139124533 Pressure-Vessels.pdf

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Civil and Structural For additional information on this subject, contact File Reference: CSE-110.02 PEDD Coordinator on 874-6556

Engineering Encyclopedia Saudi Aramco DeskTop Standards

ASME PRESSURE VESSELS

Engineering Encyclopedia Analysis and Design of Tanks, Vessels and Piping

ASME Pressure Vessels

Saudi Aramco DeskTop Standards i

MODULE COMPONENT PAGE

INTRODUCTION............................................................................................................. 7

IDENTIFYING TYPES, COMPONENTS, AND USES OF PRESSURE VESSELS........................................................................................................................ 8

Background .......................................................................................................... 8 Vertical Pressure Vessels..................................................................................... 8

Components .............................................................................................. 8 Towers/Columns........................................................................................ 9 Vertical Drums ......................................................................................... 11 Vertical Reactors ..................................................................................... 12

Horizontal Pressure Vessels............................................................................... 13 Horizontal Drums ..................................................................................... 13 Shell-and-Tube Heat Exchangers ............................................................ 14

Spherical Pressure Vessels................................................................................ 15 Spherical Reactors................................................................................... 15 Spherical Storage Vessels....................................................................... 16

Uses ................................................................................................................... 17 Towers ..................................................................................................... 17 Drums ...................................................................................................... 17 Reactors .................................................................................................. 17 Shell-and-Tube Heat Exchangers ............................................................ 17 Spherical Storage Vessels....................................................................... 18

IDENTIFYING APPLICABLE CODES AND STANDARDS FOR PRESSURE VESSELS...................................................................................................................... 19

ASME, Section VIII, Boiler and Pressure Vessel Code ...................................... 20 Division 1 ................................................................................................. 20 Division 2 ................................................................................................. 20

API Standard 510, Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair and Alteration ........................................................... 21 Standards of the Tubular Exchanger Manufacturers Association ....................... 21



Saudi Aramco DeskTop Standards ii

API Standard 660, Shell-and-Tube Heat Exchangers for General Refinery Services ............................................................................................... 21 SAES-A-004, Pressure Testing .......................................................................... 21 SAES-B-057, Safety Requirements: Refrigerated and Pressure Storage Vessels ................................................................................................. 22 SAES-C-001, Design Criteria - Columns ............................................................ 22 SAES-D-001, Design Criteria for Pressure Vessels ........................................... 22 SAES-D-004, Sizing of Unfired Pressure Vessels .............................................. 22 SAES-E-001, Basic Design Criteria for Unfired Heat Transfer Equipment .......................................................................................................... 23 32-SAMSS-004, Pressure Vessels..................................................................... 23

DISTINGUISHING AMONG TYPES, MECHANICAL PROPERTIES, AND ALLOWABLE STRESSES OF STEELS THAT ARE USED FOR PRESSURE VESSELS...................................................................................................................... 24

Types of Steels................................................................................................... 24 Mechanical Properties of Steels ......................................................................... 26 Allowable Stress ................................................................................................. 26 Corrosion Resistance ......................................................................................... 35 Toughness.......................................................................................................... 35 Fabricability ........................................................................................................ 36

Post-Weld Heat Treatment ...................................................................... 36

CALCULATING CIVIL/MECHANICAL LOADS ON PRESSURE VESSELS.................. 38

Thickness Estimates For Vessel Shell Components .......................................... 39 Dead Weight....................................................................................................... 41 Hydrostatic Test Weight ..................................................................................... 43 Pressure: Design Versus Operating .................................................................. 45

Background.............................................................................................. 45 Saudi Aramco Standards ......................................................................... 45 Procedure ................................................................................................ 45

Temperature: Design versus Operating............................................................. 47 Background.............................................................................................. 47 Saudi Aramco Standards ......................................................................... 47 Procedure ................................................................................................ 48



Saudi Aramco DeskTop Standards iii

Wind ................................................................................................................... 49 Background.............................................................................................. 49 Saudi Aramco Standards ......................................................................... 51 Formulas.................................................................................................. 51 Transverse Wind Loading ........................................................................ 52

Earthquake ......................................................................................................... 56 Earthquake Loads.................................................................................... 56 Background.............................................................................................. 56 Saudi Aramco Standards ......................................................................... 56 Formulas.................................................................................................. 57

Appurtenances ................................................................................................... 59

IDENTIFYING MECHANICAL CONSIDERATIONS FOR STRUCTURAL SUPPORT AND/OR ATTACHMENTS TO PRESSURE VESSELS .............................. 60

Background ........................................................................................................ 60 Temperature ....................................................................................................... 60 Skirt Support....................................................................................................... 61 Support Legs ...................................................................................................... 62 Support Lugs ...................................................................................................... 65 Saddle Supports ................................................................................................. 66 Support Structures.............................................................................................. 67 Structural Attachments ....................................................................................... 68

Considerations Regarding Repairs and Alterations to Pressure Vessels .................................................................................................... 69

IDENTIFYING VARIOUS TYPES OF FOUNDATIONS THAT ARE USED FOR PRESSURE VESSELS......................................................................................... 70

General............................................................................................................... 70 Spread Footings ................................................................................................. 71 Mat Foundations................................................................................................. 73 Concrete Ring Foundations ................................................................................ 74 Pile Foundations With a Concrete Cap............................................................... 75

CALCULATING ANCHOR BOLT STRESSES AND BASEPLATE WIDTH AND THICKNESS FOR PRESSURE VESSELS, GIVEN A SPECIFIC LOADING ...................................................................................................................... 77



Saudi Aramco DeskTop Standards iv

Background ........................................................................................................ 77 General Rules..................................................................................................... 77 Anchor Bolts ....................................................................................................... 77 Baseplates.......................................................................................................... 79 Summary ............................................................................................................ 81

WORK AID 1: PROCEDURES AND REFERENCE MATERIAL FOR CALCULATING LOADS ON PRESSURE VESSELS............................ 82

Work Aid 1A: Procedure for Calculating the Dead Weight ................................. 82 Work Aid 1B: Procedure for Calculating Hydrostatic Weight.............................. 84 Work Aid 1C: Procedure for Calculating Design Pressure Based on

Operating Pressure...................................................................... 87 Work Aid 1D: Procedure for Calculating Design Temperature Based

on Operating Temperature........................................................... 88 Work Aid 1E: Procedure and Reference Material for Calculating Wind

Loading........................................................................................ 89 Work Aid 1F: Procedure and Reference Material for Calculating

Earthquake Loading..................................................................... 92 Work Aid 1G: Procedure for Calculating Loads from Insulation and

Appurtenances............................................................................. 97 Insulation ................................................................................................. 97 Appurtenances......................................................................................... 99

WORK AID 2: PROCEDURES AND ADDITIONAL INFORMATION FOR CALCULATING ANCHOR BOLT STRESSES AND BASEPLATE WIDTH AND THICKNESS FOR PRESSURE VESSELS, GIVEN A SPECIFIC LOADING......................................... 100

Work Aid 2A: Procedure and Additional Information for Calculating Anchor Bolt Requirements ......................................................... 100

Work Aid 2B: Procedure and Additional Information for Calculating Baseplate Requirements for Support Skirts ............................... 104

GLOSSARY ................................................................................................................ 108



Saudi Aramco DeskTop Standards v

LIST OF FIGURES

Figure 1. Tower ............................................................................................................ 10 Figure 2. Vertical Drum ................................................................................................ 11 Figure 3. Vertical Reactor............................................................................................. 12 Figure 4. Horizontal Drum ............................................................................................ 13 Figure 5. Shell-and-Tube Heat Exchanger ................................................................... 14 Figure 6. Spherical Reactor.......................................................................................... 15 Figure 7. Spherical Storage Vessel .............................................................................. 16 Figure 8. Acceptable Material for Carbon and Low-Alloy Steel Vessels....................... 25 Figure 9a. Maximum Allowable Stress Values S For Ferrous Materials...................... 27 Figure 10. Dead And Hydrostatic Test Weight Sample Problems ................................ 43 Figure 11. Wind Force on Tall Vessel .......................................................................... 49 Figure 12. Wind Force on Horizontal Vessel ................................................................ 50 Figure 13. Wind Sample Problem Tower...................................................................... 54 Figure 14. Earthquake Loading on a Simple Tall Tower............................................... 57 Figure 15. Support Skirts.............................................................................................. 61 Figure 16. Angle Supports for Small Vessels ............................................................... 63 Figure 17. Orientation of I Supports for Large Vessels ................................................ 63 Figure 18. Loads on Support Legs ............................................................................... 64 Figure 19. Support Lugs............................................................................................... 65 Figure 20. Saddle Supports.......................................................................................... 66 Figure 21. Support Clips............................................................................................... 68 Figure 22. Spread Footings And Pedestals.................................................................. 72 Figure 23. Mat Foundation and Pedestals.................................................................... 73 Figure 24. Concrete Ring Foundation .......................................................................... 74 Figure 25. Pile Foundations With Concrete Cap .......................................................... 76 Figure 29. Height and Gust Correction Factors............................................................ 90 Figure 30. Shape Factor............................................................................................... 91 Figure 31. Effective Diameter, De ................................................................................ 91



Saudi Aramco DeskTop Standards vi

Figure 32. Site Soil Coefficients ................................................................................... 93 Figure 33. Typical Insulation Densities......................................................................... 97 Figure 34. Number of Anchor Bolts ............................................................................ 101 Figure 35. Allowable Anchor Bolt Stress .................................................................... 102 Figure 36. Anchor Bolt Data ....................................................................................... 103 Figure 37. Allowable Bearing Stress for Concrete...................................................... 105 Figure 38. Baseplate Dimensions............................................................................... 106



Saudi Aramco DeskTop Standards 7

INTRODUCTION

CSE 110.02, ASME Pressure Vessels, provides civil and mechanical engineers with an overview of the civil and mechanical engineering aspects that govern the analysis and design of pressure vessels.

CSE 110.02 identifies various pressure vessels, their components and uses, and applicable codes and standards. It identifies the common types of steels that are used to construct pressure vessels, their mechanical properties, and the allowable stresses. It also describes and demonstrates how to calculate the loads on pressure vessels. In addition, it identifies the mechanical considerations for additions to or modifications of attachments to pressure vessels; it identifies the types and uses of foundations for pressure vessels; and it describes and demonstrates how to calculate the anchor bolt stresses and baseplate width and thickness for pressure vessels, given a specific loading.




IDENTIFYING TYPES, COMPONENTS, AND USES OF PRESSURE VESSELS

Background

This section discusses the following primary types and components of pressure vessels:

• Vertical • Horizontal • Spherical

This section concludes with a description of the uses of specific types of pressure vessels.

Pressure vessels are pressurized containers that are used in all stages of processing in the petroleum and petrochemical industry. Typical pressures for Saudi Aramco applications are between 103 and 21,000 kPa (15 and 3,000 psig). The pressure for most vessels is less than 7,000 kPa (1,000 psig). Temperatures range from -29°C to 815°C (-20°F to 1,500°F). The majority of vessels have temperatures less than 537°C (1,000°F). Pressure vessels are supplied in a wide range of sizes to meet the application needs. Some large refinery vessels are greater than 10 m (30 ft.) or more in diameter and greater than 60 m (200 ft.) high.

Pressure vessels accomplish fluid separations, chemical conversions, and pressurized storage. These processes are described further in later paragraphs for specific pressure vessel types.

Vertical Pressure Vessels

This section discusses the following types of vertical pressure vessels:

• Towers/columns • Drums • Reactors

Components

A vertical pressure vessel consists of the following parts:

• Top head • Shell




• Bottom head • Support skirt

Towers/Columns

A tower or column is a slender vessel (its height is greater than its diameter). A tower may have multiple diameters. Figure 1 illustrates a tower and its primary components. Normally, pressures are not very high, but temperatures can reach 400°C (750°F). The temperature depends upon the use of the tower.




Nozzle

Nozzle

Nozzle

Nozzle

Shell

Shell

Tray

Tray

Head

Head

Skirt support

Cone

Figure 1. Tower




Vertical Drums

Vertical drums are small pressure vessels of a single diameter. They are located at grade and may be supported by legs, lugs or a skirt. Figure 2 illustrates a vertical drum and its primary components. The drum length-to-diameter ratio is usually less than 5. The distinction between tall towers and vertical drums is the diameter-to-length ratio of five.

