aramco 1

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

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready 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, EngineeringServices, Saudi Aramco.

Chapter : Civil For additional information on this subject, contactFile Reference: CSE11001 J.H. Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

API Storage Tank

Engineering Encyclopedia Civil

API Storage Tank

Saudi Aramco DeskTop Standards

CONTENTS PAGE

TYPES, COMPONENTS, AND USES OF STORAGE TANKS.............................. 1

Background .................................................................................................... 1

API Atmospheric Storage Tanks .................................................................... 1Supported Cone Roof Tank ................................................................. 2Self-Supporting Fixed Roof Tank....................................................... 3Floating Roof Tank............................................................................. 4Fixed Roof with Internal Floating Roof Tank..................................... 5

API Low-Pressure Storage Tanks................................................................... 7Single-Walled, Low-Pressure Tank .................................................... 8Double-Walled, Low-Pressure Tank................................................... 9Spheroidal Low-Pressure Tank........................................................... 10Spherical Low-Pressure Tank ............................................................. 12

Other Storage Tanks....................................................................................... 13

APPLICABLE CODES AND STANDARDS FOR SELECTEDSTORAGE TANKS................................................................................................... 14

Codes and Standards for API Atmospheric Storage Tanks ............................ 14API Standard 650, Welded Steel Tanks for Oil Storage..................... 14API 653, Tank Inspection, Repair, Alteration and Reconstruction ............................................................................... 15SAES-A-004, Pressure Testing........................................................... 16SAES-B-005, Spacing and Diking for Atmospheric and Low- Pressure Tanks ............................................................................... 16SAES-B-007B, Air Foam Systems for Storage Tanks........................ 16SAES-D-100, Atmospheric and Low-Pressure Tanks ........................ 16SAES-D-108, Storage Tank Integrity ................................................. 1632-SAMSS-005, Atmospheric Steel Tanks ........................................ 16Codes and Standards for API Low-Pressure Storage Tanks ............... 17API Standard 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks ........................................... 17SAES-A-004, Pressure Testing........................................................... 17SAES-B-005, Spacing and Diking for Atmospheric andLow-Pressure Tanks............................................................................ 17SAES-D-100, Atmospheric and Low-Pressure Tanks ........................ 1832-SAMSS-006, Large Welded Low-Pressure Tanks ........................ 18


API Storage Tank


TYPES, MECHANICAL PROPERTIES, AND ALLOWABLE STRESSESOF STEELS COMMONLY USED FOR STORAGE TANKS ................................. 19

Background .................................................................................................... 19Design Metal Temperature.................................................................. 19Minimum Tensile Strength ................................................................. 20Minimum Yield Strength .................................................................... 20Allowable Stresses .............................................................................. 20

Allowable Types of Steels.............................................................................. 21Atmospheric Storage Tanks................................................................ 21Low-Pressure Storage Tanks .............................................................. 26

CALCULATING CIVIL/MECHANICAL LOADS FOR ATMOSPHERICSTORAGE TANKS................................................................................................... 27

Background .................................................................................................... 27

Estimating the Minimum Acceptable Thickness of Tank Components.......... 27API One-Foot Method ........................................................................ 28Minimum Shell Course Thicknesses for Construction Purposes ........ 29Minimum Thicknesses for Tank Bottoms........................................... 30

Weight Loads ................................................................................................. 31Background......................................................................................... 31Procedures........................................................................................... 31

Total Pressure and Equivalent Liquid Height................................................. 34Background......................................................................................... 34Procedures........................................................................................... 37

Roof Live Load .............................................................................................. 38Background......................................................................................... 38Procedure ............................................................................................ 38

Wind Loads .................................................................................................... 39Background......................................................................................... 39

Saudi Aramco Standards ................................................................................ 40

Formulas......................................................................................................... 41

Wind Loads .................................................................................................... 41


API Storage Tank


Wind Roof-Lift Loading................................................................................. 44

Earthquake Base-Overturning Moment.......................................................... 46Background......................................................................................... 46Saudi Aramco Standards..................................................................... 46Seismic Zones ..................................................................................... 46Formulas ............................................................................................. 47

Appurtenances................................................................................................ 50

MECHANICAL CONSIDERATIONS FOR ADDITIONS ORMODIFICATIONS TO THE APPURTENANCES ON A STORAGE TANK ......... 51

Background .................................................................................................... 51

Membrane Stress ............................................................................................ 51

Bending Stress................................................................................................ 51

Peak Stresses .................................................................................................. 53

Changes in Dead Weight ................................................................................ 54

Attachments.................................................................................................... 54Ladders or Spiral Stairways ................................................................ 54Platforms............................................................................................. 55Accesses.............................................................................................. 55Supports .............................................................................................. 56

USES OF VARIOUS TYPES OF FOUNDATIONS FOR STORAGE TANKS....... 57

General ........................................................................................................... 57

Soil ................................................................................................................. 58Preloading ........................................................................................... 58Compaction......................................................................................... 58Excavation and Backfill ...................................................................... 59

Types of Foundations ..................................................................................... 59Compacted Earth with Oiled Sand Pad............................................... 59Ringwalls ............................................................................................ 60Concrete Pad....................................................................................... 62Piled Foundation................................................................................. 63


API Storage Tank


Saudi Aramco Requirements .......................................................................... 6432-SAMSS-005, Atmospheric Steel Tanks ........................................ 6432-SAMSS-006, Large, Low-Pressure Storage Tanks ....................... 67SAES-D-108, Storage Tank Integrity ................................................. 67SAES-M-100, Saudi Aramco Building Code ..................................... 67SAES-Q-005, Concrete Foundations .................................................. 67

EFFECTS OF TYPES OF SETTLEMENT ON STORAGE TANKS ....................... 70

Background .................................................................................................... 70

Types of Settlement........................................................................................ 70Uniform............................................................................................... 70Planar Tilt ........................................................................................... 71Deviation from Planar Tilt .................................................................. 73Center-to-Edge.................................................................................... 74Local Shell or Bottom......................................................................... 75

Evaluation of Tank Settlement ....................................................................... 76

CALCULATING TANK SETTLEMENT ................................................................. 77

WORK AID

WORK AID 1: PROCEDURES AND DATABASES FOR CALCULATING CIVIL/MECHANICAL LOADS FOR ATMOSPHERIC STORAGE TANKS........................................................................ 79

Work Aid 1A: Procedure for Calculating Weight Loads .............................. 79Work Aid 1B: Procedure for Calculating Total Pressure .............................. 82Work Aid 1C: Procedure for Calculating Roof Live Load............................ 83Work Aid 1D: Procedure and Database for Calculating Wind Loads ......................................................................... 84Work Aid 1E: Procedure and Databases for Calculating Earthquake Base-Overturning Moment................................ 86Work Aid 1F: Procedure for Calculating Live Loads for Appurtenances....................................................................... 91

WORK AID 2: PROCEDURE FOR CALCULATING TANK SETTLEMENT...... 92

GLOSSARY .........................................................................................................................94


API Storage Tank


LIST OF FIGURES

Figure 1. Cone Roof Tank .................................................................................................. 2Figure 2. Geodesic Dome Fixed Roof Tank....................................................................... 3Figure 3. Floating Roof Tank ............................................................................................. 4Figure 4. Fixed Cone Roof with Internal Floating Roof Tank ........................................... 6Figure 5. Single-Walled, Low-Pressure Tank .................................................................... 8Figure 6. Double-Walled, Low-Pressure Tank................................................................... 9Figure 7. Spheroidal Low-Pressure Tanks.......................................................................... 11Figure 8. Spherical Low-Pressure Tank ............................................................................. 12Figure 9. Material Groups .................................................................................................. 22Figure 10. Minimum Permissible Design Metal Temperatures for Plates Used In Tank Shells Without Impact Testing............................................................... 23Figure 11. Permissible Plate Materials and Allowable Stresses (psi) ................................... 25Figure 12. Minimum Shell Thicknesses for API 650 Tanks ................................................ 29Figure 13. Hydrostatic Pressure vs. Depth Below Liquid Surface ....................................... 35Figure 14. Effect of Vapor Pressure on the Total Pressure at a Given Depth Below the Surface................................................................................................ 36Figure 15. Wind Base-Shear Force, FW, and Wind Base-Overturning Moment, MW ......... 39Figure 16. Wind Roof-Lift Load, LW ................................................................................... 40Figure 17. Bending Stress..................................................................................................... 52Figure 18. Stress Concentration............................................................................................ 53Figure 19. Compacted Earth with Oiled Sand Pad ............................................................... 59Figure 20. Crushed Stone Ringwall...................................................................................... 61Figure 21. Concrete Ringwall............................................................................................... 62Figure 22. Concrete Pad ....................................................................................................... 62Figure 23. Piled Foundation with Concrete Slab.................................................................. 64Figure 24. Required Number of Reference Points................................................................ 66Figure 25. Required Settlement Readings ............................................................................ 66Figure 26. Safety Factors for Foundations ........................................................................... 68Figure 27. Uniform Settlement............................................................................................. 71Figure 28. Planar Tilt Settlement.......................................................................................... 72Figure 29. Deviation From Planar Tilt Settlement............................................................... 73Figure 30. Center-To-Edge Settlement................................................................................ 74Figure 31. Local Shell or Bottom Settlement ....................................................................... 75Figure 32. Current and Initial Tank Elevation Readings ...................................................... 78Figure 39. Height-Correction and Gust Response Factors ................................................... 84Figure 40. Factor k ............................................................................................................... 86Figure 41. Weight Coefficients ............................................................................................ 87Figure 42. Site Amplification Factor .................................................................................... 88Figure 43. Height Coefficients ............................................................................................. 89Figure 44. Graph for Plotting Data....................................................................................... 92Figure 45. Example of a Plot ................................................................................................ 93


API Storage Tank

Saudi Aramco DeskTop Standards 1

TYPES, COMPONENTS, AND USES OF STORAGE TANKS

Background

This section discusses the types, components, and uses of the following general types ofstorage tanks:

• API atmospheric

• API low-pressure

• Other

API Atmospheric Storage Tanks

API atmospheric storage tanks store crude oil, petroleum products, chemicals, and water.These tanks are the most common type of storage for petroleum products.

An API atmospheric storage tank consists of a:

• Conical steel bottom resting directly on the ground or on a prepared foundation

• Vertical, cylindrical steel shell

• Roof (The type of roof used depends on the liquid being stored.)

This section discusses the following types of API atmospheric storage tanks:

• Supported cone roof

• Self-supporting fixed roof

• Floating roof

• Fixed roof with internal floating roof


API Storage Tank


Supported Cone Roof Tank

A supported cone roof tank has a fixed roof in the shape of a cone that is supported by rafterson girders or by rafters on roof trusses. The girders or trusses are in turn supported bycolumns resting on the tank bottom.

The supported cone roof tank cannot withstand any significant pressure or vacuum. The roofmust be equipped with an open vent, a pressure-actuated vent, or a "frangible joint". Afrangible joint is a weak welded seam at the roof-to-shell junction. The weld is designed tofail before any major rupture can occur in the tank’s shell. Without proper venting, vaporpressure changes sufficient to damage the roof or shell may result from daily temperaturefluctuations, normal filling and emptying cycles, or from vapor generation due to a fire in thevicinity of the tank.

Components - Figure 1 shows a supported cone roof tank and its primary components.

Usage - Supported cone roof tanks are used when floating roof tanks are not required or arenot more economical. Supported cone roof tanks can be larger in diameter than self-supporting, fixed roof tanks.

Shell

Top angle

(For small diameter tanks without spiral stairway)

Gauge hatch

Spiral stairway

Shell nozzles

Ladder

Open vent (if pressure/vacuum vent not used)

Nozzle

Roof manhole

Shell manhole

Foam connection

Sump

Water draw-off

Pressure vacuum vent

Nozzle

Access platform

Bottom

Roof truss

Roof support column

Figure 1. Cone Roof Tank


API Storage Tank


Self-Supporting Fixed Roof Tank

The roof of a self-supporting, fixed roof tank is supported completely from the shell withoutsupplementary structural members. Therefore, it provides all of its own structural support.The roof may be either conical or dome-shaped. A dome-shaped roof can support itself at alarger diameter than a cone-shaped roof. The self-supporting, fixed roof tank has the samecharacteristics and usages as the supported cone roof tank, except for its roof support details.

Components - Figure 2 shows a geodesic dome fixed roof tank and its primary components.

Usage - Self-supporting, fixed roof tanks are practical only where relatively small fixed rooftanks are required.

Roof

Bottom

Platform

Ladder}AppurtenancesShell

Access hatch

Center dome vent

Figure 2. Geodesic Dome Fixed Roof Tank


API Storage Tank


Floating Roof Tank

A floating roof tank has an open top and a movable roof that floats on top of the liquid beingstored. A space between the floating roof and the tank shell allows the roof to move freely asliquid is added to or withdrawn from the tank. To minimize evaporation losses and reduce therisk of fire, a flexible sealing device is attached to the floating roof. This sealing device canmove freely up and down the tank shell and closes off the space between the rim of the roofand the tank shell.

By virtually eliminating the vapor space above the liquid, the floating roof tank greatlyreduces:

• Evaporative losses• Fire danger• Corrosion caused by the presence of air

Components - Figure 3 shows the features of a floating roof tank that distinguish it from afixed roof tank.

Usage - Saudi Aramco Standard SAES-D-100 specifies that floating roof tanks must be usedto store petroleum products with flash points below 54°C (130°F) or if the flash point is lessthan 8°C (15°F) higher than the storage temperature. Examples of these products are gasolineand naphtha. SAES-D-100 also specifies that floating roof tanks should not to be used tostore products that tend to boil under atmospheric conditions.

