Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not.

Chemical Engineering Design Pressure Vessel Design A pressure vessel is any vessel that falls under the definition laid down in the ASME Boiler and Pressure Vessel Code, Section VIII, Rules for the Construction of Pressure Vessels (ASME BPV Code Sec. VIII)A pressure vessel is any vessel that falls under the definition laid down in the ASME Boiler and Pressure Vessel Code, Section VIII, Rules for the Construction of Pressure Vessels (ASME BPV Code Sec. VIII) The definition applies to most process reactors, distillation columns, separators (flashes and decanters), pressurized storage vessels and heat exchangersThe definition applies to most process reactors, distillation columns, separators (flashes and decanters), pressurized storage vessels and heat exchangers Source: UOP © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design Isn’t This Something to Leave to the Mechanical Engineers? Chemical engineers are usually not properly trained or qualified to carry out detailed mechanical design of vessels. Most mechanical designs are completed by specialists in later phases of designChemical engineers are usually not properly trained or qualified to carry out detailed mechanical design of vessels. Most mechanical designs are completed by specialists in later phases of designBut The process design engineer needs to understand pressure vessel design in order to generate good cost estimates (e.g. in Aspen ICARUS)The process design engineer needs to understand pressure vessel design in order to generate good cost estimates (e.g. in Aspen ICARUS) Costs can vary discontinuously with vessel designCosts can vary discontinuously with vessel design A 10  C change in temperature could double the vessel cost if it causes a change in code!A 10  C change in temperature could double the vessel cost if it causes a change in code! Adding a component could cause a change in metallurgy that would mean moving to a more expensive code designAdding a component could cause a change in metallurgy that would mean moving to a more expensive code design The process engineer will end up specifying the main constraints on the vessel design: if you don’t know how to do this properly, you can’t really design anythingThe process engineer will end up specifying the main constraints on the vessel design: if you don’t know how to do this properly, you can’t really design anything

Chemical Engineering Design Pressure Vessel Design Pressure Vessel Design CodesPressure Vessel Design Codes Vessel Geometry & ConstructionVessel Geometry & Construction Strength of MaterialsStrength of Materials Vessel SpecificationsVessel Specifications Materials of ConstructionMaterials of Construction Pressure Vessel Design RulesPressure Vessel Design Rules Fabrication, Inspection and TestingFabrication, Inspection and Testing

Chemical Engineering Design ASME Boiler and Pressure Vessel Code ASME BPV Code is the legally required standard for pressure vessel design, fabrication, inspection and testing in North AmericaASME BPV Code is the legally required standard for pressure vessel design, fabrication, inspection and testing in North America Section IRules for construction of power boilers IIMaterials IIINuclear power plant components IVRules for construction of heating boilers VNondestructive examination VIRecommended rules for the care and operation of heating boilers VIIRecommended guidelines for the care of power boilers VIIIRules for the construction of pressure vessels Division 1 Division 2Alternative rules Division 3Alternative rules for the construction of high pressure vessels IXWelding and brazing qualifications XFiber-reinforced plastic vessels XIRules for in service inspection of nuclear power plant components XIIRules for construction and continued service of transport tanks Most chemical plant vessels fall under Sec. VIII D.1 or D.2 Allowable stresses are given in Sec. II Often used for bio-reactors

Chemical Engineering Design Advantages of Designing to Code The Code is a consensus best practiceThe Code is a consensus best practice It is usually required by lawIt is usually required by law –Local requirements may vary (particularly overseas), but ASME code is usually recognized as acceptable –Always check for local regulations that may require stricter standards Code rules are often applied even for vessels that don’t require construction to codeCode rules are often applied even for vessels that don’t require construction to code –Savings of not following code rules are negligible as vessel shops are set up to do everything to code

Chemical Engineering Design ASME BPV Code Sec. VIII Divisions Division 1 Rigorous analysis of local thermal and fatigue stresses not requiredRigorous analysis of local thermal and fatigue stresses not required Safety factor of 3.5 against tensile failure and 1.25 for 100,000 hour creep ruptureSafety factor of 3.5 against tensile failure and 1.25 for 100,000 hour creep rupture Limited to design pressures below 3000 psi (but usually costs more than Div.2 above about 1500 psi)Limited to design pressures below 3000 psi (but usually costs more than Div.2 above about 1500 psi) Division 2 Requires more analysis than Div.1, and more inspection, but allows thinner walled vesselsRequires more analysis than Div.1, and more inspection, but allows thinner walled vessels Safety factor of 3.0 against tensile failureSafety factor of 3.0 against tensile failure Limited to design temperatures less than 900  F (outside creep range)Limited to design temperatures less than 900  F (outside creep range) More economical for high pressure vessels, but fewer fabricators availableMore economical for high pressure vessels, but fewer fabricators available Either Division of the Code is acceptable, but provisions cannot be mixed and matchedEither Division of the Code is acceptable, but provisions cannot be mixed and matched