Head

Head

Shell

Nozzle

Support leg

Figure 2. Vertical Drum




Vertical Reactors

Vertical reactors are cylindrical in shape and contain one or more internal catalyst beds. Figure 3 illustrates a vertical reactor and its primary components. The lowest catalyst bed is supported from the bottom head, and any upper beds are supported on a steel grid structure.

Inlet nozzle

Outlet nozzle

Head

Head

Lower catalyst bed

Upper catalyst bed

Support skirt

Outlet collector

Shell

Catalyst bedsupport grid

Inert fill

Figure 3. Vertical Reactor




Horizontal Pressure Vessels

This section describes the following types of horizontal pressure vessels:

• Horizontal Drums • Shell-and-tube heat exchangers

Horizontal Drums

Horizontal drums have a wide range of diameters and lengths. Some horizontal drums are up to 4.25 m (14 ft.) in diameter and over 30 m (100 ft.) long. A drum consists of the following parts:

• A shell • Two heads • Saddle supports

Figure 4 illustrates a horizontal drum and its primary components.

Nozzle

Head Head

A

AShell

Saddle support

Section A-A

Figure 4. Horizontal Drum




Shell-and-Tube Heat Exchangers

A shell-and-tube heat exchanger is a special purpose pressure vessel. It consists of the following parts:

• A shell • Heads • A tube bundle • Saddle supports

Figure 5 illustrates a shell-and-tube heat exchanger and its primary components.

Lifting lug

Stationary head nozzle

Pass partition

Shell nozzle

Tubes

Shell

Tie-rods and spacers

Transverse baffles or support plates

Vent connection

Drain connection

Support saddle

Support saddle

Channel cover Tubesheet

Floating head

Shell coverChannel

Source: © 1988 by Tubular Exchanger Manufacturers Association.

Figure 5. Shell-and-Tube Heat Exchanger




Spherical Pressure Vessels

Spherical pressure vessels are used for pressurized storage or as reactors. A spherical pressure vessel is a hollow sphere with nozzles, internal components, and support based on the application.

Spherical Reactors

Typically, spherical reactors have only one catalyst bed supported directly from the vessel shell. Figure 6 illustrates a spherical reactor and its primary components.

Inlet nozzle

Shell

Outlet collector

CatalystSupport skirt

Outlet nozzle

Thermowell nozzle

Manway

Catalyst pump nozzle

Catalyst sampler

Figure 6. Spherical Reactor




Spherical Storage Vessels

Spherical storage vessels are designed as pressure vessels when the design pressure exceeds 103 kPa (15 psig). Spherical pressure storage vessels are supported on legs, with cross-bracing for increased stability under wind and earthquake loading conditions. Figure 7 illustrates a spherical storage vessel and its primary components.

Shell

Support leg

Cross bracing

Figure 7. Spherical Storage Vessel




Uses

Towers

Towers separate hydrocarbon streams into different fractions that are required at other points in the process. Frequently, the hydrocarbons are separated based on the different boiling points of the hydrocarbon fractions. Trays or packing materials throughout the height of the tower control flow distribution and velocity to aid the separation process. Nozzles located along the length of the tower extract the fluid at particular elevations.

Drums

Either horizontal or vertical drums are used when fluid separation is required and when there is a small volume storage application. The selection of a horizontal or a vertical drum depends on the process requirements. For example, a drum may be required to separate two liquids of different densities or a vapor from a liquid. Some drums are used as filters, and others are used as volumes to absorb liquid flow surges from another part of the process system.

Reactors

A chemical reaction takes place inside a reactor in the presence of a catalyst. A chemical reaction converts one hydrocarbon form into another hydrocarbon form that is required at a later stage of the processing operation. Depending on the process, operating temperatures can approach 537°C (1,000°F) at pressures over 7,000 kPa (1,000 psig). Spherical reactors perform similar functions as vertical reactors. The selection of a spherical reactor or a vertical reactor is based on process and volume considerations.

Shell-and-Tube Heat Exchangers

A shell-and-tube heat exchanger is a special purpose pressure vessel that transfers heat from one fluid to another. The exchanger may warm or cool a liquid or gas. Most shell-and-tube heat exchangers are oriented horizontally, but they can be oriented vertically for some applications.




Spherical Storage Vessels

Spherical storage vessels at atmospheric temperature store hydrocarbon liquids under pressure. The liquid may be the result of an intermediate refining step or a final product. The pressure in the vapor space above the liquid in the sphere results from either the vapor pressure of the liquid at ambient temperature or pressurization from an outside source.




IDENTIFYING APPLICABLE CODES AND STANDARDS FOR PRESSURE VESSELS

This section summarizes the scope of the following codes and standards that apply to pressure vessels:

• ASME, Section VIII, Boiler and Pressure Vessel Code

• API Standard 510, Pressure Vessel Inspection Code: Maintenance, Inspection, Rating, Repair and Alterations

• Standards of the Tubular Exchanger Manufacturers Association (TEMA)

• API Standard 660, Shell-and-Tube Heat Exchangers for General Refinery Services

• SAES-A-004, Pressure Testing

• SAES-B-057, Safety Requirements: Refrigerated and Pressure Storage Vessels

• SAES-C-001, Design Criteria - Columns

• SAES-D-001, Design Criteria for Pressure Vessels

• SAES-D-004, Sizing of Unfired Pressure Vessels

• SAES-E-001, Basic Design Criteria for Unfired Heat Transfer Equipment

• 32-SAMSS-004, Pressure Vessels




ASME, Section VIII, Boiler and Pressure Vessel Code Division 1

Division 1 of Section VIII of the ASME Code provides requirements for the design, fabrication, inspection, and testing of pressure vessels, except for the following:

• Fired process tubular heaters • Pressure containers that are integral parts of

rotating or reciprocating mechanical devices. • Piping systems and piping components • Vessels containing water under pressure to serve

strictly as a cushion • Vessels that have an internal or external operating

pressure less than 103 kPa (15 psig). • Vessels that have an inside diameter, width, height,

or cross-section diagonal less than 150 mm (6 in.). • Vessels for human occupancy

Division 1 applies to vessels with pressures greater than 103 kPa (15 psig), but less than 20,685 kPa (3,000 psig). For vessels with pressures greater than 20,685 kPa (3,000 psig), Division 1 provides the basic rules, but deviations from and additions to the rules of Division 1 are usually necessary.

Division 2

Division 2 of Section VIII of the ASME Code is an alternative to the minimum construction requirements of Division 1. The allowable design stress for a Division 2 pressure vessel is greater than a Division 1 vessel, and usually results in thinner components. However, Division 2 requires more stringent design standards for components. These more stringent standards include the following:

• More precise stress calculations • More stringent quality control, fabrication, and

inspection requirements for materials • Additional restrictions on permissible design details

A Division 2 design becomes economically attractive for higher design pressure especially when alloy material is required. In these cases, the reduced costs of the smaller quantity of material is greater than the increased costs of the areas where Division 2 is more stringent.




API Standard 510, Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair and Alteration

This standard covers requirements for inspection, rerating, repair, and alteration of pressure vessels that are used by the petroleum and chemical industry. The standard applies to pressure vessels that have been designed, fabricated, inspected, and tested in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2. However, this Code does not apply to vessels until they have been put into service.

Standards of the Tubular Exchanger Manufacturers Association

The Standards of the Tubular Exchange Manufacturers Association (TEMA) provide recommendations for specification, design, fabrication, inspection testing and installation of shell-and-tube heat exchangers.

API Standard 660, Shell-and-Tube Heat Exchangers for General Refinery Services

API Standard 660 provides requirements for shell-and-tube heat exchangers. These requirements supplement the TEMA standards. The requirements cover requirements for design, fabrication, inspection and testing.

SAES-A-004, Pressure Testing

SAES-A-004 provides the pressure testing requirements and procedures for all plant equipment and piping that is subject to pressure or vacuum from fluid, gas, or air. This standard applies to both new and existing equipment.




SAES-B-057, Safety Requirements: Refrigerated and Pressure Storage Vessels

SAES-B-057 provides the design and safety requirements for:

• Low-pressure dome roof tanks that store refrigerated liquefied gases up to 17 kPa (2.5 psig) design pressure

• Spheres and spheroids that store flammable materials with design pressures equal to and greater than 17 kPa (2.5 psig)

• Horizontal pressure drums that store NGL with design pressures equal to or greater than 69 kPa (10 psig)

This standard provides the requirements either specifically or by reference for the following subjects:

• Foundations and grading • Diking and drainage • Tank and vessel spacing

SAES-C-001, Design Criteria - Columns

SAES-C-001 provides mandatory requirements either specifically or by reference for the process design and installation of trayed and packed columns or towers.

SAES-D-001, Design Criteria for Pressure Vessels

SAES-D-001 provides additional requirements for vessels covered by ASME, Section VIII. This standard applies to all pressure vessels except when both the vessel's design pressure is less than 1,380 kPa (200 psig) and its volume is less than 2 cu meters (70 cu. ft.).

SAES-D-004, Sizing of Unfired Pressure Vessels

SAES-D-004 establishes design requirements for preparation of process specifications for vessels.




SAES-E-001, Basic Design Criteria for Unfired Heat Transfer Equipment

SAES-E-001 provides requirements for the design of the following heat transfer equipment:

• Air-cooled heat exchangers • Double-pipe heat exchangers • Shell-and-tube heat exchangers

32-SAMSS-004, Pressure Vessels

32-SAMSS-004 specifies additional requirements for the purchase of vessels that are designed to meet the ASME code and is typically a part of the pressure vessel purchase order. This specification excludes vessels with both a design pressure less than or equal to 1,380 kPa (200 psi) and a volume less than or equal to 2 cu. meters (70 cu. ft.).

32-SAMSS-004 specifies requirements for internal appurtenances, preparation for shipment, vendor responsibilities, and modifications to Section VIII of the ASME Code. It lists acceptable materials that are based on the type of component and the vessel service classification. While the listed materials are suitable for a wide range of applications, other material types may be required for particular applications. When appropriate, the vessel vendor may also propose alternative materials for Saudi Aramco consideration.




DISTINGUISHING AMONG TYPES, MECHANICAL PROPERTIES, AND ALLOWABLE STRESSES OF STEELS THAT ARE USED FOR PRESSURE VESSELS

This section describes the types of steels that are used for pressure vessels. It also provides information on their mechanical properties and allowable stresses of these steels.

Types of Steels

Saudi Aramco Materials System Specification 32-SAMSS-004 specifies the acceptable “first choice” material specifications. Figure 8 lists these acceptable materials according to type of component and type of service. Particular applications may require other material types that are not listed. When appropriate, the vessel vendor may also propose alternative materials for Saudi Aramco consideration. In either of these cases, Saudi Aramco must review and approve the material proposed.




VESSEL SERVICE CLASSIFICATION

GENERAL SERVICE

HIGH TEMPERATURE

SERVICE

LOW TEMPERATURE

SERVICE

WET SOUR

SERVICE

VESSEL COMPONENT

0° to 350°C (32° to 650°F) Note (1)

351° to 455°C (651° to 850°F) Note (1)

0° to -46°C (+32° to -50°F)

Note (1)

-47° to -101°C (-51° to -150°F)

Note (1)

To 205°C (400°F)

Note (1)+ Plate for Shells, Heads, Rolled Nozzles, Reinforcing pads, and Stiffeners

SA-516 SA-442

SA-515 +++ SA-387 Cl 1, Gr. 2

SA-516 SA-537 SA-662

SA-203, Gr. D SA-203, Gr. E

SA-516 (normalized) SA-737 SA-515

Pipe for Nozzles SA-53 Gr. B (seamless) SA-106, Gr. B

SA-106 SA-335, Gr. P11

SA-333, Gr. 6 SA-333, Gr. 3 SA-333, Gr ,1 or 6 SA-106, Gr. B

Flanges and Forgings

SA-105 SA-181, Gr. II

SA-105 SA-182, Gr. F11

SA-350-LF2 SA-350-LF3 SA-105 SA-350-LF1 or LF2

Fittings++ SA-234 WPB SA-234 WPB or WP11

SA-420, Gr. WPL6 SA-420, Gr. WPL3 SA-234 WPB or WPC

Bolts Nuts

SA-193-B7 SA-194-2H

SA-193-B7 or B16 SA-194-2H or 6

SA-320-L7 SA-194, Gr. 2H

SA-320, Gr. L43 SA-194, Gr. 7

SA-193, Gr. B7MSA-320, Gr. L7MSA-194, Gr. 2HM

Supports and attachments

SA-283, Gr. C Note (2) SA-285, Gr. C Note (2) SA-36 Note (3)

External: Note (2) SA-283, Gr. C SA-36 Note (3)

SA-516 SA-537 SA-662 Note (2)

SA-203, Gr. D SA-203, Gr. E Note (2)

External: SA-283, Gr. C Note (2) SA-36 Note (3) SA-285, Gr. C Note (2)

Internal: Note (2) SA-285, Gr. C SA-516 SA-387, Cl. 1, Gr. 2

Internal: Note (2)A-576, Gr 1018-1025 Note (4) Shell Plate

+ Sour service above 205°C (400°F) is not within scope of this Specification. ++ Grade of material must be the same classification as pipe and plate for the indicated service. +++ Avoid prolonged exposures to temperature above 425°C (800°F), as the carbide phase of carbon steel may be

converted to graphite. NOTES: (1) These temperatures are limiting design temperatures and are not operating temperatures. (2) That section of attachments extending 305 mm(12 in) or less from the shell head or pressure containing part of any

Division 2 pressure vessel or low temperature service vessel shall be of the same material as the item to which it is attached. Beyond the 305mm(12 in) or any attachments to Division 1 pressure vessels, the material may be as shown in this figure.