Continuous fabric seal

Wind girder

Tank bottom

Gauge hatch

Roof supports

Automatic bleeder vent

Deck Screen

Roof supports

Check valve

Articulated pipe drain

Deck manhole

Emergency drain Pontoon

manhole

Pontoon

Tank shell

Figure 3. Floating Roof Tank


API Storage Tank


Fixed Roof with Internal Floating Roof Tank

A fixed roof with internal floating roof tank is either a self-supporting roof tank or asupported cone roof tank with an internal floating roof inside. The internal floating roof floatson top of the liquid being stored. A flexible sealing device closes off the space between therim of the internal floating roof and the tank shell.

The internal floating roof is usually constructed of materials other than steel, such asaluminum or polyurethane. Usually, the internal floating roof is designed to be assembledwithin a completely constructed tank. The internal floating roof functions the same way asthe floating roof in the floating roof tank: it virtually eliminates the vapor space above theliquid.

Components - Figure 4 shows a fixed roof with internal floating roof tank and its primarycomponents.

Usage - This type of tank typically is used when the service of an existing fixed roof tank ischanged and a floating roof tank should be used for the new service. The tank is prepared forthe new service by adding the internal floating roof inside the existing tank. This type of tankalso may be required when a floating roof tank needs a fixed roof for environmentalprotection or product quality. In this case, a fixed roof is often added to an existing floatingroof tank. A fixed roof with internal floating roof tank has the same usage as a floating rooftank.


API Storage Tank


3

115

2

14

813

7

4

1

6 129

15

10

LEGEND

1. Peripheral roof vent 9. Column negotiating device2. Center roof vent 10. Support legs3. Roof hatch 11. Vacuum-relief device4. Antirotation device 12. Internal floating roof5. Overflow vent 13. Antistatic grounding6. Seal 14. Gauge funnel7. Manway 15. Pontoon8. Gauge flotewell

Figure 4. Fixed Cone Roof with Internal Floating Roof Tank


API Storage Tank


API Low-Pressure Storage Tanks

API low-pressure storage tanks store the following:

• Gases

• Liquids that require a small amount of pressurization

• Liquids that require containment of the vapor as well as the liquid

This section discusses the following types of API low-pressure storage tanks:

• Single-walled

• Double-walled

• Spheroidal

• Spherical


API Storage Tank


Single-Walled, Low-Pressure Tank

The single-walled, low-pressure tank only uses one layer of steel on the shell of the tank tocontain the liquid and vapor.

Components - Figure 5 shows a single-walled, low-pressure tank and its primarycomponents.

3/4 Page Graphic

(6.5" X 5.375")

Roof manholePressure safety valve/

vacuum vent

Bottom

Shell

Roof

Ladder

Compression ring

Figure 5. Single-Walled, Low-Pressure Tank


API Storage Tank


Double-Walled, Low-Pressure Tank

The double-walled, low-pressure tank uses two layers of steel for the shell of the tank tocontain the liquid and vapor.

Usage - This type of tank is used for refrigerated storage. Insulation is installed between theinner and outer layers of the shell. The space between the shells generally is maintained at aslightly positive pressure by a gas, such as nitrogen, that will not liquefy at the storagetemperature.

Components - Figure 6 shows one design of a double-walled, low-pressure tank and itsprimary components.

Cone Roof

Ceiling hangers

Safety valve Fill/discharge nozzle

Insulated suspended ceiling

Inner shell

Insulation

Outer shell

Bottom

Insulation

Figure 6. Double-Walled, Low-Pressure Tank


API Storage Tank


Spheroidal Low-Pressure Tank

The spheroidal low-pressure tank approximates the ideal shape of a free-standing liquiddroplet in which the shell stresses are theoretically equal in all directions. The normal designpressure of spheroidal tanks ranges from 17 kPa(ga) - 103 kPa (ga) (2.5 to 15 psig). Althoughcommercially available, spheroidal tanks are not used widely since spheres are generally moreeconomical to build.

Components - Figure 7 shows two typical spheroid and low-pressure tank designs and theirprimary components.


API Storage Tank


Ordinary Spheroid

Noded Spheroid

Supports

Supports

Sand cushion

Sand cushion

Section

Section

Tie

Truss

Elevation

Elevation

Figure 7. Spheroidal Low-Pressure Tanks


API Storage Tank


Spherical Low-Pressure Tank

The most common type of pressure storage, the spherical low-pressure tank, is a sphere onindividual support columns. It provides the maximum volume of storage for the amount ofwall material used.

Components - Figure 8 shows a spherical low-pressure tank and its components.

Figure 8. Spherical Low-Pressure Tank


API Storage Tank


Other Storage Tanks

Other general types of storage tanks exist. Generally, these tanks are small tanks or tanksbuilt with special storage requirements, including shop-built tanks and chemical storage tanks.Normally they provide atmospheric pressure storage only.


API Storage Tank


APPLICABLE CODES AND STANDARDS FOR SELECTED STORAGE TANKS

This section discusses the codes and standards that apply to storage tanks.

Codes and Standards for API Atmospheric Storage Tanks

The following codes and standards apply to API atmospheric storage tanks:

• API Standard 650, Welded Steel Tanks for Oil Storage

• API Standard 653, Tank Inspection, Repair, Alteration and Reconstruction

• SAES-A-004, Pressure Testing

• SAES-B-005, Spacing and Diking for Atmospheric and Low-Pressure Tanks

• SAES-B-007B, Air Foam Systems for Storage Tanks

• SAES-D-100, Atmospheric and Low-Pressure Tanks

• SAES-D-108, Storage Tank Integrity

• 32-SAMSS-005, Atmospheric Steel Tanks

API Standard 650, Welded Steel Tanks for Oil Storage

This standard provides the requirements for vertical, cylindrical, aboveground, carbon-steelstorage tanks. This standard applies to the following tanks:

• Tanks with internal pressures from atmospheric pressure to 17 kPa (ga) (2.5psig)

• Tanks that are nonrefrigerated

• Tanks with design temperatures less than 260°C (500°F)

• Tanks that store petroleum, other liquid products, or water

This standard covers material, design, fabrication, erection, and testing.

The appendices in this standard cover:

• Optional design basis for small tanks

• Recommendations for design and construction of foundations for above groundoil storage tanks

• External floating roofs


API Storage Tank


• Technical inquiries

• Seismic design of storage tanks

• Design of tanks for small internal pressures

• Structurally supported aluminum dome roofs

• Internal floating roofs

• Undertank leak detection and subgrade protection

• Shop-assembled storage tanks

• Example of application of the variable-design-point procedure to determineshell plate thicknesses

• API Standard 650 storage tank data sheets

• Requirements for tanks operating at elevated temperatures

• Use of materials that are on hand but are not identified as complying with anylisted specification

• Recommendations for underbottom connections

• Allowable external loads on tank shell openings

API 653, Tank Inspection, Repair, Alteration and Reconstruction

This standard covers requirements for inspection, repair, alteration and reconstruction of API650 (and its predecessor API 12C) atmospheric storage tanks that have already been placed inservice. The standard includes the following sections:

• Suitability for Service

• Brittle Fracture Considerations

• Inspection

• Materials

• Design Considerations for Reconstructed Tank

• Tank Repair and Alteration


API Storage Tank


• Dismantling and Reconstruction

• Welding

• Examination and Testing

• Marking and Reconstruction

This standard also has appendices that cover evaluation criteria for tank bottom settlement andchecklists for tank inspection.

SAES-A-004, Pressure Testing

This standard provides the pressure-testing requirements for storage tanks.

SAES-B-005, Spacing and Diking for Atmospheric and Low-Pressure Tanks

This standard provides the spacing and diking requirements for aboveground storage tanks.

SAES-B-007B, Air Foam Systems for Storage Tanks

This standard provides the basic requirements for the installation of air foam fire protectionsystems for large, atmospheric storage tanks.

SAES-D-100, Atmospheric and Low-Pressure Tanks

This standard provides the requirements for the selection, design, and installation of carbon-steel, stainless-steel, and fiberglass storage tanks. The standard applies to the following tanks:

• Tanks that store crude oils, petroleum products, water, and other liquids

• Tanks with internal operating pressures not greater than 103 kPa (ga) (15 psig)

• Tanks with design temperatures between -168°C and +260°C (-270°F and+500°F)

SAES-D-108, Storage Tank Integrity

This standard provides the requirements for testing and inspecting welded steel tanks thathave already been put into service and does not apply to the initial construction of tanks. Thisstandard parallels the API 653 Standard and covers additions and exceptions to the API-653Standard.

32-SAMSS-005, Atmospheric Steel Tanks

This specification covers modifications and additions to API Standard 650. The specificationis included with the purchase order supplied to the tank vendor.


API Storage Tank


Codes and Standards for API Low-Pressure Storage Tanks

The following codes and standards apply to API low-pressure storage tanks:

• API Standard 620, Design and Construction of Large, Welded, Low-PressureStorage Tanks

• SAES-A-004, Pressure Testing

• SAES-B-005, Spacing and Diking for Atmospheric and Low-Pressure Tanks

• SAES-D-100, Atmospheric and Low-Pressure Tanks

• 32-SAMSS-006, Large Welded Low-Pressure Tanks

API Standard 620, Design and Construction of Large, Welded, Low-PressureStorage Tanks

This standard provides the requirements for aboveground tanks with a single vertical-axis-of-revolution. The standard applies to the following tanks:

• Tanks with internal pressures greater than 3.4 kPa (ga) (0.5 psig) but notgreater than 103 kPa (ga) (15 psig)

• Tanks with metal temperatures from -168°C to +120°C (-270°F and +250°F)

• Tanks that are large enough to require field erection

• Tanks that store liquid or gaseous petroleum products, water, and other liquids

Specifically excluded from this standard are small shop-built tanks, tanks covered by APIStandard 650, and “lift-type” gas holders.

SAES-A-004, Pressure Testing

This standard provides the pressure-testing requirements for storage tanks.

SAES-B-005, Spacing and Diking for Atmospheric and Low-Pressure Tanks

This standard provides the spacing and diking requirements for aboveground storage tanks.


API Storage Tank


SAES-D-100, Atmospheric and Low-Pressure Tanks

This standard provides the requirements for the selection, design, and installation of carbon-steel, stainless-steel, and fiberglass storage tanks. The standard applies to the following tanks:

• Tanks that store crude oils, petroleum products, water, and other liquids

• Tanks with internal operating pressures not greater than 103 kPa (ga) (15 psi)

• Tanks with design temperatures between -168°C and +260°C (-270°F and+500°F)

32-SAMSS-006, Large Welded Low-Pressure Tanks

This specification covers modifications and additions to API Standard 620. The specificationis limited to single-walled, aboveground, low-pressure tanks. The specification excludesspheres and spheroids. The specification is included with the purchase order supplied to thetank vendor.


API Storage Tank


Types, Mechanical Properties, and allowable stresses of steels commonlyused for storage tanks

Background

This section discusses the types of steels commonly used for storage tanks. The sectionprovides information on the mechanical properties and allowable stresses of these steels.

The following factors are important in selecting the steel for a storage tank:

• Design metal temperature

• Minimum tensile strength

• Minimum yield strength

• Allowable stresses

Design Metal Temperature

Since most steels become brittle at low temperature and lose their strength at elevatedtemperatures, it is important to select a steel that is appropriate for the range of temperaturesat the tank site and for the necessary storage conditions of the contained fluid. For storagetanks a (minimum) design metal temperature and a maximum operating temperature is usuallyspecified. According to API Standard 650, unless experience or special local conditionsjustify another assumption, the (minimum) design metal temperature is assumed to be 8.3°C(15°F) above the lowest one-day mean ambient temperature in the location where the tank isto be installed.

The (minimum) design metal temperature for refrigerated tanks may also be determined bythe temperature being maintained by refrigeration. This temperature will usually be lowerthan 8.3°C(15°F) above the one-day mean ambient temperature.

The maximum operating temperature may also be important in the design of storage tanksused for heated fluids if the temperature is above 93°C (200°F). Above 93°C (200°F) API650 Standard requires a reduction in the allowable stress used in the tank’s design.


API Storage Tank


Minimum Tensile Strength

Adequate assurance that a tank will not rupture under normal operating loads is required;therefore, it is important to select a steel that has sufficient tensile strength. Tensile strength isthe maximum stress to which a material can be subjected without rupturing. The MinimumTensile Strength is the minimum value of the tensile strength required by the applicablematerial standard which governs the manufacture of the steel.

Minimum Yield Strength

A tank must keep its shape and not permanently deform under normal operating loads;therefore, it is important to select a steel that has a sufficient yield strength. Yield strength isthe amount of stress a material can undergo before there is a relatively large plasticdeformation for small increases in stress. If the stress in the tank is kept below this value, thetank will not suffer any permanent deformation. The Minimum Yield Strength is theminimum value of the yield strength required by the applicable materials standard whichgoverns the manufacture of the steel.

Allowable Stresses

The thickness of tank components such as the shell and roof must be determined usingformulas contained in the applicable API Standard. Typically, these formulas use anallowable stress that depends upon the materials of construction and may depend on themaximum operating temperature. The allowable design stress is based on applying a factor ofsafety to the material’s minimum tensile and yield strength. In the case of API 650 tanks, amaximum Allowable Product Design Stress, Sd, and a maximum Allowable Hydrostatic TestStress, St, is used in the formulas to determine tank shell thickness. The Allowable ProductDesign Stress is used when normal operating fluid is contained. During hydrostatic test, aslightly higher allowable Hydrostatic Test Stress is permitted because this is a controlledsituation. Sd is limited to 40% of the minimum tensile strength or 2/3 of the minimum yieldstrength. St is limited to 3/7 of the minimum tensile strength and 3/4 of the minimum yieldstrength.


