Pressure-deficient Conditions Research Articles

Uncertainty quantification of pressure-driven analysis for water distribution network modeling

The hydraulic analysis of water distribution networks (WDNs) is divided into two approaches: namely, a demand-driven analysis (DDA) and a pressure-driven analysis (PDA). In the DDA, the basic assumption is that the nodal demand is fully supplied irrespective of the nodal pressure, which is mainly suitable for normal operating conditions. However, in abnormal conditions, such as pipe failures or unexpected increase in demand, the DDA approach may cause unrealistic results, such as negative pressure. To address the shortcomings of DDA, PDA has been considered in a number of studies. For PDA, however, the head-outflow relation (HOR) should be given, which is known to contain a high degree of uncertainty. Here, the DDA-based simulator, EPANET2 was modified to develop a PDA model simulating pressure deficient conditions and a Monte Carlo simulation (MCS) was performed to consider the quantitative uncertainty in HOR. The developed PDA model was applied to two networks (a well-known benchmark system and a real-life WDN) and the results showed that the proposed model is superior to other reported models when dealing with negative pressure under abnormal conditions. In addition, the MCS-based sensitivity analysis presents the ranges of pressure and available discharge, quantifying service reliability of water networks.

Optimal sensor locations for contamination detection in pressure-deficient water distribution networks using genetic algorithm

A new sensor placement problem is formulated to cover two objectives of: (1) assuring quality of water delivered to consumers; and (2) detection of any contamination event at the earliest so as to minimize its consequences, through maximization of: (1) demand coverage; and (2) time-constrained detection likelihood for pressure deficient networks. The network may become pressure-deficient owing to continued use of water distribution network beyond its design life. The two objectives are combined using weights. Genetic algorithm is used to obtain optimum sensor locations. The methodology is applied to a pressure- deficient network in the Dharampeth zone of Nagpur city (India). The pressure- dependent analysis is carried out using WaterGem v8i to simulate the system hydraulics. Performance objectives are evaluated considering the availability of flows at nodes and velocity of flow in pipes under pressure-deficient conditions. Comparison of optimal sensor network design is carried out with that obtained by demand-dependent analysis.

Extended Period Simulation of Pressure-Deficient Networks Using Pressure Reducing Valves

Generally, the water distribution networks are designed for peak demands so that under normal operating conditions pressure is adequate to meet the nodal demand however, due to this, the pressure remains excessive when the consumption is low. This leads to huge water leakages and pipe bursts due to excess available pressure. These excess of pressure can be reduced by operating pressure reducing valves. Moreover, the pressure deficient conditions of water distribution network depends on many factors such as excess demand (i.e., fire fighting demand), location and elevation of nodes, location and pressure setting values of pressure reducing valve, ageing of pipes (i.e., increase the pipe roughness) etc. In exceptional situations, pressure deficient condition also may occur when there is a fire demand while pressure reducing valves are in operating condition. The available literature of the pressure deficient condition as well as optimal location, number and their pressure setting values of pressure reducing valves are analysed by mathematical programming or optimization methods. Normally, it is cumbersome to the field engineers to use the any toolkit utility functions. Hence in this study, the pressure deficient condition is analysed via the method of modified pressure-deficient network algorithm without using EPANET toolkit functions which are coupled with pressure reducing valve operation. A benchmark problem is analysed and compared for the proposed method. Further, a real water distribution networks is analysed introducing fire demand at junction/distribution node using fixed and diurnal nodal demand pattern coupled with and without pressure reducing valve operation in a single hydraulic simulation.

Simulation of Water Distribution Network under Pressure-Deficient Condition

Pressure deficient condition occurs in the water distribution network (WDN) when the nodal demands are in excess of the design discharge as in the case of fire demand, pump failure, pipe breaks, valve failure etc. It causes either no-flow or partial-flow depending upon the available pressure head at the nodes. To evaluate the nodal flows in such condition, node flow analysis (NFA) gives reasonable results in comparison to demand-driven analysis (DDA) and head-dependent analysis (HDA). The NFA works on the predefined pressure-discharge relationship to evaluate the nodal flows. However, this approach requires human intervention and hence cannot be applied to large WDN. Recently, modified pressure-deficient network algorithm (M-PDNA) has been developed by Babu and Mohan (2012) for pressure-deficient analysis with EPANET toolkit. However, it requires modification of the source code of EPANET. In this study a relationship with the M-PDNA and node flow analysis (Gupta and Bhave 1996) has been investigated and it is found that M-PDNA is the simplified version of NFA. Further, the working principle of M-PDNA has been investigated with suitable examples of Babu and Mohan (2012). The theoretical basis of M-PDNA has not been investigated in terms of head-discharge relationship. Herein, a head-discharge relationship based on the working principal of M-PDNA is proposed. Some of the toolkits are also readily available to modify demand driven solver of EPANET 2 to suit for the pressure-driven analysis and then it can be used for analysing pressure deficient network. Also in this study, a modification in M-PDNA approach is proposed which does not require the use of EPANET toolkit which is found to be capable of simulating both pressure-sufficient and pressure-deficient conditions in a single hydraulic simulation. Using the proposed approach, pressure-deficient condition is analysed with constant and variable demand pattern.

