sulphuric acid storage tank

DESIGN OF AN ABOVE GROUND SULPHURIC

ACID STORAGE TANK

Final Year Project Report

Group: 53 Batch: 2009-2010

Muhammad Anas

ME-09139

S.M Ali Asad Jafri

Mirza Noman Baig

ME-09185

ME-09322

Irfan Khan Lodhi ME-09070

Internal Advisor:

Mr. Akhlaque Ahmed

Assistant Professor

Department of Mechanical Engineering

Reference#: 53/2013

CERTIFICATE

It is to certify that the following students have completed their project Design of an above ground

Sulphuric acid tank" satisfactorily.

Group: 53 Batch: 2009-2010

Name Seat No.

Muhammad Anas

ME-09139

S.M Ali Asad Jafri

Mirza Noman Baig

ME-09185

ME-09322

Irfan Khan Lodhi ME-09070

Internal Advisor

Mr. Akhlaque Ahmed

Assistant Professor

Project Coordinator

Dr. Muhammad Shakaib

Associate Professor

DEPARTMENT OF MECHANICAL ENGINEERING

NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

DEPARTMENT OF MECHANICAL ENGINEERING

NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

ii

ACKNOWLEDGEMENTS

The special thank goes to our helpful teacher, Mr. Akhlaque Ahmed -Assistant Professor

Mechanical Department (NEDUET).The supervision and support that he gave truly help

the progression and smoothness of the Storage Tank Design. The co-operation is much

indeed appreciated.

Our grateful thanks also go to Mr. Ahmed Mustafa Manager at Engro Polymer . A big

contribution and hard worked from him during the project is very great indeed. All practical

concepts and knowledge during the project would be nothing without the enthusiasm and

imagination from him. Besides, this project makes us realized the value of codes & standards

in designing of mechanical elements, which challenges us every minute. Not forget, great

appreciation go to the rest of classmates that help us from time to time during the project. The

whole project really brought us together to appreciate the true value of friendship and respect

of each other.

Last but not least we would like to thank Mr. Ahmed Mustafa Manager at Engro

Polymer for providing free demo version of Etank2000 that truly helps in verification of our

results.

Finally, Special thanks also to Dr. Muhammad Shakaib Project Coordinator, that have

encouraged, support and help us in completing this course successfully.

iii

ABSTRACT

This project represents the designing of storage tank. It includes sufficient information and

knowledge on how a storage tank is designed and which parameters are considered while

designing a storage tank according to codes and standard.

Storage tanks have been widely used in many industrial particularly in the oil refinery and

petrochemical industry which are to store a multitude of different product with Sulphuric acid

as one of it.

There are various industrial code and standard available for the basic requirement for tank

design and construction. Design and safety concern has been a great concern for the

increasing case of fire and explosion due the tank failure.

Although every effort has been made to obtain the most accurate solutions, it is the nature of

engineering that certain simplifying assumption is made. Solutions achieved should be

viewed in this light, and where judgment is required, they should be made with due

concentration.

This project describes different classification of storage tank followed by the description of

major components of a storage tank. The calculations are made as per API 650 code.

iv

TABLE OF CONTENTS

Contents Page No

Acknowledgements ii

Abstract iii

List of figures x

List of Tables xii

2.2 Types of Storage Tanks...5

2.2.1 Classification Based on Internal Pressure..6

2.2.1.1 Atmospheric Tanks..6

2.2.1.2 Low Pressure Tanks6

2.2.1.3 High Pressure Tanks6

2.2.2 Classification Based on Roof Type8

2.2.2.1 Fixed Roof Tanks...8

2.2.2.1.1 Cone Roof Tanks10

2.2.2.1.2 Umbrella Roof Tanks.11

2.2.2.1.3 Dome Roof Tanks..11

2.2.2.2 Floating Roof Tanks...12.

2.2.2.2.1 Internal Floating Roof ....12

2.2.2.2.2 External Floating Roof. 13

2.3 Process Description and Requirements....14

2.4 Design Factors considered in the Design of Storage Tanks....15

2.4.1 Metal Temperature...15

2.4.2 Pressure ...15

2.4.3 Specific Gravity.16

2.4.4 Corrosion Allowance..17

2.4.5 Other Loads....18

2.5 Mechanical Design.....19

2.6 Mechanical Design Considerations....20

2.7 Tank Assembly...22

v

2.7.1 Shell Attachments...22

2.7.1.1 Permanent Attachments..22

2.7.1.2 Temporary Attachments.....22

2.7.2 Tank Venting..22

2.7.3 Wind Girder....23

2.7.3.1 Secondary Wind Girder..23

2.7.4 Clean-out Doors.....23

2.7.5 Stairways and Handrails.24

2.7.6 Drainage arrangement....24

2.7.7 Nozzles...25.

2.7.8 Manholes....25

2.7.9 Anchor Bolts..26

3. INTRODUCTION TO API...27

3.1 Introduction..27

3.2 Standards and certification...27

3.3 API 650 (Welded steel tanks for oil storage)28

3.3.1 Scope.....29

3.3.2 Materials...29

3.3.3. Design...30

3.3.3.1 Welded Joints...30

3.3.3.1.1 Double-welded butt joint..30

3.3.3.1.2 Single-welded butt joint with backing..30

3.3.3.1.3 Double-welded lap joint .....30

3.3.3.1.4 Single-welded lap joint....30

3.3.3.1.5 Butt-weld. ..30

3.3.3.1.6 Fillet weld..30.

3.3.3.1.7 Full-fillet weld...30

3.3.3.1.8 Tack weld .30

3.3.3.2 Weld Size....30

3.3.3.3 Restrictions on Joints...30

vi

3.3.3.4 Typical Joints.........31

3.3.3.4.1 Vertical Shell Joints31

3.3.3.4.2 Horizontal Shell Joints31

3.3.3.4.3 Lap-Welded Bottom Joints.31

3.3.3.4.4 Butt-Welded Bottom Joints....31

3.3.3.4.5 Bottom Annular-Plate Joints...31

3.3.3.4.6 Shell-to-Bottom Fillet Welds..31

3.3.3.4.7 Wind Girder Joints..31

3.3.3.4.8 Roof and Top-Angle Joints.32

3.3.3.5 Loads..32

3.3.3.5.1 Dead load (DL) ...32

3.3.3.5.2 Stored liquid (F).....32

3.3.3.5.3 Hydrostatic test (Ht)...32

3.3.3.5.4 Minimum roof live load (Lr).32

3.3.3.5.5 Snow (S)................... 32

3.3.3.5.6 Wind (W) .....32

3.3.3.5.7 Design internal pressure (Pi) 32

3.3.3.5.8 Design external pressure (Pe)32

3.3.3.5.9 External Pressure......32

3.3.4 Marking.....33

3.3.4.1 Nameplates...33

4. TANK DESIGN34

4.1 Introduction...34

4.2 Shell Design...34

4.3 Calculating Shell Thickness...35

vii

4.3.1 One-Foot Method...35

4.3.2 Variable Design Point Method...35

4.4 Shell Design by one foot method36

4.4.1 Longitudinal Stress..37

4.4.2 Circumferential Stress ....37

4.4.3 Longitudinal Stress versus Circumferential Stress..38

4.4.4 Circumferential Stress Thickness Equation and 1-Foot Method 39

4.5 Top Stiffener and Intermediate Wind Girder Design..39 4.5.1 Top Stiffener/ Top Wind Girder..39

4.5.2 Intermediate Wind Girder40

4.6 Bottom Plate Design.43

4.7 Roof Design..44

4.8 Overturning Stability against Wind Load.45.

4.9 Seismic Design..47

4.9.1 Overturning Stability against seismic load...47

4.10 Anchorage requirement.48

.

