aramco 1

Engineering Encyclopedia Saudi Aramco DeskTop Standards

API Storage Tank

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

Chapter : Civil File Reference: CSE11001

For additional information on this subject, contact J.H. Thomas on 875-2230

Engineering Encyclopedia

Civil API Storage Tank

CONTENTS

PAGE

TYPES, COMPONENTS, AND USES OF STORAGE TANKS.............................. 1 Background .................................................................................................... 1 API Atmospheric Storage Tanks .................................................................... 1 Supported Cone Roof Tank ................................................................. 2 Self-Supporting Fixed Roof Tank....................................................... 3 Floating Roof Tank............................................................................. 4 Fixed Roof with Internal Floating Roof Tank..................................... 5 API Low-Pressure Storage Tanks................................................................... 7 Single-Walled, Low-Pressure Tank .................................................... 8 Double-Walled, Low-Pressure Tank................................................... 9 Spheroidal Low-Pressure Tank........................................................... 10 Spherical Low-Pressure Tank ............................................................. 12 Other Storage Tanks....................................................................................... 13 APPLICABLE CODES AND STANDARDS FOR SELECTED STORAGE TANKS ................................................................................................... 14 Codes and Standards for API Atmospheric Storage Tanks ............................ 14 API Standard 650, Welded Steel Tanks for Oil Storage..................... 14 API 653, Tank Inspection, Repair, Alteration and Reconstruction ............................................................................... 15 SAES-A-004, Pressure Testing........................................................... 16 SAES-B-005, Spacing and Diking for Atmospheric and LowPressure Tanks ............................................................................... 16 SAES-B-007B, Air Foam Systems for Storage Tanks........................ 16 SAES-D-100, Atmospheric and Low-Pressure Tanks ........................ 16 SAES-D-108, Storage Tank Integrity ................................................. 16 32-SAMSS-005, Atmospheric Steel Tanks ........................................ 16 Codes and Standards for API Low-Pressure Storage Tanks ............... 17 API Standard 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks ........................................... 17 SAES-A-004, Pressure Testing........................................................... 17 SAES-B-005, Spacing and Diking for Atmospheric and Low-Pressure Tanks............................................................................ 17 SAES-D-100, Atmospheric and Low-Pressure Tanks ........................ 18 32-SAMSS-006, Large Welded Low-Pressure Tanks ........................ 18

Saudi Aramco DeskTop Standards



TYPES, MECHANICAL PROPERTIES, AND ALLOWABLE STRESSES OF STEELS COMMONLY USED FOR STORAGE TANKS ................................. 19 Background .................................................................................................... 19 Design Metal Temperature.................................................................. 19 Minimum Tensile Strength ................................................................. 20 Minimum Yield Strength .................................................................... 20 Allowable Stresses .............................................................................. 20 Allowable Types of Steels.............................................................................. 21 Atmospheric Storage Tanks................................................................ 21 Low-Pressure Storage Tanks .............................................................. 26 CALCULATING CIVIL/MECHANICAL LOADS FOR ATMOSPHERIC STORAGE TANKS ................................................................................................... 27 Background .................................................................................................... 27 Estimating the Minimum Acceptable Thickness of Tank Components.......... 27 API One-Foot Method ........................................................................ 28 Minimum Shell Course Thicknesses for Construction Purposes ........ 29 Minimum Thicknesses for Tank Bottoms........................................... 30 Weight Loads ................................................................................................. 31 Background......................................................................................... 31 Procedures........................................................................................... 31 Total Pressure and Equivalent Liquid Height................................................. 34 Background......................................................................................... 34 Procedures........................................................................................... 37 Roof Live Load .............................................................................................. 38 Background......................................................................................... 38 Procedure ............................................................................................ 38 Wind Loads .................................................................................................... 39 Background......................................................................................... 39 Saudi Aramco Standards ................................................................................ 40 Formulas......................................................................................................... 41 Wind Loads .................................................................................................... 41




Wind Roof-Lift Loading................................................................................. 44 Earthquake Base-Overturning Moment.......................................................... 46 Background......................................................................................... 46 Saudi Aramco Standards..................................................................... 46 Seismic Zones ..................................................................................... 46 Formulas ............................................................................................. 47 Appurtenances ................................................................................................ 50 MECHANICAL CONSIDERATIONS FOR ADDITIONS OR MODIFICATIONS 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.................................................................................................... 54 Ladders or Spiral Stairways ................................................................ 54 Platforms............................................................................................. 55 Accesses.............................................................................................. 55 Supports .............................................................................................. 56 USES OF VARIOUS TYPES OF FOUNDATIONS FOR STORAGE TANKS....... 57 General ........................................................................................................... 57 Soil ................................................................................................................. 58 Preloading ........................................................................................... 58 Compaction......................................................................................... 58 Excavation and Backfill...................................................................... 59 Types of Foundations ..................................................................................... 59 Compacted Earth with Oiled Sand Pad............................................... 59 Ringwalls ............................................................................................ 60 Concrete Pad ....................................................................................... 62 Piled Foundation ................................................................................. 63




Saudi Aramco Requirements.......................................................................... 64 32-SAMSS-005, Atmospheric Steel Tanks ........................................ 64 32-SAMSS-006, Large, Low-Pressure Storage Tanks ....................... 67 SAES-D-108, Storage Tank Integrity ................................................. 67 SAES-M-100, Saudi Aramco Building Code ..................................... 67 SAES-Q-005, Concrete Foundations .................................................. 67 EFFECTS OF TYPES OF SETTLEMENT ON STORAGE TANKS ....................... 70 Background .................................................................................................... 70 Types of Settlement........................................................................................ 70 Uniform............................................................................................... 70 Planar Tilt ........................................................................................... 71 Deviation from Planar Tilt .................................................................. 73 Center-to-Edge.................................................................................... 74 Local 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: Work Aid 1B: Work Aid 1C: Work Aid 1D:

Procedure for Calculating Weight Loads .............................. 79 Procedure for Calculating Total Pressure .............................. 82 Procedure for Calculating Roof Live Load............................ 83 Procedure and Database for Calculating Wind Loads ......................................................................... 84 Work Aid 1E: Procedure and Databases for Calculating Earthquake Base-Overturning Moment................................ 86 Work Aid 1F: Procedure for Calculating Live Loads for Appurtenances....................................................................... 91 WORK AID 2: PROCEDURE FOR CALCULATING TANK SETTLEMENT...... 92 GLOSSARY .........................................................................................................................94




LIST OF FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 39. Figure 40. Figure 41. Figure 42. Figure 43. Figure 44. Figure 45.

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




TYPES, COMPONENTS, AND USES OF STORAGE TANKS Background This section discusses the types, components, and uses of the following general types of storage 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


1



Supported Cone Roof Tank A supported cone roof tank has a fixed roof in the shape of a cone that is supported by rafters on girders or by rafters on roof trusses. The girders or trusses are in turn supported by columns resting on the tank bottom. The supported cone roof tank cannot withstand any significant pressure or vacuum. The roof must be equipped with an open vent, a pressure-actuated vent, or a "frangible joint". A frangible joint is a weak welded seam at the roof-to-shell junction. The weld is designed to fail before any major rupture can occur in the tank’s shell. Without proper venting, vapor pressure changes sufficient to damage the roof or shell may result from daily temperature fluctuations, normal filling and emptying cycles, or from vapor generation due to a fire in the vicinity 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 are not more economical. Supported cone roof tanks can be larger in diameter than selfsupporting, fixed roof tanks.

