Industrial electronic devices as a cost-saving measure in water distribution and treatment

Transcription

1 Industrial electronic devices as a cost-saving measure in water distribution and treatment Pascal BONNEFOI ECOLE SUPERIEURE D ELECTRICITE Engineer - Water Segment Manager Schneider Electric Industries SA, Grenoble, France Contents Synopsis 1 1. Introduction 2 2. Motor starter or electronic variable speed drive? Economic advantages of an electronic starter Economic advantages of an electronic variable speed drive 4 3. Specific application examples A pumping station Two wastewater treatment stations 6 4. Seeking greater overall efficiency 7 5. Conclusion 8 Acknowledgements 8 Synopsis The important part of electrical energy, within the cost of water distribution and treatment, requires water stations to be optimized and/or modernized. Industrial electronic devices intended for control and monitoring, such as starters and electronic variable speed drives, allow between 25 and 30% of electrical energy cost to be saved and add many functions. Moreover, the expansion of automation, which is linked to the expansion of applications through the Internet, offers new management and savings. Schneider Electric Industries Page 1 of 8

2 Industrial electronic devices as a cost-saving measure in water distribution and treatment Reducing the cost of electrical energy has become an imperative for managers and operators of water distribution systems and treatment stations. Industrial electronic devices, the use of which is growing in this sector, offer a practical solution to these requirements and one that has already been proven in numerous installations. 1. Introduction In all industrialized countries, the distribution and treatment of water is now extremely reliant on electrical energy. It has been found that water management involves the control of three key balance sheet items which, according to the country, are divided more or less equally. These items are: electricity consumption, purchase of treatment products (production and sewerage), and the personnel delivering the services. And electricity consumption often accounts for 30% of the total. Reducing this item has therefore become an imperative for operators and managers of water distribution systems. Within this context, industrial electronic devices are increasingly being used in this sector: To adjust the power factor in order to avoid penalties from the electrical utility and also to reduce initial investment (size of equipment) For management purposes, to reduce billing costs (choosing the cheapest tariff periods) To control motors (on pumps, agitators, etc.) as a means of reducing consumption and extending the service life of equipment To replace traditional control methods such as motor-driven valves, which lead to unnecessary energy consumption by pump motors In each of these cases the savings, depending on the load level and the size of the installation, can be substantial. Motor control offers particularly great potential, however, as it allows a direct reduction in consumption, in other words in the number of kvah to be paid for. That is why the remainder of this article is primarily devoted to this issue. 2. Motor starter or electronic variable speed drive? In a pumping station for crop irrigation, for example, the requirement is to start up the motor in the evening at full load and to stop it at dawn, for which purpose a simple starter is sufficient. In another scenario, managing the variable flow rates of water jets in a fountain will require a variable speed drive in order to achieve an animated effect. In the same way, to obtain a constant water pressure in the showers on every floor of a hotel simultaneously, regardless of the flow rate, the pressure will be regulated by means of a variable speed drive. An electronic starter gradually revs the motor up to its nominal speed, but does not provide the option of operating at other speeds. It is suitable for any type of simple application, including pumps. This electronic solution is replacing the conventional star-delta reduced-voltage pump controller. It is recommended for use in centrifugal pump applications where the aim is to limit the starting current, and to this end this solution is suitable for quadratic torque type applications. It is the most economical solution for controlling the acceleration and deceleration of the machine (see inset 1 opposite). Schneider Electric Industries Page 2 of 8

3 A variable speed drive controls the power supplied to the motor to adjust its speed to the mechanical requirement. The speed reference comes from a process regulator (flow or pressure). This is a proportional-integral type regulator, which can either be external or built into the drive. Preset speeds and a faster/slower function are sometimes also used in order to work with logic control commands. A combination of variable speed drive + asynchronous motor can usefully replace other mechanical solutions based on control valves, which operate by reducing the effective cross-section of the pipe (speed adjusted to flow rate). Of the many advantages of speed variation (see inset 2), this change is the most promising in terms of energy savings, and the elimination of control valves greatly simplifies hydraulic installations. What is more, these valves are a major source of pressure losses. The difference in usage between a starter and a variable speed drive therefore lies not in the simple transmission of start/stop commands, which both perform equally well, but in the additional functions that are offered by a variable speed drive but not by a starter and that are necessary and/or desirable for controlling motors Economic advantages of an electronic starter Its main advantage lies in the suppression of pressure shocks (water hammer) during starting and stopping: it forms the ideal partner to the water hammer arresters fitted in distribution systems. In this way it helps to extend the service life of hydraulic equipment and hence to reduce overall costs. A further advantage is the reduction in the starting current, which in the case of large equipment helps to cut installation costs. Inset 1 Advantages of electronic starters (e.g. the Altistart 48 from Schneider Electric) Improved management of hydraulic transients (suppression of water hammer) Simple pump regulation (regardless of load status) Protection against underload (running dry), phase failure or phase reversal, and in the event of blockage Thermal protection of motor Reduction of variations in electricity supply (current peak at start-up and voltage dips) Limitation of energy losses and temperature rises in the electrical cabinet by bypassing the starter Option of remote communication via communication bus Reduction in operating costs (electricity consumption and maintenance) Inset 2 Advantages of electronic variable speed drives (e.g. the Altivar 38 from Schneider Electric) in addition to the advantages of electronic starters: Adjustment of flow rate or pressure to required level Greater efficiency in continuous duty Elimination of control valves Fully automatable Simple and reliable control Substantial reduction in civil engineering costs Significant energy savings Schneider Electric Industries Page 3 of 8

