QUALIFICATION METHODOLOGIES FOR MECHANICAL …

XA0300614

QUALIFICATION METHODOLOGIESFOR MECHANICAL COMPONENTS,

I&C, PIPING USING ON-SITE TESTING

Maurizio ZolaISMES LMC, Italy

WORKSHOP ON"SEISMIC DESIGN ASSESSMENT BY EXPERIMENTAL METHODS"

NUCLEAR POWER IN~STITUTE OF CHINA (NPIC)CHENGDU, CHINA, 10-14 SEPTEMBER 2001

QUALIFICATION METHODOLOGIES FOR MECHANICALCOMPONENTS, I&C, PIPING USING ON-SITE TESTING

NATIONAL WORKSHOP

"Seismic Design Assessment by Experimental Methods"

NPIC - CHENGDU

September 10-14, 2001


Author: Maurizio ZOLA - ISMES LMC


1. Equipment qualification: a qualification procedure shall confirm that the equipment is capableof meeting, throughout its design operational life, the requirements for performing safetyfunctions while subject to the environmental conditions prevailing at the time of need. ("TheSafety of Nuclear Power Plant: Design" IAEA Safety Guide).

2. IEC 780 gives the following definition: Qualification is the generation and maintenance ofevidence to ensure that the equipment will operate on demand to meet the system performancerequirements.

3. When it should be used on-site testing?"Seismic design and qualification for nuclear power plants" IAEA Safety Guide is requiringperiodic safety review. (Maintenance throughout the design operational life)"Inspection and Testing for Acceptance" AEA Q4 Safety Guide requires that in somecircumstances final acceptance of a supplied item is only possible after it has been installed.(Generation of evidence)

4. Testing methodology: low impedance tests

* mechanical excitation

* impact

* soil blast.

4.1. Mechanical excitation: small exciters (electro-dynamic shakers or servo-hydraulicactuators) are used.

4.1.1. Electro-dynamic shakers:- they are used for small excitation forces from 1 N up to 1000 N;- inertial mounting is preferred by suspension (easy to do and to use);- no restraint is added to the component;- sinusoidal sweeping gives valuable information on the whole frequency range,because the energy distribution can be easily controlled;- random vibrations excitation can be used to distribute energy in a way similar to theseismic excitation and to produce simultaneous excitation of different vibration modes;

- non linear behaviour of the component can be studied by changing the excitationlevel or by performing up and down sweeping;

4.1.2. Servo-hydraulic actuators:- they can be used to generate high level forces from 100 N up to 250 kN;- they need a reaction frame;- care should be taken when performing analysis because of the added restraint to theexcited structure;- besides sinusoidal and random vibrations excitation, earthquake reproduction ispossible due to the available long stroke;- non linear behaviour of the component can be studied by changing the excitationlevel or by performing up and down sweeping.



4.2. Impact:- very easy to use;- many different points of the component can be excited;- very low excitation forces can be generated to avoid local damage of the component;- the energy distribution can not be easily controlled;- the method can give information about natural frequencies and vibration modes;- it is difficult to evaluate non linear behaviour.

4.3. Soil blast:

* very expensive;

• it requires a complex test management;

• it gives valuable information not only about mechanical components inside theplant, but also about foundation, soil, civil structures and interaction between soiland buildings and between supporting structures and supported components;

• floor response spectra can be effectively evaluated;

* the transmission of the excitation from the foundations to the differentcomponents is the same as in the case of seismic excitation.

5. Impulse technique for structural frequency response testing.

5.1. Impulse technique is the simplest and fastest of the various techniques commonly used.

5.2. Impulse technique requires special signal processing techniques, if accurate frequencyresponse measurements are to be obtained.

5.3. Impulse technique falls into the class of transient excitation.

5.4. If the noise-to-signal ratio at the input measurement point is much less than 1, the measuredfrequency response will closely approximate the desired true frequency response function.

5.5. Bias and random errors

5.6. The greater the measurement noise, the greater the number of averages required toapproach the expected value of the cross-spectrum between input and the outputmeasurement signals.

5.7. The coherence function is a measure of the contamination of the two signals in terms ofnoise and non-linear effects: values close to indicate with very low contamination.

5.8. With real normal modes at the resonance frequency, each point on a structure is eitherexactly in-phase or exactly 180 degrees out-of-phase with any other point.

5.9. Certain types of damping which are very often encountered in practice will cause theeigenvectors to have nonzero imaginary components, resulting in complex mode shapes:the relative phase associated with a point on a structure is some value other than 0 or 180degrees.