Open vent (if pressure/vacuum vent not used) Pressure vacuum vent Roof truss

Nozzle

Roof Gauge manhole Access hatch platform

Nozzle

Shell

Roof support column

Water draw-off

Sump

Top angle Foam connection Ladder Spiral (For small diameter tanks stairway without spiral stairway) Shell nozzles

Shell manhole

Bottom

Figure 1. Cone Roof Tank


2



Self-Supporting Fixed Roof Tank The roof of a self-supporting, fixed roof tank is supported completely from the shell without supplementary 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 a larger diameter than a cone-shaped roof. The self-supporting, fixed roof tank has the same characteristics 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 roof tanks are required.

Center dome vent Roof

Access hatch

}

Platform

Shell

Ladder

Appurtenances

Bottom

Figure 2. Geodesic Dome Fixed Roof Tank


3



Floating Roof Tank A floating roof tank has an open top and a movable roof that floats on top of the liquid being stored. A space between the floating roof and the tank shell allows the roof to move freely as liquid is added to or withdrawn from the tank. To minimize evaporation losses and reduce the risk of fire, a flexible sealing device is attached to the floating roof. This sealing device can move freely up and down the tank shell and closes off the space between the rim of the roof and the tank shell. By virtually eliminating the vapor space above the liquid, the floating roof tank greatly reduces: • • •

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

Tank shell

Wind girder Continuous fabric seal Gauge Automatic hatch bleeder vent Roof supports Deck

Pontoon Tank bottom

Emergency drain Roof Pontoon supports manhole Deck manhole Screen

Check valve

Articulated pipe drain

Figure 3. Floating Roof Tank Saudi Aramco DeskTop Standards

4



Fixed Roof with Internal Floating Roof Tank A fixed roof with internal floating roof tank is either a self-supporting roof tank or a supported cone roof tank with an internal floating roof inside. The internal floating roof floats on top of the liquid being stored. A flexible sealing device closes off the space between the rim of the internal floating roof and the tank shell. The internal floating roof is usually constructed of materials other than steel, such as aluminum or polyurethane. Usually, the internal floating roof is designed to be assembled within a completely constructed tank. The internal floating roof functions the same way as the floating roof in the floating roof tank: it virtually eliminates the vapor space above the liquid. Components - Figure 4 shows a fixed roof with internal floating roof tank and its primary components. Usage - This type of tank typically is used when the service of an existing fixed roof tank is changed and a floating roof tank should be used for the new service. The tank is prepared for the new service by adding the internal floating roof inside the existing tank. This type of tank also may be required when a floating roof tank needs a fixed roof for environmental protection or product quality. In this case, a fixed roof is often added to an existing floating roof tank. A fixed roof with internal floating roof tank has the same usage as a floating roof tank.


5



2 1

3

4 6

5 8

7

9

10

11

13 12

14

15

LEGEND 1. 2. 3. 4. 5. 6. 7. 8.

Peripheral roof vent Center roof vent Roof hatch Antirotation device Overflow vent Seal Manway Gauge flotewell

9. 10. 11. 12. 13. 14. 15.

Column negotiating device Support legs Vacuum-relief device Internal floating roof Antistatic grounding Gauge funnel Pontoon

Figure 4. Fixed Cone Roof with Internal Floating Roof Tank


6



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


7



Single-Walled, Low-Pressure Tank The single-walled, low-pressure tank only uses one layer of steel on the shell of the tank to contain the liquid and vapor. Components - Figure 5 shows a single-walled, low-pressure tank and its primary components.

Pressure safety valve/ vacuum vent

Roof

Roof manhole

Compression ring

3/4 Page Graphic (6.5" X 5.375")

Shell

Ladder

Bottom

Figure 5. Single-Walled, Low-Pressure Tank


8



Double-Walled, Low-Pressure Tank The double-walled, low-pressure tank uses two layers of steel for the shell of the tank to contain the liquid and vapor. Usage - This type of tank is used for refrigerated storage. Insulation is installed between the inner and outer layers of the shell. The space between the shells generally is maintained at a slightly positive pressure by a gas, such as nitrogen, that will not liquefy at the storage temperature. Components - Figure 6 shows one design of a double-walled, low-pressure tank and its primary components. Safety valve Ceiling hangers

Fill/discharge nozzle

Cone Roof

Insulated suspended ceiling Inner shell Insulation

Insulation Outer shell Bottom

Figure 6. Double-Walled, Low-Pressure Tank


9



Spheroidal Low-Pressure Tank The spheroidal low-pressure tank approximates the ideal shape of a free-standing liquid droplet in which the shell stresses are theoretically equal in all directions. The normal design pressure of spheroidal tanks ranges from 17 kPa(ga) - 103 kPa (ga) (2.5 to 15 psig). Although commercially available, spheroidal tanks are not used widely since spheres are generally more economical to build. Components - Figure 7 shows two typical spheroid and low-pressure tank designs and their primary components.


10



Ordinary Spheroid

Supports Elevation Section

Sand cushion

Noded Spheroid

Tie Truss

Supports Elevation Section Sand cushion

Figure 7. Spheroidal Low-Pressure Tanks


11



Spherical Low-Pressure Tank The most common type of pressure storage, the spherical low-pressure tank, is a sphere on individual support columns. It provides the maximum volume of storage for the amount of wall material used. Components - Figure 8 shows a spherical low-pressure tank and its components.

Figure 8. Spherical Low-Pressure Tank


12



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


13



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-steel storage tanks. This standard applies to the following tanks: •

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

•

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 ground oil storage tanks

•

External floating roofs


14



•

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 determine shell 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 any listed 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 API 650 (and its predecessor API 12C) atmospheric storage tanks that have already been placed in service. The standard includes the following sections: •

Suitability for Service

•

Brittle Fracture Considerations

•

Inspection

•

Materials

•

Design Considerations for Reconstructed Tank

•

Tank Repair and Alteration


15



•

Dismantling and Reconstruction

•

Welding

•

Examination and Testing

•

Marking and Reconstruction

This standard also has appendices that cover evaluation criteria for tank bottom settlement and checklists 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 protection systems 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 carbonsteel, 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 that have already been put into service and does not apply to the initial construction of tanks. This standard parallels the API 653 Standard and covers additions and exceptions to the API-653 Standard. 32-SAMSS-005, Atmospheric Steel Tanks This specification covers modifications and additions to API Standard 650. The specification is included with the purchase order supplied to the tank vendor. Saudi Aramco DeskTop Standards

16



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-Pressure Storage 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-Pressure Storage Tanks

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

Tanks with internal pressures greater than 3.4 kPa (ga) (0.5 psig) but not greater 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 API Standard 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.


17



SAES-D-100, Atmospheric and Low-Pressure Tanks This standard provides the requirements for the selection, design, and installation of carbonsteel, 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 specification is limited to single-walled, aboveground, low-pressure tanks. The specification excludes spheres and spheroids. The specification is included with the purchase order supplied to the tank vendor.


18



Types, Mechanical Properties, and allowable stresses of steels commonly used for storage tanks Background This section discusses the types of steels commonly used for storage tanks. The section provides 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 elevated temperatures, it is important to select a steel that is appropriate for the range of temperatures at the tank site and for the necessary storage conditions of the contained fluid. For storage tanks a (minimum) design metal temperature and a maximum operating temperature is usually specified. According to API Standard 650, unless experience or special local conditions justify 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 is to be installed. The (minimum) design metal temperature for refrigerated tanks may also be determined by the temperature being maintained by refrigeration. This temperature will usually be lower than 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 tanks used for heated fluids if the temperature is above 93°C (200°F). Above 93°C (200°F) API 650 Standard requires a reduction in the allowable stress used in the tank’s design.


