MECHANICAL ENGINEERING LIBRARY

Download Mechanical Engineering Text books, Notes, Study Materials and many more for free exclusive at MECHANICALIBRARY

Pressure Vessels Design and Practice by Somnath Chattopadhyay Free Download PDF

PRESSURE VESSELS DESIGN AND PRACTICE

BY SOMNATH CHATTOPADHYAY

FREE DOWNLOAD PDF




CONTENTS
  1. OVERVIEW OF PRESSURE VESSELS
  2. PRESSURE VESSEL DESIGN PHILOSOPHY
  3. STRUCTURAL DESIGN CRITERIA
  4. STRESS CATEGORIES AND STRESS LIMITS
  5. DESIGN OF CYLINDRICAL SHELLS
  6. DESIGN OF HEADS AND COVERS
  7. DESIGN OF NOZZLES AND OPENINGS
  8. FATIGUE ASSESSMENT OF PRESSURE VESSELS
  9. BOLTED FLANGE CONNECTIONS
  10. DESIGN OF VESSEL SUPPORTS
  11. SIMPLIFIED INELASTIC METHODS IN PRESSURE VESSEL DESIGN
  12. CASE STUDIES


Overview of Pressure Vessels

Introduction

Vessels, tanks, and pipelines that carry, store, or receive fluids are called pressure vessels. A pressure vessel is defined as a container with a pressure differential between inside and outside. The inside pressure is usually higher than the outside, except for some isolated situations. The fluid inside the vessel may undergo a change in state as in the case of steam boilers, or may combine with other reagents as in the case of a chemical reactor. Pressure vessels often have a combination of high pressures together with high temperatures, and in some cases flammable fluids or highly radioactive materials. Because of such hazards it is imperative that the design be such that no leakage can occur. In addition these vessels have to be designed carefully to cope with the operating temperature and pressure. It should be borne in mind that the rupture of a pressure vessel has a potential to cause extensive physical injury and property damage. Plant safety and integrity are of fundamental concern in pressure vessel design and these of course depend on the adequacy of design codes. When discussing pressure vessels we must also consider tanks. Pressure vessels and tanks are significantly different in both design and construction: tanks, unlike pressure vessels, are limited to atmospheric pressure; and pressure vessels often have internals while most tanks do not (and those that do are limited to heating coils or mixers). 

Pressure vessels are used in a number of industries; for example, the power generation industry for fossil and nuclear power, the petrochemical industry for storing and processing crude petroleum oil in tank farms as well as storing gasoline in service stations, and the chemical industry (in chemical reactors) to name but a few. Their use has expanded throughout the world. Pressure vessels and tanks are, in fact, essential to the chemical, petroleum, petrochemical and nuclear industries. It is in this class of equipment that the reactions, separations, and storage of raw materials occur. Generally speaking, pressurized equipment is required for a wide  range of industrial plant for storage and manufacturing purposes. The size and geometric form of pressure vessels vary greatly from the large cylindrical vessels used for high-pressure gas storage to the small size used as hydraulic units for aircraft. Some are buried in the ground or deep in the ocean, but most are positioned on ground or supported in platforms. Pressure vessels are usually spherical or cylindrical, with domed ends. The cylindrical vessels are generally preferred, since they present simpler manufacturing problems and make better use of the available space. Boiler drums, heat exchangers, chemical reactors, and so on, are generally cylindrical. Spherical vessels have the advantage of requiring thinner walls for a given pressure and diameter than the equivalent cylinder. Therefore they are used for large gas or liquid containers, gas-cooled nuclear reactors, containment buildings for nuclear plant, and so on. Containment vessels for liquids at very low pressures are sometimes in the form of lobed spheroids or in the shape of a drop. This has the advantage of providing the best possible stress distribution when the tank is full. 



