VARIOUS APPROACHES USED IN THE SEISMIC QUALIFICATION OF THE PIPING SYSTEMS IN NUCLEAR FACILITIES. Introduction

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1 VARIOUS APPROACHES USED IN THE SEISMIC QUALIFICATION OF THE PIPING SYSTEMS IN NUCLEAR FACILITIES A. Musil, P. Markov Stevenson&Associates, Pilsen, Czech Republic Introduction In the firm Stevenson&Associates the computational system (see Fig. 1) enhancing the efficiency of modeling and evaluation of the piping systems according to the most commonly applied international codes, which are the ASME Boiler and Pressure Vessel Code (ASME BPVC) and the PNAE G in the case of the WWER-type reactors, is being developed at the current time. The paper presents the step-by-step description of the approach applied in the developed computational system when the differences between the versions for the ASME BPVC and the PNAE Code are mentioned. Preprocessing of a pipeline model and computation of the response for the load cases required by the applied code is performed by using ANSYS software. A 3D beam FEM model of the evaluated pipeline is created by using Piping Models module incorporating the macros developed to automate the individual stages of model preprocessing and response computation. Model geometry and response results are exported into data files which are then imported into the evaluation program set up in Mathcad software. Thus the program receives complete information on the FEM model necessary to carry out automatic evaluation in which the stress at all nodes of the model is determined following the guidelines of the applied code. Subsequently, the obtained values are compared with the stress allowable values specified for the material of the pipeline to calculate the safety factor and based on its value to identify the most critical locations within the qualified pipeline. Fig. 1. Scheme of the computation system for the evaluation of piping systems Such procedure makes development of a pipeline model significantly faster, automates the subsequent evaluation process, and minimizes the risk of a human-factor error. FEM Model Numbers of specialized program modules which can have variously close relation to general FEM systems or can be absolutely autonomous have been developed for modeling of piping systems. An example of such modeler is called Piping Models and is incorporated in ANSYS computer code. Such solution is very convenient since the computation models developed by using Piping Models module are standard 3D computational models built-up from one-dimensional finite elements (PIPE16, PIPE18). Piping Models module enables comfortable modeling of straight sections, curved sections, miter bends, valves, flanges, reducers, flexible supports, and supports with gap. In order to model tee connections according to the provisions ASME Code macro tee.mac was developed. It enables to create the model of the tee connection of UFT or WT type into a

2 chosen node of a pipeline, in a similar manner as standard TEE procedure of Piping Models module which models tee connections simpler than the code [1] requires. Modeling of a tee connection in accordance with the PNAE G Code, Appendix 5, is realized by application of standard TEE procedure with certain modifications. The other macros which have been created are beg.mac, rst.mac, and rse.mac. Macro beg.mac serves for proper definition of all input parameters of preprocessor for modeling and computation analysis. It contains material and section properties of the part of the pipeline which will be modeled next. This macro must me modified prior to its execution to correspond to the currently modeled pipeline. After the execution of macro beg.mac the model of the entire piping system can be built using ANSYS Piping Models module interactively in a standard way. Only for tee connection development when the ASME BPVC is applied, macro tee.mac shall be executed instead of standard TEE procedure of Piping Models module. Modeling of Tee Connection According to the Provisions of the ASME Code The ASME Code classifies two types of tee connections (or tee branches) in the piping design. For the computation models of welding tees (WT) it is required that both the run pipe and the branch pipe are extended to the intersection of their centerlines. The intersection is assumed stiff as well as the length of the branch pipe between the intersection and the run pipe surface. The basic model of unreinforced fabricated tee (UFT) is identical. Only an element of negligible length with local bending flexibility shall be added on the surface of the run pipe. For local flexibility both in bending in the plane defined by the run pipe and the branch pipe centerlines and in the plane perpendicular such that: φ = kmd / EI b, (1) where: Φ local rotation of the branch pipe due to acting moment M, k flexibility of the wall of the branch pipe in the defined direction, M bending moment in the defined direction (see Fig. 2), d outer diameter of the branch pipe, E Young elasticity modulus I b cross-sectional moment of inertia of the branch pipe, T b nominal thickness of the branch pipe defined for various types of reinforcement. The values of k x3 related to moment M x3 which acts perpendicularly to the plane defined by the axes of the run pipe and the branch pipe are defined as [1]: 1/ 2 ' [ ] ( T T ) 1.5 ( D / T ) ( T / t )( d / D) k / x = r r n b r (2) and the values of k z3 related to moment M z3 which acts in the plane defined by the axes of the run pipe and the branch pipe are calculated using:

3 1/ 2 ' [ ] ( T T ) ( D / T )( T / t )( d / D) k / z = r r n b r (3) where: D outside diameter of the run pipe, d outside diameter of the branch pipe, T r thickness of the run pipe, t n thickness of the branch pipe with reinforcement according to [1], T b thickness of branch pipe according to [1].. Fig. 2. Tee connection with displayed moments In case of 3D beam elements, the first requirement (rigid connection at the intersection of the run pipe and the branch pipe centerlines) is satisfied automatically due to basic properties of beam elements. In order to satisfy the second and the third requirement it was necessary to develop a specialized procedure. It was realized by creation of macro tee.mac. In this macro the data describing the material properties and geometry of the run pipe and the branch pipe are read as first. After their reading TEE command from Piping Models module is executed to provide the creation of three new elements with a shared node at the intersection of the run pipe and the branch pipe centerlines. These new elements form one PIPE17 element. PIPE17 element is a combination of three uniaxial elastic straight pipe elements (PIPE16) arranged in a tee configuration with tension-compression, torsion, and bending capabilities. One of the parameters specifies whether the branch connection is of UFT type or WT type. The following parameters define the length of the inserted elements modeling the run pipe and the branch pipe. The length of each run pipe element is equal to two diameters of the run pipe which is in accordance with the default value in TEE command. The length of the branch pipe value is set to be equal to one half of that diameter. Thus a new node on the surface of the branch pipe is created which provides that the second requirement of the code is satisfied. Then the properties of the new branch element must be modified. Since it must be rigid, it was chosen E = and ρ = 0. Thus it is achieved that the model becomes even more realistic than is required by the code because any other value of ρ than zero would bring additional mass compared to the real mass of a pipeline. The third requirement was solved by inserting an element of BEAM4 type into the new node on the surface of the run pipe. The length of this element was set equal to one tenth of the diameter d of the branch pipe which is in agreement with the study [2].The inserted BEAM4 element is of rectangular cross section with moments of inertia in bending: I = I (4) b y / k x3 and I = I (5) h y / k z3

4 where I y is cross-sectional moment of inertia in bending of the pipe branch (if it was modeled by PIPE16): I y 4 4 ( D d )/ 6 = π. (6) The computational model of the branch pipe developed by this procedure fully satisfies the requirements of the code [1]. Macro tee.mac will create it at any part of a pipeline where a branch pipe is modeled. The purpose of the use of BEAM4 element is to model the stiffness reduction of branch pipe due to effect of its attachment (weld) to a thin wall of the run pipe. Scheme of UFT type of tee connection according to the provisions of the ASME BPVC is shown in Fig. 3 and the example of the model of such tee connection created by using macro tee.mac is shown in Fig. 4. Fig. 3. Scheme of the model of UFT type of tee connection according to the provisions of the ASME BPVC Fig. 4. Model of UFT type of tee connection created by using macro tee.mac Modeling of Tee Connection According to the Provisions of the PNAE Code As already mentioned, the model of tee connection according to the provision of the PNAE G Code [3] can be developed by using standard TEE procedure from Piping Models module of ANSYS. Since tee connection is evaluated in the sections A, V, B as it is displayed on the scheme in Fig. 5, the only modification is the relocation of the edge nodes of both elements of the run pipe to the location of the sections. This shortens the length of the run pipe elements to be equal to the length of the branch pipe radius. The branch pipe element is modeled to connect the tee intersection with the run pipe surface. Fig. 5 Scheme of tee connection according to the provisions of the PNAE G Code Computation Analyses The resulting product of Piping Models module is a standard computational model of ANSYS code like the example in Fig. 6. After the setting of boundary conditions and static or dynamic loadings, nodal displacements, forces, and moments are computed. Static analyses are performed in a standard way, seismic analyses are performed by automating procedures developed by Stevenson&Associates. For the analysis of the pipeline response on the seismic excitation by using Response Spectrum Method [4] there are several versions of macro rsm.mac available depending on the applied method for the mode response combination, the most typically it can be Complete Quadratic Combination (CQC) or Square-Root-of-Sums-of-

