Pvs

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Pressure Vessels

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Purpose

Introduction of the governing codes and basic considerations and concepts of pressure vessel design, fabrication, inspection, and modification.

Attendees will understand the basis for a vessel’s design conditions and will be able to determine the required thickness for internal pressure.

Attendees will also understand the purpose of common refinery pressure vessels and the function of their internals.

Finally, attendees will have a general understanding of vessel fabrication and repair or alteration.

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Contents

Codes and StandardsPressure Vessel Shapes, Types, and UsesInternalsDesign ConditionsDesignPressure testingEvaluation, Repair, and Alteration

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Codes and Standards

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Codes and Standards

The purpose of this module is to identify the principle Codes and Standards that govern the design and fabrication of pressure vessels and to provide guidelines for finding the appropriate documents

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Design Codes

Provide rules for the design of equipment adequate for design conditions determined by others.

Do not provide rules or guidance for the determination of design conditions, materials of construction or corrosion allowance.

Defined scope of most design codes includes new construction only, not revamps, repairs, or rerates.

Documents referenced by the Code are treated as part of the Code.

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Design Codes

Code, in its entirety, and all of its provision must be used.

Laws and regulations in force at the site determine the Code (and perhaps the edition) that must be used and could limit use of referenced or auxiliary documents (e.g., Code Cases).

Calculations must be stamped by an experienced, licensed, professional engineer.

Fabrication should be performed in a shop with an ASME Code Stamp, signifying certification by ASME (and successful inspection by ASME).

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Codes and Standards

ASME Section I “Rules for Construction of Power Boilers”– Used for steam generating equipment and

certain auxiliary equipment and piping

– Often used for power plants that cannot afford to be “down”; therefore, design a little more conservatism into them

– Uses factor of safety of 3.5 for tensile failure

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Codes and Standards

ASME Section VIII Division 1 “Rules for Construction of Pressure Vessels”– Used for most unfired refinery equipment

– Uses factor of safety of 3.5 against tensile failure

– Required for external pressure and internal pressure greater than 15psig (1.05kg/cm2(g)). Limited to 3000 psig (210kg/cm2(g)) internal pressure (less as a practical matter)

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Codes and Standards

ASME Section VIII Division 2 “Rules for Construction of Pressure Vessels – Alternative Rules”– Used for high pressure refinery equipment

– Uses factor of safety of 3 against tensile failure, resulting in thinner vessels (compared to Division 1)

– Not permitted in the creep range of materials

– Requires additional design analysis (e.g., local and thermal stress, fatigue) and quality control (e.g., full X-ray, stringent material requirements)

– Usually becomes economical if the shell thickness (per Division 1) exceeds 4 inches (100mm)

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Codes and Standards

ASME Section II, Part D “Materials – Properties”– Lists allowable stresses for use with other Code

Sections– Provides other useful properties such as yield and

ultimate tensile strength– Properties listed as a function of temperature

ASME Section V “Non Destructive Examination”

ASME Section IX “Welding and Brazing Qualifications”

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Codes and StandardsASME Code Cases and Interpretations

ASME Boiler and Pressure Vessel Code Cases and Interpretations– Code Cases are auxiliary to the Pressure

Vessel Code. If accepted by the local governing body they carry the legal weight and authority of the Code.

– Interpretations are committee responses to questions but carry no legal weight. They exist for many Sections of the Code.

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Codes and Standards

TEMA (Tubular Exchanger Manufacturers Association)– Applies to shell and tube exchangers.– Provides some additional criteria, otherwise

references the pressure vessel Code.

Company Standards often supplement the Code, imposing additional requirements.

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Codes and Standards

Other counties have similar Codes and Standards applicable to pressure vessel design. Europe has the new Pressure Equipment Directive (PED) as an “umbrella” procedure.

– Primarily addresses risk, safety (e.g., Essential Safety Requirements (ESR)), and traceability issues

– Requires approval by the Notified Body, 3rd Party Organization, or User Inspector

– ASME vessel (and piping design procedures meet the requirements, materials do not

– ASME is working towards compliance

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Codes and Standards

Other, more specialized, Codes and Standards are mentioned in the following sections of this seminar.

An extensive list of applicable documents issued by API, ASME, PIP, and others is included as Attachment A to the handout.