Chemical Engineering Design Vessels Specifically Excluded by ASME BPV Code Sec. VIII Div 1 Vessels within the scope of other sections of the BPV code. For example, power boilers (Sec. I), fiber-reinforced plastic vessels (Sec. X) and transport tanks (Sec. XIII).Vessels within the scope of other sections of the BPV code. For example, power boilers (Sec. I), fiber-reinforced plastic vessels (Sec. X) and transport tanks (Sec. XIII). Fired process tubular heaters.Fired process tubular heaters. Pressure containers that are integral parts of rotating or reciprocating devices such as pumps, compressors, turbines or engines.Pressure containers that are integral parts of rotating or reciprocating devices such as pumps, compressors, turbines or engines. Piping systems (which are covered by ASME B31.3 – see Chapter 5).Piping systems (which are covered by ASME B31.3 – see Chapter 5). Piping components and accessories such as valves, strainers, in-line mixers and spargers.Piping components and accessories such as valves, strainers, in-line mixers and spargers. Vessels containing water at less than 300 psi (2 MPa) and less than 210ºF (99ºC).Vessels containing water at less than 300 psi (2 MPa) and less than 210ºF (99ºC). Hot water storage tanks heated by steam with heat rate less than 0.2 MMBTU/hr (58.6 kW), water temperature less than 210ºF (99ºC) and volume less than 120 gal (450 liters).Hot water storage tanks heated by steam with heat rate less than 0.2 MMBTU/hr (58.6 kW), water temperature less than 210ºF (99ºC) and volume less than 120 gal (450 liters). Vessels having internal pressure less than 15 psi (100 kPa) or greater than 3000 psi (20 MPa).Vessels having internal pressure less than 15 psi (100 kPa) or greater than 3000 psi (20 MPa). Vessels of internal diameter or height less than 6 inches (152 mm).Vessels of internal diameter or height less than 6 inches (152 mm). Pressure vessels for human occupancy.Pressure vessels for human occupancy.

Chemical Engineering Design ASME Code Stamp Name Plate Can only be used if vessel is designed, inspected and tested under the supervision of a Certified Individual employed by the manufacturerCan only be used if vessel is designed, inspected and tested under the supervision of a Certified Individual employed by the manufacturer The code stamp must be clearly visible on the vesselThe code stamp must be clearly visible on the vessel

Chemical Engineering Design Other Related Codes Storage tanks are usually not designed to BPV CodeStorage tanks are usually not designed to BPV Code –API Standard 620, Large low pressure storage tanks, Pressure 0.5 to 15 psig –API Standard 650, Welded storage tanks, Pressures up to 0.5 psig Fittings are covered by other ASME codesFittings are covered by other ASME codes –ASME B16.5, Pipe flanges and flanged fittings –ASME B16.9, Factory-made wrought buttwelding fittings –ASME B16.11 Forged fittings, socket welding and threaded –ASME B16.47, Large diameter steel flanges NPS26 Through NPS60 Piping is covered by a different ASME codePiping is covered by a different ASME code –ASME B16.3, Process piping Heat exchangers have additional codes set by TEMAHeat exchangers have additional codes set by TEMA

Chemical Engineering Design Use of Design Codes & Standards The latest version of the design code should always be consulted as regulations changeThe latest version of the design code should always be consulted as regulations change –Example: new version of ASME BPV Code Sec. VIII Div. 2 will allow for thinner walls on high pressure vessels All the information given in this presentation is from the 2004 editionAll the information given in this presentation is from the 2004 edition

Chemical Engineering Design Pressure Vessel Shape What shape of pressure vessel uses the least amount of metal to contain a given volume, pressure?What shape of pressure vessel uses the least amount of metal to contain a given volume, pressure? A sphere! Why is this shape not more widely used?Why is this shape not more widely used? –Usually need to have an extended section of constant cross- section to provide support for vessel internals, trays, distributors, etc. –It is much easier to obtain and maintain uniform flow in a cylindrical bed of catalyst or packing than it is in a non-uniform cross-section –A cylinder takes up a lot less plot space for the same volume –A sphere is more expensive to fabricate