Pressure Vessels Design Philosophy

General overview

Engineering design is an activity to ensure fitness for service. Within the context of pressure vessel design, this primarily involves strength considerations. The ‘‘total design’’ is a topic with far-reaching ramifications. It might include aspects of fuel system design, reactor design, or thermal hydraulic design. In our subsequent discussions, the underlying philosophy, decisions and calculations related solely to the strength design are referred to the ‘‘pressure vessel design.’’ For certain pressure vessels and related equipment, preliminary design may still be governed by heat transfer and fluid flow requirements. Although the aspect of thermal hydraulic design is intricately related to the structural design, especially for thermal transient loadings, we will not be discussing them in any detail. It will be assumed that the temperature distribution associated with a particular thermal transient has already been evaluated in a typical design application. However, in these cases the designer still has to consider how the desired configurations of the vessel are to be designed from a structural standpoint and how these designs will perform their intended service. The role of engineering mechanics in the pressure vessel design process is to provide descriptions of the pressure vessel parts and materials in terms of mathematical models, which can be analyzed in closed form in a limited number of situations and mostly have to be solved numerically. Even the so-called simple models that can be solved in closed form might involve fairly complex mathematics. In a few isolated instances, intelligent applications of well-known principles have led to simplifying concepts. These concepts have generally eased the designer’s task. However, in a majority of cases, especially when advanced materials and alloys are at a premium, there is a need to make the optimum use of the materials necessitating application of advanced structural analysis. As the complexity of the analysis increases, the aspect of interpretation of the results of the analysis becomes increasingly extensive. Furthermore, a large number of these models approximate the material behavior along with the extent of yielding. As we understand material behavior more and more, the uncertainties and omitted factors in design become more apparent. The improvement will continue as knowledge and cognizance of influencing design and material parameters increase and are put to engineering and economic use.



The safety demands within the nuclear industry have accelerated studies on pressure vessel material behavior and advanced the state of the art of stress analysis. For instance, the nuclear reactor, with its extremely large heavy section cover flanges and nozzle reinforcement operating under severe thermal transients in a neutron irradiation environment, has focused considerable attention on research in this area which has been directly responsible for improved materials, knowledge of their behavior in specific environments, and new stress analysis methods. High-strength materials created by alloying elements, manufacturing processes, or heat treatments, are developed to satisfy economic or engineering demands such as reduced vessel thickness. They are continually being tested to establish design limits consistent with their higher strength and adapted to vessel design as experimental and fabrication knowledge justifies their use. There is no one perfect material for pressure
vessels suitable for all environments, but material selection must match application and environment. This has become especially important in chemical reactors because of the embrittlement effects of gaseous absorption, and in nuclear reactors because of the irradiation damage from neutron bombardment. Major improvements, extensions and developments in analytical and experimental stress analysis are permitting fuller utilization of material properties with confidence and justification. Many previously insoluble equations of elasticity are now being solved numerically. These together with experimental techniques are being used to study the structural discontinuities at nozzle openings, attachments, and so on. This is significant because 80 percent of all pressure vessel failures are caused by highly localized stresses associated with these ‘weak link’ construction details. It is therefore apparent that the stress concentrations at vessel nozzle openings, attachments, and weldments are of prime importance, and methods for minimizing them through better designs and analyses are the keys to long pressure vessel life. Control of proper construction details results in a vessel of balanced design and maximum integrity. In the area of pressure vessel design there are important roles played by the disciplines of structural mechanics as well as material science. As  mentioned earlier, we try to provide a description of pressure vessel components in terms of mathematical models that are amenable to closedform solutions, as well as numerical solutions. The development of computer methods (sometimes referred to as computer-aided design, or CAD) has had a profound impact on the stress and deflection analysis of pressure vessel components. Their use has been extended to include the evaluation criteria as well, by a suitable combination of post processing of the solutions and visual representation of numerical results. In a number of cases advanced software systems are dedicated to present animation that aids the visualization and subsequent appreciation of the analysis. A number of design and analysis codes have been developed that proceed from the conceptual design through the analysis, sometimes modeling the nonlinear geometric and material behavior. Results such as  temperatures, deflections and stresses are routinely obtained, but the analysis often extends to further evaluations covering creep, fatigue, and fracture mechanics. With the advent of three-dimensional CAD software and their parametric, feature-driven automated design technology, it is now possible to ensure the integrity of designs by capturing changes anywhere in the product development process, and updating the model and all engineering deliverables automatically. Pressure vessel designs that once averaged 24 hours to finish are completed in about 2 hours. Such productivity gains translate into substantial savings in engineering labor associated with each new pressure vessel design. The typical design of a pressure vessel component would entail looking at the geometry and manufacturing construction details, and subsequently at the loads experienced by the component. The load experienced by the vessel is related to factors such as design pressure, design temperature, and mechanical loads (due to dead weight and piping thermal expansion) along with the postulated transients (typically those due to temperature and pressure) that are anticipated during the life of the plant. These transients generally reflect the fluid temperature and pressure excursions of the mode of operation of the equipment. The type of fluid that will be contained in the pressure vessel of course is an important design parameter, especially if it is radioactive or toxic. Also included is the information on site location that would provide loads due to earthquake (seismic), and other postulated accident loads. 


