PRESSURE VESSELS, Part II: Pressure Vessel Design .

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PRESSURE VESSELS, Part II:Pressure Vessel Design, ExternalPressure, Flat Covers and SelfNozzles.STUDY NOTESInstructor: Javier Tirenti

TrainingProjectsConnecting DotsTable of contentsIntroduction . 41.Design codes . 81.1)2.3.4.5.6.Codes comparison . 8ASME BPVC – Boiler and pressure vessel code . 102.1)Historical review . 102.2)BPVC Sections . 112.3)Section VIII Divisions. 12ASME BPVC Section VIII, Div.1 . 143.1)Scope . 143.2)Code organization . 153.3)ASME stamp . 183.4)Code revisions . 19Design conditions. 214.1)Temperature. 214.2)Pressure . 214.3)Loads . 234.4)Corrosion allowance . 244.5)Wind, earthquake and snow . 244.6)Hydro test . 244.7)Multi-compartment equipment . 244.8)Reference design books . 25Material selection . 265.1)Corrosion . 265.2)Essential properties of materials . 315.3)Technical-economical selection . 355.4)Materials designation . 365.5)Most used materials . 37Joint efficiency . 406.1)Welded joints. 416.2)Welded joint evaluation . 456.3)Joint efficiency value . 466.4)Selection charts . 47PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 2

TrainingProjectsConnecting Dots6.5)7.The full or spot dilemma . 48Design of parts under external pressure . 497.1)Cylindrical shells . 507.2)Spherical shells. 577.3)Heads and conical transitions . 588.Flat heads. 629.Self-reinforced and Integral nozzles . 639.1)Nozzle attachment . 659.2)Nozzle neck. 659.3)Reinforcement . 679.4)Flanges. 7210.References . 74PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 3

TrainingProjectsConnecting DotsIntroductionA pressure vessel is considered as any closed vessel that is capable ofstoring a pressurized fluid, either internal or external pressure, regardlessof their shape and dimensions. The cylindrical vessels, to which we refer inthis volume, are calculated on the principles of thin-walled cylinders.The first step in designing a container is choosing the best type for the servicefor which it is intended. The factors influencing the choice of type are thefunction of the container, the location, the nature of the fluid that has tobe stored, the temperature and operating pressure and their ability to storethe volume needed by the process.Pressure vessels can be classified according to their intended service,temperature and pressure, materials and geometry. Different types of pressurevessels can be classified as follows:PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 4

TrainingProjectsConnecting DotsAccording to the intended use of the pressure vessel, they can be dividedinto storage containers and process vessels.The first classes are only used for storing fluids under pressure, and inaccordance with the service are known as storage tanks.Process pressure vessels have multiple and varied uses, among them we canmention heat exchangers, reactors, fractionating towers, distillation towers,etc.According to the shape, pressure vessel may be cylindrical or spherical.The former may be horizontal or vertical, and in some cases may have coilsto increase or lower the temperature of the fluid.Spherical pressure vessels are usually used as storage tanks, and arerecommended for storing large volumes.Since the spherical shape is the "natural" form bodies adopt whensubjected to internal pressure, this would be the most economical way tostore pressurized fluids. However, the manufacture of such containers ismuch more expensive compared with cylindrical containers.Pressure vessel partsThe following two sample vessels are presented: vertical and horizontal. Inboth cases the main parts are shown:PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 5

TrainingProjectsConnecting DotsGeometry definitionTo define the geometry of a pressure vessel, the inner diameter of theequipment and the distance between tangent lines is used.The inner diameter should be used, since this is a process requirement.PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 6

TrainingProjectsConnecting Dots Welding line: point at which the head and shell are welded Tangent line: point at which the curvature of the head beginsDepending on the head fabrication method, heads come with a straight skirt.To set the length of the pressure vessel (regardless the type of heads), thedistance between tangent lines is used since this distance is notdependent on the head manufacturing method. It is very rare that the weldand tangent lines coincide.Manufacturing sequencePRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 7

TrainingProjectsConnecting Dots1.Design codesThe purpose of using design codes is to avoid disasters that can affecthumans. Therefore, they comprise a range of experiences and good practices.While there are several rules that apply, developed by countries withrecognized technical expertise in the subject, the code that is the mostinternationally recognized and the most used is Section VIII "PressureVessels" part of the Boiler and Pressure Vessel Code (BPVC) of theAmerican Society of Mechanical Engineers (ASME).Other than the code above, the most commonly codes used for pressurevessels are: Europe: EN-13445 Germany: A. D. Merkblatt Code United Kingdom: British Standards BS 5500 France: CODAP China: GB-150The rules found in the design codes represent many years of experience. Ifused wisely, the code requirements can: Communicate design requirements Utilize know-how and technology Keep equipment costs low Reduce insurance costs Provide rules for the design of equipment adequate for designconditions determined by others. Do not provide rules or guidance for the determination of designconditions. Do not provide rules or guidance for the determination of the requiredmaterial(s) of construction or corrosion allowance. Design scope of most design codes includes new construction only, notrevamps, repairs or rerates.1.1)Codes comparisonProvisions of a design code are an interrelated set of design, fabrication,inspection, and testing requirements. For example, the use of a higher designstress may depend upon use of stringent material, analysis, examination, andtesting requirements.Therefore, different codes can arrive at different resulting wall thickness yethave equivalent degrees of reliability.PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 8