19



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 is the maximum stress to which a material can be subjected without rupturing. The Minimum Tensile Strength is the minimum value of the tensile strength required by the applicable material 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 is the amount of stress a material can undergo before there is a relatively large plastic deformation for small increases in stress. If the stress in the tank is kept below this value, the tank will not suffer any permanent deformation. The Minimum Yield Strength is the minimum value of the yield strength required by the applicable materials standard which governs the manufacture of the steel. Allowable Stresses The thickness of tank components such as the shell and roof must be determined using formulas contained in the applicable API Standard. Typically, these formulas use an allowable stress that depends upon the materials of construction and may depend on the maximum operating temperature. The allowable design stress is based on applying a factor of safety to the material’s minimum tensile and yield strength. In the case of API 650 tanks, a maximum Allowable Product Design Stress, Sd, and a maximum Allowable Hydrostatic Test Stress, St, is used in the formulas to determine tank shell thickness. The Allowable Product Design Stress is used when normal operating fluid is contained. During hydrostatic test, a slightly higher allowable Hydrostatic Test Stress is permitted because this is a controlled situation. Sd is limited to 40% of the minimum tensile strength or 2/3 of the minimum yield strength. St is limited to 3/7 of the minimum tensile strength and 3/4 of the minimum yield strength.


20



Allowable Types of Steels Atmospheric Storage Tanks API Standard 650 permits the use of several specifications of steel plates for atmospheric storage tank construction. API Standard 650 also identifies permissible specifications for structural shapes, piping and forgings, flanges, bolting, and for welding electrodes. These specifications 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 alloy content, manufacturing process, yield strengths and toughness. The types of steels permitted by API Standard 650 are divided into eight groups according to the steel manufacturing process used for each material. Figure 9 identifies the material groups. Figure 10 shows the (minimum) design metal temperature permitted for each material group without requiring impact testing based on plate thickness. Figure 11 provides the allowable stresses from API 650 for particular material specifications.


21



Group I As Rolled, Semikilled

Group II As Rolled, Killed or Semikilled

Group III As Rolled, Killed Fine-Grain Practice

Group IIIA Normalized, Killed Fine-Grain Practice Material

Notes

A 131 CS A 573-58 A 516-55 A 516-60 G40.21M-260W Fe 42 D Grade 41

10 10 10 9, 10 4, 9, 10 5, 9, 10

Material

Notes

Material

Notes

Material

A 283 C A 285 C A 131 A A 36 Fe 42 B Grade 37 Grade 41

2 2 2 2, 3 4 3, 5 6

A 131 B A 36 A 442-55 A 442-60 G40.21M-260W Fe 42 C Grade 41

7 2, 6

A 573-58 A 516-55 A 516-60 G40.21M-260W Fe 42 D Grade 41

4 5, 8

Notes

9 4, 9 5, 9

Group IV As Rolled, Killed Fine-Grain Practice

Group IVA As Rolled, Killed Fine-Grain Practice

Group V Normalized, Killed Fine-Grain Practice

Material

Material

Notes

Material

Notes

A 662 C A 573-70 G40.21M-300W G40.21M-350W

11 9, 11 9, 11

A 573-70 A 516-65 A 516-70 G40.21M-300W G40.21M-350W

10 10 10 9, 10 9, 10

A 573-65 A 573-70 A 516-65 A 516-70 A 662 B G40.21M-300W G40.21M-350W Fe 44 B, C, D Fe 52 C, D Grade 44

Notes

9 9 4, 9 9 5, 9

Group VI Normalized or Quenched and Tempered, Killed Fine-Grain Practice Reduced Carbon Material Notes A 131 EH 36 A 633 C A 633 D A 537 I A 537 II A 678 A A 678 B A 737 B

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


22


Civil

Design metal temperature (°F)

API Storage Tank

60

60

50

50 pI ou Gr

40

40

V up I Gro

30

30 20

VA I up I pI Gro ou r A G I pI ou r G

10 0 -10

20

pV Grou

G

See Note 1

u ro

pI

10

II

0 -10

VI Group

-20

-20

-30

-30 Group IIIA

-40 -50 -60

-40

See Note 2

0.25

-50

0.50

0.75

1.00

1.25

1.50

-60

Thickness, including corrosion allowance (inches) Notes: 1. 2. 3. 4. 5.

The Group II and Group V lines coincide at thicknesses less than 1/2 inch. The Group III and Group IIIA lines coincide at thicknesses less than 1/2 inch. The materials in each group are listed in Table 2-3. This figure is not applicable to controlled-rolled plates (see 2.2.7.4). 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 mm multiply the thickness in inches by 25.4.

Figure 10. Minimum Permissible Design Metal Temperatures for Plates Used In Tank Shells Without Impact Testing


23



Cost and acceptability of the material, at the specified design metal temperature and required thickness, determine the selection of the steel specification. In general, higher strength steels cost more per pound. Note that the principal difference between “structural” steels, such as ASTM A36, and most other specifications permitted by API Standard 650 is that the structural steel has a higher minimum design metal temperature and may not be able to be used without impact testing if the required thickness is too large (refer to Figure 11).


24



Plate Specification

Grade

Minimum Yield Strength

A 283 A 285 A 131 A 36 A 131

C C A, B, CS -EH 36

30,000 30,000 34,000 36,000 51,000

A 442 A 442 A 573 A 573 A 573

55 60 58 65 70

A 516 A 516 A 516 A 516

Minimum Tensile Strength

Product Design Stress Sd

Hydrostatic Test Stress St

55,000 55,000 58,000 58,000 71,000a

20,000 20,000 22,700 23,200 28,400

22,500 22,500 24,900 24,900 30,400

30,000 32,000 32,000 35,000 42,000

55,000 60,000 58,000 65,000 70,000a

20,000 21,300 21,300 23,300 28,000

22,500 24,000 24,000 26,300 30,000

55 60 65 70

30,000 32,000 35,000 38,000

55,000 60,000 65,000 70,000

20,000 21,300 23,300 25,300

22,500 24,000 26,300 28,500

A 662 A 662 A 537 A 537

B C 1 2

40,000 43,000 50,000 60,000

65,000 70,000a 70,000a 80,000a

26,000 28,000 28,000 32,000

27,900 30,000 30,000 34,300

A 633 A 678 A 678 A 737

C, D A B B

50,000 50,000 60,000 50,000

70,000a 70,000a 80,000a 70,000a

28,000 28,000 32,000 28,000

30,000 30,000 34,300 30,000

G40.21M G40.21M G40.21M G40.21M

260W 300W 350WT 350W

37,700 43,500 50,800 58,800

23,800 26,100 27,900 26,100

25,500 28,000 29,800 28,000

37 41 44

30,000 34,000 36,000

20,000 22,700 24,000

22,500 25,000 26,800

B, C B, C C, D

34,000 35,500 48,500

22,700 23,700 28,400

25,500 26,600 30,400

ASTM Specifications

CSA Specifications 59,500 65,300 69,600a 65,300

National Standards 52,600 58,300 62,600

ISO 630 Fe42 Fe44 Fe52

60,000 62,500 71,000a

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


25



Low-Pressure Storage Tanks API Standard 620 permits the use of several specifications of steel plates for low-pressure storage tank construction. API Standard 620 also identifies permissible specifications for structural shapes, piping and forgings, flanges, bolting, and for welding electrodes. These specifications are based on ASTM, CSA, and ISO specifications. The ASTM, CSA, and ISO specifications classify steels based on alloy content, manufacturing process, yield strengths and 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 of API 620. Also allowable stresses and weld-joint efficiency are specified in Table 3-1 and Table 3-2 of the standard.