API Storage Tank


Allowable Types of Steels

Atmospheric Storage Tanks

API Standard 650 permits the use of several specifications of steel plates for atmosphericstorage tank construction. API Standard 650 also identifies permissible specifications forstructural shapes, piping and forgings, flanges, bolting, and for welding electrodes. Thesespecifications are based on the American Society for Testing and Materials (ASTM),Canadian Standards Association (CSA), and International Organization for Standardization(ISO) specifications. The ASTM, CSA, and ISO specifications classify steels based on alloycontent, manufacturing process, yield strengths and toughness.

The types of steels permitted by API Standard 650 are divided into eight groups according tothe steel manufacturing process used for each material. Figure 9 identifies the materialgroups. Figure 10 shows the (minimum) design metal temperature permitted for eachmaterial group without requiring impact testing based on plate thickness. Figure 11 providesthe allowable stresses from API 650 for particular material specifications.


API Storage Tank


Group IAs Rolled, Semikilled

Group IIAs Rolled, Killed or

Semikilled

Group IIIAs Rolled, Killed

Fine-Grain Practice

Group IIIANormalized, KilledFine-Grain Practice

Material Notes Material Notes Material Notes Material NotesA 283 C 2 A 131 B 7 A 573-58 A 131 CSA 285 C 2 A 36 2, 6 A 516-55 A 573-58 10A 131 A 2 A 442-55 A 516-60 A 516-55 10

A 36 2, 3 A 442-60 G40.21M-260W 9 A 516-60 10Fe 42 B 4 G40.21M-260W Fe 42 D 4, 9 G40.21M-260W 9, 10

Grade 37 3, 5 Fe 42 C 4 Grade 41 5, 9 Fe 42 D 4, 9, 10Grade 41 6 Grade 41 5, 8 Grade 41 5, 9, 10

Group IVAs Rolled, Killed

Fine-Grain Practice

Group IVAAs Rolled, Killed

Fine-Grain Practice

Group VNormalized, KilledFine-Grain Practice

Group VINormalized orQuenched and

Tempered, KilledFine-Grain Practice

Reduced CarbonMaterial Notes Material Notes Material Notes Material NotesA 573-65 A 662 C A 573-70 10 A 131 EH 36A 573-70 A 573-70 11 A 516-65 10 A 633 CA 516-65 G40.21M-300W 9, 11 A 516-70 10 A 633 DA 516-70 G40.21M-350W 9, 11 G40.21M-300W 9, 10 A 537 IA 662 B G40.21M-350W 9, 10 A 537 II

G40.21M-300W 9 A 678 AG40.21M-350W 9 A 678 B

Fe 44 B, C, D 4, 9 A 737 BFe 52 C, D 9Grade 44 5, 9

Notes:

1. Most of the listed material specification numbers refer to ASTM specifications (including Grade or Class); there are,however, some exceptions: G40.21M (including Grade) is a CSA specification; Grades Fe 42, Fe 44, and Fe 52(including Quality) are contained in ISO 630; and Grade 37, Grade 41, and Grade 44 are related to national standards(see 2.2.5).

2. Must be killed or semikilled.3. Thickness ² 0.50 inch.4. Maximum manganese content of 1.5 percent.5. Thickness 0.75 inch maximum when controlled-rolled steel is used in place of normalized steel.6. Manganese content shall be 0.80-1.20 percent by heat analysis for all thicknesses.7. Thickness ² 1 inch.8. Must be killed.9. Must be killed and made to fine-grain practice.10. Must be normalized.11. Must have chemistry (heat) modified to a maximum carbon content of 0.20 percent and a maximum manganese content

of 1.60 percent (see 2.2.6.4).

Source: ANSI/API Standard 650, Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993, Table 2-3.Reprinted courtesy of the American Petroleum Institute.

Figure 9. Material Groups


API Storage Tank


60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-600.25 0.50 0.75 1.00 1.25 1.50

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-60

Group IGroup IV

Group IVA

Group IIA Group II

Group V

Group III

Group VI

Group IIIA

See Note 2

See Note 1

Thickness, including corrosion allowance (inches)

Des

ign

met

al t

emp

erat

ure

(°F

)

Notes:

1. The Group II and Group V lines coincide at thicknesses less than 1/2 inch.2. The Group III and Group IIIA lines coincide at thicknesses less than 1/2 inch.3. The materials in each group are listed in Table 2-3.4. This figure is not applicable to controlled-rolled plates (see 2.2.7.4).5. Use the Group IIA curve for pipe and flanges (see 2.5.5.2 and 2.5.5.3).

Source: ANSI/API Standard 650, Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993, Figure 2-1.Reprinted courtesy of the American Petroleum Institute.

Note: To convert °F to °C subtract 32°F from the temperature in degrees F and multiply by 5/9. To convert inches to mmmultiply the thickness in inches by 25.4.

Figure 10. Minimum Permissible Design Metal Temperaturesfor Plates Used In Tank Shells Without ImpactTesting


API Storage Tank


Cost and acceptability of the material, at the specified design metal temperature and requiredthickness, determine the selection of the steel specification. In general, higher strength steelscost more per pound.

Note that the principal difference between “structural” steels, such as ASTM A36, and mostother specifications permitted by API Standard 650 is that the structural steel has a higherminimum design metal temperature and may not be able to be used without impact testing ifthe required thickness is too large (refer to Figure 11).


API Storage Tank


PlateSpecification Grade

MinimumYield

Strength

MinimumTensile

Strength

Product DesignStress Sd

HydrostaticTest

Stress St

ASTM SpecificationsA 283 C 30,000 55,000 20,000 22,500A 285 C 30,000 55,000 20,000 22,500A 131 A, B, CS 34,000 58,000 22,700 24,900A 36 -- 36,000 58,000 23,200 24,900A 131 EH 36 51,000 71,000a 28,400 30,400

A 442 55 30,000 55,000 20,000 22,500A 442 60 32,000 60,000 21,300 24,000A 573 58 32,000 58,000 21,300 24,000A 573 65 35,000 65,000 23,300 26,300A 573 70 42,000 70,000a 28,000 30,000

A 516 55 30,000 55,000 20,000 22,500A 516 60 32,000 60,000 21,300 24,000A 516 65 35,000 65,000 23,300 26,300A 516 70 38,000 70,000 25,300 28,500

A 662 B 40,000 65,000 26,000 27,900A 662 C 43,000 70,000a 28,000 30,000A 537 1 50,000 70,000a 28,000 30,000A 537 2 60,000 80,000a 32,000 34,300

A 633 C, D 50,000 70,000a 28,000 30,000A 678 A 50,000 70,000a 28,000 30,000A 678 B 60,000 80,000a 32,000 34,300A 737 B 50,000 70,000a 28,000 30,000

CSA SpecificationsG40.21M 260W 37,700 59,500 23,800 25,500G40.21M 300W 43,500 65,300 26,100 28,000G40.21M 350WT 50,800 69,600a 27,900 29,800G40.21M 350W 58,800 65,300 26,100 28,000

National Standards37 30,000 52,600 20,000 22,50041 34,000 58,300 22,700 25,00044 36,000 62,600 24,000 26,800

ISO 630Fe42 B, C 34,000 60,000 22,700 25,500Fe44 B, C 35,500 62,500 23,700 26,600Fe52 C, D 48,500 71,000a 28,400 30,400

a By agreement between the purchaser and the manufacturer, the tensile strength of these materials may be increased to 75,000pounds per square inch minimum and 90,000 pounds per square inch maximum (and to 85,000 pounds per square inch minimumand 100,000 pounds per square inch maximum for ASTM A 537, Class 2, and A 678, Grade B). When this is done, the allowablestresses shall be determined as stated in 3.6.2.1 and 3.6.2.2.

Source: ANSI/API Standard 650, Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993, Table 3-2.Reprinted courtesy of the American Petroleum Institute.

SI Note: To convert allowable stresses in psi to MPa multiply by 6.895 x 10-3

Figure 11. Permissible Plate Materials and Allowable Stresses(PSI)


API Storage Tank


Low-Pressure Storage Tanks

API Standard 620 permits the use of several specifications of steel plates for low-pressurestorage tank construction. API Standard 620 also identifies permissible specifications forstructural shapes, piping and forgings, flanges, bolting, and for welding electrodes. Thesespecifications are based on ASTM, CSA, and ISO specifications. The ASTM, CSA, and ISOspecifications classify steels based on alloy content, manufacturing process, yield strengthsand toughness.

Similar to API 650, the API Standard 620 requirements for selection of steel are based on(minimum) design metal temperature and plate thickness and are presented in Table 2-1 ofAPI 620. Also allowable stresses and weld-joint efficiency are specified in Table 3-1 andTable 3-2 of the standard.


API Storage Tank


CALCULATING CIVIL/MECHANICAL LOADS for atmospheric storage tanks

Background

This section discusses and demonstrates how to calculate the civil/mechanical loads imposedon atmospheric storage tanks. Civil/mechanical loads are loads with which a civil ormechanical engineer would be concerned when designing a tank or its foundation. Thefollowing types of loads are covered:

• Weight Loads

• Total Pressure

• Roof Live Load

• Wind Loads

• Earthquake Base-Overturning Moment

• Live Loads on Appurtenances

Before the weight loads acting on a tank or its foundation can be calculated, the thickness ofthe various components of which a tank is comprised must be known or estimated. The nextsection discusses how to estimate the thickness of tank components if these weight loads arenot known.

Estimating the Minimum Acceptable Thickness of Tank Components

API 650 and API 620 have many criteria for determining the minimum thicknesses of tankcomponents. In this section we will discuss only one of the methods given in API 650 fordetermination of the minimum thickness of the tank shell courses. We will also indicate theAPI 650 minimum thickness requirements for tank bottoms and roofs.

Corrosion allowances, if required, should be added to the minimum thicknesses that arecalculated by the API method that is described later in this module, or the minimumthicknesses specified in API Standards. Corrosion allowances are usually specified by themetallurgical engineer and any further discussion is outside the scope of this course.

The API 650 method and minimum thickness requirements presented in this module can beused for initial thickness estimates for the main components of a tank for the civil/mechanicaldesign. Note that the specific methods and requirements of the applicable API standard shouldbe used in any definitive work. PEDP course MEX 203 is recommended if the Participant isinterested in a more in-depth treatment of API 650 and API 620 requirements.


API Storage Tank


API One-Foot Method

While the liquid contents at the top of a storage tank are essentially at atmospheric pressure,the pressure increases with the depth below the liquid's surface due to the weight of the liquidabove. Therefore, the lower shell courses of a tank are usually thicker than the upper shellcourses to withstand the greater pressure. To account for the increase in pressure thethickness of each shell course must be calculated using an appropriate method. The API 650One-Foot Method is one method that can be used to estimate the thicknesses of API 650 tankshell courses.

In the One-Foot Method, the thickness of each shell course is determined based on limitingthe circumferential membrane stress in the shell at a point that is one foot above the lowestpoint of each shell course to be below an allowable stress. (Hence the name for the method.)The other method presented in API 650 is called the Variable-Design-Point Method. TheVariable-Design-Point Method is much more complex than the One-Foot Method, anddiscussion of it is outside the scope of this course. The Variable Design Point Method is aniterative method that uses the shell course thicknesses determined by the One-Foot Method asits initial starting point. Therefore, the Variable-Design-Point Method can be considered as ameans to "fine-tune" the shell thicknesses of each course. The Variable-Design-Point Methodusually results in slightly thinner and hence more economical tank shells. It should be notedthat the One-Foot Method is actually limited by API 650 to be used only for tanks under 60 m(200 ft.) in diameter and that the Variable-Design-Point Method must be used for larger tanks.However, the One-Foot Method can still be used as a initial estimating tool.

In the One-Foot Method, the minimum thickness of the shell is determined as the larger oftwo quantities, td or tt, as described below:

td = C1d(H-C2)G/Sd + CA

tt = C1d(H-C2)/St

td = Minimum thickness of the shell based on design conditions in inches(millimeters).

tt = Minimum thickness of the shell based on hydrostatic test conditions ininches (millimeters).

C1 = Constant, which accounts for the density of water and the dimensional unitsystem used, equal to 2.6 for U.S. units and 4.9 for S.I. units.

C2 = Constant, equal to 1 foot for U.S. units and 0.3 meters for S.I. units.


API Storage Tank


d = Nominal diameter of the tank in feet (meters).

H = Design liquid level of the tank in feet (meters).

G = Specific gravity of the liquid stored with respect to water (dimensionless).

CA = Corrosion allowance, if required, in inches (millimeters).

Sd = Allowable Product Design Stress for design conditions (Figure 11) in psi(MPa).

St = Allowable Hydrostatic Test Stress for hydrostatic test conditions (Figure11) in psi (MPa).

Note that the above equation is based on the 1993 edition of API 650, in which the weld-jointefficiency of the tank's vertical seams is assumed to be 1.0. In re-evaluating an existing tank,the allowable stresses, Sd and St, may have to be multiplied by a weld-joint efficiency, E,equal to 0.7 or 0.85, depending on the degree of radiography used in the original construction.

Minimum Shell Course Thicknesses for Construction Purposes

API 650 also specifies minimum thicknesses for shell courses for construction purposes basedon tank diameter. These minimum thicknesses are indicated in Figure 12 and may govern thethicknesses of the upper shell courses. Note that the corrosion allowance, if required, shouldbe added to the plate thicknesses shown in Figure 12.

