1,720,995 research outputs found
An overview of water demand: Volume vs. Pressure based demands
No full textWhile there is no question about the fact that demands in a water distribution system can be dependent on pressure, the importance of considering pressure based demands in modeling has been widely discussed. This discussion often centres on whether demands are pressure or volume based. This, however, is the wrong question. Demands are BOTH pressure and volume based and demands are really a sum of several difference components of demand including volume based demand, controlled pressure based demand, uncontrolled pressure based demand and leakage. Each of these components is described in the paper with some special consideration of the case where pressure approaches zero and the implications of considering these components
Hydraulic and economic analysis of real time control
No full textThe aim of this paper is to analyze the hydraulic and economic effectiveness of the real-Time control (RTC) of pressure reducing valves (PRVs), for the purpose of managing pressure to reduce leakage and pipe bursts. In their conventional use, a PRV automatically reduces a higher inlet pressure to a lower downstream set-point pressure (steady-state), regardless of changing flow rate or varying inlet pressure. In local RTC, instead, the set-point can be adjusted in real time to accommodate the variations in the water discharge though the valve. In detail, local RTC is performed thanks to a programmable logic controller (PLC), which can use the water discharge measurement received from an electromagnetic flowmeter in proximity to the valve, to calculate a new valve setting. The initial applications of the work will concern the extended period simulation of a pressure zone, in which the locally controlled PRVs is installed in the feed pipe that connects the source to the pressure zone. In this case study, nodal demands are reconstructed stochastically through the bottom-up approach and the relationship between downstream set-point pressure and water discharge through the device is derived in such a way as to meet users' pressure requirements at all nodes. The subsequent sections describe the assessment of the total cost of the controlled system, including the installation cost of the control device, the flow-dependent operation and maintenance (O&M) cost, and the pipe burst repair cost over the planning horizon. The total cost of the local RTC will be compared with that of three other scenarios: 1) no control, 2) conventional PRV and 3) remote RTC under various conditions of system size, demand pattern and leakage. The analysis shows that conventional PRV's are most appropriate for small pressure zones, with limited leakage and low-cost water while real time control is needed in larger zones, with high leakage and high-cost water
Demand Components in Water Distribution Network Analysis
No full textSolving water distribution network hydraulics depends to a great extent on demand representation in the related simulation models. The classical approach of simulation models for water distribution networks (WDNs) is described as demand-driven. The demands are fixed a priori in the model as an assumption or from field observations. Recently a more realist approach to predict the hydraulic system behavior, described as head/pressure-driven, better accounts for the fact that the demands depend in some ways on head status of the network. Thus, this paper presents a comprehensive view of demands in the enhanced WDN simulation models including considerations of human-based, volume-based, uncontrolled orifice-based and leakage-based demands as distinct types of network outflows. The paper proposes and discusses the representation of each type of demand in a comprehensive framework which is consistent with the hydraulic principles and the specific working condition
Taking account of uncertainty in demand growth when phasing the construction of a water distribution network
No full textAs is well known, water systems grow gradually over long periods of time, and the life of piping tends to be much longer than the planning horizon used for pipe sizing. Furthermore, the uncertainty about future demands grows with the length of the time horizon. The design of water-distribution systems should therefore be performed in phases, to follow the gradual network growth, and taking account of the uncertainty connected with demand growth. The design approach proposed in this paper to consider these aspects is able to identify, on prefixed time steps or intervals, the necessary upgrades of the construction where each upgrade consists of installing pipes in new sites or in parallel to pipes that already exist, in order to render the network able to satisfy user demand with acceptable service pressure over the different phases of its life. Uncertainty in demand growth is considered by expressing the growth rate by means of a discrete random variable with assigned probability mass function. Optimization of phasing of construction is then performed by considering two objective functions: present-worth cost of the construction (to be minimized), and minimum-pressure surplus over time (to be maximized), which is represented as a discrete random variable with a derived probability distribution as a consequence of the assumption made on the water demand, which randomly grows from phase to phase of the construction. Within this framework, a specific criterion to rank discrete random variables is presented here. The application of the methodology to a case study shows that optimizing phasing of construction while accounting for uncertainty in demand growth leads to the network being sized more conservatively, so that the network construction obtained turns out to be more flexible to adapt itself to various conditions of demand growth over time
