Facilitator :

Shikha Nagpal

PRESENTATION ON ATMOSPHERIC TANKS

• Tank may be defined as a large vessel for storing fluid.

• Storage tanks are very thin cylinders with extremely large ratio of diameter to thickness, tanks having dia. > 15m are considered as large dia. storage tanks.

• Earlier tanks were rivetted, subsequently these were replaced with double lap welded shells and now with availability of high tensile grades of steel & better welding technology ,full fusion butt welding are employed.

ATMOSPHERIC TANKS

CLASSIFICATION OF STORAGE TANK

ON THE BASIS OF TEMPERATURE :• Ambient Temperature• Elevated Temperature• Low Temperature (cryogenic)

ON BASIS OF LOCATION :• Above Ground• Under Ground• Mounded

ON THE BASIS OF SHAPE :• Cylinder Vertical • Cylinder Horizontal• Spherical

CLASSIFICATION OF CYLINDRICAL VERTICAL TANKS Fixed Roof Tanks Roofs on structural supports (truss & rafter supported

without columns) Roofs on structural supports with columns Self-supported roofs (Dome/ Cone)

Floating Roof : (a) Internal Floating

Pan Type / Pontoon Type steel decks Aluminium Floating Deck

(b) External Floating

Single Deck Double Deck

Low Temperature & Cryogenic Tanks

Single Wall Double Wall

The various codes referred for the design of atmospheric tanks are as

follows :

IS – 803 : Used for Vertical Mild Steel Cylindrical Oil Storage Tanks

BS – 2654 : Used for Vertical Steel Non –Refrigerated Tanks

API – 650 : Welded Steel Tanks (internal P < 18kpa)

API – 620 : For internal P > 18 kpa & refrigerated Tanks

Design codes for vertical storage tanks

API 650 establishes minimum requirements for material, design, fabrication, erection, and testing for vertical, cylindrical, aboveground, closed and open top, welded carbon or stainless steel storage tanks in various sizes and capacities for internal pressure approximating atmospheric pressure (internal pressure not exceeding the weight of roof plates), The maximum internal pressure for closed-top API 650 tanks may be increased to the maximum internal pressure of 18 kpa when the additional requirements of Appendix F of API 650 are met.

This standard applies only to tanks whose entire bottom is uniformly supported and to tanks in non-refrigerated service that have a maximum design temperature of 93ºC (200ºF) or less.For maximum design temperature above 93ºC (200ºF) but not exceeding 260ºC (500ºF) , Appendix M is referred.

Appendix C provides minimum requirements for single-deck pontoon-type

and double-deck-type external floating roofs.

Appendix H provides minimum requirements that apply to a tank with an

internal floating roof and a fixed roof at top of the tank shell,and to the

tank appurtenances.

Section 4 – MaterialsMaterials used in the construction of tanks shall confirm to thespecifications listed in this section subject to modifications and limitations indicated in this standard.

Section 5 - DesignDesign considerations : LOADS

Dead Load (DL)– The weight of the tank or tank component, including corrosion allowance unless otherwise noted.

Design External Pressure (Pe)– shall not be less than 0.25 kpa (1”of water). (Appendix V for the external pressure exceeding 0.25kpa but not exceeding 6.9kpa)

Design Internal Pressure (Pi)– shall not exceed 18 kpa (2.5 lbf/in2),(internal pressure more than the weight of the roof plate ) the max. internal pressure for closed-top API 650 std tanks may be increased to max. internal pressure permitted when requirements of Appendix F are met.

Hydrostatic Test (Ht)– The load due to filling the tank with water to the design liquid level.

Minimum Roof live load (Lr) – 1.0 kpa (20 lb/in2) on the horizontal projected area of the roof.

Seismic (E) – Seismic loads determined in accordance with Appendix E.

Snow (S) – The ground snow load shall be determined as per local regulation.

Stored Liquid (F) – The load due to filling the tank to the design liquid level with liquid with the design gravity specified by the purchaser.

Test Pressure (Pt) – As required by code.(F.4.4 or F.7.6)

Wind (W) – Wind load to be considered as per as per local regulation.

Bottom PlatesAll bottom plates shall have a minimum nominal thickness of 6 mm

(1/4 in.) exclusive of any corrosion allowance specified by the purchaser for

the bottomplates.

Annular Bottom PlateThe bottom annular plate is butt-welded outer ring of plates to which thetank shell is welded.

When the bottom shell course is designed using the allowable stress formaterial group IV,IVA,V or VI, butt-welded annular bottom plates shall be used.

When the bottom shell course is of material in group IV,IVA,V or VI andthe maximum product stress for the first shell course is less than or equalto 160 Mpa or the or the maximum hydrostatic test stress for the first

shellcourse is less than or equal to 172 Mpa, lap-welded bottom plates be usedin lieu of butt-welded annular bottom plates.

As per Appendix M, tanks with diameters exceeding 30m (100ft) shallhave butt-welded annular bottom plates.

Annular bottom plates shall have a radial width that provides at least

600mm (24 in.) between the inside of the shell and any lap welded joint in

the remainder of the bottom.

