- Email: [email protected]
Assessing the Operation of the Cooling Water System of a Hydro-Power Plant Using EPANET
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 112 (2017) 51 – 57
Sustainable Solutions for Energy and Environment, EENVIRO 2016, 26-28 October 2016, Bucharest, Romania
Assessing the Operation of the Cooling Water System of a HydroPower Plant Using EPANET Diana Maria Bucura, Costin Ioan Cosoiub, Raluca Gabriela Iovanela, Alina Alexandrina Nicolaea, Sanda-Carmen Georgescua,* b
a Power Engineering Faculty, University “Politehnica” of Bucharest, 313 Spl. Independentei, Bucharest 060042, Romania Hydraulics and Environmental Protection Dept, Technical University of Civil Engineering Bucharest, 124 Lacul Tei, Bucharest 020396
Abstract This paper focuses on the Cooling Water System (CWS) of Vidraru Hydro-Power Plant (HPP) a 220 MW underground HPP, on the Arges River in Romania. It is equipped with 4 high head vertical Francis turbines of 55 MW each, 4 hydropower generators of 61 MVA each, and 7 step-up transformers of 40 MVA each. The goal is to model the above CWS and to analyse the system response under normal and critical operating scenarios. The numerical model built in EPANET is complex: the cooling water network is normally fed by pumping from the underground tailrace (using up to 3 pumps of 110 kW each), but there is a backup supplying circuit fed directly from the penstock bifurcation pipe (using pressure reducing valves). Within the power plant, the following equipments need cooling water: air-water heat exchangers (related to generators and HVAC system); oil-water heat exchangers (related to bearings, speed governors and transformers); coolers of the turbines seals. Since EPANET lacks such physical components, they are artificially replaced by throttle control valves set with equivalent loss coefficients, established upon pressure drop values available for each equipment. The computed flow rate and pressure values match the existing recordings for normal operation. The numerical model also provides data on the system behaviour under critical operation conditions, e.g. when the HPP is operating at full capacity with CWS fed only by 2 pumps, out of 3 available pumps. © 2017 2017Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy 2016. and Environment and Environment 2016 Keywords: EPANET; Hydro-Power Plant; heat exchanger; cooler; cooling water system; throttle control valve
* Corresponding author. Tel.: +4-072-362-4418; fax: +4-021-318-1015. E-mail address: [email protected]
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy and Environment 2016 doi:10.1016/j.egypro.2017.03.1058
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Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
1. Introduction Due to operation safety, the cooling water system is an important issue within any Hydro-Power Plant (HPP) [1]÷[3]. In this paper, the case study focuses on the cooling water system of Vidraru Hydro-Power Plant, a high head HPP placed on Arges River in Romania. Vidraru HPP was commissioned in December 1966. It is an underground power plant, built at 105 m below the ground level. The installed power reaches 220 MW, the maximum site gross head is of 324 m (the minimum gross head value is 272 m), the installed flow rate equals 90 m3/s, and the average produced energy is of 400 GWh per year. A multi-annual average flow rate of 19.7 m3/s feeds Vidraru Lake a reservoir of 465 million m3 total volume, which mainly supplies the water distribution system of Bucharest; it also supplies other urban water distribution and irrigation systems, as well as the cascade of 17 Hydro-Power Plants built on Arges River, Vidraru HPP being the upstream-one and the biggest. Vidraru Hydro-Power Plant is equipped with [4]: x 4 vertical axis Francis turbines, built in 1965; for each turbine, the rated output is 55 MW (the maximum power is 56.5 MW); the head varies from 242 to 290 m; the flow rate varies from 18 to 22.5 m 3/s; the runner diameter equals 2.4 m; x 4 synchronous hydro generators, each of 61 MVA apparent power, 55 MW active power, 10500r5% V output voltage, 3350 A output current, 50 Hz frequency and a rated speed of 428.6 rpm; x 7 step-up transformers of 40 MVA 220/10.5/10.5 kV each. The cooling water system of Vidraru HPP is fed from a water tank [4] (a concrete tank built at the level of the machine hall). The water tank is normally filled by pumping from the underground tailrace (figure 1), through up to 3 centrifugal pumps (of 110 kW each at 0.134 m3/s rated flow rate and 58 m pumping head; the pumps speed is constant: 2900 rpm). If necessary, the tank can be filled directly from the penstock bifurcation pipe, through 4 parallel pipes with Pressure Reducing Valves (PRV); the capacity of each PRV branch is 0.1 m 3/s; the maximum inlet pressure is 40 bar, and the output pressure is 5.5 bar. The emptying pipe of the tank is connected to the tailrace. The cooling water flows from the tank to the consumers, then it is discharged into the tailrace.