In assessing the structural integrity of the pressure vessel and associated equipment, an elastic analysis, an inelastic analysis (elastic–plastic or plastic) or a limit analysis may be invoked. The design philosophy then is to determine the stresses for the purpose of identifying the stress concentration, the proximity to the yield strength, or to determine the shakedown limit load. The stress concentration effects are then employed for detailed fatigue evaluation to assess structural integrity under cyclic loading. In some situations a crack growth analysis may be warranted, while in other situations, stability or buckling issues may be critical. For demonstrating adequacy for cyclic operation, the specific cycles and the associated loadings must be known a priority. In this context, it is important for a pressure vessel designer to understand the nature of loading and the structural response to the loading. This generally decides what type of analysis needs to be
performed, as well as what would be the magnitude of the allowable stresses or strains. Generally the loads acting on a structure can be classified as sustained, deformation controlled, or thermal. These three load types may be applied in a steady or a cyclic manner. The structure under the action of these loads may respond in a number of ways 

  • When the response is elastic, the structure is safe from collapse when the applied loading is steady. When the load is applied cyclically a failure due to fatigue is likely; this is termed failure due to high cycle fatigue.
  • When the response is elastic in some regions of the structure and plastic in others, there is the potential to have an unacceptably large deformation produced by both sustained and deformation-controlled loads. Cyclic loads or cyclic temperature distributions can produce plastic deformations that alternate in tension and compression
  • and cause fatigue failure, termed low cycle fatigue. Such distribution of loads could be of such a magnitude that it produces plastic deformations in some regions when initially applied, but upon removal these deformations become elastic, and subsequent loading results in predominantly elastic action. This is termed shakedown. Under cyclic loading fatigue failure is likely and because of elastic action, this would be termed as low cycle fatigue.
  • When the sustained loading (due to bending or tension) is such that the entire cross-section becomes plastic, gross collapse of the structure takes place.
  • Ratcheting is produced by a combination of a sustained extensional load and either a strain-controlled cyclic load or a cyclic temperature distribution that is alternately applied and removed. This produces cycling straining of the material which in turn produces incremental growth (cyclic) leading to what is called an incremental collapse. This can also lead to low cycle fatigue. 
  • Sustained loads in brittle materials or in ductile materials at low temperatures could result in brittle fracture, which is a form of structural collapse. 



Structural Design Criteria

Modes of failure

Two basic modes of failure are assumed for the design of pressure vessels. These are: (a) elastic failure, governed by the theory of elasticity; and (b) plastic failure, governed by the theory of plasticity. Except for thick-walled pressure vessels, elastic failure is assumed. When the material is stretched beyond the elastic limit, excessive plastic deformation or rupture is expected. The relevant material properties are the yield strength and ultimate strength. In real vessels we have a multiaxial stress situation, where the failure is not governed by the individual components of stress but by some combination of all stress components. 

Theories of failure

The most commonly used theories of failure are:

Maximum principal stress theory
Maximum shear stress theory



Fatigue assessment of pressure vessels

Fatigue has been recognized as a major failure mode in pressure vessels, and specific rules for its prevention appear in design codes. Stated simply, fatigue failure is caused by the cyclic action of loads and thermal conditions. In many design situations, the expected number of cycles is in millions and for all practical purposes can be considered as infinite. Accordingly, the concept of endurance limit has been employed in a number of design rules. Endurance limit is the stress that can be applied for an infinite number of cycles without producing failure. However, the typical number of stress cycles rarely exceeds 100,000 and frequently only a few thousand. Therefore, fatigue analysis requires somewhat more involved concepts than just the endurance limit. Fatigue refers to the behavior of material under repeated loads, which is distinct from the behavior under monotonically applied loading. There is a progressive localized permanent deformation under fluctuating loads that culminates in cracks and complete fracture after a sufficient number of cycles. The fatigue process itself occurs over a period of time. However, failure may occur suddenly and without prior warning, in which case the damage mechanisms may have been operating since loading was first introduced. This period of time is often  eferred to as the usage period. The fatigue process appears to initiate from local areas that have high stresses. These highly stressed regions are due to abrupt changes in geometry leading to high stress concentrations, due to temperature differentials, imperfections, or the presence of residual stress. The failure takes place when the crack after repeated cycling grows to a point at which the material can no longer withstand the loads and a complete separation occurs. Metallurgical defects such as a void or an inclusion often act as sites for fatigue crack initiation. The fatigue process consists of crack initiation, crack propagation, and eventual fracture. Another way to look at the process is to postulate it in terms of initiation of microcracks, coalescence of these microcracks into macrocracks, followed by growth to unstable fracture. Fatigue has been classified as one of high cycle and low cycle. High cycle fatigue involves very little plastic action. The low cycle fatigue failure involves a few thousand cycles and involves strains in excess of yield strain. Fatigue damage in the low cycle has been found to be related to plastic strain and fatigue curves for use in this region should be based on strain ranges. For the high cycle fatigue cases, the stress ranges can be used. The procedure of using strain amplitude as a function of number of cycles forms the cornerstone of fatigue analysis. The design curve is based on strain-controlled data. The best-fit curves were reduced by a factor of 2 on stress and 20 on cycles to account for environment, size effect, and scatter of data. The basic elements of the fatigue evaluation in pressure vessels rests on the use of maximum shear theory of failure (Tresca criterion), with the assumptions of linear elastic behavior along with the use of Miner’s rule for estimating the cumulative effect of stress cycles of varying amplitude. Fatigue failure typically occurs at structural discontinuities which give rise to stress concentration. The stress concentration factors are generally based on theoretical analysis involving statically applied loads. These are directly applicable to fatigue analysis only when the nominal stress multiplied by the stress concentration factor is below the material yield strength. When it exceeds the yield strength, there is a redistribution of stress and strain. For sharp geometries, using values of stress concentrations obtained elastically leads to underprediction of fatigue lives when compared with the actual test data. In fact under no circumstances does a value of more than 5 need be applied, and it is observed that the value of these factors do not vary with the magnitude of cyclic strain and associated fatigue life. In the design of pressure vessels, the stress concentration which is really the strain concentration – is limited to 5 and for most discontinuities such as grooves and fillets no more than a value of 4 is used 




USEFUL LINKS








MECHANICAL METALLURGY BY GEORGE E DIETER


MECHANICS OF MATERIALS BY FERDINAND P. BEER,E. RUSSELL JOHNSTON JR,JOHN T. DEWOLF, DAVID F. MAZUREK

Basic and Applied Thermodynamics by P.K.Nag Free Download PDF

BASIC AND APPLIED THERMODYNAMICS

BY P.K.NAG

FREE DOWNLOAD PDF




CONTENTS
  1. INTRODUCTION
  2. TEMPERATURE
  3. WORK AND HEAT TRANSFER
  4. FIRST LAW OF THERMODYNAMICS
  5. FIRST LAW APPLIED TO FLOW PROCESSES
  6. SECOND LAW OF THERMODYNAMICS
  7. ENTROPY
  8. AVAILABLE ENERGY, ENERGY IRREVERSIBILITY
  9. PROPERTIES OF PURE SUBSTANCES
  10. PROPERTIES OF GASES AND GAS MIXTURES
  11. THERMODYNAMIC RELATIONS, EQUILIBRIUM AND THIRD LAW
  12. VAPOUR POWER CYCLES
  13. GAS POWER CYCLES
  14. REFRIGERATION CYCLES
  15. PSYCHROMETRICS
  16. REACTIVE SYSTEMS
  17. COMPRESSIBLE FLUID FLOW
  18. ELEMENTS OF HEAT TRANSFER
  19. STATISTICAL THERMODYNAMICS
  20. IRREVERSIBLE THERMODYNAMICS
  21. KINETIC THEORY OF GASES AND DISTRIBUTION OF MOLECULAR VELOCITIES
  22. TRANSPORT PROCESSES IN GASES


