ML20113F622
| ML20113F622 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 01/31/1985 |
| From: | GEORGIA POWER CO. |
| To: | |
| Shared Package | |
| ML20113F618 | List: |
| References | |
| NUDOCS 8501240079 | |
| Download: ML20113F622 (7) | |
Text
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RECOMMENDED PIPING SYSTEM DAMPING VALUES
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FOR THE E.I. HATCH NUCLEAR PLANT UNITS 1 AND 2 JANUARY, 1985 e
4 8501240079 850116
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1.0 INTRODUCTION
The currently used damping values for seismic analysis of piping systems were developed in the early 1970's. Since that time, a much larger body of-test data has been developed which identifies significant conservatism in the current design basis. The conservatism inherent in the current seismic analysis damping values, given in Plant Hatch Unit 1 FSAR Table
-12.3-2 and Unit 2 FSAR Table 3.7A-1, results in a large numter of unnecessary piping system seismic supports which reduces thermal flexibility of the supported system during normal operation. Therefore, Georgia Power Company (GPC) Sas established a revised position on~ damping values for Plant Hatch seismic analysis based on the latest test data available. The revised position increases the Plant Hatch safety margins on piping systems for earthquake events, since the piping seismic loads are reduced and the support system configuration has not changed. The GPC position is described herein, along with a brief summary of.the technical Lasis..
2.0 TECHNICAL POSITION
.The GPC position on damping values for seismic analysis applies to Seismic Category I piping systems in Plant Hatch. The position is
~ ' illustrated in Figure 1 and utilizes, for response spectrum analysis of-piping systems, the frequency dependent approach established by the Pressure Vessel Research Cour.cil (PVRC) Technical Committee on Piping Systems (TCPS). J All i 6 -
r 3.0- TECHNICAL BASIS The frequency dependent' damping methodology was derived by the TCPS Task Group on Damping Values. Formal publication of this methodology is given in the Welding Research Council (WRC) Bulletin Number 300 (I)
The American Society of Mechanical Engineers (ASME) has issued the draft Code Case N-411 that permits the use of the frequency dependent damping values for piping analysis as an alternative to the damping values of non-mandatory Appendix N of Section III of the ASME Boiler and Pressure
-Vessel (B&PV) Code. The Appendix N damping values are identical to those of Regulatory Guide 1.61(2)
It is expected that the Code Case will obtain final approval from the ASME and be published sometime during the first quarter of 1985.
The WRC Bulletin Number 300 mentioned above contains, in addition to the technical position on damping, an outline of the purpose, scope, and approach used in evaluating the damping data used to formulate the position, as well as a description of the data collection methods and 33urces.
The collected damping Jata is from a wide spectrum of sources, including laboratory and in-plant tests of piping with different sizes, types of l -
supports and types.of excitation. The test results are, for the most part, from piping systems already installed in nuclear power plants, the exceptions being prototype and scale model tests of nuclear piping systems.
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The damping data, which varies widely in format and content, required an evaluation process that would result in rational, technically defendable damping 'value recomendations. The combination of multiple regression analyses and engineering judgment used to assess the damping data showed a strong relationship between decreasing frequency and increasing
' damping, independent of the pipe diameter.
The PVRC Task Group on damping performed a series of regression analyses in an. attempt to determine the parameters of a piping system which may contribute significantly to damping. The parameters that were examined
' include the number and type of pipe supports, the frequency at which the system vibrates, system configuration, presence or absence of insulation, pipe material, and pipe thickness. The goal of the analyses was to aid in developing and examining empirical relationships between the above parameters and damping using the collected data.
The use of multiple regression analyses in the evaluation of damping data requires, theoretically, a homogeneous data base acquired from a set of carefully designed tests. The data base used for performing this analysis was not as rigidly structured. To the extent that the data base is limited, engineering judgment was applied in selecting, sorting, weighting, adjusting, and manipulating the data. This was done by graphical representation of the major parameters which affect system damping values (graphs of damping values versus each major parameter) with evaluation based on developed judgment.
In this way, the major parameters were examined on a consistent basis and the results of the regression analysis critically evaluated. The dependency of the results on any one variable, as well as the range through which the dependency is reliable, was readily seen.. In this way, the frequency-dependent damping value recomendation was based on mean values from the graphical representation of the test data. '
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An assessment of the impact of the new damping values on the actual seismic safety margins was performed by Lawrence Livermore National Laboratory (LLNL). This study showed that the use of Code Case N-411 damping provided a better comparison with the best estimate time history analysis than did the use of Regulatory Guide 1.61 damping values. LLNL performed a series of comparative analyses on three ZION Unit #1 piping systems using the PVRC recommended frequency dependent damping values and
'the Regulatory Guide 1.61 damping values. These analyses are reported in
'NUREG/CR-3526, " Impact of Changes in Damping and Spectrum Peak Broadening on the Seismic Response of Piping Systems"(3)
A number of other independent evaluations for typical piping systems hava assessed the potential benefits resulting from use of the frequency dependent damping values for the seismic analysis of piping.
In addition to LLNL, these assessments have been performed by Duke Power Company, Tennessee Valley Authority, Westinghouse, and General Electric Company.
The overall conclusion of these studies is that the use of the PVRC frequency dependent damping values results in reduced piping response, therefore decreasing pipe stresses and support loads.
In summary, it can be concluded that PVRC frequency dependent damping values are a more realistic estimate of piping system damping than that previously allowed and that the use of the PVRC frequency dependent damping values reduces the seismic response of piping systems.
Consequently, for a piping system that maintains its existing. support configuration, this reduced response results in decreased pipe stresses and support loadings when compared to the original analysis and therefore the design margin is increased. Also, the PVRC damping values, being more realistic, will maintain the intended safety margin for Plant Hatch when used for new or replacement piping systems and load reconciliation work. JANi6
4.0 CONCLUSION
It has been demonstrated that the damping values defined in Section 2.0 are a more realistic estimate of damping for piping systems than the damping values used.in the past.
In addition, the application of these damping values will at least maintain the intended seismic safety margins for Plant Hatch. It is concluded, therefore, that use of the damping values of Section 2.0 does not constitute an unreviewed safety issue.
The GPC revised position is to use these damping values for new or replacement piping systems and load reconciliation work at Plant Hatch.
5.0 REFERENCES
4 1.
Welding Research Council Bulletin Number 300: Technical Position on Criteria Establishment; Technical Position on Damping Values for l
' Piping -Interim Summary Report; Technical Position on Response Spectra Broadening, and Technical Position on Industry Practice; 1984.
2.
" Damping Values for Seismic Design of Nuclear Power Plants", USNRC Regulatory. Guide 1.61, October,1973.
3.
" Impact of Changes in Damping and Spectrum Peak Broadening on the-Seismic Response of Piping Systems", S. C. Lu and C. K. Chou for
~ Lawrence Livermore National Laboratory, NUREG/CR-3526, UCRL-53491. JANi6
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