Author: Maurizio ZOLA - ISMES LMC 1


5.10. The usefulness of the impulse technique lies in the fact that the energy in an impulse isdistributed continuously in the frequency domain rather than occurring at discrete spectrallines as in the case of periodic signals.

5.1 1. The useful frequency range of an impulse is also a function of the shape of the impulse.

5.12. Non-linearity in structures.

5.12.1. Excitation of a non-linear system by a pure random signal will yield the bestestimate, in the mean square sense, of the linear response.

5.12.2. Excitation by a pure sine wave is also useful for studying non-linear system becauseit allows precise control of the input spectrum level.

5.12.3. The impulse technique, because of its very high ratio of peak level to total energy, isparticularly ill-suited for testing non-linear systems.

5.13. The impulse technique is especially susceptible to noise and truncation errors.

5.14. The usable frequency range for an impulse is depending upon the shape and the durationof the force signal:

5.14.1. the first zero crossing of the Fourier Transform of the force signal should be wellabove the maximum frequency of interest;

5.14.2. the square wave pulse has the lowest zero crossing frequency and is equal to theinverse of the time duration;

5.14.3. a good rule of thumb is to choose a sampling frequency greater than ten times theinverse of the time duration of the pulse in order to sample in a good way the forcepulse and then to stop the frequency analysis to a frequency equal to the inverse of thepulse duration.

5.15. The duration of the pulse is very short relative to the sample length:

5.15.1. the sample length should be chosen to have sufficient frequency resolution and

5.15.2. to capture the whole damped response oscillation;

5.15.3. then a long queue of noise is present in the sample of the force signal.

5.16. Special time-sample windows have been developed for the impulse technique:

5.16.1. a good compromise for the force signal is a time window with unity amplitude forthe duration of the pulse and a cosine taper from unity to zero;

5.16.2. for the response signal an exponential window can be used which decaysexponentially from 1 to 0.05 in the sample time.

5.17. Equipment requirements:

5.17.1. all elements of the measuring chains should be linear and have low noise;

5.17.2. weight and tip hardness should be adequate to the particular application: themagnitude and time duration of the force pulse depend upon the local characteristics ofthe structure as well as the hammer characteristics;



5.17.3. the analysis system should have suitable dynamic range to reduce the noise to theminimum (number of bits for the analogue-to-digital converter).

5.18. Frequency response testing:

5.18.1. the first step is to make a frequency response measurements to identify importantresonances;

5.18.2. the locations of the stationary transducer should be chosen to measure the responsewhereas the impact is applied at suitable points;

5.18.3. multiple impacts should be avoided because the resulting frequency spectrum couldhave zero points;

5.18.4. coherence function should be evaluated to monitor the quality of the signals;

5.18.5. the quadrature (imaginary part) component of the frequency response can be used toidentify mode shapes.

5.19. The impulse technique:

5.19.1. is used when moderately accurate estimates of modal parameters and modal shapesare required

5.19.2. does not produce results of sufficient accuracy to develop simulation models.

6. Example No 1: IN-SITU DYNAMIC TESTS ON REACT'OR TANK.

6.1. In the framework of the seismic assessment of the PEG reactor core on site test wereperformed on the reactor tank.

6.2. The block scheme of the seismic analysis was the following:

6.2.1. definition of the seismic design response spectrum

6.2.2. generation of soil acceleration time histories

6.2.3. numerical analysis of the seismic response of the reactor building

6.2.4. linear model of the core

6.2.5. linear analysis of the reactor block

6.2.6. non-linear analysis of the reactor core

6.2.7. definition of a linear equivalent model of the core

6.2.8. analysis of the response by means of the equivalent model: the process is terminatedif the solution is convergent, otherwise a new non-linear analysis is performed.

6.3. To define the excitation at the supporting grid of the core the frequency transfer functionbetween the supporting points of the reactor tank and the grid should be determined.

6.4. To reduce uncertainties which could impair the core analysis, on site dynamic tests wereperformed.

6.5. A mechanical exciter was fixed to the grid and a sinusoidal excitation was generatedstudying the response of the tank.

Author: Maurizio ZOLA - ISMES LMC1


6.6. The excitation was applied not simultaneously along the three orthogonal axes of the tank:vertical and two horizontal.

6.7. Special care was given to the choice of the excitation level in order to avoid any damage tothe grid, to the tank and to the supporting members.

