Optimization of Water Tank Operation in Water Supply Distribution Systems – Užice Waterworks Case Study

Aleksandar Daničić1, Maja Pražić1

 

1 Institute for the Development of Water Resources "Jaroslav Černi", Jaroslava Cernog st. 80, Belgrade, Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

This paper presents the results of a study of the optimization of the water distribution network in the city of Užice (Serbia), with particular focus on the water tanks within the system. A methodological approach is presented which includes the assessment of the present condition, distribution system modeling, model calibration, analyses of alternative solutions and defining of priority actions. Recommendations regarding water tank operation, together with other measures are provided and elaborated.

Keywords: waterworks, water distribution system, tank, pressure, model, calibration.

Introduction

The need for balancing water input and water demand (consumption) in a water supply distribution system represents one of the basic conditions for its regular operation. In contrast to variability of consumption over time, water input into the distribution system is usually more or less constant during a day. However there are many practical examples where variable water production and input into the system takes place, in order for water input to meet the actual water demand (consumption). Such cases in practice are characterized by a lack of, or insufficient, water tank volume in the system, which inevitably leads to the need to provide water input that meets peak demand directly from the water source. Variable water production contributes to a range of operational problems for the water source, especially in cases of well sources. Variable water production from well sources shortens their life span, with occasional additional adverse effects such as accelerated equipment wear and tear, increased electricity consumption, etc.

On the other hand, the absence of a water tank in the distribution network also implies the absence of a point with a fixed piezometric level which influences the pressure throughout the system. In systems without a water tank, the required pressure is maintained by pumps. However, comparison of stability of operation of a water supply network without and with a water tank, results in the unquestionable advantage of the latter. Assuming that the network is characterized by sufficient flow capacity, the variations of pressure in the system with a water tank, corresponds mainly to the water depth in the tank (on the order of several meters). In the case of water input through the pump station into the system without a water tank, under similar circumstances, the variable flow rates correspond to pressure changes that can vary within an interval up to a few bars (a few tens of meters of water column - calculating the energy per unit weight, which is expressed in meters). Despite a low level of service, increased pressure variations in the system are followed by a series of adverse effects, which are reflected in accelerated material fatigue and increased risk for pipe bursts and water leakage, increased risk of low pressure occurrences in the critical areas of the system (distant areas and/or areas on higher elevations), especially during daily and seasonal peaks of consumption (WSA/WCA, 1994).

But the presence of a tank in the system is not sufficient in itself. For stable system operations, in addition to adequate tank volume, the proper choice of location and the number of these facilities in the system, as well as the optimal relationship of mutual effects between tanks, or pumping stations and tanks, are necessary. Due to the specific features of each individual water supply system, consumption and a set of control conditions (recommendations), relating to the problem of selecting the number and location of vital system structures, cannot be of universal character. The current recommendations are modest in scope and too general in nature, to be used in practice for precise analysis of the current state of the system, or the planning of its development. This probably represents one of the reasons for the phenomenon of inadequate tank volume in a significant number of water supply systems in operation, which was observed by the authors. There are a number of cases where sufficient tank capacity in the system exists, but its utilization is insufficient, which appears as a result of its inadequate involvement in the system operations. This can produce far-reaching adverse effects on the operation of the water supply system. The objective of this paper is to provide methodological and practical guidance for analyses of water distribution systems, especially related to utilization of water tank volumes in order to provide a stable and efficient operation of a water distribution system.

 

The City of Užice Water Supply Distribution System

A methodological approach and steps in the analyses of the system will be demonstrated on the City of Užice Water Distribution System (Daničić and Pražic, 2008). Preparation of the Master Plan included the analysis of the existing and future water consumption/demand, analysis and diagnosis of the current state of the water supply system, identification of operational problems and defining recommendations for immediate actions including several alternative solutions for the system development until the end of the design period, evaluation and selection of the best alternative by using a set of technical and economic criteria and defining the phasing of the system development. The focus of the Master Plan were activities dedicated to establishing and developing numerous simulation models, which have supported the analyses of the system under the existing and future (designed) conditions. Model calibrations that were carried out were of crucial importance in demonstrating a cause and effect relationships in the system. Thorough analyses and diagnostics of the current system state have been conducted by using calibrated steady-state models of the existing system under maximum daily water consumption/demand conditions. These calibrated models were then further developed by including planned improvements of the current system and future system development and conditions (including both regular and irregular system operations).

