Overview of Tank Design

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> RESOURCE PROJECTS > TECHNOLOGY > INTEGRATED SERVICES > Structural > Mechanical > Design > Verification > Project

Management

INSIGHTS SESSION

OVERVIEW OF TANK DESIGN

> Structural > Mechanical > Design > Verification > Project Management > Structural > Mechanical > Design > Verification > Project Management

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Content of Information Session

> Introduction

> Samples

> Applicable Design Codes

> Scope of API 650

> Loads / Wind / Seismic

> Floor & Shell Design

> Stiffening Rings

> Roof & Roof Framing Design

> Nozzles & Venting

> Tank Anchoring

> Tank Installation Methods

> Tank Repairs

> Other Important Topics

> Conclusion


Introduction

Purpose of Presentation

To highlight some of the important aspects of designing steel tanks,

with particular emphasis on cylindrical vertical tanks to API 650

Types of Tanks

> Vertical – cylindrical or square, liquid retaining

> Horizontal – cylindrical, saddle-supported on steel or ground

> Silos/Bins – typically cylindrical, supported on steel or ground

> Hoppers – typically square, supported on or under steelwork

> Thickeners & Clarifiers – conical bottom, column supported

> Vessels – require specialist design to AS1210; must be 3rd Party

verified and registered with WorkSafe WA


Samples

Acid Tank - RNP

Diesel Storage Tank - RNP


Samples (cont..)

Worsley Alumina Water Treatment Plant – Water Tanks


Samples (cont..)

Typical

Horizontal

Tank


Applicable Design Standards

> API 650 Welded steel tanks for oil storage (flat-bottomed;

liquid only)

> API 653 Tank Inspection, Repair, Alteration, and

Reconstruction

> API 620 Design & construction of large, welded, low-

pressure storage

> API 2000 Tank venting

> AS 1940 The storage and handling of flammable and

combustible liquids

> AS 1692 Steel tanks for flammable and combustible liquids

> AS 3774 Loads on bulk solids containers

> AS 1170 Loading code (general, dead, live, wind, seismic,

etc.

> AS 1210 Pressure vessels

> PD 5500 Specification for unfired, fusion welded pressure

vessels (formerly BS 5500)


Scope of API 650

> Section 5 – for carbon steel, non heated tanks (up to 930C),

based on 100% shell utilisation

> Appendix A – for ‘small’ tanks, based on 70% shell utilisation

using SG = 1.0, no spot radiography required

> Appendix F – for tanks with up to 18kPa internal pressure

> Appendix J – for shop assembled tanks up to 6m diameter

> Appendix M – for tanks at temperature, 930C < t < 2600C

> Appendix S – for Austenitic Stainless Steel tanks

> Appendix X – for Duplex Stainless Steel (Cr, Mo, Ni) tanks


Loads

> Hydrostatic pressures - p.g.h

> Pressure from bulk solids - AS 3774

> Nozzle Loads - From client specifications

> Live - API 650 section 5.2 (1.0 kPa for roof)

- Stair way, ladder access and cross

over bridges to AS 1657, agitators,

launders, etc..

> Wind - AS 1170.2. API 650 sections 5.2, 5.11

> Seismic - AS 1170.4, API 650 Appendix E


Wind

> AS 1170.2 and API 650 sections 5.2, 5.11 which has been recently

revised

> Design wind speed per API 650 is 190 km/hr (53m/s). This is

‘permissable’ 3-sec gust wind speed, based on 2% annual

probability of being exceeded [R=50].

> Design wind pressure =

0.86 kPa x (V/190)2 on

vertical projected areas

of cylindrical surfaces

> Design uplift = 1.44 kPa x

(V/190)2 on horizontal

projected areas of

conical /curved surfaces

> V is the 3-sec gust wind

speed determined as per

AS 1170.2.


Seismic

> AS 1170.4, API 650 Appendix E

> Seismic load is the resultant of Vi & Vc where Vi = Ai (Wsolids) and

Vc = Ac (Wconvective)

> API 650 is a permissible stress method based on earthquake

ground motion due to an event with 2% probability of

exceedance in a 50-year period [R=2,500]

> Previously API 650 seismic was based on 10% probability of

exceedance in 50 years [R=500]

> AS 1170.4 results in ULS load so divide by 1.4 to get permissible

value


Seismic (cont..)

> The lateral seismic force consists of impulsive and convective

components

> Impulsive component relates to the inertia of a portion of the

liquid along the walls and bottom which moves in unison with

the tank as a rigidly attached mass

> Convective component is caused by the dynamic movement of

the remaining fluid inside the tank

> Low H/D ratios → convective forces predominant (see Fig EC-8)

> High H/D ratios → impulsive forces predominant (see Fig EC-8)

> Period for impulsive force typically a fraction of a second while

convective period is several seconds long


Seismic (cont..)