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Pressure Vessel Shapes, Types, and Uses

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Vessel Types and Uses

In this Section, the primary shapes, types and uses of refinery pressure vessels will be described.

The attendee will be able to recognize common shapes and orientations of pressure vessels, and will understand the primary uses of pressure vessels in refineries.

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Pressure Vessels

Pressure Vessel definition - A container in which an occurrence takes place at a different pressure than atmospheric.

In refineries, pressure vessels may up to 200 feet (60M) tall or 50 feet (15M) in diameter. They are designed for pressures up to 3000psig (210kg/cm2(g)) or temperatures of 1600ºF (870ºC).

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Overall Geometry

The sphere is the most economical shape forpressure retention, but is rarely used– Used for some gas storage vessels, particularly

high pressureThe cylinder is the shape of nearly all refinery pressure vessels– Easy to fabricate– Constant cross section (eases internal

fabrication and installation, distribution and collection of process fluids, and process control)

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Overall Geometry

Most vessels are oriented vertically unless there is a specific (process) reason to be placed horizontally (e.g., gravity separators)– Simplifies support– Minimizes “footprint”– Allows gravity to aid process

• Dense fluids fall, light fluids are displaced upwards

– Easier to modify volume by changing the length

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Overall Geometry

Vessel dimensions (diameter and length) and orientation are controlled by process requirements (e.g., space velocity, fluid distribution, catalyst contact, residence time, tray design and spacing, etc.)

Cylinder length to inside diameter ratio of 3 or 4 is typically used– Provides good mix of inside volume, cross-section

area, and vessel cost (e.g., wall thickness)

Inside diameter and length dimensions are set to increments of 6 inches or 100 mm– Matches commonly available head sizes and “can”

lengths for the shell

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Tangent and Weld Lines

Tangent Line– Point at which the head curvature begins

Weld Line– Point at which the head and shell are welded

together

The weld line is very rarely the same point as the tangent line. This moves the weld to a point where fit is easier (e.g., both sections are cylindrical) and away from any stress concentrations present at the geometrical joint.

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Tangent and Weld LinesOverview

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Common Refinery Pressure Vessels

Fractionators– Separates fluids by boiling/condensation point– Primary method of separation in a refinery– Requires energy (heat) input– Tall, vertical vessels

Separators/Receivers– Separate fluids of different densities (e.g., vapor and liquid)– Does not require energy input– May be horizontal or vertical

Surge Drums– Provide volume to absorb variations in fluid flow,

providing a smooth downstream fluid flow– Allows continued short term operation if feed is interrupted– May be horizontal or vertical

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Common Refinery Pressure Vessels

Reactors– Contain chemical reactions that are the heart of the

process– Nearly always vertical– Contain a catalytic material (generally granular)– Most severe operating conditions in the process unit

Regenerators– Regenerate catalyst in circulating Catalyst Systems

Miscellaneous– Heat Exchangers– Pump casings

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Common Head Styles

HemisphericalEllipticalConicalFlanged and DishedTorisphericalFlat

Hemispherical and 2:1 Elliptical are the most common.

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Internals

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Internals

This section describes common internals and their function.

The attendee will be able to recognize commonly encountered internals and understand their function.

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Internals

Internals are normally used to control the flow of fluids within a pressure vessel. Proper distribution, collection, and mixing of fluids is critical to the performance of the process.

Other internals are used for auxiliary purposes, such as support of catalyst beds.

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Fractionation

Rising vapor contacts with descending liquid.The vapor strips light components from the liquid.The liquid absorbs heavy components from the vapor.Internals maximize the vapor/liquid contact and distribute the vapor through the liquid.

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All the liquid goesdown each downcomer

Downcomer Seal Area

Odd and even trays are identical but are rotated 180º from each other

Active Areaor Bubbling

Area

Downcomer Area

Fractionation Devices - Trays

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Fractionation Devices

Trays– Sieve

• Simplest, most common style• 2 to 1 operating range

– Valve• Cost about 20% more than sieve tray• 5 to 1 operating range• Subject to plugging

– Bubble Cap• Cost may be 3 times valve tray• Good if no weeping is critical

– Cartridge (an assembly of several trays inserted as a unit into small diameter vessels

The number of trays is a process variable, determined by the boiling point differences, the number of products sought, and the purity required