(3) Shall not be welded directly to shell. (4) Nonresulfurized, special quality only. Merchant quality ("M" grades) not permitted.

Figure 8. Acceptable Material for Carbon and Low-Alloy Steel Vessels




Mechanical Properties of Steels

In selecting the steel for a pressure vessel, the following qualities of the steel are important:

• Allowable stress

• Corrosion resistance

• Toughness

• Fabricability

Allowable Stress

Figures 9a and 9b are from the ASME Code Section II-D and give the maximum allowable tensile stress at specific temperatures of some carbon and low-alloy steel plate specifications. Older editions of the ASME Code Section VIII, Division 1 or 2, contain similar tables for allowable stress, but for new construction the latest code values should be used. The maximum allowable longitudinal compression stress cannot be greater than the maximum allowable tensile stress, but it can be less than the maximum allowable tensile stress, depending upon the situation. The material allowable stress is based on various measures of the material's strength properties. These properties include its yield strength, tensile strength, creep and rupture resistance. To ensure that the material does not fail, appropriate safety factors are applied to these strength properties to give the allowable stresses. As the temperature increases, both the allowable stress and the related material strength decrease.




Table 1A 1995 SECTION II

Source: ASME Boiler and Pressure Vessel Code, Section II-D, 1993 Addendum, with permission

from the American Society of Mechanical Engineers. pp. 18.

Figure 9a. Maximum Allowable Stress Values S For Ferrous Materials




PART D - PROPERTIES Table 1A



Figure 9a. Maximum Allowable Stress Values S For Ferrous Materials, Cont'd







Figure 9b. Maximum Allowable Stress Values S For Ferrous Materials, Cont'd




Corrosion Resistance

Corrosion deteriorates metals by chemical action. Corrosion is probably the single most important factor in material selection. A slight change in the chemical composition of the environment, such as temperature, can significantly change the corrosion resistance of a particular metal. As the temperature increases, the corrosion rate increases. Corrosion rates for various metals in well-known process environments are based on experience. Laboratory tests can determine corrosion rates for new process environments. Typically, a “corrosion allowance” for a pressure vessel is based on the anticipated corrosion rate and vessel design life. The corrosion allowance is added to the vessel component required thicknesses for the specified load conditions to give the total required vessel thickness. The corrosion allowance compensates for the metal thinning that occurs during operation.

Toughness

The toughness of a material is its ability to resist brittle fracture. Pressure vessel components constructed of ferrous material occasionally have failed at pressures well below their design pressure. These failures generally occur at low temperatures and these failures are brittle rather than ductile. A brittle fracture shows no leak or warning before failure and is characterized by a lack of deformation and yielding. A ductile failure is characterized by deformation and yielding before failure.

For a brittle fracture to occur, the following conditions must occur simultaneously at a location in a pressure vessel:

• The material must have an insufficient fracture toughness at that temperature.

• There must be enough stress in the component to cause a crack to initiate and grow.

• There must be a defect of critical size in the component, such as at a weld, to act as a local stress concentration point and site for crack initiation.

The brittle fracture occurs without warning the first time the component is exposed to the above combination of low temperature, high stress, and critical size defect. Materials for




low temperature service applications must be sufficiently tough to resist brittle fracture at the minimum design temperature.

Fabricability

A material with outstanding strength, corrosion resistance, and toughness is useless if it cannot be made into the pressure vessel component. For example, plate material must have sufficient ductility to permit rolling, and it must be weldable to allow assembly into the required shapes. Also, engineers must consider the effect of welding on material properties.

Welders must use welding procedures that ensure acceptable strength and quality of the welded joints. To maintain corrosion resistance, the material chemistry of the weld area must be equivalent to that of the base material. In all cases, the ASME Code requires that every welding procedure be specified in written form and be tested for acceptability. All welders must pass capability tests. Only capable welders using qualified welding procedures are permitted to fabricate ASME Code equipment.

Post-Weld Heat Treatment

Another consideration when selecting materials is the potential need for post-weld heat treatment (PWHT). PWHT heats the pressure vessel to a high temperature (after completing the welding) and holds it there for a specified time. Material specification and thickness determine the temperature and holding time required. PWHT might be required for one or more of the following reasons:

• Stress relief

• Hardness reduction

• Process considerations

Welding is done at extremely high temperatures. When the welding is completed, the weld and adjacent base material contract during cooling. This contraction causes stresses in the component because of constraint in the overall structure. To ensure that these stresses do not cause a vessel failure, the welds may require PWHT. The ASME Code contains rules that govern PWHT for this purpose.




The welding process may produce locally hard regions in the weld and adjacent area. These locally hard regions occur in materials with low chrome content. Such materials are less ductile and more prone to cracking. PWHT reduces these hard areas in certain materials and restores ductility. Since the ASME Code does not specify weld hardness requirements, engineers must specify PWHT for weld hardness reduction.

The last reason for PWHT is related to the previous one, but goes further. Some process environments, such as high caustic concentrations, may cause cracking at highly stressed welds in carbon steel. The residual stresses remaining after welding are sufficient to cause cracking in these environments. PWHT relieves the weld stresses to prevent cracking. Again, the ASME Code does not require PWHT for this purpose, and it must be specified by the user.




CALCULATING CIVIL/MECHANICAL LOADS ON PRESSURE VESSELS

This section describes and demonstrates how to calculate the civil/mechanical loads on pressure vessels.

This section covers the following types of loading:

• Dead weight • Hydrostatic test weight • Pressure: Design versus operating • Temperature: Design versus operating • Wind • Earthquake • Appurtenances (live load and dead load)

Saudi Aramco Materials System Specification 32-SAMSS-004 requires that the following loading conditions be used for designing pressure vessels, their supports, and associated foundations:

• Design wind load or seismic load, plus all dead loads excluding operating fluid. The design wind load is based on a wind velocity of 85 mph. This loading condition accounts for the situation where the vessel is totally erected, but is not yet in operation (no design pressure or design temperature) nor filled with the operating fluid. It is assumed that the vessel can be exposed to either the design wind velocity of 85 mph or earthquake conditions, but these will not occur simultaneously.

• Internal or external design pressure, plus total operating weight and design wind load or seismic load. This loading condition accounts for the normal operating condition of the vessel. The vessel is totally erected and is in operation at its normal design pressure and temperature with the operating fluid. It is assumed that the vessel can be exposed to either the design wind velocity of 85 mph or earthquake conditions, but these will not occur simultaneously.

• Test pressure, hydrostatic test weight, plus 30 mph wind. This loading condition accounts for the




situation where the vessel is being hydrostatically tested with water after erection in the field. The vessel may be exposed to a wind velocity of 30 mph during the hydrotest. It is assumed that a hydrotest would not be done if the wind velocity is higher than 30 mph, and that it would not be done at all during an earthquake.

Thickness Estimates For Vessel Shell Components

The ASME Code, Section VIII, Division 1, has many requirements and equations for designing a pressure vessel. These equations and requirements determine the minimum required thickness to withstand the pressures or forces acting on the vessel and to allow for corrosion in service. In this section, a simple equation is described that can estimate the thickness of common pressure vessel shell components. This equation is based on simplifying one of the ASME Code equations that determines the thickness of a cylindrical shell. Applicable Code equations and requirements should be used for definitive work. PEDP course MEX 202 covers the ASME Code pressure vessel design equations and requirements in greater detail.

In the approximate method, the minimum thickness of a shell component is determined from the following equation:

t =PDK2SE

+ CA

t = Minimum shell thickness which is based on design pressure, mm (in.)

P = Design pressure at the bottom of the shell section, kPa (psi)

D = Nominal diameter of the vessel, mm (in.)

K = 1.0 for cylindrical shell

0.5 for hemispherical head

1.0 for 2:1 elliptical head

S = Allowable stress for design conditions from the ASME Code Section IID, kPa (psi)

E = Weld Joint Efficiency, assume the following:




= 1.0 if full or 100% radiography is specified

= 0.85 for spot radiography, if specified or unknown

= 0.7 for no radiography (seldom specified)

CA = Corrosion allowance, mm (in.)

SI Note: 1 psi = 6.895 kPa; 1 in. = 25.4 mm

In the above equation, a corrosion allowance has been added to the minimum shell thickness that is required for design pressure. Corrosion allowance, CA, is part of the vessel design requirements, and is based on the expected corrosion that occurs during the life of the vessel.

The weld joint efficiency, E, accounts for the expected quality of the welded joints in the vessel shell. Specific values for weld joint efficiency are specified in the ASME Code, based on the weld joint details and the amount of specified radiographic inspection.

Sample Problem 1: Estimating Thicknesses of Pressure Vessel Components

In this example problem we will estimate the thicknesses of a cylindrical shell, an ellipsoidal head, and a hemispherical head due to internal pressure.

Given :

• A diameter of 8 ft. • A design pressure of 200 psig • A design temperature of 650°F • Material is A516, Gr 70 • Corrosion Allowance of .0625 in. • Spot radiography




Solution:

From Figure 9a for A516, Gr 70, S equals 17,500 psi.

For the cylindrical shell and ellipsoidal head:

t = PDK2SE + CA

t = 200( ) 96( ) 1.0( )2( ) 17, 500( ) .85( ) + 0.0625

= .7079 in.

For a hemispherical head:

t =200( ) 96( ) 0.5( )

2( ) 17,500( ) .85( ) + 0.0625

= .3852 in.

For the cylindrical shell and ellipsoidal head, a plate thickness equal to the next nominal thickness (3/4 inch) could be used for further estimating work.

For the hemispherical head, a plate thickness equal to the next nominal thickness (7/16 inch) could be used for further estimating work.

Dead Weight

The design engineer must design the foundation and support structure for the weight of the vessel and its contents, internals, and attachments.

Work Aid 1A provides the procedure for calculating the dead weight of a vessel.




Sample Problem 2: Dead Weight

Calculate the dead weight for the pressure vessel shown in Figure 10.

Given:

• Cylindrical shell: 2.5 in. thick, 10 ft. I.D., 70 ft. high • Hemispherical heads: 1.25 in. thick, 10 ft. I.D. • Skirt: 1.25 in. thick, 25 ft. high

Solution:

Use Work Aid 1A.

In Step 1:

Vshs = π 2( )d 2t

Vshs = π 2( )× 10( )2 × 1.25 12( )= 16.4 ft 3

Vsc = πdht

Vsc = π ×10 × 70 × 2.5 12( )= 458.1 ft 3

Vskirt = πdht

Vskirt = π ×10 × 25 × 1.25 12( )= 81.8 ft 3

In Step 2:

05/01/95

Vs= 2 � Vshs + Vsc + Vskirt

Vs= 2 � 16.4 + 458.1 + 81.8 = 572.7 ft3

In Step 3: Ws = Vs � �st

Ws= 572.7 � 490 = 280,623 lb.

Answer:

The dead weight of the pressure vessel is approximately 280,600 lb., excluding any internals or appurtenances.




10 ft I.D.

5 ft

70 ft

5 ft

10 ft I.D.