Chemical Engineering Design Pressure Vessel Shape Most pressure vessels are at least 2:1 cylinders: 3:1 or 4:1 are most common:Most pressure vessels are at least 2:1 cylinders: 3:1 or 4:1 are most common: Distillation columns are obviously an exception: diameter is set by flooding correlations and height by number of traysDistillation columns are obviously an exception: diameter is set by flooding correlations and height by number of trays 2:1 3:1 4:1 (To scale)

Chemical Engineering Design Vessel Size Restrictions Diameter gets very expensive if > 13.5 ft. Why?Diameter gets very expensive if > 13.5 ft. Why? Height (length) gets very expensive if > 180 ft. Why?Height (length) gets very expensive if > 180 ft. Why? Roughly 50 cranes can lift > 180 ft Only 14 can lift > 240 ft Vessels that can’t be transported have to be fabricated on siteVessels that can’t be transported have to be fabricated on site

Chemical Engineering Design Vessel Orientation Usually verticalUsually vertical –Easier to distribute fluids across a smaller cross section –Smaller plot space Reasons for using horizontal vesselsReasons for using horizontal vessels –To promote phase separation Increased cross section = lower vertical velocity = less entrainmentIncreased cross section = lower vertical velocity = less entrainment Decanters, settling tanks, separators, flash vesselsDecanters, settling tanks, separators, flash vessels –To allow internals to be pulled for cleaning Heat exchangersHeat exchangers

Chemical Engineering Design Head (Closure) Designs HemisphericalHemispherical –Good for high pressures –Higher internal volume –Most expensive to form & join to shell –Half the thickness of the shell EllipsoidalEllipsoidal –Cheaper than hemispherical and less internal volume –Depth is half diameter –Same thickness as shell –Most common type > 15 bar TorisphericalTorispherical –Part torus, part sphere –Similar to elliptical, but cheaper to fabricate –Cheapest for pressures less than 15 bar

Chemical Engineering Design Tangent and Weld Lines Tangent line is where curvature beginsTangent line is where curvature begins Weld line is where weld is locatedWeld line is where weld is located Usually they are not the same, as the head is fabricated to allow a weld away from the geometrical jointUsually they are not the same, as the head is fabricated to allow a weld away from the geometrical joint

Chemical Engineering Design Welded Joints Some weld types are not permitted by ASME BPV CodeSome weld types are not permitted by ASME BPV Code Many other possible variations, including use of backing strips and joint reinforcementMany other possible variations, including use of backing strips and joint reinforcement Sec. VIII Div. 1 Part UW has details of permissible joints, corners, etc.Sec. VIII Div. 1 Part UW has details of permissible joints, corners, etc. Welds are usually ground smooth and inspectedWelds are usually ground smooth and inspected –Type of inspection depends on Code Division Butt weld Single fillet lap weld Double fillet lap weld Double fillet corner joint Double welded butt weld

Chemical Engineering Design Gasketed Joints (a)Full face gasket (b)Gasket within bolt circle (c)Spigot and socket (d)O-ring Used when vessel must be opened frequently for cleaning, inspection, etc.Used when vessel must be opened frequently for cleaning, inspection, etc. Also used for instrument connectionsAlso used for instrument connections Not used at high temperatures or pressures (gaskets fail)Not used at high temperatures or pressures (gaskets fail) Higher fugitive emissions than welded jointsHigher fugitive emissions than welded joints

Chemical Engineering Design Nozzles Vessel needs nozzles forVessel needs nozzles for –Feeds, Products –Hot &/or cold utilities –Manways, bursting disks, relief valves –Instruments Pressure, Level, ThermowellsPressure, Level, Thermowells Sample pointsSample points More nozzles = more costMore nozzles = more cost Nozzles are usually on side of vessel, away from weld lines, usually perpendicular to shellNozzles are usually on side of vessel, away from weld lines, usually perpendicular to shell Nozzles may or may not be flanged (as shown) depending on joint typeNozzles may or may not be flanged (as shown) depending on joint type The number & location of nozzles are usually specified by the process engineerThe number & location of nozzles are usually specified by the process engineer

