Abstract. Operational Aspects of Cycling. Introduction

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1 Impact of frequent start / stop & Cyclic Loading on the life of pressure part components using CREEP-FATIGUE Analysis Arnab Bhattacharya, DGM -O&M Sipat Yogesh Kumar Sharma, DGM -CenPEEP Abstract As the Indian Power industry acclimates to new challenges, there is a significant increase in the number of plants those do not operate in traditional base load mode. The factor contributing to this trend include fuel price change & the increased mandated use of renewable energy. Increased flexible operation in the form of cycling, load following & peaking increases stress on critical components of steam turbine & steam generator. These unforeseen stresses introduced because of cycling operation causes negative long and short term effect on equipment reliability & availability. Operational Aspects of Cycling Cycling means a unit frequently going off-line to meet load demand. This mode induces higher mechanical & thermal stresses relative to base load operation. Load Following: Unit operating over a greater range of MW output compared to base load unit. It induces thermal stresses because of steam temperature variation due to fast ramp up & ramp down. Peaking & Low load also fall under load following. Even though these operating modes does not involve speed cycles but thermal stresses significantly degrades the critical components of machines. Creep fatigue application of PADO is capable of evaluating the impact of cyclic operation and thus helps in better planned outage, reduce damage to equipment and also can avoid catastrophic failures. This paper presents a number of case studies that shows how creep fatigue application software can be used successfully to quantify the extent of damage caused to boiler pressure part components due to parameter aberrations occurring during cycling. This paper will help in understanding the impact of cycling on these critical Power plant equipment s which in-turn will avoid undue forced outage & help in preserving equipment functionality. This paper also uses example to demonstrate the essence of quality DCS control system to achieve faster ramp rates without causing damage to capital equipment s. Introduction The fundamental requirement for the machine elements on which their efficiency depends are strength, stiffness, wear and corrosion resistance. Wear can be minimized by good design. Selection of good materials and surface protection will help in preventing the corrosion of machine surfaces. The other factor that can cause failure of machine part is its excessive deformation. Deformation means change of shape due to elastic strain, plastic strain and fracture. Steam generator and turbine components that are exposed to high temperatures and pressures suffer serious material degradation during their lifetime. Pressure induced stress increases creep and fluctuations of temperature and pressure causes fatigue failure in these components. Depending on material properties, under certain stress conditions creep increases strongly with temperature and decreases with wall thickness, whereas fatigue due to temperature induced stress develops in thick walled components. During the design of the component both types of stress is taken into consideration for failure. Because the actual operating condition deviates from design conditions, the material degradation is not proportional to operating time and therefore an analytical tool is required to estimate the extent of deterioration in high pressure components to prevent catastrophic failures.

2 The unfavorable effects (Fast life consumption) that are due to cycling includes: Thermal Fatigue Creep & Creep Fatigue Mechanical Fatigue Erosion/ Corrosion Controls & Valve wear & tear Some or all the above modes will be significant depending on the operating mode of machine. Fatigue & creep interaction can result in cracking of Turbine components. The critical components include: High pressure rotor region:- Steam seals near high temp steam inlets, blade attachments & center bore. HP Casings:- Horizontal joint bolt hole ligaments, transition region between casing & shell mounted control valve casing, diaphragm & nozzles. The thermal stresses in rotors & thick walled casings results from starting, loading & unloading transients. During loading the surfaces of the components respond to heat transfer from the surrounding steam & started heating up. The cooler inner regions of components restrict the growth of outer components forcing the surface to compression & inner regions into tension & if the rate of change of steam temperature is severe it will cause creep damage. The creep damage is the function of operating temperature & time. The amount of fatigue damage is dependent on the how severe the loading rate & frequency of cycles. Development of Thermal Cracks Hostile start/stop & loading leads to low cycle fatigue cracks. Thinner sections will heat up faster than the surrounding material. Thus thicker section prevents thinner one to expand thus developing stress. During unloading & cooling an opposite condition develops & surrounding heavy mass will restrict contraction of thinner section thus leading to tensile stresses. These stresses can spread surface cracks in the material. The magnitude of these thermal stresses because of frequent starting / stopping / fast loading/ unloading cycles is temperature dependent change/ rate of change. The operator in fact has control over only one or two parameters: Speed of loading /unloading & Total Temperature change: Cold/warm/ hot startup Application of CREEP-FATIGUE Software The Creep Fatigue application software is used to monitor thermal stress and creep and evaluate cumulative damage. The system operates by interfacing with the plant DCS to collect necessary data from the components to be monitored. Component and geometry dependent stress transfer functions, specifically developed for each component, convert the collected instrument data to creep and fatigue stresses versus time. These stresses are then used to predict creep-fatigue damage accumulation and crack growth for known or postulated defects in the monitored components. Monitored damage and crack growth rates are projected into the future to estimate the time to reach critical damage i.e., the remaining life. Figure 1 and figure 2 displays stepwise creep and fatigue crack growth for unit#1 and unit#2 from October, 2015 onwards. Each step increase corresponds to parameter aberrations causing creep and/or fatigue stress beyond critical stress and results in increase in crack size in final SH O/L header. Usually parameter aberrations are attributed to reducing start-up time and oil consumption, too much excess air and faster ramp rates. Following case studies demonstrate how creep fatigue application can be used to analyze the quality of start-up shutdown and cycling load operations in supercritical units.