5. 0 DESIGN CALCULATIONS50

5.1 Material Selection .50

5.2 Design Specifications.51

5.3 Basic Calculation....52

5.4 Shell Design53

5.5 Bottom Plate Design .......56

5.6 Annular Plate Design...56

5.7 Intermediate Wind Girder58

5.8 Roof Design (Supported Conical Roof )..61

viii

5.8.1 Roof Plate Design.61

5.9 RAFTER DESIGN ..............63

5.10 COLUMN DESIGN.64

5.11 Tank Overturning Stability...68

5.12.1 RESISTANCE TO OVERTURNING (per API-650 5.11.2) .. 71

5.12.2 Stability of Tank against Seismic Load .... 73

5.12.3 SEISMIC VARIABLES .74

5.12.4 STRUCTURAL PERIOD OF VIBRATION..76

6.0 RESISTANCE TO DESIGN LOAD .77

6.1 EFFECTIVE WEIGHT OF PRODUCT .....77

6.1.1 DESIGN LOADS 77

6.2.1 CENTER OF ACTION FOR EFFECTIVE LATERAL FORCES ..78

6.2.2 CENTER OF ACTION for RINGWALL OVERTURNING MOMENT...78

6.2.3 CENTER OF ACTION for SLAB OVERTURNING MOMENT .79

6.2.4 Dynamic Liquid Hoop Forces .79

6.2.5 Overturning Moment ...80

6.2.6 RESISTANCE TO DESIGN LOADS.....80

6.3 ANCHOR BOLT DESIGN..83

6.4 CAPACITIES and WEIGHTS .....87

7.0 DEVELOPMENT OF STORAGE TANK DESIGN SOFTWARE &

VERIFICATION OF RESULTS USING ETANK 2000.93

7.1 Overview....93

7.2 Design Capabilities.93

7.3 Key Features 93

7.4 Application Areas.93

ix

7.5 Visual Basic Programming for Shell Design.94

7.6 VERIFICATION OF RESULTS..101

CONCLUSION..103

REFERENCES...104

APPENDIX105

Appendix C Shell Design....107

Appendix D Bottom and annular Plate Design...110

Appendix E Intermediate wind girder ...112

Appendix F Roof Design114

Appendix G Tank overturning stability..117

x

LIST OF FIGURES

Figure 1 Types of Tank ............................................................................................. 4

Figure 2 Tanks on the basis of internal pressure ..................................................... 7

Figure 3 Fixed roof tank ........................................................................................... 8

Figure 4 Fixed roof tank ........................................................................................... 9

Figure 5 Steel tank with cone roof .......................................................................... 10

Figure 6 Cone roof tank with column supports ...................................................... 10

Figure 7 Umbrella Roof Tank11

Figure 8 Dome roof tank ........................................................................................ 11

Figure 9 Internal floating roof tank........................................................................ 12

Figure 10 External floating roof tank ....................................................................... 13

Figure 11 Storage tank capacities and levels shell .................................................. 14

Figure 12 Hydrostatic pressure in a storage tank .................................................... 16

Figure 13 Corrosion allowance in a tank shell ........................................................ 17

Figure 14 Wind and earthquake loads ..................................................................... 18

Figure 15 Tank exploding ......................................................................................... 20

Figure 16 Loading diagram on a tank shell ............................................................. 21

Figure 17 Tank Venting....22

Figure 18 Wind Girder placement on shell.. 23

Figure 19 Cleanout Door...23

Figure 20 Stairways and handrails.....24

Figure 21 Nozzle....25

Figure 22 Manholes......25

Figure 23 Anchor Bolts........26

Figure 24 Name Plates of storage tanks....33

Figure 25 Diagramatic variation of stress in a shell...34

Figure 26 Thickeness of tank shell courses...35

Figure 27 Longitudinal forces acting on thin cylinder....37

Figure 28 Circumferential forces acting on thin cylinder .........38

Figure 29 Stiffener rings.........39

Figure 30 Wind girders.41

Figure 31 Transformed shell and intermediate wind girder..42

Figure 32 Bottom Layout for tank.......43

Figure 33 Cross joints in bottom plates.............44

xi

Figure 34 Overturning moment against wind load...45

Figure 35 Shell out of roundness caused by wind.45

Figure 36 Roof Segments.......62

Figure 37 Compression ring at shell to roof joint.64

Figure 38 Selected Plate Size76

Figure 39 Surface development of shell ....77

Figure 40 Construction of upper shell course- inside tank.....78

Figure 41 Construction of upper shell course- outside tank ..78

Figure 42 Bottom plate arrangement on foundation80

Figure 43 Arrangement of bottom plates81

Figure 44 3D model of storage tank.. .83.

Figure 45 3D model of storage tank ....84

Figure 46 Arrangement of stiffener at roof.85

Figure 47 Snapshot of software87.

Figure 48 Design report showing hydroststic and design thickness..87

Figure 49 Message box indicating compeltion of calcuklations..93

Figure 50 Output Results..94

Figure 51 Verification of results thriugh E-Tank.95

xii

LIST OF TABLES

Table 1: Pepsi can and storage tank comparison table..19

Table 2: Anchorage ratio criteria [API 650,2007]..48

Table 3: BOQ for shell arrangement79

Table 4: BOQ for bottom plate arrangement..82

1

CHAPTER 1: INTRODUCTION

1.1 Rationale

Storage tanks have been widely used over the world in many industries. They are designed,

fabricated and tested to code and standard. There are a variety of codes and standards stating

the similar common minimum requirements and some additional requirements from company

standards or specifications.

Engineer or tank designer who do the preliminary and detail design are normally not familiar

or not exposed to the actual site condition. Their designs are basically based on the code and

standard requirements and basic theory from reference book. Some would only rely on the

commercial software for the basic design, they have limited knowledge on the actual tank

operation which limit them on cost effectiveness and even safety detail design.

There is limited procedure and rules in design the fixed and floating roof tanks. These had

resulted lots of roof failure in the industry. Hence industry, tank owner and also the tank

designer or engineer need to have a simple rules and formula to ensure the roof is adequately

designed and strong enough for the various loading during operation.

Beside of the procedures and rules, understanding of the stresses behave in the tank material

is essential for a complete safe design.

Hence it is essential for the engineers or tank designer to know how and what effects each

inter discipline design would have on ones tank that affected the tank integrity, and taking all these consideration into his design.

1.2 Project Goal

1.2.1Project Aim

The aim of this project is to follow basic rules and procedures, highlighting the concerns in

designing of a fixed roof tank.

1.2.2Project Objective

The main objective of this project is To design an above ground Suphuric acid storage tank.

2

1.3 Project Methodology

1.3.1 Literature Review

Literature review is conducted to study the basic design and requirement of the fixed roof

storage tank in the storage tank design code (API 650 Welded Steel Tanks for Oil Storage).

1.3.2Design Approach

Upon completion of the literature review, design approach is then developed. The storage

tank design consists of two major designs, that is (1) the shell design analysis and (2) the roof

design.

In the shell design analysis, shell stress design will be performed taking into consideration of

all the considerably loading including hydrostatic pressure, wind loading and seismic loading.

In the roof design, it consists of two sections, that is (1) roof stress design and the (2) roof

fitting and accessories design.

1.4 Perspective view on Standards and Codes

Industries that require the storage of flammable and combustible liquids face a complex array

of codes and standards with which they must comply. Besides adhering to environmental

regulations, underground and aboveground tank systems must be sited and operated in

accordance with local building and fire codes.

1.4.1 What is a standard?

A standard is a series of requirements that tell you how to do something. A standard tends not

to have any enforcement requirements. A standard becomes an enforceable document when it

is adopted by reference in a code.

1.4.2 What is a code?

A code is a set of regulations that tells you when to do something. A code will have

requirements specifying the administration and enforcement of the document.

1.4.3How Does a Code or Standard Become Enforceable?

A code or standard becomes enforceable when it is adopted by reference through local, state,

or federal government legislative process, such as an ordinance, statute, or bill. The law must

adopt a specific edition (year of publication) of a code or standard, and may include

amendments to specific portions of the code or standard being adopted.

3

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction

Storage tanks have been widely used in many industrial established particularly in the

processing plant such as oil refinery and petrochemical industry. They are used to store a

multitude of different products. They come in a range of sizes from small to truly gigantic,

product stored range from raw material to finished products, from gases to liquids, solid and

mixture thereof.

Liquids and vapors in the petroleum industry, usually called petroleum liquids or vapors

generally are mixtures of hydrocarbons having dissimilar true vapor pressures. Examples

would include jet fuel diesel, gasoline and crude oil.

Liquidsandvaporsinthechemicalindustry,usuallycalledvolatileorganicliquidsand vapor,

are composed of pure chemicals or mixtures of chemicals with similar true vapor

pressures. Examples would include benzene, styrene, and alcohols.

There are a wide variety of storage tanks, they can be constructed above ground, in ground

and below ground. In shape, they can be in vertical cylindrical, horizontal cylindrical,

spherical or rectangular form, but vertical cylindrical are the most usual used.

In a vertical cylindrical storage tank, it is further broken down into various types, including

the open top tank, fixed roof tank, external floating roof and internal floating roof tank.

The type of storage tank used for specified product is principally determined by safety and

environmental requirement. Operation cost and cost effectiveness are the main factors in

selecting the type of storage tank.

.

.

4

2.2 Types of Storage Tanks

Figure 1Types of storage tank

Types Of Storage Tanks

Based On Internal Pressure

Atmospheric Tanks

Low Pressure Tanks

High Pressure Tanks

Based On Roof type

Fixed Roof Tanks

Cone Roof

Umbrella Roof

Dome Roof

Floating Roof Tanks

Internal Floating Roof

External Floating Roof

5

2.2.1 Classification Based on Internal Pressure In the case the internal pressure reacts on the tank during storage, it is possible to classify the

tanks based on this level of pressure. This pressure effect depends directly of the size of the

tank. The larger the tank, the more severe effect of pressure is on the structure. This

classification is commonly employed by codes, standards and regulations all over the world.

