VIBRATION MEASUREMENT PLAN FOR FEED-WATER PIPING SYSTEM FOR THE APR1400 COMPREHENSIVE VIBRATION ASSESSMENT PROGRAM Doyoung Ko Korea Nuclear & Hydro Power Co., Ltd. Central Research Institute, 70, 1312-gil, Yuseong-daero, Daejeon, 34101, Republic of Korea email: doyoung.ko@khnp.co.kr The applicant or licensee should evaluate the comprehensive vibration assessment program (CVAP) on pressure fluctuations and vibration at piping and components in nuclear power plants, including the reactor coolant, steam, feed-water, and condensate systems according to the U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.20 (Rev.3). The Advanced Power Reactor 1400 (APR1400) is classified as a non-prototype category 1 type. The CVAP for the non-prototype category 1 type should be conducted by stress and vibration analysis and either extensive measurements or full inspection. However, the vibration measurement program related to the evaluation of the potential adverse flow effect from pressure fluctuations and vibrations in piping systems should not be omitted. This paper describes the vibration measurement plan for the feed-water piping system of the APR1400 CVAP. We expect that the vibration measurement plan proposed in this paper will be used as one of the valuable materials for pre-operational and initial startup tests for the APR1400 CVAP in Korea. 1. Introduction The U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.20 [1] presents guidance for establishing comprehensive vibration assessment program (CVAP) for reactor vessel internals. It also provides helpful information on methods for evaluating the potential adverse effects of flow-induced pressure fluctuations and vibrations in piping systems and steam generators. Therefor the feed-water piping system is newly added as system for the CVAP from U.S. NRC RG 1.20. This paper suggests an account of vibration measurement plan for the Advanced Power Reactor 1400 (APR1400) CVAP of the feed-water piping systems attached to the secondary side of the steam generators. This paper will focus on identification of pressure pulsation and acoustic resonance, instruments types for vibration measurement, and choice of locations for vibration instrumentation. This is a portion of the overall CVAP which includes the measurement program; the CVAP consists of stress and vibration analysis, measurement program, and inspection program. There is an interface between the feed-water system presented by this paper and the steam generator internals since the effects of significant pressure pulsations from the piping systems must be considered for possible interaction with steam generator internals [2]. The feed-water piping system is Class 2 piping [3] from the steam generator nozzles (three for each steam generator) to after the containment isolation valves. Piping systems after that point are designed to Class 3 ASME Code rules or ASME B31.1 [4]. The vibration testing of the feed-water piping system is a subset of the overall pre-operational and initial startup test programs addressed by U.S. NRC RG 1.68 [5]. The additional testing performed based on U.S. NRC RG 1.20 will be done in conjunction with other pre-operational and startup testing, including steady state and transient power conditions. For the feed-water vibration 1
testing, the flow testing at significant steam flow rates will require actual operation of the reactor during startup tests for production of heat. 2. Identification of acoustic resonance The following described the three major sources of acoustic vibration: standpipes, control valves, and pumps. The conventional methods used to identify and diagnose these sources of acoustic vibration are provided. During pre-operational testing phase, if an acoustic resonance is identified in the piping system, then measures will be taken to mitigate the resonance. 2.1 Acoustic resonance caused by flow past standpipes Acoustic resonance from flow past a standpipe, a side branch attached to the main piping, or a dead leg is well-understood. Its occurrence at a power plant is accompanied by pure-tone, single frequency, and high amplitude excitation. The noise generated from such an event is an identifying characteristic of a problem due to flow-induced instability where pulsations are generally in excess of 100 Hz. In addition, the vibration and noise tend to increase significantly as the load is increased, with the amplitude changing rapidly with flow rate as the critical flow rate is approached from above or below. The critical flow rate for the initiation of resonance is a function of the standpipe dimensions; 'd' (inside diameter of side branch entrance) and 'L' (branch stub length or standpipe height for safety relief valve) [6-7]. 2.2 Acoustic resonance caused by control valves A potential source of high-frequency vibration is vortex shedding and high-frequency pressure fluctuations caused by throttling at control valves. The dynamic coupling effects between valve and fluid system may intensify the amplitude of vibration [8]. The approach for mitigating valveinduced resonance will verify that the natural frequency of the control valve is not near an acoustic resonance frequency of the downstream piping either by design, or by providing damping [9-11]. Another best practice for the arrangement of control valves is to avoid the use of back-to-back fittings, such as an elbow immediately downstream of a valve, which can increase flow turbulence and vibration. 