BASIC AND APPLIED THERMODYNAMICS BY P.K.NAG 

Thermodynamics is the science of energy transfer and its effect on the physical properties of substances.  It is based upon observations of common experience which have been formulated into thermodynamic laws.  These principles of energy conversion.  The applications of ho thermodynamic laws and principles are found in all fields of energy technology,  notably in steam and nuclear power plants,  internal combustion engines,  gas turbines,  airconditioning,  refrigeration,  process plants,  and direct energy conversion devices,  

Macroscopic vs Microscopic Viewpoint : There are two points of view from which the behaviour of matter can be studied:  the macroscopic and the microscopic.  In the macroscopic approach,  a certain quantity of matter is considered,  without the events occurring at the molecular level being taken into account.  From the microscopic point of view,  matter is composed of myriads of molecules.  If it is a gas,  each molecule at a given instant has a certain position,  velocity,  and energy,and for each molecule these change very frequently as a result of collisions.  The behaviour of the gas is described by summing up the behaviour of each molecule.  Such a study is made in microscopic or statistical thermodynamics.  Macroscopic thermodynamics is only concerned with the effects of the action of many molecules,  and these effects can be perceived by human senses.  For example,  the macroscopic quantity,pressure,  is the average rate of change of momentum due to all the molecular collisions made on a unit area.  The effects of pressure can be felt.  The macroscopic point of view is not concerned with the action of individual molecules,  and the force on a given unit area can be measured by using,  e.g.,  a pressure gauge.  These macroscopic observations are completely independent of the assumptions regarding the nature of matter. All the results of classical or macroscopic thermodynamics can however be derived from the microscopic and statistical study of matter.


Thermodynamic Properties,  Processes and Cycles:

Every system has certain characteristics by which its physical condition may be described,  e.g.,  volume,  temperature,  pressure,  etc.  Such characteristics are called properties of the system.  These are all macroscopic in nature.  When all the properties of a system have definite values,  the system is said to exist at a definite state.  Properties are the coordinates to describe the state of a system.  They are the state variables of the system.  Any operation in which one or more of the properties of a system changes is called a change of state.  The succession of states passed through during a change of state is called the path of the change of state.  When the path is completely specified,  the change of state is called a process,  e.g.,  a constant pressure process.  A thermodynamic cycle is defined as a series of state changes such that the final state is identical with the initial state.  Properties may be of two types.  Intensive properties are independent of the mass in the system,  e pressure,  temperature,  etc.  Extensive properties are related to mass,  e.g.,  volume,  energy,  etc.  If mass is increased,  the values of the extensive properties also increase.  Specific extensive properties,  i.e.,  extensive properties per unit mass, are intensive properties example specific volume, specific energy , density etc

Homogeneous and heterogeneous Systems : A quantity of matter homogeneous throughout in chemical composition and physical structure is called a phase.  Every substance can exist in any one of the three phases,  viz,  solid,  liquid and gas.  A system consisting of a single phase is called a homogeneous system,  while a system consisting of more than one phase is known as a heterogeneous system.  