2.2.1.1 Atmospheric Tanks These tanks are the most common. Although they are called atmospheric, they are usually operated at internal pressure slightly above atmospheric pressure. The fire codes define an

atmospheric tank as operating from atmospheric up to 3.5 kN/m2above atmospheric

pressure.

2.2.1.2 Low-Pressure Tanks Within the context of tanks, low pressure mean that tanks are designed for a pressure higher than atmospheric tanks. Tanks of this type are designed to operate from atmospheric pressure

up to about 100 kN/m2.

2.2.1.3 Pressure Vessels (High-Pressure Tanks)

Since high-pressure tanks are really pressure vessels, the term high-pressure tank is not

frequently used; instead they are called only vessels. Because these kinds of tanks are usually

built underground, they are treated separately from other tanks by all codes, standards, and

regulations.

6

Figure 2Tanks on the basis of Internal Pressure

7

2.2.2 Classification based on Roof Type

2.2.2.1 Fixed Roof Tanks

Fixed Roof Tanks can be divided into cone roof, umbrella roof and dome roof types. They

can be self-supported or rafter/ trusses supported depending on the size.

Of currently used tank designs, the fixed-roof tank is the least expensive to construct and is

generally considered the minimum acceptable equipment for storing VOL's (volatile organic

liquids).A typical fixed-roof tank consists of a cylindrical steel shell with a cone- or dome-

shaped roof that is permanently affixed to the tank shell. Most recently built tanks are of all-

welded construction and are designed to be both liquid-and vapor-tight.

For fixed-roof tanks, the nominal capacity is the geometric volume from the bottom of the

tank up to the cur bangle, which is a metallic angle that is welded along the periphery at the

top of the cylindrical portion of the tank.

Figure3FixedRoofTank

8

Figure4FixedRoofTank

9

2.2.2.1.1Cone Roof Tanks

Cone-roof tanks have also cylindrical shells in the lower part. These are the most widely

used tanks for storage of relatively large quantities of fluid. They have a vertical axis of

symmetry; the bottom is usually flat land the top is made in the form of shallow cone. They

are economical to build and the economy supports a number of contractors capable of

building them. Cone-roof tanks typically have roof rafters and support columns excepting

very small-diameters tanks.

Figure5Steel Tank with cone-roof

Figure 6Cone-rooftankwithcolumnsupports

10

2.2.2.1.2 Umbrella-Roof Tanks

They are very similar to cone-roof tanks, but there of looks like an umbrella. They are

usually constructed with diameters not much larger than 20 m. Another difference is that

the umbrella-roof does not have to be supported by columns to the bottom of the tank, so

that they can be a self-supporting structure.

Figure7UmbrellaRoofTank

2.2.2.1.3 Dome-Roof Tanks

This type has almost the same shape of the umbrella type except that the dome approximate

a spherical surface more closely than these gentled sections of an umbrella-roof. There are

several ways to fabricate such tanks. One of the known as the tank airlift method, "in which

the roof and the upper course of shell are fabricated first, then lifted by air that is blown in to

the tanks as the remaining lower courses of steel shell are welded into place.

Figure8DomeRoofTank

11

2.2.2.2 Floating Roof Tanks

Floating roof tanks is which the roof floats directly on top of the product.

There are 2 types of floating roof:

2.2.2.2.1 Internal Floating Roof Internal floating roof tank is the one in which the roof floats on the product in a fixed roof

tank.

Figure9InternalFloatingRoof Tank

Peripheral roof vents Fixed-roof center vent

Fixed roof

column

supported

Tank shell

Gage head

Fixed roof

Supported column

Deck drain Access hatch

Deck leg

Ladder

Vacuum breaker

Sample port

Rim seal

12

2.2.2.2.2External Floating Roof External floating roof tank is the one in which the roof floats on the product in an open tank

and the roof is open to atmosphere.

Figure10ExternalFloatingRoofTank

13

2.3 Process Description and Requirements

Capacity determination is the one of the first steps in designing the tank. Only after the

capacity is known, the tank can be sized up. The definition of the maximum capacity can be

explained easily in figure below.

Figure 11 Storage Tank Capacities and Levels shell

The maximum or total capacity is the sum of the inactive capacity (minimum operating

volume remaining volume in tank), actual or net-working capacity and the-overfill protecting

capacity.

The net-working capacity is the volume of available product under normal operating

conditions, which is between the low liquid level (LLL) and the high liquid level (HLL).

14

2.4 Design Factors that are considered in the Mechanical Design of Storage

Tanks

This section discusses the primary factors that are considered in the mechanical design of

storage tanks. These factors are as follows:

1. Metal temperature

2. Pressure

3. Specific gravity of the stored liquid

4. Corrosion allowance

5. Other loads

2.4.1 Metal Temperature

The metal temperature of storage tank components is determined by the operating

requirements of the stored liquid, and by the ambient temperature at the tank location.

The operating requirements and operating conditions of the stored liquid are

determined by process engineers. The mechanical design of storage tank components

must consider both the highest and the lowest temperatures to which the tank can be

exposed.

The maximum operating temperature determines the allowable stress that is used for

the mechanical design of storage tank components. The allowable stress of each

specific material is constant for all temperatures up to 93C (200F); however, the

allowable stress of each material decreases for temperatures that are above 93C

(200F). API-650 Appendix M contains additional design criteria that must be

followed for tanks that have maximum operating temperatures over 93C (200F).

The tank could experience a permanent deformation or a ductile fracture if the design

requirements of API-650 Appendix M are not followed.

2.4.2 Pressure

The internal pressure at which a storage tank will operate determines which API

standard is to be used for the mechanical design of the tank and its associated

components. API-650, Welded Steel Tanks for Oil Storage, is the design standard for

tanks that operate at internal pressures approximating atmospheric pressure. API-650

may also be used for tanks that operate at internal pressures up to 17 kpa; however,

additional design requirements that are contained in API-650 Appendix F must be

followed if the internal pressure exceeds atmospheric pressure.

15

2.4.3 Specific Gravity of Stored Liquid

The specific gravity of the liquid that is being stored, (G), in conjunction with the

depth of the liquid determines the hydrostatic pressure of the liquid. The total

hydrostatic pressure at a given elevation in a tank must be considered in determining

the required thickness of the tank shell.

Figure 12 Hydrostatic Pressure in a Storage Tank

Storage tanks must be designed for the specific gravity of water (1.0), because the

tanks are filled with water for testing purposes after they are constructed. If the

specific gravity of the liquid that is to be stored exceeds 1.0 (water), the tank must be

designed for the higher specific gravity.

The required shell thickness is directly proportional to the specific gravity of the

stored liquid. If the specific gravity is not correctly specified, the calculated shell

thickness will be incorrect. In extreme cases, the shell can become permanently

deformed if it is too thin, and a ductile fracture may occur.

It may be desirable for operational reasons to change the liquid that is being stored

after the tank has been in service for some period of time. In situations where the

stored liquid is changed, the tank must be evaluated based on the specific gravity of

the new liquid.

16

2.4.4 Corrosion Allowance

The components of a storage tank may lose metal due to corrosion that is caused by

the stored liquid. To compensate for this metal loss, a "corrosion allowance" (CA)

may be added to the metal thickness that is required for strength. This "corrosion

allowance" offsets the expected deterioration of the tank components while they are in

service. When needed, the corrosion allowance is typically added to the calculated

required thicknesses of the shell, internal components, and structural members that

may be used to support a fixed roof. A corrosion allowance is typically not added to

the required thicknesses of the roof itself or the bottom.

Figure 13 Corrosion Allowance in Tank Shell

Where

t = Minimum required shell thickness

T = Total required shell thickness

CA = Corrosion allowance

17

2.4.5 Other Loads

The mechanical design of a storage tank must also consider loads other than pressure.

These other loads include wind and earthquake, loads that are imposed by connected

piping systems (and other attachments) on nozzles, and rainwater accumulation on the

roof of tanks.

Figure 14Wind and Earthquake Loads

Where

Fw= Base shear force due to wind

M = Overturning moment due to wind or earthquake

18

2.5 Mechanical Design

Stress design and analysis of the storage tank is the greatest concern to engineer as it provides

the basic for the tank stability and integrity.

The basic stress analyses to be taken care in tank design are as follow:

Tank shell wall due to internal and external loading Bottom plate/ Tank flooring

Storage tanks always look big and strong, and there are also often being referred as tin can. Some simple comparison in term of their sizes and strength is made here.

Table 1 Pepsi Can and Storage Tank Comparison Table

From the Table 1, it can be seen found the tank ratio (t/D) is 4 times less than the typical bean

can which show that how relatively flimsy the shell of the tank it would be if it is subjected

to partial vacuum. Figure 19 shows tank exploding due to vacuum loading.