2.3 Acoustic resonance caused by control pumps Due to the interaction that occurs between the impeller vane and the pump volute lip, hydraulic pulses are always generated at vane passing frequencies or a harmonic of vane passing (i.e., multiples of the pump speed). Pulsations originate at the pump and travel throughout the entire discharge piping. A certain level of pressure fluctuations is unavoidable and has no detrimental effects. Excessive pressure fluctuations, however, can excite pump and pipe vibrations and might even cause damage when the pulse frequency corresponds to an acoustic resonance frequency or structural frequency of the piping. Acoustic resonance in centrifugal pumps is caused by an interaction of the pump dynamic characteristics and internal flow passages [12]. In multistage pumps with back-to-back impeller design, a resonance condition can occur between standing waves in the long cross-over and the blade passing frequency. 3. Instrument types for vibration measurement The vibration test for the feed-water piping system will be performed as part of initial startup test, since the reactor will be needed to fully test vibration of the feed-water piping system with feed flow conditions. A vibration measurement system will be placed on portions that are most likely to develop acoustic resonance. The acoustic resonance detectors will be placed in piping in that is relatively close to the steam generators, so that greatest probability exists to identify any pulsations that could 2 ICSV23, Athens (Greece), 10-14 July 2016
affect the steam generators. While the mechanical vibration measurements that are performed as part of the ASME OM will detect beam mode mechanical vibration effects, they do not necessarily detect acoustic and shell mode vibrations. Therefore, instruments will be added to monitor for acoustic noise in the fluid pulsations (filtered up to 300 Hz) using strain gauge sets, on selected valves using tri-axial accelerometers (up to 5 khz) to monitor for excessive vibrations, and selected strain gauges from the strain gauge sets will be filtered at up to 20 khz to monitor for shell mode vibrations [13]. 3.1 Strain gauge for dynamic pressure monitoring Strain gauge is relatively simple and reliable instrument to install. They have a distinct advantage of being non-intrusive to the pressure boundary but they require careful preparation of the surfaces and the availability of an appropriate access to the desired locations. They have the capability to sense rapid transients up to 100 khz [14]. A monitoring system will be established that uses strain gauges to measure dynamic changes in pressure inside the pipe. The system will measure frequencies up to 300 Hz. The strain gauges will be oriented such that they measure hoop strain in the pipe. At a given longitudinal (axial) location, four gauges will be placed around the circumference of the pipe every 90 degree angle. Two sets of four strain gauges will be placed at two axial locations along a run of straight pipe. The axial distance between these stations will be less than half the wavelength of the upper limit frequency [15]. This is calculated by the following equation: D sd = c 2f where D sd = maximum allowable distance from the companion set of strain gauges, c = speed of sound inside the pipe, f = frequency upper limit (300 Hz for steam). This frequency is maximum recommended for feed-water to achieve a practical axial separation distance for the strain gauge sets, which for the feed-water line is approximately 2.08 m. Table 1 shows the effect of maximum monitored frequency on spacing of sensors at the feed-water system. Table 1: Effect of maximum monitored frequency on spacing of sensors. Acoustic speed (m/s) Upper limit frequency monitored (Hz) Separation distance (m) 1250 200 3.13 1250 250 2.50 1250 300 2.08 3.2 Accelerometer for excessive vibration monitoring Mechanical vibration testing for the feed-water piping in the APR1400 will be performed in accordance with ASME OM-2012, Part 3, Vibration Testing of Piping Systems, [3]. This standard identifies three vibration monitoring groups VMG 1, VMG 2, and VMG 3. For the APR1400, the feed-water piping system will be classified as VMG 2. The judgment that none of the system need to be considered as VMG 1 is based on no past experience of problems with shell mode with vibration for the piping in the feed-water piping system. The majority of piping vibration response occurs at frequencies lower than 10 Hz; therefore, instrumentation capable of low-frequency measurements is required. The VMG 2 piping will be measured using displacement transducers, such as a linear-variable differential transformer (LVDT) or lanyard potentiometers where possible. An LVDT can measure an appropriate frequency range to greater than 200 Hz. Its main disadvantage is that one end of the transducer must be attached to a non-moving reference structure (e.g., building), since an LVDT measures relative displacement ICSV23, Athens (Greece), 10-14 July 2016 3
