Hossam Saadeldin AbdelMeguid

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

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, leakage 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.

Combined Energy and Pressure Management in Water Distribution Systems

In this paper a method is proposed for combined energy and pressure management via integration and coordination of pump scheduling with pressure control aspects. The proposed solution involves: formulation of an optimisation problem with the cost function being the total cost of water treatment and pumps energy usage, utilisation of an hydraulic model of the network with pressure dependent leakage, and inclusion of a PRV model with the PRV set-points included as a set of decision variables. Such problem formulation led to the optimizer attempting to reduce both energy usage and leakage. The developed algorithm has been integrated into a modelling, simulation and optimisation environment called FINESSE. The case study selected is a major water supply network, being part of Yorkshire Water Services, with a total average demand of 400 l/s.

PRESSURE AND LEAKAGE MANAGEMENT IN WATER DISTRIBUTION SYSTEMS VIA FLOW MODULATION PRVS

Globally, water demand is increasing while the recourses are diminishing therefore the leakage reduction in water distribution systems (WDSs) becomes an important objective for the water industry. The benefits of applying pressure control policy in WDS in order to reduce the leakage has been discussed in (Ulanicki et al. 2008). The pressure management via flow modulation has been applied and its benefits has been documented e.g. in (Yates and MacDonald 2007). Flow modulation PRVs can be operated either hydraulically (AbdelMeguid et al. 2009) or electronically to modulate the outlet pressure according to the demand level and required pressure at critical nodes. In this paper a genetic algorithm (GA) is used to calculate the coefficients of second order relationship between the flow and the optimal outlet pressure for a PRV. The method is implemented in Matlab linked to the EPAnet hydraulic simulator. The obtained curve can be subsequently implemented using a flow modulation controller (AbdelMeguid et al. 2009). The results of optimal PRV flow modulation via GA has been compared with the time schedule approach using a non-linear programming method described in (Ulanicki et al. 2008). The results of both techniques are very close to each other and resulted in almost the same amount of leakage reduction. The main advantage of the flow modulation in comparison to time schedules is that the modulation curve is calculated once and operates robustly over a wide range of demands. Although, the flow modulation is getting popular in the UK for single inlet district metering areas (DMAs), a special care must be taken for multi-inlet DMAs where interactions between inconsistent flow modulation curves may lead to hunting phenomena.

FEEDBACK RULES FOR OPERATION OF PUMPS IN A WATER SUPPLY SYSTEM CONSIDERING ELECTRICITY TARIFFS

The cost of energy used for pumping water constitutes a large proportion of operational expenditure for a water utility. Energy saving measures in water supply systems can be realized in different ways, by design of the system to be energy efficient, by proper maintenance of equipment especially pumps and by optimal control of the system. The cost of the pumping is a product of energy consumption and an electricity tariff. The energy can be reduced by pumping less water, lowering the head against which the water is pumped and by operating pumps near peak efficiency. The cost can also be reduced by re-scheduling the pumping from expensive to cheap tariff periods. Typically the real time control for time varying tariffs is implemented in a predictive control fashion, in which an optimal time schedule is calculated ahead over 24 hours period by a solver and recalculated at regular intervals e.g. 1 hour. In order to operate the scheme in real time the solver must be sufficiently fast and this may not always be possible for big water supply systems. In this paper a method to synthesize feedback control rules is proposed taking into account a time varying tariff. The rules are calculated off-line and then implemented in local PLCs or in a control room. Once the rules are implemented the response to the changing state of the water system is instantaneous. In this paper the feedback rules are calculated by a genetic algorithm. Each pump station has a rule described by two water levels in a downstream reservoir and two values of pump speed, for each tariff period. The lower and upper water levels of the downstream reservoir correspond to the pump being ON or OFF. The approach has been applied to a large scale water supply system and compared with the traditional time schedule approach. The achieved cost for the feedback control is only slightly higher than that for the time schedule approach. However, the feedback control by its nature is more robust and performs well in the presence of uncertainty in water demands and in inaccuracy of hydraulic models.