25 ft

Figure 10. Dead And Hydrostatic Test Weight Sample Problems

Hydrostatic Test Weight

When designing supports and foundations, the design engineer must consider the weight of the empty vessel and the maximum weight of its contents. Since petroleum products are typically lighter than water, the heaviest content load usually occurs during hydrostatic testing. The total weight must include the weight of all vessel internals, attachments, etc.

Work Aid 1B provides the procedure for calculating hydrostatic test weight.




Sample Problem 3: Hydrostatic Test Weight

Calculate the hydrostatic test weight for the pressure vessel shown in Figure 10.

Solution:

Use Work Aid 1B.

In Step 1:

Vhsphere = π / 12 d 3

Vhsphere = π / 12 × 10( )3 = 261.8 ft 3

Vcylinder = π / 4( )d 2h

Vcylinder = π / 4 × 10( )2 ×70 = 5,498 ft 3

In Step 2: Vv = 2 × Vhsphere + Vcylinder

Vv = 2 � 261.8 + 5,498 = 6,022 ft3

In Step 3: Ww = Vv × γw

Ww = 6,022 × 62.4 = 375,773 lb.

In Step 4: WH = Ws + Ww

WH = 280,600 + 375,773 = 656,373 lb. = 656, 400 lb.

Answer:

The hydrostatic test weight of the pressure vessel is approximately 656,400 lb., excluding any internals and appurtenances.




Pressure: Design Versus Operating

Background

When designing a pressure vessel, two pressure values may be indicated: the operating pressure and the design pressure. The operating pressure is the pressure expected during normal operation. The design pressure is the maximum pressure that the vessel is designed to withstand. The design pressure provides a safety margin to account for fluctuations that occur during normal operations. The design pressure of a vessel is based on the pressure at the top of the vessel. This pressure in most cases is equal to the safety valve set pressure. The safety valve protects the vessel from over pressure. If the vessel contains a liquid, the lower sections of the vessel must be designed for the top head design pressure plus an additional pressure due to the static head of liquid above the section being designed. Therefore, when reviewing contractor calculations, it is common to find that the bottom head and lower shell courses of a very tall vessel are designed for higher pressure than the top head or upper shell courses. In addition, the vessel may be required to be designed for a negative gage pressure or full vacuum, depending on the operation of the vessel. The design of large low pressure vessels may be governed by the full or partial vacuum case.

Saudi Aramco Standards

SAES-D-001 requires that vessels be designed for an internal pressure equal to the maximum operating pressure plus 103 kPa (15 psi) or 10% of the maximum operating pressure, whichever is greater. For vessels in vacuum service, the design must be based on an external pressure of 103 kPa (15 psi) or 25% more than the maximum external pressure, whichever is smaller. If a vessel can be steamed out during cleaning or precommissioning and it is not already designed for vacuum conditions, the vessel design must be based on an external pressure of 52 kPa (7.5 psi) at 150°C (300°F). Normally, the process design engineer uses the above bases when determining the design pressure and he sets the vessel (top head) design pressure. The mechanical design engineer then uses the top head design pressure plus the static head pressure to determine the design pressure at the bottom of each vessel section.

Procedure




Work Aid 1C provides the procedure for calculating the design pressure based on the expected operating pressure.

Sample Problem 4: Determine Design Pressures

Determine the design pressure at the top and bottom heads of a vessel with a maximum internal operating pressure of 75 psig, that is 100 ft. high and may be filled with oil having a specific gravity of 0.9. The vessel may also be steamed out prior to startup.

Solution: Use Work Aid 1C.

Step 1: Calculate the Top Head Pressure.

PDtop = Po + 15

Since Po < 150 psig; PDtop =75 + 15 = 90 psig

Step 2: Calculate the Static Head Pressure.

P sh = H ×γ w

C.F. × G

Psh = 100 × 62.4144

× 0.9 = 39 psi

Step 3: Calculate the Bottom Head Pressure.

PDbot = P Dtop + Psh

PDbottom = 90 + 39 = 129 psig

Step 4: Determine whether or not the vessel should be designed for external pressure. Since the vessel can be steamed out, the vessel should also be designed for 7.5 psi external pressure at 300 °F.

Answer:

The design pressure of the vessel must be at least 90 psig at the top head. The bottom head should be designed for a minimum of 129 psig. The entire vessel should also be designed for 7.5 psi external pressure.




Temperature: Design versus Operating

Background

When designing a pressure vessel, two design temperatures are indicated: the design temperature and the Minimum Design Metal Temperature. The design temperature is the maximum design temperature and is set slightly above the maximum operating temperature that may occur during normal operation. The maximum design temperature is the maximum temperature that the vessel is designed to withstand and includes a safety margin for fluctuations that occur during normal operations. Engineers use the maximum design temperature to determine the allowable stress of the vessel materials.

The Minimum Design Metal Temperature (MDMT) or Critical Exposure Temperature (CET) is the minimum metal temperature coincident with a pressure greater than 25% of the design pressure. When setting the minimum design temperature, process engineers usually consider the lowest one day mean ambient temperature and the temperature during hydrostatic testing and must also consider auto-refrigeration, when applicable. The vessel material must have sufficient toughness to resist brittle fracture at the minimum design temperature. The maximum and minimum design temperatures are usually determined by the process engineer.


SAES-D-001 requires that the design temperature for vessels with operating temperatures at or above -18°C (0°F) be not less than the maximum operating temperature plus 28°C (50°F). SAES-D-001 requires that vessels with an operating temperature below -18°C (0°F) be designed for a temperature not more than 14°C (25°F) below the minimum operating temperature.

SAES-D-001 also defines a Minimum Design Metal Temperature (MDMT) or Critical Exposure Temperature (CET) as the lowest temperature that a pressure vessel can be exposed to a pressure more than 25% of the design pressure. The CET may be based on meteorological site data such as the lowest one day mean temperature, or the CET may be based on a temperature of 17°C (30°F) below the lowest expected hydrotest temperature.




Procedure

Work Aid 1D provides the procedure for calculating the design temperature based on the expected operating temperature.

Sample Problem 5: Determine Design Temperatures

Determine the design temperatures for a vessel with a minimum operating temperature of 32°F and a maximum operating temperature of 500°F.

Solution: Use Work Aid 1D.

Since To max ≥ 0°FD.T.≥ 500 + 50D.T.≥ 550°FSince To min ≥ 0°FM.D.M.T.= To min = 32°F

Answer:

As a minimum, the vessel must be designed for a minimum metal temperature of 32°F and a maximum metal temperature of 550°F.




Wind

Background

If a pressure vessel and its supports are not properly designed, a strong wind can blow the vessel over or cause its anchor bolts to break. Therefore, pressure vessel designers must consider the forces exerted by winds. Figures 11 and 12 provide diagrams of wind force for typical pressure vessel configurations.

De

60 ft

50 ft

40 ft

30 ft

25 ft

20 ft

15 ft

F6

F5

F4

F3F2

F1

VwindMwind

F7

F8

Figure 11. Wind Force on Tall Vessel




Le

FL

B

De

FT

B

Figure 12. Wind Force on Horizontal Vessel





SAES-D-001 requires that the design of pressure vessels and supports be based on a wind loading due to a 137 km/h (85 mph) wind at a reference height of 10 m (33 ft.) above grade. During hydrotest, the design of a pressure vessel and its supports should be based on a wind loading due to a 48 km/h (30 mph) wind at the reference elevation.

Formulas

The following equations are based on equations in ANSI/ASCE 7-88 (formerly ANSI A58.1).

The wind pressure at the reference elevation, qr, can be calculated from the following equation:

qr = C.F. x Vr2

where:

qr = Wind pressure at the reference elevation, Pa (lb./ft.2)

Vr = Wind velocity at the reference elevation km/h (mph)

C.F. = A constant and conversion factor which depends on the density of air at sea level and the units used in the equation. C.F. equals 0.00256 for U.S. units and 0.0473 for SI units given above.

Based on the design wind speed of 137 km/h (85 mph), qr is equal to 888 Pa (18.5 lb./ft.2).

During hydrotest, with a 48.3 km/h (30 mph) wind speed, qr is equal to 110 Pa (2.3 lb./ft.2).

The wind pressure at a given height, qh, can be calculated from the following formula:

qh = KhG qr




where:

qh = Wind pressure at a given height above the reference elevation, Pa (lb./ft2)

kh = Height correction factor which varies with height above the reference elevation, (dimensionless)

G = Gust response factor based on the maximum height of the structure (dimensionless)


Kh and G are found in ASCE 7-88 based on the height and exposure classification of the location. Excerpts of these tables are presented in Work Aid 1E for exposure classification C.

Transverse Wind Loading

The transverse wind loading on the vessel is proportional to the wind pressure at a given elevation, the effective area of the vessel and the drag coefficient, Cs. The drag coefficient, Cs, is given in ASCE 7-88 for structures with varying proportions and degrees of surface roughness. An abbreviated table in Work Aid 1E applies to most cylindrical pressure vessels.

The transverse wind load on a vessel section can be expressed as:

F = AKhGCsqr

where:

F = Transverse wind load, N (lbs.)

A = Effective area of the vessel section, m2 (ft.2)

Cs = Surface drag coefficient (dimensionless)

Kh = Height correction factor which varies with height above the reference elevation, (dimensionless)

G = Gust response factor based on the maximum height of the structure (dimensionless)





When calculating the effects of wind on a pressure vessel, an engineer must calculate the forces caused by the wind and the resulting bending moment at the base. To do this, the engineer must calculate the projected windward area of the vessel. The calculation must either include every attachment to the pressure vessel or an estimate of the attachments’ effects. To estimate the attachments’ effects, an effective diameter, De, which accounts for the attachments (that is, piping, ladders, platforms, etc.), can be calculated. A method for calculating De is in Work Aid 1E. Using an effective diameter gives sufficiently accurate results in most cases.

When the wind load on a tall multidiameter vessel is calculated, the calculation is usually more accurate if the vessel is divided into a number of sections. The formulas for the base shear force and the base overturning moment as a result of wind are included in Work Aid 1E.

Sample Problem 6: Wind Loadings

Calculate the wind base shear force and overturning moment for a tower.

Given:

A tower (refer to Figure 13) with the following construction:

• Hemispherical heads

• Skirt support attachment 25 ft. above the ground

• Total tower height of 105 ft

• Top 35 ft. of the shell 15 ft. in diameter

• Bottom 30 ft. of the shell 20 ft. in diameter

• Shell-reducer section 7-1/2 ft. long

• Attached piping, ladders and platforms

• The surface reoughness D'/D ≈ .01




15 ft

7-1/2 ft

35 ft

7-1/2 ft

30 ft

10 ft

20 ft

25 ft

Figure 13. Wind Sample Problem Tower




Solution:

Use Work Aid 1E.

1. Vwind

Determine G based on maximum value of H from Figure 29

For H = 105, G = 1.16 Determine Cs based on linear interpolation from

Figure 30 H/D = 105/15 = 7 Since D'/D = 0 < D'/D ≈ .01 < D'/D � .02

C s = 0.6 + 0.8

2 = 0.7

Determine De based on D for tower with attached piping and ladders from Figure 31.

For D > 10 with attached piping and platforms, De = 1.4 x D

For 20 ft. diameter section, De = 1.4 x 20 = 28 ft.

For reducer section, De = 1.4 x (20+15)/2 = 24.5 ft.

For 15 ft. diameter section, De = 1.4 x 15 = 21 ft.

To calculate Vwind using Eqn. 2, assume the tower is divided into 6 sections.

Vwind = (((0.8)(15-0)(28))+

((0.87)(25-15)(28))+

((1.06)(55-25)(28))+

((1.18)(62.5-55)(24.5))+

((1.29)(97.5-62.5)(21))+

((1.40)(105-97.5)(21))) x ((1.16)(0.7)(18.5))

Vwind = 42,900 lbs.




2. To calculate Mwind using Eqn. 3, assume the tower is divided into 6 sections.

Mwind = (((0.8)(15-0)(28)(15+0)/2)+

((0.87)(25-15)(28)(25+15)/2)+ ((1.06)(55-25)(28)(55+25)/2)+ ((1.18)(62.5-55)(24.5)(62.5+55)/2)+ ((1.29)(97.5-62.5)(21)(97.5+62.5)/2)+ ((1.40)(105-97.5)(21)(105+97.5)/2)) x

((1.16)(0.7)(18.5)) Mwind = 2,312,000 ft.-lbs.

