Session: 08-11: Computational Fluid Dynamics (CFD) and Applications - XI

Paper Number: 135693

135693 - Fluid-Induced Vibration Assessment in Main Steam Pipe Using Computational Fluid Dynamics 

Abstract: 

Flow-induced Acoustic Resonance in piping system with closed side branches can cause premature wear and damage of equipment in many engineering applications. Especially in the Nuclear Power Plant (NPP) main steam lines consist of many side branches and flows high temperature and high pressure steam at rapid flow rate. Under these conditions, a phenomenon known as "lock-in" can occur when the frequency of vortex shedding from the closed side branch overlaps with the acoustic natural frequency of the branch pipe itself. In such cases, unstable vibration amplitude can significantly increase, posing a threat to the structural integrity of the main steam line and its equipment.

This study performed computational fluid dynamics (CFD) simulations to predict vortex shedding frequencies in the Main Steam Atmosphere Dump Valve (MSADV) branch pipe and the Main Steam Safety Valve (MSSV) branch pipe within the NPP main steam line. The CFD-based evaluation method has been validated through measurements of branch pipe vibrations during 100% power operation of the NPP, as well as by comparing the results with the predicted values. The simulations using the commercial CFD code ANSYS CFX 2022 R1, with a flow domain generated based on NPP design data. The grid was generated to ensure an accurate prediction of the viscous boundary layer by maintaining the y+ value lower than 1. The grid was created for ensure accurate prediction of the viscous boundary layer by satisfying the y+ value lower than 1. The Steam properties with IAPWS-IF97 look-up table are used to simulate real steam properties. For turbulence analysis of inside branch pipes, the Scaling Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is used. Transient analysis was performed ensuring that the RMS courant number below 1. The pressure fluctuation data in the time history of MSADV and MSSV branch pipe, generated through transient analysis, were transformed into amplitude data in the frequency domain using Fast Fourier Transformation (FFT) for identification of dominant Vortex Shedding frequencies at branch pipe, all dominant frequencies were predicted 171.4 Hz and 185.7 Hz.

To validate these predictions, vibration measurements were performed with 3-axis accelerometer and portable vibration monitoring equipment. The accelerometer was installed on the MSADV and MSSV body and These measurement recorded acceleration during 30 second, capturing 122,880 data. The vibration measurement results confirmed dominant frequencies were 172.8 Hz and 193.2 Hz in both MSADV and MSSV branch pipe. Upon comparing the CFD predictions with the NPP site measurements, a minor deviation of approximately 3.9% was observed.

Possible sources of this deviation include discrepancies in the internal geometry of the branch pipes between the design drawings and the actual installation conditions due to allowable construction tolerance. Additionally, differences in vibration measurement locations and methods between CFD analysis and experimentation contributed to the observed deviation.

In conclusion, this study combines CFD simulations and experimental measurements to evaluate fluid-induced vibration in NPP main steam lines. The results demonstrate the effectiveness of CFD simulations in predicting vibration frequencies, with a discrepancy of 3.9% compared to experimental measurements. The identified sources of discrepancies include differences in geometry representation, installation conditions, measurement location and methods. Minimizing these sources of discrepancies will enhance the accuracy of CFD analyses and contribute to the safety assessment of NPP main steam pipes.

Presenting Author: HAEIN LEE KEPCO-ENC

Presenting Author Biography: Haein Lee is Senior Engineer at KEPCO-ENC, South Korea.
He has experience in system engineering and O&M of the Nuclear Power plants
also design of mechanical equipment, heat exchanger, pressure vessels for Nuclear Power Plant.
He holds a B.S in Aircraft system Engineering from Korea Aerospace University,
M.S in Mechanical Engineering for Kyungpook National University.

Authors:

HAEIN LEE KEPCO-ENC
Ahram Lee KEPCO-ENC
Younho Won KEPCO-ENC
Gon Hwangbo KEPCO-ENC