4 2.2. Economic advantages of an electronic variable speed drive As any expert in the field knows, in order for a device to be cost-efficient it needs to operate at close to optimum efficiency for the maximum possible time. Figure 1 shows an efficiency curve for a centrifugal pump, the most commonly used pump. The normal utilization zone is the crosshatched zone in which: the mechanical stresses on the hydraulic components are normal and balanced overall energy efficiency is satisfactory a specific point known as the best efficiency point (BEP) is located. This is the point at which the pump returns the maximum absorbed energy to the liquid. And for many pumps this is the minimum wear point (minimum stress and vibration). Efficiency (%) BEP Q (m3/h) Fig. 1. The BEP is the point at which the pump returns the maximum absorbed energy to the liquid. However, this approach requires the addition of the characteristics of the hydraulic system and the operating constraints (flow and pressure) in order to obtain an operating point (Figure 2). Manometric lift (=Pressure) characteristic Operating point => The actuating pressure and the receiving pressure are balanced, the fluid has reached its speed. Hydraulic system characteristic The pump is delivering a pressure greater than the needs of the application => the liquid accelerates. Flow rate(m 3 /h) The pump is delivering a pressure below the needs of the application => the liquid cannot be accelerated. Fig. 2. The operating point of a centrifugal pump The inclusion of a variable speed drive can satisfy these requirements by eliminating the use of control valves, which operate by reducing the effective cross-section of the pipe (Figure 3). Schneider Electric Industries Page 4 of 8

5 a/ b/ Pressure Hmax Pmin Operating point, valve almost closed Intermediate operating point characteristic Operating point, valve open Pressure Hmax Pmin characteristic at N4 characteristic at N5 characteristic at N6 characteristic at N7 Hydraulic system Qmax Flow rate Qmax Flow rate Fig. 3. Varying the flow rate, a/ using valves, b/ using speed control Pressure is then adjusted to flow rate in optimum conditions with no waste of energy (Figure 4). Fig 4a: Regulation by valve Fig 4b: Regulation by variable speed drive Pressure Pressure Hm Hydraulic system Valve partially closed at nominal speed Hydraulic system Valve open Hm at variable speed Medium flow rate Flow rate Hydraulic power = Pressure x Flow rate Medium flow rate Flow rate Fig. 4. Power consumption at medium flow rate, clearly favoring the variable speed drive 3. Specific application examples 3.1. A pumping station Example of the new collection and booster station in Royan, France. The town s water consumption exhibits a seasonal fluctuation due to tourism. The requirement was for a control system that would guarantee an appropriate pressure for consumers, whatever the conditions of use. The proposed solution combines automation, start-up, control and monitoring of the motors for a subsurface pump at a depth of 200m in a 350m well and for a 450m 3 /h flow booster, using variable speed drives. Schneider Electric Industries Page 5 of 8

6 a/ b/ Fig. 5. a/ Overall solution combining electricity distribution and pump control in a prefabricated concrete housing b/ View of the variable speed drive and the control cabinet 3.2. Two wastewater treatment stations Example of biological process optimization at the sewage treatment plant in Limoges, France, which is designed to treat the effluent corresponding on average to 236,000 inhabitants. There were a number of objectives behind the extension and modernization of this station: To restrict the discharge of wastewater directly into the River Vienne To extend its sewage treatment capacity To introduce complementary nitrogen and phosphorus treatment The biological process is to be optimized by controlling the flow of air and in particular by providing stable oxygenation of the tanks by means of speed-controlled compressors. Fig. 6. Treatment tank, compressor, and control cabinet Schneider Electric Industries Page 6 of 8

7 Example of the modernization and extension of the north Budapest plant in Hungary, which treats up to 200,000m 3 of wastewater per day. To meet changes in European standards and to safeguard the environment, the operator had to modernize the existing infrastructure. The solution for the full automation of the plant is based upon a network of PLCs and includes a control and monitoring system. Site operations have been optimized by the use of an Ethernet network and the installation of a web server. The operation of the installation can be continuously monitored either on site or remotely, by any authorized person. Data analysis and data processing take place in real time, allowing faster and more relevant decisionmaking, with a reduction in energy and maintenance costs. Fig. 7. The wastewater treatment plant in Budapest 4. Seeking greater overall efficiency Efficiency nowadays is rarely measured at an equipment level only but more often in terms of an installation or a complex infrastructure. When considering the choice of component for an assembly it is therefore helpful to take into account not only electrical distribution but also all automation functions and finally all remote functionalities: remote monitoring, remote control and remote management. That is why Schneider Electric frequently becomes involved in research into such assemblies. Recent technological innovations in this area are introducing new services with novel implementation options, in particular the possibilities provided by the Ethernet standard and the Internet. For example, from an office computer equipped with an Internet browser an operator can access web pages showing an animated model of the pumping station and of each motor-driven pump, along with a display of its various parameters (speed, flow rate, input and output status), allowing diagnostics and remote adjustment in the event of a fault. Monitoring is simplified, and the time savings when intervention is required are obvious. Schneider Electric Industries Page 7 of 8

8 Fig. 8. Example of the Transparent Factory application proposed by Schneider Electric 5. Conclusion Of the various ways in which this maximum global efficiency can be achieved, having the support of a manufacturer that designs motor control and remote control assemblies and also understands the problems inherent in the distribution and treatment of water is a guarantee of success. More information is available on the Internet: In the section on electrical engineering solutions for hydraulic applications: Acknowledgements Pascal BONNEFOI, ECOLE SUPERIEURE D ELECTRICITE Engineer Water Segment Manager within Schneider Electric since He joined Merlin Gerin in 1986 to work on dependability solutions. In 1990 he was made responsible for training customers in the use of electrical installation dimensioning tools. He was subsequently involved in the international development of a number of businesses, including Software, Services (maintenance and modernization) and Medium Voltage. Schneider Electric Industries Page 8 of 8