api.620.2002

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THE UNITED STATES OF AMERICA

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HEED THIS NOTICE: Criminal penalties may apply for noncompliance.

e

Document Name: API 620: Design and Construction of Large Welded Low
Pressure Storage Tanks

CFR Section(s): 49 CFR 195.264(e)(3)

Standards Body: American Petroleum Institute

Official Incorporator:
THE EXECUTIVE DIRECTOR

OFFICE OF THE FEDERAL REGISTER
WASHINGTON, D.C.



Design and Construction of Large,
Welded, Low-Pressure Storage
Tanks

Downstream Segment
API STANDARD 620
lENTH EDITION, FEBRUARY 2002

American
Petroleum
Institute

Helping You
Get The Job
Done Right;M

SPECIAL NOTES

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Copyright '0 2002 American Petroleum Institute

FOREWORD

This standard is based on the accumulated knowledge and experience of purchasers and
manufacturers of welded, low-pressure oil storage tanks of various sizes and capacities for
intemal pressures not more than 15 pounds per square inch gauge. The object of this publi-
cation is to provide a purchase specification to facilitate the manufacture and procurement of
such storage tanks.

If tanks are pmchased in accordance with the specifications ofthis standard, the purchaser
is required to specify certain basic requirements. The purchaser may desire to modify, delete,
or amplity sections of this standard, but reference shall not be made to this standard on the
nameplate or manufacturer's certification for tanks that do not fulfill the minimum require-
ments or that exceed the limitations of this standard. It is strongly recommended that such
modifications, deletions, or amplifications be made by supplementing this standard, rather
than by rewriting or incorporating sections of it into another complete standard.

Each edition, revision, or addenda to this API standard may be used beginning with the
date of issuance shown on the cover page for that edition, revision, or addenda. Each edition,
revision, or addenda to this API standard becomes effective six months after the date of issu-
ance for equipment that is certified by the manufacturer as being designed, fabricated, con-
stmcted, examined, and tested per this standard. During the six-month time between the date
of issuance of the edition, revision, or addenda and the effective date, the purchaser and man-
ufacturer shall specify to which edition, revision, or addenda the equipment is to be built.

The design mles given in this standard are minimum requirements. More stringent design
mles specified by the purchaser or furnished by the manufacturer are acceptable when mutu-
ally agreed upon by the purchaser and the manufacturer. This standard is not to be inter-
preted as approving, recommending, or endorsing any specific design, nor as limiting the
method of design or construction.

This standard is not intended to cover storage tanks that are to be erected in areas subject
to regulations more stringent than the specifications of this standard. When this standard is
specified for such tanks, it should be followed insofar as it does not conflict with local
requirements.

After revisions to this standard have been issued, they may be applied to tanks to be com-
pleted after the date of issue. The tank nameplate shall state the date of the edition and any
revision to that edition to which the tank is designed and constmcted.

API publications may be used by anyone desiring to do so. Every effort has been made by
the Institute to assure the accuracy and reliability ofthe data contained in them; however, the
Institute makes no representation, warranty, or guarantee in connection with this publication
and hereby expressly disclaims any liability or responsibility for loss or damage resulting
from its use or for the violation of any federal, state, or municipal regulation with which this
publication may conflict.

Suggested revisions are invited and should be submitted to the standardization manager,
American Petrolellm lnstitute, 1220 L Street, N.W., Washington, D.C. 20005.

iii









































5-2 API STANDARD 620

5.4 LOADINGS ential direction (determined in accordance with
5.5.4.2 for the thickness-to-radius ratio involved),
The following loadings shall be considered in the design of
large, low-pressure storage tanks: Sta allowable tensile stress, in Ibf/in.2; Sta lower than
Sts because of the presence of a coexistent com-
a. The internal pressure as specified in 5.3 and any partial pressive stress perpendicular to it,
vacuum resulting from operation.
b. The weight of the tank and specified contents, from empty allowable compressive stress, in Ibt/in.1; Sea is
to filll, with or without the maximum gas pressure specified. lower than Scs because of the presence of a coexist-
c. The supporting system, both localized and general, includ- ent tensile or compressive stress perpendicular to it,
ing the c1Tect that is predictable from the nature of the
foundation conditions (see Appendices C and D). Stc computed tensile stress, in Ibllin.2, at the point
d. Superimposed loading, such as platforms and brackets for under consideration,
stairways and, where climatic conditions warrant, excessive
snow (sec Appendix E). computed compressive stress, in Ibffin.2, at the
e. Wind loads or, when specified, earthquake loadings (see point under consideration,
5.5.6).
f. Loads resulting from connected piping. general variable for indicating a tensile stress, in
g. The weight orany insulation and linings. Ibli'in.2, which may be either an allowable or com-
puted value depending on the context in which the
5.5 MAXIMUM ALLOWABLE STRESS FOR variable is used,
WALLS15
general variable for indicating a compressive
5.5.1 General stress, in Ibf/in.2, which may be either an allow-
able or computed value depending on the context
Higher localized shear and secondary bending stresses in which the variable is used,
may exist in the walls of tanks designed and fabricated
according to this standard, and the prescribed test loadings N ratio of the tensile stress, St, to the maximum
may result in some localizcd reshaping. This is pem1issible, allowable stress for simple tension, Sts,
since localized reshaping is expected as part of a legitimate
labrication operation, if the reshaping is not so severe that M ratio of the compressive stress sc, to the maximun1
upon release of the test pressure, plastic straining occurs in allowable compressive stress, Scs (see Figure F-l).
the opposite direction. This would tend to develop continuing
plastic straining in subsequent normal operation. 5.5.2.2 The term tank wall is defined in 3.3. Unless other- I 01
wise stipulated in this standard, the stresses in nozzle and
5.5.2 Nomenclature manway necks, reinforcing pads, flanges, and cover plates
shall not exceed the values that apply lor the walls of the tank.
5.5.2.1 Variables relating to stresses common to the
requirements of 5.5.3 through 5.5.5 and Figure 5-1 are 5.5.3 Maximum Tensile Stresses
defined as lollows:
5.5.3.1 The maximum tensile stresses in the outside walls
thickness of the wall, in in., of a tank, as determined for any of the loadings listed in 5.4 or
any concurrent combination of such loadings that is expected
R radius of the wall, in in., to be encountered in the specified operation, shall not exceed
the applicable stress values determined in accordance with
c corrosion allowance, in in., provisions described in 5.5.3.2 and 5.5.3.3.