TrainingProjectsConnecting DotsPRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 9

TrainingProjectsConnecting Dots2.ASME BPVC – Boiler and pressure vessel code2.1)Historical reviewBy the end of the 18th century the use of boilers operating at pressures muchhigher than the atmospheric and the need to protect staff from faults, grewrapidly. In many cases, the result of operating boilers and vessels in thoseconditions were catastrophic.Several attempts were made to standardize the design criteria andcalculation, but in 1911, due to the lack of uniformity in boiler fabrication,manufacturers and users of boilers and pressure vessels requested advice tothe ASME association to correct this situation.Finally, in 1915, that association published the first ASME Boiler Code (nowSection I) in the United States of America. The codes were established toprovide manufacturing methods, records and report design data.Until 1930, when the first welded vessel was manufactured, pressurevessels and boilers were riveted. Joints were made "overlapping" theplates or using strips, placed on the joints, drilling and tightening the rivets.It was estimated that each rivet added pressure in a certain influence area, thusguaranteeing the integrity of the equipment.Historically, engineers have applied the traditional strength of materials rulesfor designing pressure vessels. They are still used today, but combined with: Nondestructive examination (NDE)PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 10

TrainingProjectsConnecting Dots Safety coefficients Lessons learnedThe aforementioned includes the classic design of pressure vessels; nowadayschecks according to Finite Element Analysis (FEA) are more and more extendedwith outstanding results, therefore we have to consider using this powerfultool in our designs if necessary.2.2)BPVC SectionsThe ASME BVPC code is a set of standards, specifications, and designrules based on many years of experience, all applied to the design,fabrication, installation, inspection, and certification of pressure vessels.It was created in the United States of America; several insurance companiesdemanded a design code in order to reduce losses and casualties. The ASMEBoiler and Pressure Vessel is divided into the following sections:Those shown in the figure above are the twelve sections of the code. Toproperly design a pressure vessel, it is necessary to understand Section VIII ofcourse, and additionally, the designer will need to be familiar with Sections II, Vand IX.According to the scope of each section, the 12 parts can be grouped asfollows: Construction codes: Sections I, III, IV, VIII, X & XII Reference codes: Sections II, V, IXPRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 11

TrainingProjectsConnecting Dots 2.3)Rules for operating, inspection and in service maintenance: Section VI &VII.Section VIII DivisionsThe ASME Code Section VIII is a fabrication code. It contains mandatoryrequirements, specific prohibitions, and rules of construction and nonmandatory appendices. The code does not cover all possibilities related tothese activities, therefore, aspects not specifically mentioned should not beconsidered prohibited.Anyway, who writes the design codes? The code is written by recognizedpeople from different areas: academics, inspection agencies, owners,users, manufacturers of pressure vessels and notified bodies amongothers.What is the most important aspect of the code? The most important aspectof the code is to know its organization and to know where to look things up.WHAT IS EVEN MORE IMPORTANT IS TO READ UNTIL THE END OF THEPARAGRAPH; EVEN THOUGH IT SEEMS AS IF WE HAVE FOUND WHAT WEWERE LOOKING FOR.There are 3 divisions in ASME Section VIII: divisions 1, 2 and 3. Division 3is used for the calculation and design of high pressure equipment, around10,000 psig (703 kg/cm2g), while Div.2 and Div.1 are used for the rest ofapplications.Equipment design according to Div.1 is based on rules that do not requirea detailed assessment of all stresses. There are high secondary stressesand also bending stresses present, but since safety factors areconservative, stresses will be compensated.When designing according to Div.2 a more detailed analysis is performed,that allows the designer to consider higher allowable stresses and thusget much more real, economic and reliable results.Another difference between the ASME VIII Div.2 and Div.1 lies in failure theoryused to establish the calculation equations. While Div.1 is based on the theoryof normal stress, Div.2 is based on the theory of maximum distortion energy(Von Misses).Additionally, the calculation approach is different between divisions; for Div.1the design is by rules, whereas for Div.2 the design is by analysis, a muchmore exact method.The most important limitations of Div.1 are regarding pressure; when thedesign pressure exceeds 3000 psi (210 Kg/cm2); design according to Div.2 isrequired. Additionally, Div 1 cannot be used for pressures below 15 psi (1.054kg/cm2).PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 12