Answer:

The base shear force is 42,900 lb., and the overturning moment at the base is 2,310,000 ft.-lb.

Earthquake Earthquake Loads Background

Pressure vessels must also be designed for earthquake loads for vessels in seismically active areas. The ground movement beneath the vessel during an earthquake induces transverse shearing forces and an overturning moment in the vessel, its support, and foundation.


SAES-D-001, Design Criteria for Pressure Vessels, requires that earthquake loads be considered for all pressure vessels and refers to SAES-M-100, The Saudi Aramco Building Code, to determine the applicable seismic zone. SAES-M-100 states that seismic loads do not apply to Aramco Eastern Province operating areas; however, seismic loads, based on a seismic Zone 1, apply to the Yanbu NGL plant installation and the Royal Commission tract at Yanbu. For areas other than the above mentioned, the manager of the Consulting Services Department should be contacted prior to the start of design. SAES-M-100 is a listing of modifications to the U.S. Uniform Building Code (UBC); therefore, the seismic loads should be calculated in accordance with the UBC Code.




Formulas

The actual response of a tall multidiameter pressure vessel that may have non-uniformly distributed masses can be quite complex, but is easily calculated with computer methods. However, simple formulas can be developed for a tall, single diameter, vertical pressure vessel with a uniformly distributed mass and wall thickness. The procedures in Work Aid 1F are based on the UBC procedures and these simplifying assumptions.

Work Aid 1F provides the procedure and reference material for calculating earthquake loads.

Ft

h

x

Fe(x)

MearthquakeVearthquake

Figure 14. Earthquake Loading on a Simple Tall Tower




Sample Problem 7: Earthquake Loadings

Calculate the earthquake base shear force and overturning moment for a vertical tower shown in Figure 14.

Given:

A single diameter vertical tower with the following parameters:

• Attachment of the skirt support to the shell 25 ft. above the ground

• 75 ft. from the skirt attachment point to the top • Weight of 825,000 lb. when operating • Located in seismic Zone 1 • Located on rock • Vessel Outside Diameter, 10 ft • Thickness of shell and skirt, 1.25 in

Solution:

Use Work Aid 1F.

Step 1: T = 2.65 × 10−5( ) 100

10( )2 8,250( )101.25

0.5

T = .681 seconds

Step 2: S = 1.0

Step 3: C =

1.25( ) 1( ).68( )2 3

C = 1.615

Step 4:

CR w

= 1.6154 =.404 ≥ .075∴ OK

Use C = 1.615




Step 5: V =

0.075( ) 1( ) 1.615( ) 825, 000( )4

V = 24,982 lbs.

Step 6: Ft = 0 (since T < 0.7 seconds)

Ft = 0

Step 7: M = 100 (2 (24,982) + 0)/3

M = 1,666,000 ft.-lbs.

Answer:

The base shear force is 24,982 lb., and the overturning moment is 1,666,000 ft.-lbs.

Appurtenances

When designing or modifying a pressure vessel or designing a foundation, the design engineer must allow for the weight and forces that are exerted by any attached appurtenances to the pressure vessel.

The primary load of the pressure vessel appurtenances is their weight. When the engineer does not know the actual weights of the specific items, he may estimate the weight of all vessel appurtenances with the guidelines provided in Work Aid 1G. The engineer should also add the weight of any external insulation or internal lining. A particular appurtenance may also create a bending moment on the vessel that must be considered in the local mechanical design of the vessel. The "Identifying Mechanical Considerations for Structural Support and/or Attachments to Pressure Vessels" section describes this topic. Some appurtenances, such as stairs, ladders, and platforms, also have live loads that engineers must consider in the detailed design. However, design engineers normally do not need to consider such localized live loads when designing the vessel supports and foundation.

Work Aid 1G provides the formulas for calculating or estimating the dead loads and live loads of appurtenances.




IDENTIFYING MECHANICAL CONSIDERATIONS FOR STRUCTURAL SUPPORT AND/OR ATTACHMENTS TO PRESSURE VESSELS

Background

The primary pressure-retaining shell and heads of a pressure vessel are designed with adequate wall thickness to withstand pressure, weight, wind, earthquake, and other imposed loads. External attachments to the vessel create additional localized loads. Typical attachments to pressure vessels include vessel supports, nozzles, platforms, ladders, and piping supports. These local loads cause localized stresses in the vessel shell that also must be kept within allowable limits. This section describes the considerations when evaluating these structural supports and attachments.

Temperature

A change in temperature changes the size of a material. Most materials expand as their temperature increases and contract as their temperature decreases. The components of a pressure vessel and the support for the pressure vessel change dimensions as the vessel and the support change temperature. The pressure vessel support, any associated structure, and foundation stay near the ambient temperature, while the temperature of the vessel metal changes. Therefore, the design of the support structure and the attachments to a pressure vessel must allow for the expansion and contraction of the pressure vessel. This allowance is accomplished either by ensuring that the pressure vessel is free to undergo unrestrained thermal movement, or by ensuring that restrained thermal movement does not result in excessive stress in the vessel shell and support. Local thermal stresses at vessel attachment points are minimized by the following:

• Reduction of local thermal gradients

• Use of the same material for the attachment as for the vessel

• Use of design details that reduce abrupt geometric transitions

• Use of appropriate insulation details




Skirt Support The skirt support is the most common design of support used for tall, vertical pressure vessels. Skirts are economical because they generally transfer the loads from the vessel to the foundation uniformly around the circumference. Skirts also transfer the loads to the foundation through anchor bolts and bearing plates. Figure 15 shows the two most common skirt details and the attachment of skirts to the vessel.

Butted weld blends smoothly into head contour

Straight Flared15° MAX

Figure 15. Support Skirts

The straight skirt in Figure 15 is most often used. The shell and skirt centerlines are nearly coincident, and the skirt is butt welded to the bottom head and blended smoothly into the head contour. The flared skirt in Figure 15 is used when a high bending moment at the base requires more and/or larger diameter anchor bolts than can fit around the circumference of a straight skirt at the base. Differential thermal expansion is normally not a factor with skirt-supported vessels. The skirt is typically long enough and flexible enough to accommodate the differential radial thermal expansion between a hot vessel shell and relatively cool skirt without causing excessive thermal stresses. However, special consideration is necessary in this area for heavy wall [over 50 mm (2 in.) thick] pressure vessels operating at high temperatures [over 260°C (500°F)], because sharp thermal gradients can exist at the skirt-to-shell junction area. In these cases, special insulation details are often used to keep thermal stresses within allowable limits.




The following additional considerations affect skirt design: • The skirt must be designed with adequate thickness

to resist the operating weight load and imposed bending moment due to either wind or earthquake (whichever is greater). It must also be designed for the hydrotest load, plus a 48 Km/h (30 mph) wind.

• The skirt must be designed considering compressive loads and buckling in accordance with the ASME Code, Section VIII, Division 1.

• The maximum stress in the skirt-to-head weld, accounting for the type of weld and the degree of inspection, must be kept within allowable limits.

• For tall towers, the skirt thickness is generally chosen to be at least the thickness of the corroded bottom shell plate.

• If a large access or pipe opening is located in the skirt shell, the opening may require reinforcement.

Support Legs Uniformly spaced columns, which are called support legs, usually support small and medium-sized vertical vessels at ground level. Pressurized storage spheres are also typically supported on legs. To allow easy access under the vessel, the number of support legs is usually four, unless a larger number of legs are required to distribute the loadings. Pressurized storage spheres are typically very large in diameter and require more than four support legs.

The most often used structural shapes for support legs are equal leg angles and I-shapes. The support legs usually have diagonal bracing for lateral loads due to wind or earthquake. Figure 16 shows the two different ways to weld the angle supports to the vessel shell. Angles are used for short support legs when the loads are low. As Figure 17 shows, I-shapes are used for larger and heavier vessels. The I-shapes oriented as shown in Detail B of Figure 17 are easier to weld to the shell, but the orientation shown in Detail A can carry much heavier eccentric external loads. Round steel pipes often support pressurized storage spheres. Round pipe is especially suitable for a column, since it has a large radius of gyration in all directions and good buckling and torsional resistance. The centroidal axis of pipe columns are set to coincide with the centerline of the vessel shell, and eliminate any eccentricity in the column and baseplate calculations.




A B

Figure 16. Angle Supports for Small Vessels

Figure 17. Orientation of I Supports for Large Vessels Differential thermal expansion is normally not a factor with leg-supported vessels. The legs are usually long enough and flexible enough to accommodate the differential radial thermal expansion of the vessel without causing high thermal stresses. Figure 18 shows vertical and horizontal loads that are imposed on support legs due to weight and wind or seismic forces. The wind load is horizontal and is assumed to act at the centroid of the exposed surface. The earthquake load also acts horizontally, at the center of gravity of the vessel. Compression and tension reactions act on the columns and their support bases due to the vessel weight and the overturning moment due to wind or earthquake. The columns are designed so that their stresses are within allowable limits, and the columns do not buckle under the compressive loads. The local details at the leg attachment to the vessel shell are also designed to keep the local vessel stresses within allowable limits. To limit the local vessel stresses, it is sometimes necessary to add a reinforcing pad to the vessel shell at the support attachment point.




Direction of wind or earthquakeW = Wo or WT

P = Pw or Pe

H

L

P

Pw = Wind loadPe = Earthquake load

Wo = Operating weightWT = Test weight

Figure 18. Loads on Support Legs




Support Lugs

Figure 19 shows support lugs normally limited to vertical pressure vessels with small to medium diameters of .3 to 3 m (1 to 10 ft.) and moderate height-to-diameter ratios (5-to-2). Support lugs are normally supported by structural steel girders and columns. Slotted holes for the lug anchor bolts permit differential radial thermal expansion of the vessel shell, which eliminates the potential for high local thermal stresses in the vessel shell. In situations with high weight loads at the lugs, low friction bearing pads (for example, Teflon or polished stainless steel) under the lugs reduce the frictional load and allows for easier unrestrained radial thermal expansion. The support structure must be designed for the lateral friction force as well as the other loads from the vessel.

Top bar

Base plate

Gusset Neutral axis

Detail of support lug

Support lugs

Front view Side view

Center of support area

Figure 19. Support Lugs




The detailed design of the support lugs may include a top bar, multiple vertical gusset plates, and/or a reinforcing plate welded to the vessel shell. In extreme cases, complete circumferential reinforcing rings are welded to the vessel shell to further distribute the imposed loads around the circumference of the vessel shell. The detailed approach depends on the magnitude of the loads.

Saddle Supports

As Figure 20 shows, most horizontal cylindrical vessels are supported by two saddle-type supports. The steel saddle plates are typically welded to the vessel shell over a contact angle of at least 120°. One saddle support is fixed to its pedestal while the other has slotted anchor bolt holes that permit free longitudinal movement of the vessel. This arrangement allows for longitudinal thermal expansion of the vessel. Low friction bearing plates are used at the supports in situations where there are especially heavy loads. The support pedestals must be designed for the weight, wind, earthquake, and friction loads that are imposed by the vessel.

A

A

A-A

>120°

Figure 20. Saddle Supports




In most situations, wind or earthquake loading on a horizontal vessel are not design-governing factors and can be ignored in designing the vessel. Frictional loads must be considered in the detailed design of the support pedestal, foundation, and the saddle support. The design pressure conditions normally set the overall wall thickness of the vessel shell and heads.

However, the weight loads of a horizontal vessel can significantly affect the detailed design of the saddle supports. The saddle reaction loads due to the vessel weight are concentrated and induce high localized stresses in the shell. The stress distribution is also a function of the distance between the saddle support and the vessel end closure. Furthermore, the shell deformation that occurs over the saddle reduces the effective stiffness of the vessel shell at the saddles. This reduced stiffness makes the full cross section of the shell less effective in carrying the imposed weight loads.

A generally accepted procedure called a “Zick Analysis” is used to evaluate the design of the vessel and saddle from a weight support standpoint. The original paper that discussed this procedure was published in the Welding Journal Research Supplement in 1951. The author of the paper was L. P. Zick, and the title was "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports". The procedure is discussed in many available pressure vessel texts, and is highlighted in MEX 202. A Zick Analysis determines the stresses in the vessel shell at midspan and near the saddles, including the localized stresses over the saddles. These stresses are compared to allowable values to determine their acceptability. In some cases, it may be necessary to widen or relocate the saddle supports and/or add circumferential stiffener rings at the saddles to keep the stresses within allowable limits.