The Municipality of Užice is located in the western part of Serbia, 190 km south west of Belgrade and has a population of about 85,000. The City's water supply distribution system supplies a total of approximately 55,000 inhabitants and some industrial facilities located in the City and its surrounding areas, as well as settlements in the eastern part of the municipality. This system consists of an impressive number of vital facilities - 32 tanks, 36 pumping/booster stations and a total of over 220 km of main distribution pipelines. In order to avoid any confusion, it is emphasized that we are referring to a unique water supply system, encompassing a very dispersed area with a significant number of diverse facilities operating within the network, supplying consumers at elevations ranging from 350 and 750 masl, within 8 elevation zones. The area covered by the existing City system includes 42 subsystems in total. The extremely complex and dispersed characteristics of this water supply distribution system make this system highly challenging in terms of planning and management. Due to these complexities, this paper focuses only on part of the system which includes three central and several peripheral subsystems. The selected part of the system is characterized by the presence of a large number of vital facilities, the mutual impact of which is analyzed in the Master Plan (Daničić and Pražić, 2008) and represents the subject of this paper.

A section of the Užice system, spanning the downtown area and neighborhoods of Krčagovo and Sevojno, is the focus of analysis in this paper. A schematic representation of this part of the City system is shown in Fig 1. The main water supply direction is defined with the position of the three serially connected subsystems, with gravity flow from the highest to the lowest point. The main water flow through each of these subsystems is defined by the position of the corresponding upper and lower water tank. Each of the subsystems, located in the direction of the main flow, is characterized by a considerable difference in height between the upper (upstream) and lower (downstream) tanks. This produces the need for a water level controller/regulator at the entrance to each of the downstream tanks, in order to avoid uncontrolled tank overflows. Fig. 1 shows six tanks, as vital structures, which operate in subsystems 1 – 3. These water tanks, numbered [1] - [6], have the following main characteristics:

[1] – Tank "Cerovića Brdo" (T Cerovića Brdo – in the text below) – downstream point from the water treatment plant "Cerovića Brdo" (WTP): BL/OL [Bottom Level / Overflow Level] = 500/504 masl, V (volume) = 1,300 m3;

[2] – Tank "Belo Groblje" - new (T Belo Groblje N): BL/OL = 479.9/484 masl, V = 1,400 m3;

[3] – Tank "Belo Groblje" - old (T Belo Groblje O): BL/OL = 474,6/478,8 masl, V = 1,200 m3;

[4] – Tank "Vujića Brdo" (T Vujića Brdo): BL/OL = 467/474.5 masl, V = 1,000 m3;

[5] – Tank "Dovarje" (T Dovarje): BL/OL = 453/460 masl, V = 1,000 m3;

[6] – Tank "Sevojno" (T Sevojno): BL/OL = 430/437 masl, V = 2,000 m3.

 

fig01
Figure 1: Scheme of the existing system in the treated area.

 

Consumer elevation data and maximum daily water demands are:

  • Subsystem 1: 408 – 480 masl, gross Q max day = 130 l/s;
  • Subsystem 2: without consumers
  • Subsystem 3: 360 – 430 masl, gross Q max day = 125 l/s.

Subsystem 1 was constructed in the central city area, between T Cerovića Brdo and T Vujića Brdo. This subsystem is supplied by T Cerovića Brdo, which is located downstream from the same-named water treatment plant. The connecting line between T Cerovića Brdo and T Vujića Brdo is a ϕ600/ϕ500 gravitational pipeline which is the main distribution line in the subsystem. The elevation difference between water levels in T Cerovića Brdo and T Vujića Brdo define the gross available energy for the flow through the ϕ600/ϕ500 pipeline, which is exclusively unidirectional (from T Cerovića Brdo to T Vujića Brdo).

The ϕ400 pipeline is the main distribution line for the northern sections of subsystem 1 and represents the largest profile connected to the subsystem's main ϕ600/ϕ500 line. Tanks "Belo Groblje" – old/new are located at a downstream point from the ϕ400 pipeline which acts as a unique inlet/outlet medium. This, along with the fact that the altitude of the "Belo groblje – old/new tanks being located midway between T Cerovića Brdo and T Vujića Brdo leading to a "side type connection" of tanks "Belo Groblje" – old/new, in relation to subsystem's main pipeline ϕ600/ϕ500.

