*Corresponding author

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1 2016 International Conference on Applied Mechanics, Mechanical and Materials Engineering (AMMME 2016) ISBN: Vibration Analysis of High Pressure Cylinder Steam Inlet Pipes During Turbine Startup in Nuclear Power Plant Zhi-lin CHEN 1, Xiang-xiang DONG 2, Lei LIN 1,*, You WU 1 and Fei XUE 1 1 Suzhou Nuclear Power Research Institute, Ltd, China 2 Yangjiang Nuclear Power Plant, China *Corresponding author Keywords: High pressure cylinder, Pipe, Vibration, Alternating stress. Abstract. Vibration and dynamic strain measurement of high pressure cylinder (HPC) steam inlet pipes(sip) is carried out during turbine startup in a nuclear power plant. The operation state of the pipes was analyzed and evaluated combined with the unit electric power, the turbine speed and the opening of control valves. The result shows that the pipe vibration velocity exceeds the allowable limit during the turbine startup and low electric power platform. And pipe vibration is synchronous to the control valves quick operation. The alternating stress is lower than the allowable value during the whole process. Pipe vibration is mainly caused by the steam flow excitation when the control valves operate quickly Introduction Vibration of HPC SIPs and high level bearing vibration has been an important problem during turbine-generator starts up in a nuclear power plant in China. High level vibration can cause bearing damage, turbine trip and other malfunctions. Turbine vibration is mainly caused by the operation itself or the steam admission mode. Xiuzhu Huang [1] studied the root cause of turbine vibration at DAYA BAY unit2 during start up and electric loading through imbalance analysis, bearing resonance, generator rotor thermal variables, and radial friction between rotary and static parts, etc. Pipeline vibration is relative with the fluid condition, the dynamic characteristics of the pipeline and the vibration characteristics of the connecting equipment. Wachel J.C s [2] research showed that the vibration induced by fluid was one important cause of pipe vibration. Koichi Yonezawa [3] studied main steam control valve(mscv) vibration at different boundary conditions with unsteady fluid, which showed that the unsteady fluid in valves would cause random pressure pulsation in pulse waves. David Galbally [4] studied pressure pulsation and vibration response of the safe-relief valve of main stream system in BWR unit, which shown that the valve vibration response could reach up to 20g, besides, there was a non-periodic pulse-vibration behavior inside the valve at a sinusoidal pressure pulsation caused by internal acoustic resonance. In this paper, the vibration of HPC SIPs are evaluated based on the acceleration and dynamic strain measurement, combined with the change trend of rotating speed, electric power, valve opening angle. The relationship between pipe vibration and control valve opening, electric power, rotating speed, bearing vibration are analyzed. Experiment Method There are 4 steam inlet pipes(named as SIP1, SIP2, SIP3, and SIP4) of high pressure cylinder, with one control valve arranged on each pipe (named as MSV1,MSV2, MSV3, and MSV4). The 765[mm] 45[mm] pipeline material is 16Mo3. The design temperature and design pressure are 316[ C] and 8.5[[MPa]]. 28 right-angle strain rosettes are used to measure the dynamic strain at welds, elbows and straight pipe. At the same time, the vibration acceleration of pipes was measured by accelerometers mounted at axial position (X), horizontal lateral (Y) and vertical direction (Z) during turbine startup to electric power rising. The arrangement of measurement points is shown in Figure 1. Principal stress of measuring points could be calculated as follows:

2 = ± (1) is the maximum/minimum principal stress, is the elasticity modulus, is the Poisson ratio, ε, ε, ε is the dynamic strain at 0, 45 and 90. The maximum alternating stress amplitude of the pipe is calculated as : =Max /2 (2) LMS SCM205/VB8-II acquisition system and KH G4-11 strain gages were used to measure the dynamic strain. The portable vibration test analyzer COCO80, B&W accelerometer and PCB-422E02 charge amplifier were used for vibration measurement. Figure 1. Strain and vibration measurement point layout. Result Analysis Time-domain Vibration Features Analysis (1) Turbine startup process analysis( ) The trend of vibration velocity RMS value at V3 point, turbine rotational speed and MSV opening angle during turbine startup ( ) are shown in Figure 2 (The other points have similar trend as V3 point.). It can be seen from Figure.2 that vibration velocity amplitude changes with the unit conditions, especially with the MSV opening angles. The opening angles of MSV1 to MSV4 change range are respectively 0~1.07%, 0~1.39%, 0 ~3.78%, and 0~1.05% during turbine startup. As can be seen from Figure 2, vibration level changes synchronously with the MSVs operation, especially MSV3. The main features are as follows, SIPs vibration step change appears when MSV3 opening and closing quickly. SIPs vibration increase to the maximum value when turbine rotation speed changes rapidly (35 to 398 and 398 to 1500), along with the rapid opening of MSV3. The opening speeds of MSV1 to MSV4 are slow during turbine startup, which has little effect on SIPs vibration.

3 Figure 2. Operating parameters trend during turbine startup. (2) Electric power raising process analysis (0-485[MW]) Vibration velocity RMS value trend along with turbine speed and MSVs opening angles during electric power rising from turbine startup to electric power up to 485[MW] is shown in Figure 3. The opening angles of MSV1 to MSV4 are respectively 0~11.13%, 0~11.14%, 0~13.02%, and 0~11.12%. The main pipes vibration features are as follows. SIPs vibration step change appears when MSV3 opening and closing quickly. SIPs vibration step change appears during the electric power raising from 0 to 50[MW] at a rate 50[MW]/min after generator is in parallel, although there is no quick opening or closing of the four MSVs. The opening angles of MSV1, MSV2, and MSV4 are relative small, and the opening and closing has minor effect on the SIPs vibration. Figure 3. Operating parameters trend from turbine startup to electric power 485[MW]. Frequency Domain Analysis The frequency spectrum curves are obtained through FFT and show the main vibration frequencies information, which can used together with the excitation and structural natural dynamic characteristics to analyze the vibrational properties. Figure 4 shows the x-direction vibration acceleration auto-power spectrum along with the turbine speed and electric power. It indicates that, During the speed raising period, the main vibration frequency bands are 405~515[Hz], 729.7[Hz] and 1080[Hz];

4 After power grid, as electric power increases, the vibration frequency changes from high to low. The vibration frequency changes from 1080[Hz] before 50[MW] to 723.9[Hz] at 100[MW]. At 470[MW], the vibration frequencies decrease to 421.9[Hz]. (a) Figure.4. X direction vibration acceleration auto-power spectrum of V3 vs turbine rotational speed (a) and electric power (b). Vibration Velocity and Alternating Stress Intensity Assessment for SIPs Pipe vibration peak velocity limit is given in ASME OM-S/G-2007 Part3 [5]. However, there are many transients during turbine startup which will cause high transient vibration velocity and lead to over conservative. So the RMS vibration velocity assessment method [6] is adopted. The allowable vibration velocity RMS value of the SIPs is calculated as 35.54[mm/s]. The maximum vibration velocity RMS values from the first startup to the last startup of the four SIPs are shown in Table 1. From Table 1 it shows that the vibration levels of four SIPs all exceed the allowable value during the process of startup to 90[MW], and the vibration level of SIP3 is much higher than the other pipes. Especially, the vibration level is relatively high at the periods of 25 to 400 and 398 to At higher electric power platform, the vibration level decreases below the allowable limit. Table 1. Maximum vibration velocity RMS values for four SIPs [mm/s]. (b) Measurement Point Unit condition [MW] 90 [MW] 485 [MW] 768 [MW] SIP SIP SIP SIP The alternating stress intensity criteria [5] is, / (3) Where S is the measured maximum alternating stress amplitude. S =0.8S, where S =86[MPa] and =1.3 according to reference [5]. Thus / =52.92 MPa. The variation property of alternating stress amplitudes is similar but with smaller extent to that of unit parameters such as rotational speed or electric power. As an example, Figure.7 shows the strain curves and alternating stress amplitude curve of point L17 from 20 to 90[MW]. It is obviously that the strain and alternating stress amplitude will fluctuate when the unit condition changes instantaneously and the strain and stress amplitudes maintain steady when the unit condition is in steady state. It is shown from Figure.7 that the alternating stress amplitude at higher

