Welded Steel Tanks for Oil Storage

SECTION 3—DESIGN

3.1 JOINTS

3.1.1 Definitions

The definitions in 3.1.1.1 through 3.1.1.8 apply to tank

joint designs (see 7.1 for definitions that apply to welders and

welding procedures).

3.1.1.1 double-welded butt joint:

A joint between two

abutting parts lying in approximately the same plane that is

welded from both sides.

3.1.1.2 single-welded butt joint with backing:

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.

3.1.1.3 double-welded lap joint:

A joint between two

overlapping members in which the overlapped edges of both

members are welded with fillet welds.

3.1.1.4 single-welded lap joint:

A joint between two

overlapping members in which the overlapped edge of one

member is welded with a fillet weld.

3.1.1.5 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.

3.1.1.6 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.

3.1.1.7 full-fillet weld:

A fillet weld whose size is equal

to the thickness of the thinner joined member.

3.1.1.8 tack weld:

A weld made to hold the parts of a

weldment in proper alignment until the final welds are made.

3.1.2 Weld Size

3.1.2.1

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).

3.1.2.2

The size of an equal-leg fillet weld shall be based

on the leg length of the largest isosceles right triangle that can

be inscribed within the cross section of the fillet weld. The

size of an unequal-leg fillet weld shall be based on the leg

lengths of the largest right triangle that can be inscribed

within the cross section of the fillet weld.

3.1.3 Restrictions on Joints

3.1.3.1

Restrictions on the type and size of welded joints

are given in 3.1.3.2 through 3.1.3.5.

3.1.3.2

Tack welds shall not be considered as having any

strength value in the finished structure.

3.1.3.3

The minimum size of fillet welds shall be as follows:

On plates 5 mm (

in.) thick, the weld shall be a full-fillet

weld, and on plates more than 5 mm (

in.) thick, the weld

thickness shall not be less than one-third the thickness of the

thinner plate at the joint and shall be at least 5 mm (

in.).

3.1.3.4

Single-welded lap joints are permissible only on

bottom plates and roof plates.

3.1.3.5

Lap-welded joints, as tack-welded, shall be lapped

at least five times the nominal thickness of the thinner plate

joined; however, with double-welded lap joints, the lap need

not exceed 50 mm (2 in.), and with single-welded lap joints,

the lap need not exceed 25 mm (1 in.).

3.1.4 Welding Symbols

Welding symbols used on drawings shall be the symbols of

the American Welding Society.

3.1.5 Typical Joints

3.1.5.1 General

Typical tank joints are shown in Figures 3-1, 3-2, 3-3A, 3-

3B, and 3-3C. The wide faces of nonsymmetrical V- or U-butt

joints may be on the outside or the inside of the tank shell at

the option of the manufacturer. The tank shell shall be

designed so that all courses are truly vertical.

3.1.5.2 Vertical Shell Joints

a. 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 to meet

the requirements of 5.2.1 and 5.2.3. The suitability of the

plate preparation and welding procedure shall be determined

in accordance with 7.2.

b. Vertical joints in adjacent shell courses shall not be aligned

but shall be offset from each other a minimum distance of 5

where

is the plate thickness of the thicker course at the point

of offset.

3.1.5.3 Horizontal Shell Joints

a. 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. The

suitability of the plate preparation and welding procedure

shall be determined in accordance with 7.2.

b. Unless otherwise specified, abutting shell plates at horizontal

joints shall have a common vertical centerline.

3.1.5.4 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. Three-plate laps in tank

3-2 API S

TANDARD

650

bottoms shall be at least 300 mm (12 in.) from each other,

from the tank shell, from butt-welded annular-plate joints,

and from joints between annular plates and the bottom. Lapping

of two bottom plates on the butt-welded annular plates

does not constitute a three-plate lap weld. When annular

plates are used or are required by 3.5.1, they shall be buttwelded

and shall have a radial width that provides at least 600

mm (24 in.) between the inside of the shell and any lapwelded

joint in the remainder of the bottom. Bottom plates

need to be welded on the top side only, with a continuous fullfillet

weld on all seams. Unless annular bottom plates are

used, the bottom plates under the bottom shell ring shall have

the outer ends of the joints fitted and lap-welded to form a

smooth bearing for the shell plates, as shown in Figure 3-3B.