In addition, three subsystems are connected to the main ϕ600/ϕ500 distribution line. Two of these are used for the transport of water towards the peripheral subsystems, while the third feeds subsystem 1:

  • Subsystem "Zlatiborski put – Zabučje", is supplied via the booster station "Zlatiborski put," the first in a series of structures of this type, where water from subsystem 1 is pumped in a high area of Zabučje, with a ϕ200 pump station suction line, which is connected to the main distribution line of subsystem 1 in its downstream section (ϕ500).
  • Subsystem "Velika brana – Surduk", with PS Velika brana as an upstream facility, for water being pumped into a series of subsystems, located in the southwestern part of the municipality (pumping being the dominant water transport mode).
  • The subsystem of the Turica source water well (Q = 70 l/s), which feeds subsystem 1 is also connected to the main line of distribution subsystem 1 at a downstream point/location from a ϕ300 pipeline, on a pressured part of the "Turica" pump station at its ϕ600 to ϕ500 reduction point.

Subsystem 1, with T Cerovića Brdo (500/504 masl) transmits pressure to the distribution network and supplies consumers in the central city area located at altitudes between 408 and 480 masl. This leads to excessive pressure in the lower part of the distribution network located near T Vujića Brdo. In order to reduce the effects of water pumping from T Cerovića Brdo, a pressure reducing valve (PRV) was installed in the upstream part of the main distribution ϕ600 pipeline. This reduces high pressures in the lower parts of the distribution network, but produces operational problems due to reduced pressure in the ϕ600 part of the pipeline that occurs immediately downstream from the PRV and in the nearby secondary distribution network.

Subsystem 2, defined between T Vujića Brdo and T Dovarje, represents a small portion of the system without consumer connections, that is located on the lower part of central city area. Water from T Vujića Brdo (downstream of subsystem 1 and upstream from subsystem 2) is transported to T Dovarje through a ϕ450 pipeline, which is the only pipeline in subsystem 2. As the altitude difference between T Cerovića Brdo (upstream of subsystem 1) and T Dovarje (upstream of subsystem 3) is approximately 50 m, which corresponds to the height of one supply zone (in a densely populated urban area), subsystem 2, located between subsystems 1 (central part of the city) and 3 (settlements of Krčagovo and Sevojno), represents a medium for water transfer from the upper to the lower consumer zone, realized through 2 steps (2 tanks).

Subsystem 3, which is located in the Krčagovo and Sevojno settlements, represents the lowest section of the water distribution system. Water is transported by gravity from T Dovarje into the subsystem 3 network, through the main ϕ450 distribution line. Pipeline ϕ450, which connects T Dovarje with T Sevojno, has been designed and built to transfer peak water demand into the Krčagovo and Sevojno settlement areas. T Sevojno, located downstream of subsystem 3, with an altitude that is about 30 m below T Dovarje, was designed to operate as a water balancing tank located on the opposite side of the water supply zone from the water input (hereinafter "opposing tank").

 

Analysis of the Existing System Operations

The above described complex system of pipelines and other water supply facilities of the central part of the system, produces a complex operation which is burdened by some serious problems which shall be described below. Analysis of the existing water supply system was carried out on a calibrated model, as well as the "so-called" alternative models, that had been formed to simulate extraordinary circumstances of system use, designed to examine the system's behavior under extreme conditions.

First of all, comparing the maximum daily gross consumption (demand) of subsystem 1 (130 l/s) and subsystem 3 (125 l/s) with corresponding volumes of the water tanks in each subsystem (3.900 m3 in subsystem 1 and 3.000 m3 in subsystem 3), percentage of daily water demand covered by the tank volume (Ct), for maximum water consumption conditions, are as follows:

  • Ct (subsystem 1) = 33 %
  • Ct (subsystem 3) = 28 %

Although slightly lower than those recommended (35 - 40%, Daničić and Pražić, 2008), the above given values may be deemed acceptable.

 

Operation of the "Sevojno" tank (Subsystem 3)

T Sevojno has been designed to operate as the opposing tank in subsystem 3, in relation to the position of T Dovarje (the subsystem's main tank), which had been located at the upstream point of the subsystem. Therefore the ϕ450 pipeline, being the main distribution line in subsystem 3, represents the only inlet/outlet medium for filling and emptying the Sevojno water tank.

Generally, one of main benefits of having opposing water tanks in the system is that the required diameter of the main distribution pipeline can be reduced since the consumers are supplied from both water input points and from the (opposing) tank during peak demand. Reducing the required diameter of the main pipeline may bring significant investment savings when distances between the main tank and the network are long, as in subsystem 3 (L (T Dovarje – T sevojno) = 6,7 km).