Chemical Engineering Design Nozzle Reinforcement Shell is weakened around nozzles, and must also support eccentric loads from pipesShell is weakened around nozzles, and must also support eccentric loads from pipes Usually weld reinforcing pads to thicken the shell near the nozzle. Area of reinforcement = or > area of nozzle: see Code requirementsUsually weld reinforcing pads to thicken the shell near the nozzle. Area of reinforcement = or > area of nozzle: see Code requirements

Chemical Engineering Design Swaged Vessels Vessel does not have to be constant diameterVessel does not have to be constant diameter It is sometimes cheaper to make a vessel with several sections of different diameterIt is sometimes cheaper to make a vessel with several sections of different diameter Smaller diameters are usually at the top, for structural reasonsSmaller diameters are usually at the top, for structural reasons ASME BPV Code gives rules for tapered sectionsASME BPV Code gives rules for tapered sections

Chemical Engineering Design Vessel Supports Supports must allow for thermal expansion in operationSupports must allow for thermal expansion in operation Smaller vessels are usually supported on beams – a support ring or brackets are welded to the vesselSmaller vessels are usually supported on beams – a support ring or brackets are welded to the vessel Horizontal vessels often rest on saddlesHorizontal vessels often rest on saddles Tall vertical vessels are often supported using a skirt rather than legs. Can you think why?Tall vertical vessels are often supported using a skirt rather than legs. Can you think why?

Chemical Engineering Design Vessel Supports Note that if the vessel rests on a beam then the part of the vessel below the support ring is hanging and the wall is in tension from the weight of material in the vessel, the dead weight of the vessel itself and the internal pressureNote that if the vessel rests on a beam then the part of the vessel below the support ring is hanging and the wall is in tension from the weight of material in the vessel, the dead weight of the vessel itself and the internal pressure The part of the vessel above the support ring is supported and the wall is in compression from the dead weight (but probably in tension from internal pressure)The part of the vessel above the support ring is supported and the wall is in compression from the dead weight (but probably in tension from internal pressure)

Chemical Engineering Design Jacketed Vessels Heating or cooling jackets are often used for smaller vessels such as stirred tank reactorsHeating or cooling jackets are often used for smaller vessels such as stirred tank reactors If the jacket can have higher pressure than the vessel then the vessel walls must be designed for compressive stressesIf the jacket can have higher pressure than the vessel then the vessel walls must be designed for compressive stresses –Internal stiffening rings are often used for vessels subject to external pressure –For small vessels the walls are just made thicker

Chemical Engineering Design Vessel Internals Source: UOP Most vessels have at least some internalsMost vessels have at least some internals –Distillation trays –Packing supports –Distribution grids –Heating or cooling coils These may require support rings welded to the inside of the vesselThese may require support rings welded to the inside of the vessel The internals & support rings need to be considered when calculating vessel weights for stress analysisThe internals & support rings need to be considered when calculating vessel weights for stress analysis

Chemical Engineering Design Stress and Strain Stress  = force divided by area over which it is appliedStress  = force divided by area over which it is applied –Area = original cross section in a tensile test –Stress can be applied directly or can result from an applied strain –Examples: dead weight, internal or external pressure, etc. Strain ε = distortion per unit lengthStrain ε = distortion per unit length –Strain = elongation divided by original length in tensile test –Strain can be applied directly or can result from an applied stress –Example: thermal movement relative to fixed supports F F Cross-sectional area A L0L0  = F / A ε = (L – L 0 )/L 0

Chemical Engineering Design Typical Stress-Strain Curve for a Mild Steel

Chemical Engineering Design Creep At high temperatures, strain can continue to increase over time under constant load or displacementAt high temperatures, strain can continue to increase over time under constant load or displacement –Creep strain = increase in strain at constant load –Creep relaxation = reduction in stress at constant displacement Accumulated creep strain can lead to failure: creep ruptureAccumulated creep strain can lead to failure: creep rupture Time Stress or Strain Stress Strain Time Stress or Strain Stress Strain Fracture Low Temp High Temp

Chemical Engineering Design Principle Stresses & Maximum Shear Stress For a two-dimensional system the principal stresses at any point are:For a two-dimensional system the principal stresses at any point are: The maximum shear stress is half the algebraic difference between the principal stresses:The maximum shear stress is half the algebraic difference between the principal stresses: Compressive stresses are taken as negative, tensile as positiveCompressive stresses are taken as negative, tensile as positive Normal stresses  x,  y Shear stress τ xy τ xy xx yy xx yy  1,  2 = ½(  x +  y ) ± ½  [(  y -  x ) 2 + 4τ xy 2 ] Maximum shear stress = ½(  1 -  2 ) For design purposes, often just use  1 -  2