19

Figure 15Tank Exploding

2.6 Mechanical Design Consideration

The principal factors in determine the shell thickness is the loads, the primary loading to

determine the basic shell thickness is as follow:

The internal loading due to the head of liquid The pressure in the vapor space

Other external loading shall be taken into consideration are:

External pressure-Vacuum condition Wind loading Seismic Loading Localized loads resulting from nozzles, attachments, ladder/ stair and platform etc.

20

Figure 16Loading Diagram on a Tank Shell

The internal pressure exerted on the tank shell is the product liquid head; the pressure is at the

highest at the tank shell bottom and decreases linearly along its height. External loading of

wind and seismic act on the tank shell and create an overturning moment about the shell to

bottom joint, this result in the uplift reaction of the tank and affected the tank stability.

The various stresses to which the shell of a tank is subjected are:

Hoop tension This is caused by the head of product in the tank, together with any over pressure in the roof

space of a fixed roof tank.

Axial compression This comes from the tank self-weight, internal vacuum, wind and seismic loading acting on

the shell which causes an overturning effect.

Vertical bending

This is due to the expansion of shell under normal service loading.

21

2.7 TANK ASSEMBLY

2.7.1 Shell Attachments

2.7.1.1 Permanent Attachments

Permanent attachments are items welded to the shell that will remain while the tank is in its

intended service. These include items such as wind girders, stairs, gauging systems, davits,

walkways, tank anchors, supports for internal items such as heating coils and other piping

supports, ladders, floating roof supports welded to the shell, exterior piping supports,

grounding clips, insulation rings, and electrical conduit and fixtures. Items installed above the

maximum liquid level of the tank are not permanent attachments.

2.7.1.2 Temporary Attachments

Temporary attachments are items welded to the shell that will be removed prior to the tank

being commissioned into its intended service. These include items such as alignment clips,

fitting equipment, stabilizers, and lifting lugs.

2.7.2 Tank Venting

Suitable vents shall be provided to prevent overstressing of the roof deck or seal membrane.

Vents, bleeder valves, or other suitable means shall be adequate to evacuate air and gases

from underneath the roof during initial filling Tanks designed in accordance with this

standard and having a fixed roof shall be vented for both normal conditions (resulting from

operational requirements and atmospheric changes) and emergency conditions (resulting

from exposure to an external fire). Normal venting shall be adequate to prevent internal or

external pressure from exceeding the corresponding tank design pressures. Emergency

venting requirements are satisfied if the tank is equipped with a weak roof-to-shell

attachment (frangible joint).

Figure 17 Tank Venting

22

2.7.3 Wind Girder

Tank shall be provided with stiffening rings to maintain roundness when the tank is subjected

to wind loads. The stiffening rings shall be located at or near the top of the top course,

preferably on the outside of the tank shell. Stiffening rings may be made of structural

sections, formed plate sections, sections built up by welding. The outer periphery of

stiffening rings may be circular or polygonal Rings that may trap liquid shall provided with

adequate drain holes.

Figure 18 Wind girder placement on shell

2.7.3.1 Secondary wind girders

Tank may require secondary rings to maintain roundness over the full height of the tank shell

under wind and/or vacuum conditions. There are basically, additional stiffening rings.

Continuous welding (full penetration butt welds) shall be used for all connections of the

secondary wind girders.

2.7.4 Clean-out doors

If required for tanks made of carbon steel, clean out doors shall be designed and fabricated.

This is more for sludge removal and to allow entry of a conveyor belt, if required.

Figure 19 Clean-out doors

23

2.7.5 Stairways and handrails

Vertical tanks should be provided with spiral stairways. An exception may be made for

groups of tanks of less than 12.5 m diameter sited close together and connected by walkways

at roof level. In such groups, two tanks at opposite ends of each group shall be provided with

stairways, so that each tank in that group will then have at least two escape routes from the

roof.

Handrails shall be provided at the edge of the roof for full circumference of all fixed roof

tanks and to the center of the roof on all tanks exceeding 12.5 m diameter. Handrails shall be

provided on the outside of all spiral stairways. For open top tanks, the inside of the staircases

shall also be provided with a handrail in the immediate vicinity of the top landing. Handrails

shall be provided on both sides of all walkways between tanks. Stairways shall be provided

with the specified lighting facilities.

Figure 20Stairways and handrails

2.7.6 Drainage arrangement Water draw (Center drains or side drains)

In operation, tank bottoms should normally slope down towards the center and be fitted with

center sumps.

Large tanks (>50 m diameter) may also be provided with additional side drain sumps, the

nozzles of which may be blinded off after the water test.

However, for products with temperature exceeding 100 , the tank bottom slope up towards the centre in order to prevent corrosion caused by rain water penetrating under the bottom.

24

2.7.7Nozzle

When considering the nozzle, one might think that it has to do something with change in

velocity and pressure which is the application of Bernoullis equation and continuity equation as well. In storage tanks, the nozzle is nothing but just an opening through which either the

fluid can enter or leave the storage tank.

Figure 21 Nozzle

2.7.8Manholes

Manholes are nothing but big nozzles of sizes more than 16 inch .Usually the pipe section

used for manholes will be fabricated pipe from plate. Therefore, for vessels with joint

efficiency 1, the long seem weld is category A butt, and hence calls for 100% radiography

prior to fit up of the circumferential joint between the flange and the neck. For manholes with

fabricated neck, the reinforcement pad is essential and will be provided as per design. In case

there is any practical difficulty and placement of reinforcement pad prior to the welding

manhole to the shell, it can be inserted into halves provided there are at least two tapped

telltale holes on both halves of the pad. A part from these, manufactures of manhole is the

same as that of the nozzle.

Figure 22Manholes

25

2.7.9Anchor bolts

An anchor bolt is used to attach objects or structures to concrete. There are many types of

anchor bolts, consisting of designs that are mostly propriety to the manufacturing companies.

All consist of a threaded end, to which a nut and washer can be attached for the external load.

Anchor bolts are extensively used on all types of projects, from standard building to dams

and nuclear power plants.

The simplest anchor bolt is a cast-in-place anchor. Most designs consist of standard bolt with

a hexagonal head , which is cast in the wet concrete before it sets .There are other designs,

some consisting of a bent bolt with a hook on the end, or some other sort of bending.

Figure 23 Anchor bolts

26

CHAPTER 3: INTRODUCTION TO API STANDARDS

3.1 INTRODUCTION

American Petroleum Institute (API) Standards 650, 653 and 620 are the primary industry

standards by which most aboveground welded storage tanks are designed, constructed and

maintained. These standards address both newly constructed and existing aboveground

storage tanks used in the petroleum, petrochemical and chemical industries. API Standards

650, 653, 620, and some related Recommended Practices that have been made over the years

or are being developed to improve the standards with respect to leak detection and spill

prevention. API and other standards and practices that should be followed to reduce the risk

of spills and leaks. API has published standards for the construction of aboveground storage

tanks since the mid- 1930s. API Standards for aboveground storage tanks designed for atmospheric pressures, up to a maximum of 2.5 psig. API Standard 620 is applicable to tanks

and vessels designed for low-pressure storage, ranging from about 2.5 psig to 15 psig.

The First Edition of API 650 was published in 1961, but its predecessor, API 12C, had been

in use since 1936, when welding began to replace riveting as the preferred construction

method. Both API 12C and API 650 address only newly constructed tanks. It was not until

the late 1980s that API began development of a new standard to address specific maintenance and inspection issues for existing aboveground storage tanks. This standard is

API 653, Tank Inspection, Repair, Alteration and Reconstruction. Since the publication of API Standard 653 in 1991, the tank inspection, repair, alteration and reconstruction methods

prescribed therein have - when properly applied significantly improved the safety and reliability of existing tanks. This standard and other API standards are continuously being

improved to incorporate new technology and to reflect the actual experiences of owners and

operators of aboveground storage tanks.

3.2Standards and Certification

The publications, technical standards, and online products are designed, according to API

itself, to help users improve the efficiency and cost-effectiveness of their operations, comply

with legislative and regulatory requirements, and safeguard health, ensure safety, and

(perhaps most controversially) "protect the environment". Each publication is overseen by a

committee of industry professionals, mostly member company engineers.

These technical standards tend to be uncontroversial. For example, API 610 is the

specification for centrifugal pumps, API 675 is the specification for controlled volume

positive displacement pumps, both packed-plunger and diaphragm types are included.

Diaphragm pumps that use direct mechanical actuation are excluded. API 677 is the standard

for gear units and API 682 governs mechanical seals.

27

API provides vessel codes and standards for the design and fabrication of pressure vessels

that help safeguard the lives of people and environments all over the world.