Fig. 1. Cooling water system feeding [4].
Fig. 2. Consumers that need cooling water [4].
As drawn in figure 2, the cooling water supplies the consumers through 3 main branch-pipes: a branch feeds the transformers (this pipe splits up according to the existing 2 power lines, mounted along the transformers, on each side of the access corridor); a small branch feeds the HVAC system of the powerhouse; the main branch feeds the Hydro-Power units (a HP-unit consists of a turbine and its generator). The existing HP-units are labelled HPU1÷ HPU4 in figure 2. The consumers that need cooling water are the following equipments and lubrication systems [4]:
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x air-water heat exchangers related to hydro generators (there are 6 sets of 3 air coolers around a generator); x oil-water heat exchangers related to bearings a heavy-duty thrust bearing (requesting 7 l/s of cooling water at 4 bar, ensured through 4 coolers) is placed at the upper part of the generator, while a guide bearing (requesting 3 l/s of cooling water at 4 bar, ensured through 6 smaller coolers) is placed at the lower part of the generator; another guide bearing (with one cooler) is attached to the hydraulic turbine; x oil-water heat exchangers related to step-up transformers (2 coolers are attached to each transformer; the oil flows outside the water pipes, being pumped through oil pumps; the oil pressure is kept greater than the water pressure); x oil-water heat exchangers related to speed governors; x coolers of the turbine main shaft axial seals; x air-water heat exchangers related to the HVAC system of the powerhouse. From the consumers, the water is discharged into the tailrace as following [4]: from each HP-unit, the cooling water is discharged near each turbine, through a check valve; from all transformers and from the HVAC system, the cooling water is collected into a pipe and finally discharged near HPU4. Along the cooling water circuit, all pipes are thermally insulated. The incoming cooling water, of 15qC temperature, is filtered (e.g. for each HP-unit, there are 2 filters, each of 0.06 m3/s capacity). The goal of this study is to model the above Cooling Water System (CWS) and to analyse the system response under normal and critical operating scenarios. To achieve that, we built in EPANET a numerical model that simulates Vidraru CWS operation for different working scenarios. EPANET software is commonly used to model complex water distribution systems [5]÷[9] and other hydraulic systems [10]÷[13]. Despite the fact that EPANET lacks physical components like coolers and filters [14], [15], the software capabilities can be extended to cooling water systems, if the CWS numerical model can be calibrated upon recordings [3], [16]. Thus, knowing that important minor losses are attached to coolers and filters [17], [18], one can replace them in the numerical model by Throttle Control Valves [3] that can be set with equivalent loss coefficients. 2. Numerical model built in EPANET The numerical model built in EPANET for Vidraru CWS (figure 3) is equivalent to the real hydraulic system from figures 1 and 2. The pipes length and diameters match the existing geometric configuration [4]; the model incorporates the existing valves pressure reducing valves (PRV), shutoff valves, discharge control valves and check valves, as well as the existing pumps. The curves of the pumps, namely pumping head H in meters versus flow rate Q in m3/s, and efficiency K in percents versus Q in m3/s, are defined by the following polynomials: 2 ° H H Q 106 2660Q ® 2 3 ° ¯K K Q 1282Q 7296Q 12903Q .
(1)
The cooling water sources, namely the tailrace and the penstock bifurcation pipe (figure 1), are modelled as reservoirs, which represent infinite sources of water in EPANET [15]. The cooling water tank (figures 1 and 2) is modelled in EPANET as a storage tank, which fills and empties upon consumption; an equivalent diameter is attached to that tank, to obtain the rated volume between the minimum and maximum water levels. The coolers, described above as consumers, as well as the HP-unit's filters are replaced on the network map by 27 Throttle Control Valves (TCV) coloured in red in figure 3; the loss coefficients of those equivalent TCVs are established upon pressure drop values available for each equipment [4]. To simplify the numerical model, for each HP-unit, all coolers are replaced by 6 TCVs, namely one equivalent TCV per group of coolers, listed in figure 2 from top to bottom as: speed governor; heavy-duty thrust bearing; generator's air coolers; generator guide bearing; turbine guide bearing; turbine seals. All heat exchangers related to the step-up transformers [19] are replaced by two equivalent TCVs, namely one on each branch that feeds the transformers (figure 2 and 3). The small branch that feeds the HVAC system (figure 2) splits up, in fact, in 4 smaller pipes that feed the air-coolers, but to simplify the model, the HVAC system is unified within a single end-consumer, represented by one equivalent TCV.