Chemical Engineering Design Failure of Materials Failure of materials under combined tensile and shear stresses is not simple to predict. Several theories have been proposed: Maximum Principal Stress TheoryMaximum Principal Stress Theory –Component fails when one of the principal stresses exceeds the value that causes failure in simple tension Maximum Shear Stress TheoryMaximum Shear Stress Theory –Component fails when maximum shear stress exceeds the shear stress that causes failure in simple tension Maximum Strain Energy TheoryMaximum Strain Energy Theory –Component fails when strain energy per unit volume exceeds the value that causes failure in simple tension BPV Code gives values for maximum allowable stress for different materials as a function of temperature, incorporating a safety factor relative to the stress that causes failure (ASME BPV Code Sec. II)BPV Code gives values for maximum allowable stress for different materials as a function of temperature, incorporating a safety factor relative to the stress that causes failure (ASME BPV Code Sec. II) Failure in compression is by buckling, which is much harder to predict than tensile failure. The procedure in the Code is iterative. This should definitely be left to a specialistFailure in compression is by buckling, which is much harder to predict than tensile failure. The procedure in the Code is iterative. This should definitely be left to a specialist

Chemical Engineering Design Loads Causing Stresses on Pressure Vessel Walls Internal or external pressureInternal or external pressure Dead weight of vesselDead weight of vessel Weight of contents under normal or upset conditionsWeight of contents under normal or upset conditions Weight of contents during hydraulic testingWeight of contents during hydraulic testing Weight of internalsWeight of internals Weight of attached equipment (piping, decks, ladders, etc)Weight of attached equipment (piping, decks, ladders, etc) Stresses at geometric discontinuitiesStresses at geometric discontinuities Bending moments due to supportsBending moments due to supports Thermal expansion, differential thermal expansionThermal expansion, differential thermal expansion Cyclic loads due to pressure or temperature changesCyclic loads due to pressure or temperature changes Wind & snow loadsWind & snow loads Seismic loadsSeismic loads Residual stresses from manufactureResidual stresses from manufacture Loads due to friction (solids flow)Loads due to friction (solids flow) All these must be combined to determine principal stresses

Chemical Engineering Design Example: Wind Load Wind exerts a pressure on one side of the vesselWind exerts a pressure on one side of the vessel Resulting force acts like a uniform beam load and exerts a bending moment on vesselResulting force acts like a uniform beam load and exerts a bending moment on vessel Windward wall is placed in tension, leeward in compressionWindward wall is placed in tension, leeward in compression Vortex shedding can cause vibrationVortex shedding can cause vibration –Hence spirals on chimneys –Usually not needed for columns due to ladders, pipes, decks, etc. Wind Bending moment

Chemical Engineering Design Thin Cylinder Subject to Internal Pressure Forces due to internal pressure are balanced by shear stresses in wallForces due to internal pressure are balanced by shear stresses in wall Horizontal section:Horizontal section: Vertical section:Vertical section: Similar equations can be derived for other geometries such as heads (see Ch 13)Similar equations can be derived for other geometries such as heads (see Ch 13) HH LL Longitudinal stress,  L Hoop stress,  H Inside diameter, D Wall thickness, t Height, h

Chemical Engineering Design Vessel Specifications Set By the Process Engineer The process engineer will usually specify the following parameters based on process requirements:The process engineer will usually specify the following parameters based on process requirements: –Vessel size and shape (volume, L and D) –Vessel orientation and elevation –Maximum and minimum design pressure –Maximum and minimum design temperature –Number of nozzles needed (& location) –Vessel internals And often also: –Material of construction –Corrosion allowance There is often a lot of dialogue with the mechanical engineer to set the final specificationsThere is often a lot of dialogue with the mechanical engineer to set the final specifications

