Investigation of Fully Welded Ball Valves’ Seismic Behavior via Spectrum Analysis

Abstract

To evaluate the seismic performance of fully welded ball valves, this study employs the modal decomposition response spectrum method, combined with actual earthquake data from the PEER seismic database, to analyze the valve's response under lateral, longitudinal, vertical, and combined seismic loads. The results indicate that under lateral, longitudinal, and combined loads, the connection between the valve body and the pipeline is the most critical area. Under vertical loads, the maximum stress occurs at the connection between the ball and the support plate. According to the JB 4732 standard, the critical sections were evaluated and confirmed to meet safety requirements under seismic conditions. It is recommended to adopt a limit state design approach and to closely monitor lateral and vertical displacements at critical locations during operation and maintenance.

Introduction

Earthquakes are a major cause of natural gas pipeline failures. As critical safety components of pipeline systems, line shut-off valves must demonstrate reliable seismic performance. For example, the "Sichuan Gas Eastward Transmission" project includes 74 shut-off valve chambers, making the seismic reliability of its core component—the fully welded ball valve—particularly critical. Substantial progress has been made in the seismic analysis of valves. For example, Xuelian Dong et al. evaluated the seismic response of nuclear-grade double-disc gate valves using the equivalent static method and the SRSS combination method, ensuring compliance with ASME standards.

Ping Liu et al. combined theoretical analysis with finite element simulations to optimize the structure of a main water supply isolation gate valve, enhancing its seismic performance. The results were validated through experimental testing. Shuxun Li et al. proposed a predictive model for assessing the ultimate load capacity of ball valves subjected to geological hazards, with its accuracy verified via numerical simulations.

Common seismic analysis methods include:

The equivalent static method is simple and conservative but cannot capture the structure’s dynamic response.

Time history analysis offers high accuracy but is computationally intensive, making it more suitable for critical structures.

The mode decomposition response spectrum method offers a balance between accuracy and efficiency, making it widely used in engineering seismic analysis.

Given these advantages, this study adopts the mode decomposition response spectrum method to analyze the seismic response of a fully welded ball valve produced by a specific manufacturer. The research focuses on the valve’s natural vibration characteristics and stress distribution under seismic loads, aiming to provide insights for future design optimization, as well as operational and maintenance strategies.

1. Modal Decomposition Response Spectrum Method

The modal decomposition response spectrum method calculates the seismic response of a multi-degree-of-freedom system by applying the seismic acceleration design response spectrum of a single-degree-of-freedom system, utilizing modal decomposition and the principle of modal orthogonality. In this study, the method is applied with consideration of torsional coupling. The seismic response corresponding to each vibration mode is calculated separately and subsequently combined using suitable modal superposition techniques to derive the overall seismic response of the system. The unidirectional seismic force exerted by a structural vibration mode on a mass point is calculated using the following equations:

Where:

Fij is the unidirectional seismic inertial force acting on mass point jjj in the iii-th vibration mode;

Mij is the bending moment at mass point jjj in the iii-th vibration mode;

ai is the seismic influence coefficient corresponding to the natural period of the iii-th vibration mode, obtained from the design response spectrum curve in the seismic design code;

Xij is the unidirectional displacement of mass point jjj in the iii-th vibration mode;

rj​ is the radius of inertia of mass point jjj about the axis of rotation in the given direction;

θij​ is the relative rotational angle of mass point jjj in the iii-th vibration mode;

Gj is the mass of mass point jjj.

Using Equations (1) and (2), the seismic response of each mass point in the structure can be calculated for each vibration mode. These responses are then applied to the structural model to determine the internal forces and displacements at each point. From these, the overall stress and displacement distributions of the structure under seismic loading can be determined. It is important to note that the accuracy of the modal decomposition response spectrum method depends heavily on the number of vibration modes included in the analysis. To ensure accurate results, the cumulative mass participation of the selected vibration modes should be close to 1.

