ANSALDO ENERGIA STEAM TURBINE OPERATION IN FLEXIBLE COMBINED CYCLE. Authors:

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1 ANSALDO ENERGIA STEAM TURBINE OPERATION IN FLEXIBLE COMBINED CYCLE Authors: Carlo Bima Sandro Zanier (Ansaldo Energia) (Ansaldo Energia) 1

2 INDEX SCENARIO PLANT FLEXIBILITY FLEXIBLE STEAM TURBINE MECHANICAL FEATURES FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES CONCLUSION 2

3 SCENARIO The liberalization of the energy market in the EU has led to local energy exchange market, where power production and consumption quantities are exchanged following the energy demand in a competitive environment, bringing to a floating production demand as a consequence. Combined cycle power plants have to be modified to take into account the up-to-date requests of the power market tending to: FLEXIBILITY 3

4 SCENARIO MARKET FLEXIBILITY REQUIREMENTS Base load operation with high efficiency and low environmental emissions (NO x, CO) Partial load operation with high efficiency and low environmental emissions (NO x, CO) Primary and secondary grid frequency control (istantaneous load step) Two shift operation 4

5 FLEXIBILITY FLEXIBILITY means: Plant load profile in cycling mode (Daily/Weekly Plant Start up / Shut down) Repeatable start-up Reduced start-up times Typical start-up distribution: 245/year daily start up (hot conditions) 45/year weekly start up (warm conditions) 10/year cold start up 4 full load trips 5

6 volume [MW] SCENARIO energy demand provision (May 2009, 15th) data from RWE Transportnetz Strom GmbH hours Germany: RWE GmbH Italy: Terna S.p.A. Spain:Red Eléctrica de Espana France: RTE 6

7 INDEX SCENARIO PLANT FLEXIBILITY FLEXIBLE STEAM TURBINE MECHANICAL FEATURES FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES CONCLUSION 7

8 PLANT FLEXIBILITY There are two possible combined cycle plant operating modes: 1) two shift operation : where the plant is at full load, or partial load depending on the market requests, during the daytime and shutdowns every night and weekend. 2) minimum load operation : where the plant runs during the weekdays and shutdowns only during weekend. 8

9 PLANT FLEXIBILITY Two shift operation mode is the best solution because the fuel consumption costs overlook the power production gains because of the low power price per MWh in the night period In the two shift operation the only negative net is in the start-up and shut-down intervals Two shift operation mode involves: a major plant main components life consumption, as they suffer higher thermal stresses for the frequent starts; cycling capability, reduced start-up times, reliable and iterative start-up/shut-down procedures. 9

10 PLANT FLEXIBILITY Cycling mode operation The main items to be considered are: GT and ST improvements Thermal cycle components / systems requirements Start up and shutdown optimized procedures Evaluation of the main components life and subsequent measures to be taken to fulfill the plant expected life and the cycling operation Components / Systems requirements The main results and evaluations of Ansaldo Energia studies are: Need to maintain condenser vacuum Need to maintain HRSG drum pressure as high as possible HRSG admissible HP drum temperature gradient optimisation Attemperators at SH and HRH outlet to control Steam temperature gradients 10

11 PLANT FLEXIBILITY 11

12 INDEX SCENARIO PLANT FLEXIBILITY FLEXIBLE STEAM TURBINE MECHANICAL FEATURES FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES CONCLUSION 12

13 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES The steam turbine is subjected to many hot and warm start-up; each startup produces a fatigue stress damaging the rotor after some years. The target is to design the turbine in order to limit the low cycle fatigue and to reduce the start-up time. Improvement may be obtained acting on: - Suitable materials to reduce the creep and fatigue. - Inner casing without big flanges but jointed with shrinking rings to reduce strains/forces due to asymmetrical thermal expansion. - The inner connection for the steam is symmetrical and distributes flow at the complete radial section of the turbine. 13

14 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Inner casing without big flanges but jointed with shrinking rings to reduce strains/forces due to asymmetrical thermal expansion. SHRINKING RINGS FLANGES 14

15 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES The inner connection for the steam is symmetrical and distributes flow at the complete radial section of the turbine. STEAM ADMISSION STEAM ADMISSION 15

16 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Analysis The steam turbine rotor hot parts are under low cycle fatigue stress and creep. The fatigue is generated by the steam temperature variations with relevant steam flow, as during start-up phase or very high load variations (more than 50%). The creep occurs in components subjected to high temperature stresses (typically more than 500 C). Low cycle fatigue is the most important for steam turbine in combined cycle, as the request start-ups during the operation is very high. The inlet steam temperatures in HP and IP turbine sections have to follow the expected curves within tolerance of ±10 C. The steam temperature control is obtained by steam final desuperheaters, located between the HRSG and the steam turbine. 16