Designed operation of opposing tank includes its filling during periods of low water consumption (system pressure higher then water level in opposing tank). During this phase, water input into the water supply zone is higher than the water demand within the zone and surplus water is being discharged into the opposing tank.

Contrary to the above, during peak consumption, pressure in the system decreases to the value below the water level in the opposing tank. This induces water flow from the opposing tank into the network, i.e. this phase is characterized by simultaneous transport of water to the water supply zone from both the water input point and from the opposing tank. Such water input into the network from two opposite directions during peak water consumption reduces the required size of the main distribution pipeline.

However, in the case of T Sevojno, consumption in the Krčagovo and Sevojno areas had never increased sufficiently to cause a decrease in the piezometric level in the subsystem to values that are below the water level in T Sevojno (Fig. 2). In addition, the designed and realized altitude difference between T Dovarje and T Sevojno amounts to δH min/max = 16 - 30 m, is so great, that expectations that consumption would increase to values which would decrease pressure in the system for that amount, were totally unrealistic.

 

fig02
Figure 2: T Sevojno out of order.

 

Water demand analysis and projections that had been performed for purposes of preparing the Master Plan (Danićić and Pražić, 2008) revealed a significant reduction of consumption Krčagovo and Sevojno areas covered by subsystem 3. A separate technical water system has been constructed for the Krčagovo aluminum rolling mill, which is the largest individual consumer in the system (average consumption = 30 l/s). Following its separation from the public system, the aluminum mill's average net annual consumption from the drinking water supply system had decreased to a value less than 1 l/s (sanitary water use only), which significantly reduced the total consumption of subsystem 3. This illustrates, that under current circumstances, it is not possible to achieve water input from T Sevojno into the distribution network, and therefore this facility is permanently out of function.

 

Operation of the "Belo Groblje" tanks – old/new (subsystem 1)

The so-called lateral connection of the "Belo Groblje" tanks – old/new to the ϕ600/ϕ500 main distribution pipeline in subsystem 1 excludes most of the volume of these tanks from the system operations. During periods of increased water transport through the subsystem, the piezometric level at the point of the ϕ400 lateral pipeline connection to the main ϕ500 pipeline (point "B" –Fig. 3), may be reduced to a value that is much lower than the water level in the treated tanks, and than tanks will not receive any water flow. In contrast to this case, during low water consumption, flow through the main ϕ600/ϕ500 pipeline can produce a piezometric value at point "B" (Fig. 3) which is higher than the level in these tanks, which would prevent any water flow from the tanks to the distribution network.

 

fig03
Figure 3: Operation of "Belo Groblje" tanks – old/new.

 

The problem of the "Belo Groblje" tanks' operation, lies in the fact that the role and operation of these tanks completely depends on external factors and the varying water levels in these tanks do not produce a feedback of the system. Namely, the piezometric level at point "B" (Fig. 3), which directly dictates the mode of water input/output to/from the "Belo Groblje" tanks, almost exclusively depends on the altitude difference between the water levels in T Cerovića Brdo (upstream from subsystem 1) and T Vujića Brdo (downstream from subsystem 1). The reason for this is that consumption along the lateral ϕ400 pipeline is considerably lower than the amount of flow which is transported through the main ϕ600/ϕ500 pipeline. It is not due to the fact that a much larger section of subsystem 1 is being supplied by the ϕ600/ϕ500 pipeline, it is also related to the fact that this pipeline transports quantities of water needed for consumption by the downstream subsystem 3 (gross Q max day = 125 l/s). This means that the WAY in which the "Belo Groblje" tanks – old/new are incorporated into the distribution system leads to system operations instability, which is why these facilities are out of order.

In general, the above described issues relating to the inclusion of the "Sevojno" tanks (opposing tank in subsystem 3) and "Belo Groblje" – old/new (laterally connected to the main distribution line in subsystem 1) into the distribution system, represent the same problem. In both cases, the expected (designed) operation of these tanks includes their filling during periods of lower water demand/consumption, as well as tank emptying during peak water demand/consumption. The main difference is increased complexity of the for the lateral tank connection (at least 3 tanks per system), in comparison with the presence of an opposing tank in the system (two tanks in the system). In both cases, the designed operation of the tanks (opposing tank, laterally connected tank) is based on predictions, related to projections of future water demand.

However, future water demand/consumption represents a stochastic variable which cannot be predicted with a high degree of certainty. In both of the above described cases, the achieved water consumption values (as well as its spatial and temporal variations) had greatly differed from the design predictions. This has left three tanks (T Sevojno, T Belo Groblje – old/new) out of system operations.

