Hydraulic and Economic Analysis of Real Time Control

Transcription

1 Hydraulic and Economic Analysis of Real Time Control Tom Walski 1, Enrico Creaco 2 1 Bentley Systems, Incorporated, 3 Brian s Place, Nanticoke, PA, USA 2 Dipartimento di Ingegneria Civile ed Architettura, Università degli Studi di Pavia 1 Tom.Walski@bentley.com ABSTRACT The aim of this paper is to analyze the hydraulic and economic effectiveness of the real-time control (RTC) of pressure reducing valves (PRVs), for the purpose of managing pressure to reduce leakage and pipe bursts. In their conventional use, a PRV automatically reduces a higher inlet pressure to a lower downstream set-point pressure (steady-state), regardless of changing flow rate or varying inlet pressure. In local RTC, instead, the set-point can be adjusted in real time to accommodate the variations in the water discharge though the valve. In detail, local RTC is performed thanks to a programmable logic controller (PLC), which can use the water discharge measurement received from an electromagnetic flowmeter in proximity to the valve, to calculate a new valve setting. The initial applications of the work will concern the extended period simulation of a pressure zone, in which the locally controlled PRVs is installed in the feed pipe that connects the source to the pressure zone. In this case study, nodal demands are reconstructed stochastically through the bottom-up approach and the relationship between downstream set-point pressure and water discharge through the device is derived in such a way as to meet users pressure requirements at all nodes. The subsequent sections describe the assessment of the total cost of the controlled system, including the installation cost of the control device, the flow-dependent operation and maintenance (O&M) cost, and the pipe burst repair cost over the planning horizon. The total cost of the local RTC will be compared with that of three other scenarios: 1) no control, 2) conventional PRV and 3) remote RTC under various conditions of system size, demand pattern and leakage. The analysis shows that conventional PRV s are most appropriate for small pressure zones, with limited leakage and low-cost water while real time control is needed in larger zones, with high leakage and high-cost water. Keywords: Pressure reducing valves, real time control, economic analysis 1 Background Reducing pressure in a water distribution system can reduce leaks and pipe breaks. There are several levels of controls that can be established to control pressure in closed pressure zones. These range from 1. No control, 2. Control with conventional Pressure Reducing Valves (PRVs) with fixed pressure setting. This consists of a single PRV usual installed in a vault. 3. Real time control based on measured values from the control valve location (Local RTC). This consists of a control valve with inlet and outlet pressure sensors, a flow meter and a control device (usually a Programmable Logic Controller (PLC)) in a single vault.

2 4. Real time control based on measured values from remote locations (Remote RTC). This consists of a control point vault(s) with a pressure sensor and communication equipment to transmit pressure readings to the control valve and a control valve vault with the control valve, communication equipment and a PLC. The decision to select one approach over another depends on the estimated effectiveness as compared with the estimated cost. This paper describes an extensive analysis of the economics of the different approaches based on overall life cycle costs over a wide range of sizes, demand patters, value of water, leakage rates and break rates. 2 Approach The decision as to the best type of pressure control is based on the present worth of 1. Cost to install control measures, 2. Cost of water loss due to leakage, 3. Cost of repair of pipe breaks. Some of the parameters that were held constant during the analysis were: 1. Cost estimate for each type of control, 2. Operating cost for each type of control is small relative to other costs, 3. Interest rate = 3% 4. Study period = 40 years 5. No storage downstream 6. Inlet HGL = 40 m 7. Target HGL = 25 m 8. Leak exponent = 1 9. Cost of break = 900 euros 10. Fraction of break pressure dependent = Number of nodes in model = The utility already runs a reasonably thorough leak detection and repair program 13. Reduction in pressure not large enough to adversely affect fire flow or service 14. Reduction in pressure will not significantly affect customer satisfaction 15.The analysis is for a fixed sized system with no long-term growth trend. The range of conditions for which the present worth was calculated was: 1. System: Larger zone average flow = 50 L/s ; Smaller zone = 5 L/s 2. Operating and Maintenance (O&M) unit cost for water; to 1.0 euro/1000l 3. Leak rate initial: Low = 4%, Medium = 15%, High = 30% 4. Demand pattern: Peaked = Peak:Average ~ 3, Smooth = Peak:Average ~ 1.5 Details of the formulation and cost calculations and literature review are provided in Creaco and Walski [1] and Creaco and Walski [2]. 3 Results