3.1.5.5 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. Typical

permissible bottom butt-welds without a backing strip are the

same as those shown in Figure 3-1. The use of a backing strip

at least 3 mm (

in.) thick tack welded to the underside of the

plate is permitted. Butt-welds using a backing strip are shown

in Figure 3-3A. If square grooves are employed, the root openings

shall not be less than 6 mm (

in.). A metal spacer shall

be used to maintain the root opening between the adjoining

plate edges unless the manufacturer submits another method

of butt-welding the bottom for the purchaser’s approval.

Three-plate joints in the tank bottom shall be at least 300 mm

(12 in.) from each other and from the tank shell.

3.1.5.6 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.

3.1.5.7 Shell-to-Bottom Fillet Welds

a. For bottom and annular plates with a nominal thickness

12.5 mm (

in.), 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. The size of each weld shall not be more than 12.5 mm

(

in.) and shall not be less than the nominal thickness of the

thinner of the two plates joined (that is, the shell plate or the

bottom plate immediately under the shell) or less than the following

values:

Nominal Thickness

of Shell Plate

Minimum Size of

Fillet Weld

(mm) (in.) (mm) (in.)

5 0.1875 5

> 5 to 20 > 0.1875 to 0.75 6

> 20 to 32 > 0.75 to 1.25 8

> 32 to 45 > 1.25 to 1.75 10

●

Figure 3-1—Typical Vertical Shell Joints

Figure 3-2—Typical Horizontal Shell Joints

􀀀􀀀

􀀀 􀀀 􀀀

Single-V butt joint

Single-U butt joint

Double-V butt joint

Square-groove butt joint Double-U butt joint

Note: See 3.1.5.2 for specific requirements for vertical shell joints.

Optional

outside angle

Angle-to-shell

butt joint

complete penetration

Alternative

angle-to-shell joint

Square-groove

butt joint

complete penetration

Single-bevel

butt joint

complete penetration

Double-bevel

butt joint

complete penetration

Note: See 3.1.5.3 for specific requirements for horizontal

shell joints.

ELDED

TEEL

ANKS

FOR

TORAGE

3-3

b. For annular plates with a nominal thickness greater than

12.5 mm (

in.), the attachment welds shall be sized so that

either the legs of the fillet welds or the groove depth plus the

leg of the fillet for a combined weld is of a size equal to the

annular-plate thickness (see Figure 3-3C), but shall not

exceed the shell plate thickness.

c. Shell-to-bottom fillet welds for shell material in Groups

IV, IVA, V, or VI shall be made with a minimum of two

passes.

d. Shell-to-bottom fillet weld around low-type reinforcing

pads shown in Figure 3-5, Details a and b or around shell

insert plates that extend beyond the outside surface of the

adjacent tank shell shall be sized as required by paragraphs a,

b or c.

e. The bottom or annular plates shall be sufficient to provide

a minimum 13 mm (

in.) from the toe of the fillet weld referenced

in 3.1.5.7d to the outside edge of the bottom or

annular plates.

3.1.5.8 Wind Girder Joints

a. Full-penetration butt-welds shall be used for joining ring

sections.

b. Continuous welds shall be used for all horizontal top-side

joints and for all vertical joints. 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.

3.1.5.9 Roof and Top-Angle Joints

a. 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.

b. Roof plates shall be attached to the top angle of a tank

with a continuous fillet weld on the top side only, as specified

in 3.10.2.5.

c. The top-angle sections for self-supporting roofs shall be

joined by butt-welds having complete penetration and fusion.

Joint efficiency factors need not be applied in conforming to

the requirements of 3.10.5 and 3.10.6.