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Valves, Bubble Cap, Sieve

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Tray Concerns

Flooding– Vapor velocity entrains too much liquid to tray above– Tray hydraulics and downcomer size do not allow all

of the liquid to flow to tray below• High tray pressure drop or downcomer exit loss• High downcomer velocity may entrain vapor

Weeping, i.e., loss of liquid through the vapor holesTurndown capacityCorrosion, debris, dislodged traysFouling/plugging of vapor openings

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Fractionation Devices - Packing

Packing– Random (dumped)

• Raschig Rings• Pall rings• Raschig Super-Ring• IMPT (Koch-Glitsch)• Nutter Rings (Sulzer)

– Structured - Koch-Glitsch, Sulzer, Others.– Cost may be 5 times sieve tray– Pressure drop may be 1/5 that of sieve tray

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Raschig ProductsPhotos From Raschig

Super-Ring Various Products

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VIDEO

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Separators

Commonly used for desalters, separators, receivers, coalesces, knockout drums, etc

Separates 2 or 3 fluids (vapor and liquid)

Separates by means of density – closer densities are more difficult to separate– Also depends upon droplet size, material

viscosity, and drag coefficient

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Exercise

Why are separators used? Give two or three uses. What are some examples in a refinery?

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Separators

Separates by means of one or more of the following methods– Momentum – Denser materials respond more

slowly to sudden changes flow of direction– Settling – Heavier materials settle to the

bottom, displacing lighter materials upwards– Coalescing – combines small droplets into

larger droplets to promote settling

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Horizontal and Vertical Separators

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Separators

Settling is governed by terminal velocity, i.e., how fast the dispersed material moves relative to the continuous phase material

Vessel is sized so that separation occurs before the continuous fluid leaves the vessel– Targeting separation of smaller droplets

increases the size of the vessel– Different fluids (e.g., vapor or liquid) may be

the continuous fluid in different areas

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Types of VesselsVapor/Liquid Separator

CFBA

D

Tangent Length

M.R. M.R.

Min.Min.

I.D.

Min.

CL

Example:Overhead Receiver Liquid Out

Vapor Out

Inlet Distributor

150

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Note: Use side inlet and horizontal distributor if liquid volume flows are approximately equal.

Types of VesselsLiquid/Liquid Separator

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Types of VesselsVapor/Liquid/Liquid Separator

Low Heavy Liquid Rate

EB

F

6"

Tangent Length

M.R.

Min.

I.D.

Min.

Example: HP Separator

D

Heavy Liquid Out

Light Liquid Out

FeedA

Vapor Out

Inlet Distributor

M.R.

CL

G

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Equivalent Light and Heavy Liquid Rates

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Low Light Liquid Rate

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Separators

Separator Internals– Inlet distributors

• Provide momentum separation• Deposit each material in the portion of the

vessel where it is predominant (e.g., liquid on the bottom)

– Vortex breakers• Eliminate liquid vortexes at outlets• Vortex may reentrain/mix fluids

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Vertical Distributorsfor Horizontal Vessels

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Vortex Breakers

4"

8" Dia.for 3"6" Dia.under 3"

3 - 3/4" x 1/4" BarsSpaced at 120º

3" and Under

2D

D

D

3"

D

Four WayBaffle

2D

D

4" Through 8" 10" and Over

2"

8"

4"2" H

4" Pipe

D

D Pipe

2D

D

2" H

D

D Pipe

2DFour Way

Baffle

3" H

D

3" and Under 4" Through 8" 10" and Over

H = as specified

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Separators

Mist Eliminators– Used to “dry” vapor streams– Coalesce small liquid droplets together– Promotes gravity separation of very small droplets– Creates a complex, labyrinth, path forcing droplets

to collide with each other or parts of the device– Types

• Vanes• Centrifugal Elements (e.g., cyclones), separate

by momentum differences• Filters

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Vane Mist Eliminators

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Centrifugal Elements

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Separators

– Types• Electrostatic precipitators (separate via attraction

to/repulsion from electrically charged plates• Mesh blankets

– The most common type.– Consist of a ‘maze’ of small, intertwined wires.

Usually 6-12 inches (150-300mm) thick in the direction of flow.

– High efficiency for a range of velocities, processes, droplet sizes, fluids. Wide turndown (or turnup) flexibility.