On the other hand, exclusion of the "Sevojno" and "Belo Groblje" (old/new) tanks from system operations, completely devalues the above information on the formal sufficiency of tank volume in the observed part of the City's water distribution system. Under the given circumstances, this figure is not of great significance, whereas in a real system, corresponding compensation is performed by T Cerovića Brdo, which is a treated water tank located within the water treatment plant (Fig. 4).

 

fig04
Figure  4: Model calibration: measured and calculated water depths in T Cerovića Brdo.

 

Exclusion of the "Sevojno" and "Belo Groblje"- old/new tanks from system operations also means that equalization of consumption variations is performed by T Cerovića Brdo only. This can be observed by the significant variations in the T Cerovića Brdo water levels, with several charge/discharge cycles per day. This results in variable water production at the plant, which can have a very negative impact on the water treatment process. It is noted that the diagram in Fig 4. refers to the day when calibration measurements were performed (beginning of May), which is characterized by considerably lower water consumption than that of peak consumption. The model, which had been developed to analyze system operations during maximum daily demand/consumption, produced a diagram of the water levels in T Cerovića Brdo with an even larger number of charge/discharge cycles.

 

Pressure reducing valve in subsystem 1

The pressure reducing valve (PRV) has been installed in subsystem 1 - on the upstream part of the main ϕ600 distribution line, in order to reduce excessive high pressures, which appeared on the downstream parts of subsystem 1. However, downstream pressure is not the only value that has been reduced by the use of a PRV – use of this device has also reduced the transport capacity of subsystem 1.

In order to analyze the impacts of PRV use on the flow capacity of subsystem 1, two alternative models were developed (it is noted that, alongside the calibrated model and the model that supports system operations during maximum daytime consumption, a set of alternative models, which are dedicated to analysing certain characteristics of the system, or its elements, during extreme water use conditions and represent an indispensable instrument for defining the cause - effect relationships in the existing system):

  • Alt. Model 1: subsystem 1, without consumers, without a PRV, without a level regulator (LR) at the entrance to T Vujića Brdo,
  • Alt. Model 2: subsystem 1, without consumers, with a PRV and LR.

Analysis performed on Alt. Model 1, completely supporting free flow between T Cerovića Brdo and T Vujića Brdo, was used to examine the flow capacity of the ϕ600/ϕ500 pipeline. In this case, which is characterized by fast emptying of T Cerovića Brdo and fast filling of T Vujića Brdo, the flow is a function of the height difference between the water level in these tanks. On the basis of the geometrical characteristics of the serially connected ϕ600 pipelines (L = 1,280 m) and ϕ500 (L = 1.800 m) and the assumed roughness value of k = 1.0 mm (old steel pipes), two extreme cases can be recognized:

  • min δH (R Cerovića Brdo – R Vujića Brdo) = 25.5 m ⇒ Q min (ϕ600/ϕ500) = 450 l/s
  • max δH (R Cerovića Brdo – R Vujića Brdo) = 37.0 m ⇒ Q max (ϕ600/ϕ500) = 550 l/s

Analysis performed on the Alt. Model 2, which supported the use of flow regulating devices in subsystem 1, was aimed at defining a decreased amount of flow capacity in the main distribution pipeline, resulting from PRV use. Analysis performed on this model was focused on point "C" (see Fig. 5), as the most vulnerabele point of subsystem 1. Point "C" is located at the highest altitude, on the part of the main pipeline, built downstream of the PRV. "C" and represents the point with the lowest pressure in the subsystem, where a partial vacuum at point C, produced by the use of a PRV, can be expected in real system operations (Fig. 5).

On the other side of the subsystem, there is point "D", located near T Vujića Brdo, at the subsystem's lowest elevation point (408 masl). Without the use of a PRV, pressure at point D may reach the value of 96 m (hydrostatic level). In order to properly simulate control conditions from the real system, the PRV opening in the model was fixed at a value that produces pressure at point D on the upper end of the acceptable range (a value that justifies the use of a PRV):

p ("D") = 60 m

A fully open level regulator (LR), located at the entrance to T Vujića Brdo, alows maximum flow through the ϕ600/ϕ500 pipeline. However, in this case a partial vacuum occurs at point C. Since a partial vacuum is an unacceptable condition in any water supply distribution system, the PRV opening in the simulation had been progressively reduced to a value that would enable a positive pressure at point C. At the same time, a gradual reduction in the regulator opening produced a decrease in flow through the main distribution pipeline. In this regard, flow through the main ϕ600/ϕ500 pipeline, depends on pressure at point C:

Q (ϕ600/ϕ500) = f (p("C"))

Following the above rules, the maximum flow through the ϕ600/ϕ500 pipeline corresponds to the minimum opening of the LR, which enables a non-negative pressure at point C:

max Q (ϕ600/ϕ500) = Q (p("C") = 0)

All the, above mentioned, conditions that were incorporated into Alt. Model 2, have resulted in a modest maximum flow through the ϕ600/ϕ500 pipeline, which amounted to:

max Q (ϕ600/ϕ500) = 200 l/s

 

fig05
Figure 5: Subsystem 1 transportation (flow) capacity.

 

Reduced flow capacity of the main distribution pipeline in subsystem 1, produces large-scale operational problems. Namely, the capacity of the system's largest water source – the Vrutci reservoir, is estimated to be 800 l/s, which represents twice the value of the current water intake and treatment capacity. Under the present conditions, in addition to the intake and treatment of 400 l/s from the Vrutci reservoir at the WTP Cerovića Brdo, there are other water sources in the system whose total capacities amount to 120 l/s. Therefore, the current maximal daily water demand of 370 l/s in the area covered by the system (city of Užice with surrounding settlements in the eastern part of the municipality), can easily be met by the existing sources, even with the additional spare capacity of about 150 l/s.

However, the unresolved issues of water supply to remote areas in the western part of the municipality and in the neighboring municipalities, produces constant pressure on the waterworks company to expand the Užice system. Therefore, there are ambitious plans that forecast the gradual expansion of the City's system in the western part of the municipality. Plans for the more distant future include the expansion of this system to the neighboring municipalities (which will enable this system to take on a regional character), with adequate increases to the system's inlet and treatment capacity up to the above mentioned value of 800 l/s.

It should be noted that the T Cerovića Brdo, as a downstream point from the treatment plant and the entry point for the distribution system, is located within the central city area, far away from the peripheral parts of the existing and future network. The ϕ600/ϕ500 discharge pipeline from T Cerovića Brdo, which is also the main distribution line for subsystem 1 and the main supply line for a number of peripheral subsystems, has the largest flow capacity in the system. In this regard, a reduction in pipeline flow capacity, from a value of 450 - 550 l/s, to only 200 l/s, represents a serious limitation and a problem of strategic character.

 

Double regulation in subsystem 1

Double regulation in the area of subsystem 1, formed by applying a pressure reducing valve (PRV), on the upstream part of the ϕ600 pipeline and a level regulator (LR), located at the entrance to the T Vujića Brdo, has a negative impact on the distribution network operation. A high flow capacity value of the main ϕ600/ϕ500 distribution line produces rapid reactions of the LR. This means that higher flow occurence through the ϕ600/ϕ500 pipeline (in accordance with the flow conditions, that enable higher flow rates), is followed by a rapid water level increase in T Vujića Brdo and corresponding fast closing of the LR at the entrance point to R Vujića Brdo. Closing of the LR is induced by a sudden increase of upstream pressure and a flow decrease toward T Vujića Brdo. The increased pressure in the sub-system is followed by a PRV reaction, located at an upstream part of the network. The PRV responds to any increase in pressure by reducing downstream pressure to a targeted level. The reduction of downstream pressure, resulting from the PRV response, produces a fast water level decrease in T Vujića Brdo, followed by the opening of the LR. This results in an increased flow towards T Vujića Brdo and this cycle is repeated indefinitely.

In the diagrams below (Fig. 6, 7, 8 and 9 ), it is evident that the duration of each phase of pressure increasing and decreasing amounts to 1 – 2 hours, which indicates an inherent instability in the operation of subsystem 1, with serious impacts of double regulation on the operation of subsystem 1. Constant fluctuations in system pressure/flow and tank levels, not only create unstable water supply conditions, but also can lead to accelerated pipe material fatigue, more frequent pipe bursts and increasing water losses.

 

fig06
Figure 6: T Vujića Brdo inflow calibration.

 

fig07
Figure 7: Water level in T Vujića Brdo calibration.

 

fig08
Figure 8: Water level in T Dovarje calibration.

