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1、第四章 压力容器设计CHAPTER 4Design of Pressure VesselMechanical Design of Process EquipmentMechanical Design of Process EquipmentOur topic covers those aspects of the mechanical design of chemical plant that are of particular interest to chemical engineers. The main topic considered is the design of pressure
2、 vessels. The design of storage tanks，centrifuges and heat-exchanger tube sheets are also discussed briefly. The chemical engineer will not usually be called on to undertake the detailed mechanical design of a pressure vessel. Vessel design is a speciafised subject，and will be carried out by mechani
3、cal engineers who are conversant with the current design codes and practices，and methods of stress analysis. However，the chemical engineer will be responsible for developing and specifying the basic design information for a particular vessel，and needs to have a general appreciation of pressure vesse
4、l design to work effectively with the specialist designer.The basic data neededThe basic data needed by the specialist designer will be： 1.Vessel function. 2.Process materials and services. 3.Operating and design temperature and pressure. 4.Materials of construction. 5. Vessel dimensions and orienta
5、tion. 6. Type of vessel heads to be used. 7. Openings and connections required. 8. Specification of heating and cooling jackets or coils. 9. Type of agitator. 10. Specification of internal fittings. Them is no strict definition of what constitutes a pressure vessel，but it is generally accepted that
6、any closed vessel over 150 mm diameter subject to a pressure difference of more than 1 bar should be designed as a pressure vessel. It is not possible to give a completely comprehensive account of vessel design in one chapter. The design methods and data given should be sufficient for the preliminar
7、y design of conventional vessels. Sufficient for the chemical engineer to check the feasibility of a proposed equipment design；to estimate the vessel cost for an economic analysis；and to determine the vessels general proportions and weight for plant layout purposes. For a more detailed account of pr
8、essure vessel design the reader should refer to other useful books on the mechanical design of process equipment. An elementary understanding of the principles of the “Strength of Materials”（Mechanics of Solids）will be needed to follow this chapter. Readers who are not familiar with the subject shou
9、ld consult one of the many textbooks available；such as those by Faupel and Fisher（1981）. The book by Faupel and Fisher is particularly recommended as a general introduction to mechanical design for chemical engineers.Classification of pressure vesselsFor the purposes of design and analysis， pressure
10、 vessels are sub-divided into two classes depending on the ratio of the wall thickness to vessel diameter：thin-walled vessels，with a thickness ratio of less than 1：10；and thick-walled above this ratio. The principal stresses acting at a point in the wall of a vessel，to a pressure load，are shown in F
11、igure 3.1. If the wall is thin，the radial stress 3 will be small and can be neglected in comparison with the other stresses， and the longitudinal and circumferential stresses1 and2 can be taken as constant over the wall thickness，In a thick wall，the magnitude of the radial stress will be significant
12、 ，and the circumferential stress will vary across the wall. The majority of the vessels used in the chemical and allied industries are classified as thin-walled vessels. Thick- walled vessels are used for high pressures.Membrane stresses in shells of revolutionA shell of revolution is the form swept
13、 out by a line or curve rotated about an axis.（A solid of revolution is formed by rotating an area about an axis.）Most process vessels are made up from shells of revolution： cylindrical and conical sections；and hemispherical， ellipsoidal and torispherical heads.The walls of thin vessels can be consi
14、dered to be“membranes”；supporting loads without significant bending or shear stresses；similar to the walls of a balloon. The analysis of the membrane stresses induced in shells of revolution by internal pressure gives a basis for determining the minimum wall thickness required for vessel shells. The
15、 actual thickness required will also depend on the stresses arising from the other loads to which the vessel is subjected. Consider the shell of revolution of general shape shown in Figure ，under a loading that is rotationally symmetric；that is，the load per unit area（pressure）on the shell is constant round the circumference，but not necessarily the same from top to bottom.第一、第二曲率半径Let P = pressure， t = thickness of shell， = the meridional（longitudinal）stress，the stress acting along a meridian， = the circumferential or tangential stress，th