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Fig. 3. Numerical model of the cooling water system of Vidraru HPP the consumers (equivalent TCVs) are coloured in red.
The CWS numerical model presented in figure 3 consists of: 2 reservoirs and one storage tank, 84 pipes, 146 junctions, 3 constant speed centrifugal pumps and 70 valves (4 valves are set as PRVs, while the remaining valves are set as TCVs). Among all TCVs, 27 valves are the equivalent consumers described above: they are set open or closed, depending on the CWS working scenario; 39 valves are shutoff valves. Two shutoff valves, placed before and after the PRV system, as well as all 4 PRVs are set normally-closed; they are open only when the tank must be fed from the penstock bifurcation pipe. The tank can be fed simultaneously from the tailrace (by pumping) and from the penstock (through the PRV system), or it can be fed using only one of those two water sources; the tailrace source is the preferred option. On the discharge pipe of each pump, there is a TCV and a check valve (CV); the discharge TCV is set open or closed, upon the working scenario (within the simulation, a pump operates at a certain working point or it is shutdown); the CV is set as pipe-property [14], [15] in EPANET. The remaining shutoff valves are active, e.g. being set normally-open when all HP-units are working. 3. Numerical results Based on the proposed numerical model, we modelled the operation of the cooling water system of Vidraru HPP for several scenarios, where a different number of hydro-power units are working at their rated power, for various options of cooling water supply (the storage tank can be fed both from the penstock and from the tailrace/ or only from the tailrace/ or only from the penstock; when the water is pumped from the tailrace, a different number of pumps are open). Among the multitude of possible cases, we present further the following 4 working scenarios: c all 4 HP-units in operation; tank fed by all 3 pumps (PRV system closed); the cooling water total flow rate Qt reaches its maximum value: Qt Qt max ; d all 4 HP-units in operation; tank fed from the penstock (pumps closed); here also Qt Qt max ; e all 4 HP-units in operation; tank fed only by 2 pumps (PRV system closed; one pump kept closed); the condition Qt Qt max depends on the initial water level in the storage tank; f 2 HP-units in operation (HPU4 and HPU3 shutdown); half capacity transformers' consumers; tank fed by one pump (PRV system closed; 2 pumps kept closed); CWS operation depends on the initial water level in the tank.
Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
For the selected scenarios, the corresponding flow rate distribution on pipes (in m3/h) and cooling water pressure distribution in nodes (in mH2O) are presented in figures 4÷7. Since the numerical model was calibrated upon in-situ recordings for all consumers (through equivalent TCVs, which loss coefficients ensure minor losses equal to the real pressure drop values), the computed cooling water flow rate and pressure values match the requested values.
Fig. 4. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario c: all HP-units are working, tank fed by 3 pumps.
Fig. 5. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario d: all HP-units are working, tank fed from the penstock.
55
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Fig. 6. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario e: all HP-units are working, tank fed by 2 pumps.
Fig. 7. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario f: HPU1 and HPU2 in operation; HPU3 and HPU4 shutdown; tank fed by a single pump.
For critical scenarios, like scenarios e and f, the requested CWS operation is ensured only when the initial water level in the storage tank is near the upper level limit. It is recommended to monitor the cooling water consumption, by keeping or restoring the water level in the tank as close as possible to the maximum level.
Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
4. Conclusions We built in EPANET a numerical model to study the cooling water system of Vidraru Hydro-Power Plant a 220 MW underground HPP. Since EPANET lacks physical components as coolers and filters, they are artificially replaced within the network map by throttle control valves, set with equivalent loss coefficients, computed upon insitu recordings concerning pressure drop values for coolers and filters. Simulations were performed for different working scenarios, combining the operation of the hydro-power units and adjacent equipments, on one hand, and the feeding options available for the cooling water storage tank, on the other hand. As expected, the numerical model calibration ensured the match between computed hydraulic parameters and corresponding recordings for normal working scenarios. The model flexibility allows gaining insights on the system behaviour during critical working scenarios. Thus, the storage tank operation can be tested numerically, to check the cooling water system response, in order to prevent any real improper filling or emptying of the tank, which can disrupt the power plant operation. Acknowledgements Thanks are due to Curtea de Arges Hydropower Branch of the Romanian company Hidroelectrica SA, for data availability [4]. This work has been supported by the Executive Agency for Higher Education, Research, Development and Innovation, PN-II-PT-PCCA-2013-4, ECOTURB Project. References [1] Dunca G, Bucur DM, Isbasoiu EC, Calinoiu C, Ghergu C. Vibration level analysis during the operation of a high head Hydro Power Plant. UPB Sci. Bull, Series D 2012;74(1):59-66. [2] Dunca G, Isbasoiu EC, Calinoiu C, Bucur DM, Ghergu C. Vibrations level analyse during pumping station Gâlceag operation. UPB Sci. Bull, Series D 2008;70(4):181-190. [3] Georgescu S-C, Georgescu A-M, Jumara A, Piraianu V-F, Dunca G. Numerical simulation of the cooling water system of a 115 MW HydroPower Plant. Energy Procedia 2016;85:228-234. [4] Apostu (Nicolae) AA. Hydraulic analysis of the cooling water system of Vidraru Hydro-Power Plant (in Romanian). Internship Report. Vidraru Hydro-Power Plant, Curtea de Arges Hydropower Branch, Hidroelectrica SA, Romania; 2014. [5] Georgescu S-C, Georgescu A-M, Madularea RA, Piraianu V-F, Anton A, Dunca G. Numerical model of a medium-sized municipal water distribution system located in Romania. Procedia Eng 2015;119:660-8. [6] Alves Z, Muranho J, Albuquerque T, Ferreira A. Water distribution network's modeling and calibration. A case study based on scarce inventory data. Procedia Eng 2014;70:31-40. [7] Georgescu S-C, Georgescu A-M. Application of HBMOA to pumping stations scheduling for a water distribution network with multiple tanks. Procedia Eng 2014;70:715-723. [8] Georgescu A-M, Georgescu S-C. Numerical modelling of chlorine distribution in an urban water supply system. Environ Eng Manag J 2013; 12(4):657-664. [9] Georgescu A-M, Georgescu S-C. Chlorine concentration decay in the water distribution system of a town with 50000 inhabitants. UPB Sci. Bull, Series D 2012;74(1):103-114. [10] Georgescu A-M, Georgescu S-C, Cosoiu CI, Hasegan L, Anton A, Bucur DM. EPANET simulation of control methods for centrifugal pumps operating under variable system demand. Procedia Eng 2015;119:1012-9. [11] Georgescu S-C, Georgescu A-M. Pumping station scheduling for water distribution networks in EPANET. UPB Sci. Bull, Series D 2015; 77(2):235-246. [12] Georgescu A-M, Cosoiu C-I, Perju S, Georgescu S-C, Hasegan L, Anton A. Estimation of the efficiency for variable speed pumps in EPANET compared with experimental data. Procedia Eng 2014;89:1404-1411. [13] Georgescu S-C, Popa R, Georgescu A-M. Pumping stations scheduling for a water supply system with multiple tanks. UPB Sci. Bull, Series D 2010;72(3):129-140. [14] Georgescu S-C, Georgescu A-M. EPANET Manual (in Romanian). Bucharest: Printech Press; 2014. [15] Rossman L. EPANET 2 Users Manual. U.S. Environmental Protection Agency: EPA/600/R-00/057. Cincinnati, OH, USA; 2000. [16] Dunca G, Bucur DM, Jonsson P, Cervantes MJ. Discharge measurements using the pressure-time method: different evaluation procedures. UPB Sci. Bull, Series D 2014;76(4):195-202. [17] Darie G, Petcu HI. Methodology and software for prediction of cogeneration steam turbines performances. In: Plesu V., Agachi PS, editors. Computer-Aided Chemical Engineering. Elsevier Science Ltd; 2007;24:1103-1108. [18] Necula H, Ghizdeanu EN, Darie G. An experimental analyse of the heat transfer for a tube bundle in a transversally flowing air. WSEAS Transactions on Heat and Mass Transfer 2006;1(3):349-355. [19] Costinas S, Georgescu S-C, Opris I. Smart solutions for the auxiliary power supplies schemes in hydropower plants. UPB Sci. Bull, Series C 2014;76(3):245-253.
