Analysis of stress design of petrochemical pipeline

As an organic component of petrochemical transportation network, petrochemical pipelines are connected with petrochemical devices to transport media, and their performance, service life, etc. have a very important relationship with stress. Stress design needs to be highly valued to ensure the safety of pipeline operation. Based on this, the article first summarizes the stress, summarizes the main stress types, analyzes the impact of pipeline stress on the pipeline, and finally puts forward a series of key points of pipeline stress design, which provides a reference for optimizing the pipeline stress design.

The design of pipeline stress is an important organic part of petrochemical pipeline design. Pipeline stress analysis refers to the analysis of pipeline stress on the basis of scientific calculation, so that the safety requirements of pipeline and other equipment can be met [1]. In pipeline design, designers should put an end to subjective assumptions, fully consider the stress state of the pipeline, and pay attention to rationality and design efficiency in the design process.

1. Stress overview

Stress refers to the force that when an object is deformed due to the influence of external factors such as force, temperature field, humidity, etc., the internal force of the interaction between various parts of the object resists the external effect and restores the object to the position before deformation [2]. The petrochemical pipeline will be affected by the properties of the fluid in the pipeline and the use environment of the pipeline during its use. The factors affecting the pipeline structure of petrochemical enterprises include the thermal expansion force of the pipeline, the sustained external load, the cold contraction force of the pipeline structure, the internal pressure of the pipeline, etc. [3]. During the operation of petrochemical pipeline, if the pipeline bears more than the maximum bearing capacity of the pipeline structure design, the pipeline will be deformed. Therefore, when designing the pipeline structure of petrochemical enterprises, it is necessary to fully consider the influence of different types of stress on the pipeline structure, and formulate the design scheme of pipeline stress in combination with the conditions of stress generation and the degree of pipeline damage caused by different stresses.

2. Stress classification

In the current pipeline design, the stress conditions usually include three kinds, namely, primary stress condition, secondary stress condition and peak stress condition.

2.1 Primary stress

The primary stress in the pipeline structure design process is mainly caused by external load conditions, such as pressure, gravity and other external force conditions, which will have a direct impact on the stability of the pipeline structure [4]. The primary stress is non-self-limiting and can be balanced with the applied load. When the external force increases, the primary stress will also increase. If the increased external force is greater than the yield limit of the pipe material, the pipe structure will have plastic deformation, resulting in pipe damage. The common primary stress in pipeline design includes the weight pressure, internal pressure and self-gravity of the medium in the pipeline.

2.2 Secondary stress

When the pipeline structure is deformed, the stress caused by the constraint is secondary stress. The secondary stress cannot be directly balanced with the external force of the pipeline. Self-limiting is an important feature of secondary stress [5]. If the pipe structure is subject to local yield or small deformation, its stress will be reduced. When the secondary stress of pipeline structure is too large, fatigue damage will occur. The main factors causing the secondary stress of the pipeline include the displacement of the breakpoint, the expansion and contraction of the heat and the cold.

2.3 Peak stress

The peak stress is generally due to the thermal effect or discontinuous type of force in the pipe accessories, pipe structure and other parts, which is superimposed with the primary stress or secondary stress, resulting in the peak stress under the stress increment. The peak stress is characterized by strong concealment, and the structural deformation under the peak stress is not significant [6]. In addition, based on the influence of peak stress, the peak stress is easy to cause fatigue crack, brittle failure and other problems of the pipeline. The peak stress generally occurs in the position where the weld is not fully penetrated in the pipeline, and the position of the small radius fillet of the pipe structure attachment.

3. Stress analysis of petrochemical pipeline

Stress analysis includes dynamic analysis and static analysis.

