Since this is a thin wall with a small t, t2 is smaller and can be neglected such that after simplification be neglected such that after simplification. Note that for the spherical pressure vessel, the hoop and axial stresses are equal and are one half of the hoop stress in the cylindrical pressure vessel. This makes the spherical pressure vessel a more “efficient” pressure vessel geometry. 15.2.2.2 Mohr Circle Determination 51
Sum forces in the horizontal direction is as: In plane Mohr’s circle its just a point In conclusion “thin wall stress analysis” is not completely accurate but allows certain simplifying assumptions to be made while maintaining a fair degree of accuracy. The main simplifying assumptions are that the stress is biaxial and that the stress are uniform across the shell wall. For thin-walled vessels these assumptions have proven themselves to be reliable. 15.3 Pressure vessel Engineering Drawing/PV Elite Software PV-Elite is a graphical based, easy-to-use software program that provides engineers, designers, estimators, fabricators and inspectors with complete design capabilities of tall towers, horizontal vessels, individual vessel and heat exchangers. It’s a complete solution for analysis and evaluation. Users of PV Elite have designed equipment for 52
the most extreme uses and have done so quickly, accurately and profitably. I did not get to learn the software, but I did handle it and I am aware of its great use in fabrication calculations. FIG 27: PV elite Features: • Vessel Design and Analysis, Rectangular and Non-Circular Vessel Analysis, Individual Component Analysis • Comprehensive Error Checking, Saddle, Leg and Skirt Design • Analysis for Horizontal Shipping of Vertical Vessels, Extensive Material Databases, Steel Databases and Modelling The design reports for the following have been compiled on PV-Elite 2008, by the company and are quite beyond the scope of my internship report. However, the calculations I studied are about internal pressure calculation, external pressure calculation, nozzle, flange calculations, wind load calculation, seismic calculation, 53
detail and element weights and centre of gravity calculations, dished end calculations, shell design, PWHT, Impact test, Hydrostatic test pressure calculations and others, based on the design Codes. FIG 28: Engineering Drawing 54
Chapter 16: THEORIES OF FAILURE 16.1 FAILURE IN PRESSURE VESSELS Categories of Failures: • Material - Improper Selection of materials; defects in material. • Design—Incorrect design data; inaccurate or incorrect design methods; inadequate shop testing. • Fabrication – Poor quality control; improper or insufficient fabrication procedures including welding; heat treatment or forming methods. TYPES OF FAILURES • Elastic deformation—Elastic instability or elastic buckling, vessel geometry, and stiffness as well as properties of materials are protecting against buckling. • Brittle fracture—Can occur at low or intermediate temperature. Brittle fractures have occurred in vessels made of low carbon steel in the 40-50 F range during hydrotest where minor flaws exist. • Excessive plastic deformation—The primary and secondary stress limits as outlined in ASME Section VIII, Division 2, are intended to prevent excessive plastic deformation and incremental collapse. • Stress rupture—Creep deformation as a result of fatigue or cyclic loading, i.e., progressive fracture. Creep is a time-dependent phenomenon, whereas fatigue is a cyclic-dependent phenomenon • Plastic instability—Incremental collapse; incremental collapse is cyclic strain accumulation or cumulative cyclic deformation. Cumulative damage leads to instability of vessel by plastic deformation. • High Strain—Low cyclic fatigue is strain-governed and occurs mainly in lower strength/high-ductile materials. • Stress corrosion—It is well known that chlorides cause stress corrosion cracking in stainless steels; likewise caustic service can • cause stress corrosion cracking in carbon steel. Materials selection is critical in these services. • Corrosion fatigue—Occurs when corrosive and fatigue effects occur simultaneously. Corrosion can reduce fatigue life by pitting the surface and propagating cracks. Material selection and fatigue properties are the major considerations. 16.2 CAUSES OF FALIURE 55
1. Cracking After welding or heat treating when the material cools down suddenly cracking occurs into it. We can also correlates water hammering with cracking. Cracking may occur due to fast quenching. When the progressive loading causes failure it reaches the point where microscopic holes or cavities begin to appear on surface these are cracks, sharp edges elevate local stresses that initiates the cracks Fig 29: Cracking 2. Explosion failure If the liquid or gas present in the pressure vessel is not in correct proportion or its temperature and temperature is not accurate the explosion occurs into the vessel that weakens its material and may cause fracture. 3. Plastic instability Incremental cyclic stress accumulates and cause instability of pressure vessel due to plastic deformation. This instability occurs in spherical pressure vessel during symmetric loading. 4. Corrosion cracking Disintegration of metal into its constituents due to some chemical reaction is called corrosion. It is obvious that corrosion weakens the material of equipment and hence it could not stress or pressure and can cause failure. 5. Fatigue Due to repetitive cyclic loading fatigue occurs into the metal. It is progressive damage caused by fluctuating stress and strain in the material. Avoiding 56