Figure 3-3A—Typical Roof and Bottom Joints

ROOF-PLATE JOINT

ROOF-TO-SHELL JOINTS

ALTERNATIVE ROOF-TO-SHELL JOINT

(SEE NOTE 2)

BOTTOM-TO-SHELL JOINT

BOTTOM-PLATE JOINTS

Single-welded

full-fillet lap joint

Single-welded butt joint

with backing strip

Optional

V groove

Bottom or annular Inside

bottom plate

1.75

t ≤ R ≤ 3t

Inside of shell

Optional

outside angle

Inside

Tack weld

Notes:

1. See 3.1.5.4 through 3.1.5.9 for specific requirements for roof

and bottom joints.

2. The alternative roof-to-shell joint is subject to the limitations of

3.1.5.9, item f.

Figure 3-3B—Method for Preparing Lap-Welded

Bottom Plates Under Tank Shell (See 3.1.5.4)

Shell plate

Bottom plate

●

3-4 API S

TANDARD

650

d. At the option of the manufacturer, for self-supporting

roofs of the cone, dome, or umbrella type, the edges of the

roof plates may be flanged horizontally to rest flat against the

top angle to improve welding conditions.

e. Except as specified for open-top tanks in 3.9, for self-supporting

roofs in 3.10.5 and 3.10.6, and for tanks with the

flanged roof-to-shell detail described in item f below, tank

shells shall be supplied with top angles of not less than the

following sizes: for tanks with a diameter less than or equal to

11 m (35 ft), 51

・~

4.8 mm (2

・~

in.); for tanks

with a diameter greater than 11 m (35 ft) but less than or

equal to 18 m (60 ft), 51

・~

6.4 mm (2

・~

in.); and

for tanks with a diameter greater than 18 m (60 ft), 76

・~

9.5 mm (3

・~

in.). At the purchaser’s option, the outstanding

leg of the top angle may extend inside or outside the

tank shell.

f. For tanks with a diameter less than or equal to 9 m (30 ft)

and a supported cone roof (see 3.10.4), the top edge of the

shell may be flanged in lieu of installing a top angle. The

bend radius and the width of the flanged edge shall conform

to the details of Figure 3-3A. This construction may be used

for any tank with a self-supporting roof (see 3.10.5 and

3.10.6) if the total cross-sectional area of the junction fulfills

the stated area requirements for the construction of the top

angle. No additional member, such as an angle or a bar, shall

be added to the flanged roof-to-shell detail.

3.2 DESIGN CONSIDERATIONS

3.2.1 Loads

Loads are defined as follows:

a. Dead load (

): the weight of the tank or tank component,

including any corrosion allowance unless otherwise noted.

b. Stored liquid (

): the load due to filling the tank to the

design liquid level (see 3.6.3.2) with liquid with the design

specific gravity specified by the purchaser.

c. Hydrostatic test (

): the load due to filling the tank with

water to the design liquid level.

d. Minimum roof live load (

): 1.0 kPa (20 lbf/ft

) on the

horizontal projected area of the roof.

e. Snow (

): The ground snow load shall be determined from

ASCE 7, Figure 7-1 or Table 7-1 unless the ground snow load

that equals or exceeds the value based on a 2% annual probability

of being exceeded (50 year mean recurrence interval) is

specified by the purchaser. 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. The design snow load shall be reported

to the purchaser.

f. Wind (

): The design wind speed (

) shall be 190 km/hr

(120 mph), the 3 second gust design wind speed determined

from ASCE 7, Figure 6-1, or the 3-second gust design wind

speed specified by the purchaser [this specified wind speed

shall be for a 3-second gust based on a 2% annual probability

of being exceeded (50-year mean recurrence interval)]. The

design wind pressure shall be 0.86 kPa [

/190]

, [(18 lbf/

)(

/120)

] on vertical projected areas of cylindrical surfaces

and 1.44 kPa (

/190)

, [(30 lbf/ft

)(

/120)

] uplift (see

item 2) on horizontal projected areas of conical or doubly

curved surfaces, where

is the 3-second gust wind speed.

The 3-second gust wind speed used shall be reported to the

purchaser.