• (1) Dynamic analysis. It is to strengthen the dynamic analysis of the fluid during the analysis of the actual prestress of the pipeline, so that the pipeline stress can be accurately and scientifically evaluated and the basis for stress design can be provided. First, chemical and petroleum pipelines transport fluid media. Due consideration shall be given to the properties of different fluids and different properties in the transportation medium during the design of the pipeline, so as to ensure that the resonance phenomenon can be avoided during the transportation medium of the oil pipeline, increase the corresponding prestress and extend the service life of the pipeline. The second is the analysis of external joint forces in the design of oil pipeline stress [7]. The external combined force has an important influence on the stress of petrochemical pipeline. The external power will also vary greatly with the actual working environment. Therefore, the external conditions should be fully considered in the actual use process, and should be considered as a factor affecting the overall stress design of the pipeline.
• (2) Static analysis. The pre-stress is a very critical detection standard in the formulation of the standard for the service condition of the pipeline. During the production or transportation of oil, it needs to be transported by pipeline transportation. The strength and bearing capacity of pipeline will directly affect the safety and timeliness of oil transportation. The prestress bearing capacity of the pipeline will also directly affect the bearing capacity of the oil pipeline maintenance work, which has an important impact on ensuring the efficient development of the oil pipeline maintenance work. In the original data measurement of oil pipeline, the measured maximum defect depth, maximum defect length and the ultimate tensile length of oil pipeline need to be strictly calculated. These basic data have a very important impact on the maintenance of oil pipelines. Especially in the maximum allowable operation of submarine pipeline, the pressure the pipeline bears will directly affect the safety of oil pipeline maintenance. During maintenance, the pressure borne by the pipeline shall be reduced to the greatest extent to avoid serious safety accidents during maintenance. After ensuring that the working pressure is relatively stable, it is necessary to clean up the sundries in the anti-corrosion and thermal insulation layer in the pipeline, and continue to put it into use after inspection. In addition, in addition to the force of prestress, the pipeline is affected by the thermodynamic principle, and its thermal expansion and contraction will also cause the deviation of the pipeline, resulting in problems in the actual use of the pipeline.

4. Design points of petrochemical pipeline stress

4.1 Pay high attention to the flexible design of petrochemical pipelines

During the design process, the designers should attach great importance to the flexible design of the pipeline, use the flexible design concept, and optimize the stress design, so that the petrochemical pipeline can effectively cope with the stress in the process of use, in order to prevent the pipeline from being damaged due to stress in the process of operation. Petrochemical pipelines are usually characterized by flexibility. In the design process, a reasonable flexibility coefficient is adopted to meet the requirements of the pipeline for flexibility design, so as to effectively improve the adaptability of the pipeline and ensure the stability of the pipeline in use [8].
When designing the stress, designers should pay attention to the flexible design to make the flexible effect fully play. When designing the stress flexibility, designers should first attach great importance to the thermal expansion and contraction performance of petrochemical pipelines to ensure that the pipeline can meet the subsequent transportation requirements during operation and avoid the pipeline damage caused by temperature factors.
The flexible design should also pay attention to the structure of the pipeline itself, and pay attention to the reasonable application of the relevant supports. Through the application of the supports, the applicability of the pipeline can be improved, and the damage caused by fatigue during the operation can be avoided, thus effectively reducing the various deformations of the petrochemical pipeline.

4.2 Attach great importance to natural compensation in the process of petrochemical pipeline design

The petrochemical pipeline itself has the compensation function, which can reduce the deformation of the pipeline under the influence of stress. However, if the pipeline has excessive external or internal load, it will increase the natural compensation. At present, “mine compensator” is a widely used natural compensation method with high compensation efficiency. In the process of conveying medium with too high dimension, if the medium quality is relatively large, the application of “ore compensator” will be limited, and the compensation requirements cannot be met [9]. The specific installation process of the natural compensator is as follows.

• (1) Define the type of natural compensator. Determine the type of natural compensator in combination with the temperature of the transmission medium of the petrochemical pipeline and the pipe diameter of the pipeline.
• (2) Calculate the pipe expansion. Expansion=expansion coefficient × Pipe length.
• (3) Determine the position of the natural compensator. Generally, the compensation position is at the midpoint of two fixed positions of petrochemical pipelines.
• (4) Check the stress. Ensure that the safe and reliable operation of petrochemical pipeline is guaranteed after increasing natural compensation. Although the effect of applying natural compensator is relatively good, the process is relatively complex and the cost is relatively high, so the selection needs to be considered comprehensively.

4.3 Scientific selection of pipeline materials

In the process of designing stress, we need to attach great importance to the selection of materials. The pipeline material is the basis for the subsequent stable and reliable work of the pipeline, so the designer needs to optimize the selection of materials. During the selection of pipeline materials, designers should conduct a comprehensive and in-depth analysis of the mechanism and impact of different materials, so as to ensure the selection of the best transportation materials and avoid the pipeline quality problems caused by unreasonable material selection. With the continuous development of science and technology and the continuous emergence of various new materials, designers need to comprehensively evaluate various key indicators and make scientific choices [10]. For example, the subsequent stress effect of petrochemical pipeline is directly affected by the expansion coefficient of the material. Based on this, starting from the action mechanism of stress generation, the pipeline material is reasonably selected to ensure the stability of the pipeline under the condition of thermal expansion and cold contraction, and to prevent the pipeline deformation. While paying attention to the expansion coefficient, the designer should also pay attention to the elastic modulus, flexibility coefficient and other indicators of the pipeline, and select the best material in combination with the nature and environment of the transportation medium.