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ScienceDirect Energy Procedia 112 (2017) 51 – 57
Sustainable Solutions for Energy and Environment, EENVIRO 2016, 26-28 October 2016, Bucharest, Romania
Assessing the Operation of the Cooling Water System of a HydroPower Plant Using EPANET Diana Maria Bucura, Costin Ioan Cosoiub, Raluca Gabriela Iovanela, Alina Alexandrina Nicolaea, Sanda-Carmen Georgescua,* b
a Power Engineering Faculty, University “Politehnica” of Bucharest, 313 Spl. Independentei, Bucharest 060042, Romania Hydraulics and Environmental Protection Dept, Technical University of Civil Engineering Bucharest, 124 Lacul Tei, Bucharest 020396
Abstract This paper focuses on the Cooling Water System (CWS) of Vidraru Hydro-Power Plant (HPP) a 220 MW underground HPP, on the Arges River in Romania. It is equipped with 4 high head vertical Francis turbines of 55 MW each, 4 hydropower generators of 61 MVA each, and 7 step-up transformers of 40 MVA each. The goal is to model the above CWS and to analyse the system response under normal and critical operating scenarios. The numerical model built in EPANET is complex: the cooling water network is normally fed by pumping from the underground tailrace (using up to 3 pumps of 110 kW each), but there is a backup supplying circuit fed directly from the penstock bifurcation pipe (using pressure reducing valves). Within the power plant, the following equipments need cooling water: air-water heat exchangers (related to generators and HVAC system); oil-water heat exchangers (related to bearings, speed governors and transformers); coolers of the turbines seals. Since EPANET lacks such physical components, they are artificially replaced by throttle control valves set with equivalent loss coefficients, established upon pressure drop values available for each equipment. The computed flow rate and pressure values match the existing recordings for normal operation. The numerical model also provides data on the system behaviour under critical operation conditions, e.g. when the HPP is operating at full capacity with CWS fed only by 2 pumps, out of 3 available pumps. © 2017 2017Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy 2016. and Environment and Environment 2016 Keywords: EPANET; Hydro-Power Plant; heat exchanger; cooler; cooling water system; throttle control valve
* Corresponding author. Tel.: +4-072-362-4418; fax: +4-021-318-1015. E-mail address: [email protected]
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the international conference on Sustainable Solutions for Energy and Environment 2016 doi:10.1016/j.egypro.2017.03.1058
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Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
1. Introduction Due to operation safety, the cooling water system is an important issue within any Hydro-Power Plant (HPP) [1]÷[3]. In this paper, the case study focuses on the cooling water system of Vidraru Hydro-Power Plant, a high head HPP placed on Arges River in Romania. Vidraru HPP was commissioned in December 1966. It is an underground power plant, built at 105 m below the ground level. The installed power reaches 220 MW, the maximum site gross head is of 324 m (the minimum gross head value is 272 m), the installed flow rate equals 90 m3/s, and the average produced energy is of 400 GWh per year. A multi-annual average flow rate of 19.7 m3/s feeds Vidraru Lake a reservoir of 465 million m3 total volume, which mainly supplies the water distribution system of Bucharest; it also supplies other urban water distribution and irrigation systems, as well as the cascade of 17 Hydro-Power Plants built on Arges River, Vidraru HPP being the upstream-one and the biggest. Vidraru Hydro-Power Plant is equipped with [4]: x 4 vertical axis Francis turbines, built in 1965; for each turbine, the rated output is 55 MW (the maximum power is 56.5 MW); the head varies from 242 to 290 m; the flow rate varies from 18 to 22.5 m 3/s; the runner diameter equals 2.4 m; x 4 synchronous hydro generators, each of 61 MVA apparent power, 55 MW active power, 10500r5% V output voltage, 3350 A output current, 50 Hz frequency and a rated speed of 428.6 rpm; x 7 step-up transformers of 40 MVA 220/10.5/10.5 kV each. The cooling water system of Vidraru HPP is fed from a water tank [4] (a concrete tank built at the level of the machine hall). The water tank is normally filled by pumping from the underground tailrace (figure 1), through up to 3 centrifugal pumps (of 110 kW each at 0.134 m3/s rated flow rate and 58 m pumping head; the pumps speed is constant: 2900 rpm). If necessary, the tank can be filled directly from the penstock bifurcation pipe, through 4 parallel pipes with Pressure Reducing Valves (PRV); the capacity of each PRV branch is 0.1 m 3/s; the maximum inlet pressure is 40 bar, and the output pressure is 5.5 bar. The emptying pipe of the tank is connected to the tailrace. The cooling water flows from the tank to the consumers, then it is discharged into the tailrace.