API also defines and drafts standards for measurement for manufactured products such as:

Precision thread gauges

Plain plug and ring gauges

Thread measuring systems

Metrology and industrial supplies

Measuring instruments

Custom gauges

Precision machining and grinding

3.3 API 650 (WELDED STEEL TANKS FOR OIL STORAGE)

API 650 covers material, design, fabrication, erection and testing requirements for

aboveground, vertical, cylindrical, closed and open-top, welded steel storage tanks in various

sizes and capacities. This standard applies to tanks with internal pressures approximating

atmospheric pressure, but ranging as high as 2.5 psig. This standard applies to newly

constructed tanks before they have been placed in service.

General contents are:

1. Scope

2. Materials

3. Design

4. Fabrication.

5. Erection

6. Methods Of Inspecting Joints

7. Welding Procedure And Welder Qualifications

8. Marking

28

3.3.1 Scope

This standard covers material design, fabrication, erection, and testing requirements for vertical, cylindrical, aboveground, closed- and open-top, welded steel storage

tanks in various sizes and capacities.

Internal pressures approximating atmospheric pressure (internal pressures not exceeding the weight of the roof plates), but a higher internal pressure is permitted

when additional requirements are met.

This standard applies only to tanks whose entire bottom is uniformly supported and to tanks in no refrigerated service that have a maximum design temperature of 93C.

This standard is designed to provide the petroleum industry with tanks of adequate safety and reasonable economy.

This standard does not present or establish a fixed series of allowable tank sizes; instead, it is intended to permit the purchaser to select whatever size tank may best

meet his needs.

This standard has requirements given in two alternate systems of units, that is SI Units and US Customary Units. System can be choose by the mutual agreement

between manufacturer and purchaser but the condition is that units must be

consistent.

The appendices of this standard provide a number of design options requiring decisions by the purchaser, standard requirements, recommendations, and

information that supplements the basic standard.

An appendix becomes a requirement only when the purchaser specifies an option covered by that appendix.

3.3.2Materials

Materials used in the construction of tanks shall conform to the specifications listed in this standard.

Conditions of usage of material

i. Approved by the purchaser.

ii. Material should certify to meet all of the requirements of a material specification listed in this standard.

New or unused plates should be completely identified by records.

29

Materials of construction are used that are certified to two or more material specifications.

Plates for shells, roofs and bottoms may be ordered on an edge thickness basis or on a weight(kg/m2 or lb/ft2)

Shell plates are limited to a max thickness of 45mm (1.75in).

Plates used as flanges may be thicker than 45mm (1.75in).

3.3.3Design

3.3.3.1 Welded Joints

3.3.3.1.1 Double-welded butt joint :A joint between two abutting parts lying in

approximately the same plane that is welded from both sides.

3.3.3.1.2 Single-welded butt joint with backing :A joint between two abutting parts lying in

approximately the same plane that is welded from one side only with the use of a strip bar or

another suitable backing material.

3.3.3.1.3 Double-welded lap joint: A joint between two overlapping members in which the

overlapped edges of both members are welded with fillet welds.

3.3.3.1.4 Single-welded lap joint: A joint between two overlapping members in which the

overlapped edge of one member is welded with a fillet weld.

3.3.3.1.5 Butt-weld: A weld placed in a groove between two abutting members. Grooves

may be square, V-shaped (single or double), or U-shaped (single or double), or they may be

either single or double beveled.

3.3.3.1.6 Fillet weld: A weld of approximately triangular cross section that joins two surfaces

at approximately right angles, as in a lap joint, tee joint, or corner joint.

3.3.3.1.7 Full-fillet weld: A fillet weld whose size is equal to the thickness of the thinner

joined member.

3.3.3.1.8 Tack weld: A weld made to hold the parts of a element in proper alignment until

the final welds are made.

3.3.3.2 Weld Size:

The size of a groove weld shall be based on the joint penetration (that is, the depth of

chamfering plus the root penetration when specified).

3.3.3.3 Restrictions on Joints:

There are also some restrictions in welded joints mentioned in standards according to the

thicknesses and position of sheets/plates.

30

3.3.3.4 Typical Joints

3.3.3.4.1 Vertical Shell Joints:

Vertical shell joints shall be butt joints with complete penetration and complete fusion

attained by double welding or other means that will obtain the same quality of deposited weld

metal on the inside and outside weld surfaces. Vertical joints in adjacent shell courses shall

not be aligned but shall be offset from each other a minimum distance of 5t. Where, t is the

plate thickness of the thicker course at the point of offset.

3.3.3.4.2 Horizontal Shell Joints:

Horizontal shell joints shall have complete penetration and complete fusion; however, as an

alternative, top angles may be attached to the shell by a double-welded lap joint. Unless

otherwise specified, abutting shell plates at horizontal joints shall have a common vertical

centerline.

3.3.3.4.3 Lap-Welded Bottom Joints:

Lap-welded bottom plates shall be reasonably rectangular. Additionally, plate may be either

square cut or may have mill edges. Mill edges to be welded shall be relatively smooth and

uniform, free of deleterious deposits, and have a shape such that a full fillet weld can be

achieved. Lapping of two bottom plates on the butt-welded annular plates does not constitute

a three-plate lap weld.

3.3.3.4.4 Butt-Welded Bottom Joints:

Butt-welded bottom plates shall have their parallel edges prepared for butt welding with

either square or V grooves. Butt-welds shall be made using an appropriate weld joint

configuration that yields a complete penetration weld.

3.3.3.4.5 Bottom Annular-Plate Joints:

Bottom annular-plate radial joints shall be butt-welded in accordance with 3.1.5.5 and shall

have complete penetration and complete fusion. The backing strip, if used, shall be

compatible for welding the annular plates together.

3.3.3.4.6 Shell-to-Bottom Fillet Welds:

For bottom and annular plates with a nominal thickness 12.5 mm (1/2in.), and less, the

attachment between the bottom edge of the lowest course shell plate and the bottom plate

shall be a continuous fillet weld laid on each side of the shell plate.

3.3.3.4.7 Wind Girder Joints:

Full-penetration butt-welds shall be used for joining ring sections. Horizontal bottom-side

joints shall be seal-welded if specified by the purchaser. Seal-welding should be considered

to minimize the potential for entrapped moisture, which may cause corrosion.

31

3.3.3.4.8 Roof and Top-Angle Joints:

Roof plates shall, as a minimum, be welded on the top side with a continuous full-fillet weld

on all seams. Butt-welds are also permitted. Roof plates shall be attached to the top angle of a

tank with a continuous fillet weld on the top side only.

3.3.3.5 Loads

Loads are defined as follows:

3.3.3.5.1 Dead load (DL): The weight of the tank or tank component, including any

corrosion allowance.

3.3.3.5.2 Stored liquid (F): The load due to filling the tank to the design liquid level with

liquid with the design specific gravity.

3.3.3.5.3 Hydrostatic test (Ht): The load due to filling the tank with water to the design

liquid level.

3.3.3.5.4 Minimum roof live load (Lr): 1kPa (20 lbf/ft) on the horizontal projected area of

the roof.

3.3.3.5.5 Snow (S): The design snow load shall be 0.84 times the ground snow load.

Alternately, the design snow load shall be determined from the ground snow load in

accordance with ASCE 7.

3.3.3.5.6 Wind (W): The design wind speed (V) shall be 190 km/hr (120 mph), the 3 second

gust design wind speed determined from ASCE 7.

3.3.3.5.7 Design internal pressure (Pi): Shall not exceed 18 kPa (2.5lbf/in2).

3.3.3.5.8 Design external pressure (Pe): Shall not be less than 0.25kPa (1 in. of water) and

shall not exceed 6.9 kPa(1.01 lbf/in2).

3.3.3.5.9 External Pressure: Tanks that meet the requirements of this standard may be

subjected to a partial vacuum of 0.25 kPa (1 in. of water), without the need to provide any

additional supporting calculations.

32

3.3.4 Marking

3.3.4.1 Nameplates

A tank made in accordance with this standard shall be identified by a nameplate. The nameplate shall be attached to the tank shell adjacent to a manhole or to a

manhole reinforcing plate immediately above a manhole.

A nameplate that is placed directly on the shell plate or reinforcing plate shall be attached by continuous welding or brazing all around the nameplate.

When a tank is fabricated and erected by a single organization, that organizations name shall appear on the nameplate as both fabricator and erector.

When a tank is fabricated by one organization and erected by another, the names of both organizations shall appear on the nameplate, or separate nameplates shall

be applied by each.

Figure 24 Name plate for a tank

33

CHAPTER 4: TANK DESIGN

4.1 Introduction

Storage tank design consists of 2 main sections Shell Design and Roof Design. The shell design include the shell stress design which is to size up the shell wall thickness, top and

intermediate stiffener ring, stability check against the wind and seismic load and sizing up the

anchor bolt. The roof design will consist of roof stress design, and the roof accessories and

fitting design.

