single-phase design and the multi-phase design lies in the fact that whereas the former entails optimizing a single construction phase at a time, i.e. the current construction phase, the latter is based on the phasing of construction and then is aimed at optimizing the current construction phase and all the subsequent phases, included inside a certain temporal horizon, simultaneously. The demand feedback is here used as a pragmatic tool for updating the forecast at some specific time instant of the future demand growth: such an update is performed by setting the future demand growth equal to that really observed in the previous time phase. Alternatively, the predicted demand growth rate at the generic time instant can be kept equal to the value assumed at the time instant when the generic node appears, without taking account of the demand variation really observed in time in the node (absence of demand feedback). Applications to a real case study show that the multi-phase design with the demand feedback is the most reliable because it makes it possible to reduce the overall construction costs while attenuating the occurrence of pressure deficits in the various construction phases of the network. Optimal design for a single phase will virtually guarantee a sub-optimal solution over the long run

Accounting for phasing of construction within the design of water distribution networks

The traditional optimization approach for water distribution mains is that of considering a single design scenario with prefixed nodal demands representing the peak values at the end of the life cycle of the construction. Instead, this paper presents a different approach for the design of water distribution mains aimed at considering the phasing of construction. It makes it possible to identify, on prefixed time steps or intervals (for instance 25 years), the upgrade of the construction rendering the network able to satisfy, during the expected life of the system, growing nodal demands related to the increment in the population served. In order to show the benefits of this approach in comparison to using a single design flow, an optimization methodology, aimed at introducing new pipes in the network as needed at each time step, was set-up and applied to a simple case-study, where two different scenarios were considered concerning the growth of the network. Results showed that this approach is able to yield better results when compared with the single flow design, because it enables short term construction upgrades to be performed while keeping a vision of the expected long term network growth

Direct computation of variable speed pumps for water distribution system analysis

The paper introduces the Variable Speed Pump coefficient direct computation that has been implemented in the latest water distribution modelling software. The water distribution simulation packages are often based upon EPANET hydraulic solver that has been developed by employing the Global Gradient Algorithm (GGA). Originally, the computation of the Variable Speed Pumps (VSP) coefficient was an iterative procedure, which could lead to convergence problem. In order to overcome this problem, a direct com-putation algorithm of the VSP coefficient was incorporated into the GGA, giving rise to a complex non-symmetric problem, which was solved by partitioning the original system matrix to lead to the solution of a large symmetrical problem, which size remains more or less the number of demand driven nodes, plus a small non-symmetrical problem, which size equals the number of VSPs. The results of the approach were tested on the benchmark examples. After exhaustive debugging and testing, the algorithm was finally introduced in WaterCAD and WaterGEMS for the purposes of water distribution system analysis

Automatic Parameter Estimation Extension for Variable Speed Pumps

A method for estimating the relative speed factor parameter for a variable speed pump in a hydraulic network sufficient to maintain a fixed pressure at a control node is provided. A desired operating characteristic is determined and this is inserted into a matrix of equations describing the hydraulic system. The largely symmetrical matrix includes certain aspects of the system representing the variable speed pumps which are non-symmetrical. Non-symmetrical portions of the matrix are separated out and solved using an LU factorization technique. Non-sparse, non symmetric matrices are generated and the difference in head correction is solved for to compute the updated nodal head vector and ultimately determine the variable pump speed parameter. The invention allows estimation of the variable speed factor for a variable speed pump drive sufficient to maintain a fixed pressure at a control node. The system fully integrates variable pump speed operation with the status and control capabilities of known hydraulic network solvers

Multiobjective evolutionary algorithms applied to the rehabilitation of a water distribution system: a comparative study

Abstract. Recognising the multiobjective nature of the decision process for rehabilitation of water supply distribution systems, this paper presents a comparative study of two multiobjective evolutionary methods, namely, multiobjective genetic algorithm (MOGA) and strength Pareto evolutionary algorithm (SPEA). The analyses were conducted on a simple hypothetical network for cost minimisation and minimum pressure requirement, treated as a two-objective problem. For the application example studied, SPEA outperforms MOGA in terms of the Pareto fronts produced and processing time required.