INFLUENCE OF PREHEATER LOSS IN REGENERATION EFFICIENCY ON THE SECONDARY CIRCUIT

Transcription

1 U.P.B. Sci. Bull., Series C, Vol. 7, Iss. 1, 010 ISSN x INFLUENCE OF PREHEATER LOSS IN REGENERATION EFFICIENCY ON THE SECONDARY CIRCUIT Ilie PRISECARU 1, Iulian Pavel NIŢĂ, Daniel DUPLEAC 3 Lucrarea prezintă rezultatele analizelor funcționarii circuitului secundar al unității de la Centrala Nuclearoelectrică Cernavodă în regimuri anormale produse prin defectarea preîncălzitoarelor regenerative. Lucrarea a avut ca scop optimizarea procedurilor de exploatare în scopul minimizării pierderilor de putere electrică care apar ca urmare a funcţionării anormale a centralei produsă de defectarea unui/unor preîncălzitoare. S-a realizat o metodă de calcul simplificat pentru un preîncălzitor de apă de alimentare în regimuri anormale. S-au simulat regimurile nenominale ale centralei produse de evenimentul postulat, folosind metoda simplificată. Analiza rezultatelor a demonstrat că este posibilă modificarea procedurii de operare actuale cu menţinerea reactorului la putere nominală şi minimizarea scăderilor de putere la bornele generatorului electric. Metodă de calcul rapid a preîncălzitoarelor poate fi utilizată de asemenea şi pentru determinarea disfuncţionalităţilor funcționale ale acestora. This paper presents the results of operating analyses in secondary circuit of Cernavoda NPP - Unit, in abnormal regimes, due to the failure of a feedwater preheater from the regenerative chain. Based on these analyses, operating solutions are proposed in order to minimize power losses from the normal regime. In this paper we developed a simplified method for calculating the operating parameters of the heat exchangers in abnormal regimes. Also this method allows quick detection of the abnormally operating heat exchanger. Keywords: NPP operation, abnormal regimes, minimized losses, heat exchangers, plant efficiency 1. Introduction In this paper one proposes optimizing the operating procedures in event of preheater unavailability. Current procedure, in case of isolation of a preheater, requires a reactor power step back of 10% of the rated value, what reduces the generated electric power by about 70 MWe. 1 Prof., Power Plant Engineering Department, University POLITEHNICA of Bucharest, Romania Eng., Center of Engineering and Technology for Nuclear Projects POB 504-MG-4, 409 Atomistilor Street Magurele - Ilfov, Romania 3 Prof., Power Plant Engineering Department, University POLITEHNICA of Bucharest, Romania, danieldu@cne.pub.ro

2 154 Ilie Prisecaru, Iulian Pavel Niţă, Daniel Dupleac The paper analyzes the operating regimes and thermal-hydraulic and economical implications which can occur in case of reactor power maintained at its rated value. One considers the process and instrumentation diagram a CANDU 6 Unit of Cernavoda NPP (Fig. 1). The upstream and downstream limits of the analysis are main condenser nozzles and steam generator nozzles, respectively. The simulation used as input data the result from stationary hydraulic analyses performed for Main Condensate System and FeedWater System. These analyses were based on the computational code PIPENET. In order to model and simulate the thermal hydraulics of regenerative chain we used the computational code MMS (Fig. and Fig. 3). There were examined the following possible transient regimes: Normal operating regime, used for model calibration; The regime of preheater by-pass, with reactor power stepped back to 90% of rated power (RP) for the cases: by-pass of a LP1; by-pass of the a LP and LP3 bank ; by-pass of a HP 5 The regime of by-pass of a preheater with reactor power maintained to 100% of RP for the cases: by-pass of a LP1; by-pass o a LP and LP3 bank ; by-pass of a HP 5 The results obtained allowed the determination of electric power value produced by the power plant in regimes and 3. In case of operating the reactor at 100% RP when a LP1 is by-passed, the Unit power is reduced by %; when a LP and LP3 bank is by-passed and when a HP5 is by-passed the Unit power is reduced by 4%. When the rector is stepped back to 90% RP the Unit power reduces by 11-1%. It leads us to idea of a new operating procedure with the reactor power maintained to RP, in event of unavailability of any preheater. It will result in a gain of the turbo-generator group power of at least 8%, that is about 50MW.