4.2 Shell Design

For practical reasons, it is necessary to build up the shell from a number of fairly small

rectangular pieces of plate, butt welded together. Each piece will be cylindrically curved and

it is convenient to build up the shell in a number of rings, or courses, one on top of the other.

This technique provides, at least for deeper tanks, a convenient opportunity to use thicker

plates in the lower rings and thinner plates in the upper rings.

The lowest course of plates is fully welded to the bottom plate of the tank providing radial

restraint to the bottom edge of the plate. Similarly, the bottom edge of any course which sits

on top of a thicker course is somewhat restrained because the thicker plate is stiffer.

Figure 25Diagrammatic variation of stress in a shell

34

4.3Calculating Shell Thickness

API 650 gives two methods for calculating the required plate thickness for each shell course

:the one-foot method and the variable design point method.

4.3.1 One-Foot Method

The one-foot method is based on limiting the approximate membrane stress to the

allowable stress at a location that is 1 ft. above the bottom of the course being

considered. The required shell thickness is then determined based on that stress. A

distance of 1 ft. above the bottom of the course is assumed to be the location of

maximum membrane stress.

Thismethodshallnotbeusedfortankslargerthan200ft.indiameter.Anassumptionismadetha

teachshellcourseisstiffenedeitherbythetankbottomorthethicker

shellcourseimmediatelybelow.Therefore,ineffect,eachshellcourseisreinforcedatitslower

circumferentialseam,andthemaximumstressthatoccursinashell

courseisshiftedabovethecircumferentialseam.Thedistancethatthemaximum

stressisshiftedisconservativelysetatonefoot,whichgivesthedesignmethodits name.

4.3.2 Variable Design Point Method

Design by the variable design point method gives shell thicknesses at design points that

result in the calculated stresses being relatively closed to the actual circumferential shell

stresses this method may only be used when the purchaser has not specified that the one-

foot method be used and when the following is true.

Whereas per API 650 sec 5.6.4

L = (500Dt)0.5

, in mm

D = tank diameter, in m

t = bottom course shell thickness excluding any corrosion allowance, in mm

H = design liquid level ,in m

Figure 26 Tank Shell Courses thicknesses

35

4.4 Shell Design by One foot method

The required minimum thickness of shell plates shall be the greater of the value computed as

followed [API 650, 2007 sec 5.6.3]:

Design shell thickness

Hydrostatic test shell thickness

Where

td = design shell thickness, mm

tt = hydrostatic test shell thickness, mm

D = nominal tank diameter, m

H = design liquid level, m

G = design specific gravity of the liquid stored

C.A = corrosion allowance, mm

Sd = allowable stress for the design condition, MPa

St = allowable stress for the hydrostatic test condition, MPa

The equation in the API 650 (2007) 1-Foot Method can be derived from the basic membrane

theory, the two main stresses exerting on the cylindrical shell due to the internal pressure are

longitudinal stress and circumferential stress. Lets look into each stress individually by analyzing the stresses in the thin-walled cylindrical shell which an internal pressure exerted

on it.

CAS

GHDt

d

L

d

)3.0(9.4 1

t

L

tS

HDt

)3.0(9.4 1

36

4.4.1 Longitudinal Stress

Figure shows a thin walled cylindrical in which the longitudinal force FL resulted from

the internal pressure, Pi, acting on the thin cylinder of thickness t, length L, and diameter

D.

Figure 27 Longitudinal forces acting on thin cylinder under internal pressure

Longitudinal force =

Area resisting Fl , A = as shaded in above diagram

Longitudinal stresses =

Sl =

=

37

4.4.2Circumferential Stress

Similarly Figure 2.2 considers the circumferential stresses caused by internal pressure, Pi,

acting on the thin cylinder of thickness t, length L, and diameter D.

Figure 28 Circumferential forces acting on thin cylinder under internal pressure

Circumferential force =

Area resisting Fc , A = as shaded in above diagram

circumferential stress Sc =

Sc=

=

4.4.3Longitudinal Stress versus Circumferential Stress

Comparing the both thickness equations due to the longitudinal stress and circumferential

stress, with a specific allowable stress, pressure and fixed diameter, the required wall

thickness to withstand the internal pressure, Pi, for circumferential stress will twice that

required for the longitudinal stress. Circumferential stress in the thin wall will be the

governing stress and hence the Circumferential Stress Thickness Equation (tc) is used.

38

4.4.4 Circumferential Stress Thickness Equation and 1-Foot Method

From the Circumferential Stress Thickness Equation, replace the internal pressure, pi to

the hydrostatic pressure due to product liquid head , consider the effective head at 0.3 m height (H 0.3), and consider the corrosion allowance (C.A) by adding in to the equation of circumferential stress. The minimum required thickness from the 1-Foot

method can be now be derived.

Circumferential Stress Thickness equation to 1-Foot method equation

4.5Top Stiffener and Intermediate Wind Girder Design

4.5.1 Top Stiffener/ Top Wind Girder

Stiffener rings of top wind girder are to be provided in an open-top tank to maintain the

roundness when the tank is subjected to wind load. The stiffener rings shall be located at

or near the top course and outside of the tank shell. The girder can also be used as an

access and maintenance platform. There are five numbers of typical stiffener rings

sections for the tank shell given in API 650 (2007) and they are shown in Figure 30 [API

650, 2007].

Figure 29 Stiffener ring

39

The requirement in API 650 (2007) stated that when the stiffener rings or top wind girder

are located more than 0.6 m below the top of the shell, the tank shall be provided with a

minimum size of 64 x 64 x 4.8 mm top curb angle for shells thickness 5 mm, and with a

76 x 76 x 6.4 mm angle for shell more than 5 mm thick. . The top wind girder is designed

based on the equation for the minimum required section modules of the stiffener ring

[API 650, 2007, sec 5.9.6].

Where

Z = Minimum required section modulus, cm

D = Nominal tank diameter, m

H2= Height of the tank shell, in m, including any freeboard provided above themaximum

filling height

V = design wind speed (3-sec gust), km/h

4.5.2 Intermediate Wind Girder

The shell of the storage tank is susceptible to buckling under influence of wind and

internal vacuum, especially when in a near empty or empty condition. It is essential to

analysis the shell to ensure that it is stable under these conditions. Intermediate stiffener

or wind girder will be provided if necessary.

To determine whether the intermediate wind girder is required, the maximum height of

the un-stiffened shell shall be determined. The maximum height of the un-stiffener shell

will be calculated as follows [API 650, 2007,sec 5.9.7.1]:

Where

H1 = Vertical distance, in m, between the intermediate wind girder and top wind girder

t = Thickness of the top shell course, mm

D = Nominal tank diameter, m

V = design wind speed (3-sec gust), km/h

As stated in earlier section, the shell is made of up diminishing thickness and it makes

the analysis difficult. The equivalent shell method is employed to convert themulti-

40

thickness shell into an equivalent shell having the equal thickness as to the top shell

course.

The actual width of each shell course in changed into a transposed width of each shell

course having the top shell course thickness by the following formula [API 650,

2007,sec 5.9.7.2]:

Where

W tr= Transposed width of each shell course, mm

Wt = Actual width of each shell course, mm

tuniform= Thickness of the top shell course, mm

tactual= Thickness of the shell course for which the transpose width is being calculated, mm

Figure 30 Wind Girders

Situations exist where just a top wind girder alone will not provide enough shell stiffness for

a given combination of tank height, tank diameter, and tank shell course thicknesses. Put in

simple terms, the distance between the top wind girder and the tank bottom is too large, in

41

these situations, to resist wind-induced shell deformation. Installation of an intermediate wind

girder at a location between the top wind girder and the tank bottom reduces the un stiffened

length of the shell, and is required in order to prevent shell deformation in these cases.

Intermediate wind girder design calculations in accordance with API-650 requirements

consist of the following general steps:

Determine if an intermediate wind girder is needed, based on design wind velocity, tank diameter, and shell course thicknesses.

Locate the intermediate wind girder. Calculate the minimum required section modulus of the intermediate wind

girder and select a standard structural shape that provides this section

modulus.

The API-650 procedure for locating the intermediate wind girder considers the variation in

shell course thickness. As illustrated in Figure 32, the API-650 procedure mathematically

converts the actual tank shell height to a "transformed shell" height. As detail in design

calculation chapter the shell transformation is done by accounting for the actual individual

course thicknesses. The transformed shell then has the same stiffness throughout its height

Locating the intermediate wind girder at the mid-height of the transformed shell results in

equal shell stiffness both above and below the intermediate wind girder. The intermediate

wind girder is then located on the actual tank shell in the same course and in the same relative

position within that course as it is on the transformed shell. Using this approach, the

intermediate wind girder is located much higher than the mid-height on the actual tank shell.