sharp surfaces or discontinuities and reducing tensile residual stresses can prevent fatigue failures. 6. Stress rupture When creep results progressive loading and fatigue, creep is time dependent, and fatigue is cycle dependent. 7. High strain failure Low cycle fatigue gives strain mainly in less ductile material. 8. Creep Progressive deformation into the material due to the high temperature is called creeping. When we apply constant load on the material and measure the strain over the specific time interval it gives creep rate. In the stage I resistance against creep increases then in stage II steady creep will occur then in stage III cross sectional area will decrease due to necking. FIG 30: Creep Strain-Time graph 9. Stress corrosion cracking The failure due to tensile stresses in the susceptible material in addition with crack promoting environment causes stress corrosion cracking. We can prevent stress corrosion cracking by using appropriate metal, by changing the design that reduces tensile stress and by eliminating the particles that promote cracks. 57
FIG 31: Stress corrosion cracking 10. Ductile and brittle fractures Ductile materials undergo observable plastic deformation before fracture while brittle materials undergo little plastic deformation. Brittle fracture travels rapidly without showing significant deformation and occur at low temperature and ductile metals show significant cleavage or tearing of metal, it occurs due to overloading and discontinuities. FIG 32: Ductile and brittle fractures 11. Wear failure When we remove material from the surface by mechanical means or rubbing hard particles onto the surface the damage occurs into it that causes gradual destruction. FIG 33: Wear Failure 12. Hydrogen embitterment The process in which metal become brittle and fracture when hydrogen atom diffuses into the metal and produces internal pressure and reduce its ductility and tensile strength until cracks become visible. FIG 34: Hydrogen Embitterment 58
13. Erosion Erosion failure occurs when material is removed onto the surface when fluid passes through it. In the pressure vessels erosion takes place where the fluid velocity increases (at inlet due to expansion from nozzle or at outlet). Erosion degrades the material of the pressure vessels and make its walls this. 14. Welding failures Welding failure occurs in overstressed faulty welds. Pressure vessels involve welding during its fabrication (welding of metal sheets join to form vessel). When weld starts to solidify any residual loading may cause to break it before it solidifies completely and form cracks into the weld and impurities in the solidified weld are trapped at the centre all these factors causes failure in the weld. Maximum stresses generate at these welding portions and hence cracks and impurities cause failures. FIG 35: Welding joint 15. Thermal shocks When a body expands to different extent at different parts due to its temperature gradient it is called thermal shock. As thermal stress is different at different point, when it exceeds its limit of strength at some point cracks will generate. 59
Chapter17: CONCLUSION My training has brought me to realize the engineering problem solving skills that are called for in the fabrication of boilers and pressure vessels. Though I was more involved with studying the Codes and trying to replicate the manufacturing assignments that Driplex has previously handled—only the design aspects—a considerable amount of time was also spent studying the skills of the workers which are required for fabrication and the thought process of the management in arranging for the required resources. The ASME BPVC is a very comprehensive code and can be understood only through regular practice and execution of it in real life terms. AutoCAD – a mechanical 3D CAD program is utilised. The design aspects if done manually would require superhuman skills, so there are software, exempli gratia – PV Elite, which have pre-feeded standardized values and entering particular data generates the missing data. Training courses to learn the codes are offered by ASME and several other organizations. A Design Engineer has to attend these conferences and the company should keep updated copies of the ASME BPVC, every three years; the ASME BPVC 2021 would be the latest, offering after the 2019 version, by ASME. Third party inspections are done by companies offering conformity assessment and certification services, and others, for Testing, Inspection and Certification (TIC) purpose of the assignments SUEZ receives from their clientele. A project requires thorough co-operation from all departments to be executed. No single NDT technique serves all the purpose. The choice of a particular technique depends on specimens’ properties, accessibility & nature of defects expected in the specimen often various techniques are used in conjunction. In the markets – Global, National or Local companies have trusted partners. Besides offering services and products to clients, companies strive to develop innovative solutions to reduce risks, improve performance and promote sustainable development. 60
Chapter 17: REFERENCES • Company Profile of SUEZ Available at - https://www.suezwatertechnologies.com/ • ASME Boiler and Pressure Vessel Code (BPVC) Available at - https://www.asme.org/codes-standards • Automatic Steel Vessel Head Manufacturing Line Available at – www.industrialmontagegrup.ro • Pressure vessel design Available at – https://www.processingmagazine.com • PV-Elite Available at - http://www.coade.com/products/pv-elite 61
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