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4 P4WE P0WE D1 P01WE S0 WE P 01 P 0 P 03 S 0 D1WE P03WE P3WE P3 P4 M4NL M3NL P04WE P05WE M4 M3 P 04 P05 INTR_DEG01 INTR_DEG011 IN TR_DEG 01 INT R_DEG 013 INTR_DEG014 INTR _D EG 015 S09WE S10WE S 09 P 55 AC9 AC6 P55WE S 10 D EBIT_POM PA_ SP DEBIT_P OM PA_P V P 14 P 15 P 16 P10 P 04 LC V WE DEBIT_ POMPA_ P07WE P16WE P09WE A8IN J41 LCV A8 J41W E1 LCVIN P15WE P1 4WE P07 P09 P04WE J41WE9 P1 P1WE P10WE I01 I4 I43 J01WE1 LCV136_3IN L CV 136_3 A11 J01WE9 LCV_136_IN J01 L CV _136_ P10WE A P 10 AIN NI V_PIP5A S06WE S06 LCV 136_3WE P11WE NIV_PIP5AS P A11IN P11 LCV_136_W E NIV_PIP5B SETAV0 S03WE S 03 P1WE P13WE NIV_PIP5BSP SETAV03 P1 P1 3 NIV_PIP5A PV NIV_PIP5BPV OCOL IREWE I05 I04 P07 P01 PIP5BWE P 18WE P 0WE PIP5AW E P 18 P 0 AC15 AC16 PIP5B PIP5A PIP5BSE P 01WE PIP5AS E NIV_G EN5SP N IV _GEN5P V P19WE PIP5BDE I0 P1 9 NIV_GEN5 PIP5AD E P19WE P 17WE P07WE A7IN P3WE I03 P3 P19 OCOLIRE P 17 A7 OC OLIRE IN J0WE1 J0WE9 J0 P6WE P6 J03WE1 J03WE8 J03 P59WE P5 9 S04WE S 04 P54WE S07WE S07 S05 S05WE P3WE P54 P41WE P3 P4WE P8WE P4 P4WE P5WE P4 P41 P8 P5 AC 17 SE TAV06 AC18 SETAV05 AC 5 NIV_GEN SP NIV_GEN PV SETAV07 AC19 NI V_G EN1SP NIV_G EN3SP S ETAV 08 NIV_G EN1P V NIV_GEN NIV_G EN3PV NIV_GEN4SP NIV_GEN4 PV NIV_ GEN1 LCV309WE NIV _GEN3 LC V303WE LCV311WE LCV 306WE NIV_GEN4 A4IN A3I N A5I N LCV311 A6IN LCV306 LCV 309 LCV 303 A5 A3 A6 LC V306IN LCV3 09IN LC V311IN A4 LCV303IN RSG WE RSG WE RSG 1WE RSG3WE RSG RSG3 RS G RS G1 156 Ilie Prisecaru, Iulian Pavel Niţă, Daniel Dupleac FEED WATER SYSTEM FROM MAIN CONDENSATE SYSTEM MSR DRAIN BLEED STEAM DEAERATOR HP5 DRAIN SG3 LEVEL CONTROL VALVE SG 3 Point 4 Graphic MAIN PUMP RECIRCULATE HP5A - HP5B LEVEL CONTROL VALVES Point 6 Graphic FEAD WATER HEADER SG 1 Point 5 Graphic HP5 BLEED STEAM HP5 SG 4 FEED WATER PUMPS SG BY-PASS HP5 Fig. 3 Computational diagram of the Feedwater System. Simulating and modeling of transient regimes of regenerative preheating system Regenerative chain of a NPP (nuclear power plant) includes low pressure reheaters (LP), high pressure reheaters, (HP) a deaerator (D), feed water pumps (PA), and pipes (see Fig. 1). Regenerative preheating modeling was done modular. Each type of equipment is modeled separately. One interconnects modules by transferring the output values in a way become input into values for the next module. In the whole model there were introduced modules for preheaters, valves and automation parts. Computational diagram of the simulation of the regenerative chain using MMS is shown in Fig. and 3. Mathematical models underlying the complete module simulation are described in [3] and were confirmed and applied in simulation work for Cernavoda NPP systems [4, 5.7] One pursued the development parameters of the preheating circuit regenerative procedures referred to. In the analysis have sought answers to questions: - The system's ability to maintain power reactor at face value?; - Development of system parameters (pressure, temperature, etc.) Endanger the safe operation of the plant? In the review one considered all existing automation and functional facility that is level regulators of each preheater (LP1, LP, LP3, HP5 and deaerator) and flow regulators of steam generators. One considered for the changes of field of the