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Separators

– Mesh Blankets• Low cost• Easily installed or replaced (even as a retrofit• Low ∆P and operating costs• Plugs easily, not suitable for fouling services

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Typical Horizontal Installationof a Sectional Mesh Blanket

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Surge Drums

Surge drums are “wide spots in the line”Sized to provide a desired residence, or retention, time– Required volume is a function of the flow rate– Set by process or operation considerations– Usually between 5 and 30 minutes– Has few internals– May be a dedicated vessel or part of another

vessel, like a separator or fractionator

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Reactors

Flow may be axial or radialRequired internals may be complexContain catalyst to promote the desired reaction– Internals contain and support the catalyst– Catalyst may have an inert material below (and

above) to help contain the small diameter catalyst– In circulating catalyst systems the internals

control the catalyst flow

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Reactors

Internals evenly distribute, collect, and mix flow. Maldistribution has severe process consequences.Due to the severe design/operating conditions the internals ore often made of a high alloy.Damage to internals affects process performance and may allow catalyst to escape and damage downstream equipment.

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Downflow Reactor

Inlet Distributor

V/L Tray

19 MM Balls

6 MM Balls

Catalyst

3 MM Balls

19 MM Balls6 MM Balls

Tray Manway

Unloading Sleeve

Outlet Basket

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Radial Flow Reactor

Catalyst Bed Coverplate

Scallop Shield

Centerpipe Shroud

Outer Screen

Catalyst Bed (Concentrically Loadedaround Centerpipe)

Centerpipe)

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Design Conditions

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Design Conditions

In this section the pressure and temperature conditions used for pressure vessel design will be defined and the means of determining them outlined

The attendees will be able to understand the basis for and meaning of the design conditions specified for a pressure vessels

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Process Design ConsiderationsInternal Pressure

Maximum Operating– Highest operating pressure foreseen for all

applicable cases (normal, turndown, startup shutdown)

Design Pressure– Maximum operating pressure plus a safety

margin (a margin is not mandated by the Code)

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Process Design ConsiderationsInternal Pressure

Internal Design Pressure – Typically the greater of

• 110 percent of the maximum operating pressure• maximum operating pressure plus 15 to 25 psi

(1.05 to 1.75 kg/cm2)• A minimum value set by the owner

– Must accommodate the relief valve set point and reclosure pressure (usually about percent below the set point)

– More responsive relief valves (e.g., pilot operated valves) may permit a smaller margin, e.g., design for 105 percent of the operating pressure

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Process Design ConditionsExchanger Design Pressure

To avoid the need for an additional relief valve, the low pressure side may be designed for 10/13 of the high pressure side design pressure

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Vacuum (external pressure) conditions are specified for– Equipment that operates under vacuum

(including startup and shutdown)– Equipment is subject to vacuum during drainage– Where loss of reboiler or other heat to a gas with

a resultant cooling, even condensation, can result in a vacuum

Design may be for full vacuum or a specified partial vacuumDesign of all vessels for a minimum vacuum (e.g., 2.5psig (0.175 kg/cm2(g)) is common

Process Design ConditionsVacuum Design

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Process Design Considerations

Design pressure is defined to be at the top of the vessel in its operating position

Mechanical design conditions at the bottom should consider:– Liquid head– Upflow or downflow pressure drop– Hydrostatic test conditions

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Process Design ConditionsDesign Temperature

Normal Operating– Highest temperature expected during the

equipment’s operating cycle, including start and end of run.

Design Temperature– Normal operating temperature plus a margin (a

margin is not mandated by the Code)– May set a minimum value (e.g., based on steamout)

If operation is cryogenic (cold), the margin is a minus value. Alternative margins may be considered where the metallurgy is affected

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Design Temperature

Cold Wall Design– Internally insulated vessels allow lower shell design

temperature and possibly a lower and less expensive metallurgy

Flange Classes– Watch the effect on the flange class when setting the

design temperatureConsider the effect of elevated design temperature on the allowable design stress. Due to creep considerations, the allowable stress can drop rapidly at elevated temperatures.