Support Structures

Sometimes the vessel support (skirt, legs, or saddles) is mounted directly on a concrete foundation. In other cases, the vessel support may be on a support structure of a steel framework or reinforced concrete. Lug-supported vessels are usually supported in structural framing consisting of beams and columns and sometimes bracing.




Structural Attachments

External loads on pressure vessel nozzles or other attachments on the vessel shell induce local stresses in the vessel shell. Loads on vessel nozzles are typically imposed by weight and the thermal expansion of connected piping systems.

These loads may consist of forces and bending moments in all three coordinate directions. Figure 21 shows structural attachments to a pressure vessel shell that may support items such as pipe, ladders, platforms, and other equipment. These attachments usually impose a weight load and a bending moment on the vessel shell.

Support clip

Figure 21. Support Clips

The external loads at nozzles and structural attachments cause higher localized stresses in the vessel shell. If these loads are high enough, additional reinforcement may be provided at the nozzle or attachment to keep the local stresses within allowable limits.




Considerations Regarding Repairs and Alterations to Pressure Vessels

After pressure vessels have been in service, repairs may be required due to metal corrosion, over-pressure or over-temperature which distort or crack the components. Modifications may also be required to install a new nozzle, process control instrument or attachment clip for a ladder or platforms. All repairs or modifications to a pressure vessel should be carefully engineered before any work is done to ensure that the work meets the latest design, fabrication, inspection and testing standards of the Code and standards for original construction. In addition, the Pressure Vessel Inspection Code, API 510, can usually be used to assess and give guidance on repairs and alterations. Refer to PEDP MEX 202, Evaluating Pressure Vessel Designs and Installations, for a more in-depth treatment about the various code requirements applicable to repairs and alterations.




IDENTIFYING VARIOUS TYPES OF FOUNDATIONS THAT ARE USED FOR PRESSURE VESSELS

General

A foundation transfers the loads from a pressure vessel and support structure to the underlying soil or rock. Foundations may be placed at the ground surface, but more often, they are placed at some depth below the surface into firm soil or rock to provide additional stability and to reach below the level of frost penetration. A properly designed foundation accomplishes the following:

• Limits settlement to amounts that can be tolerated by the vessel and associated piping.

• Prevents overturning of the vessel. • Prevents sliding of the vessel.

An improperly designed or constructed foundation can cause a pressure vessel to do the following:

• Break its connecting pipes. • Leak • Rupture • Tilt or turn over

The following sections discuss these common types of foundations:

• Spread footings • Mat foundations • Concrete ring foundations • Pile foundations with a concrete cap




Spread Footings

A spread footing is a concrete pad that supports a single load, such as a tall vertical pressure vessel. In some cases, spread footings may support individual legs of a multi-leg supported vessel if the legs are widely spaced. However, consideration must then be given to the effect of differential settlement of the footing under each leg.

Spread footings are generally square or rectangular but may be circular or octagonal. The base of the spread footing must be located below the frost line and at a level where the soil or rock has the required strength and settlement characteristics. When the required depth of the spread footing exceeds 3 m (10 ft.), other foundation types, such as piles, may be appropriate. Typically, a concrete pedestal transfers the load from the pressure vessel support structure to the footing located at some nominal depth below final grade. Figure 22 shows two typical spread footings and pedestals.




��

��

Pedestal

Pedestal

Pedestal

Elevation

Elevation Plan

PanSpread footing

Spread footing

Rectangular

Octagonal

Figure 22. Spread Footings And Pedestals




Mat Foundations

A mat foundation is a single concrete pad that covers the entire area beneath the pressure vessel structure and supports all of the vessel legs or columns. Whenever the sum of the areas of individual spread footings exceeds over half of the area beneath the vessel, mat foundations are usually more economical. A mat foundation is commonly used when closely spaced legs support a pressure vessel.

As with spread footings, mat foundations are located at a nominal depth below frost level on soil and rock that has the required strength and settlement characteristics. Typically, pedestals transfer loads from the vessel support structure to the mat foundation. Figure 23 illustrates a mat foundation and pedestals.

Pedestal

Elevation Plan

Mat foundation

Mat foundation

Figure 23. Mat Foundation and Pedestals




Concrete Ring Foundations

A concrete ring foundation consists of a continuous strip of concrete shaped in a circle. A concrete ring foundation is used when a large pressure vessel is supported by a skirt or a circle of closely spaced legs. The base of the concrete ring is located below the frost depth on soil or rock with the required strength and settlement characteristics. Figure 24 illustrates a concrete ring foundation.

Paving

Reinforced concrete

Top view

Figure 24. Concrete Ring Foundation




Pile Foundations With a Concrete Cap

Pile foundations are used when the subsurface soil supporting spread footings, mat, or concrete ring foundations is too weak or too compressible to provide adequate support for the pressure vessel. Piles transfer the loads to more suitable material at a greater depth. Pile foundations also resist high lateral loads or overturning loads, such as from wind or earthquake.

Piles are structural members with a small cross-sectional area compared to their length. Piles are usually constructed of steel or concrete. A structural concrete slab or cap constructed over the piles distributes the loads from the pressure vessel supports to the piling system. When the bottom of the pile cap is in the ground, it should be below the level of frost penetration. Concrete pedestals transfer the loads from the vessel legs to the pile cap and piles. Figure 25 illustrates pile foundations with a concrete cap.




Support pedestals

Soil

Concrete cap

Concrete cap

Soil

Paving

Paving

Piles

Piles

Figure 25. Pile Foundations With Concrete Cap




CALCULATING ANCHOR BOLT STRESSES AND BASEPLATE WIDTH AND THICKNESS FOR PRESSURE VESSELS, GIVEN A SPECIFIC LOADING

Background

Anchor bolts and a baseplate attach a pressure vessel to its foundation. The anchor bolts and baseplate must be designed to secure the pressure vessel under operating and test conditions, including exposure to the maximum design wind or earthquake load, whichever is greater. If the anchor bolts are too close to each other, the ability of the concrete to hold each anchor bolt is reduced, and the design is compromised.

General Rules

The following general rules apply to the design of anchor bolts and selection of baseplates:

• The number of anchor bolts in a baseplate should be a multiple of four, for example, 4, 8, 12, 16.

• Anchor bolts should not be placed closer than 0.5 m (18 in.) apart (centerline to centerline). A closer spacing reduces their holding strength in a concrete foundation.

• No fewer than eight anchor bolts should secure a tall vessel.

• The area within the root of the bolt threads (the root area) is the effective area of the anchor bolts. The anchor bolt root area is used to determine the anchor bolt stress under the applied loads, and the bolt number and size that are required to keep the bolt stress within allowable limits.

Anchor Bolts

Work Aid 2A contains the procedure and additional information needed to calculate anchor bolt requirements.

Sample Problem 8: Anchor Bolt Calculation

Determine the number of anchor bolts needed and calculate the size of each bolt for a vertical tower.

Given:

A vertical tower with the following parameters:




• Empty weight of 10,000 lb. • Earthquake moment of 1,110,000 ft.-lb. • Design wind moment of 4,000,000 ft.-lb. • Bolt circle diameter of 11 ft. • A-325 anchor bolts.

Solution:

Use Work Aid 2A.

In Step 1:

M earthquake = 1,110,000ft.lbs.

M wind = 4,000, 000ft.lbs.∴M wind governs

In Step 2:

A B = π4 d abc

2

A B = π4( )112

A B = 95 ft.2

In Step 3:

C B = π × d abcC B = π × 11C B = 34.55 ft.

In Step 4:

T = MAB

– WCB

T = 4,000,000( )/ 95( )− 10,000 / 34.55( )T = 41, 815 lb./ linear ft.

In Step 5: Since 9.55 ft. < dabc < 11.5 ft., N = 20

In Step 6:

BA =

TCBSBN

BA = (41,815 x 34.55)/(44,000 x 20)

BA = 1.642 in.2/bolt

In Step 7: Since 1.642 < 1.744 � Bolt size is 1-3/4 in.

Answer: The tower requires 20 anchor bolts of 1-3/4 in. diameter.




Baseplates

After determining the number and size of anchor bolts, calculate the width of the baseplate. The width of the baseplate and its area must be sufficient to keep the bearing stress on the concrete foundation within acceptable limits. There must be sufficient distance from the bolt hole to the edge of the baseplate to provide enough bearing surface for the hold-down nut. There must be sufficient distance from the leg or skirt to the bolt holes to permit access to tighten the hold-down nut.

Work Aid 2B provides the procedures and additional information required to calculate baseplate width.

Sample Problem 9: Baseplate Calculation

For the vertical tower described in the previous sample problem (Anchor Bolt Calculation), determine the minimum required width of the support baseplate.

Given:

• Vessel weight during hydrotest is 500,000 lb.

• 4,000 psi ultimate strength concrete is being used for the foundation.

• Skirt outer diameter is 126 in.

• The skirt thickness is 0.75 in.

Solution:

Use Work Aid 2B.

In Step 1:

A S = π4

D sk2

A S = π4 10.5( )2

A S = 86.6 ft.2

In Step 2:

C S = πDskC S = π × 10.5C S = 33.0 ft.




In Step 3: P C = M

AS+ W

CS

PC = (4,000,000)/86.5) + (500,000/33.0)

PC = 61,400 lb./Linear ft.

In Step 4:

L min =PCf b

× C.F.

f b = 1,400

L min = 61, 4001, 400( ) 12( )

L min = 3.65 in.

In Step 5: L1 =

d abc – D sk( )2

Since the skirt outside diameter is 126 in. and the bolt circle

diameter was specified as 11 ft.= 132 in., L1 =

132 − 126( )2 = 3 in .

In Step 6: From Figure 36, the minimum L2min must be 2.25 in. and

L3min = 1.75. in.

L1min = 2.25 + 0.7(.75) = 2.775 in.

Since L1 > L1min use L1 In Step 7: LACT = L1 + L3 + tsk + L4

LACT = 3 + 1.75 + .75 + .75 = 6.25 in.

LACT > Lmin � OK

Answer:

The baseplate width must be at least 3.65 inches wide and will probably be over 6-1/4 inches wide to extend inside of the skirt to permit welding the skirt to the baseplate ring with fillet welds both inside and outside the skirt.




Summary

This module has provided an overview of ASME pressure vessels. The Participant should be able to identify the general types of pressure vessels and the rules that govern their fabrication, design, inspection, and testing. In addition, the Participant should understand the effect of adding or modifying attachments to a pressure vessel and the physical structure of a foundation that supports a pressure vessel. Also, the Participant should be able to calculate some of the common loadings on a pressure vessel and the minimum baseplate dimensions.




WORK AID 1: PROCEDURES AND REFERENCE MATERIAL FOR CALCULATING LOADS ON PRESSURE VESSELS

Work Aid 1A: Procedure for Calculating the Dead Weight

1. Using the appropriate formula(s) from the following list, calculate the volume of the shell of the pressure vessel:

For the volume of the shell of a sphere:

Vss = πd2t (Eqn. 1)

where:

Vss = Volume of the shell of a sphere, m3 (ft.3)

d = Inside diameter of the sphere, m (ft.)

t = Thickness of the sphere’s shell, m (ft.)

For the volume of the shell of a hemisphere:

Vshs =π2

d2t (Eqn. 2)

where:

Vshs = Volume of the shell of a hemisphere, m3 (ft.3)

d = Inside diameter of the hemisphere, m (ft.)

t = Thickness of the hemisphere’s shell, m (ft.)

For the volume of the shell of a cylinder:

Vsc = πdht (Eqn. 3)

where:

Vsc = Volume of the shell of a cylinder, m3 (ft.3)

d = Inside diameter of the cylinder, m (ft.)

h = Height or length of the cylinder, m (ft.)

t = Thickness of the cylinder’s shell, m (ft.)




For the volume of the shell of a conical section:

Vscone = πt

d 1 + d 2( )2

d 1 – d 2( )2

4 + h 2 (Eqn. 4)

where:

Vscone = Volume of the shell of a conical section, m3 (ft.3)

t = Thickness of the shell of the section, m (ft.)

d1 = Inside diameter of the larger end of the section, m (ft.)

d2 = Inside diameter of the smaller end of the section, m (ft.)

h = Height of the section, m (ft.)