Figure31 Transformed shell and intermediate wind girder

42

4.6 Bottom Plate Designing

For oil storage tanks, steel bottom plates are specified, laid and fully supported on a prepared

foundation.

Since it is in common practice if the diameter of tank is greater is than 12m than we have to

incorporate annular plate in bottom plate to make it more strengthen. According to API 650

5.4 all bottom plates shall have a minimum nominal thickness of 6mm exclusive of any

corrosion allowance specified by the purchaser for the bottom plates.

The bottom is made up of a number of rectangular plates, surrounded by a set of shaped

plates, called sketch plates, to give a circular shape, as shown in Figure 33. The plates slightly

overlap each other and are pressed locally at the corners where three plates meet (see Figure

34). Lapped and fillet welded joints are preferred to butt welded joints (which must be

welded onto a backing strip below the joint) because they are easier and cheaper to make.

Figure 32Bottom layout for tank

See figure 34

details A and B

43

Figure 33 Cross joints in bottom plates

4.7 Roof Design

We have selected conical roof with self-supported at its periphery since it is easy to construct

and design comparing with other types of roof. But it is strongly dependent to customer that

what type of roof selected for storage tank.

Fixed roofs of cylindrical tanks are formed of steel plate and are of either conical or domed

(spherically curved) configuration. The steel plates can be entirely self-supporting (by

'membrane' action), or they may rest on top of some form of support structure.

Membrane roofs are more difficult to erect - they require some temporary support during

placing and welding and are usually found only on smaller tanks. Permanent support steelwork for the roof plate may either span the complete diameter of the

tank or may in turn be supported on columns inside the tank. The use of a single central

column is particularly effective in relatively small tanks (15-20 m diameter), for example.

The main members of the support steelwork are, naturally, radial to the tank. They can be

simple rolled beam sections or, for larger tanks, they can be fabricated trusses.

Roof plates are usually lapped and fillet welded to one another. For low pressure tanks, they

do not need to be welded to any structure which supports them, but they must normally be

welded to the top of the shell.

44

4.8 Overturning Stability against Wind Load

The overturning stability of the tank shall be analyzed against the wind pressure, and to

determine the stability of the tank with and without anchorage. The wind pressure used in the

analysis is given as per API 650 (2007, sec 5.11). The design wind pressure on the vertical

projected areas of cylindrical surface area shall be 0.86 kPa (V/190) and 1.44 kPa (V/190)

uplift on horizontal projected area of conical surface. These design wind pressure are in

accordance with American Society of Civil Engineer - ASCE 7 for wind exposure Category

C [ASCE 7, 2005]. The loading diagram due to the wind pressure on the roof tank is shown

in following Figures.

Figure 34 Overturning moment against wind load

Figure 35 Shell Out of Roundness Caused By Wind

45

The wind load (Fs) on the shell is calculated by multiplying the wind pressure (ws) to the

projected area of the shell.

As per API 650 (2007), the tank will be structurally stable without anchorage when the below

uplift criteria are meet [API 650, 2007, sec 5.11.2].

Where

Mpi= moment about the shell-to-bottom from design internal pressure (Pi) and it can be

calculated by the formula

Mw= Overturning moment about the shell-to-bottom joint from horizontal plus vertical wind

pressure and is equal to Fr. Lr + Fs. Ls. Fr and Fs is the wind load acting on the roof and shell

respectively and Lr and Ls is the height from tank bottom to the roof center and shell center

respectively.

MDL= Moment about the shell-to-bottom joint from the weight of the shell and roof supported

by the shell and is calculated as 0.5 D.WDL.

MF= Moment about the shell-to-bottom joint from liquid weight and is equal to

46

4.9 Seismic Design

The seismic design of the storage tank is accordance to API 650 (2007) Appendix E. There are two major analyses to be performed in the seismic design, and they are:

i)Overturning Stability check - The overturning moment will be calculated and check for the

anchorage requirement. The number of anchor bolt required and the anchor bolt size will also

be determined based on the overturning moment.

ii)Maximum base shear

4.9.1Overturning Stability against seismic load

The seismic overturning moment at the base of the tank shall be the square root of sum of

squares (SRSS) summation of the impulsive and convective components multiply by the

respective moment arms to the center of action of the forces.

For tanks supported by the concrete ring wall, the equation for calculating the ring wall

moment, Mrw is as follow [API 650, 2007, sec E.6.1.5]:

Where

Ai = Impulsive design response spectrum acceleration coefficient, %g

Ac = Convective design response spectrum acceleration coefficient, %g

Wi = Effective impulsive portion of liquid weight, N

Ws = Total weight of the tank shell and appurtenances, N

Wr = Total weight of fixed tank roof including framing, knuckles, any permanent attachments

and 10% of the roof design snow load, N

Wc = Effective convective (sloshing) portion of liquid weight, N

Xi = Height from the bottom of the tank shell to the center of action of the lateral seismic

force related to the impulsive liquid force for ring wall moment, m

Xs = Height from the bottom of the tank shell to the shells center of gravity, m

Xr = Height from the bottom of the tank shell to the roof and roof appurtenances center of

gravity, m

Xc = Height from the bottom of the tank shell to the center of action of the lateral seismic

force related to the convective liquid force for ring wall moment, m

47

This overturning moment is important for the mechanical to design the anchorage

requirement and determine the minimum the number and size of the anchor bolt for the

storage tank. It is also important to the civil engineer to design the tank foundation in which

the tank is being supported.

4.10 Anchorage requirement

The resistance to the design ring wall overturning moment at the base of the shell will be

provided by the weight of the tank shell, weight of the roof reaction, Wrs, by the weight of a

portion of the tank contents adjacent to the shell for unanchored tanks or provided by the

mechanical anchorage devices.

The anchorage requirement is checked by the Anchorage Ratio, J, and the anchorage ratio

criteria in Table will determine whether the tank can be self-anchored or mechanically

anchored.

Table 2Anchorage Ratio Criteria [API650, 2007, table E-6]

48

The anchorage ratio, J is determined as follow [API650, 2007, sec E.6.2.1.1.1]:

Where

wt = Weight of tank shell & portion of roof supported by shell and is define as

wa = Resisting force of annulus which is defined as

Where

Fy= Min. specified yield strength of bottom annulus

H = Maximum design product level

Ge = Effective specific gravity including vertical seismic effect

= G(1 - 0.4 Av) ; G = 1, Specific gravity

Av = Vertical earthquake acceleration coefficient

Wint= Uplift due to product pressure

wrs= Roof load acting on shell, including 10% of specified snow load

49

CHAPTER:05 DESIGN CALCULATIONS

5.1 Material selection

The first step is to select material for shell, bottom plate, annular plate, roof plate etc.

The factors that govern that material selection are as follows:

Cost

Availability

Strength

Inertness

ASTM 283 Grade C is selected as shell, bottom, annular and roof plate material. The yield

strength of the material is given as 205 MPa. it belongs to group 1 material as listed in table

4-3a of API 650.

Material used in the construction of tanks shall conform to the specification listed in the API

sec 4, subject to the modification and limitation indicated in this standard material produced

to specification other than those listed in this section may be employed, provided that the

material is certified to meet all of the requirements listed in this standard and the material is

approved by the purchaser the manufacturer proposal shall identify the material specification

to be used.

50

5.2 Design specifications

Design Code API 650, 11th

Edition

Fluid Sulphuric acid

Material SA 283 Gr. C(m = 7850 kg/m3)

Specific Tank Diameter 5.6 meters

Tank Height 11.0 meters

Geometrical Capacity 270 m3

Design Liquid Level 10.49 m

gravity of contents G = 1.94

Materials yield strength dy = 205 MPa

Design Pressure (+ve) 3.0 IN WC (0.1083 psig

Design Pressure (-ve) 1.0 IN WC (0.036 psig)

Design Temperature (max) 50oC

Design Temperature (min) 5 oC

Roof Type Structurally Supported Fixed Cone Roof

Roof Slope 1:16

Bottom Type Flat non-annular with center sump

Bottom Slope 1:48

Allowable Product Design stress Sd = 137.0 Mpa

Allowable Hydrostatic test stress St = 154 MPa

Corrosion allowance

Bottom = 1.5mm

Shell = 1.5 mm

Roof = 1.5mm

Joint Efficiency 0.85

Wind Speed 120 mph

Seismic Zone 2B

Plate Size 1500 mm x 6000 mm

Modulus of Elasticity E = 199000 MPa

Yield strength of steel structure (Stiffeners) Fy = 250 MPa

51

Material

Shell Material HR-235

Roof Material HR-235

Bottom Material HR-235

Structure Material ASTM A-36

Pipe Material A-106 Gr. B

Flange Material A-105, 150#, Welding Neck

Gasket Material Spiral Wound

Anchor Bolt Material ASTM A-36

Venting Type Vent to closed loop/ vapor return

Painting Yes (Only External)

Internal Lining Yes (Rubber lining 3mm thick compatible

with H2SO4)

Insulation Yes

Insulation Thickness 2 Inch

Insulation Density 50 kg/m3

Tank Foundation Ring Wall

52

5.3 Basic Calculations

Height = H= 11.0 m (Given)

Diameter =D = 5.6 m (Given)

Aspect Ratio =

=

= 1.96

Total working capacity =

=

= 270

= 9567.84

5.4 Shell Design

As per API 650 5.6.3

Calculations of Shell Thicknesses

The required shell plates thickness shall be greater of the values computed by following

formulas.