26



CALCULATING CIVIL/MECHANICAL LOADS for atmospheric storage tanks Background This section discusses and demonstrates how to calculate the civil/mechanical loads imposed on atmospheric storage tanks. Civil/mechanical loads are loads with which a civil or mechanical engineer would be concerned when designing a tank or its foundation. The following 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 of the various components of which a tank is comprised must be known or estimated. The next section discusses how to estimate the thickness of tank components if these weight loads are not known. Estimating the Minimum Acceptable Thickness of Tank Components API 650 and API 620 have many criteria for determining the minimum thicknesses of tank components. In this section we will discuss only one of the methods given in API 650 for determination of the minimum thickness of the tank shell courses. We will also indicate the API 650 minimum thickness requirements for tank bottoms and roofs. Corrosion allowances, if required, should be added to the minimum thicknesses that are calculated by the API method that is described later in this module, or the minimum thicknesses specified in API Standards. Corrosion allowances are usually specified by the metallurgical 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 be used for initial thickness estimates for the main components of a tank for the civil/mechanical design. Note that the specific methods and requirements of the applicable API standard should be used in any definitive work. PEDP course MEX 203 is recommended if the Participant is interested in a more in-depth treatment of API 650 and API 620 requirements.


27



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 liquid above. Therefore, the lower shell courses of a tank are usually thicker than the upper shell courses to withstand the greater pressure. To account for the increase in pressure the thickness of each shell course must be calculated using an appropriate method. The API 650 One-Foot Method is one method that can be used to estimate the thicknesses of API 650 tank shell courses. In the One-Foot Method, the thickness of each shell course is determined based on limiting the circumferential membrane stress in the shell at a point that is one foot above the lowest point 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. The Variable-Design-Point Method is much more complex than the One-Foot Method, and discussion of it is outside the scope of this course. The Variable Design Point Method is an iterative method that uses the shell course thicknesses determined by the One-Foot Method as its initial starting point. Therefore, the Variable-Design-Point Method can be considered as a means to "fine-tune" the shell thicknesses of each course. The Variable-Design-Point Method usually results in slightly thinner and hence more economical tank shells. It should be noted that 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 of two 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 in inches (millimeters).

C1

= Constant, which accounts for the density of water and the dimensional unit system 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.


28



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 (Figure 11) in psi (MPa).

Note that the above equation is based on the 1993 edition of API 650, in which the weld-joint efficiency 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 based on tank diameter. These minimum thicknesses are indicated in Figure 12 and may govern the thicknesses of the upper shell courses. Note that the corrosion allowance, if required, should be added to the plate thicknesses shown in Figure 12. Nominal Tank Diameter

Nominal Plate Thickness

meters

ft.

mm

in.

<15.25

<50

4.5

3/16

15.25 to 36.5

50 to 120

6.0

1/4

36.5 to 61.0

120 to 200

7.5

5/16

>61

>200

9.0

3/8

Figure 12. Minimum Shell Thicknesses for API 650 Tanks


29



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 plate thicknesses, 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 tank

API-650

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 the previous 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 if the specific gravity of the oil was greater than 0.8, the design case would have governed rather than the hydrotest case, and the minimum thickness for the shell would then have been based on the design case.


30



Weight Loads Background When designing a tank and its foundation, the design engineer must consider the weight loads which are the weight of the tank and the maximum weight of its contents. Since most petroleum products are lighter than water, the heaviest weight load occurs during hydrostatic testing, which is done using water. If a tank and its foundation are designed to withstand the total hydrostatic test weight, WT, the tank foundation should also be able to withstand the weight load imposed during normal operation 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 or appurtenances during the test. The tank dead weight empty, WD, is equal to the weight of the tank bottom, Wb, the weight of the shell, Ws, the weight of the roof(s), Wr, the weight of any appurtenances, Wa, and the weight of any insulation, Wi, that may be installed at the time of the hydrostatic test. Note that insulation is usually not installed at the time of test and the live loads on the roof and appurtenances are usually small compared to other loads involved, and may be considered 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 hydrostatic test 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 the following course thicknesses: -

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

-

Second course, 1-1/8 in.


31



•

-

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 of the 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 =¹/4d2HLγw WH =¹/4 x (300)2 x 45 x 62.4 WH =~198,500,000 lb. In Step 2, calculate the weight of the tank bottom, Wb: π Wb = 4 d2tγ st Wb =(¹/4 x (300)2 x 0.25/12) x 490 Wb = ~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 of the 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.


32



Using values given for this problem: Ws = Ws = Ws =

¹dth x Vst ¹ x 300 x 490 x 48 x 0.0686 ~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 Wr Wr

π = 4 d2t × γst x D.F. = (¹/4 x (300)2 x 3/16 x 490 x (1 + 0.20) 12 = ~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 values calculated in Steps 2 through 4: Wa Wa Wa

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

In Step 6, calculate the tank dead weight empty, WD: W D = W s + Wb + Wr + Wa + Wi WD = 1,520,000 + 721,600 + 649,400 + 57,820 WD = ~2,949,000 lb. In Step 7, calculate the total hydrostatic test weight, WT: WT =WH + WD WT =198,500,000 + 2,949,000 WT =~201,450,000 lb. Answer: The hydrostatic test water weight is approximately 198,500,000 lb. The tank dead weight empty is approximately 2,949,000 lb. The total hydrostatic test weight is approximately 101,400,000 lb.


33



Total Pressure and Equivalent Liquid Height Background Different pressure loads act on the tank bottom, tank shell, and tank roof that are sometimes used 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 highest hydrostatic pressure occurs during hydrostatic testing. The vapor pressure, PV, is a function of the volatility of the liquid contained in the tank at its storage temperature. The process engineer determines the vapor pressure for which the tank should be designed. The total pressure, PT, to which a component is subjected is equal to the sum of the individual pressures. The highest total pressure may occur during the normal operation due to the vapor pressure 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.


34



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

Pressure (psi) 0

0

5

10

15

20

25

30

16

24

40

48

= ity rav cg cifi spe

32

) er at (w 1 = ity 5 av 0.7 gr = ity ific av ec gr sp ific 0 ec 0.5 sp

Liquid Depth (ft)

8

56

Figure 13. Hydrostatic Pressure vs. Depth Below Liquid Surface SI Note:

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


35



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

0

0

5

Pressure (psi) 15 20

10

25

30

35

Liquid depth (ft)

8 16 no

va rp re ss = 10 ps i( sp ec ifi =

=

80

ity

ity

0.

av

=

av

gr

gr

ity

c

av

56

ific

gr

ec

ic

sp

cif

i(

pe

ps

(s

5

e

=

ur

e

e

ur

ss

48

ur

ss

re

40

po

re

rp

rp

po

32

po

va

va

24

)

)

80

80

SI Note:

0.

0.

)

Figure 14. Effect of Vapor Pressure on the Total Pressure at a Given Depth Below the Surface

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 concept of equivalent liquid height will be introduced. The equivalent liquid height is equal to the total pressure divided by the specific gravity of the liquid stored in the tank. This equivalent liquid height is then used in the API 650 equations to determine the thickness of the shell course instead of the actual fill height.


36



Procedures The procedure for calculating total pressure and the equivalent liquid height is provided in Work Aid 1B. Sample Problem 3: Total Pressure Calculate the total pressure at the bottom of a cone roof tank and the equivalent liquid height due 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 2.5 psig. 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 1/144 PH = 25.0 psig In Step 2, calculate the total pressure, PT: PT PT PT

= = =

P H + PV 25.0 + 2.5 ~27.5 psig

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

= = =

PT x C.F. /γ 27.5 x 144 / (62.4 x 0.9) 70.5 ft.

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


37



Roof Live Load Background The roof live load consists of the weights of items on the roof that are not a part of the permanent 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 roof live load. A minimum required live load of 122 Kg/m2 or 1.2 kN/m2 (25 lb./ft.2) is specified in 32-SAMSS-006 for low-pressure tanks. The same minimum live load is specified in API Standard 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 heavy personnel 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 vapor pressure, 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 the minimum roof live load is applicable. Solution: Use Work Aid 1C, to calculate the roof live load, LRLL: LRLL =

π 2 4 d × 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.


