pressure vessel total calculation

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pressure vessel total calculation

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INTRODUCTION
Chemical engineering involves the application of sciences to the process industries, which are primarily concerned, with the conversion of one material into another by ahemical or physical means. These processes require the handling or storing of large quantities of materials in containers of varied constructions, depending upon the existing state of the material, it's physical and chemical properties and the required operations, which are to be performed. For handling such liquids and gases, a container or vessel is used. It is called a pressure vessel, when they are containers for fluids subjected to pressure. They are leak proof containers. They may be of any shape ranging from types of processing equipment. Most process equipment units may be considered as vessels with various modifications necessary to enable the units to perform certain required functions, e.g. an autoclave may be considered as high-pressure vessel equipped with agitation and heating sources.
Pressure vessels are in accordance with ASME code. The code gives for thickness and stress of basic components, it is up to the designer to select appropriate analytical as procedure for determining stress due to other loadings. The designer must familiarize himself with the various types of stresses and loadings in order to accurately apply the results of analysis. Designer must also consider some adequate stress or failure theory in order to confine stress and set allowable stress limits.
The methods of design are primarily based on elastic analysis. There are also other criteria such as stresses in plastic region, fatigue, creep, etc. which need consideration in certain cases. Elastic analysis is developed on the assumption that the material is isotropic and homogeneous and that it is loaded in the elastic region. This analysis is not applicable in the plastic range. Under cyclic variation of load causing plastic flow, the material to hardens and the behavior of material becomes purely elastic. This is a phenomenon called shakedown or cessation of plastic deformation under cyclic loading.
Elastic analysis is therefore in most important method of designing pressure vessel shells and components beyond the elastic limit, the material yields and the plastic region (spreads with increased value of load. The load for which this occurs is called collapse load rusting pressure.
Limit analysis is concerned with calculating the load or pressure at which flow of jfitructure material occurs due to yielding. However, this method is not usually applied to Resign of pressure vessels. When vessels are subjected to cyclic loading, it is necessary to consider requirements for elastic cycling of the material and the effects of this on component behavior. In the case of a discontinuity of shape, load may give rise to plastic cycling. Under these conditions, shakedown with occur. Maximum shakedown load is twice the first yield load. Therefore, an elastic analysis is valid up to the range of load, under cyclic loading conditions. A factor of safety on the stress or a factor of safety of twenty is applied on the numbers cycles. Design stress is accepted as the lower value.

SCOPE OF THE PROJECT
--In sophisticated pressure vessels encountered in engineering construction; high pressure, extremes of temperature and severity of functional performance requirements pose exciting design problems. The word "DESIGN" does not mean only the calculation of the detailed dimensions of a member, but rather is an all-inclusive term, incorporating:
1. The reasoning that established the most likely mode of damage or failure;
2. The method of stress analysis employed and significance of results;
.. 3. The selection of materials type and its environmental behaviour.
I The ever-increasing use of vessel has given special emphasis to analytical and experimental methods for determining their emphasis to analytical and experimental methods for determining their operating stresses. Of equal importance is the appraising the significance of these stresses. This appraisal entails the means of determining the values and extent of the stresses and strains, establishing the behaviour of the material involved, and evaluating the compatibility of these two factors in the media or environment to which they are subjected. Knowledge of material behaviour is required not only to avoid failures, but also equally to permit maximum economy of material choice and amount used.

DESIGN CRITERIA

3.1 FACTORS INFLUENCING THE DESIGN
[Regardless of the nature of application of the vessels, a number of factors usually must be considered in designing the unit. The most important consideration often is the selection of the type of vessel that performs the required services in the most satisfactory manner. In developing the design, a number of othercriteria must be considered such as the properties of material used, the induced stresses, the elastic stability, and the aesthetic appearance of the unit. The cost of fabricated vessel is also important in relation to its service and useful life.

3.2 DESIGN OF PRESSURE VESSELS TO CODE SPECIFICATION
American, Indian, British, Japanese, German and many other codes are available for design of pressure vessels. However the internationally accepted for design of pressure vessel code is American Society of Mechanical Engineering (ASME).
Various codes governing the procedures for the design, fabrication, inspection, testing and operation of pressure vessels have been developed; partly as safety measure. These procedures furnish standards by which, any state can be assured of the safety of pressure vessels installed within its boundaries. The code used for unfired pressure vessels is Section VII of the ASME boiler and pressure vessel code. It is usually necessary that the pressure vessel equipment be designed to a specific code in order to obtain insurance on the plant in which the vessel is to be used. Regardless of the method of design, pressure vessels with in the limits of the ASME code specification are usually checked against these specifications.

3.3 DEVELOPMENT AND SCOPE OF ASME CODE
In 1911, American Society of Mechanical Engineering established a committee to formulate standard specifications for the construction of steam boilers and other pressure vessels. This committee reviewed the existing Massachusetts and Ohio rules and eonducted an extensive survey among superintendents of inspection departments, Engineers, fabricators, and boiler operators. A number of preliminary reports were issued and revised. A final draft was prepared in 1914 and was approved as a code and copy righted in 1915.
The introduction to the code stated that public hearings on the code should be held every two years. In 1918, a revised edition of the ASME code was issued. In 1924, the code was revised with the addition of a new section VII, which represented a new code for unfired pressure vessels.

3.4 THE API-ASME CODE
In 1931, a joint API-ASME committee on unfired pressure vessels was appointed to prepare a code for safe practice in the design, construction, inspection and repair of unfired pressure vessels.