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Specified Design Conditions

The specified design conditions are those resulting in the most severe head/shell requirements– Generally the greatest temperature and greatest

pressureIf the greatest temperature and pressure do not act simultaneously, the governing case may not include either or bothDifferent portions of the equipment may have different design conditions– Consider need to accommodate pressure testing

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Low Temperature Requirements

At low temperatures, many materials may become brittle– ASME Code contains additional requirements

for these materials depending upon the applicable MDMT

MDMT stands for Minimum Design Metal Temperature– Is the lowest mean temperature of the metal (not

the internal fluid) considering many factors, including operating temperature, low ambient temperature, and auto refrigeration

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Low Temperature Requirements

Application of additional requirements (e.g., impact testing) depends upon the material, MDMT, and thickness

Figure UCS-66 of ASME Section VIII Division 1 is used to determine if Charpy V-notch testing is required

Low temperature considerations (the MDMT) may lead to a change in the metallurgy.

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Name Plate

Name plates must be prominently affixed to all vessels and contain the information called for by the Code in Section UG-116. See the following slide.

The maximum allowable working pressure is the maximum permissible pressure at the top of the vessel. It’s based on the actual metal thickness minus the corrosion allowance. The design pressure is frequently (and conservatively) used.

The Code Stamp indicates Code used and signifies that the fabricator abides by ASME guidelines and has passed their inspection.

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W (if arc or gas welded)RT (if Radio graphed)

HT (if Postweld heat treated)

Name of Manufacturer

psi at °FMax. Allowable Working Pressure

Min. Design Metal Temperature

Manufacturer’s Serial Number

Year Built

°F at psi

Name Plate

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Design

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Design

This section introduces the basic methods and considerations for determination of pressure vessel shell and head thicknesses. The means of accommodating openings (nozzles) is also presented.

Attendees will be able to design simple vessels for internal pressure and understand some of the other factors influencing the shell thickness. They will also understand comment methods of providing nozzle reinforcement.

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Design

Loadings causing stress in the shell– Internal or external pressure– Wind and seismic– Vessel and contents (internals, catalyst, fluids) weight– Attached equipment and piping– Thermal gradients– Impact– Residual stresses– Local stress concentrations due to discontinuities– Cyclic loads (pressure or thermal)

This following discussion focuses on internal pressure, and touches on external pressure and wind/seismic concerns

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Stress Analysis of Pressure Vessels

Derives the basic formulas for shell stress from internal pressure

Presents the ASME Code pressure design equations

Vessel design calculations must be performed by an experience, licensed, professional engineer

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Stress Analysis of Pressure Vessels

Thicknesses determined from the presented methods do not include the corrosion allowance. The final thickness will be greater to include the corrosion allowance and to round to the next available plate thickness.

A glossary of terms is included in Appendix B.

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ASME Code Design ThicknessEquations for Shells

Section VIII, Division 1

Cylindrical Shells– Circumferential stress (longitudinal joints)

t PRSE P

=− 0 6.

Limits Rt 12

P 0.385SE

≤

≤

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ASME Code Design Thickness Equations for Shells

– Longitudinal Stress (circumferential joints)

– For circumferential stress (longitudinal joints), based on the outside radius

t PRSE P

=+2 0 4.

Limits Rt 12

P 1.25SE

≤

≤

t PRSE P

O=+ 0 4.

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Common Head Styles

HemisphericalEllipticalConicalFlanged and DishedTorisphericalFlat (rare due to its large thickness for all but very small vessels)

Hemispherical and 2:1 elliptical are by far the most common styles

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Hemispherical– Optimal pressure containing shape– Half as thick as the shell– No sharp radius bends (e.g. knuckles) or stress

concentration points– Joint with the shell is more complex– More difficult to form or fabricate, fewer

potential vendors– Suitable for thick shells (> 2 inches (50mm))

(from a cost viewpoint)

Hemisphericalversus 2:1 Elliptical Heads

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Hemispherical vs. 2:1 Elliptical Heads

2:1 elliptical– Most common head style– Three dimensional elliptical geometry with a

depth equal to half the vessel radius– Same thickness as the shell– Easy butt weld detail at joint with the shell– Commonly available– Less volume and surface area than hemispherical– Knuckles are in hoop compression– Suitable for thin shells (< 2 inches (50mm)) (from

a cost viewpoint)

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ASME Code Design Thickness Equations for Shells and Heads

Spherical Shells

Spherical shells based upon the outside radius

t PRSE P

=−2 0 2.

Limits t 0.356R P 0.665SE

≤≤

t PRSE P

O=+2 0 8.