5 power platform is larger than that of lower power platform and larger after unit grid than before unit grid. The variation trend of alternating stress amplitude is similar to that of the strain. Figure 7. Strain of point L17 during unit startup. Figure 8. Alternating stress amplitude of point 17 during startup. Table 2 shows the maximum alternating stress amplitudes of SIP1 to SIP4 at variant conditions. The data indicates that the alternating stress amplitudes are higher at startup process and at 90[MW] and higher electric power platform than the other conditions. And the maximum alternating stress amplitude during the startup and power raising process is lower than the allowable value. Unit condition Table 2. The maximum alternating stress amplitudes of SIP1 to SIP4 [MPa] [RP M] 1500[RP M] 0-50[MW] 50-60[ MW] 90[M W] [M W] SIP number SIP1 L SIP2 L SIP3 L SIP4 L Summary Vibration acceleration and dynamic strain of four SIPs on HPC are measured during start-up of a nuclear power plant. Conclusions can be drawn as follows, a) During the period from start-up to 90[MW], the maximum velocity RMS values of the four SIPs exceed the allowable value. While when the electric power arrives at 485[MW] and higher level, the vibration level of the SIPs reduce significantly. b) The alternative stress amplitudes of the SIPs have similar trend as vibration velocity and lower than the allowable value. c) The vibration frequencies are high during turbine startup. Main frequencies are 405~515[Hz], 729.7~732.8[Hz], 1080~1085[Hz]. During electric power rising, SIPs vibration frequency decrease and the main frequencies are 421.9[Hz], 160.0[Hz], and 231.3[Hz]. d) Pipe vibration level is affected directly by the MSV opening angle, especially by MSV3. At the time the MSV3 open or close quickly, step changes of vibration acceleration and strain occur. This is because steam impacts the valve or pipe instantly when valve open or close quickly and the huge impacts force the pipe to vibrate severely. The vibration level tends to decrease steadily after synchronization during which period the valve opening angle adjusts slowly. Acknowledgement This research was financially supported by the CGNPC Rush Plan Foundation

6 References [1] Huang Xiuzhu, Kou Shengli. Diagnosis and Troubleshooting of Turbine Vibration at Unit No.2, Daya Bay Nuclear Power Plant[J]. Thermal Power, 1996(1): [2] Wachel J C, Smith D R. Vibration Troubleshooting of Existing Piping Systems[R], Houston: Engineering Dynamics Incorporated. July [3] Koichi Yonezawa, Ryohei Ogawa, etc. Flow-induced vibration of a steam control valve[j]. Journal of Fluids and Structures, 2012(35): [4] David Galbally, Gonzalo García, etc. Analysis of pressure oscillations and safety relief valve vibrations in the main steam system of a Boiling Water Reactor[J]. Nuclear Engineering and Design, 2015(293): [5] The American Society of Mechanical Engineers, ASME OM-S/G-2007 PART 3, Standards and Guides for Operation and Maintenance of Nuclear Power Plants[S]. New York: The American Society of Mechanical Engineers, [6] Sébastien Caillaud, Didier Briand. Correction Factors for ASME ANSI-OM3 Stress/Velocity Relationship With Respect to Static Design[C], Transactions of the 17th International Conference on Structural Mechanics in Reactor Technology (SMiRT 17). Prague: International Association for Structural Mechanics in Reactor Technology