17 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves Turbine start-ups can be classified into various categories according to the thermal condition of the turbine at the start-up time. The true ruling criteria are the thermal temperatures to which the various components have cooled. The typical start-up categories are: cold, warm and hot. To optimize the turbine start-up time Ansaldo has introduced four steam turbine thermal state: cold, warm-cold, warm and hot. 17

18 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves START-UP SHUTDOWN TIME METAL TEMPERATURE COLD HOURS >72 TEMP<150 C WARM-COLD 48<HOURS< C<TEMP<190 C WARM 8<HOURS< C<TEMP<320 C HOT HOURS<8 TEMP>320 C 18

19 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves Cold Metal Temp.< 150 C [%] Time [min] 19

20 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves Cold-Warm 150 C <Metal Temp.< 190 C [%] Time [min] 20

21 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves Warm 190 C < Metal Temp. < 320 C [%] Time [min] 21

22 FLEXIBLE STEAM TURBINE MECHANICAL FEATURES Start-up Curves Hot 320 C < Metal Temp. [%] Time [min] 22

23 Shut-down Analysis FLEXIBLE STEAM TURBINE MECHANICAL FEATURES It is important to maintain the heat during the shut-down phase in order to perform the start-ups after night standstill or weekend standstill limiting the start-up time and the related fatigue stress. The recommended solution is to reduce the steam flow (maximum 10%/minute) by closing the turbine valves while the gas turbine still produces hot burnt gases, maintaining the vacuum in the condenser and in the gland steam after turbine trip; hence it is possible to reduce the heat lost by the IP rotor to the condenser 23

24 Shut-down Curves FLEXIBLE STEAM TURBINE MECHANICAL FEATURES 24

25 INDEX SCENARIO PLANT FLEXIBILITY FLEXIBLE STEAM TURBINE MECHANICAL FEATURES FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES CONCLUSION 25

26 ST automatic start-up Scope: Repeatable start-up Reduced start-up times How: FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES To operate in automatic mode, the control functions related to the process systems of the steam turbine are divided in hierarchically way: Turbine Run up (TRU) is the general sequence that manages the start up and shut down of the machine. Functional group (FG) collects open and close loop of the same auxiliary process system. Drive control level. 26

27 ST RSE Improvement Scope: FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES Minimize the stresses on equipment brought by the frequent changes in operating conditions Optimize operating cycle conditions in order to reduce startup times How: Systematic correlation of material stress/strain with fatigue and crack initiation The «rotor stress evaluator» program include an on-line life cycle counter New approaches to turbine modeling Material of construction High temperature Geometry of components 27

28 FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES ST start-up Curves Development Reducing the time of start-up curves in cold mode. Supplying different modes in the selection of start-up curves: the first one named normal, to be used when it is possible to measure the time to be previsioned by the operator to guarantee the start-up is finished at a scheduled time; the second one named fast has to be used when operator needs to provide power in a very short time. Each start-up is associated with an Equivalent Operating Hours Calculator to know in real time which is the life consumption of the steam turbine. Providing a continuous modification of acceleration during run-up and gradient during load-up, instead of having a selection based on discrete values. Improving the reliability of the temperature measure used as input for the calculation of steam turbine thermal stress as steam temperature upstream stop valves. Realizing the loading-up in closed loop to follow strictly the start-up curve imposed. They are flexible solutions can be mixed together according to Customer requirements. 28

29 FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES ST Participation to Frequency Control Scope: Reduce the plant load gap to maintain the reserve for the primary frequency control when GT slow gradient near the rated load (plant load >98.5% during under-frequency errors) How: Operating ST in modified steam sliding pressure mode HRSG HP SH / IP RH sections pressurisation through ST inlet valves throttling 29

30 FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES ST Contribution to Minimum Load Operation Scope: Reduce the plant minimum load operation below 45% without increase of NOx and CO due to not requiring further GT power decrease How: Operating ST in load control loop. In this operating mode it can be selected the frequency control on the steam turbine leaving the gas turbine working at its minimum load without the disturbances generated by the grid The steam produced by HRSG and not used by the steam turbine is diverted to the condenser through the by-pass systems. 30

31 INDEX SCENARIO PLANT FLEXIBILITY FLEXIBLE STEAM TURBINE MECHANICAL FEATURES FLEXIBLE STEAM TURBINE FUNCTIONAL FEATURES CONCLUSION 31

32 CONCLUSION Ansaldo Energia has kept pace with the new demands for involving the generating units in the flexible behaviour requested by the market. A flexible steam turbine can help us to approach the most strict requirements of the combined cycle in terms of frequent and fast start up operation and for the grid frequency control. The best solution is to manage the combined cycle in two shift operation philosophy; the more start-up foreseen will not cause damages for thermal fatigue if the plant, if the steam turbine will follow the start-up curves. In case of minimum environmental load operation the steam turbine stress will be lower than in two shift operation, only when the steam temperature is maintained about at nominal value. 32

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