The following equation approximates the volume of the shell of a 2:1 semi-elliptical head:

Vsell = 1.084 x d2t (Eqn. 5)

where:

Vsell = Volume of the shell of a 2:1 semi-elliptical head, m (ft.)3

d = Inside diameter of the major axis, m (ft.)

t = Thickness of the head, m (ft.)

2. Using the following formula calculate the total volume of steel in the vessel shell:

Vs = ΣVheads + ΣV shell + V skirt (Eqn. 6)

where:

Vs = Total volume of steel in the vessel shell, m3 (ft.)3

∑Vheads = Volume of steel in the vessel heads, m3 (ft.)3

∑Vshell = Volume of steel in the vessel shells, m3 (ft.)3

Vskirt = Volume of steel in the skirt, m3 (ft.)3




3. Using the following formula calculate the dead weight of the pressure vessel:

W s = V s × γ st (Eqn. 7)

where:

Ws = Dead weight of the pressure vessel, kN (lb.)

Vs = Total volume of the steel in the pressure vessel, m3 (ft.)3

�st = Weight density of steel, 77 kN/m3 (490 lb./ft.3)

Work Aid 1B: Procedure for Calculating Hydrostatic Weight

1. Using the appropriate formula(s) below, calculate the volume of the pressure vessel :

For the volume of a sphere:

Vsphere = π

6 d 3 (Eqn. 8)

where:

Vsphere = Volume of a sphere, m3 (ft.3)

d = Inside diameter of the sphere, m (ft.)

For the volume of a hemisphere:

Vhsphere =π

12d3

(Eqn. 9)

where:

Vhsphere = Volume of a hemisphere, m3 (ft.3)

d = Inside diameter of the hemisphere, m (ft.)




For the volume of a cylinder:

Vcylinder = π4 d 2h (Eqn. 10)

where:

Vcylinder = Volume of a cylinder, m3 (ft.)

d = Inside diameter of the cylinder, m (ft.)

h = Height or length of the cylinder, m (ft.)

For the volume of a conical section:

Vconical reducer = πh

12 d 12 + d 2

2 + d 1d 2( ) (Eqn. 11)

where:

Vconical reducer = Volume of the conical reducer section, m3 (ft.3)

h = Height of the conical section, m (ft.)

d1 = Inside diameter of the larger end of the section, m (ft.)

d2 = Inside diameter of the smaller end of the section, m (ft.)

For the volume of a 2:1 semi-elliptical head:

Vell = π24 d3 (Eqn. 12)

where:

Vell = Volume of the 2:1 semi-elliptical head, m3 (ft.3)

d = Inside diameter of the major axis of the head, m (ft.)




2. Using the following formula calculate the total internal volume of the pressure vessel:

Vv = ΣVheads + ΣVshells (Eqn. 13)

where:

Vv = Total internal volume in the pressure vessel, m3 (ft.3)

∑Vheads = Total internal volume in the vessel heads, m3 (ft.3)

∑Vshells = Total internal volume in the vessel shells, m3 (ft.3)

3. Using the following formula calculate hydrostatic test water weight of the vessel shell:

Ww = Vv × γw (Eqn. 14)

where:

Vv = Total internal volume in the pressure vessel, m3 (ft.3)

�st = Weight density of water, 9.81 kN/m3 (62.4 lb./ft.3)

4. Calculate the hydrostatic test weight by adding the hydrostatic test water weight to the dead weight of the shell using the following formula:

WH = Ws + Ww (Eqn. 15)

where:

WH = Hydrostatic test weight, kN (lb.)

Ws = Dead weight of the shell, kN (lb.)

Ww = Hydrostatic test water weight, kN (lb.)




Work Aid 1C: Procedure for Calculating Design Pressure Based on Operating Pressure

1. Using the appropriate formula below, calculate the top head design pressure based on the operating pressure.

When Po < 1034 kPa (150 psi):

PDtop = Po + 15 psig (Eqn. 16 U.S.)

PDtop = Po + 103 kPa (Eqn. 16 SI)

When Po ≥ 1034 kPa (150 psi):

PDtop = 1.1 Po (Eqn. 17)

where:

PDtop = Design pressure, kPa ga (psig)

Po = Maximum normal internal operating pressure, kPa ga (psig)

2. The static pressure head of the vessel shell section which is below the top head can be determined from the following formula:

Psh = H ×

γ wC.F. × G (Eqn. 18)

where:

Psh = Pressure due to static head effects, kPa (psi)

C.F. = Conversion factor 1 for SI units, 144 for US units

H = Depth below surface, m (ft.)

�w = Weight density of water, 9.81 kN/m3 (62.4 lb./ft.3)

G = Specific gravity (dimensionless)




3. The design pressure of a vessel section below the top head can be found from the following formula:

PDbot = PDtop + Psh (Eqn. 19)

where:

PDbot = Design pressure at the bottom of shell section kPa (psig)

PDtop = Top head design pressure, kPa (psig)

Psh = Pressure due to static head effects, kPa (psi)

4. Determine if the vessel can be subjected to external pressure or vacuum.

(a) If the vessel is in vacuum service, design for an external pressure equal to the smaller of 103 kPa (15 psi) or 25% more than the maximum possible external pressure.

(b) If the vessel can be steamed out and is not already designed for vacuum, design for an external pressure of 52 kPa (7.5 psi) at 150°C (300°F).

Work Aid 1D: Procedure for Calculating Design Temperature Based on Operating Temperature

1. Using the appropriate formulas below, calculate the design temperatures based on the operating temperatures.

When Tomax ≥ -17.8°C (0°F):

D.T. ≥ Tomax + 28°C (Eqn. 20 SI)

D.T. ≥ Tomax + 50°F (Eqn. 20 U.S.)

When Tomax < -17.8°C (0°F):

D.T. ≥ Tomax (Eqn. 21)

where:

Tomax = Maximum operating temperature, °C (°F)

D.T. = Maximum Design Temperature, °C (°F)




When Tomin < -17.8°C (0°F):

M.D.M.T. ≤ Tomin - 14°C (Eqn. 22 SI)

M.D.M.T. ≤ Tomin - 25°F (Eqn. 22 U.S.)

When Tomin ≥ -17.8°C (0°F):

M.D.M.T. ≤ Tomin (Eqn. 23)

where:

M.D.M.T. = Minimum design metal temperature, °C (°F)

Tomin = Minimum expected operating temperature, °C (°F)

Work Aid 1E: Procedure and Reference Material for Calculating Wind Loading

1. Using the following formula, calculate the base shear force:

Vwind = (∑Kh(hh - hl)(De)) x GCsqr (Eqn. 24)

where:

Vwind = Base shear force, N (lb.)

Kh = Height correction factor from Figure 29

hh = Highest point on the pressure vessel within the height range, m (ft.)

hl = Lowest point on the pressure vessel within the height range, m (ft.)

De = Effective diameter of the pressure vessel within the height range from Figure 31, m (ft.)

G = Gust factor for the maximum height of vessel from Figure 29

Cs = Shape factor from Figure 30

qr = Wind pressure at reference elevation, 888 Pa (18.5 lb./ft.2) for a design wind speed of 137 km/h (85 mph)




HEIGHT ABOVE GRADE (ft) Kh G0 to 15 0.80 1.32

20 0.87 1.29

25 0.93 1.27

30 0.98 1.26

40 1.06 1.23

50 1.13 1.21

60 1.19 1.20

70 1.24 1.19

80 1.29 1.18

90 1.34 1.17

100 1.38 1.16

120 1.45 1.15

140 1.52 1.14

160 1.58 1.13

180 1.63 1.12

200 1.68 1.11

250 1.79 1.10

Note: Kh varies with height but G is at maximum height.

Source: Based on ANSI/ASCE 7-88 Tables 7 and 8

Figure 29. Height and Gust Correction Factors




Type of Vessel

Cs for H/D values 1 7 25

Round cross section moderately smooth 0.5 0.6 0.7

Round cross section rough (D'/D �0.02) 0.7 0.8 0.9

Round cross section very rough (D'/D �0.08) 0.8 1.0 1.2

Source: Based on ANSI/ASCE 7-88 Table 12

Figure 30. Shape Factor

where:

H = Total height in m (ft.)

D = Diameter m (ft.)

D' = Depth of protruding element, m (ft.)

D = [VESSEL DIAMETER+ (2 • INSULATION

THICKNESS)], m (ft.)

ATTACHED PIPING BUT

WITHOUT LADDERS

ATTACHED PIPING,LADDERS, AND

PLATFORMS

Š 1 (3) De = 1.6 D De = 2.0 D1 - 3 (3 - 10) De = 1.4 D De = 1.6 D

> 3 (10) De = 1.2 D De = 1.4 D

Figure 31. Effective Diameter, De




2. Using the following formula, calculate the base overturning moment :

M wind = ΣKh h h − h l( )De

h h + h l2

× GCsq r (Eqn. 25)

where:

Mwind = Overturning moment, N-m (ft.-lb.)

Kh = Height correction factor from Figure 29

hh = Highest point on the pressure vessel within the height range, m (ft.)

hl = Lowest point on the pressure vessel within the height range, m (ft.)

De = Effective diameter of the pressure vessel within the height range from Figure 31, m (ft.)

G = Gust factor for the maximum height of the vessel from Figure 29

Cs = Shape factor from Figure 30

qr = Wind pressure at reference elevation, 888 Pa (18.5 lb./ft.2) for a design wind speed of 137 km/h (85 mph)

Work Aid 1F: Procedure and Reference Material for Calculating Earthquake Loading

1. Determine the first natural period of vibration of the vessel, T.

a) For a tall cylindrical pressure vessel with a uniform thickness and mass distribution:

T = K1(H/D)2(wD/t)0.5 (Eqn. 26)




where:

K1 = A constant and conversion factor which depends on the units. K1 equals 2.65 x 10-5 for U.S. units used below and 1.11 x 10-5 for S.I. units used below

T = The first mode period of vibration in seconds

H = Height of the vessel in feet (meters)

D = Outside diameter of the vessel in feet (meters)

w = Weight per unit height of the vessel in lb./ft. (N/m)

t = Thickness of the vessel in inches (millimeters)

b) For other vessels, T can be calculated by the Raleigh-Ritz procedure, a finite element method computer program or another suitable procedure.

2. Determine the site soil coefficient, S, from Figure 32.

Type Soil Profile S

S1 Rocklike material characterized by a shear-wave velocity greater than 2,500 ft. per second or by other suitable means of classification or Stiff or dense soil where the soil depth is less than 200 ft.

1.0

S2 Dense or stiff soil where the soil depth exceeds 200 ft. 1.2

S3 Soil 40 ft. or more deep containing more than 20 ft. of soft to medium stiff clay, but not more than 40 ft. of soft clay

1.5

S4 Soil containing more than 40 ft. of soft clay 2.0

Unknown soil conditions. When the soil profile is not known in sufficient detail assume soil profile S3 unless S4 is specified by Consulting Services Department.

Source: Based on Uniform Building Code 1991 Edition, Table 23J

Figure 32. Site Soil Coefficients




3. Determine the seismic response coefficient, C:

C =

1.25ST

23

but no more than 2.75 (Eqn. 27)

where:

S = Site soil coefficient defined above

T = The natural period of vibration of the tower, seconds

4. Determine the ratio of C/Rw. In further calculations, this ratio shall not be less than 0.075:

where:

C = Seismic response coefficient defined above

Rw = Numerical coefficient which depends on the lateral load resisting system used in the structure. Rw equals 4, for skirt supported vessels. For other structures see UBC Table 23-Q.

5. Determine the total base shear force, V, from the following equation:

V = ZICW

Rw (Eqn. 28)

where:

Z = Seismic zone factor per 1991 UBC for the location (note the seismic zone factor per the 1991 UBC is equal to the seismic probability coefficient Z per ANSI/ASCE 7-88 divided by 2.5). Z = 0.0 for seismic Zone zero, Z = 0.075 for seismic Zone 1, and Z = 0.15 for seismic Zone 2.

I = Importance factor based on occupancy category. I is equal to 1.0 for most pressure vessels, and equal to 1.25 for Hazardous Facilities (pressure vessels storing toxic or explosive substances), or Essential Facilities (pressure vessels storing fire fighting liquids).

C = Seismic response coefficient defined above.

