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Strength Analysis of Ball Valve Body Withstand External Loads

June 17, 2018

1 Overview

Pipeline ball valves are used in large-scale long-distance oil and gas pipelines. Their quality is directly related to the safety of the transmission medium. Therefore, the requirements for long-distance pipeline valves are more stringent than those of general-purpose valves, which require that the valves have high sealing performance and adequate strength and toughness. After the pipeline ball valve is installed in the pipeline, as a component of the pipeline system, in addition to the internal medium pressure, it also has to bear the influence of external loads. The structural strength of the valve body under the composite action of the medium pressure and the external load was analyzed. The safety of the structural strength of the valve body could only be verified by finite element simulation. Based on the finite element analysis, a test platform for the development of a bending load simulation test was established. The actual stress state of the valve body under the bending load and the function of the valve were tested to verify the correctness of the finite element numerical analysis. Through analysis and research, it provides a reliable theoretical basis for the design and production of products and ensures the reliable operation of pipeline ball valves on long-distance pipelines. 

2 Technical analysis

After the valve is installed in the pipeline, in addition to the pressure of the internal medium, the bending moment M due to the settlement of the foundation and its own weight, as well as the external pressure load Q1 and the external tension load Q2 due to installation and temperature changes, it may also be suffered. In the analysis and design, it is assumed that M, Q1, and Q2 are respectively applied to the ball valve. Therefore, only the composite action of an external force and medium pressure is considered. According to the relevant provisions of the ASME QME-1 on the end loading test, the structural strength of the valve body under the joint action of the medium pressure and the external moment load is analyzed.

The test valve assembly is mounted in the test fixture, and the end piece transfers the end of the test load. The test arrangement is such that a constant torque is applied over the entire length of the valve. When the full test pressure is applied simultaneously in the valve and the connecting pipe, the valve body bears at least the normal axial pull force generated in the end cap. When the test torque is applied, it has the most adverse effect on the operability of the test valve assembly as much as possible. For most gate and ball valves, it is generally considered that the stem plane and the centerline of the pipe, that is, the bending moment is applied to the center plane of the valve stem, tension and pressure are applied to the centerline of the pipe. 

The external load of the valve is generated after the valve is installed in the pipeline. The maximum external load applied at the end is defined as the ultimate load that can be applied to the valve without yielding the pipe. External bending moment M, pressure Q1 and tension Q2 are

In the formula,

Fb —Structure section modulus

S—The maximum stress that may occur in the connecting pipe, taking the yield strength of the pipe material σS

Fa—structure section area

S1—Maximum pressure that may occur in connecting pipes

S2—Maximum tensile that may occur in the connecting pipe 

In the valve body structure, its neck opening structure is a typical overall structure discontinuity, causing local stress concentration. In ASME VIII-2, analytical design methods are used, and the qualification of the design is checked by examining the structural behavior under various external loads. According to the discontinuous theory of elastic shell, the elastic stress field is decomposed into three different stresses:

Primary stress (primary total film stress Pm, primary local film stress PL, primary bending stress Pb), secondary stress Q, and peak stress F, etc., and then evaluate the structural strength with the corresponding stress intensity limit. 

Since the load applied to the valve body is aperiodic, the primary and secondary stresses are caused by the failure of the valve body, and the influence on the peak stress can be ignored. According to the requirements of ASME VIII-2, only three stresses need to be checked.

The primary total film stress, and the calibration criteria is Pm ≤ Sm.

The primary local film stress + primary bending stress, and the calibration criterion is PL+Pb≤1.5Sm.

The primary local film stress + primary bending stress + secondary bending stress strength. The calibration criterion is PL+Pb+Q≤3Sm (Sm is the allowable stress intensity for a given material).

The primary total film stress, and the calibration criteria are Pm ≤ Sm.

The primary local film stress + primary bending stress, and the calibration criterion is PL+Pb≤1.5Sm.

The primary local film stress + primary bending stress + secondary bending stress strength.