Chemical Engineering Design Design Pressure Normal operating pressureNormal operating pressure The pressure at which you expect the process to usually be operatedThe pressure at which you expect the process to usually be operated Maximum operating pressureMaximum operating pressure The highest pressure expected including upset conditions such as startup, shutdown, emergency shutdownThe highest pressure expected including upset conditions such as startup, shutdown, emergency shutdown Design pressureDesign pressure Maximum operating pressure plus a safety marginMaximum operating pressure plus a safety margin Margin is typically 10% of maximum operating pressure or 25 psi, whichever is greaterMargin is typically 10% of maximum operating pressure or 25 psi, whichever is greater Usually specify pressure at top of vessel, where relief valve is locatedUsually specify pressure at top of vessel, where relief valve is located The BPV Code Sec. VIII Div. 1 doesn’t say much on how to set the design pressureThe BPV Code Sec. VIII Div. 1 doesn’t say much on how to set the design pressure “..a pressure vessel shall be designed for at least the most severe condition of coincident pressure and temperature expected in normal operation.”“..a pressure vessel shall be designed for at least the most severe condition of coincident pressure and temperature expected in normal operation.”

Chemical Engineering Design Design for Vacuum The minimum internal pressure a vessel can experience is full vacuum (-14.7 psig)The minimum internal pressure a vessel can experience is full vacuum (-14.7 psig) Vacuum can be caused by:Vacuum can be caused by: –Intentional process operation under vacuum (including start-up and shutdown) –Cooling down a vessel that contains a condensable vapor –Pumping out or draining contents without allowing enough vapor to enter –Operator error Vacuum puts vessel walls into compressive stressVacuum puts vessel walls into compressive stress What happens if vessel is not designed for vacuum conditions?What happens if vessel is not designed for vacuum conditions?

Chemical Engineering Design Vessel Subjected to Excess Vacuum Normal practice is to design for vacuum if it can be expected to occurNormal practice is to design for vacuum if it can be expected to occur

Chemical Engineering Design Design Temperatures Maximum:Maximum: –Highest mean metal temperature expected in operation, including transient conditions, plus a margin –Margin is typically plus 50  F MinimumMinimum –Lowest mean metal temperature expected in operation, including transient conditions, upsets, auto-refrigeration, climatic conditions, anything else that could cause cooling, minus a margin –Margin is typically -25  F –MDMT: minimum design metal temperature is important as metals can become brittle at low temperatures Designer should allow for possible failure of upstream equipment (e.g., loss of coolant on upstream cooler)Designer should allow for possible failure of upstream equipment (e.g., loss of coolant on upstream cooler)

Chemical Engineering Design Design Temperature Considerations Due to creep, maximum allowable stress drops off rapidly at higher temperaturesDue to creep, maximum allowable stress drops off rapidly at higher temperatures –Forces designer to use more expensive alloys BPV Code Sec. VIII Div.2 cannot be applied for design temperatures > 900  F (no creep safety factor in Div.2)BPV Code Sec. VIII Div.2 cannot be applied for design temperatures > 900  F (no creep safety factor in Div.2) The Code allows design of vessels with different temperature zonesThe Code allows design of vessels with different temperature zones –Very useful for high temperature vessels –Not usually applied to medium temperature vessels such as heat exchangers, distillation columns

Chemical Engineering Design Design Temperature & Pressure Exercise 1 What is the design pressure?What is the design pressure? 120 + 25 = 145 psig What is the design temperature?What is the design temperature? 340 + 50 = 390  F 100 psig 180 F 120 psig 340 F

Chemical Engineering Design Design Temperature & Pressure Exercise 2 What is the shell-side design pressure?What is the shell-side design pressure? 588 + 58 = 646 psig What is the tube-side design temperature?What is the tube-side design temperature? 482 + 50 = 532  F Oil 400 psig 120 F 390 psig 450 F Steam 40 barg 482 F

Chemical Engineering Design Materials Selection Criteria SafetySafety –Material must have sufficient strength at design conditions –Material must be able to withstand variation (or cycling) in process conditions –Material must have sufficient corrosion resistance to survive in service between inspection intervals Ease of fabricationEase of fabrication Availability in standard sizes (plates, sections, tubes)Availability in standard sizes (plates, sections, tubes) CostCost –Includes initial cost and cost of periodic replacement