6.8. Moreover special attention was paid to cleanliness and to avoid any damage to the surfaceswhen fixing both the mechanical exciter and the measuring instruments.

6.9. Due to the low level excitation very sensitive transducers were needed, then seismomneterswere used.

6.10. Conclusions:

6.10.1. The cost of the tests was justified by the importance to demonstrate the capability ofthe plant to withstand relatively high intensity earthquakes typical of that region.

7. Example No 2: N-SITU DYNAMIC TESTS ON CABINETS.

7.1. During the seismic assessment of the NPP` in Paks (Hungary) funded with a contract by theEuropean Community in the framework of the technical assistance to the East EuropeCountries, some electrical components were identified as outliers. In order toexperimentally verify the suitability of the experimental/numerical approach to the seismicqualification of these safety related components inside cabinets in-situ testing wasperformed on some cabinets.

7.2. The qualification approach was articulated in the following steps:

7.2.1. impact hammer tests on instrumented cabinets

7.2.2. data processing to get the transfer functions between the response of the cabinet andthe excitation force in order to determine the first mode of vibration

7.2.3. estimate of the natural frequencies and relevant mode shapes and associated dampingfactor

7.2.4. numerical generation of the transmissibility function between base acceleration andresponse acceleration in correspondence of the locations of the components to bequalified

7.2.5. generation of an acceleration time history starting from the FRS corresponding to thebuilding elevation where the cabinets are installed

7.2.6. computation of the acceleration response time history by the convolution of the floormotion and the transmissibility of the cabinet

7.2.7. computation of the Shock Response Spectrum of the obtained acceleration for eachcomponent to be used as Required Response Spectrum for seismic testing and analysisof the component.

7.3. Three different types of cabinets were tested with the following instrumentation:

7.3.1. impact hammer with force transducer

7.3.2. piezo-resistive accelerometers



7.3.3. a PC based data acquisition system.

7.4. For each cabinet a measurement position grid covering the structural member in elevationwas adopted and different impact points were chosen taking into account both the positionof the components and the structural typology of the cabinet.

7.5. The following acquisition parameters were adopted:

7.5.1. number of impacts: 6

7.5.2. sampling frequency: 500 Hz (to get a time resolution of 2 ins)

7.5.3. sample duration: 20 s (to get a frequency resolution of 0,05 Hz)

7.5.4. cut-off frequency of the low-pass filters: 40 Hz.

7.6. The steps of the data processing were following:

7.6.1. base-line correction of each force and acceleration time history .

7.6.2. windowing of the base-line corrected time histories

7.6.3. FFT of the sample time histories

7.6.4. computation of the transfer function between force and acceleration response

7.6.5. computation of the coherence function (a value greater than 0,7 is consideredacceptable)

7.7. The estimation of the modal parameters was performed on the obtained transfer function:

7.7.1. as natural frequency was assumed the value in correspondence of the maximum peak

of the transfer function

7.7.2. the mode shape was derived from the values of the peak of the transfer function

7.7.3. the damping factor was taken from the transfer function with the half-powerbandwidth rule.

7.8. Transmissibility functions determination

7.8.1. the cabinet was represented as a single degree of freedom system (SDOF) taking intoaccount that:

7.8.1.1. the side-to-side stiffness is very higher than that front-to-back because thecabinet are connected together side-by-side and thus the natural frequencies withmodal shapes having important deformations in side-to-side directions fall in thehigh frequency range where the seismic FRS has low energy contents;

7.8.1.2. the same argument hold for the vertical direction;

7.8.1.3. the first bending mode in front-to-back direction is the main mode because ithas associated the most part of the modal mass;

7.8.2. the transmissibility function is then given by the product between a participationfactor due to base excitation and the frequency response function of a single degree offreedom system; the parameters are following:

Author: Maurizio ZOLA - ISMES LMC d1~ cC!


7.8.2.1. first natural frequency

7.8.2.2. associated damping factor

7.8.2.3. associated mode shape of vibration

7.8.2.4. mass matrix (spatial distribution of the equipment mass)

7.9. Computation of the acceleration responses in correspondence of the different positions ofthe components:

7.9.1. the acceleration time history was computed by the convolution of the floor motionand the different transmissibility functions.

7.10. Computation of the Shock Response Spectrum for the seismic qualification tests:

7.10.1. the obtained acceleration for each component was processed to get the Test RequiredResponse Spectrum in side-to-side direction

7.10.2. for side-to-side and vertical direction the FRS was adopted with no modifications inaccordance to the hypothesis that the cabinet is not amplifying the base motion in thosedirections.