Design Shell Thickness (As per API 5.6.3)

Hydrostatic Test Thickness (As p (as per API 5.6.3)

Height for each shell course HLi= HL(i-1) - (i-1)W

G = Specific Gravity of fluid to be stored

D = Nominal dia. of tank (m)

HL1 = Design liquid level (m) for course under consideration.

CA = Corrosion allowance. = 1.50 mm

td = Design shell thickness (mm)

CAS

GHDt

d

L

d

)3.0(9.4 1

t

L

tS

HDt

)3.0(9.4 1

53

tt= Hydrostatic test shell thickness (mm)

HLo = Total Height of the Tank Shell = 11.00 m

W = Width of the Plate (Course) = 2.45 m

i = Shell Course Number = 1 to 7

1st Shell Course

Width of 1st. Course W1 =1.5 m

Design height for 1st shell Course HL1 = 11.0 m

Required Shell Thickness tt =2.15mm

Required Shell Thickness td= 3.92mm

Shell thickness selected t1= 6.00 mm

2nd

Shell Course

Width of 2nd

Course W2 =1.50 m

Design height for 2nd

shell Course HL2=9.5 m



Shell thickness selected t2= 6.00 mm

3rd

Shell Course

Width of 3rd

Course W3 =1.50 m

Design height for 3rd

shell Course HL3 =8.0 m

Required Shell Thickness tt =1.52 mm


Shell thickness selected t3 = 6.00 mm

54

4th Shell Course

Width of 4th Course W4 =1.50 m

Design height for 4th shell Course HL4 =6.5 m


Required Shell Thickness td= 2.86 mm


5th Shell Course






6th Shell Course


Design height for 6th

shell Course HL6 =3.5 m




7th Shell Course






55

8th Shell Course



Required Shell Thickness tt = 0 mm



Shell Thickness & Weight Summary

Shell Course # 1 2 3 4 5 6 7 8

Shell width (m) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 0.5

Shell Thick, corroded (mm) 6.0 6.00 6.00 6.00 6.00 6.00 6.00 6.00

Shell Weight (KN) 12.17 12.17 12.17 12.17 12.17 12.17 12.17 4.05

Shell Weight(corroded) (KN) 9.13 9.13 9.13 9.13 9.13 9.13 9.13 3.04

Total Shell Weight = 12.17(7)+4.05=89.24KN

Total Shell Weight (corroded) = 9.13(7)+ 3.04=66.95KN

56

5.5 Bottom Plate Design

As per API 650 5.4.1

All bottom plates shall have minimum nominal thickness of 6mm, exclusive of any corrosion

allowance as per API 650 5.4.1

Required Bottom Plate Thickness tr= 6 + CA mm

= 6+1.5 = 7.5 mm

Selected bottom plate thickness tb = 8.0 mm

Weight of Btm. Plate +Annular plate

For corroded weight

Weight of Btm. Plate +Annular plate

Weight of Btm. Plate +Annular plate = 7907.57 kg =>15.72 KN

Weight of Btm. Plate +Annular plate = 64.24.90 kg =>12.78KN (Corroded)

5.6 Annular Plate Design

(As per API 650 Sec. 5.5)

Product stress 137.00Mpa

Hydrostatic test stress 154.00 MPa

Annular bottom plate thickness t = 6 mm (As per API 650 Table 5-1 a)

Including corrosion allowance 7.50 mm

Used annular plate thickness tb = 8.00 mm

Max. design liquid level HL1= 10.5 m

222

1.01.044

1.0act

mbmbact WDDttWD

222

1.01.04

)(

4

)(1.0act

mbmbact WDDCAtCAtWD

57

5.7 Intermediate Wind Girder Design

(As per API 650 Sec. 5.9.7)

The maximum height of the Un-stiffened shell shall be calculated as follows:

H1 = 9.47 t(

)

(

)


Where

H1= Vertical distance, in m, between the intermediate wind girder and the top angle of the

shall or the top wind girder of an open top tank.

t = Thickness of the top shell course = 6 mm

D = Nominal tank diameter (m) = 5.76 m

H1 =30.68 m

Modification as per API 650 (5.9.7.1 Note 2, a)

H1 = (

)xH1

= 17.64 m

Transformed Shell thicknesses

Wtr(i) = Wi x (

)


Where,

Wtr(i) = Transformed Shell Thickness

Wi =Width of Courses

(W1 to 7= 1.50 m ; W8 = 0.50 m)

ttop = Shell Thickness of 8th

Course

ti = Thickness Of the Course that is consider

i = Shell Course Number = 1 to 8

58

1st Course

Thickness of first course t1= 6.00 mm

Wtr1 = W1 x(

)

= 1499.61 mm

2nd Course

Thickness of 2nd course t2= 6.00 mm

Wtr2 = W2 x (

)

= 1499.61mm

3rd Course

Thickness of 3rd course t3 = 6.00 mm

Wtr3 = W3 x (

)

= 1499.61 mm

4th Course

Thickness of 4th course t4= 6.00 mm

Wtr4 = W4 x (

)

= 1499.61 mm

5th Course

Thickness of 5th course t5 = 6.00 mm

Wtr5 = W5 x (

)

= 1499.61 mm

59

6th Course


Wtr6 = W6 x (

)

= 1499.61mm

7th Course


Wtr7 = W7 x (

)

= 1499.61mm

8th Course


Wtr7 = W7 x (

)

= 489.87mm

Height of transformed shell, Htr = Wtr1+Wtr2+Wtr3+Wtr4+Wtr5+Wtr6+Wtr7+ Wtr8

Height of transformed shell, Htr =10987.14 mm or 10.98 m

As

Htr< H1

10.98 < 17.64

Intermediate Wind Girder is Not Required.

60

5.8 Roof Design (Supported Conical Roof) As per API 650(5.10.5)

Minimum roof thickness is 5mm (As per API 650(5.10.5)

Roof plate thickness = 6.5 mm (with corrosion allowance)

Selected Roof Plate thickness= th= 7 mm

= slope of roof = 3.57

Do= Outer Diameter= 5.6 m

= 850 kg/m3

Vertical Projected Area of Roof =

pt = 0.75 in/ft (Cone Roof Pitch)

Horizontal Projected Area of Roof (Per API-650 5.2.1.f):

Xw = Moment Arm of UPLIFT wind force on roof

=

=

Ap = Projected Area of roof for wind moment

=

R = 9.184 ft

Dead Load = Insulation + Plate Weight + Added Dead Load

Roof Loads (per API-650 Appendix R)

61

e.1b = DL + MAX(Sb,Lr) + 0.4 Pe

e.2b = DL + Pe + 0.4 MAX(Sb,Lr)

T = Balanced Roof Design Load (per API-650 Appendix R)

= MAX(e.1b,e.2b)

e.1u = DL + MAX(Sb,Lr) + 0.4 Pe

e.2u = DL + Pe + 0.4 MAX(Su,Lr)

U = Unbalanced Roof Design Load (per API-650 Appendix R)

= MAX(e.1u,e.2u)

= 33.833

P = Max. Design Load = Lr1

l = Maximum Rafter Spacing (Per API-650 5.10.4.4)

62

MINIMUM # OF RAFTERS

FOR OUTER SHELL RING:

l = 84 in. since calculated l > 84 in. (7 ft)

Minimum roof thickness based on actual rafter spacing:

RLoad

= Maximum Roof Load based on actual rafter spacing

Let

Max

= RLoad

P

(Vacuum limited by actual rafter spacing)

= -0.3336 PSI or -9.25 IN. H2O

Pa

= P

= -0.3336 PSI or -9.25 IN H2O.

t

=0.2465 in.

63

5.9 RAFTER DESIGN

Maximum Rafter Span = 9.184 ft

Average Rafter Spacing on Shell = 6.282 ft

Average Plate Width=

=3.141 ft

M

= Maximum Bending Moment

M

=

where,

l = (9.184)(12)

= 110.21 in.

Mmax = (9.66)(110.21)

Z req'd = Mmax/23,200

W

(Max. stress allowed for each rafter in ring 1)

Max

(Max. Load allowed for each rafter in ring 1)

64

Let

Max

= Max

144

P

(Vacuum limited by Rafter Type)

= -1

sulphuric acid storage tank

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