TrainingProjectsConnecting DotsOther than the pressure limitation, the scope of both Div.1 and Div.2 is thesame, and the main differences are: Allowable stresses Stress calculations Cyclic service design General design Quality control Inspection and fabricationNow, can we establish a clear rule that tells us when to use each division?The answer is NO. Each case is different and the designer must analyze alldesign conditions to determine which division to use. Whenever thespecialist deems appropriate, design shall be carried out according to bothdivisions to compare results. Even though a general rule cannot be listed, achart containing some tips is presented below:It can be said that Div.1 specifies general conservative design criteria, whileDiv.2 provides a better design, using stresses close to the real ones. The lattercombined with more stringent non-destructive examination, results in aneconomically more efficient design.PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 13

TrainingProjectsConnecting Dots3.ASME BPVC Section VIII, Div.13.1)ScopeThe extent of coverage of VIII-1 is defined in section U-1. The word “Scope”actually refers to two terms: the type of equipment considered as well asthe geometry of the pressure vessel.Before any design, it is recommended that the designer carefully reviews theparagraph U-1, to determine whether the equipment can be designedaccording to the code and its implications. The main considerations included inthe scope of the code are summarized below: U-1(a) (2) pressure vessels are defined as containers for the"containment" of internal or external pressure. This definition appliesto a very wide range of pressure vessels, some of which have not beenconsidered in the development of the rules. In order to avoid confusionabout what kind of pressure vessel is covered by VIII-1, the Committeepreferred to list the equipment "not covered in the development ofthe rules” instead of making a list of the ones considered. U-1(c)(2) indicates that VIII-1 is not applicable for the followingpressure vessels:(a) Those included in the scope of other sections of the ASME Code.(b) Process tubular heaters.(c) Pressure containers that are integral part of machines (rotatingequipment) U-1(c)(2)(d) piping systems are excluded from the scope of VIII1.Establishing the difference between a piping system and a pressurevessel sometimes can be complex:(a) If the main purpose of the pressure vessel is to transfer fluid fromone point to another in the system, then it could be consideredas a pipeline, and it must comply with other code’s requirements.(b) The main purpose of internal components such us fractionatingtrays or demisters is not to transfer fluid, they are installed forprocess reasons, therefore they must be included in the scope ofVIII-1. U-1(c) (2) (h) defines the scope towards pressure. If a vessel has aninternal or external pressure less than 100 kPa can be consideredoutside the scope of VIII-1.PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 14

TrainingProjectsConnecting Dots U-1(c) (2) (i) pressure vessels smaller than 152mm regardinginternal diameter, width, height or diagonal, are considered out ofthe scope of VIII-1, independently from its length or design pressure. U-1(c) (2) (j) Pressure vessels for Human Occupancy (PVHO) areoutside the scope of VIII-1. The design code in this case is ANSI/ASMEPVHO-1. U-1(g) some equipment to generate steam are included in the scopeof VIII-1.(a) U-1(g)(1): Unfired Boilers can be fabricated according to Section Ior Section VIII-1 (see UG-125(b) and UW-2(c))(b) U-1(g)(2) the following pressure vessels, in which steam isgenerated, are included in the scope of VIII-1. U-1(g) (2) (a): pressure vessels know as evaporators or heatexchangers. U-1(g) (2) (b): pressure vessels in which steam is generated dueto heat present in a system or process.U-1(e) defines the scope in terms of geometry. The restrictions andlimits are according to the following:(c) Welded nozzles (no flange): the first circumferential joint in thenozzle neck. The limit might be located in the vessel.(d) Threaded nozzles (no flange): the first thread.(e) Flanged Nozzles: the first flange face.3.2)Code organizationSection VIII, division 1 is organized and divided according to the following:PRESSURE VESSELS ONLINE COURSE, Part II – Instructor Javier Tirenti Page 15

TrainingProjectsConnecting Dots3.2.1)Sub-section A: general requirementsPart UGGeneral Requirements for all construction methods and all materials.Paragraphs go from UG-1 to UG-137.Since they are general requirements, they are the most important part ofall. If the goal is to create safe and technically and economically feasibledesigns, the designer should be familiar with all paragraphs and figuresA simplified summary of the division of this part is as follows:UG-4 to UG-15: MaterialsUG-16 to UG-55: DesignUG-36 to UG-45: Openings and reinforcementsUG-75 to UG-85: FabricationUG-90 to UG-103: Inspection a

There are 3 divisions in ASME Section VIII: divisions 1, 2 and 3. Division 3 is used for the calculation and design of high pressure equipment, around 10,000 psig (703 kg/cm2g), while Div.2 and Div.1 are used for the rest of applications. Equipment design according to Div.1 is based on rules that do not require a detailed assessment of all .

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