5 Influence of preheater loss in regeneration efficiency on the secondary circuit 157 plant, that the most important parameters are flow rate and fluid temperature throughout the plant. MMS computational code was used for transient operating in order to obtain outcomes of the system and new stationary regimes that occur after system stabilization, thereby determining the operating parameters. They were presented in detail in ref. [10]. This work aims to present a simplified calculation method of heat exchanger. The method is illustrated on a feedwater preheater, i.e. HP5. 3. Simplified calculation of a regenerative preheater The normal operation of the installation of regenerative preheating water supply from thermal power stations and nuclear power at part load turbogenerators group is a constant concern of the project designers and field operators [1,5,7,8]. Feedwater preheater behavior can be determined if one known in detail heat transfer process that occurs in these devices, when the load of turbo-generator is changing. Complete calculations of heat balance and heat transfer arrangements were made for 100%, 80%, 60% and 40% of the flow rate of steam generator - schemes presented in HB by GE and recalculated in [11.1]. One performed the exact calculation of heat transfer using MATHCAD program. The results of this calculation were summarized in two diagrams and system of mathematical equations used in computer processing. Using this simplified method of calculating the behavior of regenerative preheater abnormal regimes, with a minimum of input data (load power in the % RP and temperature of input water preheater) may cause exit temperatures, temperature, pressure device in the condensation and heat load. This simplified method has the next advantages: quickness and calculation handiness; it can be done during operation, in order to evaluate the state of the heater through comparing data from design with exploitation; very good evaluation comparing with exact solution. Exact calculation was performed for the four base regimes that define the curves from diagrams namely regimes 40%, 60%, 80%, and 100% MCR (main continuous rate) for flows and pressures at turbine bleed that HP5 preheaters. Convection heat transfer coefficients were calculated separately for each of the working areas of equipment HP5 namely the condensation: the steam condensing zone: the water supply; subcooled and condensed area: the condensed steam; subcooled area condensed: the water supply.

6 158 Ilie Prisecaru, Iulian Pavel Niţă, Daniel Dupleac For the four areas above were used the following relations for convection calculation: 1 3 g 1 a) = 0.95 λ ρ G' s η N α (1) b) Nu α = λ * ; Nu = 0.01 Re Pr fl c di () c) Nu α = λ * ; Nu = 0.01 Re Pr fl c d (3) i d) Nu η fl α = λ * ; Nu = 0.01 Re Pr fl d e η p (4) The values of global heat transfer coefficients for the 3 areas indicated above on are shown in Table 1: Table 1 The values of global heat transfer coefficients for the three area HP5 Regime heat transfer coefficient 100% 80% 60% 40% condensing area subcool area In Fig. 4 one presented the heat flux of a HP 5 versus turbo aggregate load. Fig. 4 was obtained using terminal temperature HP 5 (see Fig. 5 and 6), determined with simplified procedure. Sarcinca termica [MW] Heat Sarcina Load termica of the equipment a aparatului Power Nivel Level putere [%] Fig. 4. Heat load of equipment HP5 according to plant load

7 Influence of preheater loss in regeneration efficiency on the secondary circuit 159 For user-friendly computer codes in the analytical correlation for heat by power load is: ( x) 9,96 x + 141,84 x [ ] Q = + MW t (5) where x is the percentage of power. 4 3 ( x) = 5,083 x 6,0417 x + 4,0417 x 0,4083 x [ C] (6) dt ( x) = 5,86 x 0,4764 x [ C] (7) DT where x is the percentage of power. 6 Diferentele Terminal temperature finale de temeperatura difference pentru for HP5 PIP5 5 Diferentele finale de temperatura [ C] 4 3 DT dt 1 0 0% 0% 40% 60% 80% 100% Power Level Nivel putere [%] Fig. 5. Terminal temperature difference DT and dt for HP5 equipment according to the power load Once the details made above were done, one can proceed to establish parameters and equipment operators working in particular: - bleeding steam at outlet steam turbine - Feedwater

8 160 Ilie Prisecaru, Iulian Pavel Niţă, Daniel Dupleac ΔT T T' 1 T'' 1 = t' 1 T'' t'' 1 dt T' = t'' t' S Fig. 6. Temperature final differences heat exchanger equipment Known variables: a) Q - heat load of the equipment from Fig. 5 b) t' - feedwater temperature upstream of HP5 c) g apa = required feedwater Based on known input data, one could determine output temperature without appealing to transcendental equations of heat transfer: Q T' ' = + t' [ C] (8) g c apa pm Once determined the size T'' and known as t' resulting: T = T '' + Dt [ ] (9) = ] (10) ' 1 C t '' 1 t' + dt [ C Using diagram from Fig. 5 or analytical relations [6] and [7] for partial load which would calculate. Having the value of T' 1 that represents the steam condensation temperature (saturation temperature) we can determine operating pressure from condensing zone. The method indicated above was used for all regenerative preheater system. Having appropriate diagrams available, it will provide equipment characteristics used to simplify calculations of energy balance throughout the regenerative circuit.

