Su dağıtım şebekelerinde en uygun basınç yönetimi metodunun belirlenmesi
Determination of the optimal pressure management method in water distribution networks
Yazar: MEHMET MELİH KOŞUCU
Danışman: DOÇ. DR. MEHMET CÜNEYD DEMİREL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Hidrolik ve Su Kaynakları Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Değişken akım = Varied flow ; Hidrolik = Hydraulics ; Hidrolik analiz = Hydraulic analysis ; Hidrolik modelleme = Hydraulic modelling ; Kararsız akım = Unsteady flow ; Su dağıtım sistemleri = Water distribution systems ; Su dağıtım şebekeleri = Water distribution networks

Özet:
Su dağıtım şebekelerinde meydana gelen yüksek basınçlar, hem su kayıplarını, hem de boru arızalarının meydana gelme sıklığını arttırmaktadır. Su kayıplarını ve boru arızalarının miktarını azaltmak, ve su dağıtım şebekelerinin faydalı ömrünü uzatmak için yüksek olan basıncı düşürmek gerekmektedir. Fakat şebekeden su alan tüketicilerin tamamının basınçlı su kaynağından istifade edebilmesi için basıncın şebekede izin verilen minimum değerden daha düşük olmaması gerekmektedir. Bu sebeple basıncın hassas bir şekilde yönetilip su kayıpları ve boru arızalarının azaltılması, ve abonelerin suya erişiminin aksamaması önemlidir.  
Pratikte dört farklı basınç yönetimi metodunun var olduğu bilinmektedir. Bunlar Sabit Çıkışlı, Zaman Ayarlı, Debi Ayarlı ve Kritik Nokta Ayarlı Basınç Yönetimi metodlarıdır. Sabit Çıkışlı Basınç Yönetimi'nde şebekenin girişindeki PRV'nin (Pressure Reducing Valve = Basınç Düşürücü Vana) sabit bir çıkış basıncı vardır ve bu basıncın değeri zamana veya debiye göre değişmez. Zaman Ayarlı Basınç Yönetimi tatbik edilirken PRV çıkışında, gece saatlerinde gündüz saatlerine göre daha düşük çıkış basıncı verilmektedir. Böylece su tüketiminin az ve basıncın yüksek olduğu gece saatlerinde meydana gelen su kayıpları azaltılmış olur. Debi Ayarlı Basınç Yönetimi'nde PRV'nin çıkış basıncı, PRV'den geçen debiye göre sürekli güncellenir. Bu yöntemde debi arttığında çıkış basıncı artar, debi azaldığında da PRV'de yük kaybı arttırılır ve böylece çıkış basıncı azalır. Kritik Nokta Ayarlı Basınç Yönetimi'nde ise su dağıtım şebekesindeki kritik nokta basıncının şebekede izin verilen minimum değerde sabit tutulması hedeflenir. Burada kritik nokta, şebekedeki en düşük basınçların meydana geldiği noktadır.  
Bir su dağıtım şebekesinde en uygun basınç yönetimi metodunun belirlenebilmesi için hidrolik olarak modellenmesi gerekmektedir. Dört farklı basınç yönetiminin hidrolik olarak modellenmesi EPANET 3 adlı açık kaynak kodlu yazılımda gerçekleştirilmiştir. EPANET 3'ün mevcut versiyonu hidrolik çözücü olarak yarı-kararlı akımların hidroliğini esas alan Küresel Gradyan Algoritması'nı (GGA) kullanmaktadır. Bu çalışma kapsamında basınç yönetimi yapılırken ise sıkıştırılamaz değişken akımların hidroliğini esas alan Rijit Su Sütunu Küresel Gradyan Algoritması (RWC-GGA) kullanılmıştır.
Büyüklüğü, tüketim paterni ve su kaybı miktarı farklı olan 18 su dağıtım şebekesi üzerinde yapılan basınç yönetimi hidrolik simülasyonları sonucunda hangi basınç yönetimi metodunun hangi tip şebekede ve hangi birim su maliyetinde en uygulanabilir olduğu belirlenmiştir. Bu analizlere göre birim su maliyetinin yüksek olduğu durumlarda Kritik Nokta Ayarlı, düşük olduğu durumlarda da Sabit Çıkışlı veya Zaman Ayarlı Basınç Yönetimi metodlarının malî açıdan en avantajlı seçenekler olduğu anlaşılmıştır.