5 Squares (SRSS). After the completion of the computation analyses, macros rst.mac (for static analysis) or rse.mac (seismic analysis) are executed to generate the output data files containing the model data such as element types, model geometry and results of the computed responses (nodal displacements, forces, and moments). Subsequently, these files are imported into Piping Evaluation program in which the evaluation of the pipeline is carried out in terms of compliance with the acceptance criteria of the applied code. Fig. 6. Computational model of a pipeline Piping Evaluation Piping Evaluation program set up in Mathcad software consists of a number of separated, consequential modules which serve for realization of individual steps during the piping evaluation process. Thanks to the capability of Mathcad to read data form various types of data files (xls, txt, LOTUS, etc.), it is possible to automate the import of the data generated by ANSYS. At the same time, Mathcad also enables to create these types of data files which is utilized both during the data transfer between individual modules and during the generation of output data files containing the results of evaluation. The function of the first module Pipeline Reading is to read the data of all elements of the pipeline FEM model and on the basis of these data (Element Name, Real Constant, KeyOpt) to sort and classify the elements into the groups representing the standardized types of pipeline components (straight pipe, curved pipe, tee connection, flange connection, etc.). At the same time, the elements of the FEM model representing different types of elements which will not be further evaluated (SPRING, LINK, etc.) are filtered out and are not proceeded to the following evaluation steps. The separate tables of the evaluated types of pipeline components containing all data essential for the following evaluation steps are created and exported in the form of Excel sheets. The subsequent module in Piping Evaluation program is Pipeline Response module. This module imports the data files with the response results for the applied loads computed in the FEM model in ANSYS. The evaluation based on the ASME BPVC is carried out for nodal values of moments. The responses on individual seismic load cases are combined the most commonly by applying the SRSS rule and the resultant nodal values of moment then computed. The resultant moments are calculated identically for static loads and the resultant nodal moments for both static and seismic loads are then exported to Stress Evaluation module. For the pipeline evaluation following the PNAE code different version of Pipeline Response module was developed since the stress evaluation is based on the nodal values of Fz (axial force) and moment components Mx, My (bending moments), and Mz (torsional moment). Stress Evaluation module reads both files with the element data of all evaluated types of pipeline components imported from Pipeline Reading module and files with the response results generated by Pipeline Response module. For the case of the pipeline evaluation according to the ASME BPVC code, primary stress intensity S n is determined for the required load case (Service Level) at all nodes of the evaluated types of pipeline components. Provided

6 the PNAE code is applied for the pipeline evaluation, reduced stress (σ) 2 is determined. The results listed in the Excel tables are passed to the last phase of the pipeline evaluation which is performed in Final Evaluation module. Module Allowable Stress contains the procedures for determination of the values of material allowable stress defined by the acceptance criteria of the applied code corresponding to the required load combination. The determined allowable value is exported to the last module which is Final Evaluation. In this module the values of stress computed in Stress Evaluation module are compared with the allowable stress values determined in Allowable Stress module. In Final Evaluation module the parameter called Safety Factor is calculated for all nodes of the evaluated pipeline elements. Its value expresses the safety reserve of the investigated node in the pipeline model under the applied seismic loading compared to the allowable stress value. On the basis of this value, the critical locations of the pipeline are identified. The pipeline satisfies in terms of compliance with the acceptance criteria provided the value of Safety Factor is not lower than 1. If this condition is not met, the seismic reinforcement provisions shall be proposed to achieve the sufficient seismic strength of the pipeline. The results produced by this module are exported as an output file of seismic qualification of the pipeline. Conclusion The presented computational system is prepared for the evaluation of piping according to the ASME BVPC Subsection NB (piping systems of the primary circuit) and Subsection NC (piping systems of the other nuclear facilities) pro Service Levels C, D (normal operation loads + seismic loads). It remains to extend the program to cover the piping evaluation for Service Levels A and B (inclusion of the thermal gradients on elastic stress). Parallelly, the version for the evaluation based on the PNAE G Code [3] has been developed and implemented. This Code is expected to be applied for the piping evaluations in the project of the construction of the 3 rd and 4 th Unit of the Mochovce Nuclear Power Plant in Slovakia. The aim of the current efforts is to develop the complex, universal and effective computational system for the pipeline qualifications with regard to the current needs of the nuclear industry. References 1. ASME Boiler & Pressure Vessel Code, Section III, Division 1. New York, USA, Stress Intensification Factors and Flexibility Factors for Unreinforced Branch Connections. EPRI Report No. TR , Palo Alto, USA, Strength Design Code for Components and Pipelines of Nuclear Power Facilities (PNAE G ). Gosatomenergonadzor of USSR, Energoatomizdat, Moscow, Russia, Gupta, A. K. Response Spectrum Method in Seismic Analysis and Design of Structures. Blackwell scientific Publications, Cambridge, USA, 1990.

RF-PIPING. Add-on Modules for PIPELINES. Program Description

RF-PIPING. Add-on Modules for PIPELINES. Program Description Version February 2016 Programs RF-PIPING Add-on Modules for PIPELINES Program Description All rights, including those of translations, are reserved. No portion of this book may be reproduced mechanically,