CHAPTER 1 INTRODUCTION TO PIPING SYSTEM

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1 1 CHAPTER 1 INTRODUCTION TO PIPING SYSTEM 1.1 INTRODUCTION Piping systems in any industry are similar to the arteries and veins in the human body. Pipelines are common in almost all industries. They carry crude oil from oil wells to tank farms for storage or to refineries for processing (Figure 1.1). The pipe lines are also used for transportation and distribution of natural gas. The piping systems in thermal power plants convey high-temperature and high-pressure steam to generate electricity. In a power plant, the piping system is used to transport low and high-pressure water, chemicals, low-pressure steam and condensate. In food processing plants, chemical plants, paper mills and other similar industrial establishments, the piping systems are utilized to carry vapours, liquids, chemicals, mixtures, gases and solids from one location to another. The fire protection piping networks in industrial, residential, commercial and other buildings carry fire suppression fluids, such as gases, water and chemicals to provide protection to life and property. The design, operation, construction and maintenance of various piping systems involve an understanding of piping fundamentals, generic and specific design considerations, materials, fabrication, testing and inspection requirements, installation in addition to the state, local and federal regulations. Piping for most process industries represents the major item of

2 2 unit investment. Typical total erected piping cost ranges from 25 to 50 percent of the total cost of a plant. Figure 1.1 Plant layout arrangements in process plant (Saipem India Projects Ltd) In all applications, by virtue of physical conditions pipes may cause serious damage when exposed to the atmosphere. Failure of these components will also cause plant shutdowns and maintenance. Hence the integrity of piping components is of great importance in industrial applications (Figure 1.2). The structural integrity and cost of piping system are of major concern in oil and gas, chemical and other industries. Piping systems can be subjected to severe thermal, seismic, pressure and other mechanical loads and for this reason, an increasing amount of attention is required to retain the integrity of piping components. The structural integrity of piping system relies heavily on the correctness of design codes and practices, which can only be achieved

3 3 through a thorough understanding of the behaviour of piping components and systems under different types of load. Consequently, the piping designer often faces the necessity of making careful and realistic compromises between cost and design features without sacrificing the safety standards. Figure 1.2 Piping systems in membrane unit of HRU (Saipem India Projects Ltd) 1.2 PIPING DESIGN Piping Piping includes pipe, gaskets, valves, flanges, bolting, fittings and pressure containing portions of other piping components (Figure 1.3). It also includes pipe supports and hangers and other items necessary to prevent overstressing and pressurization of the pressure-containing components. It is evident that a pipe is either one element or a part of piping. Therefore, pipe sections that are connected with valves, fittings and other mechanical equipment and properly supported by supports and hangers are called piping.

4 4 Figure 1.3 Piping components Pipe fittings include pipe bends, reducers, tees, branch connections and integrally reinforced connections Ovality Ovality refers to a deviation of the structure of the pipe from an ideal round shape to an elliptical shape caused by shape irregularities during manufacturing (Figure 1.4). It is a measure of the difference between the maximum and minimum outside diameters of a circular pipe. Figure 1.4 Pipe bend cross section The percent ovality is resolute by the deviation of major and minor diameters divided by the nominal diameter of the pipe bend (Mohinder L 2000).

5 5 1.3 REFORMER The reformer is used in the production of synthesis gases from the natural gas. In a reformer, steam is mixed with the natural gas and the combined stream is further heated and routed through tubes in a reforming furnace containing nickel oxide catalyst. The reforming reaction occurs in which methane in the natural gas gets partially converted to carbon dioxide, hydrogen and carbon monoxide. The reformer is one of the main critical equipment in methanol and ammonia production plants. The cost of the reformer is a substantial part of the investment of the complete plant. A study is required for maintaining high levels of safety, reliability and structure integrity of the reformer system with concentration on the construction materials, damage mechanisms and their mitigation for the radiant section (catalyst tube, inlet and outlet pigtail, inlet and outlet manifold) Inlet Pigtail Pipe Bends Inlet pigtail pipe bends are curved bars with an annual cross section, whose reaction to external loading is complex in reformer application. The reformer arrangement with inlet pigtail pipe bends is indicated in Figure 1.5. These are often considered to be the critical components of the piping system, because of the critical cross section of the pipe bend. It is used to change the direction of the pipe and also absorbs the force and moment in the piping system. The rigidity of the pipe bend is lower than the connected straight pipe and as a result, the bend fails earlier than the straight pipe and it becomes the weakest part of the piping system. For design and structural integrity assessment, information on elastic stresses and evaluation of limit loads are necessary. This, in turn, will help to know about the maximum load carrying capacity of the inlet pigtail pipe bends for the specified operating conditions.