38



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 tank appurtenances, such as platforms and ladders, can overload these appurtenances or their attachments to the tank. The pressure due to the wind varies around the circumference of the tank from a high pressure on the windward side to a low pressure (vacuum) on the leeward side. The effects of wind increase with increasing height above grade. Wind blowing over the top of the tank can also cause a negative pressure or vacuum to act on the tank roof. In tank design, the primary loads that concern a civil/mechanical engineer are the wind baseshear 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 calculation of these loads is provided in Work Aid 1D. Figures 15 and 16 provide diagrams of these wind forces.

Wind

Mw

Fw

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


39



Lw

Wind Wind

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 surface area of the tank and of wind drag or lift coefficients. The wind pressure increases with increasing velocity and increasing height. The wind drag coefficient is a function of the H/d ratio of the tank. The loads on the tank increase with increasing height and diameter of the tank 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, of 137 km/h (85 mph) which is measured at 10 m (33 ft.) above grade. SAES-D-100 requires that the tanks be designed for wind loads in accordance with ANSI/ASCE 7-88 (formerly ANSI Standard A58.1) for exposure Level C.


40



Formulas The following equations are based on equations presented in ANSI/ASCE 7-88 (formerly ANSI Standard A58.1). The wind pressure increases with increasing velocity. The wind pressure at the reference elevation, qr, can be calculated from the following equation: qr

= 0.0473 Vr2

(S.I. Units)

= 0.00256 Vr2 (U.S. Units)

where: qr

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

Vr

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

(Eqn. 1 SI) (Eqn. 1 US)

Based on the design wind velocity of 137 Km/h (85 mph) indicated in SAES-D-100, qr is equal 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 which increases as the elevation increases, the projected area of a portion of the tank or an appurtenance, 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)

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

=

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).

where:

Height-correction factor which varies with height above the reference elevation, (dimensionless).


41



The wind force increases as the height above the reference elevation increases. In order to determine the wind force at a higher elevation, a height correction factor, Kh, and a gust response factor, G, are used. Kh and G that are found in ANSI/ASCE 7-88 are based on the height and exposure classification of the location. Excerpts of these tables are presented in Work Aid 1D for exposure classification C. The wind drag coefficient, Cf, is given in ANSI/ASCE 7-88 for typical structures with various proportions. For most tanks the H/D (Height/Diameter) ratio is less than one and the surface roughness of the tank is relatively smooth. Therefore, a typical value of Cf that would be used for a tank is 0.5. The wind drag coefficient, Cf, for an appurtenance is a function of the appurtenance's shape and solidity (net area/gross area) ratio. The value of Cf for appurtenances ranges from approximately 0.7 to 2.0 depending on the shape and solidity ratio. The Participant should reference ANSI/ASCE 7-88 directly if detailed calculations are to be made of wind load on appurtenances. When determining the effective projected area of a tank, the designer can include the wind force on every appurtenance in the calculation or the designer can estimate the effect of the appurtenances by assuming the tank has an effective diameter, D, slightly larger than its actual outside diameter. However for this course, the wind load on the tank will be approximated by using the nominal tank diameter or the nominal tank diameter plus two times the insulation thickness (if any), and the effect of the wind load on the appurtenances will be ignored. Since the wind load increases with height above the reference elevation, it is typical to assume that the tank is divided up into a number of height ranges. The wind loads acting in each height range are calculated assuming that Kh elevated at the midpoint of the range applies over the whole range. The wind forces acting on each height range are then summed up to determine the total loads acting on the tank base. With these approximations, the formulas for the wind base-shear force and wind baseoverturning moment can be readily calculated. For wind base-shear force: F w = (∑ Kh (hh − hl ))× DGCf qr

(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.).


42



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 externally insulated, 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 Saudi Aramco locations.

For wind base-overturning moment:   h + hl   × DGCfqr Mw =  ∑ Kh (hh − hl ) h  2   (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.)

D

=


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 Saudi Aramco locations.

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


43



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 gust response, 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 wind roof-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 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.

=

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 the wind 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


44



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 F w = ~ 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 Lw

= =

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

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


45



Earthquake Base-Overturning Moment Background The Eastern Province of Saudi Arabia is relatively safe from earthquakes. However, parts of the western area of the Kingdom potentially may experience earthquakes. The design engineer must make sure that tanks in these areas are designed to withstand certain earthquake loads. An earthquake can cause a tank to overturn, to slide, or to be deformed permanently. It can also cause attached piping and appurtenances to rupture or tear off the tank. In an open-top tank, an earthquake may cause the contents of a tank to slosh over the top. With petroleum tanks, all these events pose a risk of fire. When an engineer designs a tank to withstand earthquakes, he considers the following two response 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 that moves in the fundamental sloshing mode Saudi Aramco Standards

SAES-D-100 and 32-SAMSS-005 require that the tank be designed for seismic loads in accordance with API 650 Appendix E. The seismic zone should be indicated on the Tank Data 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 seismic zone represents the relative risk of earthquake damage and determines the amount of seismic resistance required in structural design. Low numbers represent low risk; high numbers represent high risk. Zone 0 requires no earthquake design. Zone 2 is the highest classification for any Saudi Aramco location.


46



Formulas If we assume that the seismic zone is Zone 1 and the tank is at least 30 m (100 ft.) in diameter and less than 15 m (50 ft.) in height, the API 650 Appendix E formula for the earthquake base-overturning moment is as follows:   S   M E = ZI  0.24 W s H scg + 0.24 W r Ht + 0.24K w1W CK 1H L + K 3  2 K W2 W C K 2 HL   k d   (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 maximum design liquid height, HL

Wc

=

Weight of the tank liquid contents, N (lb.) equal to the hydrostatic test water 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 the maximum design liquid height, HL

HL

=

Maximum design liquid level, m (ft.)

S

=

Site amplification factor


47



k

=

Factor, based on the ratio of the diameter of the tank, d, to the design maximum liquid height, HL

d

=

Diameter of the tank, m (ft.)

KW2

=

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

K2

=

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

K3

=

Coefficient which is a function of the first sloshing mode of the tank equal to 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 units except for K3.

The first term in the equation approximates the response of the tank shell to the lateral ground motion. The second term in the equation approximates the response of the roof to the lateral ground motion. The third term in the equation approximates the response of the liquid contents that move in unison with the shell. The fourth term in the equation approximates the response of the liquid contents that slosh. Work Aid 1E provides the procedures and databases for calculating the earthquake baseoverturning 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.


48



•

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 = k =

300/45 = 6.67 ~0.83

In Step 2, determine factors Kw1 and Kw2: Kw1 Kw2

= =

~0.18 ~0.78

In Step 3, determine factor S: S

=

1.5

In Step 4, determine factors K1 and K2: K1 K2

= =

~0.38 ~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.


49



Appurtenances When designing or modifying a tank or designing a foundation, the design engineer must allow for the weight and the forces exerted by the appurtenances. The primary loads contributed by tank appurtenances are their weight. In the absence of the actual weight of the specific items involved, the weight of all tank appurtenances may be estimated based on the total tank weight, excluding the weight of the contents. Depending on the particular appurtenance involved and how it is attached to the tank, the weight of the appurtenance may also impose a bending moment on the tank, which the design engineer may need to consider. The design considerations for appurtenances will be highlighted in a later section that covers tank attachments. Some appurtenances, such as stairs, ladders, and platforms, 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 on appurtenances.


50



MECHANICAL CONSIDERATIONS FOR ADDITIONS OR MODIFICATIONS TO THE 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 the greatest 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. In addition, 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 an unusually large number of heavy attachments in a relatively small area. High membrane stresses 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 creates stresses within the part. When the bending increases, the stress also increases. Normally, the stresses are highest in the area of the applied load. Local bending stress in the material caused by 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 bending stresses in the junction. For instance, if the shell is loaded by a ladder clip near the bottom of the tank, the bending applies additional loads on the weld between the shell and the tank bottom. The addition of reinforcing plates or pads where attachments are made to the tank reduces the bending stress. These plates or pads distribute the applied loads over a wider area of the tank and reduce the localized stresses.