Nominal Tank Diameter Nominal Plate Thickness

meters ft. mm in.

<15.25

15.25 to 36.5

36.5 to 61.0

>61

<50

50 to 120

120 to 200

>200

4.5

6.0

7.5

9.0

3/16

1/4

5/16

3/8

Figure 12. Minimum Shell Thicknesses for API 650 Tanks


API Storage Tank


Minimum Thicknesses for Tank Bottoms

Per API 650, the minimum new nominal thickness of the tank bottom plates is 6 mm (1/4 in.)excluding corrosion allowance. The minimum new nominal thickness of roofs is 4.5 mm(3/16 in.) excluding corrosion allowance. Note that API uses the terminology nominal platethicknesses, since the normal tolerances on plate materials is 0.01 inch or 0.25 mm.

Sample Problem 1: Estimating the thicknesses of the lowest shell course of an API-650tank

Given :

A floating roof tank with:

• A diameter of 200 ft.

• A design liquid storage height of 64 ft.

• An eight-foot shell course height

• Material is A516 Gr 65

• Corrosion Allowance of 0.125 in.

• The tank contains oil with a specific gravity of 0.75

Solution:

From Figure 11 for A516 Gr 65, Sd equals 23300 psi and St equals 26300 psi and using theprevious equations:

td = (2.6 x 200 x (64-1) x 0.75)/23300 + 0.125 = 1.180 in.

tt = (2.6 x 200 x (64-1))/26300 = 1.24 in.

A plate thickness equal to the next nominal thickness (1-1/4 inches) would probably be used.Note that the minimum thickness required for hydrotest governs the design. Also note that ifthe specific gravity of the oil was greater than 0.8, the design case would have governedrather than the hydrotest case, and the minimum thickness for the shell would then have beenbased on the design case.


API Storage Tank


Weight Loads

Background

When designing a tank and its foundation, the design engineer must consider the weight loadswhich are the weight of the tank and the maximum weight of its contents. Since mostpetroleum products are lighter than water, the heaviest weight load occurs during hydrostatictesting, which is done using water.

If a tank and its foundation are designed to withstand the total hydrostatic test weight, WT, thetank foundation should also be able to withstand the weight load imposed during normaloperation when lighter weight crude oils or petroleum products are stored.

The total hydrostatic test weight, WT, is equal to the sum of the hydrostatic test water weight,WH, the tank dead weight empty, WD, and any live loads acting on the tank roof orappurtenances during the test. The tank dead weight empty, WD, is equal to the weight of thetank bottom, Wb, the weight of the shell, Ws, the weight of the roof(s), Wr, the weight of anyappurtenances, Wa, and the weight of any insulation, Wi, that may be installed at the time ofthe hydrostatic test.

Note that insulation is usually not installed at the time of test and the live loads on the roofand appurtenances are usually small compared to other loads involved, and may beconsidered negligible for the purposes of estimating the total hydrostatic test weight, WT.

Procedures

The procedures for calculating weight loads are provided in Work Aid 1A.

Sample Problem 2: Calculating Weight Loads

Calculate the hydrostatic test water weight, the tank dead weight empty, and total hydrostatictest weight of a floating roof tank.

Given:

A floating roof tank with:

• A diameter of 300 ft.

• A designed liquid storage height of 45 ft.

• A tank shell consisting of six, 8 ft. high courses of steel plates with thefollowing course thicknesses:

- First course (bottom course), 1-3/8 in.

- Second course, 1-1/8 in.


API Storage Tank


- Third course, 15/16 in.

- Fourth course, 11/16 in.

- Fifth course, 7/16 in.

- Sixth course (top course), 3/8 in.

• A floating roof:

- That is 3/16 in. thick

- With pontoons and other support structure that add 20% to the weight ofthe roof

• A bottom that is 1/4 in. thick

• The appurtenances on the tank add 2% to the weight of the tank

Solution:

Use Work Aid 1A.

In Step 1, calculate the hydrostatic test water weight, WH:

WH =¹/4d2HLγwWH =¹/4 x (300)2 x 45 x 62.4WH =~198,500,000 lb.

In Step 2, calculate the weight of the tank bottom, Wb:

Wb = π4 d2tγst

Wb =(¹/4 x (300)2 x 0.25/12) x 490Wb = ~721,600 lb.

In Step 3, calculate the weight of the tank shell, Ws:

Since all of the tank shell courses have the same height, the average thickness ofthe shell courses can be computed and used to simplify the calculation:

tavg = (1-3/8 + 1-1/8 + 15/16 + 11/16 + 7/16 + 3/8)/6 = 0.8232 in. = 0.0686 ft.


API Storage Tank


Using values given for this problem:

Ws = ¹dth x VstWs = ¹ x 300 x 490 x 48 x 0.0686Ws = ~1,520,000 lb.

In Step 4, calculate the weight of the floating roof, Wr:

Using the values given for this problem, including 20% for roof structure:

Wr = π4 d2t ×

γst x D.F.Wr = (¹/4 x (300)2 x 3/16 x 490 x (1 + 0.20)

12Wr = ~649,400 lb.

In Step 5, calculate the weight of the appurtenances, Wa:

Given that the appurtenances are 2% of the tank weight, and using the valuescalculated in Steps 2 through 4:

Wa = (Ws + Wb + Wr) x 0.02Wa = (1,520,000 + 721,600 + 649,400) x 0.02Wa = ~57,820 lb.

In Step 6, calculate the tank dead weight empty, WD:

WD = Ws + Wb + Wr + Wa + WiWD = 1,520,000 + 721,600 + 649,400 + 57,820WD = ~2,949,000 lb.

In Step 7, calculate the total hydrostatic test weight, WT:

WT =WH + WDWT =198,500,000 + 2,949,000WT =~201,450,000 lb.

Answer:

The hydrostatic test water weight is approximately 198,500,000 lb. The tank dead weightempty is approximately 2,949,000 lb. The total hydrostatic test weight is approximately101,400,000 lb.


API Storage Tank


Total Pressure and Equivalent Liquid Height

Background

Different pressure loads act on the tank bottom, tank shell, and tank roof that are sometimesused in the design of these components.

Sources of Pressure on a Tank Bottom - The total pressure on a tank bottom is due to:

• Hydrostatic pressure

• Vapor pressure

Sources of Pressure on a Tank Shell - The sources of pressure on a tank shell are as follows:

• Hydrostatic pressure

• Vapor pressure

• Wind pressure effects

Sources of Pressure on a Tank Roof - The sources of pressure on a tank roof are as follows:

• Vapor pressure

• Wind pressure effects

The hydrostatic pressure, PH, increases with the depth below the liquid surface. The highesthydrostatic pressure occurs during hydrostatic testing.

The vapor pressure, PV, is a function of the volatility of the liquid contained in the tank at itsstorage temperature. The process engineer determines the vapor pressure for which the tankshould be designed.

The total pressure, PT, to which a component is subjected is equal to the sum of the individualpressures. The highest total pressure may occur during the normal operation due to the vaporpressure in addition to the hydrostatic pressure of the liquid being stored.

The effects of wind pressure on the tank shell and roof will be covered in a later section.


API Storage Tank


Effect of Depth and Liquid Density on Hydrostatic Pressure - Figure 13 shows that thehydrostatic pressure increases as the depth below the liquid surface increases, and as theliquid density or specific gravity increases.

specific gravity = 1 (water)

Pressure (psi)

Liq

uid

Dep

th (

ft)

0 5 10 15 20 25 300

8

16

24

32

40

48

56

specific gravity = 0.75

specific gravity = 0.50

Figure 13. Hydrostatic Pressure vs. Depth Below LiquidSurface

SI Note: To convert psi to kPa multiply by 6.895 kPa/psi.To convert feet to meters multiply by .3 m/ft.


API Storage Tank


Effect of Vapor Pressure - Figure 14 shows the effect of increasing vapor pressures on thetotal pressure at a given depth in a tank. Note that the vapor pressure of API 650 tanks islimited to be below 17 kPa (2.5 psig) and that the vapor pressure of API 620 tanks is limitedto 103 kPa(15 psig) at the top of the tank.

0 5 10 15 20 25 30 350

8

16

24

32

40

48

56

no vapor pressure (specific gravity = 0.80)

vapor pressure = 5 psi (specific gravity = 0.80)

vapor pressure = 10 psi (specific gravity = 0.80)Pressure (psi)

Liq

uid

dep

th (

ft)

Figure 14. Effect of Vapor Pressure on the Total Pressure at aGivenDepth Below the Surface

SI Note: To convert psi to kPa multiply by 6.895 kPa/psi.To convert feet to meters multiply by .3 m/ft.

Equivalent Liquid Height - In order to account for the effects of vapor pressure the conceptof equivalent liquid height will be introduced. The equivalent liquid height is equal to thetotal pressure divided by the specific gravity of the liquid stored in the tank. This equivalentliquid height is then used in the API 650 equations to determine the thickness of the shellcourse instead of the actual fill height.


API Storage Tank


Procedures

The procedure for calculating total pressure and the equivalent liquid height is provided inWork Aid 1B.

Sample Problem 3: Total Pressure

Calculate the total pressure at the bottom of a cone roof tank and the equivalent liquid heightdue to vapor pressure in the tank.

Given:

The tank has a designed liquid storage height of 64 ft. The vapor pressure of the liquid is 2.5psig. The specific gravity of the oil is 0.9.

Solution:

Use Work Aid 1B.

In Step 1, calculate the hydrostatic pressure, PH:

PH = γh x C.F.PH = (62.4 x 0.9) x 64 x 1/144PH = 25.0 psig

In Step 2, calculate the total pressure, PT:

PT = PH + PVPT = 25.0 + 2.5PT = ~27.5 psig

In Step 3, calculate the equivalent liquid height, Heq:

Heq = PT x C.F. /γHeq = 27.5 x 144 / (62.4 x 0.9)Heq = 70.5 ft.

Answer:

The total pressure at the bottom of the tank is approximately 27.5 psig. The equivalent liquidheight that can be used for design of the shell course is 70.5 ft.


API Storage Tank


Roof Live Load

Background

The roof live load consists of the weights of items on the roof that are not a part of thepermanent structure. Some examples are as follows:

• Personnel• Equipment• Rainwater• Sand or dust

The roofs, tank and its foundation must be designed with the capability to support the rooflive load. A minimum required live load of 122 Kg/m2 or 1.2 kN/m2 (25 lb./ft.2) is specifiedin 32-SAMSS-006 for low-pressure tanks. The same minimum live load is specified in APIStandard 650 for atmospheric tanks. If more than this minimum live load must be supported,then a higher load should be specified. Higher live loads, such as those due to heavypersonnel traffic, heavy equipment, heavy rains or heavy accumulations of sand or dust,should be indicated by the process engineer. If the tank is designed for a positive vaporpressure, the roof must be designed for this also.

Procedure

The procedure for calculating roof live load is provided in Work Aid 1C.

Sample Problem 4: Roof Live Load

Calculate the roof live load for a flat roofed tank that is 100 ft. in diameter. Assume that theminimum roof live load is applicable.

Solution:

Use Work Aid 1C, to calculate the roof live load, LRLL:

LRLL =π4 d2 × RL

LRLL = ¹/4 d2RL

LRLL = ¹/4 (100)2 x 25

LRLL = ~196,350 lb.

Answer:

The roof live load is approximately 196,350 lb.


API Storage Tank


Wind Loads

Background

A strong wind can overturn or slide a tank off its foundation or cause a tank wall to collapse.Empty tanks are especially vulnerable to wind forces. Wind forces acting on tankappurtenances, such as platforms and ladders, can overload these appurtenances or theirattachments to the tank. The pressure due to the wind varies around the circumference of thetank from a high pressure on the windward side to a low pressure (vacuum) on the leewardside. The effects of wind increase with increasing height above grade. Wind blowing overthe top of the tank can also cause a negative pressure or vacuum to act on the tank roof. Intank design, the primary loads that concern a civil/mechanical engineer are the wind base-shear force, FW, the wind base-overturning moment, MW, and the wind roof-lift load, LW.These loads are discussed in detail later in this section and the procedures for the calculationof these loads is provided in Work Aid 1D. Figures 15 and 16 provide diagrams of thesewind forces.

Wind

Fw

Mw

Figure 15. Wind Base-Shear Force, FW, and Wind Base-Overturning Moment, MW


API Storage Tank


WindWind

Lw

Figure 16. Wind Roof-Lift Load, LW

The following factors affect the wind load on a tank:

• Wind velocity (V)

• Tank diameter (d)

• Tank height (H)

• Tank height to tank diameter ratio (H/d)

• The number, size, and characteristics of appurtenances

The wind loads on the tank are a cumulative effect of the wind pressure acting over a surfacearea of the tank and of wind drag or lift coefficients. The wind pressure increases withincreasing velocity and increasing height. The wind drag coefficient is a function of the H/dratio of the tank. The loads on the tank increase with increasing height and diameter of thetank and with the number and size of appurtenances.

Saudi Aramco Standards

SAES-D-100 requires that all tanks be designed to withstand a reference wind velocity, Vr, of137 km/h (85 mph) which is measured at 10 m (33 ft.) above grade. SAES-D-100 requiresthat the tanks be designed for wind loads in accordance with ANSI/ASCE 7-88 (formerlyANSI Standard A58.1) for exposure Level C.