Fig. 1. Cooling water system feeding [4].
Fig. 2. Consumers that need cooling water [4].
As drawn in figure 2, the cooling water supplies the consumers through 3 main branch-pipes: a branch feeds the transformers (this pipe splits up according to the existing 2 power lines, mounted along the transformers, on each side of the access corridor); a small branch feeds the HVAC system of the powerhouse; the main branch feeds the Hydro-Power units (a HP-unit consists of a turbine and its generator). The existing HP-units are labelled HPU1÷ HPU4 in figure 2. The consumers that need cooling water are the following equipments and lubrication systems [4]:
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x air-water heat exchangers related to hydro generators (there are 6 sets of 3 air coolers around a generator); x oil-water heat exchangers related to bearings a heavy-duty thrust bearing (requesting 7 l/s of cooling water at 4 bar, ensured through 4 coolers) is placed at the upper part of the generator, while a guide bearing (requesting 3 l/s of cooling water at 4 bar, ensured through 6 smaller coolers) is placed at the lower part of the generator; another guide bearing (with one cooler) is attached to the hydraulic turbine; x oil-water heat exchangers related to step-up transformers (2 coolers are attached to each transformer; the oil flows outside the water pipes, being pumped through oil pumps; the oil pressure is kept greater than the water pressure); x oil-water heat exchangers related to speed governors; x coolers of the turbine main shaft axial seals; x air-water heat exchangers related to the HVAC system of the powerhouse. From the consumers, the water is discharged into the tailrace as following [4]: from each HP-unit, the cooling water is discharged near each turbine, through a check valve; from all transformers and from the HVAC system, the cooling water is collected into a pipe and finally discharged near HPU4. Along the cooling water circuit, all pipes are thermally insulated. The incoming cooling water, of 15qC temperature, is filtered (e.g. for each HP-unit, there are 2 filters, each of 0.06 m3/s capacity). The goal of this study is to model the above Cooling Water System (CWS) and to analyse the system response under normal and critical operating scenarios. To achieve that, we built in EPANET a numerical model that simulates Vidraru CWS operation for different working scenarios. EPANET software is commonly used to model complex water distribution systems [5]÷[9] and other hydraulic systems [10]÷[13]. Despite the fact that EPANET lacks physical components like coolers and filters [14], [15], the software capabilities can be extended to cooling water systems, if the CWS numerical model can be calibrated upon recordings [3], [16]. Thus, knowing that important minor losses are attached to coolers and filters [17], [18], one can replace them in the numerical model by Throttle Control Valves [3] that can be set with equivalent loss coefficients. 2. Numerical model built in EPANET The numerical model built in EPANET for Vidraru CWS (figure 3) is equivalent to the real hydraulic system from figures 1 and 2. The pipes length and diameters match the existing geometric configuration [4]; the model incorporates the existing valves pressure reducing valves (PRV), shutoff valves, discharge control valves and check valves, as well as the existing pumps. The curves of the pumps, namely pumping head H in meters versus flow rate Q in m3/s, and efficiency K in percents versus Q in m3/s, are defined by the following polynomials: 2 ° H H Q 106 2660Q ® 2 3 ° ¯K K Q 1282Q 7296Q 12903Q .
(1)
The cooling water sources, namely the tailrace and the penstock bifurcation pipe (figure 1), are modelled as reservoirs, which represent infinite sources of water in EPANET [15]. The cooling water tank (figures 1 and 2) is modelled in EPANET as a storage tank, which fills and empties upon consumption; an equivalent diameter is attached to that tank, to obtain the rated volume between the minimum and maximum water levels. The coolers, described above as consumers, as well as the HP-unit's filters are replaced on the network map by 27 Throttle Control Valves (TCV) coloured in red in figure 3; the loss coefficients of those equivalent TCVs are established upon pressure drop values available for each equipment [4]. To simplify the numerical model, for each HP-unit, all coolers are replaced by 6 TCVs, namely one equivalent TCV per group of coolers, listed in figure 2 from top to bottom as: speed governor; heavy-duty thrust bearing; generator's air coolers; generator guide bearing; turbine guide bearing; turbine seals. All heat exchangers related to the step-up transformers [19] are replaced by two equivalent TCVs, namely one on each branch that feeds the transformers (figure 2 and 3). The small branch that feeds the HVAC system (figure 2) splits up, in fact, in 4 smaller pipes that feed the air-coolers, but to simplify the model, the HVAC system is unified within a single end-consumer, represented by one equivalent TCV.