6 6 Figure 1.5 Reformer piping arrangement with inlet pigtail pipe bends Inlet pigtail pipe bends are designed to accommodate the effect of a wide range of loads resulting from internal, external pressure and bending including the deformation during installation and operation. At an elevated temperature during plant operation, the bend section may be a potential source of failure in the reformer piping arrangement due to ovality and wall thickness variation. 1.4 MOTIVATION OF THE RESEARCH WORK Pipe bends provide additional flexibility in the piping network by absorbing thermal expansion or contraction. Pipe bends employed in a reformer are subjected to the various degradation mechanisms, resulting in failure of pipe bend region. Failure of an inlet pigtail pipe bend has a catastrophic effect at extrados as depicted in Figure 1.6. This failure is

7 7 identified at the site during operation and it requires shutting down the plant. It is necessary to identify the potential cause of this failure in pipe bend so that suitable solution, considering minimum design requirement can be provided. Figure 1.6 Failure in inlet pigtail pipe bends During plant operation, the piping system is exposed to thermal and mechanical loads due to internal pressure, axial tension, global bending moment, combined internal pressure and bending and these induced loads are transmitted to the curved region causing high-stress level when compared to a straight pipe. In the manufacturing process of pipe bends, it is challenging to avoid thickening on the inner radius and thinning on the outer radius. The cross section of the bend becomes non-circular and its acceptability is based on the induced level of shape irregularity. The pipe bends with ovality are subjected to higher stresses during the operation of a plant. The shape irregularity in pipe bends and the components to which they are attached beyond a certain level will damage the piping system. Hence a thorough knowledge of the stresses induced in pipe bends under various loading conditions is more essential, to determine the acceptability of a pipe bend

8 8 with ovality. Besides shape irregularities, pipe diameter, wall thickness and bend radius affects the load carrying capacity of a pipe bend. The study on ovality effect in inlet pigtail pipe bend under various loading conditions is the main objective of this research. 1.5 APPROACHES AND METHODOLOGIES A better understanding and quantification of the effect of geometric irregularity would enable a piping designer to select the most efficient manufacturing process consistent with acceptable stress behaviour in the piping system. In this research work, various possible reasons for the failure in extrados of reformer inlet pigtail pipe bends were studied. The literature related to analytical, numerical and experimental analysis in pipe bend design was reviewed. Flexibility analyses were performed using CAESAR II to study the stress distribution in the piping system network in reformer including inlet pigtail pipe bend. Based on support arrangements and the thermal expansion of the piping system, an inlet pigtail pipe bend experiences higher stresses, identified as potential source of failure in the reformer piping system. A three-dimensional finite element analysis were carried out using CATIA and ANSYS-workbench to model and analyze a standalone long radius inlet pigtail pipe bend with attached straight pipe considering ovality to study the stress distribution in pipe bend geometry. The experimental setup was developed to study the behaviour of pipe bend and the possible internal and external loads acting in pipe bend. The applied load vs deflection in the pipe bend geometry was reported. The FEA results were compared with experimental evaluation and proposed limit

9 9 load solutions for the pipe bend considering ovality to avoid failure in bend geometry. The non-traditional technique of Artificial Neural Network (ANN) provides the correlation for allowable internal pressure with respect to pipe bend geometry. Based on the generic solution proposed, the geometric details of inlet pigtail pipe bend used in reformer were considered for analysis and provide the suitable solution to avoid failure. 1.6 FRAMEWORK OF RESEARCH ACTIVITY Effect of ovality in inlet pigtail pipe bend used in reformers was investigated Design of pipe bend : Code requirements, comparison with literature, finding the gap analysis, identify the reason for choosing the problem To get the optimum design solution for the limit load of inlet pigtail pipe bends with ovality considering internal pressure and in-plane bending moment loading Flexibility study performed and identified potential load of failure for inlet pigtail pipe bend using CAESAR II Software Carriedout FEA for the identified problem [ ANSYS-workbench] Using FEA results, correlation developed for internal pressure loading with other design parameters [Artificial Neural Network] Experimental evaluation performed for various pipe bends geometry and compared with numerical results Implementation in industry

10 ORGANIZATION OF THE THESIS Chapter 2: Presents the literature review on related areas of pipe bends and the research problem considered Chapter 3: Represents problem definition Chapter 4: Describes the flexibility analysis of reformer piping system and limit loads of pipe bend the pipe bend Chapter 5: Discusses the finite element modelling and analysis of Chapter 6: Presents the experimental evaluation to validate FE solutions for pipe bend Chapter 7: Deals with the mathematical approach to develop a correlation for internal pressure loading Chapter 8: Review the overall results and discussions on the effect of ovality in pipe bend Chapter 9: Discusses the industrial application for the defined problem in the research work. Chapter 10: Summarizes the research work, with conclusion, major contributions and future scope of work