API Storage Tank


Formulas

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

The wind pressure increases with increasing velocity. The wind pressure at the referenceelevation, qr, can be calculated from the following equation:

qr = 0.0473 Vr2 (S.I. Units) (Eqn. 1 SI)

= 0.00256 Vr2 (U.S. Units) (Eqn. 1 US)where:

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

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

Based on the design wind velocity of 137 Km/h (85 mph) indicated in SAES-D-100, qr isequal to 888 Pa (18.5 lb./ft.2)

Wind Loads

The wind load on the tank or an appurtenance is proportional to the wind pressure whichincreases as the elevation increases, the projected area of a portion of the tank or anappurtenance, and a wind drag coefficient.

The wind force on a portion of the tank or an appurtenance can then be expressed as:

f = AKhGCfqr (Eqn. 2)where:

f = Wind force on a portion of a tank or an appurtenance, N (lbs.)

A = Effective projected area of a portion of the tank or an appurtenance, m2 (ft.2)

Kh = Height-correction factor which varies with height above the referenceelevation, (dimensionless).

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

Cf = Surface drag coefficient (dimensionless)

qr = Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2).


API Storage Tank


The wind force increases as the height above the reference elevation increases. In order todetermine the wind force at a higher elevation, a height correction factor, Kh, and a gustresponse factor, G, are used. Kh and G that are found in ANSI/ASCE 7-88 are based on theheight and exposure classification of the location. Excerpts of these tables are presented inWork Aid 1D for exposure classification C.

The wind drag coefficient, Cf, is given in ANSI/ASCE 7-88 for typical structures with variousproportions. For most tanks the H/D (Height/Diameter) ratio is less than one and the surfaceroughness of the tank is relatively smooth. Therefore, a typical value of Cf that would beused for a tank is 0.5. The wind drag coefficient, Cf, for an appurtenance is a function of theappurtenance's shape and solidity (net area/gross area) ratio. The value of Cf forappurtenances ranges from approximately 0.7 to 2.0 depending on the shape and solidityratio. The Participant should reference ANSI/ASCE 7-88 directly if detailed calculations areto be made of wind load on appurtenances.

When determining the effective projected area of a tank, the designer can include the windforce on every appurtenance in the calculation or the designer can estimate the effect of theappurtenances by assuming the tank has an effective diameter, D, slightly larger than itsactual outside diameter. However for this course, the wind load on the tank will beapproximated by using the nominal tank diameter or the nominal tank diameter plus two timesthe insulation thickness (if any), and the effect of the wind load on the appurtenances will beignored.

Since the wind load increases with height above the reference elevation, it is typical to assumethat the tank is divided up into a number of height ranges. The wind loads acting in eachheight range are calculated assuming that Kh elevated at the midpoint of the range appliesover the whole range. The wind forces acting on each height range are then summed up todetermine the total loads acting on the tank base.

With these approximations, the formulas for the wind base-shear force and wind base-overturning moment can be readily calculated.

For wind base-shear force:

Fw = ∑ Kh hh − hl( )( )×DGCfqr (Eqn. 3)

where:

Fw = Wind base-shear force, N (lb.).

Kh = Height-correction factor evaluated at the center of the height range,(dimensionless).

hh = Highest point on the tank shell or roof within the height range, m (ft.).


API Storage Tank


hl = Lowest point on the tank shell or roof within the height range, m (ft.).

D = Effective diameter of the tank, m (ft.). If the tank is externallyinsulated, use the outside diameter of the insulation jacketing.

G = Gust-response factor based on the maximum height of the tank(dimensionless).

Cf = Wind drag coefficient (dimensionless), 0.5 for smooth tanks with H/D <1.

qr = Wind pressure at reference elevation, 888 Pa (18.5 lb./ft.2) for SaudiAramco locations.

For wind base-overturning moment:

Mw = ∑Kh hh − hl( ) hh + hl

2

× DGCfqr (Eqn. 4)

where:

Mw = Wind base-overturning moment, N-m (ft. - lb.)

Kh = Height-correction factor evaluated at the center of the height range(dimensionless)

hh = Highest point on the tank shell or roof within the height range, m (ft.)

hl = Lowest point on the tank shell or roof within the height range, m (ft.)


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

Cf = Wind drag coefficient (dimensionless), 0.5 for smooth tanks with H/D <1.

qr = Wind pressure at referenced elevation, 888 Pa (18.5 lb./ft.2) for SaudiAramco locations.

Work Aid 1D provides the procedures and databases needed to calculate the wind base-shearforce and wind base-overturning moment.


API Storage Tank


Wind Roof-Lift Loading

The wind tends to lift the roof of the tank and to lift the entire tank if the tank has a fixed roof.

ANSI/ASCE 7-88 indicates that for roofs with less than a 10° angle, a combined gustresponse, G, and pressure (lift) coefficient, Cp, should be used with the value of G × Cp = 1.2.Therefore, for a flat or cone roof tank, the following formula can be used to calculate the windroof-lift force, Lw:

Lw = π/4 d2Kh GCpqr(Eqn. 5)

where:

Lw = Wind roof-lift load, N (lb.)

d = Diameter of the tank, m (ft.)

Kh = Height-correction factor evaluated at the center of the tank roof(dimensionless)

GxCp = Combined gust and pressure (lift) coefficient, equal to 1.2 for shallowroofs with less than 10° angle

qr = Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2) for SaudiAramco locations.

Work Aid 1D provides the procedures and database for calculating the wind roof-lift load,Lw.

Sample Problem 5: Determining Wind Loads

Calculate the wind base-shear force, Fw, the wind base-overturning moment, Mw, and thewind roof-lift load, Lw, for a cone roof tank.

Given:

A cone roof tank that:

• Is 100 ft. in diameter

• Has a 48 ft. shell height

• Has a cone roof whose peak is 5 ft. above the edge of the shell


API Storage Tank


Solution:

Use Work Aid 1D.

In Step 1, calculate the wind base-shear force, Fw:

Fw = 0.8 × 16 − 0( )( )+ 0.92 × 32 − 16( )( )+ 1.06 × 48 − 32( )( )+ 1.12× 53 − 48( )( ){ }× 100 × 1.22 × 0.5 ×18.5

Fw =~ 56,500 lb.

In Step 2, calculate the wind base-overturning moment, Mw:

Mw = 0.8 16 − 0( )× 16 + 0( ) / 2( ){ + 0.92 × 32 −16( )× 32 +16( ) / 2( )+

1.06 × 48 − 32( ) 48 + 32( ) / 2( )+ 1.12 × 53 − 48( ) 53 + 48( ) / 2( )}× 100 × 1.22 × 0.5× 18.5

Mw =~ 1,600,000 ft .lb.

In Step 3, calculate the wind roof-lift load, Lw:

Lw = (π/4) (100)2 (1.12) (1.2) (18.5)Lw = ~195,300 lb.

Answer:

The wind base-shear force is approximately 56,500 lb. The wind base-overturning moment isapproximately 1,600,000 ft.-lb. The lift-wind force is approximately 195,300 lb.


API Storage Tank


Earthquake Base-Overturning Moment

Background

The Eastern Province of Saudi Arabia is relatively safe from earthquakes. However, parts ofthe western area of the Kingdom potentially may experience earthquakes. The designengineer must make sure that tanks in these areas are designed to withstand certain earthquakeloads.

An earthquake can cause a tank to overturn, to slide, or to be deformed permanently. It canalso cause attached piping and appurtenances to rupture or tear off the tank. In an open-toptank, an earthquake may cause the contents of a tank to slosh over the top. With petroleumtanks, all these events pose a risk of fire.

When an engineer designs a tank to withstand earthquakes, he considers the following tworesponse modes of the tank and its contents:

• A relatively high-frequency response to lateral ground motion of the following:

- Tank shell

- Roof

- Portion of the liquid contents that moves in unison with the shell

• A relatively low-frequency response of a portion of the liquid contents thatmoves in the fundamental sloshing mode

Saudi Aramco Standards

SAES-D-100 and 32-SAMSS-005 require that the tank be designed for seismic loads inaccordance with API 650 Appendix E. The seismic zone should be indicated on the TankData Sheet. The following procedure is based on the procedure in API 650, Appendix E.

Seismic Zones

Seismic zones are assigned whole numbers from 0 to 4. The number assigned to the seismiczone represents the relative risk of earthquake damage and determines the amount of seismicresistance required in structural design. Low numbers represent low risk; high numbersrepresent high risk. Zone 0 requires no earthquake design. Zone 2 is the highestclassification for any Saudi Aramco location.


API Storage Tank


Formulas

If we assume that the seismic zone is Zone 1 and the tank is at least 30 m (100 ft.) in diameterand less than 15 m (50 ft.) in height, the API 650 Appendix E formula for the earthquakebase-overturning moment is as follows:

ME = ZI 0.24WsHscg + 0.24WrHt + 0.24K w1W CK1HL + K 3

Sk2d

K W2WCK2HL

(Eqn. 6)where:

ME = Earthquake base-overturning moment, N-m (ft.-lb.)

Z = Seismic zone coefficient, dimensionless. For zone 0, Z = 0. For zone 1,Z = 0.1875. For zone 2, Z = 0.375.

I = Essential facilities factor, dimensionless. I = 1 for all petrochemicaltanks, unless otherwise specified by CSD.

Ws = Weight of the tank shell, N (lb.)

Hscg = Height from the base of the tank shell to the shell’s center of gravity, m(ft.)

Wr = Weight of the tank roof(s) (fixed and/or floating), N (lb.)

Ht = Total height of the tank shell, m (ft.)

KW1 = Weight coefficient, based on the ratio of the tank diameter, d, to themaximum design liquid height, HL

Wc = Weight of the tank liquid contents, N (lb.) equal to the hydrostatic testwater weight, WH, multiplied by the liquid contents specific gravity, G.

K1 = Height coefficient, based on the ratio of the diameter of the tank, d, to themaximum design liquid height, HL

HL = Maximum design liquid level, m (ft.)

S = Site amplification factor


API Storage Tank


k = Factor, based on the ratio of the diameter of the tank, d, to the designmaximum liquid height, HL


KW2 = Weight coefficient, based on the ratio of the tank diameter, d, to themaximum design liquid height, HL

K2 = Height coefficient, based on the ratio of the tank diameter, d, to themaximum design liquid height, HL

K3 = Coefficient which is a function of the first sloshing mode of the tank equalto 0.411 in SI units and 1.35 in US units.

SI Note: All constants and coefficients are suitable for use in both US and SI unitsexcept for K3.

The first term in the equation approximates the response of the tank shell to the lateral groundmotion. The second term in the equation approximates the response of the roof to the lateralground motion. The third term in the equation approximates the response of the liquidcontents that move in unison with the shell. The fourth term in the equation approximates theresponse of the liquid contents that slosh.

Work Aid 1E provides the procedures and databases for calculating the earthquake base-overturning moment, ME.

Sample Problem 6: Determining Earthquake Base-Overturning Moment

Determine the earthquake base-overturning moment for a floating roof tank.

Given:

A floating roof tank with a:

• Diameter of 300 ft.

• Height of 48 ft.

• Design liquid storage height of 45 ft.

• Weight of the stored liquid of 168,725,000 lb.

• Tank shell weight of 1,500,000 lb.


API Storage Tank


• Shell center of gravity of 17.75 ft.

• Floating roof weight of 649,000 lb.

The tank is installed in seismic Zone 1.

The tank is located on soil of unknown seismic characteristics.

Solution:

Use Work Aid 1E.

In Step 1, calculate d/HL and determine factor K:

d/HL = 300/45 = 6.67k = ~0.83

In Step 2, determine factors Kw1 and Kw2:

Kw1 = ~0.18Kw2 = ~0.78

In Step 3, determine factor S:

S = 1.5

In Step 4, determine factors K1 and K2:

K1 = ~0.38K2 = ~0.52

In Step 5, calculate the earthquake base-overturning moment, ME:

ME = (0.1875)(1.0)(0.24 × 1,500,000 × 17.75) + (0.24 × 649,000 × 48) +(0.24 × 0.18 × 168,725,000 × 0.38 × 45) + {1.35 × 1.5/((0.83)2 × 300) ×0.78 × 168,725,000 × 0.52 × 45})

ME = ~31,625,000 ft.-lb.

Answer:

The earthquake base-overturning moment is approximately 31,625,000 ft.lb.


API Storage Tank


Appurtenances

When designing or modifying a tank or designing a foundation, the design engineer mustallow for the weight and the forces exerted by the appurtenances.

The primary loads contributed by tank appurtenances are their weight. In the absence of theactual weight of the specific items involved, the weight of all tank appurtenances may beestimated based on the total tank weight, excluding the weight of the contents. Depending onthe particular appurtenance involved and how it is attached to the tank, the weight of theappurtenance may also impose a bending moment on the tank, which the design engineer mayneed to consider. The design considerations for appurtenances will be highlighted in a latersection that covers tank attachments. Some appurtenances, such as stairs, ladders, andplatforms, will also have live loads that have to be taken into consideration in the design.

Work Aid 1F provides the formulas for calculating or estimating the live loads onappurtenances.


API Storage Tank


MECHANICAL CONSIDERATIONS FOR ADDITIONS OR MODIFICATIONS TOTHE APPURTENANCES ON A STORAGE TANK

Background

This section discusses mechanical considerations for the appurtenances on a storage tank.

Attaching anything to a storage tank increases stresses in the shell. Local stresses produce thegreatest concern. Attachments cause the following local stresses:

• Membrane stresses

• Bending stresses

• Peak stresses

• Changes in dead weight

If excessive, these stresses can cause tearing, leaking, or fracturing of the storage tank. Inaddition, attachments cause changes in the dead weight of the entire storage tank.

Membrane Stress

Local loads on a tank result in changes to the membrane stress within the tank shell. Usually,the contribution of attachments to membrane stress is not a major concern with storage tanks.However, the build-up of membrane stress should not be ignored, especially if a tank has anunusually large number of heavy attachments in a relatively small area. High membranestresses can cause the tank to fail in an unexpected manner or in an unexpected area.