3 Figure 1 Figure 2 Case Study 1 (Boiler operation disturbance) On at 10:25 hrs all running CT Fans of stage-i got tripped while C&I was changing trip set point for Vibration high. Presuming fast drop in vacuum all 03 units load reduction was started. While reducing load in Unit#1, two mills were tripped manually that resulted in reduction in MS temperature, to control MS temp SH spray was reduced to minimum and that remained minimum from 10:33 hrs to 10:40 hrs and in the mean time BT was increased. Unit was in TF2 mode and actual load has gone below load set point so coal flow increased. With increased firing and spray minimum MS temp went high up to C. Again to control the MS temp feed water flow and spray flow were increased. With this action once again MS temp was reducing sharply and to control this feed water flow reduced to <600 TPH resulting furnace WW rise sharply and tripping the boiler on WW temp high. On at 00:06 hrs, Unit#1 again tripped on Generator Class-A protection (UT back up over current protection) as ID Fan-B started. Boiler MS SV power supply was kept off, therefore delay in closure of MS SV during unit tripping results in sharp drop in MS pr and temp. Parameter aberrations occurred during unit tripping and start up resulted in fatigue damage in final SH O/L header (figure 1, figure 3 and figure 4). Figure 3

4 Figure 4 Case Study 2 (Boiler operation disturbance) On , Unit#2 was on bar at 684 MW load with 06 milling system in service. At 21:15: hrs, turbine got trip on axial shift high protection. At 21:10:55 hrs hunting took place in HPH 6B level transmitters & HPH B series got bypassed. (HPH 7B & 8B was in-service and 6B was under permit). Consequently, HRH pressure shot up to 49.5 ksc and both EHVs of HRH got popped up (along with ERVs which got reset with pressure reduction) prematurely leading to reduction in unit load. EHVs remain popped up to the tune of 04 minutes (21:11 hrs to 21:15 hrs). This caused escape of huge (57 % of HRH steam flow) amount of steam to atmosphere and bypassing the IP turbine but full amount of MS steam at 247 ksc pressure was passing through the HPT and therefore the exhaust pressure got decreased to low value (~32 ksc from 48 ksc within 04 minutes). Due to HPT unbalance thrust, axial shift increased to > tripping value (2.0 mm positive, towards generator) and unit got trip on axial shift high. Thrust bearing temp increased to maximum value of C just before the unit tripping. Axial shift increased to 2.5 mm and there after value became bad. Parameter aberrations and fatigue damage represented by figure 2, 5 and 6. Figure 5

5 Case Study 3 (Boiler operation disturbance) On , Unit#2 was on bar at 690 MW load with 07 milling system in service. At 05:32: hrs, boiler got trip on MS temperature high protection. Left side MS stop valve closed due to malfunction and valve remained close for ~10 minutes. With the closure of MS stop valve MS temperature at boiler outlet started increasing from C and reached to alarm value at 05:30:20 hrs. As soon as MS stop valve (L) observed close, HP By Pass was opened and open command was issued to open MS stop valve (05:30 hrs).with the issue of open command main steam temperature started increasing steeply and reached to tripping value (592 0 C) within 2.5 minutes. Final SH tube metal temperature increased sharply after MS stop valve opening and reached to C maximum of A19/T1 assembly. Assembly A17/T1, A18/T1, A20/T1, A21/T1, A22/T1 and A23/T1 temperatures were also gone >650 0 C for few minutes. Maximum operating limit of finish SH tube temperature is C. Parameter aberrations and fatigue damage represented by figure 2,5 and 6. Figure 6 Case Study 4 (Unit trip and hot start-up) On at 16:38, Unit#3 was on bar at 660 MW and suddenly unit got trip on electrical class-a protection, GT differential relay operated due to malfunction. On at 02:32, before synchronization of Unit#3, seal oil DP was running 0.99 ksc and damper tank (EE) was overflowing, DP was adjusted to ~0.75 ksc and unit was synchronized to grid but in the mean time seal oil DP came down to 0.71 ksc and within 5 minutes damper tank level got reduce to Very Low leading to tripping of turbine on Generator mechanical protection Damper Tank level Very Low. Parameter aberrations and fatigue damage represented by figure 7,8 and 9.