51



Figure 17 shows bending stress.

Appurtenance weight

Force Pipe

Tank wall Bending moment

Force Pipe support

Exaggerated result

Figure 17. Bending Stress


52



Peak Stresses Peak stresses occur wherever a local area of material is subjected to significantly higher stress than the material in the surrounding area. This peak stress typically occurs at stress concentrations or at abrupt geometric discontinuities in the structure. Stress is concentrated at storage tank attachment points. In general, stress concentration effects need to be considered only when the loads are applied cyclically. If the combination of stress level and the number of cycles is high enough, cyclic stresses could result in a fatigue crack of the tank material or in failure of the tank. Figure 18 shows stress concentration.

Force

Tank wall Reference lines Mounting plate

Force

Force

Force

Peak stresses at corners

Result

Figure 18. Stress Concentration


53



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 the weight of the added material. When material is removed from a storage tank, the weight of the 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 as follows: •

Ladders or stairways

•

Platforms

•

Accesses

•

Piping connections

•

Supports

The following discussions of these attachments and modifications illustrate the problems that attachments and modifications may cause and the methods available for minimizing the problems. Ladders or Spiral Stairways Ladders or spiral stairways are installed on a storage tank to gain access to the tank roof for service and/or inspection. Potential Problems - When a ladder or stairway is attached to a tank, each attachment point becomes a source of bending stress and stress concentration. The attachments must be strong enough to support the weight of the ladder or stairway, the personnel who use the ladder or stairway, 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 permit some 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 attachments must not be made in areas of the tank that are already under higher stress, such as the joints between shell courses, between the shell and the bottom, or between a fixed roof and the shell. Also, the reinforcing pads or plates added at the attachment points must be large and thick enough to distribute the applied loads adequately.


54



Platforms Platforms are installed on a storage tank to provide relatively safe and convenient areas for inspection, maintenance, and/or equipment mounting. Typically, a platform is installed at the top 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 a source of bending stress and stress concentration. The attachments must be strong enough to support the weight of the platform, the personnel who use the platform, and the equipment that may be placed on the platform. Methods of Minimizing - When platforms are attached to a tank, the attachments must not be made in areas of the tank that are already under higher stress, such as the joints between shell courses, 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 a source 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 already under higher stress, if other locations are equally satisfactory. The area around the access must be reinforced and the access must be as small as possible. The access cover must be strong enough, but not excessively thick. The access must not be installed at the seam between shell courses or plates. Design details specified in the appropriate API standard that addresses these concerns must be used. Limitations - The reasons for installing an access determine the access’s location. How the access 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 of the tank, to facilitate cleaning and draining of the tank, and to provide connections for safety valves. Potential Problems - A piping connection is a source of stress concentration and bending stress due to the applied loads from the connected piping system. A piping connection causes a local weakening in the tank where the piping connection is installed.


55



Methods of Minimizing - A piping connection must not be installed in an area of a tank that is already under higher stress, if other locations are equally satisfactory. The area around the piping connection must be reinforced. API Standard 650 specifies design details and an evaluation procedure that should be followed to reduce local tank stresses. A piping connection must not be installed at a seam between shell courses, at the seam between the bottom and the shell of the tank, or at a seam between shell plates. The piping system must be provided with adequate flexibility to adjust to tank settlement and to adjust to tank expansion and contraction that results from both temperature changes and hydrostatic head. Limitations - The reasons for installing a piping connection and the layout of the piping system determine the location and size of a piping connection. Designing a piping system that allows for tank settlement may be difficult and expensive for cases of significant settlement, and the difficulty increases with the pipe diameter. Supports Supports are attached to certain types of tanks, such as spheres and spheroids, to support the weight of the tank and its contents. The support connections along with their reinforcing pads or plates must be designed to support the weight of the tank and its contents without overstressing the tank. Flat (or conical) bottom tanks are continuously supported on a foundation. Potential Problems - Support connections are a source of stress concentration and bending stress. The supports must adjust to the expansion and contraction of the tank. The supports must 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 already under higher stress. The reinforcing pads or plates must be sufficiently strong. The possible expansion and contraction of the tank during operation must be considered in the support design. Properly designed and constructed foundations can minimize tilting that results from foundation settlement. Limitations - Designing and constructing supports that properly connect to a tank and that tolerate tank expansion and contraction can be expensive. Better foundations are more expensive to construct and may not be cost effective.


56



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. The foundation 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 the bottom 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 following requirements 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. The elevation specified for the tank bottom surface should also consider the amount of total settlement that is expected.

•

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


57



•

To facilitate drainage, the finished tank grade is usually sloped upward or downward from the outer periphery to its center depending on whether an upward cone or downward cone is specified for the tank bottom. SAES-D-100 requires that the underside of the tank's bottom be coned downward in services where a water drawoff is required unless otherwise stated on the tank data sheet. When high settlement is expected at the tank center the tank bottom frequently is coned upward. The radial slope, upward or downward is, 1 in 120.

Pressurized tanks are anchored to their foundations. In the case of a flat-bottomed tank with internal pressure, anchoring helps prevent the pressure from rounding the tank’s bottom and lifting the tank off its foundation when the liquid level is low. Rounding creates stresses within the bottom of the shell and the outer edge of the bottom that could cause the tank to fail. Atmospheric storage tanks are not normally anchored, unless anchoring is needed for wind or earthquake loading. Soil Before the foundation and tank are constructed, the design engineer must estimate how much settlement will occur during the operating life of the tank. In some cases, it may be necessary to prepare the soil to better support the loads that will be placed on the soil. Common soilpreparation techniques are as follows: •

Preloading

•

Compaction

•

Excavation and backfill Preloading

Preloading the soil is the preferred method of preparation. The soil is preloaded by placing material on top of the ground that will be supporting the foundation and tank. The amount of material piled on top usually equals or exceeds the weight of the tank and foundation when the tank is filled. The material must be left in place long enough to allow the soil to compact under the weight. This time period depends on the type of soil and the rate at which it consolidates. 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, the soil may be stiffened by compaction. The soil is compacted by beating or pounding the surface with equipment specially designed for this purpose.


58



Excavation and Backfill When the existing soil does not provide the appropriate characteristics for the foundation, it may be removed and replaced by a more satisfactory soil or engineered fill. The existing soil must be removed to a depth sufficient to ensure proper support of the foundation and filled tank. The new soil or engineered fill must then be properly compacted during placement and before 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 type of foundation. This type of foundation is used for small flat-bottomed tanks constructed on stable soil. Figure 19 shows the construction of the compacted earth and oiled sand pad foundation.

Tank

Stable soil

Oiled sand layer

Stable soil Compacted earth

Figure 19. Compacted Earth with Oiled Sand Pad


59



Ringwalls A ringwall foundation consists of a ring of support material enclosing an area of compacted fill. 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 oiled sand 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 due to 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 the boundary between the ringwall and the compacted fill.

•

Ringwalls are more expensive to construct than compacted earth and oiled sand pads.


60



Crushed Stone Ringwall - Construction of the crushed stone ringwall is illustrated in Figure 20. Tank

Crushed stone

Crushed stone

Stable soil

Stable soil

Compacted fill

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 convenient anchoring than the crushed stone ringwall. When compared to the crushed stone ringwall, the concrete ringwall is more likely to have differential settlement between the ringwall and the fill inside the ringwall. Also, the concrete ringwall is more expensive to construct than the crushed stone ringwall.