Bending Stress

Applying a localized load to any part of a tank causes that part to bend. The bending createsstresses within the part. When the bending increases, the stress also increases. Normally, thestresses are highest in the area of the applied load. Local bending stress in the material causedby loads on the tank, adds to the membrane stress.

If a localized load is applied near a junction within the tank, the load may cause bendingstresses in the junction. For instance, if the shell is loaded by a ladder clip near the bottom ofthe tank, the bending applies additional loads on the weld between the shell and the tankbottom.

The addition of reinforcing plates or pads where attachments are made to the tank reduces thebending stress. These plates or pads distribute the applied loads over a wider area of the tankand reduce the localized stresses.


API Storage Tank


Figure 17 shows bending stress.

Appurtenance weight

Exaggerated result

Force

ForceTank wall

Pipe

Pipe support

Bending moment

Figure 17. Bending Stress


API Storage Tank


Peak Stresses

Peak stresses occur wherever a local area of material is subjected to significantly higher stressthan the material in the surrounding area. This peak stress typically occurs at stressconcentrations or at abrupt geometric discontinuities in the structure. Stress is concentrated atstorage tank attachment points. In general, stress concentration effects need to be consideredonly when the loads are applied cyclically. If the combination of stress level and the numberof cycles is high enough, cyclic stresses could result in a fatigue crack of the tank material orin failure of the tank.

Figure 18 shows stress concentration.

Force

Force

Force

Force

Tank wall

Reference lines

Mounting plate

Result

Peak stresses at corners

Figure 18. Stress Concentration


API Storage Tank


Changes in Dead Weight

The changes in dead weight that result from attachments or modifications are cumulative.When material is added to a storage tank, the weight of the structure is increased by theweight of the added material. When material is removed from a storage tank, the weight ofthe structure is decreased by the weight of the removed material.

Attachments

Some typical examples of attachments or modifications that are made to storage tanks are asfollows:

• Ladders or stairways

• Platforms

• Accesses

• Piping connections

• Supports

The following discussions of these attachments and modifications illustrate the problems thatattachments and modifications may cause and the methods available for minimizing theproblems.

Ladders or Spiral Stairways

Ladders or spiral stairways are installed on a storage tank to gain access to the tank roof forservice and/or inspection.

Potential Problems - When a ladder or stairway is attached to a tank, each attachment pointbecomes a source of bending stress and stress concentration. The attachments must be strongenough to support the weight of the ladder or stairway, the personnel who use the ladder orstairway, and the equipment that may be placed on or be brought up the ladder or stairway.Because of the effects of thermal expansion/contraction, the ladder attachments must permitsome small relative movement between the ladder or stairway and the tank shell.

Methods of Minimizing - When a ladder or stairway is attached to a tank, the attachmentsmust not be made in areas of the tank that are already under higher stress, such as the jointsbetween shell courses, between the shell and the bottom, or between a fixed roof and theshell. Also, the reinforcing pads or plates added at the attachment points must be large andthick enough to distribute the applied loads adequately.


API Storage Tank


Platforms

Platforms are installed on a storage tank to provide relatively safe and convenient areas forinspection, maintenance, and/or equipment mounting. Typically, a platform is installed at thetop of the ladder or stairway and near the gauging/sampling nozzles and roof-access manway.

Potential Problems - When a platform is attached to a tank, each attachment point becomes asource of bending stress and stress concentration. The attachments must be strong enough tosupport the weight of the platform, the personnel who use the platform, and the equipmentthat may be placed on the platform.

Methods of Minimizing - When platforms are attached to a tank, the attachments must not bemade in areas of the tank that are already under higher stress, such as the joints between shellcourses, between the shell and the bottom, or between a fixed roof and the shell.

Accesses

Accesses are installed in the tank shell and roof to enable inspection of the tank contents,inspection of the tank interior, and/or maintenance of the tank interior.

Potential Problems - An access is a source of stress concentration. An access may be asource of bending stress and/or a weak location in the tank.

Methods of Minimizing - Accesses must not be installed in areas of a tank that are alreadyunder higher stress, if other locations are equally satisfactory. The area around the accessmust be reinforced and the access must be as small as possible. The access cover must bestrong enough, but not excessively thick. The access must not be installed at the seambetween shell courses or plates. Design details specified in the appropriate API standard thataddresses these concerns must be used.

Limitations - The reasons for installing an access determine the access’s location. How theaccess is to be used determines its minimum size.

Piping Connections

Piping connections are installed in a tank to allow material to be transferred into and out ofthe tank, to facilitate cleaning and draining of the tank, and to provide connections for safetyvalves.

Potential Problems - A piping connection is a source of stress concentration and bendingstress due to the applied loads from the connected piping system. A piping connection causesa local weakening in the tank where the piping connection is installed.


API Storage Tank


Methods of Minimizing - A piping connection must not be installed in an area of a tank thatis already under higher stress, if other locations are equally satisfactory. The area around thepiping connection must be reinforced. API Standard 650 specifies design details and anevaluation procedure that should be followed to reduce local tank stresses. A pipingconnection must not be installed at a seam between shell courses, at the seam between thebottom and the shell of the tank, or at a seam between shell plates. The piping system mustbe provided with adequate flexibility to adjust to tank settlement and to adjust to tankexpansion and contraction that results from both temperature changes and hydrostatic head.

Limitations - The reasons for installing a piping connection and the layout of the pipingsystem determine the location and size of a piping connection. Designing a piping systemthat allows for tank settlement may be difficult and expensive for cases of significantsettlement, and the difficulty increases with the pipe diameter.

Supports

Supports are attached to certain types of tanks, such as spheres and spheroids, to support theweight of the tank and its contents. The support connections along with their reinforcing padsor plates must be designed to support the weight of the tank and its contents withoutoverstressing the tank. Flat (or conical) bottom tanks are continuously supported on afoundation.

Potential Problems - Support connections are a source of stress concentration and bendingstress. The supports must adjust to the expansion and contraction of the tank. The supportsmust be able to tolerate shifting loads that result from uneven settlement.

Methods of Minimizing - Supports must not be attached in areas of a tank that are alreadyunder higher stress. The reinforcing pads or plates must be sufficiently strong. The possibleexpansion and contraction of the tank during operation must be considered in the supportdesign. Properly designed and constructed foundations can minimize tilting that results fromfoundation settlement.

Limitations - Designing and constructing supports that properly connect to a tank and thattolerate tank expansion and contraction can be expensive. Better foundations are moreexpensive to construct and may not be cost effective.


API Storage Tank


USES OF VARIOUS TYPES OF FOUNDATIONS FOR STORAGE TANKS

General

The foundation supports the tank and prevents it from settling or sinking into the ground. Thefoundation under a tank should:

• Provide a stable surface for supporting the tank

• Limit total settlement to amounts that can be tolerated by the connecting pipes

• Limit differential settlement around the tank circumference and across thebottom to amounts that can be tolerated by the tank shell and bottom

• Provide adequate drainage

An improperly designed or constructed foundation can cause a tank to:

• Distort

• Leak

• Rupture

• Break its connecting pipes

• Have surface-water drainage problems

• Corrode on the bottom

Appendix B of API 650 gives recommendations and SAES-D-100 presents the followingrequirements for the design and construction of tank foundations:

• The grade or surface on which the tank bottom rests should be at least 0.30 m(1 ft.) above the surrounding ground surface. This grading provides drainage,keeps the bottom of the tank dry, and compensates for minor settlement. Theelevation specified for the tank bottom surface should also consider the amountof total settlement that is expected.

• Unless the foundation is concrete, the top 75 - 100 mm (3 or 4 in.) of thefinished grade should consist of sand, gravel, or small crushed stone [not morethan 25 mm (1 in.) in diameter]. The finished grade may be oiled (or stabilizedin some other manner) preserve the contour during construction and to protectthe tank bottom from moisture that will cause it to corrode.


API Storage Tank


• To facilitate drainage, the finished tank grade is usually sloped upward ordownward from the outer periphery to its center depending on whether anupward cone or downward cone is specified for the tank bottom. SAES-D-100requires that the underside of the tank's bottom be coned downward in serviceswhere a water drawoff is required unless otherwise stated on the tank datasheet. When high settlement is expected at the tank center the tank bottomfrequently is coned upward. The radial slope, upward or downward is, 1 in120.

Pressurized tanks are anchored to their foundations. In the case of a flat-bottomed tank withinternal pressure, anchoring helps prevent the pressure from rounding the tank’s bottom andlifting the tank off its foundation when the liquid level is low. Rounding creates stresseswithin the bottom of the shell and the outer edge of the bottom that could cause the tank tofail.

Atmospheric storage tanks are not normally anchored, unless anchoring is needed for wind orearthquake loading.

Soil

Before the foundation and tank are constructed, the design engineer must estimate how muchsettlement will occur during the operating life of the tank. In some cases, it may be necessaryto prepare the soil to better support the loads that will be placed on the soil. Common soil-preparation techniques are as follows:

• Preloading

• Compaction

• Excavation and backfill

Preloading

Preloading the soil is the preferred method of preparation. The soil is preloaded by placingmaterial on top of the ground that will be supporting the foundation and tank. The amount ofmaterial piled on top usually equals or exceeds the weight of the tank and foundation whenthe tank is filled. The material must be left in place long enough to allow the soil to compactunder the weight. This time period depends on the type of soil and the rate at which itconsolidates. The preload time could be six months or more.

Compaction

When there is insufficient time to preload the soil and the existing soil is to be maintained, thesoil may be stiffened by compaction. The soil is compacted by beating or pounding thesurface with equipment specially designed for this purpose.


API Storage Tank


Excavation and Backfill

When the existing soil does not provide the appropriate characteristics for the foundation, itmay be removed and replaced by a more satisfactory soil or engineered fill. The existing soilmust be removed to a depth sufficient to ensure proper support of the foundation and filledtank. The new soil or engineered fill must then be properly compacted during placement andbefore the foundation and tank are constructed.

Types of Foundations

The following sections discuss these types of foundations:

• Compacted earth with oiled sand pad

• Ringwalls

- Crushed stone ringwall

- Concrete ringwall

• Concrete pad

• Piled foundation

Compacted Earth with Oiled Sand Pad

The compacted earth with oiled sand pad foundation is the simplest and least expensive typeof foundation. This type of foundation is used for small flat-bottomed tanks constructed onstable soil. Figure 19 shows the construction of the compacted earth and oiled sand padfoundation.

Tank

Oiled sand layerStable soil Stable soil

Compacted earth

Figure 19. Compacted Earth with Oiled Sand Pad


API Storage Tank


Ringwalls

A ringwall foundation consists of a ring of support material enclosing an area of compactedfill.

Ringwalls are used for the following:

• Larger tanks

• Tanks with high shells

• Tanks with skirts

• Tanks built on soil that is likely to erode

Ringwalls help prevent shell distortion in floating roof tanks. When compared to an oiledsand pad, ringwalls provide the following advantages:

• Better distribution of the shell load

• A level, solid starting plane for construction of the shell

• Better means of leveling the tank grade

• Preservation of the tank grade contour during construction

• Retention of the fill under the tank bottom and prevention of material loss dueto erosion

• Minimal moisture under the tank

The disadvantages of ringwalls are as follows:

• A different material is used in the ringwall and the compacted fill. As a result,the compacted fill can settle, creating stresses on the bottom of the tank at theboundary between the ringwall and the compacted fill.

• Ringwalls are more expensive to construct than compacted earth and oiled sandpads.


API Storage Tank


Crushed Stone Ringwall - Construction of the crushed stone ringwall is illustrated in Figure20.

Crushed stone

Tank

Compacted fill

Crushed stone

Stable soil Stable soil

Figure 20. Crushed Stone Ringwall

Concrete Ringwall - Construction of the concrete ringwall is illustrated in Figure 21.

When a tank needs to be anchored, the concrete ringwall provides a more convenientanchoring than the crushed stone ringwall.

When compared to the crushed stone ringwall, the concrete ringwall is more likely to havedifferential settlement between the ringwall and the fill inside the ringwall. Also, the concreteringwall is more expensive to construct than the crushed stone ringwall.


API Storage Tank


Reinforced concrete

Tank

Compacted fillGrade

Figure 21. Concrete Ringwall

Concrete Pad

The concrete pad is used with tall, small-diameter tanks. The concrete pad is a solid,reinforced-concrete slab placed directly on the soil. The concrete pad provides a means ofanchoring the tank. Figure 22 illustrates a concrete pad.

Soil

Tank

Concrete slab

Figure 22. Concrete Pad


API Storage Tank


Piled Foundation

A piled foundation for a tank consists of a concrete slab or pile cap on which the tank restsand piles (columns) embedded into the soil below the slab.

The pile material may be either reinforced concrete, steel, or timber. The size, length, andnumber of piles depends on soil conditions and on the size and weight of the tank. Ageotechnical specialist usually determines the pile requirements based on the results of a soilinvestigation program.

A piled foundation is used where the following conditions exist:

• Unstable soil

• Tank weight may cause soil to push out from under the tank

• Too much settlement may result from excessive compression of soil under thetank

The advantages of a piled foundation are as follows:

• It may be used with any size of tank

• It provides convenient anchoring for the tank

Disadvantages are as follows:

• It is the most difficult foundation to correct if problems occur

• It is the most expensive foundation to construct

A piled foundation gets its supporting capacity from the piles driven into the ground. The twosources of the vertical load supporting capacity for a pile are (1) the friction along the length(sides) of the pile and (2) the bearing capacity at the bottom end of the pile. Figure 23illustrates a piled foundation.