> Wc = Effective convective

(sloshing) portion of the

liquid weight

> Wi = Effective impulsive

portion of the liquid weight

> The defining consideration in the analysis of the tank is whether

the overturning moment is large enough to result in significant

uplift of one side of the tank wall leading to compressive

buckling on the opposite side


Seismic (cont..)


Floor & Shell Design

> Tank capacity typically dictated by process requirements

> Based on capacity, layout the shell plates to minimise the total

number used – always check standard plate sizes available and

roughly layout the plates to achieve and optimal solution

> Shell design – API 650 section 5.6

> Base design – API 650 sections 5.4, 5.5

> Internal floor plates can be lapped together or lapped with

‘annular plates’ for thick walled bottom strake tanks or to

increase seismic capacity

> Subtle variations in tank diameter and height can prevent tank

overturning


Floor & Shell Design (cont..)

Typical

Floor & Shell

Layout

Drawing


Stiffening Rings

> Stiffening rings or ‘wind girders’ are required to stiffen: ─ The top of the shell against lateral loads (if open)

─ The top of the shell against roof loads (if closed)

─ The shell against buckling from lateral and vertical loads

> Design according to API 650 section 5.9 which is based on the

‘transformed’ shell method and then compared to an allowable

height of unstiffened shell

> Intermediate stiffening rings become particularly important for

large diameter tanks (bigger diameter = reduced buckling

capacity as each plate in plan becomes ‘flatter’)


Roof & Roof Framing Design

> Roof framing is one of the most complex components of tank

design, particularly with large diameter tanks. Various roof types: ─ Self supporting cone, dome, umbrella for small to 10m in diameter

─ Internal / external floating for nearly any size

─ Supported cone

• Rafters only up to 25m in diameter

• Rafters + central column up to 35m in diameter

• Rafters + concentric columns any size

• Trusses any size

> Roof design – API 650 section 5.10. Maximum spacing between

rafters at shell = 2100mm, leading to ‘spider web’ arrangement.

Self-supporting roofs are limited by the roof pitch, maximum

allowable thicknesses and diameter

> Roof plate typically lapped but can be in ‘petals’ (umbrella)

> Important to size top stiffening ring correctly when internal

columns are used to support roof


Nozzles & Venting

Venting required to allow the tank to ‘breathe’. Calculations to size

the vents are given in AS 1940 Appendix I

> Normal venting – required for temperature differences, filling

and emptying

> Emergency venting – for explosion/fire. Not required if the roof is

‘frangible’

> Vents can be ‘free’ vents or purchased products as shown

below. These restrict vapours and product loss where free vents

will not


Nozzles & Venting (cont..)

> Other nozzles – inlet, outlet, manways, level sensing. Sizes

typically process driven

> Standard nozzle details given in API 650 sections 5.7 & 5.8. Non

standard nozzles are allowed but must be proven by

calculations, typically by FEA

> Internal floating suction nozzles

> The ‘Bunbury Job’ – all vents sealed!

> Tall vents – see video feed


Tank Anchoring

> Anchors are required if the tank is not stable (overturning or

sliding) and can represent a large increase in cost if it has not

been considered in the costing. Overturning is typically caused

by wind loads when empty or seismic loads when full. A ‘self

anchored’ tank is stable without hold down bolts – typically

large diameter, open top, low height tanks

> Design according to API 650 sections 5.11, 5.12 and Appendix E

> Minimum anchor diameter is 1” (M24), excluding corrosion

allowance

> Maximum anchor spacing is 3m

> FEA sometimes required to confirm stresses at shell from anchor

attachments are within the acceptable limits


Tank Installation Methods

> Full scaffold

> Temporary platforms

> Lifting plates or ‘cans’

> Roof lifting

> Tank jacking (not shown)


Tank Installation Methods (cont..)

> Dogs, wedges

& strongbacks

> Automatic

welding


Tank Repairs

> API 653 provides information on acceptable details for tank

repairs. It also provides details on acceptable levels of shell and

floor settlement

> Most fuel tanks require repairs to the floor and first strake – the

areas where water and other contaminants collect

> Tank jacking typically used for replacement of entire tank

bottom. Slightly different to new tank erection by jacking


Other Important Topics

Some important topics for brief consideration:

> Welding requirements -API 650 sections 5.1, 5.7, 7 & other sub

sections

> Testing & inspection – API 650 section 8

> Bund requirements for flammable and combustible liquids

> Tank programs – eg Etank, Coade – be weary of the results

> Footings


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Overview of Tank Design

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