WQNetXL: A MS-excel Water Quality System Tool for WDNs

WQNetXL is a system tool for water quality analyses of water distribution networks (WDNs). WQNetXL is a collection of MS-Excel add-ins. The user-friendly environment allows just in time research transfer to students and technicians, sometimes anticipating research publications. Being water quality analyses based on network hydraulics, WQNetXL embeds the recent advances in hydraulic analyses already implemented in WDNetXL (the system tool for analysis, planning and management of WDNs). Thus, WQNetXL performs water quality also using the advances related to modelling of e.g.: background leakages; pressure-deficient conditions; pressure/flow controlled devices; private tanks; multi-floor orifices; etc.

A comparative study between the modified and available demand driven based models for head driven analysis of water distribution networks

Most of the currently available models for Water Distribution Network (WDN) hydraulic analysis assume that the nodal demands are independent of nodal heads and fully supplied. However, in practice, discharge of any node is proportional to its head. Considering a head–discharge relationship for each node in the Head Driven Analysis (HDA) method, provides the ability to model WDNs more accurately and realistically. An improved head–discharge relationship is developed and presented in this paper which takes into account two types of demands, volumetric demand and head dependent demand, separately. Most of the HDA-based models use the Newton–Raphson method for simulation. But considering the advantages of the gradient method, e.g. no need for any initial values for nodal heads and robust performance, it is applied in this research through a modified model capable of calculating leakage. The proposed modified model is evaluated using a sample network. Performance of this model is studied in comparison with the results of other available head driven EPANET-based models. Results confirm the ability of the modified model to perform more accurate hydraulic analysis of WDNs and leakage calculation, especially under pressure deficient conditions.

Noniterative Implementation of Pressure-Dependent Demands Using the Hydraulic Analysis Engine of EPANET 2

To analyze water distribution networks under pressure-deficient conditions, most of the available hydraulic simulators, including EPANET 2, must be either modified by embedding pressure-dependent demands in the governing network equations or run repeatedly with successive adjustments made to specific parameters until a sufficient hydraulic consistency is obtained. This paper presents and discusses a simple technique that implements the square root relationship between the nodal demand and the nodal pressure using EPANET 2 tools and allows a water distribution network with pressure-dependent demands to be solved in a single run of the unmodified snapshot hydraulic analysis engine of EPANET 2. In this technique, artificial strings made up of a flow control valve, a pipe with a check valve, and a reservoir are connected to the demand nodes before running the engine, and the pressure-dependent demands are determined as the flows in the strings. The resistance of the artificial pipes is chosen such that the demands are satisfied in full at a desired nodal pressure. The proposed technique shows reasonable convergence as evidenced by its testing on example networks.

Water quality based reliability analysis for water distribution networks

The research work in the past two decades on reliability analysis of water distribution networks (WDNs) is primarily focused on assessing the reliability from the point of view of hydraulics, that is, supplying the required quantity of water at the desired pressure, using various qualitative and quantitative measures. A minimum desired level of residual chlorine at consumer's tap is needed to supply quality water. In this article, both the quality and quantity of water is taken into account to assess the reliability of a WDN using a ‘water quality reliability factor’. The water quality reliability factor is defined as the ratio of the total quantity of desired quality of water supplied by the network to the total quantity of desired quality of water required to be supplied in the period of analysis. Pipe-break and pump-failure conditions are considered as random events. Hydraulic simulation of network under pressure-deficient conditions resulting from pipe-break and pump-failure conditions is carried out using node flow analysis. The proposed methodology is illustrated with two example networks taken from the literature.

Pressure-Dependent EPANET Extension

In water distribution systems (WDSs), the available flow at a demand node is dependent on the pressure at that node. When a network is lacking in pressure, not all consumer demands will be met in full. In this context, the assumption that all demands are fully satisfied regardless of the pressure in the system becomes unreasonable and represents the main limitation of the conventional demand driven analysis (DDA) approach to WDS modelling. A realistic depiction of the network performance can only be attained by considering demands to be pressure dependent. This paper presents an extension of the renowned DDA based hydraulic simulator EPANET 2 to incorporate pressure-dependent demands. This extension is termed “EPANET-PDX” (pressure-dependent extension) herein. The utilization of a continuous nodal pressure-flow function coupled with a line search and backtracking procedure greatly enhance the algorithm’s convergence rate and robustness. Simulations of real life networks consisting of multiple sources, pipes, valves and pumps were successfully executed and results are presented herein. Excellent modelling performance was achieved for analysing both normal and pressure deficient conditions of the WDSs. Detailed computational efficiency results of EPANET-PDX with reference to EPANET 2 are included as well.