1. These design wind pressures are in accordance with

ASCE 7 for wind exposure Category C. As an

alternative, pressures may be determined in accordance

with ASCE 7 (exposure category and importance

Figure 3-3C—Detail of Double Fillet-Groove Weld for Annular Bottom Plates With a Nominal

Thickness Greater Than 13 mm (

in.) (See 3.1.5.7, item b)

Shell plate

6 mm (

1/4 in.) minimum

13 mm (

1/2 in.) maximum

Annular bottom plate

A = B for

up to 25 mm

(1 in.) annular

plate

A + B minimum

Notes:

1. A = Fillet weld size limited to 13 mm (

1/2 in.) maximum.

2. A + B = Thinner of shell or annular bottom plate thickness.

3. Groove weld B may exceed fillet size A only when annular plate is thicker than 25 mm (1 inch).

●

ELDED STEEL TANKS FOR OIL STORAGE 3-5

factor provided by purchaser) or a national standard for

the specific conditions for the tank being designed.

2. The design uplift pressure on the roof (wind plus internal

pressure) need not exceed 1.6 times the design

pressure

P determined in F.4.1.

3. Windward and leeward horizontal wind loads on the

roof are conservatively equal and opposite and therefore

they are not included in these pressures.

4. Fastest mile wind speed times 1.2 is approximately

equal to 3 second gust wind speed.

g. Design internal pressure (

Pi): shall not exceed 18 kPa (2.5

lbf/in

2).

h. Test pressure (

Pt): as required by F.4.4 or F.7.6.

i. Design external pressure (

Pe): shall not be less than 0.25

kPa (1 in. of water) and shall not exceed 6.9 kPa (1.01 lbf/

in.

j. Seismic (

E): seismic loads determined in accordance with

Appendix E.

3.2.2 Design Factors

The purchaser shall state the design metal temperature

(based on ambient temperature), the design specific gravity,

the corrosion allowance (if any), and the maximum design

temperature.

3.2.3 External Loads

The purchaser shall state the magnitude and direction of

external loads or restraint, if any, for which the shell or shell

connections must be designed. The design for such loadings

shall be a matter of agreement between the purchaser and the

manufacturer.

3.2.4 Protective Measures

The purchaser should give special consideration to foundations,

corrosion allowance, hardness testing, and any other

protective measures deemed necessary.

3.2.5 External Pressure

See Appendix V for minimum requirements in the design

of tanks subject to partial internal vacuum exceeding 0.25 kPa

(1 in. of water). 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.

3.2.6 Tank Capacity

3.2.6.1

The purchaser shall specify the maximum capacity

and the overfill protection level (or volume) requirement (see

API Recommended Practice 2350).

3.2.6.2

Maximum capacity is the volume of product in a

tank when the tank is filled to its design liquid level as defined

in 3.6.3.2 (see Appendix L).

3.2.6.3

The net working capacity is the volume of available

product under normal operating conditions. The net

working capacity is equal to the maximum capacity (3.2.6.2)

less the minimum operating volume remaining in the tank,

less the overfill protection level (or volume) requirement (see

Appendix L).

3.3 SPECIAL CONSIDERATIONS

3.3.1 Foundation

The selection of the tank site and the design and construction

of the foundation shall be given careful consideration, as

outlined in Appendix B, to ensure adequate tank support. The

adequacy of the foundation is the responsibility of the purchaser.

3.3.2 Corrosion Allowances

When necessary, the purchaser, after giving consideration

to the total effect of the liquid stored, the vapor above the liquid,

and the atmospheric environment, shall specify the corrosion

allowance to be provided for each shell course, for the

bottom, for the roof, for nozzles and manholes, and for structural

members.

3.3.3 Service Conditions

When the service conditions might include the presence of

hydrogen sulfide or other conditions that could promote

hydrogen-induced cracking, notably near the bottom of the

shell at the shell-to-bottom connections, care should be taken

to ensure that the materials of the tank and details of construction

are adequate to resist hydrogen-induced cracking. The

purchaser should consider limits on the sulfur content of the

base and weld metals as well as appropriate quality control

procedures in plate and tank fabrication. The hardness of the

welds, including the heat-affected zones, in contact with these

conditions should be considered. The weld metal and adjacent

heat-affected zone often contain a zone of hardness well

in excess of Rockwell C 22 and can be expected to be more

susceptible to cracking than unwelded metal is. Any hardness

criteria should be a matter of agreement between the purchaser

and the manufacturer and should be based on an evaluation

of the expected hydrogen sulfide concentration in the

product, the possibility of moisture being present on the

inside metal surface, and the strength and hardness characteristics

of the base metal and weld metal.

