Qualitative Analysis

Abstract

Water losses from water distribution systems (WDSs) is a prominent feature of water operations, as they occur in all water supply systems. These losses are the total of both real and apparent losses from the water network and are comprised of pipe bursts, leakages from reservoir floors, background losses and reservoir outflows. If they remain undetected over a long period of time, they can become quite severe in terms of size. The quantity of water loss varies and depends on the physical characteristics of the pipe network (i.e. length of mains, number of service connections and the length and material of supply pipe) operating factors and parameters, and the level of technology and expertise applied to control this loss.

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The water losses can be controlled by reducing the pressure in the system and by improving the water system infrastructure. When implementing a pressure control scheme, minimizing the background leakages and meeting the minimum pressure requirements at the critical nodes in the water system, were key aspects that were considered. Furthermore, maintaining a constant pressure throughout the network was also addressed, as this would ensure that the water demand is distributed equally to all customers.

Two commonly used approaches that incorporate the use of pressure reducing valves (PRVs) have been discussed and compared to determine which method would be more suitable for managing pressure variations in WDS. The flow and pressure levels were compared after each method was applied, along with the number of mains and service breaks. Additionally, the Genetic Algorithm was proposed and investigated as an optimization method to reduce the total network costs in WDS and analyse the relationship between the flow and the outlet pressure of the PRV to minimize background losses and pipe bursts.

Introduction

In recent years, supplying water resources to meet the growing water demand of urban areas has been a major challenge. Acquiring new resources in urban developments can be both expensive and time consuming. The water demand is increasing while the recourses are diminishing on a global scale. In order to preserve valuable water resources, many water utilities have been developing new strategies to minimize losses to an economic and acceptable level. The overall water demand is compromised of residential, commercial, industrial, public water use, system loss and water leakages within the WDS. Water leakages and water losses do not create revenue towards the water utility and are a major source of costs wasted on production. To reduce the operation expenses, money is spent by utilities to detect and reduce water leakages and losses within the system, to account for the increased water production, treatment and transmission costs.

Pressure Management (PM) is a technique used in the reduction of leakages at water supply systems, by minimizing excess pressure at critical points within the WDS. As a result, the frequency of pipe bursts is significantly reduced which helps utilities make significant savings on the operation & maintenance of the WDS. Furthermore, it also enables them to provide customers with an optimum level of service that meets the high water quality standards set by the client. PM is now recognized as one of the most energy efficient and cost-effective solutions for reducing leakages in WDS. This technique involves a large amount of activities which incorporate several elements of water distribution systems (WDS) including; pump control, tank regulation & pressure reduction through the application of automatic valves and hydraulic controllers. Hence, through supervising pressure control schemes over wider areas, the background leakages and frequency of pipe bursts can be minimized, the overall water quality can be improved, and the energy demand and overall water supply costs can be reduced.

Literature Review

Approaches & Methods

Water utilities design potable WDS to provide a minimum level of service pressure throughout the day at the critical node in the network. Most water systems are designed to meet pressure requirements during peak demand periods where there are high friction losses and low inlet pressures. This means that water systems operate at pressures higher than required to accommodate for the low pressure at the critical points in the system. Therefore, when system pressures are at their highest during late evening and early morning periods, that is when there is a greater risk of major pipe bursts occurring. Since leakage increases with a rise in water pressure, the leakage levels in most systems are higher than they should be during most of the time.

Typically, PM methods incorporate the use of PRVs (Pressure Reducing Valves) which are automatic valves that regulates water flow in the system, to ensure a constant pressure is maintained throughout the WDS. One such approach is the Fixed Outlet (FO) PRV that reduces the water pressure to the required outlet pressure value to ensure that the pressure requirements within pumps and reservoirs are met. However, the major problem with this solution is that the PRV must be manually adjusted to meet the system flow requirements. Pressure Management can be applied more efficiently, if the set points of a PRV can be automatically adjusted according to the flow requirements. A hydraulic flow modulator achieves this by modulating the outlet pressure of the PRVs according to the flow. This minimizes continuous over pressurization of the mains and therefore reduces the frequency of potential pipe leaks within the system.

Results and Discussion

Through studying the results obtained from both the FO & FM methods, it was evident that the FM method was more advantageous. This is because the FM method had several key benefits including; a reduction in total network costs and better regulation of pressure levels to meet high water demand levels. In comparison, the DMA flow and pressure levels were still quite high after applying the FO method, with the number of main & service breaks being numerous over a long time period. However, to save more money using the FM method, it was noted that the average reduction in marginal water flow could be further improved by fine tuning the operation of hydraulic controllers to achieve a minimum reduction in water flow in WDS.

Optimization methods can be used to determine the lowest cost of pipe sizes and to satisfy the mass energy & conservation requirements, head losses in each pipe, acceptable pipe sizes and head pressure & flow requirements at water distribution networks. This can be achieved by determining the PRV location and the network setting. The Genetic Algorithm (GA) is a popular optimization technique that is based on generating an initial population dependent on feasible networks. The GA is most suitable for Fixed Outlet, Flow Modulated & Time Modulated PRVs as they are based on a 24-hour period.

After carefully analyzing the GA method it was determined that this algorithm was very effective in adjusting the settings of FM and TM PRVs in WDS. Compared with other optimization methods, it also achieved a greater total cost benefit by reducing the money spent on tacking network problems (i.e. leakages, pipe bursts, background losses, etc.), the nodal pressure and the total sum of pipe lengths connected to each node. However, it is not clear how effective the GA model is in producing an optimal solution, and it is more suitable for solving smaller and simpler optimization problems in WDS. Researchers may need to carry out a more detailed and prolonged study to better analyse how effective the GA method is in comparison with other optimization techniques.

Conclusions & Future Work

Due to the exponential rise of urbanization and population growth, there is a greater demand for water worldwide. In WDS, a large amount of energy is consumed for water & wastewater treatment processes and managing water supply systems. Hence the sustainable operation of WDS is of paramount importance. PRVs are not recommended for usage as pressure reduction can lead to wasted energy. As a result, this energy loss leads to a misuse in natural resources. If the right steps are taken, the energy present in a storage tank can be extracted, recovered and exported from the grid to reduce the amount of CO2 emissions. Hydropower turbines are a more sustainable option as they can utilize the excess energy to satisfy the static head and pressure requirements of the WDS.

Hydropower turbines generate electricity by converting potential energy into mechanical energy which in turn is converted into electricity. There are two types of hydropower turbines; impulse and reaction turbines. Impulse turbines extract energy from the movement of water flow whereas reaction turbines extract energy from the pressure of the water head. Figure 1 shows the different types of hydropower turbines. These technologies are promising as they can control small variations in pressure levels in WDS rapidly, since the hydraulic propeller can be easily adjusted to meet the pressure demand.

Figure 1: Types of Hydropower Turbines

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