3 The present worth of costs for the four types of controls were calculated for the two different sized systems, for three leakage rates, for two demand patterns and a range of unit O&M costs. Some typical results are shown in Figure 1. Figure 1a. Large system with smooth demand pattern, low leakage and medium costs Figure 1b. Large system, smooth demand, low cost, low leakage Several hundred of these analyses were performed and they generally showed that the unit cost of water was the most important factor in determining which control strategy worked the best. The implication is that when water is expensive (e.g. membrane processes, desalinization, extensive pumping), it is worthwhile spending a considerable amount of money to reduce leakage and breaks. On the other hand, if there is very little operating cost (e.g. mountain reservoir with no pumping and minimal treatment), there is little reason to use expensive control strategies. If pressures are so high that they are bursting pipes, then a conventional PRV is adequate.

4 The next most important parameter was the size of the zone being served. If the zone is small, then a conventional PRV could adequately control pressure. However, if the zone was large, a conventional PRV could not anticipate the head loss across the zone and the outlet setting had to be based on large head loss which resulted in higher pressures than needed at the inlet to the zone and hence higher leakage most of the time. If the existing leakage was high, then it was more worthwhile using real time control, but only if the unit cost of water was significant. To summarize the results of the many runs, Figure 2 was prepared. Each graph in the figure corresponds to a specific leakage rate and demand pattern. The graphs show the percent of cost of the do nothing (no controls) alternative that could be saved by implementing the particular control strategy. The best solution for each cost is the highest line on the graph. For low cost water, there is no line about the horizontal axis which means that the controls do not pay for themselves (as long as the pressure is not so high to damage customer pumlong). As the cost of water increases, the percent savings due to reducing leaks and breaks increases and depending on the demand pattern and system size, different controls are best. The graphs show that for small systems, when pressure control is desirable, a conventional PRV is usually adequate. However, in the large systems with peaked demands and higher costs, real time control is generally preferred. The costs of Local rear-time control were only marginally less that Remote RTC while the benefits of Remote RTC were only marginally higher than Local RTC. This shows up in the graphs as lines for the two RTC options being close to one another.

5 Figure 2a. Savings from pressure control in large system

6 Figure 2b. Savings from pressure control in small system

7 While local costs and other factors will play a part in any decisions on use of controls, the overall trend is illustrated in Figure 3. Figure 3. General conditions favoring each type of pressure control The calculations above assume that there has been a reasonable effort to detect and repair leaks. Allowing leaks to run and trying to minimize them using pressure control is uneconomical. Pressure control is meant to reduce leakage rates from small, difficult to detect background leaks. The calculations were also based on the assumption that pressure control will not significantly reduce capital expenditures. If pressure control can reduce flow to the extent that the utility can eliminate a new treatment facility or delay construction of a well, then the unit cost savings would be much larger. However, pressure control usually results in marginal changes in total demand and should not dramatically affect the selection of control strategy. Pressure management can have impacts on customer satisfaction and fire protection. The changes in pressure in this analysis were small enough so that this should not occur, but this needs to be considered in application to real systems. 4 Summary In general, sophisticated real time control measures to manage pressure are most attractive in zones where the value of variable O&M costs of water are high, systems are large, demands vary significantly and leakage is high. As zones become smaller, a conventional PRV is generally adequate. As the value of water decreases, then pressure control for break prevention becomes more important that leak reduction. The larger the system, the greater the benefits of Remote RTC over Local RTC.

8 As the paper indicates, the decision on the type of pressure management is highly site specific and depends on many factors. This paper provided some general guidelines for selecting the best type of pressure management but the final decision depends on site specific conditions. References: [1] Creaco E., and Walski T. (2017). Economic Analysis of Pressure Control for Leakage and Pipe Burst Reduction. J. Water Resour. Plng. Mgmt., 2017, 143(12): [2] Creaco E., and Walski, T. (under review) Local or Remote Real Time Control? This is the question, under review J. Water Resour. Plng. Mgmt.