Note that this equation is similar to the equation used for piping

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Pressure Vessel Heads

t

Pressure Vessel Heads− Ellipsoidal

where

For a 2:1 ellipsoidal head K=1

t PDKSE P

=−2 0 2.

or P = 2SEtKD + 0.2t

K Dh

= +

16

22

2

h

D

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ASME Code Design Thickness Equations for Heads

Pressure Vessel Heads– Conical (without transition knuckle)

( )

( )

t PDSE P

t PDSE P

O

=−

=+

2 0 6

2 0 4

cos .

cos .

α

α

Limits:Half Apex Angle, α<30°P ≤ 1.25SE

D

rα

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ASME Code Design Thickness Equations for Heads

Pressure Vessel Heads– Toriconical heads (conical portion)

( )t PD

SE Pc = −2 0 6cos .αLimits r > 0.06DO

r > 3tkmandatory if α>30°

+=

=

−=

rLM

DiL

PSEPLMtk

341

cos2

2.02

α

DL

Di

r

tK

tc

α

α

– Knuckle portion

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Pressure Vessel Heads

Pressure Vessel Heads− Torispherical

where

t PLMSE P

=−2 0 2.

or P = 2SEtLM + 0.2t

M Lr

= +

14

3 for the typical case where

r=0.06L and L=skirt OD,

t PLSE P

=−

0 8850 1

..

LD

r

t

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Welds shall be examined by full or spot radiography– Full — Radiography of the entire length of the

weld joint– Spot — Radiographic examination of one spot in

each 50 feet (15M) or fraction thereof for each welder, weld method, or type of joint

Ultrasonic examination may be substituted for radiography for the final closure seam if it is not possible to obtain interpretable radiographs

Weld Examination

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Efficiency of Welded Joints (E)(Excerpt from ASME Code Table UW-12)

Degree of RadiographicExamination

No. Type of Joint Full Spot None

1 Double-welded butt joint or single-welded butt joint with backing stripwhich does not remain in place

1.00 0.85 0.70

2 Single-welded butt joint with backingstrip which remains in place

0.90 0.80 0.65

3 Single-welded butt joint without useof backing strip

– – 0.60

Type 1 joints are generally required for refinery vessels

Hemispherical and ellipsoidal heads are either seamless or 100% radiographed and use a joint efficiency of 1

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Allowable Stress Table

•Reproduced from ASME Section II, Part D.

•“P-nos” and “Group Nos” are used to combine similar materials for the purpose of specifying postweld heat treatment and welding requirements.

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•NP = not permitted.

The referenced notes provide additional requirements, cautions, limitations, or comments.

Allowable Stress Table

(continued)

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•Values in italics are governed by time dependent (creep) effects, and may deform over time.

•At elevated temperatures, allowable stresses decline rapidly. A small temperature change may lead to a large thickness change.

Allowable Stress Table(continued)

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Exercise

What is the approximate required thickness of the following cylindrical vessel?– Material = SA 516, Grade 65

– Inside diameter = 10 feet

– Design temperature = 750 ºF

– Design pressure = 500psi

– Joint efficiency 0.85 (spot radiography)

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Nozzle Details

Although nearly any orientation is possible, for ease of design and reinforcement, nozzles should be perpendicular to the shellNozzles (and reinforcement) in heads are normally parallel to the vessel axis. To comply with the above point they are normally located within the center 80% of the head diameterAlthough not prohibited by codes, avoid locating nozzles in or near vessel weld seams– Nozzle and any reinforcement will interfere with

the ability to inspect and NDE the vessel weld

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Nozzle Details

Nozzle to shell welds are difficult to examine, especially to radiograph, because of the difficulty in accessing welds between two components at a right angle and the interference in the readings caused by the geometrical changes

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Vessel FabricationNozzles

C. Built-up Nozzles D. Integrally Reinforced Nozzles

A. Pipe Couplings - Generally Avoided B. Forged Steel Nozzles

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Flared Nozzles

For high pressure, high temperature, cyclic, and other critical services, flared nozzles as shown on the following slide are often used. They– Separate the mechanical and thermal stress zones– Remove mechanical stresses from the heat affected

zone at the weld– Improve welding access and permit the weld to be

radiographed– “Smooth” the stress transition from the heavy nozzle

forging to the shell– Are, however, expensive

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1

2

3

4

Flared Nozzles

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Nozzle opening reduces the shell strength

Replace cross-sectional area of metal removed

Available reinforcement includes excess shell and nozzle thickness

Limits of effective reinforcement defined by the code

Nozzle Reinforcement

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Nozzle Reinforcement

Additional reinforcement may be provided by surface pads, insert plates, thickened full or partial shell courses, or thickened nozzle necks (integrally reinforced)