4.4 Optimization of petrochemical pipeline layout

In the process of stress design, designers need to pay attention to optimizing the overall layout of petrochemical pipeline system. Based on the scientific layout of the pipeline, improve the adaptability of the pipeline, so that the pipeline can cope with various types of stress. When optimizing the pipeline layout, designers should start from the actual requirements of the pipeline layout and combine with the overall requirements of the petrochemical project to achieve the ideal pipeline transportation effect and ensure that the functional requirements of the petrochemical project are met. The direction of petrochemical pipeline is the key point to be considered when optimizing the layout. The reasonable direction of petrochemical pipeline should be designed, especially the optimized design of some elbow areas, in order to ensure the flexibility of the elbow and improve the ability to cope with stress. In general, the L-shape trend shall be adopted as far as possible when arranging the pipeline, so as to improve the stress effect of the pipeline, reduce the number of elbows and help to extend the service life of the pipeline.

4.5 Reasonable application of supports and hangers

In the stress design process, attention should be paid to the reasonable application of the supports and hangers. In order to improve the stability of the supports and hangers, it is necessary to ensure that there is no potential damage at the position of the supports and hangers. When selecting pipe supports and hangers, the designer should first pay attention to the flexibility characteristics of the supports and hangers, so that the flexibility coefficient of the selected supports and hangers can meet the requirements, so as to deal with the problems of thermal expansion and contraction during the operation of the pipeline, and also prevent the fatigue damage caused by stress, effectively protecting the safe operation of the pipeline. When applying supports and hangers, attention should be paid to anti-vibration performance, especially when some pipes have obvious vibration problems, the selection of supports and hangers should have strong anti-vibration ability. During the installation of pipe supports and hangers, starting from the whole system, through comprehensive analysis, determine the installation height, reasonably determine the position of supports and hangers, and provide guarantee for the stability of the whole pipeline system. For some special positions, it is usually necessary to optimize the spring supports and hangers to improve their stress response effect and enhance the durability of petrochemical pipelines.

4.6 Optimization of pipeline design process

Starting from the production system of petrochemical enterprises, on the one hand, the overall design of petrochemical pipelines should be paid attention to, and on the other hand, the local design of pipelines should be paid attention to. Because most petrochemical pipelines operate underground and are used to transport oil, natural gas and other products and bear pressure for a long time, attention should be paid to the seismic performance of pipelines in pipeline design to prevent safety problems caused by natural disasters.
The medium for pipeline transportation is mainly oil, natural gas and other products, which are explosive and flammable and belong to dangerous goods. Therefore, when designing the pipeline, it is necessary to optimize the design process, fully consider the external environmental factors, and take effective safety prevention facilities to minimize the potential safety hazards. In addition, the petrochemical pipeline shall be regularly maintained and maintained to detect the pipeline failure as early as possible, and set up early warning devices to prevent safety accidents.
The design of pipeline stress needs to pay attention to thermal insulation design. When the pipeline is in operation, it is greatly affected by temperature, which requires the designer to strictly follow the national design standards. When selecting thermal insulation materials, designers should attach great importance to the change of engineering diversion, flexibly use and master the thermal conductivity technical chart, pay attention to the change of thermal conductivity of materials, and scientifically select the thermal insulation materials of pipelines according to specific national standards, so as to improve the ability of pipelines to cope with stress and improve the safety of pipelines.

5. Conclusion

As an important part of oil pipeline design, stress design is to analyze the management stress on the premise of scientific calculation to ensure the safety of pipeline and other equipment. The design of pipeline stress directly affects the safe operation and use of petrochemical pipeline, so it needs to be paid high attention. In order to improve the pipeline stress, it is necessary to pay attention to the flexible design and natural compensation design of the pipeline during the design process, optimize the oil pipeline layout and design process, scientifically select the pipeline materials, combine the reasonable application of supports and hangers and prestress, effectively improve the design quality of the pipeline stress, and provide guarantee for the safe operation of the oil pipeline.
Author: Su Hang, Li Qian
Reference:

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