 

Improvements to the Operations of the Existing System

Analyzing and selecting proper actions aimed at improving the current system operations represents an important part of the Master Plan (Daničić and Pražić). This phase of system development is very often the most important for system operators, because it defines concrete solutions to the main existing problems within a short-term period (usually 1 - 3 years). The above-described problems, located within the central part of the Užice network, are significant, but also solvable in a very efficient way, given the presence of substantial water tank volumes, which are not being properly utilized under the current conditions. According to results of an analysis performed on the model of improvements (which was derived from the maximum present-day daily consumption model) provided below, system operations improvements have been defined with the following objectives:

  • Reduction of the pressure variations in subsystem 1;
  • Inclusion of all existing tanks into system operations by creating conditions which would enable the use of their full capacity;
  • Ensure utilization of the full flow capacity of subsystem 1.

 

fig09
Figure 9: Pressure in node 32-148 calibration.

 

Improvements to the current system operations were identified using an appropriate simulation model. Analysis performed on this model included a large number of simulations which led to the definition of gradual system improvements. The final version of this iterative process defines the set of improvement measures, described below.

 

Activation of T Sevojno

Activation of T Sevojno (inactive for several decades) is aimed at improving the system's operations given that its elevation (BE/OE = 430/437 masl) makes it suitable for supplying water to a major part of the Sevojno settlement (consumers at elevations between 360 and 410 masl). For the smaller part of Sevojno, located above 410 masl, water will be supplied using booster stations.

The technical solution of the system operations improvements in the area of Sevojno, includes the construction of a new main ϕ400 distribution pipeline, 1.5 km in length, from T Sevojno to the distribution network in the Sevojno settlement, connected to the existing network at several points. At the same time, it will be necessary to close all of the existing connections between the Sevojno network and the main ϕ450 pipeline in subsystem 3 (Fig. 10). In this way, the existing ϕ450 pipeline will become the main for T Sevojno, which means that this pipeline will be relieved of supplying peak demand to Sevojno (hourly water demand variations).

Variations of water demand/consumption will be covered by the water input from T Sevojno, which means that this tank will also act as a pressure break tank in the T Dovarje (settlement of Sevojno) direction. In this regard, T Sevojno will be included into the system operations, and upstream tank capacities will no longer serve as a flow balancing tank for for Sevojno. Additionally, this sub zoning of the Sevojno area solves the problem of excessive pressures in the lowest part of the settlement, located below 400 masl. Currently, this part of the system is directly connected to the T Dovarje (BE/OE = 453/460 masl).

 

Elimination of the pressure reducing valve from system operations

Elimination of the pressure reducing valve from the system has been suggested as a necessary measure, due to poor effects of using this device in the existing system (transportation capacity of subsystem 1 reduced from 500 l/s to 200 l/s, large pressure and flow variations due to the application of double regulation in subsystem 1, as explained above). Occurances of excessively high pressures in the lower parts of subsystem 1, that will result after PRV elimination, can be efficiently resolved in a manner described below.

 

Sub zoning in subsystem 1

Sub zoning an area of subsystem 1, as a measure of improvement is aimed at reducing pressures in the lower parts of subsystem 1 and activating T Vujića Brdo. According to the descriptions and diagrams presented above, transport of water from subsystem 1 to subsystem 3 is achieved in two steps – through two tanks: T Vujića Brdo and T Dovarje. However, the existing tank connection leaves only T Vujića Brdo at the breaking chamber level for downstream placement, without the possibility of utilizing its volume for balancing between the incoming flow and water demand.

 

fig10
Figure 10: Activation of the T Sevojno.

 

fig11
Figure 11: Activation of the T Vujića Brdo.

 

Because of the above, the technical solution for the lower part of subsystem 1, includes directly connecting the downstream part of the ϕ500 (subsystem 1) and the ϕ450 pipeline, in subsystem 2. At the same time, it will be necessary to close the connection between T Vujića Brdo and T Dovarje (by installing a new valve and closing it - see Fig. 11).

It should be noted that T Vujića Brdo and T Dovarje are located in close proximity to one another, which is why the described measures do not entail significant investments. After the realization of this measure, from the downstream points of the ϕ500 pipeline (part of the main distribution line of subsystem 1), the water will be discharged directly into the T Vujića Hill and T Dovarje, including the use of the existing level regulator, located on the entrance point to these tanks. T Dovarje will supply water to Krčagovo and Sevojno, while R Vujića Brdo will be connected to subsystem 1.

In order to activate T Vujića Brdo, the construction of a 700 m long, ϕ300 outgoing pipeline is foreseen, which will be connected to the existing distribution network in the lower part of subsystem 1 at elevations below 440 masl. This part of the network is characterized by a terrain height difference of over 60 m, compared to the altitude of T Cerovića Brdo (500/504 masl). Under the existing conditions, this part of the network (as well as the remaining network in subsystem 1) is directly connected to T Cerovića Brdo, which leads to unacceptably high pressures in this part of the system after elimination of the PRV.