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Section A-A

Vent hole

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Nomenclature and Formulasfor Reinforced Openings

rn

2.5t or 2.5tn + teUse Smaller Value

2.5t or 2.5tn Use Smaller Value

t c

h

tn

trn

tr

d

te

d or Rn + tn + tuse larger value

d or Rn + tn + t

Reinforcement zone

Dp

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A = Reinforcement area required

A1 = Area available in shell

A2 = Area available in outer nozzle

A3 = Area available in inner nozzle

A4 = Area available in welds

A5 = Area available in pad (if required)

A1 + A2 + A3 + A4 + A5 (if required) ≥ A

Opening Reinforcement

(Special Code rules apply for closely spaced nozzles)

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Computer Programs

Vessel designs are calculation intensive, and require attending to many details and Code requirements. Computer programs ease the “number crunching” burden on the engineer and accommodate Code requirements. Two programs are– COMPRESS– VCESage

Specialized finite element programs are used for detailed local analysis

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Pressure Testing

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Pressure Testing

In this section the basic Code requirements for pressure testing are outlined.

The attendee will understand the methods of pressure testing, and their strengths and weaknesses, and will also be able to determine the required test pressure for given design conditions.

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Pressure Testing

Pressure testing is required by the ASME Code

Testing confirms the vessel integrity by stressing it to a level greater than it will see in operation

Testing to be performed after all fabrication, welding, and heat treatment is completed– Testing should occur prior to any painting or priming

Testing to be observed by the authorized inspector

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Pressure Testing

Test pressure may be based upon either– The design pressure (this is the normal method)– MAWP of the full, corroded or uncorroded

thickness

Two types of pressure are accepted by the Code:– Hydrostatic– Pneumatic

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Hydrostatic Pressure Testing

Vessel is filled with water and pressured to the required valueSection VIII Division 1 minimum required test pressure at all locations = 1.3 • DP • SC/SH

Use the lowest SC/SH ratio (This factor increases the level of stress during test to the relative level seen in operation)Recommended test temperature is 30ºF (17ºC) over MDMT– Temperature is of the metal, not the test fluid– Addresses the potential for brittle fracture

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Test is safer due to incompressibility of water (or other fluid)– Little energy is stored in the test fluid under

pressureEasy to see and detect leaks; large water molecule may not reveal some small openingsMay add a dye or luminescent material to see leaks

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Adequate supply of suitable water may be difficult to obtain– For example, where stainless steel is present,

chlorides are limited to 50ppm

Some environments and internals (e.g. refractory) may make hydrostatic testing undesirable

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Pneumatic Pressure Testing

Test pressure is provided by compressing air or another gasSection VIII Division 1 minimum required test pressure at any point = 1.1 • DP • SC/SH – As with hydrostatic testing, pressure may be

based upon the design pressure or the full corroded or uncorroded thickness

– Use the lowest SC/SH ratioMetal test temperature must be at least 30°F (17ºC) over the MDMT

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Very dangerous due to stored energy in the compressed gas

Heat of compression, and subsequent cooling, may mean a loss of test pressure

Existence of a leak may be detected by a loss of (i.e. difficulty maintaining) internal pressure

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May be difficult to see leak location—colored smoke sometimes added

No extra weight or hydrostatic pressure to consider

Does not damage refractory or impact the process

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Exercise

What is the required hydrotest pressure for the following vessel?– Material = ASME SA 516 Grade 65– Design Pressure = 500psig– Design Temperature = 800ºF

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Evaluation, Repair, and Alteration

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Evaluation, Repair, and Alteration

This section introduces the concepts, common means, and Codes applicable to repairs, alterations, and the evaluation of damage to pressure vessels.

The attendee will develop an understanding of the requirements applicable to repairs and alterations and will be aware of the means of evaluating pressure vessels for continued service.