3.3.4 Weld Hardness

When specified by the purchaser, the hardness of the weld

metal for shell materials in Group IV, IVA, V, or VI shall be

evaluated by one or both of the following methods:

a. The welding-procedure qualification tests for all welding

shall include hardness tests of the weld metal and heataffected

zone of the test plate. The methods of testing and the

acceptance standards shall be agreed upon by the purchaser

and the manufacturer.

b. All welds deposited by an automatic process shall be hardness

tested on the product-side surface. Unless otherwise

●

00

05

●

3-6 API S

TANDARD 650

specified, one test shall be conducted for each vertical weld,

and one test shall be conducted for each 30 m (100 ft) of circumferential

weld. The methods of testing and the acceptance

standards shall be agreed upon by the purchaser and the

manufacturer.

3.4 BOTTOM PLATES

3.4.1

All bottom plates shall have a minimum nominal

thickness of 6 mm (

1/4 in.) [49.8 kg/m2 (10.2 lbf/ft2) (see

2.2.1.2)], exclusive of any corrosion allowance specified by

the purchaser for the bottom plates. Unless otherwise agreed

to by the purchaser, all rectangular and sketch plates (bottom

plates on which the shell rests that have one end rectangular)

shall have a minimum nominal width of 1800 mm (72 in.).

3.4.2

Bottom plates of sufficient size shall be ordered so

that, when trimmed, at least a 50 mm (2 in.) width will project

outside the shell or to meet the requirements of 3.1.5.7e,

whichever is greater.

3.4.3

Bottom plates shall be welded in accordance with

3.1.5.4 or 3.1.5.5.

3.5 ANNULAR BOTTOM PLATES

3.5.1

When the bottom shell course is designed using the

allowable stress for materials in Group IV, IVA, V, or VI, buttwelded

annular bottom plates shall be used (see 3.1.5.6).

When the bottom shell course is of a material in Group IV,

IVA, V, or VI and both the maximum product stress (see

3.6.2.1) for the first shell course is less than or equal to 160

MPa (23,200 lbf/in.

2) or the maximum hydrostatic test stress

(see 3.6.2.2) for the first shell course is less than or equal to

172 MPa (24,900 lbf/in.

2), lap-welded bottom plates (see

3.1.5.4) may be used in lieu of butt-welded annular bottom

plates.

3.5.2

Annular bottom plates shall have a radial width that

provides at least 600 mm (24 in.) between the inside of the

shell and any lap-welded joint in the remainder of the bottom.

Annular bottom plate projection outside the shell shall meet

the requirements of 3.4.2. A greater radial width of annular

plate is required when calculated as follows:

In SI units:

where

= thickness of the annular plate (see 3.5.3), in mm,

= maximum design liquid level (see 3.6.3.2), in m,

= design specific gravity of the liquid to be stored.

In US Customary units:

where

= thickness of the annular plate (see 3.5.3), (in.),

= maximum design liquid level (see 3.6.3.2), (ft),

= design specific gravity of the liquid to be stored.

3.5.3

The thickness of the annular bottom plates shall not

be less than the thicknesses listed in Table 3-1 plus any specified

corrosion allowance.

3.5.4

The ring of annular plates shall have a circular outside

circumference but may have a regular polygonal shape

inside the tank shell, with the number of sides equal to the

number of annular plates. These pieces shall be welded in

accordance with 3.1.5.6 and 3.1.5.7, item b.

3.5.5

In lieu of annular plates, the entire bottom may be

butt-welded provided that the requirements for annular plate

thickness, welding, materials, and inspection are met for the

annular distance specified in 3.5.2.

