Pressure control in district metering areas with boundary and internal pressure reducing valves

Abstract

Despite operational improvements over the last 10-15 years, water utilities still are losing a significant amount of potable water from their networks through leakage. The leakage is managed on the one hand by reactive and proactive maintenance and on the other hand by pressure control to reduce background leakage from connection and joints. This paper is based on experience from the Process Control – Water Software Systems group which was involved in many pressure control projects and the current Neptune project (www.neptune.ac.uk). A fast and efficient method to calculate time schedules and flow modulation curves is presented. Both time and flow modulation can be applied to a single inlet DMA. Time modulation can be applied to a multi-inlet district metering area (DMA) but this is not always possible for flow modulation due to the risk of hunting. It is convenient to distinguish between boundary and internal pressure reducing valves (PRVs), the decision variable for a boundary valve is a PRV set-point whereas for the internal valves it is a valve resistance. The resistance is then automatically translated into a set-point for field implementation. The time modulation methodology is based on solving a nonlinear programming problem with equality constraints represented by a hydraulic model with a pressure dependent leakage term and inequality constraints representing operational requirements (e.g. pressure at critical nodes). The cost of boundary flows which include leakage flows is minimized. An extended content model with pressure dependent leakage is simulated to provide a starting point for quick convergence. Optimal time schedules are converted into flow modulation curves by plotting scatter plots of flows against heads.The algorithm has been implemented as a module in the FINESSE package and allows complete pressure control tasks to be solved. A user needs to provide an hydraulic model, eakage information and leakage characteristic – leakage area and the exponent in the pressure power law. The program calculates time schedules and also flow modulation curves for single and multi-inlet PRVs. Evaluation of optimal control strategies and benefit analysis in terms of leakage reduction for two case studies provided by Yorkshire Water Services is included.