Practical application of the head dependent gradient method for water distribution networks

Critical pressure-deficient operating conditions frequently occur in water distribution networks. Head driven analysis (HDA) of water distribution networks allows for the pressure-dependent nature of nodal flows and, therefore, yields the actual nodal flows and heads for both normal and subnormal pressure conditions. Hence, HDA simulation models are more practical than demand driven analysis models that assume that all demands are satisfied in full even under pressure-deficient conditions. This paper describes an approach in which a new pressure-dependent demand function that has no discontinuities has been incorporated in the Gradient Method. The procedure has been tested extensively and demonstrated to be capable of effectively simulating both normal and pressure-deficient operating conditions. The algorithm's convergence is smooth and rapid thanks to a line search and backtracking procedure that ensures progress towards the solution in each iteration. The results presented herein would appear to confirm that the proposed method is robust and efficient. Results for a real network are also included.

Water Distribution Network Pressure-Driven Analysis Using the Enhanced Global Gradient Algorithm (EGGA)

Pressure-driven analysis of water distribution networks (WDNs) can realistically reproduce the actual behavior of the hydraulic system, especially with reference to leakages, which are not under human control, and, sometimes to demands in pressure-deficient conditions. Classical WDN models represent the demand and leakage outflows in terms of either prior fixed nodal discharges, in classical demand-driven analysis, or dependent on actual nodal pressures, in pressure-driven analysis. This work presents a WDN pressure-driven algorithm that allows accounting for actual leakage and demand patterns along pipes or, as a complementary feature, for network topological simplification. It is on the basis of the enhanced global gradient algorithm (EGGA) and has been introduced and discussed in comparison to classical pressure-driven GGA. Three test networks have been used to study the convergence issue of the newly proposed algorithm, and the largest network has been used to discuss its computational efficiency.

Seamless pressure-deficient water distribution system model

Pressure-deficient conditions are a common occurrence in water distribution systems. These situations require accurate modelling for timely decision making. However, the conventional demand-driven analysis approach to network modelling is unsuitable for operating conditions with insufficient pressure. Increasing emphasis is being placed on the need for water companies to satisfy stringent performance standards for the continuous supply of water to consumers and it is those pressure-deficient operating conditions which are critical in determining whether or not adequate supplies can be provided. It is therefore very unfortunate that the demand-driven analysis method becomes invalid for use in precisely those critical conditions. The aim of this paper is to present a new pressure-dependent demand function to help improve the simulation of pressure-deficient conditions. The proposed function has better computational properties than those in the literature and has been incorporated successfully in the governing equations for water distribution networks. In particular, the proposed function and its derivative do not have the discontinuities that often cause convergence difficulties in the solution of the constitutive equations. A robust Newton–Raphson algorithm was developed to model water distribution systems under both normal and pressure-deficient conditions in a seamless way. Examples which demonstrate the methodology are included.

Pseudotransient Continuation Method in Extended Period Simulation of Water Distribution Systems

This paper presents and discusses a new static solver that implements the pseudotransient continuation method for the quasi-steady state analysis, or extended-period simulation of water distribution systems. The implementation is based on the concept of virtual tanks and has a clear physical meaning. The steady state solver described in this paper can analyze a pipe network under pressure deficient conditions and is free from some convergence problems that occur in the Newton-Raphson method-based solvers when analyzing a pipe network with control devices. The numerical examples considered in the paper demonstrate the convergence of the proposed method in cases where existing static solvers (e.g., that of the EPANET 2 hydraulic simulator) fail.

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Closure to “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Closure to “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Discussion of “Solution for Water Distribution Systems under Pressure-Deficient Conditions” by Wah Khim Ang and Paul W. Jowitt

Solution for Water Distribution Systems under Pressure-Deficient Conditions

In recent years, many researchers have tried to predict the behavior of the water distribution systems under pressure-deficient conditions. The root of the problem is that under these conditions the traditional demand-driven analysis will compute heads below the minimum required for outflow to occur physically at some or all of the nodes. The purpose of this paper is to present a novel algorithm for the solution of a water distribution network under pressure-deficient conditions, and is termed the pressure-deficient network algorithm (PDNA). The proposed model progressively introduces a set of artificial reservoirs into the network to initiate nodal flows, with the ultimate replacement of such reservoirs by full demand loads once it has become clear that the nodal flow can be satisfied. The foundation for the solution methodology is established using a series network for ease of discussion. The PDNA is presented in the form for coding into a computer program. For solving the flows in a looped network, the PDNA has to be used with a hydraulic network solver, as manual computation is too time consuming. Using the EPANET 2 hydraulic network solver, the PDNA is applied to both a single-source and a multiple-source network. The results show that the behavior of a water distribution system under pressure-deficient conditions is complex and nonintuitive.

Appraisal of Source Head Methods for Calculating Reliability of Water Distribution Networks

This paper presents a new way of interpreting the results of the traditional demand-driven network analysis as an approximation to pressure-driven simulation so as to calculate the reliability of single-source networks. The approach is useful because most network modeling software packages use the demand-driven analysis approach and are incapable of simulating pressure-deficient conditions properly. The formulation draws from recent developments in pressure-driven simulation and can be used to calculate the reliability of an entire water distribution network or that of the individual demand nodes. The method herein is probabilistic in that the random nature of link failures is accounted for. Using a sample network, it is shown that the method has several advantages including simplicity and a very high computational efficiency.