Water Supply Configuration and Water Losses - Nikšić Waterworks Case Study - page 05

Assessment of the Nikšić Water Supply System Operation – Present Condition

Further activities on detection of losses in the Nikšić system, defined by widely accepted standards are certainly advised to be carried out in the future, but it is also necessary to implement other measures in this system, aimed at promoting change to the existing system concept. All analysis, carried out on the above described models, pointed to problems in system operation. The concept of the system is such that its operation produces flow and pressure instability throughout the system, which in turn leads to pipe material fatigue and increased frequency of occurring of defects in the network.

In figures 5 and 6, schematic longitudinal profiles of the Nikšić system - from the source to the distribution network, are shown. Figure 5 shows the original concept of the system, which includes R Trebjesa in the system's operation, and expected pressures in the lower supply zone.

 

Fig05
Figure 5: longitudinal profile from Vidrovan source to tank Trebjesa (R Trebjesa in operation).

 

Fig06
Figure 6: longitudinal profile from Vidrovan source to tank Trebjesa (Trebjesa out of operation).

 

The main problem of the system in Nikšić is contained in the position of the R Trebjesa, which is such that its work produces the pressure in the distribution network of 6 - 9 bar, in the hydrostatic mode. Decades-long operation of the system, within the concept shown in the above scheme, resulted with the emergence of a large number of defects in the system, burdened with constant high pressures. In order to reduce the pressure in the system, two years ago R Trebjesa was excluded from the system and pump units at Duklo BPS were replaced with new, with smaller value of outlet pressure. But decades-long use of the system according to the above scheme, had already caused damage, in the form of the above mentioned value of the degree of presence of system losses. In addition, it is emphasized that under conditions, where Duklo BPS has represented the only vital structure in the network, which has greatly expanded, reduction of pressure in the system, made two years ago, could not be drastic. Namely, in order to transportate water to distant peripheral parts of the system, the outlet pressure, produced by the work of Duklo BPS , could not have been low (currently, system operates with pressure of 5 – 6 bar, downstream of Duklo BPS).

The main problems of the current system are contained in the facts that are observable in the lower of two schemes, on the previous page:

  • Transport of almost the entire water demand through the Duklo BPS inevitably leads to system pressure variations and possible increases in water losses. In addition, electricity consumption is high. It should be noted that the Vidrovan water source is located on an elevation of H = 664 masl, while the largest part of the lower consumption zone, where the majority of consumers are located, lies between 610 and 630 masl;
  • Lack of tank space in the system, the exclusion of R Trebjesa from the system has lead to reduced reliability of the water supply and a requirement to pump all water, including peak water demand into the system. This leads to increased velocities and pressure variations in the distribution network due to water demand variations.

Analyses of the system operation in the present condition, performed on the calibrated model, revealed additional operational issues:

  • Mutual damping of flows from two sources (Vidrovan and Poklonci). The second water source (Poklonci) operates only during the summer, when the yield of the Vidrovan source declines. The pipelines, that transport water from each source to the Duklo BPS, are directly connected to each other, immediately upstream of the Duklo BPS. Under these circumstances, mutual damping emerges as a result of different conditions of flow from these two sources (gravity from Vidrovan / pumping from Poklonci) which lead to the inability to utilize available capacities of water sources and occasional water shortages during the peak summer consumption.
  • Occasional gravitational flow (flow in partially full pipe) in the upstream parts of the pipeline from the Vidrovan source, during periods of high water demand and low yield of the source. This point out to high hydraulic losses in the system and pressure drops.

Analysis of the existing system also pointed out certain issues related to the insufficient capacity of certain parts of the secondary distribution network and inadequate choice of pump units in some of the existing booster pumping stations (hydrophores) for supplying consumers at higher elevations.

 

Proposed Reconstruction of the Water Supply System

The above attached descriptions of conceptual problems in the Nikšić water supply system, observed during the process of leakage detection, were emphasized in terms of looking at the real problems in the operation of this system. This is especially important from the aspect of certain analyses that pointed to possible improvements, which would be achievable in a relatively short time and on the basis of a reasonably low investment.

The main objectives of improving the work of the existing water supply - distribution system in Nikšić are contained in the following:

  • Establishing gravity flow as the dominant scheme in the distribution network, instead of pumping;
  • Re-introduction of the existing R Trebjesa water tank (V = 7,500 m3) into the operaiton;
  • Construction of additional water storage volume. Since the minimal volume for safe operation of the system is estimated at the level of approx 40% of max daily demand, the additional tank volume required for the system is in the 7,000 to 9,000 m3 range;
  • Enabling full capacity utilization of the Vidrovan and Poklonci water sources;
  • Preventing flow in partially full pipe in the upstream sections of the main pipeline from the Vidrovan source, under all operational conditions;
  • Changing the function of the Duklo BPS, from regular system operation (present mode of operation), to backup function (for providing pressure in the system under extreme consumption conditions);
  • Reducing energy consumption by utilizing gravitational flow from the Vidrovan source to the greatest extent possible. It should be noted that the existing pipeline from the Vidrovan source has high capacity – low energy losses (diameter 1000 mm, L = 15 km, houghnes k = 1 mm, for flows 250 - 400 l/s, total head loss is 2 - 4 m);
  • To properly delineate consumption zones.