between the piping or component and a fixed reference. Where displacement transducers cannot be applied, either hand-held or mounted piezoelectric accelerometers will be used. Acceleration, velocity, and displacement can be measured with the use of accelerometers. Velocity and displacement readings are obtained through single and double integration, respectively. The advantage of accelerometers is that they measure absolute acceleration and therefore do not need to be tied back or attached to any plant structure. Accelerometers are, however, subject to affect from a noise caused by high accelerations at high frequencies, such as from sudden shocks caused by looseness in the accelerometer bracket; integration of these signals, moreover, can distort the results at low frequencies. 4. Locations for vibration measurement Measurements will be taken along the piping to measure peak deflection points and to establish node points of minimum deflection. Node points (zero deflection points) are generally found at restraint points, but could be located between constraints on long runs of piping. We will determine in the locations of maximum piping vibration, considering the piping routing, support locations, and accessibility. Measurements will be taken where accessible and conservative acceptance criteria will be used for vibration measurements at these locations. For the feed-water system, a set of hoop strain gauges will be installed in each of the economizer headers for both steam generators. Each pair of twin strain gauge sets (4 strain gauges each at two adjacent axial locations on a straight run of pipe) should be monitored for pressure pulsation by filtering at 300 Hz. For each location of the twin sets of strain gauges, at least two (2 of 8) of these strain gauges should be separately monitored (prior to the filtering at 300 Hz) up to 20 khz for vibrations at dominant frequencies that could be representative of other component vibrations such as piping shell mode vibrations or other valve vibrations. The strain gauge sets will pick up any significant acoustic signals from the feed-water system that could be passed to the steam generators. This location will also be sensitive to any significant acoustic signals that might have an effect on the feed-water isolation valve integrity. The precise locations of each strain gauge sets are flexible and should take into account the final manufacture of the piping systems and the accessibility of the piping segments. Placement of the twin sets of gauges should consider the guidance of Table 1. Figure 1: Measurement locations for CVAP of APR1400 feed-water system. 4 ICSV23, Athens (Greece), 10-14 July 2016
One tri-axial accelerometer (or equivalent to provide 3 orthogonal directions) will be placed on each of the four flow control valves (two for downcomer header flow control valves and two for economizer header flow control valves). These tri-axial accelerometers will monitor for frequencies up to 5 khz and will be able to detect any vibrations of the flow control valves and can be compared to the strain gauge data to provide an overall assessment of the monitored acoustic noise. The precise location on the valves should be close to the body of the valve but vendor input should be obtained to account for experience with monitoring locations. The measurement locations for the CVAP for the APR1400 feed-water system are shown in the Figure 1. 5. Conclusion In accordance with NRC RG 1.20(Rev. 3), the measurement plan for the feed-water piping system will be conducted as a part of the APR1400 CVAP. In this paper, we proposed the vibration measurement program for the feed-water piping system for the APR1400 CVAP. The vibration measurement plan proposed in this paper will be used as one of the valuable materials for pre-operational and initial startup tests for the APR1400 CVAP in Korea. REFERENCES 1 U.S. Nuclear Regulatory Commission, Regulatory Guide 1.20, Comprehensive Vibration Assessment Program for Reactor Internals During Preoperational and Initial Testing, Rev.3 (2007). 2 Ko, D. Y. and Kim D. H., Screening Method for Flow-induced Vibration of Piping Systems for APR1400 Comprehensive Vibration Assessment Program, Transactions of the Koran Society for Noise Vibration Engineering, 25(9), 599-605 (2015). 3 American Society of Mechanical Engineers, ASME OM-2012, Operation and Maintenance of Nuclear Power Plants (2013). 4 American Society of Mechanical Engineers, ASME Code b31-1, Power Piping (2007). 5 U.S. Nuclear Regulatory Commission, Regulatory Guide 1.68, Initial Test Programs for Water Cooled Nuclear Power Plants, Rev.4 (2013). 6 Baldwin, R. M. and Simmons, H. R., Flow Induced Vibration in Safety Relief Valves, ASME Journal of Pressure Vessel Technology, 108(3), 267-272 (1986). 7 Ziada, S. and Shine, S., Strouhal Numbers of Flow Excited Acoustic resonance of Closed Side Branches, Journal of Fluids and Structures, 13, 127-142 (1999). 8 Lynch, J., Impedance Coupled Valve and Fluid System Instability, Proceedings of the International Topical Meeting: Advances in Mathematics, Computations and Reactor Physics, 28 April - 2 May, (1991). 9 Bond, T. and Cassidy, J., Control Valve Vibration Reduction, Nuclear Plant Journal (1996). 10 Wachel, J. C., Morton S. and Atkins, K., Piping Vibration Analysis, Proceedings of the 19th Turbomachinery Symposium (1990). 11 Blevis, R. D., Formulas for Nutural Frequency and Mode Shape, Van Nostrand Reinhold Company (1979). 12 Schwartz, R. E. and Nelson, R. M., Acoustic Resonance Phenomena in High Energy Variable Speed Centrifufal Pumps, Proceedings of the 1ST International Pump Symposium, 23~28 (1984). 13 Wachel J. C. and Smith D. R., Vibration Troubleshooting of Existing Piping Systems, Engineering Dynamics Incorporated (1991). 14 Harris, C. M. and Piersol, A. G., Harris Shock and Vibration Handbook, 5th Edition, Chapter 17, McGraw Hill, New York (2002). ICSV23, Athens (Greece), 10-14 July 2016 5
15 Perrosa, A. M., Denia, F. D., Fuenmayor, F. J. and Abom, M., Experimental Determination of Acoustic Properties by a Two Source Method with Simultaneous Excitations, Proceedings of the 18th International Congress on Sound and Vibration, Rio de Janeiro, Brazil, 10 14 July, (2011). 6 ICSV23, Athens (Greece), 10-14 July 2016