Thermodynamic Equilibrium: A system is said to existing a state of thermodynamic equilibrium when no change in any macroscopic property is registered,  if the system is isolated from its surroundings.  An isolated system always reaches in course of time a state of thermodynamic equilibrium and can never depart from it spontaneously.  Therefore,  there can be no spontaneous change in any macroscopic property if the system exists in an equilibrium state.  Thermodynamics studies mainly the properties of physical systems that are found in equilibrium states.  A system will be in a state of thermodynamic equilibrium,  if the conditions for the following three types of equilibrium are satisfied: (a) Mechanical equilibrium (b)  Chemical equilibrium (c) Thermal equilibrium In the absence of any unbalanced force within the system itself and also between the system and the surroundings,  the system is said to be in a state of mechanical equilibrium.  Ifan unbalanced force exists,  either the system alone or both the system and the surroundings will undergo a change of state till mechanical equilibriumis attained.
If there is no chemical reaction or transfer of matter from one part of the system another,  such as diffusion or solution,  the system is said to exist in a state of chemical equilibrium.  When a system existing in mechanical and chemical equilibrium is separated from its surroundings by a diathermic wall(diathermic means which allows heat to flow and if there is no spontaneous change in any property of the system,  the system is said to exist in a state o is not satisfied,  the system will undergo a change of state till thermal equilibrium is restored.  When the conditions for any one of the three types of equilibrium​ are not satisfied,  a system is said to be in a non equilibrium state.  If the non equilibrium of the state is due to an unbalanced force in the interior of a system or between the system and the surrounding,  the pressure varies from one part of the system to another.  There is no single pressure that refers to the system as a whole.  Similarly,  if the non equilibrium is because of the temperature of the system being different from that of its surroundings,  there is a nonuniform temperature distribution set up within the system and there is no single temperature that stands for the system as a whole.  It can thus be inferred that when the conditions for thermodynamic equilibrium are not satisfied,  the states passed through by a system cannot be described by thermodynamic properties which represent the system as a whole.  Thermodynamic properties are the macroscopic coordinates defined for,  and significant to,  only thermodynamic equilibrium states.  Both classical and statistical thermodynamics study mainly the equilibrium states of a system.

Zeroth Law of Thermodynamics : The property which distinguishes thermodynamics from other sciences is temperature.  One might that temperature bears as important a relation to thermodynamics as force does to statics or velocity does to dynamics.  Tempera-  ture is associated with the ability to distinguish hot from cold.  When two bodies at different temperatures are brought into contact,  after some time they attain a common temperature and are then said to exist in thermal equilibrium.  When a body A is in thermal equilibrium with a body B,  and also separately with a body C,  then B and C will be in thermal equilibrium with each other.  This is known as the zeroth law of thermodynamics.  It is the basis of temperature measurement.  In order to obtain a quantitative measure of temperature,  a reference body is used,  and a certain physical characteristic of this body which changes with temperature is selected.  The changes in the selected characteristic may be taken as an indication of change in temperature.  The selected characteristic is called the thermometric property,  and the reference body which is used in the determination of temperature is called the thermometer.  A very common thermometer.  consists of a small amount of mercury in an evacuated capillary tube.  In this case the extension of the mercury in the tube is used as the thermometric property There are five different kinds of thermometer,  each with its own thermometric property.
A closed system and its surroundings can interact in two ways:  (a) by work transfer,  and (b) by heat transfer.  These may be called energy interactions and these bring about changes in the properties of the system.  Thermodynamics mainly studies these energy interactions and the associated property changes of the system.  

Work Transfer : Work is one of the basic modes of energy transfer.  In mechanics the action of a force on a moving body is identified as work.  A force is a means of transmitting an effect from one body to another.  But a force itself never produces a physical effect except when coupled with motion and hence it a is not a form of energy.  An effect such as the raising of a weight through a certain distance can be performed by using a small force through a large distance or a large force through a small distance.  The product of force and distance is the same to accomplish the same effect.  In mechanics work is defined as:  The work is done by a force as it acts upon a body moving in the direction of the force.  The action of a force through a distance(or of a torque through an angle)  is called mechanical work since other forms of work can be identified,  as discussed later.  The product of the force and the distance moved parallel to the force is the magnitude of mechanical work.  In thermodynamics,  work transfer is considered as occurring between the system and the surroundings.  Work is said to be done by a system if the sole on things external to the system can be reduced to the raising of a weight The weight may not actually be raised,  but the net effect external to the system would be the raising of e weight.  Let us consider the battery and the motor in as a system.





USEFUL LINKS







MECHANICAL METALLURGY BY GEORGE E DIETER



MECHANICS OF MATERIALS BY FERDINAND P. BEER,E. RUSSELL JOHNSTON JR,JOHN T. DEWOLF, DAVID F. MAZUREK

Kinematics and Dynamics of Machines by George Henry Martin 
Extractive Metallurgy of Rare Earths by C.K.Gupta and N Krishnamurthy 
WELDING METALLURGY BY SINDO KOU 
ADVANCED MECHANICS OF MATERIALS BY ARTHUR P. BORESI AND RICHARD J. SCHMIDT

Popular Posts