61



Tank

Compacted fill

Grade Reinforced concrete

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 of anchoring the tank. Figure 22 illustrates a concrete pad.

Tank

Soil

Concrete slab

Figure 22. Concrete Pad


62



Piled Foundation A piled foundation for a tank consists of a concrete slab or pile cap on which the tank rests and piles (columns) embedded into the soil below the slab. The pile material may be either reinforced concrete, steel, or timber. The size, length, and number of piles depends on soil conditions and on the size and weight of the tank. A geotechnical specialist usually determines the pile requirements based on the results of a soil investigation 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 the tank

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 two sources 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 23 illustrates a piled foundation.


63



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 that store oil, water, and chemicals at approximately atmospheric pressure. The standard applies to newly constructed tanks during initial test and inspection.


64



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

The tank foundation must be inspected prior to tank erection for compliance with 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 API Standard 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 tank diameter shall be referred to CSD for analysis.

-

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

-

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

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

Elevation measurements must be taken during the initial hydrostatic test:

+ For this purpose, reference points equally spaced around the circumference shall be established. The reference points shall be nuts or other similar items welded to the tank’s shell 100 mm (4 in.) above the bottom edge. One of the reference points must be placed at the catch basin. + The reference points must be placed at approximately equal distances around the circumference of the tank. The number of reference points for various tank diameters is given in Figure 24.


65



Tank Diameter 15 m (50 ft.) and less Over 15 m (50 ft.) and less than 45 m (150 ft.) 45 m (150 ft.) and over

No. of Reference Points 4 8 16

Figure 24. Required Number of Reference Points + The observed elevations must be referenced to a permanent benchmark. The instrument making the measurements must be set up at a distance from the tank of at least 1-1/2 times the tank diameter. + Six sets of settlement readings must be taken. (See Figure 25.) Reading 1 2 3 4 5 6

When To Be Taken Before the start of the hydrostatic test With the tank 1/4 full ± 0.60 m (2 ft.) With the tank 1/2 full ± 0.60 m (2 ft.) With the tank 3/4 full ± 0.60 m (2 ft.) With the water level at or above the maximum working filling height (32-SAMSS-005, par. 5.5.6 gives additional time conditions for this step for special tanks) 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 tank circumference or uniform settlement greater than 50 mm (2 in.) must be reported to the Chief Engineer. + A record of elevation observations shall be filed in the Plant Inspection Record Book by the Buyer's Inspector. -

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


66



32-SAMSS-006, Large, Low-Pressure Storage Tanks This standard contains similar requirements for the design, inspection, testing and monitoring of 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 the structural integrity of welded-steel storage tanks constructed to API 650 or to API 12C. This SAES applies to existing tanks during Test and Inspections (T&Is) and not to tanks during initial 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 affect the strength or safety of the tank.

•

They have periodic inspections as required by the Equipment Inspection Schedule. SAES-M-100, Saudi Aramco Building Code

All construction must meet the requirements of the Saudi Arabian Uniform Building Code as modified by SAES-M-100. SAES-Q-005, Concrete Foundations SAES-Q-005 specifies that the foundations for atmospheric storage tanks be constructed in accordance with the instructions in API Standard 650. In addition, the standard provides requirements 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


67



•

Soil-shearing capacity

•

Groundwater location, chemistry, and fluctuation

•

For pile-type foundations, data that establishes the minimum pile group spacing based 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 existing or finished grade surface.

•

If subject to water pressure, they be designed to resist a uniformly distributed uplift 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 Sliding For compression piles, ultimate capacity For tension piles, ultimate capacity All other conditions

1.5 1.5 2.0 3.0 2.0

Figure 26. Safety Factors for Foundations SAES-Q-005 requires that concrete ringwalls meet the requirements of foundations and that they 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 soil pressure 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.


68



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 of the concrete be at least 75 mm (3 in.)

•

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


69



Effects of types of Settlement on storage tanks Background The excessive settling of a tank can cause serious tank operating problems and lead to tank failure. Therefore, a key step in tank design is estimating the amount of settlement the tank’s shell will undergo in its lifetime. The desired maximum lifetime settlement is usually less than 0.3 m (1 ft.). When settlement exceeds 0.3 m (1 ft.), there may be serious problems with the storage tanks, 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 bottom settlement. When a tank shell settles, the settlement can be classified as uniform, planar tilt, or deviation from planar tilt. When a tank bottom settles, the settlement can be classified as center-to-edge or 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 of settlement requires correction only when the foundation or piping connections develop problems. Figure 27 illustrates uniform settlement.


70



Shell settles but remains level

Drainage problem

Potential 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 cause corrosion 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 single plane, planar tilt settlement has occurred. The bottom plane does not distort; it only tilts. Figure 28 illustrates planar tilt.


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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 of these 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 differential settlement has occurred. Figure 29 illustrates deviation from planar tilt. Shell may buckle

Differential settlement around shell

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 of the tank, center-to-edge settlement has occurred. Figure 30 illustrates center-to-edge settlement.

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 settle differently from the rest of the bottom, local shell or bottom settlement has occurred. Figure 31 illustrates local shell or bottom settlement.

Local bottom settlement

Shell settles more than tank bottom

Figure 31. Local Shell or Bottom Settlement In local shell or bottom settlement, significant stress may develop in the bottom plates, their attachment welds, the bottom-to-shell junction weld, or the lower section of the shell. This stress can cause the bottom of the tank to fail.


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Evaluation of Tank Settlement API 653, Appendix B presents criteria for determining if the settlement around the tank shell or the tank bottom is excessive. If the settlement is excessive, then repairs to the tank and/or foundation may be required. Since repairs to a tank foundation, although possible, are very expensive and time consuming, the criteria in API 653 are often used as an initial screening criterion to determine whether a more sophisticated analysis using computer modeling of the tank settlement problem is required. If the tank settlement is too large, various types of repairs to the foundations can be made. If the ringwall has suffered local differential settlement, a portion of a ringwall that has settled too much may be replaced. Releveling of the entire ringwall by using an epoxy grout is sometimes done to correct for excessive tilt. Replacing/recompacting and/or releveling the entire tank pad is sometimes done if tank bottom settlement is excessive. In most cases, these repairs are made when the tank is out-of-service at a scheduled Test & Inspection interval. In some cases, the foundation repairs are made along with repairs that are required for the tank bottom, 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 steel components, the work must be properly coordinated. If the job is to be done in an economical fashion, the civil and mechanical engineers assigned to the job must work together during the assessment of settlement, the evaluation of various repair alternatives and during the ultimate repair.


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CALCULATING TANK SETTLEMENT Settling of tanks must be measured and analyzed during the life of the tanks. Work Aid 2 provides 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 been taken on this tank. Answer: The instructor will lead the Participants through the solution of this problem during class.


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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


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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 = where:

WH d HL γw

= = = =

π 2 d HLγ w 4

(Eqn. 7)

Hydrostatic test water weight, N (lb.) Diameter of the tank, m (ft.) Design maximum height of liquid in the tank, m (ft.) 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 following formula: π Wb = 4 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 appropriate formula(s) from the following: For a flat roof:


π Wr = 4 d2tr γst x D.F.

(Eqn. 10a)

79



where:

Wr d tr γst D.F.

= = = = =

Weight of the roof, N (lb.) Diameter of the tank, m (ft.) Thickness of the roof, in meters (in feet) Weight density of steel, 77 kN/m3 (490 lb./ft.3) Design factor to account for the additional weight of the roof support structure. If not specified assume this design factor is equal to 1.20.

For a cone roof:  d2 π  W r = dtr γ st  + h 2r   4 2

1/2

× D.F.

(Eqn. 10b)

where: Wr d tr hr γst D.F.

5.