Abstract:
Pressure management is one of the prominent elements of combating water losses. Repairing pipe failures caused by high pressures in the water distribution network without intervening pressure does not reduce the possibility of recurrence. Water losses may occur as a result of pipe failures and due to poor material quality or craft at the pipe connection points. In both cases, the higher the mains pressure, the greater the water loss. For this reason, applying pressure management in water distribution networks seems essential if factors such as more efficient use of resources and facilities and minimizing the damage to water consumers and water administrations are considered.
Pressure management has many benefits apart from reducing water losses and leaks. To prevent excessive water consumption by the users, to ensure more efficient use of energy in pumped systems, to reduce bursts in the network and thus the material and moral damages caused by these bursts, to prevent administrations and municipalities that manage the network from getting a negative reputation, to extend the useful life of the network, to reduce the costs of active leakage control, to reduce user complaints caused by high pressure, and to cause fewer faults and problems in building water installations are some of the benefits that can be achieved by pressure management.
In order to obtain the desired efficiency from pressure management, water distribution networks are divided into District Meter Areas (DMA). DMAs, a part of water distribution systems, are hydraulically independent zones separated from other parts of the network by valving and plugging, fed from one or more inlets. As the number of inlet pipes of a DMA increases, it becomes more challenging to regulate the pressure of that DMA. For this reason, it is vital to feed a water distribution network from as few inlets as possible, where high pressures are desired to be reduced, to ensure hydraulic integrity and manageability. Due to this importance, the more efficient and robust management of water distribution systems by dividing them under the responsibility of municipalities and water administrations is an increasingly common trend today. If pressure management is to be implemented, it is recommended that this should only be done in a DMA-shaped water distribution network.
Commonly used four pressure management methods are Fixed Outlet (FO), Time Modulated (TM), Flow Modulated (FM), and Remote Node Modulated (RNM) pressure managements. The first one (i.e. FO) is the most preferred pressure management type. In this management approach, a constant pressure level at any time is targeted in the main supply pipe of the DMA with no consideration of flow rate or time. The second approach (TM) comprises adjusting pressure according to time. As is known, pressures are high at night, contrary to daylight hours. In the TM type pressure management method, night-time high pressures are targeted to be lower values than those in the daytime as this helps to reduce leakages significantly. The FM pressure management method ensures a pressure value for every single different flow rate entering the DMA. A pressure-flow curve generates sufficient pressure at the critical nodes. The fourth approach (RNM) aims to provide the minimum allowable pressure value at the critical node of the DMA. The Inlet pressure of the DMA is updated consistently to satisfy restrictions about minimum allowable pressure in the DMA. This method is also known as real-time pressure control. Determining the most applicable and advantageous pressure management method for a water distribution network requires modelling its hydraulics.
The hydraulic simulation of water distribution systems is achieved with Global Gradient Algorithm (GGA) in EPANET software versions 2.0 and 2.2. In this algorithm, the mass and energy of the system are conserved via a Newton-based recursive scheme. GGA method is also named "Pipe Equations" since the energy balance is provided in each pipe in the network. Among various water distribution system hydraulic analysis algorithms, the best convergence and efficiency abilities belong to GGA. Because of this, the developers of EPANET have adopted GGA as a hydraulic solver algorithm. GGA solves steady-state hydraulic equations based on an iterative approach. Under time-dependent conditions, EPANET solves successive steady-state equations in each time step. In such a way, an Extended Period Simulation (EPS) analysis could be executed. EPS analysis is pertinent through short and long-term capacity determinations, water distribution system optimizations, and water quality-related investigations. Original GGA has been consistently improved in the last decades. By eliminating the spurious oscillation problem in close tanks and adding pressure-dependent demands, the algorithm has become Generalized GGA (G-GGA). 