As the elevation of T Vujića Brdo (460/467 m) corresponds to the supply of consumers located at elevations below 440 masl, connecting this part of the network to T Vujića Brdo will resolve the problem of high pressures in subsystem 1. At the same time, it will be necessary to close all of the existing connections between the network located at elevations below 440 masl and the rest of the network in the subsystem 1. Completion of this measure and the capacity activation of T Vujića Brdo, will result in considerably reduced water level variations in T Cerovića Brdo throughout the day.

 

Activation of the "Belo Groblje" tanks – old/new

The technical solution for activating the "Belo Groblje" tanks– old/new includes a conversion of the function of these structures into water transport capacities to higher water supply zones. Namely, the tanks' lateral connections to the main ϕ600/ϕ500 distribution line of subsystem 1 (described above), prove to be disadvantageous, in terms of reverse placement from these structures into the subsystem 1 network, in all the analyzed models. Because of the permanent inability to release water into the system, the "Belo Groblje" tanks – old/new, have been disconnected from real system operations.

Under the described circumstances, the adopted solution implies inclusion of T Belo Groblje - old (lower structure) into the system, in the role of a suction tank for the pumping station serving higher elevation zones. At the same time, the tank structure of T "Belo Groblje" – new, will serve as a mechanical room for pumps and accessories. The modified function of the T "Belo Groblje" – old, within the system operations, requires installation of a check valve, on the ϕ300 inlet pipeline at the tank entrance in order to prevent back flow from T Belo Groblje – old into T Cerovica Brdo.

Figure 12 represents the distribution system scheme - after implementation of all the described measures. In addition, diagrams and simulation model results are provided below (Fig. 13, 14, 15 and 16). Results clearly demonstrate improvement in system operation, which is characterized by reduced pressures in the system and smooth and slow varying operational parameters.

 

fig12
Figure 12: System schematic layout after improvements.

 

fig13
Figure 13: System improvements – water level in T Cerovića Brdo.

 

fig14
Figure 14: System improvements – water level in tanks.

 

fig15
Figure 15: System improvements – tanks - inflow.

 

fig16
Figure 16: System improvements – pressures in subsystem 1.

 

Conclusions

A recap of the measures aimed at improving operations of the Užice Water Supply Distribution System, and achieving its defined objectives consist of two main important actions - construction of two pipelines: ϕ400 (L = 1,500 m) and ϕ300 (L = 700 m). The remaining measures (eliminating PRV from the system and closing pipe connections) do not require significant investments, and can be performed during regular system maintenance activities. The proposed measures would result in the formation/creation of two subzones, elimination of the third zone, the inclusion of existing tanks into system operations with completely modified functions in relation to their current state and the activation of existing tank volumes for flow balancing between water inputs and demand.

This paper emphasizes the importance of comprehensive analyses of the state of the system operations based on simulation modeling, including defining the cause and effect relationships in the system, as a basis for the system diagnostics, examination of various system states, both regular and irregular, establishing and analyzing alternatives for making improvements to the system, etc. However, as each system is unique, this paper does not intend to define some universal rules related to water distribution system analyses that can directly lead to defining solutions in any practical case.

The Case Study presented in this paper clearly demonstrates that water tanks located in the central part of Užice can only function in accordance with their capacity once the effects of mutual influences are mitigated. At present, there is a total of 4 water tanks in subsystem 1, with only one fully operational. Only once the system is divided into several subsystems, with preferably one or maximum two water tanks, or a water tank and a pumping station, in each subsystem, can the water tank volumes (capacities) be properly utilized to their full extent.

 

References

Daničić A. Pražić M. (2008). Study and Master Plan of City of Užice Water Supply Distribution System reconstruction and development, Technical Report, 2010, Institute "Jaroslav Cerni", Belgrade, Serbia

WSA/WCA Engineering and Operations Committee (1994). Managing Leakage: UK Water industry Managing Leakage Reports A-J: Report A – Summary Report; Report B – Reporting Comparative Leakage Performance; Report C - Setting Economic Leakage Targets; Report D – Estimating Unmeasured Water Delivered; Report E - Interpreting Measured Night Flows; Report F - Using Night Flow Data; Report F - Managing Water Pressure; Report h – Dealing With Customer's Leakage; Report J – Techniques, Technology and Training. London: WRc/WSA/WCA