Simulation tests for the operation of a water main with break pressure tanks.

Abstract

The Ashley Drive break pressure tank (BPT-20 𝑀ℓ) has been installed on Durban’s Western Aqueduct. Its purpose is to release the 20 bar gravity head of the 1.4m trunk main supply from Umgeni Water at Umlaas Road. The expected peak conditions (400 𝑀ℓ/𝑑𝑎𝑦) will only allow 14 minutes for valves to close, yet they must be moved slowly in order to avoid dynamic shock. The high pressure upstream supply is admitted to the BPT through a set of thee parallel sleeve valves, which are in a control loop to maintain level in the BPT against the downstream draw. These cavitation-resistant valves cannot be operated without electrical power, so an added complication of the design is a set of 3 hydraulically-operated globe valves which switch in at extreme tank levels. Though the commissioning of the Ashley Drive BPT is already in progress, it is important to simulate the overall operation of the system for projected future flows, in order to detect possible operational problems, and to build in solutions if necessary. Optimisations include such issues as the valve closing sequence and speeds, settling level variations, and smoothness of the draw from Umgeni Water. The simulation study involved the modelling of the trunk main, the Ashley Drive BPT, the downstream Wyebank BPT and the reservoirs drawing from the trunk main before and after these two BPTs. Data handling techniques were developed in order to formulate the daily demand profiles for each of the reservoirs. Design information was used to calculate the hydraulic parameters that featured in the simulation, and to determine the residual pressures at the inlet valve sets of the BPTs. Implicit calculations with the Newton-Raphson iterative method were employed in order to obtain a pressure distribution across the BPT valves. Simple mechanisms were built into the MATLAB® program in order to accommodate the complexities of the system, e.g. the possibility of power loss, valve or BPT chamber maintenance, or the deliberately slowed movement of the valves to avoid pressure surges within the pipeline. The analysis of the results of the simulation study involved examining the efficacy of the control set-points and valve sequencing, and determining whether these settings satisfy the design specifications. Random and anticipated scenario testing was carried out within the study in order to accommodate for situations such as electricity outages or unusual consumer demands. The BPT control system was analysed to assess its adequacy and the risks associated with the proposed staggered sleeve valve control scheme. The results of this investigation are presented as multiple time-sequence graphs depicting the results of the different scenario tests. Support for the design concept, additional recommendations and indications of adverse scenarios, have emerged from this study. The original design is found to be capable of duty within the ranges of expected normal operation in 2036, and the system was observed to be capable of conveying a throughput greater than that of the design. The normal operating level was also found to be higher than intended, and valve oscillations were deemed a significant concern. It was established that operation with just two sleeve valves active within each BPT would achieve better correspondence to the design specifications. The revised control system (Control 2.0) was found to be better suited to the application, but was also diagnosed to be too slow to react under certain circumstances.