9 Influence of preheater loss in regeneration efficiency on the secondary circuit The electric energy efficiency of secondary circuit The computation of plant efficiency has on the base heat balance relations written on complete secondary circuit of the plant. The computation was based on solving simultaneous equations of heat balance on all heat regenerative heaters from secondary circuit of the power plant. Computational relations used for each heat exchanger were presented in previous paragraph. Energy balance equations were written for each of the three unavailability's LP1, LP-LP3 and HP5, for % of nominal load. The results were compared with known values and we selected the closest ones. For each form three cases we had three abnormal regimes: α) unavailability of a LP1; β) unavailability of a bank of LP-LP3; γ) unavailability of a LP5. Combining the computation for flow rates corresponding to % from nominal power with the three cases of abnormal operation we obtained 3 x 3-9 computational cases. Stationary regime calculation Initial α β γ 96% % % Table Comparing the results we obtain 3 pairs of values: (α 98%) 98.16%; (β 96%) 96.50%; (γ 96%) 95.70% From these data we can conclude that loss of one LP1 affects not so much the global efficiency of secondary circuit. The unavailability of a HP5 of a bank of LP - LP3 leads to a loss of 4% of electrical power generated with the reactor power, the loss been twice as big as in case of unavailability of LP1. 5. Conclusions In this paper one presented a simplified method used for fulfillment of a thermal calculation if a heat exchanger for abnormal operating regimes. This method was applied to the complete regenerative chain form NPP Cernavoda in order to obtain the values of energy efficiency for regime of unavailability of a preheater. One considered a parametrical analyze function of level of reactor power after initiating event: at first one considered he reactor power lowered to

10 16 Ilie Prisecaru, Iulian Pavel Niţă, Daniel Dupleac 90% of PN (conform to actual operating procedure) and a second analyze were one considered he reactor power maintained to 100% (proposed operating procedure) Following the computation made in case of unavailability of a preheater, one observed that if the reactor power is maintain to 100% of nominal power, the maximum power surge of the plant its of about 4% (in case of HP5 or LP/3 unavailability) comparing to 1% in case of reactor power step back to 90% from its nominal power. The difference between the two cases is obvious and it's on the side of the case of maintaining the reactor power. 8% recovery from nominal power represents a very important loss for a nuclear plant, in our case of about 50MW. This method permits that starting from nominal regime of the heat transfer equipment to determine its fouling rate or its malfunction. Practical, in real time an operator can determine the installation operating condition without require a stop of the installation. As result of paper one concludes that reducing the power of the reactor from 100% to 90% of nominal power in event of unavailability of preheater is not justified either form concerns from economical or safety operation point of view. REFERENCES [1]. M. Pop, A. Leca, F. Chiriac, Procese de transfer de căldură şi masă în instalaţiile industriale, Ed. Tehnica [] A109-HB - HEAT BALANCE DURING By-pass of one LP1 (90% Load) [3]. I. Prisecaru, Modelarea proceselor dinamice din centralele clasice şi nucleare, Editura Proxima, Bucuresti, 009 [4]. I. Prisecaru, D. Dupleac, Nita Iulian, "Water Hammer in Nuclear Installations Case Study in Feed WaterSystem" Proceedings of international conference ESMc'008 [5]. M. Bigu, I. Nita, I. Prisecaru, D. Dupleac, Modelling of Preheated Regenerative Chain from NPP Cernavoda using MMS Calculation Code, International Symposium on Nuclear Energy SIEN 005 p. S1.4.1-S1.4.13, Bucuresti (Romania) 3-7 Oct 005 [6]. I. Prisecaru, D. Dupleac, CANDU Saturated Steam Turbine Modeling using COMPGEN for MMS package. În: vol.: ESMc'005 [7]. I. Nita, M. Gheorghiu, I. Prisecaru,; D. Dupleac, Simulating the transient regime for main condensate system at Cernavoda NPP. În: vol.: International Conference on Energy- Environment, CIEM 005 Bucharest [8]. I. Nita, I. Prisecaru, D. Dupleac, Cernavoda NPP feedwater system transient analysis using MMS package. SIEN 003 [9]. I. Prisecaru, D. Dupleac, A.C. Constantinescu, Modelling and simulating the transitory regimes in NPP using the MMS package programs; SIEN 003, [10]. I. Nita, I. Prisecaru, D. Dupleac, Modelling and Simulation of the CANDU 6 Feedwater System Behaviour- ESM 009 [11] A109-HB - HEAT BALANCE DURING By-pass of one LP1 (90% Load) [1] A110-HB - HEAT BALANCE DURING By-pass of one bank of LP./3 (90% Load)