Pressure Vessel 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) The definition applies to most process reactors, distillation columns, separators (flashes and decanters), pressurized storage vessels and heat exchangers Source: UOP

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 design But 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 design 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 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 anything

Pressure Vessel Design Codes Vessel Geometry & Construction Strength of Materials Vessel Specifications Materials of Construction Pressure Vessel Design Rules Fabrication, Inspection and Testing

ASME Boiler and Pressure Vessel Code
ASME BPV Code is the legally required standard for pressure vessel design, fabrication, inspection and testing in North America Section I Rules for construction of power boilers II Materials III Nuclear power plant components IV Rules for construction of heating boilers V Nondestructive examination VI Recommended rules for the care and operation of heating boilers VII Recommended guidelines for the care of power boilers VIII Rules for the construction of pressure vessels Division 1 Division 2 Alternative rules Division 3 Alternative rules for the construction of high pressure vessels IX Welding and brazing qualifications X Fiber-reinforced plastic vessels XI Rules for in service inspection of nuclear power plant components XII Rules for construction and continued service of transport tanks Allowable stresses are given in Sec. II Most chemical plant vessels fall under Sec. VIII D.1 or D.2 Often used for bio-reactors

Advantages of Designing to Code
The Code is a consensus best practice It 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 code Savings of not following code rules are negligible as vessel shops are set up to do everything to code

ASME BPV Code Sec. VIII Divisions
Rigorous 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 rupture 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 vessels Safety factor of 3.0 against tensile failure Limited to design temperatures less than 900F (outside creep range) More economical for high pressure vessels, but fewer fabricators available Either Division of the Code is acceptable, but provisions cannot be mixed and matched

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). Fired process tubular heaters. 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 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). 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 of internal diameter or height less than 6 inches (152 mm). Pressure vessels for human occupancy.

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 manufacturer The code stamp must be clearly visible on the vessel

Other Related Codes Storage 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 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 code ASME B16.3, Process piping Heat exchangers have additional codes set by TEMA

Use of Design Codes & Standards
The 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 edition

Pressure Vessel Shape 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? 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

Pressure Vessel Shape 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 trays 4:1 3:1 2:1 (To scale)

Vessel Size Restrictions
Diameter gets very expensive if > 13.5 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 site

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

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

Tangent and Weld Lines Tangent line is where curvature begins
Weld 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 joint

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

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

Nozzles Vessel needs nozzles for More nozzles = more cost
Feeds, Products Hot &/or cold utilities Manways, bursting disks, relief valves Instruments Pressure, Level, Thermowells Sample points More nozzles = more cost Nozzles 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 type The number & location of nozzles are usually specified by the process engineer

Nozzle Reinforcement Shell 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 requirements

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

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

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 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)

Jacketed Vessels Heating 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 stresses Internal stiffening rings are often used for vessels subject to external pressure For small vessels the walls are just made thicker

Vessel Internals Most 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 vessel The internals & support rings need to be considered when calculating vessel weights for stress analysis Source: UOP

Stress and Strain L0  = F / A ε = (L – L0)/L0 F F Cross-sectional area A Stress  = 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 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

Typical Stress-Strain Curve for a Mild Steel

Creep Low Temp High Temp Fracture Time Stress or Strain Stress Strain Stress Stress or Strain Strain Time At 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 rupture

Principle Stresses & Maximum Shear Stress
y τxy 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: Compressive stresses are taken as negative, tensile as positive x x 1, 2 = ½(x+ y) ± ½[(y - x)2 + 4τxy2] τxy y Normal stresses x, y Shear stress τxy Maximum shear stress = ½(1 - 2) For design purposes, often just use 1 - 2

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 Theory Component fails when one of the principal stresses exceeds the value that causes failure in simple tension Maximum Shear Stress Theory Component fails when maximum shear stress exceeds the shear stress that causes failure in simple tension Maximum 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) 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 specialist

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

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

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

Vessel Specifications Set By the Process Engineer
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 specifications

Design Pressure Normal operating pressure Maximum operating pressure
The pressure at which you expect the process to usually be operated Maximum operating pressure The highest pressure expected including upset conditions such as startup, shutdown, emergency shutdown Design pressure Maximum operating pressure plus a safety margin Margin is typically 10% of maximum operating pressure or 25 psi, whichever is greater Usually 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 pressure “..a pressure vessel shall be designed for at least the most severe condition of coincident pressure and temperature expected in normal operation.”

Design for Vacuum The minimum internal pressure a vessel can experience is full vacuum (-14.7 psig) 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 stress What happens if vessel is not designed for vacuum conditions?

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

Design Temperatures Maximum: Minimum
Highest mean metal temperature expected in operation, including transient conditions, plus a margin Margin is typically plus 50F Minimum 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)

Design Temperature Considerations
Due 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) The 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

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

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

Materials Selection Criteria
Safety 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 fabrication Availability in standard sizes (plates, sections, tubes) Cost Includes initial cost and cost of periodic replacement

Commonly Used Materials
Steels 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 steel 316 – better corrosion resistance than 304, more expensive 410 Nickel 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 details

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. Finished vessel relative costs are not the same as materials relative costs as vessel cost also includes manufacturing costs, labor and fabricator’s profit

Corrosion Allowance Wall 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) Smaller corrosion allowances are used for heat transfer equipment, where wall thickness can affect heat transfer

Determining Wall Thickness
Under 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 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

Design for Internal Pressure
ASME 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 temperature S is the maximum allowable stress E is the welded joint efficiency

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

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

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

Closures Subject to Internal Pressure
Hemispherical heads Ellipsoidal heads Torispherical heads Rc is the crown radius: see Ch 13

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? From the table, S = psi Assume double-welded butt joint with spot radiography, E = 0.85 For hoop stress For longitudinal stress So hoop stress governs, choose t = 3.25 or 3.5 inches, depending on what’s readily available as plate stock

Software for Pressure Vessel Design
Rules for external pressure, combined loads are more complex Design methods and maximum allowable stresses are coded into software used by specialist designers, such as: COMPRESS (Codeware Inc.) has free demo version Pressure Vessel Suite (Computer Engineering Inc.) PVElite and CodeCalc (COADE Inc.) Simple 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 specifications Useful for checking to see if changes to specifications give cost discontinuities Not good enough for detailed vessel design

Vessel Manufacture 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 ingots Closures are usually forged Hence restricted to increments of 6” in diameter Nozzles, support rings etc. are welded on to shell and heads

Post Weld Heat Treating (PWHT)
Forming and joining (welding) can leave residual stresses in the metal Post-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-40 PWHT requirements depend on material and thickness at weld: Over 38mm for carbon steel Over 16mm for low alloy

Pressure Vessel Design

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