The calibration criterion is PL+Pb+Q≤3Sm (Sm is the allowable stress intensity for a given material).

The primary total film stress, the primary local film stress, the primary bending stress and the secondary bending stress are all decomposed according to the stress intensity. When the three stress strength check criterion conditions are satisfied at the same time, the structural strength can be judged to be safe. 

3 Mechanical Models and Finite Element Numerical Analysis

The 3D solid modeling software Solid Works was used to model the valve body. During the modeling process, the key parts of the modeling were determined according to the analysis requirements. Under the premise of ensuring the accuracy of the model analysis, it’s the necessary structural simplification and meshing process for the original 3D solid model. Select dangerous sections to homogenize and equivalently linearize stress components along a stress treatment line.

(1) Combined action of medium pressure and external bending moment

An external bending moment load mechanics model (Fig. 1) was established and its finite element numerical analysis was performed under the combined action of internal pressure and external load (Fig. 2).

Fig.1 Mechanical model under the action of medium pressure and external bending moment load 

 

Fig. 2 Stress intensity distribution of valve and pipe under combined action of medium pressure and external bending moment 

The dangerous section was used as the characteristic section location, and the stress linearization path was selected for linearization (Fig. 3).

  

Fig. 3 Linearization of dangerous sections 

The analysis shows that the valve body has local stress concentration due to the neck opening. When the pipe reaches the maximum stress strength state, according to the test requirements of ASME QM E-1 and the structural stress intensity evaluation methods of ASM E III and ASM E VIII-2, the valve structural strength and safety can be confirmed after process. 

(2) Combined action of medium pressure and external pressure load

An external pressure load mechanics model was established (Fig. 4), and finite element numerical analysis was performed under the combined action of internal pressure and external load (Fig. 5).

Fig.4 Mechanical model under the action of medium pressure and external load

 

Fig. 5 Stress intensity of valves and pipelines under combined action of medium pressure and external pressure load. 

The analysis shows that under the combined effect of medium pressure and external pressure load, when the pipeline reaches the maximum compressive stress strength state, the method is based on the test requirements of ASM EQM E-1 and the structural stress intensity evaluation methods of ASM E III and ASM E VIII-2. , the valve structural strength and safety can be confirmed after process.

 

(3) Combined action of medium pressure and external tensile load

An external tensile load mechanics model (Fig. 6) was established and its finite element numerical analysis was performed under the combined action of internal pressure and external load (Fig. 7).

Fig. 6 Mechanical model of medium pressure and external tension 

The analysis shows that under the combined action of medium pressure and external tensile load, when the pipeline reaches the maximum tensile stress strength state, the method is based on the test requirements of ASM EQM E-1 and the structural stress strength evaluation method of ASM E III and ASM E VIII-2. , The valve body structure strength is safe. 

4 Bending load simulation test

The finite element numerical analysis effectively evaluates the integrity of the pressure boundary of the valve body and optimizes the structural design of the valve body. In order to verify the accuracy of the finite element analysis and the valve’s function, it was verified by bending simulation test. In the bending test device, check the sealing performance of the valve of the pipeline ball under the combined effect of internal pressure and bending load, and measure the switching torque of the valve and the stress of each point on the valve body within the allowable range. Comparing with the finite element analysis of the valve body strength, the accuracy of the numerical analysis program is verified and the basis for establishing an accurate numerical analysis program is provided.

 

Fig. 7 Distribution of stress intensity of valve and pipe under the combined action of medium pressure and external tension load 

5 Conclusion

Through the finite element numerical analysis and the bending load simulation test of the valve body strength under the external load of the ball valve, it is determined that the valve strength meets the corresponding requirements. After the valve is installed in the pipeline, under the combined action of the external load and internal pressure, the location where the pipeline system is prone to failure often occurs at the junction of the valve and the pipeline or on the thin-walled pipeline. In the design of the engineering system, the structural stress should be fully analyzed, then it can ensure safe and stable production.

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