Comparison of various phased approaches for the constrained minimum-cost design of water distribution networks
No full textIn 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 multiphase 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
Some explicit formulations of Colebrook–White friction factor considering accuracy vs. computational speed
No full textThe Colebrook–White formulation of the friction factor is implicit and requires some iterations to be solved given a correct initial search value and a target accuracy. Some new explicit formulations to efficiently calculate the Colebrook–White friction factor are presented herein. The aim of this investigation is twofold: (i) to preserve the accuracy of estimates while (ii) reducing the computational burden (i.e. speed). On the one hand, the computational effectiveness is important when the intensive calculation of the friction factor (e.g. large-size water distribution networks (WDN) in optimization problems, flooding software, etc.) is required together with its derivative. On the other hand, the accuracy of the developing formula should be realistically chosen considering the remaining uncertainties surrounding the model where the friction factor is used. In the following, three strategies for friction factor mapping are proposed which were achieved by using the Evolutionary Polynomial Regression (EPR). The result is the encapsulation of some pieces of the friction factor implicit formulae within pseudo-polynomial structures.</jats:p
Real time control of water distribution networks: A state-of-the-art review
No full textThis paper presents a review of the current state of the art of real time control (RTC) of water distribution networks (WDNs). After proving the basic concept and terms of RTC and presenting sensors, regulation devices and controllers typically used in WDNs, the paper goes on by describing the most frequent control objectives, which mainly include service pressure regulation, control of tank filling and energy production in each WDN district. Various control methodologies recently proposed in the scientific literature are presented and discussed, along with experimental and numerical results achieved. Also, aspects related to the cost-effectiveness of RTC are critically analyzed. The paper ends by giving an outlook into potential future developments in the area of RTC for WDNs
Modulating Nodal Outflows to Guarantee Sufficient Disinfectant Residuals in Water Distribution Networks
No full textThis paper proposes the modulation of nodal outflows in water distribution networks (WDNs) to solve the problem of low disinfectant concentrations at critical dead-end nodes, in which low flow velocities and long residence times cause excessive disinfectant decay. The slight increase in nodal outflows at these sites, which can be obtained through the opening of a blowoff at the hydrant site, can help to address this problem with no need to increase disinfectant doses at the source(s) or of install additional disinfectant booster stations. The methodology is based on the combined use of optimization and flow routing/water quality modeling of WDNs. The concentration of disinfectant at the source(s) and the values of nodal emitter coefficients at the critical dead-end nodes are the decisional variables to be optimized. Two objective functions are considered in the optimization: the total volume of water delivered in the network (inclusive of supply, leakage, and additional nodal outflow considered for fixing disinfectant residuals); and the total mass of disinfectant injected into the network. The effectiveness of the methodology was proven on a real WDN, yielding insight into the economic feasibility of the solution
Accounting for phasing of construction within the design of water distribution networks
No full textThe 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 25years), 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. 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
Modulating Nodal Outflows to Guarantee Sufficient Disinfectant Residuals in Water Distribution Networks
No full textThis paper proposes the modulation of nodal outflows in water distribution networks (WDNs) to solve the problem of low disinfectant concentrations at critical dead-end nodes, in which low flow velocities and long residence times cause excessive disinfectant decay. The slight increase in nodal outflows at these sites, which can be obtained through the opening of a blowoff at the hydrant site, can help to address this problem with no need to increase disinfectant doses at the source(s) or of install additional disinfectant booster stations. The methodology is based on the combined use of optimization and flow routing/water quality modeling of WDNs. The concentration of disinfectant at the source(s) and the values of nodal emitter coefficients at the critical dead-end nodes are the decisional variables to be optimized. Two objective functions are considered in the optimization: The total volume of water delivered in the network (inclusive of supply, leakage, and additional nodal outflow considered for fixing disinfectant residuals); and the total mass of disinfectant injected into the network. The effectiveness of the methodology was proven on a real WDN, yielding insight into the economic feasibility of the solution
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