Chemical Engineering Design Commonly Used Materials SteelsSteels –Carbon steel, Killed carbon steel – cheap, widely available –Low chrome alloys (<9% Cr) – better corrosion resistance than CS, KCS –Stainless steels: 304 – cheapest austenitic stainless steel304 – cheapest austenitic stainless steel 316 – better corrosion resistance than 304, more expensive316 – better corrosion resistance than 304, more expensive 410410 Nickel AlloysNickel Alloys –Inconel, Incolloy – high temperature oxidizing environments –Monel, Hastelloy – expensive, but high corrosion resistance, used for strong acids Other metals such as aluminum and titanium are used for special applications. Fiber reinforced plastics are used for some low temperature & pressure applications. See Ch 7 for more detailsOther metals such as aluminum and titanium are used for special applications. Fiber reinforced plastics are used for some low temperature & pressure applications. See Ch 7 for more details

Chemical Engineering Design Relative Cost of Metals The maximum allowable stress values are at 40ºC (100ºF) and are taken from ASME BPV Code Sec. II Part D. The code should be consulted for values at other temperatures. Several other grades exist for most of the materials listed.The maximum allowable stress values are at 40ºC (100ºF) and are taken from ASME BPV Code Sec. II Part D. The code should be consulted for values at other temperatures. Several other grades exist for most of the materials listed. Finished vessel relative costs are not the same as materials relative costs as vessel cost also includes manufacturing costs, labor and fabricator’s profitFinished vessel relative costs are not the same as materials relative costs as vessel cost also includes manufacturing costs, labor and fabricator’s profit

Chemical Engineering Design Corrosion Allowance Wall thicknesses calculated using BPV Code equations are for the fully corroded stateWall thicknesses calculated using BPV Code equations are for the fully corroded state Usually add a corrosion allowance of 1/16” to 3/16” (1.5 to 5 mm)Usually add a corrosion allowance of 1/16” to 3/16” (1.5 to 5 mm) Smaller corrosion allowances are used for heat transfer equipment, where wall thickness can affect heat transferSmaller corrosion allowances are used for heat transfer equipment, where wall thickness can affect heat transfer

Chemical Engineering Design Determining Wall Thickness Under ASME BPV Code Sec. VIII D.1, minimum wall thickness is 1/16” (1.5mm) with no corrosion allowanceUnder ASME BPV Code Sec. VIII D.1, minimum wall thickness is 1/16” (1.5mm) with no corrosion allowance Most pressure vessels require much thicker walls to withstand governing loadMost pressure vessels require much thicker walls to withstand governing load –High pressure vessels: internal pressure usually governs –Thickness required to resist vacuum usually governs for lower pressure vessels –For vessels designed for low pressure, no vacuum, then analysis of principal stresses may be needed –Usual procedure is to design for internal pressure (or vacuum), round up to nearest available standard size and then check for other loads

Chemical Engineering Design Design for Internal Pressure ASME BPV Code Sec. VIII D.1 specifies using the larger of the shell thicknesses calculatedASME BPV Code Sec. VIII D.1 specifies using the larger of the shell thicknesses calculated –For hoop stress –or for longitudinal stress Values of S are tabulated in ASME BPV Code Sec.II for different materials as function of temperatureValues of S are tabulated in ASME BPV Code Sec.II for different materials as function of temperature S is the maximum allowable stress E is the welded joint efficiency

Chemical Engineering Design Some Maximum Allowable Stresses Under ASME BPV Code Sec. VIII D.1, Taken From Sec. II Part D

Chemical Engineering Design Welded Joint Categories ASME BPV Code has four categories of welds: A.Longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks or nozzles. B.Circumferential welds in the main shell, necks or nozzles or connecting a formed head other than hemispherical. C.Welds connecting flanges, tubesheets or flat heads to the main shell, a formed head, neck or nozzle. D.Welds connecting communicating chambers or nozzles to the main shell, to heads or to necks.

Chemical Engineering Design Welded Joint Efficiencies Allowed Under ASME BPV Code Sec. VIII D.1

Chemical Engineering Design Closures Subject to Internal Pressure Hemispherical headsHemispherical heads Ellipsoidal headsEllipsoidal heads Torispherical headsTorispherical heads R c is the crown radius: see Ch 13

Chemical Engineering Design Example What is the wall thickness required for a 10ft diameter 304 stainless steel vessel with design pressure 500 psi and design temperature 700  F?What is the wall thickness required for a 10ft diameter 304 stainless steel vessel with design pressure 500 psi and design temperature 700  F? From the table, S = 11700 psiFrom the table, S = 11700 psi Assume double-welded butt joint with spot radiography, E = 0.85Assume double-welded butt joint with spot radiography, E = 0.85 For hoop stressFor hoop stress For longitudinal stressFor longitudinal stress So hoop stress governs, choose t = 3.25 or 3.5 inches, depending on what’s readily available as plate stockSo hoop stress governs, choose t = 3.25 or 3.5 inches, depending on what’s readily available as plate stock