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Evaluation

If damage (e.g., corrosion, thinning, cracks, bulges) is detected or metallurgical damage is suspected (e.g., cyclic or creep range operation), an evaluation for continued service may be required

Some damage may not require repair– Isolated pitting corrosion– Local thin areas if

• It is shallower than the corrosion allowance• There are no cracks, sharp corners or other stress

raisers (e.g., it’s not near a nozzle)– Thin areas deeper than the corrosion allowance may not

require repair if there are no cracks, sharp corners or stress raisers

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Evaluation

Significant damage requires repair or, alternatively, a fitness for service (FFS) evaluation

– The evaluation may determine that repair or replacement is not required, a cost and downtime savings

Consider a formal fitness for service evaluation, especially if the vessel

– operated in the creep range– is deformed– has significant corrosion damage– experienced operational upsets including overpressure or

overheating– was subjected to cyclic loading– has been damaged (e.g. cracks) or deformed (e.g. bulges)

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Evaluation

API 579 “Fitness for Service”, provides a basis for evaluation of– Brittle fracture (Section 3)– General metal loss (Section 4)– Local metal loss (Section 5)– Pitting corrosion (Section 6)– Blisters and laminations (Section 7)– Weld misalignment and shell distortion (Section 8)– Crack-like flaws (Section 9)– Creep range operation (Section 10)– Fire damage (Section 11)

API 579 considers three levels of evaluation– Level 1 – Uses simple, conservative criteria with a minimum of inspection

and component date. Performed by plant inspector or engineer.– Level 2 – uses the same input as level 1, but requires a more detailed

analysis. Performed by plant engineer of FFS expert.– Level 3 – A detailed, precise, evaluation, requiring detailed inspection

and component data. Performed by an FFS specialist.

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Evaluation

FOS calculations and their interpretation are performed by specialists using software tools such as PREFIS.

For service in the creep range, a remaining life evaluation is necessary as a minimum. The evaluation is performed by a specialist using OMEGA. The older Larsen-Miller life fraction approach may also be sufficient in some situations.

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Exercise

In each of the following cases, what is your initial evaluation of the flaw? In each case the flaw has no sharp corners or cracks.

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Scope of the familiar design codes covers new construction only– For repairs and alterations (revamps), other documents

governAs with codes for new construction, the applicable document depends upon local laws and regulationsTwo common documents are:– NB23 - National Board Inspection Code– API 510 - Pressure Vessel Inspection Code, Maintenance,

Inspection, Rating, Repair, and Alteration

Repairs and Alterations

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Determine if vessel changes (including new design conditions) qualify as a repair or an alteration– Use the applicable Code, e.g., NB-23

• Repairs are restoration to original condition using original fabrication methods

• Alterations are changes in the pressure containing shell

– Requirements applicable to alterations are more stringent than for a repair


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All modifications must be designed and performed in accordance with the governing Codes.Design should be performed by an experienced, licensed, professional engineer.All work to be observed and approved by the Authorized Inspector.Vessel is registered with, for example, the National Board. Complete National Board form R-1 for repairs and R-2 for alterations (see Appendix D).

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Rerates, i.e., evaluation for more severe operation condition(s) are a special form of alteration– Suitability for service is determined per the original code

of construction.– Suitability of the materials for the intended atmosphere

must be checked, even if it has not changed.– Thoroughly inspect the vessel by both visually and

nondestructive means.– Perform a complete metallurgical evaluation to determine

the present metallurgical condition if damage is suspected (e.g., creep, fatigue, embrittlement, etc). Removal and testing of samples may be necessary.

– Use of the new (often greater) allowable stresses may be permitted if• The vessel was built after 1968• Impact test (Charpy V-notch) data is available

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Repair methods include– Complete removal of shallow cracks, leaving a

thin area if the remaining thickness is sufficient. Contour all surfaces and junctions.

– Local thin areas and areas where deeper cracks were removed may be restored by weld buildup. PWHT may be necessary.

– Replace shell sections.• Corners to be contoured• Remaining shell to be supported and stable

during replacement• PWHT may be required

– Other repair methods (e.g., overlay plates) as described in the Piping and Valve seminar.

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Proper fabrication methods must be used for the alteration, considering that the vessel has been in service– More care may be needed to prevent damage (e.g.

maintenance of proper pre, during, and post-weld heat temperatures, sequence of welding, dehydrogenization, existence of coke)

Thoroughly inspect and possibly test the modifications

If the vessel is relocated, or the length/diameter is increased, review wind and earthquake

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FCC Repair

• Replacement of FCC cyclones, with the vessel top head, may qualify as a repair

• Cut and reweld to be in the cylindrical portion to ease fit up

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Questions

Pvs

Documents

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