3.6 SHELL DESIGN

3.6.1 General

3.6.1.1

The required shell thickness shall be the greater of

the design shell thickness, including any corrosion allowance,

or the hydrostatic test shell thickness, but the shell thickness

shall not be less than the following:

3.6.1.2

Unless otherwise agreed to by the purchaser, the

shell plates shall have a minimum nominal width of 1800 mm

(72 in.). Plates that are to be butt-welded shall be properly

squared.

Paragraph deleted.

●

05

05

05

215

(

HG)0.5

------------------

Nominal Tank Diameter

(See Note 1)

Nominal Plate Thickness

(See Note 2)

(m) (ft) (mm) (in.)

< 15 < 50 5

3/16

15 to < 36 50 to < 120 6

1/4

36 to 60 120 to 200 8

5/16

> 60 > 200 10

3/8

Notes:

1. Unless otherwise specified by the purchaser, the nominal tank

diameter shall be the centerline diameter of the bottom shell-course

plates.

2. Nominal plate thickness refers to the tank shell as constructed.

The thicknesses specified are based on erection requirements.

3. When specified by the purchaser, plate with a minimum nominal

thickness of 6 millimeters may be substituted for

1/4-inch plate.

390

(

HG)0.5

------------------

●

ELDED STEEL TANKS FOR OIL STORAGE 3-7

3.6.1.3

The calculated stress for each shell course shall not

be greater than the stress permitted for the particular material

used for the course. No shell course shall be thinner than the

course above it.

3.6.1.4

The tank shell shall be checked for stability against

buckling from the design wind speed in accordance with 3.9.7.

If required for stability, intermediate girders, increased shellplate

thicknesses, or both shall be used.

3.6.1.5

The manufacturer shall furnish to the purchaser a

drawing that lists the following for each course:

a. The required shell thicknesses for both the design condition

(including corrosion allowance) and the hydrostatic test

condition.

b. The nominal thickness used.

c. The material specification.

d. The allowable stresses.

3.6.1.6

Isolated radial loads on the tank shell, such as those

caused by heavy loads on platforms and elevated walkways

between tanks, shall be distributed by rolled structural sections,

plate ribs, or built-up members.

3.6.2 Allowable Stress

3.6.2.1

The maximum allowable product design stress, Sd,

shall be as shown in Table 3-2. The net plate thicknesses—the

actual thicknesses less any corrosion allowance—shall be

used in the calculation. The design stress basis,

Sd, shall be

either two-thirds the yield strength or two-fifths the tensile

strength, whichever is less.

3.6.2.2

The maximum allowable hydrostatic test stress, St,

shall be as shown in Table 3-2. The gross plate thicknesses,

including any corrosion allowance, shall be used in the calculation.

The hydrostatic test basis shall be either three-fourths

the yield strength or three-sevenths the tensile strength,

whichever is less.

3.6.2.3

Appendix A permits an alternative shell design

with a fixed allowable stress of 145 MPa (21,000 lbf/in.

2) and

a joint efficiency factor of 0.85 or 0.70. This design may only

be used for tanks with shell thicknesses less than or equal to

12.5 mm (

1/2 in.).

3.6.2.4

Structural design stresses shall conform to the

allowable working stresses given in 3.10.3.

3.6.3 Calculation of Thickness by the 1-Foot Method

3.6.3.1

The 1-foot method calculates the thicknesses

required at design points 0.3 m (1 ft) above the bottom of

each shell course. Appendix A permits only this design

method. This method shall not be used for tanks larger than

60 m (200 ft) in diameter.