6 Figure 7 Figure 8 Figure 9 Case Study 5 (DCS PG Test and control loop tuning) On Unit#2 DCS PG test was conducted. During the test, 5% (165 MW) load was reduced from 660 MW to 495 MW within 10 minutes and ramp up in 7 minutes with very little fluctuation in pressure and temperature (figure 10). Hence no creep fatigue damage is recorded (figure 12). From to , tuning of control loops was going on with cyclic load operation. However, since parameter deviation was within limiting value (figure 13), creep fatigue stress was well below critical value (figure 11). This demonstrates the fact that even during cyclic load, creep and fatigue damage can be prevented if control loops are functioning correctly.

7 Figure 10 Figure 11 Figure 12 Figure 13

8 Case Study 6 (Ramp down) On , UAT-1B tripped when mill-1d was started. Three running mills, both TGECW, one ACW and one PAC got tripped. Load came down to 400MW. Later, during inspection two metal pieces found inside mill. At breaker end, tracking observed in cable chamber of mill-1d breaker. Fault extended upward and made UAT-1B tripped. During fast ramp down, MS Temp(L) reduced to C from C causing fatigue damage (figure 14,15,16). Figure 14 Figure 15 Figure 16 Case Study 7 (Ramp up and Ramp down) On in Unit#2, ramp up and ramp down (figure 19) was done to perform run out PG test of TDBFP A and B at 1160 TPH,1270 TPH,1360 TPH. Run out test of CEP A,B and C also completed during the same period. Parameter aberrations during ramp up resulted in fatigue damage (figure 17,20).

9 On in Unit#2, Leakage in HRH line IPSV-1 to IPCV-1&3 at 8.5 meter from a drain root attended by clamping. During this period load reduced to 270 MW and after attending same,load ramp up done (figure 21). Due to quality ramp-up and ramp down operation, MS and HRH temp fluctuation was fatigue stress was below 100 MPA and therefore no damage observed in MS header and Y piece (figure 18,20). Load reduced from 680 MW to 270 MW with maximum variation of MS Temp limited to 20 0 C. Figure 17 Figure 18 Figure 19

10 Figure 20 Figure 21

11 Recommendations: It is always a good practice to monitor all critical temperatures & temperature differentials monitored during startup/ loading, Unloading/ stopping & fast ramp up & ramp down. It is very important to maintain proper steam temperature ramp rates & temperature differential during loading / unloading for controlling thermal fatigue. These measurements comprises: Metal temp measurement around 1 st stage, Nozzle region & downstream shell. Throttle steam Surface Temperature at horizontal joint flange of inner shell. Horizontal joint bolt closest to the 1 st stage. 1 st stage bowl steam temp SH, RH and Y piece pressure, temperature and rate of change of pressure and temperature Other than this specific recommendation from OEM or from data produced with a detailed stress analysis should be followed. Conclusion The failure of engineering materials is almost always an undesirable event. Even though the causes of failure and the behavior of materials may be known, prevention of failures is difficult to guarantee. However by adopting good operation practices, in-service failures of capital equipments can be minimized. The monitoring and prediction of the rate of accumulation of creep-fatigue damage of turbine and boiler pressure part components is helpful in developing an effective life assessment strategy to prevent catastrophic failures and to manage the remaining life of these components. Therefore, creep fatigue application software having lots of potential can prove to be a very powerful analytical tool in the process of making realistic decisions concerning operating procedures, inspection intervals, and repair and replacements schedules of critical turbine and boiler pressure part components. Acknowledgement: We are thankful to CenPEEP, NTPC Sipat C&I department & Sipat plant management team for the complete support. References Case Studies from Sipat Stage-1 PADO Creep Fatigue application software Material science and engineering an introduction by William D. Callister, Jr. Sipat Unit trip report Sipat historical data from PI server