API Storage Tank


Figure 23. Piled Foundation with Concrete Slab

Saudi Aramco Requirements

The following Saudi Aramco standards apply to foundations:

• 32-SAMSS-005, Atmospheric Steel Tanks

• 32-SAMSS-006, Large, Low Pressure Storage Tanks

• SAES-D-108, Storage Tank Integrity

• SAES-M-100, Saudi Aramco Building Code

• SAES-Q-005, Concrete Foundations

32-SAMSS-005, Atmospheric Steel Tanks

32-SAMSS-005 provides the requirements for testing and inspecting welded steel tanks thatstore oil, water, and chemicals at approximately atmospheric pressure. The standard appliesto newly constructed tanks during initial test and inspection.


API Storage Tank


32-SAMSS-005 requires the following of newly constructed tanks:

• The tank foundation must be inspected prior to tank erection for compliancewith all design requirements.

• Tank bottoms inspected must be as required in API Standard 650.

• Tank welds must be inspected in accordance with the requirements in APIStandard 650.

• Hydrostatic test water must meet the requirements of par. 5.3.6.

- Cone roof tanks must be filled to 50 mm (2 in.) above the top angle.

- Any settlement of cone roof tanks that exceeds one percent of the tankdiameter shall be referred to CSD for analysis.

- Floating roof tanks must be filled to within 450 mm (18 in.) of the topangle.

- Any floating roof tank shall be considered for jacking when excessiveovalization has occurred. Ovalization is generally excessive if thedifference between the maximum and minimum diameters at the topreaches 300 mm (12 in.). Any such tank where uneven settlementreaches 2.8 mm per meter (1 in. per 30 ft.) of circumference is to bechecked for shell-to-floating roof clearance.

• The following additions or modifications to the testing apply to tanks with acapacity over 800 cm3 (5,000 barrels):

- Elevation measurements must be taken during the initial hydrostatictest:

+ For this purpose, reference points equally spaced around the circumference shall beestablished. The reference points shall be nuts or other similar items welded to the tank’sshell 100 mm (4 in.) above the bottom edge. One of the reference points must be placedat the catch basin.

+ The reference points must be placed at approximately equal distances around thecircumference of the tank. The number of reference points for various tank diameters isgiven in Figure 24.


API Storage Tank


Tank Diameter No. of Reference Points15 m (50 ft.) and less 4Over 15 m (50 ft.) and less than 45 m (150 ft.) 845 m (150 ft.) and over 16

Figure 24. Required Number of Reference Points

+ The observed elevations must be referenced to a permanent benchmark. The instrumentmaking the measurements must be set up at a distance from the tank of at least 1-1/2 timesthe tank diameter.

+ Six sets of settlement readings must be taken. (See Figure 25.)

Reading When To Be Taken1 Before the start of the hydrostatic test2 With the tank 1/4 full ± 0.60 m (2 ft.)3 With the tank 1/2 full ± 0.60 m (2 ft.)4 With the tank 3/4 full ± 0.60 m (2 ft.)5 With the water level at or above the maximum working filling

height (32-SAMSS-005, par. 5.5.6 gives additional timeconditions for this step for special tanks)

6 After the tank has been emptied of test water

Figure 25. Required Settlement Readings

+ Any differential settlement greater than 1.5 mm per meter (1/2 in. per 30 ft.) of tankcircumference or uniform settlement greater than 50 mm (2 in.) must be reported to theChief Engineer.

+ A record of elevation observations shall be filed in the Plant Inspection Record Book bythe Buyer's Inspector.

- The tank must be continually inspected as it is filled to note any leaks or other signs ofweakness in the tank, its roof, and its foundation.


API Storage Tank


32-SAMSS-006, Large, Low-Pressure Storage Tanks

This standard contains similar requirements for the design, inspection, testing and monitoringof foundations as API 650 and 32-SAMSS-005.

SAES-D-108, Storage Tank Integrity

SAES-D-108 covers additions and exceptions to the requirements of API 653 governing thestructural integrity of welded-steel storage tanks constructed to API 650 or to API 12C. ThisSAES applies to existing tanks during Test and Inspections (T&Is) and not to tanks duringinitial construction.

SAES-D-108 requires the following of existing tanks:

• They be inspected using the standards established in the API Standard 653,Tank Inspection, Repair, Alteration and Reconstruction.

• They be inspected and tested after any repair or modification that might affectthe strength or safety of the tank.

• They have periodic inspections as required by the Equipment InspectionSchedule.

SAES-M-100, Saudi Aramco Building Code

All construction must meet the requirements of the Saudi Arabian Uniform Building Code asmodified by SAES-M-100.

SAES-Q-005, Concrete Foundations

SAES-Q-005 specifies that the foundations for atmospheric storage tanks be constructed inaccordance with the instructions in API Standard 650. In addition, the standard providesrequirements for soil analysis, foundations, concrete ringwalls, and anchor bolts.

SAES-Q-005 requires that the soil analysis include the following soil-related characteristics:

• Stratigraphy of subsurface materials

• Maximum allowable soil-bearing pressure

• Recommended depth of the bottom of the foundation

• Unit soil weight

• Internal friction angles


API Storage Tank


• Soil-shearing capacity

• Groundwater location, chemistry, and fluctuation

• For pile-type foundations, data that establishes the minimum pile group spacingbased on the type of pile and load-carrying capacity

SAES-Q-005 requires the following of foundations:

• They be founded on undisturbed soil at least 600 mm (2 ft.) below the existingor finished grade surface.

• If subject to water pressure, they be designed to resist a uniformly distributeduplift equal to the full hydrostatic pressure.

• The top of the concrete be at least 150 mm (6 in.) above the finished grade.

• They have the safety factors as shown in Figure 26.

SITUATION/CONDITION SAFETY FACTOR

Overturning during construction or erection 1.5Sliding 1.5For compression piles, ultimate capacity 2.0For tension piles, ultimate capacity 3.0All other conditions 2.0

Figure 26. Safety Factors for Foundations

SAES-Q-005 requires that concrete ringwalls meet the requirements of foundations and thatthey meet the following requirements:

• They have a minimum width of 300 mm (12 in.)

• They have an average unit soil loading under the ringwall equal to the soilpressure under the confined earth at the same depth.

• They be designed to resist horizontal active earth pressure.

• They have a concrete compression strength of at least 27,600 kPa (4,000 psi)after 28 days.


API Storage Tank


SAES-Q-005 requires the following when anchor bolts are used:

• The distance from the anchor bolts or anchor-bolt sleeves to the outer edge ofthe concrete be at least 75 mm (3 in.)

• The anchor bolts that are subject to uplift or vibration be equipped with a nutthat locks the anchor bolt.


API Storage Tank


Effects of types of Settlement on storage tanks

Background

The excessive settling of a tank can cause serious tank operating problems and lead to tankfailure. Therefore, a key step in tank design is estimating the amount of settlement the tank’sshell will undergo in its lifetime. The desired maximum lifetime settlement is usually lessthan 0.3 m(1 ft.). When settlement exceeds 0.3 m (1 ft.), there may be serious problems with the storagetanks, shell, annular plate or bottom.

Types of Settlement

The settling of a tank is classified by the type of shell settlement and the type of bottomsettlement.

When a tank shell settles, the settlement can be classified as uniform, planar tilt, or deviationfrom planar tilt. When a tank bottom settles, the settlement can be classified as center-to-edgeor local shell and bottom.

The following sections discuss these types of settlement:

• Uniform

• Planar tilt

• Deviation from planar tilt

• Center-to-edge

• Local shell or bottom

Uniform

When the tank shell remains level as the tank settles, uniform settlement has occurred.Uniform settlement does not cause significant stresses or distortions in the tank. This type ofsettlement requires correction only when the foundation or piping connections developproblems. Figure 27 illustrates uniform settlement.


API Storage Tank


Shell settles but remains level

Drainage problemPotential for overstress of piping nozzle

Figure 27. Uniform Settlement

Uniform settlement can cause the following:

• Overstressing of the connecting piping and associated tank nozzle.

• Blockage of surface water drainage from the tank pad, which could causecorrosion of the tank shell or bottom

Planar Tilt

When the tank’s shell tilts as the tank settles and the bottom of the shell remains in a singleplane, planar tilt settlement has occurred. The bottom plane does not distort; it only tilts.Figure 28 illustrates planar tilt.


API Storage Tank


Shell settles and tilts

Figure 28. Planar Tilt Settlement

As the shell tilts, stresses are introduced that change the shape of the shell. As a result ofthese stresses, the top of the tank becomes elliptical.

Planar tilt settlement can cause the following:

• Malfunction of floating roof seals

• Binding of a floating roof

• Problems with connecting pipes

• Problems with surface water drainage from the tank pad

• Buckling in flanges or webs of wind girders


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Deviation from Planar Tilt

When the shell does not remain in a plane as it settles, deviation from planar tilt or differentialsettlement has occurred. Figure 29 illustrates deviation from planar tilt.

Differential settlement around shell

Shell may buckle

Figure 29. Deviation From Planar Tilt Settlement

Deviation from planar tilt settlement can cause the following:

• Malfunction of floating roof seals

• Binding of a floating roof

• Problems with connecting pipes

• Problems with surface water drainage from the tank pad

• Buckling in flanges or webs of wind girders

• Shell buckling

• Overstress of the shell or bottom plates


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Center-to-Edge

When the support under the bottom of the tank settles more than the support under the shell ofthe tank, center-to-edge settlement has occurred. Figure 30 illustrates center-to-edgesettlement.

Figure 30. Center-To-Edge Settlement

Excessive center-to-edge settlement is most likely to cause the following:

• In tanks under 45 m (150 ft.) in diameter, buckling of the bottom shell course

• In tanks over 45 m (150 ft.) in diameter, failure in the bottom plates

• Inaccuracies in tank gauging


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Local Shell or Bottom

When the shell and bottom do not settle together or if local areas of the bottom settledifferently from the rest of the bottom, local shell or bottom settlement has occurred. Figure31 illustrates local shell or bottom settlement.

Shell settles more than tank bottom

Local bottom settlement

Figure 31. Local Shell or Bottom Settlement

In local shell or bottom settlement, significant stress may develop in the bottom plates, theirattachment welds, the bottom-to-shell junction weld, or the lower section of the shell. Thisstress can cause the bottom of the tank to fail.


API Storage Tank


Evaluation of Tank Settlement

API 653, Appendix B presents criteria for determining if the settlement around the tank shellor the tank bottom is excessive. If the settlement is excessive, then repairs to the tank and/orfoundation may be required. Since repairs to a tank foundation, although possible, are veryexpensive and time consuming, the criteria in API 653 are often used as an initial screeningcriterion to determine whether a more sophisticated analysis using computer modeling of thetank settlement problem is required.

If the tank settlement is too large, various types of repairs to the foundations can be made. Ifthe ringwall has suffered local differential settlement, a portion of a ringwall that has settledtoo much may be replaced. Releveling of the entire ringwall by using an epoxy grout issometimes done to correct for excessive tilt. Replacing/recompacting and/or releveling theentire tank pad is sometimes done if tank bottom settlement is excessive. In most cases, theserepairs are made when the tank is out-of-service at a scheduled Test & Inspection interval. Insome cases, the foundation repairs are made along with repairs that are required for the tankbottom, the annular plate or to the shell and that are caused by excessive corrosion, distortion,or cracking of the steel.

Since repairs to the tanks foundation may be carried out along with repairs to the tank steelcomponents, the work must be properly coordinated. If the job is to be done in an economicalfashion, the civil and mechanical engineers assigned to the job must work together during theassessment of settlement, the evaluation of various repair alternatives and during the ultimaterepair.


API Storage Tank


CALCULATING TANK SETTLEMENT

Settling of tanks must be measured and analyzed during the life of the tanks. Work Aid 2provides a procedure for determining the amount and kind of settlement.

Sample Problem 7: Calculate Tank Settlement

Calculate the amount and kinds of tank settlement.

Given:

A tank is 35 m in diameter. The initial readings and current readings in Figure 32 have beentaken on this tank.

Answer:

The instructor will lead the Participants through the solution of this problem during class.


API Storage Tank


READING NUMBER

ORIGINAL ELEVATION (cm)

CURRENT ELEVATION (cm)

1 205.65 151.27

2 205.65 149.97

3 205.65 151.23

4 205.65 153.74

5 205.65 152.48

6 205.66 153.76

7 205.66 162.44

8 205.66 165.03

9 205.66 163.77

10 205.66 157.62

11 205.65 157.48

12 205.65 153.72

Figure 32. Current and Initial Tank Elevation Readings


API Storage Tank


Work Aid 1: Procedures and Databases for Calculating CIVIL/MECHANICAL Loads for Atmospheric Storage Tanks

Work Aid 1A: Procedure for Calculating Weight Loads

1. Calculate the hydrostatic test water weight using the following formula:

WH =π4

d2HLγ w(Eqn. 7)

where:WH = Hydrostatic test water weight, N (lb.)d = Diameter of the tank, m (ft.)HL = Design maximum height of liquid in the tank, m (ft.)γw = Weight density of water 9.81 kN/m3 (62.4 lb./ft.3)

2. If not already known, calculate the weight of the tank bottom using the followingformula:

Wb =

π4 d2tbγst (Eqn. 8)

where:Wb = Weight of the tank bottom, N (lb.)d = Diameter of the tank, m (ft.)tb = Thickness of the tank bottom, in meters (ft.)γst = Weight density of steel, 77 kN/m3 (490 lb./ft.3)

3. If not already known, calculate the weight of the tank shell using the following formula:Ws = πdtavgh γst (Eqn. 9)

where:Ws = Weight of a tank shell, N (lb.)d = Diameter of the tank, m (ft.)tavg = Average thickness of the tank shell, in meters (feet)h = Height of the tank, m (ft.)γst = Weight density of steel, 77 kN/m3 (490 lb./ft.3)

4. If not already known, estimate the weight of the tank roof(s) using the appropriateformula(s) from the following:

For a flat roof:

Wr =

π4 d2tr γst x D.F. (Eqn. 10a)


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where:Wr = Weight of the roof, N (lb.)d = Diameter of the tank, m (ft.)tr = Thickness of the roof, in meters (in feet)γst = Weight density of steel, 77 kN/m3 (490 lb./ft.3)D.F. = Design factor to account for the additional weight of the roof

support structure. If not specified assume this design factor is equalto 1.20.