EPANET 3, an open source software, solves the hydraulics of water distribution networks with the Generalized Global Gradient Algorithm (G-GGA). Although this algorithm can model hydraulic quantities on the minute or hour scale, it falls short of modelling hydraulic developments occurring on the second scale. For this reason, it has been deemed necessary to use the Rigid Water Column Global Gradient Algorithm, which adopts incompressible unsteady hydraulics and considers the inertia effects of the flow rate varies with time, as the EPANET 3 hydraulic solver in this study. Because with this hydraulic solver, it has become possible to model valve motions and water consumption changes in the order of seconds more precisely and accurately.
In the current EPANET 3 software, pressure management can only be implemented with a PRV definition that can give a constant outlet pressure, which can be called static. Therefore, it is impossible to financially determine the most suitable pressure management method with this PRV, which can only do one of 4 different pressure management. It has been deemed necessary to add a Dynamic PRV definition to the EPANET 3 software, where the PRV outlet pressure can be continuously updated according to time, flow and critical point. In addition, even if the flow through the static PRV in the current EPANET software increases or decreases, the outlet pressure always remains constant, which harms the hydraulic reliability of the valve. Because when the flow rate through the PRV increases, the outlet pressure drops rapidly, and then the PRV restores the old outlet pressure. If the flow rate decreases, the output pressure increases and returns to its previous value. It should be noted that Dynamic PRV, introduced in the EPANET 3 source code within the scope of this study, has these capabilities.
Along with the Dynamic PRV definition on EPANET 3 software, modelling four pressure management methods became possible also. Several modifications to the EPANET 3 source code and input file have made FO PM, TM PM, FM PM, and RNM PM simulable. Classic PRV in the software takes the outlet pressure as the setting. On the other hand, PM type is the setting of the Dynamic PRV, such as FO, TM, FM, and RNM. If the PM method is chosen as FO, the pressure management setting is the fixed outlet pressure value. In the case of the TM setting, the required parameters are Day_Pressure and Night_Pressure, respectively. If FM is the setting, the coefficients of the flow modulation curve are needed. Lastly, in the RNM case, target pressure and the remote node's name are necessary for the input file.    
In order to determine the most feasible pressure management method financially, four pressure management methods have been applied to 18 networks, which differ in size (large, medium and small), demand pattern (peaked and smooth) and amount of water loss (high, intermediate and low). These networks are generated from a water distribution network in the Hadımköy district of Istanbul, Turkey. Populations of the large, medium and small systems are 36000, 16000, and 4000 people, respectively. Demand pattern types are peaked (abrupt changes between day and night demands) and smooth (mild changes between day and night demands). Different water loss ratios are 48 % (high), 24 % (intermediate), and 6 % (low). The total duration of hydraulic simulations is one week, and financial analysis has been done for 40 years.
When pressure management is not applied, water losses and pipe bursts increase. This situation causes an increase in the amount of water entering the network and, therefore, its cost, and also causes more allocations from the budget for pipe bursts. When pressure management is desired to be applied, it is inevitable to undertake construction in the water distribution networks and supply the equipment. It is clear that this construction and the supply of equipment have costs. However, water losses and pipe bursts are reduced with the application of pressure management.
It should be noted that the cost of installing the necessary equipment for pressure management can be much less than the cost of reduced water production and pipe burst repair. In this case, it can be predicted that pressure management will bring a severe economic benefit. Four pressure management methods bring different levels of economic benefits. Since it is desired to maximize the benefit obtained by the pressure management activity, it is essential to reveal which pressure management method provides the maximum benefit. 
The cost-benefit analysis of each method has been made, and the method that brings the maximum benefit (maximum cost reduction) has been determined. It has been revealed that the most applicable pressure management method according to the cost-benefit analysis varies according to the unit water cost. The RNM pressure management method is usually the most appropriate if the unit water cost is high. As the unit water cost decreases, FO pressure management or TM pressure management becomes the most appropriate pressure management method instead of RNM pressure management. If the demand pattern of the network is smooth, the most feasible method is FO pressure management, and if the demand pattern is peaked, TM pressure management is the most applicable method when the unit water cost is relatively low.