= = = = = =

Weight of the cone roof, N (lb.) Diameter of the tank, m (ft.) Thickness of the roof, in meters (in feet) Height of the peak of the roof above the tank shell, m (ft.) Weight density of steel, 77 kN/m3 (490 lb./ft.3) Design factor to account for the additional weight of the roof support structure. If not specified assume this design factor is equal to 1.20.

If not already known, estimate the weight of the appurtenances. Wa = (Ws + Wb + Wr) x D.F. Ws Wb Wr Wa D.F.

= = = = =

(Eqn. 11)

Weight of the tank shell, N (lb.) Weight of the tank bottom, N (lb.) Weight of the tank roof(s), N (lb.) Weight of the appurtenances, N (lb.) 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. W D = Ws + Wb + Wr + Wa + Wi

(Eqn. 12)

where: WD Ws Wb Wr Wa Wi 7.

= = = = = =

Tank dead weight empty, N (lb.) Weight of the tank shell, N (lb.) Weight of the tank bottom, N (lb.) Weight of the tank roof(s), N (lb.) Weight of the appurtenances, N (lb.) Weight of insulation, if any, N (lb.)

Using the following formula, calculate the total hydrostatic test weight. W T = WH + WD

(Eqn. 13)

where: WT = Total hydrostatic test weight WH = Hydrostatic test water weight WD = Tank dead weight empty


81



Work Aid 1B: Procedure for Calculating Total Pressure 1.

Using the following formula, calculate the hydrostatic pressure. PH = γ h x C.F.

2.

where: PH γ

= =

h C.F.

= =

(Eqn. 14)

Hydrostatic pressure, kPa (psi) 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 the specific weight of water by the specific gravity, G, of the liquid. Height of the liquid above the point being considered, m (ft.) Conversion factor equal to 1 kPa/1 kN/m2 (1 psi/144 lb./ft.2)

Using the following formula, calculate the total pressure. P T = PH + PV

(Eqn. 15)

where: PT PH PV 3.

= = =

Total pressure, kPa (psi) Hydrostatic pressure, kPa (psi) Vapor pressure, if not specified, assume 0 kPa (psi)

Using the following formula, calculate the equivalent liquid height. Heq = (PT x C.F.)/γ

(Eqn. 16)

where: PT γ C.F.

= = =

Total pressure, kPa (psi) Specific weight of the liquid as defined above, kN/m3 (lb/ft3) Conversion factor equal to 1 kN/m2/1 kPa (144 lb/ft2/psi)


82



Work Aid 1C: Procedure for Calculating Roof Live Load Calculate the roof live load based on the specified or minimum roof live loading and the horizontal projected area of the roof using the following formula: π LRLL = 4 d2 RL where:

(Eqn. 17)

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


83



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

Using the following formula, calculate the wind base-shear force. F w = (∑ Kh (hh − hl ))× DGCf qr where:

(Eqn. 3)

Fw = Wind base-shear force, N (lb.) Kh = Height correction factor evaluated at the center of the height range from Figure 39 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.)

D

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

G

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

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

= Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2) for Saudi Aramco locations. Height above grade in meters (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


84



2.

Using the following formula, calculate the wind base-overturning moment.   h + hl   × DGCfqr Mw =  ∑ Kh (hh − hl ) h  2   where:

3.

(Eqn. 4)

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

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.)

D

=


G

=

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

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 Saudi Aramco locations.

=

Using the following formula, calculate the wind roof-lift load. Lw = π/4 d2 Kh GCpqr

(Eqn. 5)

where: Lw d Kh GxCp qr

= Wind roof-lift load, N (lb.) = Diameter of the tank, m (ft.) = Height correction factor evaluated at the mid-point of the roof (dimensionless) from Figure 39 = Combined gust and pressure lift coefficient (dimensionless) equal to 1.2 for shallow roofs with less than 10° angle. = Wind pressure at the reference elevation, 888 Pa (18.5 lb./ft.2) for Saudi Aramco locations.


85



Work Aid 1E: Procedure and Databases for Calculating Earthquake Base-Overturning Moment 1.

Calculate the value of d/HL (d = diameter of the tank, HL = the design maximum height of the liquid contents) and determine the value for factor k from Figure 40. 1.0 0.8 k 0.6 0.5

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

d/H L Source:

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

Figure 40. Factor k


86



2.

Determine the values for the weight coefficients Kw1 and Kw2 from Figure 41. 1.0 Kw 1 0.8 0.6 0.4 Kw 2 0.2

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

d/HL

Source:


Figure 41. Weight Coefficients


87



3.

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

Soil Profile Type Soil Profile Code

Soil Characteristics

S1

Either:

Site Amplification Factor S 1.0

Rock of any characteristic, whether shale-like or crystalline in nature, characterized by a shear-wave velocity greater than 2,500 ft/s or Stiff soil less than 200 ft. deep in which the soil that overlies rock consists of stable deposits of sands, gravels, or stiff clays S2

Deep cohesionless or stiff clay, including soil that is more than 200 ft. deep in which the soil that overlies rock consists of stable deposits of sands, gravels, or stiff clays

1.2

S3

Soft to medium-stiff clays and sands characterized by 30 ft. or more of soft to medium-stiff clay with or without intervening layers of sand or other cohesionless soils

1.5

S4

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

2.0

When the soil profile is not known in sufficient detail to determine the soil profile type, assume soil profile S 3

1.5

Unknown

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


88



4.

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

or K

1.0 0.8

K2

0.6

K1

0.4 0.2 0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

d/HL Source:


Figure 43. Height Coefficients


89



5.

Using the following formula, calculate the earthquake base-overturning moment.  S   2  ME = ZI (0.24WsHscg + 0.24WrHt + 0.24Kw1WcK1HL + K3  k d  KW2WcK2HL)

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

(Eqn. 6)

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. unless otherwise specified by CSD.

I = 1 for all petrochemical tanks,

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 maximum design 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 the maximum 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 maximum liquid height, HL, from Figure 40

d

= Diameter of the tank, m (ft.)

KW2 = Weight coefficient, based on the ratio of the tank diameter, d, to the maximum design liquid height, HL, from Figure 41 K2

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

K3

= Coefficient which is a function of the first sloshing mode of the tank and equal to 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 except for K3.


90



Work Aid 1F: Procedure for Calculating Live Loads for Appurtenances Platforms, ladders and their attachments to the tank should be designed to support their own weight plus a live load equal to the greater of 4450 N (1,000 lb.) or 2.4 kPa (50 lb./ft.2) on the floor and tread areas, A, unless otherwise specified. when A < 1.85 m2 (20 ft.2): LLL = 4450 N (1,000 lb.)

(Eqn. 18)

LLL = A x 2.4 kPa in SI units

(Eqn. 19)

when A > 1.85 m2 (20 ft.2): 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)


91



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 elevation readings.

3.

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

4.

Subtract the uniform settlement from the maximum difference between the original elevation reading and the corresponding actual settlement reading. The result is the amount of planar tilt settlement.

5.

Plot the actual settlement readings around the circumferences of the tank starting with the 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


92



6.

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

Actual settlement readings High point 0°

Planar tilt Elevation

Deviation from planar tilt

0

30

60

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

Figure 45. Example of a Plot 7.

The vertical difference between the best-fit cosine curve and the plot of actual settlement readings is the deviation from planar tilt and represents the differential circumferential settlement.


93



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 metal temperature

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

frangible joint

Weak welded joint at the top of a tank that fails if the tank is overpressured.

ISO

International Organization for Standardization

km/h

Kilometers per hour

ksi

1,000 pounds per square inch

maximum operating temperature

For tankage, is the maximum temperature at which the contents of the tank is stored. If above 93°C (200°F) then additional considerations 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 the movement of the container storing the liquid.

Specific Gravity

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

small tank

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


94

aramco 1

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