For a cone roof:

Wr =π2

dtrγ st

d2

4+hr

2

1/2

×D.F.(Eqn. 10b)

where:

Wr = Weight of the cone roof, N (lb.)d = Diameter of the tank, m (ft.)tr = Thickness of the roof, in meters (in feet)hr = Height of the peak of the roof above the tank shell, m (ft.)γst = Weight density of steel, 77 kN/m3 (490 lb./ft.3)D.F. = Design factor to account for the additional weight of the roof

support structure. If not specified assume this design factor is equalto 1.20.

5. If not already known, estimate the weight of the appurtenances.

Wa = (Ws + Wb + Wr) x D.F. (Eqn. 11)

Ws = Weight of the tank shell, N (lb.)Wb = Weight of the tank bottom, N (lb.)Wr = Weight of the tank roof(s), N (lb.)Wa = Weight of the appurtenances, N (lb.)D.F. = Design factor to account for the additional weight of appurtenances.

If not specified assume this design factor is equal to 0.02.


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6. Using the following formula, calculate the tank dead weight empty.

WD = Ws + Wb + Wr + Wa + Wi (Eqn. 12)

where:WD = Tank dead weight empty, N (lb.)Ws = Weight of the tank shell, N (lb.)Wb = Weight of the tank bottom, N (lb.)Wr = Weight of the tank roof(s), N (lb.)Wa = Weight of the appurtenances, N (lb.)Wi = Weight of insulation, if any, N (lb.)

7. Using the following formula, calculate the total hydrostatic test weight.

WT = WH + WD (Eqn. 13)

where:WT = Total hydrostatic test weightWH = Hydrostatic test water weightWD = Tank dead weight empty


API Storage Tank


Work Aid 1B: Procedure for Calculating Total Pressure

1. Using the following formula, calculate the hydrostatic pressure.

PH = γ h x C.F. (Eqn. 14)

where:PH = Hydrostatic pressure, kPa (psi)γ = Specific weight of the liquid, kN/m3 (lb./ft.3). For water, the specific

weight is 9.81 kN/m3 (62.4 lb./ft.3). For other liquids multiply thespecific weight of water by the specific gravity, G, of the liquid.

h = Height of the liquid above the point being considered, m (ft.)C.F. = Conversion factor equal to 1 kPa/1 kN/m2 (1 psi/144 lb./ft.2)

2. Using the following formula, calculate the total pressure.

PT = PH + PV (Eqn. 15)

where:PT = Total pressure, kPa (psi)PH = Hydrostatic pressure, kPa (psi)PV = Vapor pressure, if not specified, assume 0 kPa (psi)

3. Using the following formula, calculate the equivalent liquid height.

Heq = (PT x C.F.)/γ (Eqn. 16)

where:PT = Total pressure, kPa (psi)γ = Specific weight of the liquid as defined above, kN/m3 (lb/ft3)C.F. = Conversion factor equal to 1 kN/m2/1 kPa (144 lb/ft2/psi)


API Storage Tank


Work Aid 1C: Procedure for Calculating Roof Live Load

Calculate the roof live load based on the specified or minimum roof live loading and thehorizontal projected area of the roof using the following formula:

LRLL =

π4 d2 RL (Eqn. 17)

where:LRLL = Roof live load, lb.d = Diameter of the roof, ft.RL = Roof live loading, 1.2 kN/m2 (25 lb./ft.2)


API Storage Tank


Work Aid 1D: Procedure and Database for Calculating Wind Loads

1. Using the following formula, calculate the wind base-shear force.

Fw = ∑ Kh hh − hl( )( )×DGCfqr (Eqn. 3)

where:Fw = Wind base-shear force, N (lb.)

Kh = Height correction factor evaluated at the center of the height rangefrom Figure 39




G = Gust response factor for maximum height of the tank (dimensionless)from Figure 39.

Cf = Wind-drag coefficient, (dimensionless), 0.5 for smooth tanks withH/D < 1.

qr = Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2) forSaudi Aramco locations.

Height above grade inmeters (ft.)

Kh(evaluated at

midpoint of range)

G(Evaluated at top of

range)

0-5 (0-16) .8 1.32

5-10 (16-32) .92 1.26

10-15 (32-48) 1.06 1.23

15-20 (48-64) 1.17 1.20

20-25 (64-80) 1.25 1.18

SI Note: Kh and G is the same for both SI and US customary units

Figure 39. Height-Correction and Gust Response Factors


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2. Using the following formula, calculate the wind base-overturning moment.

Mw = ∑Kh hh − hl( ) hh + hl

2

× DGCfqr

(Eqn. 4)

where:Mw = Wind base-overturning moment, N-m (ft.-lb.)

Kh = Height correction factor evaluated at the center of the height range(dimensionless), from Figure 39




G = Gust response factor for maximum height of the tank (dimensionless)from Figure 39.

Cf = Wind-drag coefficient, (dimensionless) 0.5 for smooth tanks withH/D < 1.

qr = Wind pressure at reference elevation, 888 Pa (18.5 lb./ft.2) for SaudiAramco locations.

3. Using the following formula, calculate the wind roof-lift load.

Lw = π/4 d2 Kh GCpqr (Eqn. 5)

where:Lw = Wind roof-lift load, N (lb.)d = Diameter of the tank, m (ft.)Kh = Height correction factor evaluated at the mid-point of the roof

(dimensionless) from Figure 39GxCp = Combined gust and pressure lift coefficient (dimensionless) equal to

1.2 for shallow roofs with less than 10° angle.qr = Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2) for

Saudi Aramco locations.


API Storage Tank


Work Aid 1E: Procedure and Databases for Calculating Earthquake Base-OverturningMoment

1. Calculate the value of d/HL (d = diameter of the tank, HL = the design maximum heightof the liquid contents) and determine the value for factor k from Figure 40.

1.0

0.8

0.6

0.50 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

d/H L

k

Source: Based on ANSI/API Standard 650, Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993,Fig. E-4.

Figure 40. Factor k


API Storage Tank


2. Determine the values for the weight coefficients Kw1 and Kw2 from Figure 41.

1.00 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0.2

0.4

0.6

0.8

1.0

Kw1

Kw2

d/HL


Figure 41. Weight Coefficients


API Storage Tank


3. Determine the value for the site amplification factor S from Figure 42.

Soil Profile TypeSite

SoilProfileCode

Soil CharacteristicsAmplification

Factor

S

S1 Either:

Rock of any characteristic, whether shale-like orcrystalline in nature, characterized by a shear-wavevelocity greater than 2,500 ft/s

or

Stiff soil less than 200 ft. deep in which the soil thatoverlies rock consists of stable deposits of sands, gravels,or stiff clays

1.0

S2 Deep cohesionless or stiff clay, including soil that is morethan 200 ft. deep in which the soil that overlies rockconsists of stable deposits of sands, gravels, or stiff clays

1.2

S3 Soft to medium-stiff clays and sands characterized by 30ft. or more of soft to medium-stiff clay with or withoutintervening layers of sand or other cohesionless soils

1.5

S4 A soil profile containing more than 40 ft. of soft clay 2.0

Unknown When the soil profile is not known in sufficient detail todetermine the soil profile type, assume soil profile S3

1.5

Source: Based on ANSI/API Standard 650, Ninth Edition, Washington, D.C., American Petroleum Institute, July 1993,Table E-3.

SI Note: The site amplification factor is dimensionless. To convert feet to meters multiply 1 m/ 3.28 ft.

Figure 42. Site Amplification Factor


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4. Determine the value for the height coefficients K1 and K2 from Figure 43.

1.00 2.0

3.04.0

5.06.0

7.08.0

0.2

0.4

0.6

0.8

1.0

K2

K1

d/HL

or

K


Figure 43. Height Coefficients


API Storage Tank


5. Using the following formula, calculate the earthquake base-overturning moment.

ME = ZI (0.24WsHscg + 0.24WrHt + 0.24Kw1WcK1HL + K3

S

k 2d

KW2WcK2HL) (Eqn. 6)where:

ME = Earthquake base-overturning moment, N-m (ft.-lb.)

Z = Seismic zone coefficient, dimensionless. For zone 0, Z = 0. For zone 1, Z =0.1875. For zone 2, Z= 0.375.

I = Essential facilities factor, dimensionless. I = 1 for all petrochemical tanks,unless otherwise specified by CSD.

Ws = Weight of the tank shell, N (lb.)

Hscg = Height from the base of the tank shell to the shell’s center of gravity, m (ft.)

Wr = Weight of the tank roof(s) (fixed and/or floating), N (lb.)

Ht = Total height of the tank shell, m (ft.)

KW1 = Weight coefficient, based on the ratio of the tank diameter, d, to the maximumdesign liquid height, HL, from Figure 41

Wc = Weight of the tank liquid contents equal to the hydrostatic test water weight,WH, multiplied by the specific gravity of the tank contents, N (lb.)

K1 = Height coefficient, based on the ratio of the diameter of the tank, d, to themaximum design liquid height, HL, from Figure 43

HL = Maximum design liquid level, m (ft.)

S = Site amplification factor from Figure 42

k = Factor, based on the ratio of the diameter of the tank, d, to the design maximumliquid height, HL, from Figure 40


KW2 = Weight coefficient, based on the ratio of the tank diameter, d, to the maximumdesign liquid height, HL, from Figure 41

K2 = Height coefficient, based on the ratio of the tank diameter, d, to the maximumdesign liquid height, HL, from Figure 43

K3 = Coefficient which is a function of the first sloshing mode of the tank and equalto 0.411 in SI units and 1.35 in US units.

SI Note: All constants and coefficients are suitable for use in either SI or US units exceptfor K3.


API Storage Tank


Work Aid 1F: Procedure for Calculating Live Loads for Appurtenances

Platforms, ladders and their attachments to the tank should be designed to support their ownweight plus a live load equal to the greater of 4450 N (1,000 lb.) or 2.4 kPa (50 lb./ft.2) on thefloor and tread areas, A, unless otherwise specified.

when A < 1.85 m2 (20 ft.2):LLL = 4450 N (1,000 lb.) (Eqn. 18)

when A > 1.85 m2 (20 ft.2): LLL = A x 2.4 kPa in SI units (Eqn. 19)

or LLL = A x 50 lb./ft.2 in US units

where:LLL = Live load on appurtenances, N (lb.)A = Total floor and tread area, m2 (ft.2)


API Storage Tank


Work Aid 2: Procedure for Calculating Tank Settlement

1. Obtain the data on the original elevation readings of the tank shell.

2. Subtract the actual settlement readings from the corresponding original elevationreadings.

3. The minimum difference between an original elevation reading and the correspondingactual settlement reading is the amount of uniform settlement.

4. Subtract the uniform settlement from the maximum difference between the originalelevation reading and the corresponding actual settlement reading. The result is theamount of planar tilt settlement.

5. Plot the actual settlement readings around the circumferences of the tank starting withthe highest point at 0°. Figure 44 provides a graph that can be used for plotting the data.

Elevation

Angle

Figure 44. Graph for Plotting Data


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6. Plot a cosine curve that most closely matches the actual settlement readings. Figure 45shows an example of a plot of data.

Deviation from planar tilt

0 30 60 90 120 150 180 210 240 290 300 330 360 (0)

Elevation

High point 0°

Actual settlement readings

Angle

Planar tilt

Figure 45. Example of a Plot

7. The vertical difference between the best-fit cosine curve and the plot of actual settlementreadings is the deviation from planar tilt and represents the differential circumferentialsettlement.


API Storage Tank


GLOSSARY

ANSI American National Standards Institute

API American Petroleum Institute

ASCE American Society of Civil Engineers

ASTM American Society for Testing and Materials

crown A rise in soil elevation toward the center of an area.

CSA Canadian Standards Association

design metaltemperature

For tankage, the design metal temperature is usually set at 8°C(15°F) above the lowest one-day mean. The design metaltemperature is not the maximum temperature but the minimumtemperature for tankage. It is used to select material withadequate toughness to prevent brittle fracture.

frangible joint Weak welded joint at the top of a tank that fails if the tank isoverpressured.

ISO International Organization for Standardization

km/h Kilometers per hour

ksi 1,000 pounds per square inch

maximum operatingtemperature

For tankage, is the maximum temperature at which the contentsof the tank is stored. If above 93°C (200°F) then additionalconsiderations are required in the design.

mph Miles per hour

periphery The outer edge of an area

psi Pounds per square inch

slosh The movement of a liquid that is not synchronous with themovement of the container storing the liquid.

Specific Gravity The ratio of the weight density of a liquid to the weight densityof water (dimensionless).

small tank Tank with a diameter of 15.25 m (50 ft.) or less.

aramco 1

Documents

api atmospheric storage

storage tank integrity

selectedstorage tanks

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oil storage

fixed roof tank

tank inspection

welded steel tanks