2. Calculation Model and Boundary Conditions

This study is based on a fully welded pipeline ball valve produced by a specific manufacturer, which serves as the foundation for the analysis and modeling. The valve mainly comprises the valve body, valve stem, ball, gland, and several other components. Structural features and components that have minimal influence on the calculation results are omitted during the modeling process. The actuator is simplified and represented as a mass concentrated at its center of gravity. To enhance the accuracy and realism of the simulation, pipeline sections measuring five times the valve diameter in length are added to both ends of the valve body. A three-dimensional model is developed and meshed using ANSYS, as shown in Figure 1. The working medium in the fully welded ball valve pipeline system operates at a pressure of 10.7 MPa and a temperature of 20 °C. The primary pressure-bearing components—including the valve cover, valve seat, valve core, and valve stem—are made from A694-F65 steel, while the pipeline is constructed of X80 steel. The physical properties of these materials are detailed in Table 1.

Figure 1. Mesh model of the fully welded ball valve

Table 1 Main material properties of fully welded ball valves

Boundary conditions have a significant impact on the natural frequencies and modal vibration characteristics of the fully welded pipeline ball valve. To ensure the accuracy and reliability of the modal analysis results, the boundary conditions are defined in accordance with the valve’s actual installation conditions. In practical applications, the valve base is bolted to the foundation, while both ends are welded to the pipeline. Accordingly, fixed constraints are applied at the valve base, while displacement constraints are imposed at both ends of the valve to restrict vertical and radial movement of the connected pipelines. Additionally, symmetry constraints are applied along the valve’s symmetry plane, with the axis of symmetry aligned in the 2-axis direction.

3. Selection of Response Spectrum

Currently, there is no dedicated seismic code for natural gas pipelines and their critical equipment; therefore, the standard response spectrum from the Code for Seismic Design of Buildings is used as a reference. To better reflect real conditions, the standard response spectrum is initially based on data from the Wenchuan earthquake, with seismic records obtained from the PEER (Pacific Earthquake Engineering Research Center) database. Real earthquake records are selected, and MATLAB is used to program and compute the three-directional standard acceleration response spectra, assuming a structural damping ratio of 0.05%. The longitudinal direction aligns with the pipeline axis, the transverse direction corresponds to the horizontal axis perpendicular to the pipeline, and the vertical direction follows the direction of gravity. The frequency-domain data needed for ANSYS simulations are obtained by applying a fast Fourier transform (FFT) to the acceleration response data using MATLAB. 

4. Mode Decomposition Response Spectrum Calculation

4.1 Analysis of Structural Natural Vibration Characteristics

The natural vibration characteristics of a system—comprising its natural frequencies and vibration modes—are fundamental to determining its seismic response. In this study, the block Lanczos algorithm in ANSYS software is employed to calculate the first 10 vibration modes of the ball valve. Table 2 presents the results, showing the first six modes. As shown in Table 2, the cumulative mass participation of the first six vibration modes reaches 1, meeting the criteria for accurate analysis and calculation. Since the natural frequencies of these modes are close, the maximum internal stresses and displacements under unidirectional seismic loads are combined using the Complete Quadratic Combination (CQC) method. Additionally, because the response spectrum curves for the three orthogonal earthquake excitations are similar, the maximum internal stresses and displacements from each mode under three-directional seismic loads are combined using the Absolute Sum (ABS) method.