Chemical Engineering Design Software for Pressure Vessel Design Rules for external pressure, combined loads are more complexRules for external pressure, combined loads are more complex Design methods and maximum allowable stresses are coded into software used by specialist designers, such as:Design methods and maximum allowable stresses are coded into software used by specialist designers, such as: COMPRESS (Codeware Inc.) has free demo version http://www.codeware.com/support/tutorials/compress_video_tutorial.htmlCOMPRESS (Codeware Inc.) has free demo version http://www.codeware.com/support/tutorials/compress_video_tutorial.html http://www.codeware.com/support/tutorials/compress_video_tutorial.html Pressure Vessel Suite (Computer Engineering Inc.)Pressure Vessel Suite (Computer Engineering Inc.) PVElite and CodeCalc (COADE Inc.)PVElite and CodeCalc (COADE Inc.) Simple ASME BPV Code Sec. VIII D.1 methods are available in Aspen ICARUSSimple ASME BPV Code Sec. VIII D.1 methods are available in Aspen ICARUS Good enough for an initial cost estimate if the process engineer puts in realistic vessel specificationsGood enough for an initial cost estimate if the process engineer puts in realistic vessel specifications Useful for checking to see if changes to specifications give cost discontinuitiesUseful for checking to see if changes to specifications give cost discontinuities Not good enough for detailed vessel designNot good enough for detailed vessel design

Chemical Engineering Design Example What is the cost of a 10ft diameter, 100ft long 304 stainless steel vessel with design pressure 500 psi and design temperature 700  F?What is the cost of a 10ft diameter, 100ft long 304 stainless steel vessel with design pressure 500 psi and design temperature 700  F?

Chemical Engineering DesignExample In Aspen ICARUS, if we just enter the dimensions and material:

Chemical Engineering Design Example Total cost $575,500

Chemical Engineering Design Example: With More Complete Specifications

Chemical Engineering Design Example: With More Complete Specifications Total cost is now $1,814,400

Chemical Engineering Design Example: ICARUS Results ICARUS finds a wall thickness of 3.308”, based on 3.183” for hoop stress and 0.125” corrosion allowanceICARUS finds a wall thickness of 3.308”, based on 3.183” for hoop stress and 0.125” corrosion allowance Vacuum design thickness is 1.05”, so internal pressure is governingVacuum design thickness is 1.05”, so internal pressure is governing Not clear why the ICARUS hoop stress calculation comes out different, but possibly due to considering combined loads under wind or seismic conditionsNot clear why the ICARUS hoop stress calculation comes out different, but possibly due to considering combined loads under wind or seismic conditions Note that proper specification of vessel design changed cost by factor 3.15Note that proper specification of vessel design changed cost by factor 3.15 Results are good enough for preliminary costing, but not for mechanical designResults are good enough for preliminary costing, but not for mechanical design

Chemical Engineering Design Vessel Manufacture Shell is usually made by rolling plate and then welding along a seam:Shell is usually made by rolling plate and then welding along a seam: –Difficult to form small diameters or thick shells by this method –Long vessels are usually made in 8’ sections and butt welded Thicker vessels are made by more expensive drum forging – direct from ingotsThicker vessels are made by more expensive drum forging – direct from ingots Closures are usually forgedClosures are usually forged –Hence restricted to increments of 6” in diameter Nozzles, support rings etc. are welded on to shell and headsNozzles, support rings etc. are welded on to shell and heads

Chemical Engineering Design Post Weld Heat Treating (PWHT) Forming and joining (welding) can leave residual stresses in the metalForming and joining (welding) can leave residual stresses in the metal Post-weld heat treatment is used to relax these stressesPost-weld heat treatment is used to relax these stresses Guidelines for PWHT are given in the ASME BPV Code Sec. VIII D.1 Part UW-40Guidelines for PWHT are given in the ASME BPV Code Sec. VIII D.1 Part UW-40 PWHT requirements depend on material and thickness at weld:PWHT requirements depend on material and thickness at weld: -Over 38mm for carbon steel -Over 16mm for low alloy

Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not.

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Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not.

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Presentation on theme: "Chemical Engineering Design © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & SinnottChemical Engineering Design only. Do not."— Presentation transcript:

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