24”Shell Plate

Annular Plate Bottom Plate

The thickness of the annular bottom plates shall not be less than the greater thickness determined using table 5-1

A greater radial width of annular plate is required when calculated as follows : 215 tb (HG)0.5

where tb = thickness of annular plate, in mm H = maximum design liquid level, in m G = design specific gravity of the liquid to be stored.

Shell designThe required shell thickness shall be the greater of the design

thickness, including any corrosion allowance, or the hydrostatic test shell

thickness,but the shell thickness shall not be less than the following :

Calculation of shell thickness by the 1- Foot method

The 1-foot method calculates the thickness required at designpoints 0.3 m (1- ft) above the bottom of each shell course. Thismethod shall not be used for tanks larger than 60m (200 ft) indiameter.

The required minimum thickness of shell plates shall be greater of the

values computed by the following formulas in S.I. units :

Td = 4.9 D (H-0.3)G + CA Sd

Tt = 4.9 D (H-0.3) St

Where, td = deisgn shell thickness, in mm tt = hydrotest shell thickness, in mm D = nominal tank diameter, m H = design liquid level, m G = design specific gravity CA = corrosion allowance, mm

The design stress (Sd) shall be either 2/3rd of the of the yield strength or2/5th of the tensile strength, whichever is less.

The hydrostatic stress (St) shall be either 3/4th of the of the yield strengthor 3/7th of the tensile strength, whichever is less.

This Procedure normally provides a reduction in shell course thickness

and total material weight, but more important is its potential to permit

construction of large diameter tanks within the maximum plate thickness

limitation.

Calculation of shell thickness by the Variable Design Point Method

Top and Intermediate Stiffening rings

An open 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.

Top wind girder is required for open tank.

Top wind girder is normally used as a walkway, minimum width of which shall not be less than 710mm (28”) clear of projections including the angle on the top of the tank shell.

TOP WIND GIRDER

The required minimum section modulus of the top wind girder shall be

determined by the following equation :

Z = D2 H2 * (V)2

17 (190)2

Z = required minimum section modulus (cm3) D = nominal tank diameter (m) H2= height of the tank shell (m) V = design wind speed (km/hr)

The available section modulus shall be equal to or more than therequired section modulus of the section.

Intermediate wind girdersThe maximum height of unstiffened shell shall be calculated as

follows

H1 = 9.47 t (t)1.5 (190)2

(D)1.5 (V)2

H1 = vertical distance, in m, between the intermediate wind girder and the top angle of the shell or the top wind girder of an open-top tank

t = as ordered thickness, unless otherwise specified, of the top shell course (mm)D = nominal tank diameter, (m)V = design wind speed, (km/hr)

After the maximum height of the unstiffened shell, H1, has beendetermined, the height of the transformed shell shall be calculated asfollows :

(a) With the following equation, change the actual width of each shell course into transposed width of each shell course having the top shell course thickness :

Wtr = W (tuniform)2.5

(tactual)2.5

Wtr = transposed width of each shell course, (mm)W = actual width of each shell course, (mm)tuniform = as ordered thickness, unless otherwise specified, of the top shell course, (mm)tactual = as ordered thickness, unless otherwise specified, of the shell course, for which the transposed width is being calculated, (mm)

(b) Add the transposed widths of the courses. The sum of transposed width of the courses will give the height of the transformed shell.

If the height of the transformed shell is greater than the maximum height H1,intermediate wind girder is required and number of secondary wind

girders required can be calculated as

N = (Total transposed height) -1 (max.unstiffened height)

The required section modulus of an intermediate wind girder shall be

determined by the following equation :

Z = D2 H1 * (V)2

17 (190)2

where Z = required minimum section modulus (cm3) D = nominal tank diameter (m) H1= vertical distance between intermediate wind girders and the top angle of the shell or the top wind girder of an open-tank (m) V = design wind speed (km/hr)

Roof DesignThe following definitions apply to roof designs :

• A supported cone roof is a roof designed to approximately the surface of a right cone that is supported principally either by rafters on girders and columns or by rafters on trusses without coulmns.

• A self-supporting cone roof is a roof formed to approximately the surface of a right cone that is supported only at its periphery.

• A self-supporting dome roof is a roof formed to approximately a spherical surface that is supported only at its periphery.

• A self-supporting umbrella roof is a modified dome roof formed so that any horizontal section is a regular polygon with as many sides as there are roof plates that is supported only at its periphery.

All roof and supporting structures shall be designed for load combinations

(a), (b), (c), (e), and (f) of appendix R.

Roof plates shall have minimum nominal thickness of 5mm exclusive of

corrosion allowance.

θ <= 37º (slope 9:12) θ >= 9.5º (slope 2:12)

Minimum Thickness = D/4.8sinθ * (T/2.2)0.5 >= 5mm Maximum Thickness = 12.5mm

D = nominal diameter of the tank shell (m), T = greater of load combinations (e)(1) and (e)(2) of Appendix

R, (kpa) θ = angle of cone elements to the horizontal (deg.)

Self - Supporting Cone Roofs

Shall confirm to the following requirements :Minimum radius = 0.8DMaximum radius = 1.2D

Minimum thickness = rr/2.4 * (T/2.2)0.5 + C.A. >=5mm rr = roof radius (m)

Self - Supporting Dome and Umbrella Roofs

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