Table 2 The first 6 modal information of the ball valve

4.2 Results of Vibration Mode Decomposition Response Spectrum Analysis

The Response Spectrum module in ANSYS is used to calculate the structural response of the ball valve under three-directional seismic and combined loads, determining the maximum stress and deformation caused by earthquake forces. At the same time, the Static Structural module calculates the maximum stress and displacement of the ball valve under internal pressure, temperature, and deadweight loads. These results are then combined directionally to determine the overall maximum stress and displacement distribution of the ball valve. Under lateral, longitudinal, and combined three-dimensional seismic loads, the maximum stress occurs at the connection between the valve body and the pipeline. According to Li Shuxun et al., this connection is the weakest part of the ball valve structure due to structural discontinuities, material properties, and other factors; therefore, the analysis results are consistent with theoretical expectations. Under vertical seismic loads, the maximum stress occurs at the connection between the ball and the support plate. This is due to the ball’s large inertia during vertical excitation, which leads to significant stress concentration at the support plate connection. To better understand the stress and displacement distribution at the valve body–pipeline connection, a path is defined circumferentially at this location to extract stress and displacement values under seismic loading. Due to the high longitudinal stiffness of the ball valve, lateral and vertical seismic loads—along with combined loads—cause greater lateral and vertical displacements, with the maximum lateral displacement exceeding both longitudinal and vertical displacements. Due to the influence of structural stiffness, lateral and vertical stresses under both unidirectional and combined seismic loads are significantly higher than those in the longitudinal direction. It is evident that combined seismic loads produce greater stresses and displacements than loads applied in any single direction, leading to more severe damage to the ball valve.

4.3 Safety Evaluation

The fully welded pipeline ball valve is a steel pressure-bearing component, with its thin-walled parts evaluated according to the requirements of JB 4732-2005 (Analysis and Design Standard for Steel Pressure Vessels). Structural stresses are classified as primary, secondary, and peak stresses based on factors such as their origins, failure modes, and distribution within the structure. Each type of stress is then evaluated according to its specific assessment criteria. The stress evaluation method for thin-walled components can be classified into point evaluation and line evaluation, depending on the selected evaluation path. Considering the characteristics of the component studied in this paper, the line evaluation method is adopted. This method linearizes the calculated stress along a defined path at the section of interest. The selection of the evaluation path is crucial; according to JB 4732, it should be perpendicular to the section and pass through the point of maximum stress.

In this study, the line evaluation method is applied to the shell-type thin-walled components, using the following criteria:

Primary membrane stress mmm_mmm​: verification limit

Primary membrane stress plus primary bending stress: verification limit

Where Sm is the allowable stress of the material, and K is the load coefficient determined according to Table 3-3 in JB 4732. For seismic loads, K=1.2K is applied. Stress linearization is carried out for the ball valve under longitudinal, transverse, and combined seismic loads, with the verification results summarized in Table 3.

Table 3: Stress Assessment of Fully Welded Ball Valves Under Seismic Loads

The support structure provides essential load-bearing and stability functions, which are critical for safety during seismic events. Analysis indicates that the support primarily resists shear forces between itself and the valve body under seismic loading. Shear forces on the support are extracted using ANSYS, and the verification results are summarized in Table 4.

Table 4: Verification Results of Support Shear Stress

5. Conclusion

This study employs the Block Lanczos algorithm to extract the modal data required for the seismic analysis of the fully welded ball valve. The cumulative mass fraction of the first six vibration modes is found to reach 1.0, meeting the requirements for accurate analysis and calculation. The seismic response of the ball valve was calculated using natural seismic waves selected from the PEER database, combined with the mode decomposition response spectrum method. The stresses and deformations caused by deadweight and internal pressure were then superimposed in the same direction as the seismic loads to obtain the overall seismic response of the ball valve under lateral, longitudinal, vertical, and combined loading conditions. The connection between the valve body and the pipeline was identified as the weak point of the valve. A stress path was defined along the critical section of the ball valve to extract the stress and displacement responses under both unidirectional and combined seismic loads. The results indicate that the ball valve exhibits pronounced sensitivity to lateral and vertical seismic loads, with the greatest structural response and potential damage occurring under combined seismic loading. The critical section was analyzed and evaluated in accordance with the JB 4732 standard, confirming that the ball valve remains structurally safe under seismic loading conditions. Based on the analysis, it is recommended to enhance the seismic resistance of the ball valve through limit-state design methods during the design process. In addition, during installation and operation, close monitoring of lateral and vertical displacements at the ball valve–pipeline connection is essential to prevent potential damage caused by excessive movement.