7.11. Conclusions

7.11.1. The simplified method for the estimation of the modal parameters could be adoptedin this case because the natural frequencies were well spaced in the frequency domain,otherwise a multi-degrees of freedom modal analysis should have been applied.

7.11.2. The impulse technique can be used also to solve the case where a multi-degrees offreedom behaviour of the cabinet is present: in this case the force pulse excitationshould be applied in correspondence of each of the measuring point close to thecomponent in order to identify the whole transmissibility function matrix of the cabinetand then to use it to determine the response in correspondence of each component withbase excitation.

8. Example No 3: IN-SITU SOIL EXPLOSION TESTS ON NPP.

8.1. Within the framework of the IAEA coordinated "Benchmark Study for the Seismic Analysisand Testing of WWER type NPPs", in-situ. dynamic structural testing activities have beenperformed at the Paks Nuclear Power Plant in Hungary.

8.2. The specific objective of the investigation was to obtain experimental data on the actualdynamic structural behaviour of the plant's major constructions and equipment undernormal operating conditions, for enabling a valid seismic safety review to be made.

8.3. For carrying out the experimental investigation, in December 1994 the Paks NPP site wassubjected to the effects of appropriately designed buried explosions, with the object ofinducing an earthquake-type excitation to the plant's structures.

8.4. A set of different successive experiments was performed with the whole nuclear powerplant under normal operating conditions. (More detailed information on the tests can befound in Reference 10.2).



8.5. A large number of seismometers and accelerometers were mounted at appropriate locationsin the nuclear power plant major buildings, for recording their structural response to theartificially produced ground motion.

8.6. For the synchronous recording of the free-field excitation data, together with the relatedstructural response signals during the low strain earthquake-type excitation experiments,use was made of advanced multi-channel data acquisition and analysis system. This PCbased data acquisition and analysis system is capable of recording simultaneously up to 52signals at a 200 kHz sampling frequency with real time analogue to digital conversion.

8.7. After some preliminary data acquisition and analysis with the plant in normal operatingconditions, it was decided to low-pass filter at 20 H1z the recordings in order to reject theambient vibrations noise.

8.8. A sampling frequency of 200 Hz was chosen to ensure a satisfactory definition of the blastinduced vibration time histories.

8.9. In order to compare homogeneous information, all data were converted into acceleration.Starting from these acceleration data the acceleration response spectra were calculated.

8.10. Conclusions

8.10.1. This type of in situ testing allows the assessment of the numerical models toevacuate the seismic response of the buildings and civil structures of existing plants;

8.10.2. Moreover valuable information about the soil-structure interaction can be derivedfrom the study of the experimental response;

8.10.3. Care should be taken with respect to the energy distribution in the frequency domain:the seismic excitation is characterized by high energy in the low frequency range,whereas explosions in soil are generating and transmitting energy in the high frequencyrange; this problem can be only partially overcome with a filtering of the signals beforeprocessing;

8.10.4. Experimental frequency response function can be used to develop floor responsespectra to be used for equipment and component qualification.

9. GENERAL CONCLUSIONS

9.1. Qualification by in-situ testing is more or less always a qualification by combination oftests and analyses.

9.2. In situ testing can be performed only with low level excitation and then very specialattention should be paid to non linear behaviour of structures and components.

9.3. When dealing with components with bolted connections the behaviour can be very differentfrom low and high level excitation both due to changing of restraint conditions and energydissipating mechanisms.

9.4. Piping restraints allowing thermal expansion can have a very different behaviour from lowto high level excitation.

10. BIBLIOGRAPHY



10.1. William G.Halvorsen, David L.Brown, "Impulse Technique for Structural FrequencyResponse Testing", Sound and Vibration, November 1977.

10.2. A.Giirpinar, M.Zola, "Benchmark Study for the Seismic Analysis and Testing ofWWER type NPPs - Overview and General Comparison for Paks NPP", SMiRT 14 PostConference Seminar No. 14 (Seismic Evaluation of Existing Nuclear Facilities, Vienna,August 1997.

10.3. Julius S.Bendat, Allan G.Piersol "Engineering Applications of Correlation andSpectral Analysis", John Wiley & Sons, New York, 1980.

10.4. Julius S.Bendat "NonlineaT System Analysis and Identification from Random Data",John Wiley & Sons, New York, 1980.


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