3.6.3.2

The required minimum thickness of shell plates

shall be the greater of the values computed by the following

formulas:

In SI units:

where

= design shell thickness, in mm,

= hydrostatic test shell thickness, in mm,

= nominal tank diameter, in m (see 3.6.1.1, Note 1),

= design liquid level, in m,

= height from the bottom of the course under consideration

to the top of the shell including the top

angle, if any; to the bottom of any overflow that

limits the tank filling height; or to any other level

specified by the purchaser, restricted by an internal

floating roof, or controlled to allow for seismic

wave action,

= design specific gravity of the liquid to be stored,

as specified by the purchaser,

Table 3-1—Annular Bottom-Plate Thicknesses

SI Units

Nominal Plate

Thickness

a of First

Shell Course (mm)

Hydrostatic Test Stress

b in First Shell Course

(MPa)

≤

190 ≤ 210 ≤ 230 ≤ 250

≤ 19 6 6 7 9

19 <

t ≤ 25 6 7 10 11

25 <

t ≤ 32 6 9 12 14

32 <

t ≤ 38 8 11 14 17

38 <

t ≤ 45 9 13 16 19

US Customary

Nominal Plate

Thickness

a of First

Shell Course (in.)

Hydrostatic Test Stress

c in First Shell Course

(lbf/in

≤

27,000 ≤ 30,000 ≤ 33,000 ≤ 36,000

≤ 0.75 1/4

/32

0.75 <

t ≤ 1.00 1/4

/32

/16

1.00 <

t ≤ 1.25 1/4

/32

/16

1.25 <

t ≤ 1.50 5/16

/16

1.50 <

t ≤ 1.75 11/32

Nominal plate thickness refers to the tank shell as constructed.

Hydrostatic test stresses are calculated from [4.9D(H – 0.3)]/t

(see 3.6.3.2).

Hydrostatic test stresses are calculated from [2.6 D(H – 1]/t

(see 3.6.3.2).

Note: The thicknesses specified in the table, as well as the width

specified in 3.5.2, are based on the foundation providing uniform

support under the full width of the annular plate. Unless the foundation

is properly compacted, particularly at the inside of a concrete

ringwall, settlement will produce additional stresses in the

annular plate.

05

05

00

4.9

D(H – 0.3)G

= --------------------------------------- +

4.9

D(H – 0.3)

= ----------------------------------

●

3-8 API S

TANDARD 650

= corrosion allowance, in mm, as specified by the

purchaser (see 3.3.2),

= allowable stress for the design condition, in MPa

(see 3.6.2.1),

= allowable stress for the hydrostatic test condition,

in MPa (see 3.6.2.2).

●

Table 3-2—Permissible Plate Materials and Allowable Stresses

Plate

Specification Grade

Minimum

Yield Strength

MPa (psi)

Minimum

Tensile Strength

MPa (psi)

Product

Design Stress

MPa (psi)

Hydrostatic

Test Stress

MPa (psi)

ASTM Specifications

A 283M (A 283) C (C) 205 (30,000) 380 (55,000) 137 (20,000) 154 (22,500)

A 285M (A 285) C (C) 205 (30,000) 380 (55,000) 137 (20,000) 154 (22,500)

A 131M (A 131) A, B, CS

(A, B, CS)

235 (34,000) 400 (58,000) 157 (22,700) 171 (24,900)

A 36M (A 36) — 250 (36,000) 400 (58,000) 160 (23,200) 171 (24,900)

A 131M (A 131) EH 36 (EH 36) 360 (51,000) 490

a (71,000a) 196 (28,400) 210 (30,400)

A 573M (A 573) 400 (58) 220 (32,000) 400 (58,000) 147 (21,300) 165 (24,000)

A 573M (A 573) 450 (65) 240 (35,000) 450 (65,000) 160 (23,300) 180 (26,300)

A 573M (A 573) 485 (70) 290 (42,000) 485

a (70,000a) 193 (28,000) 208 (30,000)

A 516M (A 516) 380 (55) 205 (30,000) 380 (55,000) 137 (20,000) 154 (22,500)

A 516M (A 516) 415 (60) 220 (32,000) 415 (60,000) 147 (21,300) 165 (24,000)

A 516M (A 516) 450 (65) 240 (35,000) 450 (65,000) 160 (23,300) 180 (26,300)

A 516M (A 516) 485 (70) 260 (38,000) 485 (70,000) 173 (25,300) 195 (28,500)

A 662M (A 662) B (B) 275 (40,000) 450 (65,000) 180 (26,000) 193 (27,900)

A 662M (A 662) C (C) 295 (43,000) 485