W = Total weight of the vessel, lb.(N)

Rw = Numerical coefficient as defined above




6. Determine the top load, Ft:

Ft = 0.07TV (Eqn. 29)

but not more than .25V and may be considered as equal to zero when T is 0.7 seconds or less.

where:

T = The natural period of the structure as calculated above

V = The total shear force as calculated above, lb. (N)

7. Determine the earthquake moment, M.

a) For a tall tower with a uniform mass distribution, the earthquake moment, M, can be calculated from the following simple equation:

M =

H 2V + Ft( )3 (Eqn. 30)

b) In other cases, the earthquake moment can be calculated assuming that the earthquake force, Fi, is distributed over the height of the structure according to the following formula:

F i =V − Ft( )w ih i

w ih i( )i=1

n∑

(Eqn. 31)

where:

V = The total base shear force due to earthquake defined above, lb. (N)

Ft = The top load defined above, lb. (N)

wi = The weight of a given section of the vessel, lb. (N)

hi = The height of the given section above the base, ft. (m)

wi = The weight of the "i"th section of the vessel, lb. (N)

hi = The height of the "i"th section above the base, ft. (m)




n = The number of section that the vessel is considered to be divided into for calculation purposes

The moment of the earthquake forces about the base can be calculated from the following formula:

M = Ft H + Fih i

i=1

n∑ (Eqn. 32)

where:

Ft = The earthquake force considered to act at the top of the tower and defined above, lb. (N)

H = The total height of the tower, ft. (N)

Fi = The earthquake force on the "i"th section determined above, lb. (N)

hi = The height of the "i"th section above the base, ft. (m)




Work Aid 1G: Procedure for Calculating Loads from Insulation and Appurtenances

In the absence of more detailed information, the weight of vessel insulation and attachments may be estimated from the following guidelines:

Insulation

For a cylindrical section:

Wi = πDoLtiγi (Eqn. 33)

where:

Wi = Insulation weight, N (lb.)

Do = Outside diameter of section, m (ft.)

L = Length of section, m (ft.)

ti = Insulation thickness, m (ft.)

�i = Insulation density, N/m3 (lb./ft.3) (Refer to Figure 33.)

Insulation Material Insulation Density, �i, N/m3 (lb./ft.3)

Calcium silicate 1.7 (11)

Mineral wool 1.25 (8)

Foamglas 1.4 (9)

Polyurethane 0.36 (2.3)

Figure 33. Typical Insulation Densities




For a conical section:

W i = π2

Do − d o( )2

4 + L2

Do + d o( )t i γ i (Eqn. 34)

where:


Do = Outside diameter of the large end of the section, m (ft.)

do = Outside diameter of the small end of the section, m (ft.)

L = Length of the section, m (ft.)



For a 2:1 semi-elliptical head:

W i = 1.084D o2t iγ i (Eqn. 35)

where:


Do = Outside diameter of the head, m (ft.)


�i = Insulation density, N/m3 (lb./ft.3) .(Refer to Figure 33.)

For a hemispherical head:

W i =

πD 02

2 t iγ i (Eqn. 36)

where:





Do = Outside diameter of the head, m (ft.)



Appurtenances

Estimate the weight of steel platforms at 1.68 kN/m2 (35 lb./ft.2) of platform area.

Estimate the weight of steel ladders at 364 N/linear meter (25 lb./linear ft.) for caged ladders and 146 N/linear meter (10 lb./linear ft.) for plain ladders.

Estimate the weight of trays in distillation columns, including liquid holdup, at 1.2 kN/m2 (25 lb./ft.2) of tray area.

Estimate the additional weight for nozzles, flanges, and other attachments at 6% of the dead weight of the vessel steel shell.




WORK AID 2: PROCEDURES AND ADDITIONAL INFORMATION FOR CALCULATING ANCHOR BOLT STRESSES AND BASEPLATE WIDTH AND THICKNESS FOR PRESSURE VESSELS, GIVEN A SPECIFIC LOADING

Work Aid 2A: Procedure and Additional Information for Calculating Anchor Bolt Requirements

1. If not known, use the following steps to determine which is greater: the overturning moment due to design wind pressure or the overturning moment due to earthquake.

• If needed, use Work Aid 1E to calculate the overturning moment for design wind pressure.

• If needed, use Work Aid 1F to calculate the overturning moment for earthquake.

• Determine which of the overturning moments is greater.

2. Using the following formula, calculate the area within the anchor bolt circle:

AB =π4

dabc2

(Eqn. 37)

where:

AB = Area of the anchor bolt circle, m2 (ft.2)

dabc = Diameter of the anchor bolt circle, m (ft.)

3. Using the following formula, calculate the circumference of the anchor bolt circle:

CB = πdabc (Eqn. 38)

where:

CB = Circumference of the anchor bolt circle, m (ft.)

dabc = Diameter of the anchor bolt circle, m (ft.)

4. Using the following formula, calculate the maximum tension in the base of the pressure vessel support:




T = M

A B− W

C B (Eqn. 39)

where:

T = Maximum tension, kN/m (lb./linear ft.)

M = Overturning moment at the base due to wind or earthquake, N-m (ft.-lb.)

AB = Area within the bolt circle, m2 (ft.2)

W = Dead weight of vessel and internals excluding operating fluid, kN (lb.)

CB = Circumference of the bolt circle, m (ft.)

5. If not given, estimate the number of anchor bolts to be used. (Refer to Figure 34.) Remember that:

• There needs to be at least .5 m (18 in.) spacing between anchor bolts.

• Anchor bolts are used in multiples of four.

• No fewer than eight bolts should secure a tall tower.

N, Number of Bolts dabc min

m (ft.)

4 8

12 16 20 24 28 32

.583 (1.91) 1.17 (3.82) 1.75 (5.73) 2.33 (7.64) 2.92 (9.56) 3.50 (11.5) 4.08 (13.4) 4.66 (15.5)

Figure 34. Number of Anchor Bolts




6. Using the following formula, calculate the required area of each anchor bolt:

BA =TCBSBN (Eqn. 40)

where:

BA = Required area of each bolt, m2 (in.2)

T = Maximum tension, kN/m (lb./ft.)

CB = Circumference of bolt circle, m (ft.)

SB = Maximum allowable stress value of bolt material, kPa (psi). (Refer to Figure 35.)

N = Number of anchor bolts

Material Specification Maximum Allowable Stress

kPa psi

A 307 137,900 20,000

A 325 303,400 44,000

A 490 372,300 54,000 Source: Based on ASCE Manual of Steel Construction

Figure 35. Allowable Anchor Bolt Stress




7. Using Figure 36, determine the size of the anchor bolts. Select the nearest size that has at least the calculated required area for each bolt.

Bolt Size (in.) Bolt Root Area (in.2)

1 0.551

1-1/8 0.693

1-1/4 0.890

1-3/8 1.054

1-1/2 1.294

1-5/8 1.515

1-3/4 1.744

1-7/8 2.049

2 2.300

2-1/4 3.020

2-1/2 3.715

2-3/4 4.618

3 5.621

(SI Note: To convert inches to mm multiply by 25.4 mm/in.)

Figure 36. Anchor Bolt Data




Work Aid 2B: Procedure and Additional Information for Calculating Baseplate Requirements for Support Skirts

1. Using the following formula, calculate the area within the skirt, AS:

AS =π4

Dsk2

(Eqn. 41)

where:

AS = Area within the skirt, m2 (ft.2)

Dsk = Outside diameter of the skirt, m (ft.)

2. Using the following formula, calculate the skirt circumference:

CS = πDsk (Eqn. 42)

where:

CS = Circumference of the skirt, m (ft.)

Dsk = Outside diameter of the skirt, m (ft.)

3. Using the following formula, calculate the maximum compression:

Pc = M

A s+ W

Cs (Eqn. 43)

where:

PC = Maximum compression, kN/m (lb./linear ft.)

M = Bending moment at the base due to the larger of either the wind or earthquake load, kN-m (ft.-lb.)

AS = Area within the skirt, m2 (ft.2)

W = Operating weight of the vessel, kN (lb.)

CS = Circumference of the skirt, m (ft.)




4. Using the following formula, calculate the approximate width of the base ring.

L min =

P cf b

C.F. (Eqn. 44)

where:

Lmin = Minimum width of the base, mm (in.)

PC = Maximum compression, kN/m (lb./ft.)

fb = Allowable bearing stress on the foundation, kPa (psi) (Refer to Figure 37.)

C.F. = Conversion factor 1 ft./12 in. in U.S. units, 1000 mm

m in SI units

28-Day Ultimate Strength of Concrete (psi) Allowable Bearing Stress, fb (psi)

3,000 1050

4,000 1400

5,000 1750

Source: AISC Manual of Steel Construction fb = .35 fc) (SI Note: To convert psi to kPa multiply by 6.895 kPa/psi)

Figure 37. Allowable Bearing Stress for Concrete

5. Using the following formula, calculate the distance between the skirt outside diameter and the bolt circle diameter and determine if it is large enough. (Refer to Figure 38.)

L1 =

d abc − D sk( )2 (Eqn. 45)

where:

L1 = Distance between the skirt outside diameter and the bolt circle, mm (in.)

dabc = Diameter of the bolt hole circle, mm (in.)

Dsk = Outside diameter of the skirt, mm (in.)




6. Determine L1min, L2min and L3min from Figure 38.

Bolt Size (in.) Dimension (in.)

L2 L3

1 1-3/8 1-1/16

1-1/8 1-1/2 1-1/8

1-1/4 1-3/4 1-1/4

1-3/8 1-7/8 1-3/8

1-1/2 2 1-1/2

1-5/8 2-1/8 1-5/8

1-3/4 2-1/4 1-3/4

1-7/8 2-3/8 1-7/8

2 2-1/2 2

2-1/4 2-3/4 2-1/4

2-1/2 3-1/16 2-3/8

2-3/4 3-3/8 2-5/8

3 3-5/8 2-7/8

L

Base plate

Skirt thickness

ts

Vessel centerline

L4

Fillet welds inside and outside. If size is unknown, assume tw = 0.7 ts

L3 L2

L1

tw

Figure 38. Baseplate Dimensions




• Figure 38 provides the minimum required values for the minimum wrench clearance L2 for a given bolt size.

• If the skirt outside fillet weld size is known, use the minimum required values of L2 and the fillet weld size, Lw, to determine L1min.

L1min = L2 + Lw (Eqn. 46a)

If the outside fillet weld size is not known, assume:

L1min = L2 + 0.7 ts (Eqn. 46b)

• Confirm that the resulting value of L1 is at least equal to the minimum required value, L1min. If it is, use L1 plus the minimum value of L3 to calculate L. If it is not, use the minimum value, L1min, and increase the bolt circle diameter as required.

7. Calculate the actual total bearing width of the plate, LACT:

LACT = L1 + L3 + ts + L4 (Eqn. 47)

where:

LACT = Actual bearing width of the baseplate, mm (in.)

L1 = Distance between the skirt outside diameter and the bolt circle, mm (in.)

L3 = Distance between the center of the bolt hole circle and the outside diameter of the baseplate, mm (in.)

L4 = Width of baseplate ring inside of skirt fillet weld toe. If not specified, assume L4 = ts

ts = Skirt thickness, mm (in.)

8. Check that:

LACT ≥ Lmin. (Eqn. 48)




GLOSSARY

anchor bolt Bolt that attaches a vessel, tank, or support structure to a foundation.

ANSI American National Standards Institute.

ASCE American Society of Civil Engineers.

API American Petroleum Institute.

appurtenance Accessory that attaches an object to a tank or vessel.

ASME American Society of Mechanical Engineers.

baseplate A metal plate that provides support. Typically, a metal plate is attached to the bottom of a tank, vessel, or support structure to connect to a foundation.

elliptical head A dished head of semi-ellipsoidal form, in which half the minor axis (inside depth of the head minus the skirt) equals one-fourth of the inside diameter of the head skirt.

hemispherical head A dished head which is formed to a hemispherical shape.

hydrotest Test performed by filling the vessel with water.

LPG Liquefied petroleum gas.

NGL Natural gas liquids.

pressure vessel A cylindrical or spherical tank constructed to hold a gas or a liquid under pressure.

torispherical head A dished head in which the central portion is formed to a spherical crown, and a transition region called a knuckle is located between the crown and cylindrical skirt. The ASME Code specifies requirements for the crown and knuckle radii.

UBC Uniform Building Code

139124533 Pressure-Vessels.pdf

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