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Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
Fig. 3. Numerical model of the cooling water system of Vidraru HPP the consumers (equivalent TCVs) are coloured in red.
The CWS numerical model presented in figure 3 consists of: 2 reservoirs and one storage tank, 84 pipes, 146 junctions, 3 constant speed centrifugal pumps and 70 valves (4 valves are set as PRVs, while the remaining valves are set as TCVs). Among all TCVs, 27 valves are the equivalent consumers described above: they are set open or closed, depending on the CWS working scenario; 39 valves are shutoff valves. Two shutoff valves, placed before and after the PRV system, as well as all 4 PRVs are set normally-closed; they are open only when the tank must be fed from the penstock bifurcation pipe. The tank can be fed simultaneously from the tailrace (by pumping) and from the penstock (through the PRV system), or it can be fed using only one of those two water sources; the tailrace source is the preferred option. On the discharge pipe of each pump, there is a TCV and a check valve (CV); the discharge TCV is set open or closed, upon the working scenario (within the simulation, a pump operates at a certain working point or it is shutdown); the CV is set as pipe-property [14], [15] in EPANET. The remaining shutoff valves are active, e.g. being set normally-open when all HP-units are working. 3. Numerical results Based on the proposed numerical model, we modelled the operation of the cooling water system of Vidraru HPP for several scenarios, where a different number of hydro-power units are working at their rated power, for various options of cooling water supply (the storage tank can be fed both from the penstock and from the tailrace/ or only from the tailrace/ or only from the penstock; when the water is pumped from the tailrace, a different number of pumps are open). Among the multitude of possible cases, we present further the following 4 working scenarios: c all 4 HP-units in operation; tank fed by all 3 pumps (PRV system closed); the cooling water total flow rate Qt reaches its maximum value: Qt Qt max ; d all 4 HP-units in operation; tank fed from the penstock (pumps closed); here also Qt Qt max ; e all 4 HP-units in operation; tank fed only by 2 pumps (PRV system closed; one pump kept closed); the condition Qt Qt max depends on the initial water level in the storage tank; f 2 HP-units in operation (HPU4 and HPU3 shutdown); half capacity transformers' consumers; tank fed by one pump (PRV system closed; 2 pumps kept closed); CWS operation depends on the initial water level in the tank.
Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
For the selected scenarios, the corresponding flow rate distribution on pipes (in m3/h) and cooling water pressure distribution in nodes (in mH2O) are presented in figures 4÷7. Since the numerical model was calibrated upon in-situ recordings for all consumers (through equivalent TCVs, which loss coefficients ensure minor losses equal to the real pressure drop values), the computed cooling water flow rate and pressure values match the requested values.
Fig. 4. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario c: all HP-units are working, tank fed by 3 pumps.
Fig. 5. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario d: all HP-units are working, tank fed from the penstock.
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Fig. 6. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario e: all HP-units are working, tank fed by 2 pumps.
Fig. 7. Flow rate (in m3/h) and pressure (in mH2O) distributions for scenario f: HPU1 and HPU2 in operation; HPU3 and HPU4 shutdown; tank fed by a single pump.
For critical scenarios, like scenarios e and f, the requested CWS operation is ensured only when the initial water level in the storage tank is near the upper level limit. It is recommended to monitor the cooling water consumption, by keeping or restoring the water level in the tank as close as possible to the maximum level.
Diana Maria Bucur et al. / Energy Procedia 112 (2017) 51 – 57
4. Conclusions We built in EPANET a numerical model to study the cooling water system of Vidraru Hydro-Power Plant a 220 MW underground HPP. Since EPANET lacks physical components as coolers and filters, they are artificially replaced within the network map by throttle control valves, set with equivalent loss coefficients, computed upon insitu recordings concerning pressure drop values for coolers and filters. Simulations were performed for different working scenarios, combining the operation of the hydro-power units and adjacent equipments, on one hand, and the feeding options available for the cooling water storage tank, on the other hand. As expected, the numerical model calibration ensured the match between computed hydraulic parameters and corresponding recordings for normal working scenarios. The model flexibility allows gaining insights on the system behaviour during critical working scenarios. Thus, the storage tank operation can be tested numerically, to check the cooling water system response, in order to prevent any real improper filling or emptying of the tank, which can disrupt the power plant operation. Acknowledgements Thanks are due to Curtea de Arges Hydropower Branch of the Romanian company Hidroelectrica SA, for data availability [4]. This work has been supported by the Executive Agency for Higher Education, Research, Development and Innovation, PN-II-PT-PCCA-2013-4, ECOTURB Project. References [1] Dunca G, Bucur DM, Isbasoiu EC, Calinoiu C, Ghergu C. Vibration level analysis during the operation of a high head Hydro Power Plant. UPB Sci. Bull, Series D 2012;74(1):59-66. [2] Dunca G, Isbasoiu EC, Calinoiu C, Bucur DM, Ghergu C. Vibrations level analyse during pumping station Gâlceag operation. UPB Sci. Bull, Series D 2008;70(4):181-190. [3] Georgescu S-C, Georgescu A-M, Jumara A, Piraianu V-F, Dunca G. Numerical simulation of the cooling water system of a 115 MW HydroPower Plant. Energy Procedia 2016;85:228-234. [4] Apostu (Nicolae) AA. Hydraulic analysis of the cooling water system of Vidraru Hydro-Power Plant (in Romanian). Internship Report. Vidraru Hydro-Power Plant, Curtea de Arges Hydropower Branch, Hidroelectrica SA, Romania; 2014. [5] Georgescu S-C, Georgescu A-M, Madularea RA, Piraianu V-F, Anton A, Dunca G. Numerical model of a medium-sized municipal water distribution system located in Romania. Procedia Eng 2015;119:660-8. [6] Alves Z, Muranho J, Albuquerque T, Ferreira A. Water distribution network's modeling and calibration. A case study based on scarce inventory data. Procedia Eng 2014;70:31-40. [7] Georgescu S-C, Georgescu A-M. Application of HBMOA to pumping stations scheduling for a water distribution network with multiple tanks. Procedia Eng 2014;70:715-723. [8] Georgescu A-M, Georgescu S-C. Numerical modelling of chlorine distribution in an urban water supply system. Environ Eng Manag J 2013; 12(4):657-664. [9] Georgescu A-M, Georgescu S-C. Chlorine concentration decay in the water distribution system of a town with 50000 inhabitants. UPB Sci. Bull, Series D 2012;74(1):103-114. [10] Georgescu A-M, Georgescu S-C, Cosoiu CI, Hasegan L, Anton A, Bucur DM. EPANET simulation of control methods for centrifugal pumps operating under variable system demand. Procedia Eng 2015;119:1012-9. [11] Georgescu S-C, Georgescu A-M. Pumping station scheduling for water distribution networks in EPANET. UPB Sci. Bull, Series D 2015; 77(2):235-246. [12] Georgescu A-M, Cosoiu C-I, Perju S, Georgescu S-C, Hasegan L, Anton A. Estimation of the efficiency for variable speed pumps in EPANET compared with experimental data. Procedia Eng 2014;89:1404-1411. [13] Georgescu S-C, Popa R, Georgescu A-M. Pumping stations scheduling for a water supply system with multiple tanks. UPB Sci. Bull, Series D 2010;72(3):129-140. [14] Georgescu S-C, Georgescu A-M. EPANET Manual (in Romanian). Bucharest: Printech Press; 2014. [15] Rossman L. EPANET 2 Users Manual. U.S. Environmental Protection Agency: EPA/600/R-00/057. Cincinnati, OH, USA; 2000. [16] Dunca G, Bucur DM, Jonsson P, Cervantes MJ. Discharge measurements using the pressure-time method: different evaluation procedures. UPB Sci. Bull, Series D 2014;76(4):195-202. [17] Darie G, Petcu HI. Methodology and software for prediction of cogeneration steam turbines performances. In: Plesu V., Agachi PS, editors. Computer-Aided Chemical Engineering. Elsevier Science Ltd; 2007;24:1103-1108. [18] Necula H, Ghizdeanu EN, Darie G. An experimental analyse of the heat transfer for a tube bundle in a transversally flowing air. WSEAS Transactions on Heat and Mass Transfer 2006;1(3):349-355. [19] Costinas S, Georgescu S-C, Opris I. Smart solutions for the auxiliary power supplies schemes in hydropower plants. UPB Sci. Bull, Series C 2014;76(3):245-253.
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