Sts maximum allowable stress for simple tension, in 5.5.3.2 Ifboth the meridional and latitudinal unit forces, 1'1
IbUin 2 , as given in Table 5-1, and T2, are tensile or if one force is tensile and the other is
zero, the computed tensile stress, St~, shall not exceed the
Ses maximum allowable longitudinal compressive applicable value given in Table 5-1.
stress, in IbUin.2, for a cylindrical wall acted upon
by an axial load with neither a tensile nor a com- 5.5.3.3 If the meridional lorce, Tl, is tensile and the coex-
pressive force acting concurrently in a circurnfer- istent latitudinal unit force, T2, is compressive or if 1'2 is ten-
sile and T\ is compressive, the computed tensile stress, Stc,
JSSee Biaxial Stress Criteria for Large Low-Pressure Tanks, written shall not exceed a value of the allowable tensile stress, Stw
by .T. .T. Dvorak and RV McGralh and published as Bulletin No. 69 obtained by multiplying the applicable stress value given in
(.Tune 1961) by the Welding Research Council, 345 East 47th Slreet,
New York, New York 10017. Table 5-1 by the appropriate value of N obtained from Figure
5-1 tor the value of compressive stress (sc = Sed and the co-

DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-3

II I

(See f\Jotes 1 and 2} II

~--16,000
i--+----+---+---+--+-4-1--I------t------+----+----+-+--+---t----+----I--II-- I'" ',,, - MS" - 13000 M
I
1- '-I--I------t---t- -----I---+---+-+-+-~1-1,'-\<;-fJ-\\I- f~I r~f~~c$~" ~-''R5,~00~0 ~ps~, ~~c~ 0 - VI /f,
-r---14,000 f- - N - -
/ ' S, -' ;o~O - :~:----[7

I / A ~L.. N -- G I/<R
1-+---+-f-t--jIL-t/ 0.30. _.+-+----+--f-------1-----1
I I12,000 ~
r------t-- Do nol e~trapolate --4.4~--I-_+__+___!--I-_+__+_--!--I-_+__+--!-+_+__+_l I II ,+-+----+--+-+-
040-
beyond lhls line

w,ooo ~-I-+-+-pl"_6~;i;;~ti;t;;~t;t=t;;~t*~;;~~*;+05';- ~~I-f---,J---.I---------II------I-----+1---------lI

-I----+___-+----l~ i"1~=I=1==I==F=l===!:=l=F1==I==F=l===!:=l=F=F+_--t-1
8,000 ~cC'''~=~t:,tj=~t:tj=~)t:t=t=~~,----=~-=t=~ 0.60I-~~+--+--+-l---!-
---, I - - "'~ 0 70 - ~+--+--+--
I ~-~ -----------
6,000 I-+--+ ,Ji I

If - I - " , ; 0,80' ~ ~-+-+-+-+----j

4,000 1----- /1 .I~ml_+-+--+----I----j

j

2,000 II 090

° I~Lt=_t_=_:j_:_:_:_:.:__:t:::t=l=~=t=I==t1=t=t===+=t=~H=+=+__+-:+-I-f-~L I
_ _ _ _ _ _ _ _ _ _ _ _ _ _ I_J.......--L---L----L------1~__.L__...L.__L~_.L...~.__L.___.l_.L......L___L_J _ I
° 0,002 0,004 0,006 0,008 0,010 0,012 0,014 0,016 0018 0,020 0,022 ___.J
_..L

(I -- c)/R ralio (see Note 3)

Notes:
I. At no time can a compressive stress ror a particular value of (I - c)/R exceed s..;.~ represented by curve
(lABe; no values of compressive stress or IV are permitled to fall to the left or above this curve.
Z, Soe Figure F·l for relationship between factors M and N,
), If compressive stress is latitudinal, use R = R]; if compress.ive "tress is meridional. use R = R20

Figure 5-1-Biaxial Stress Chart for Combined Tension and Compression, 30,000--38,000 psi Yield Strength Steels

relatcd ratio of (t -- c)/R involved, Howcver, in cases where dance with the proVIsions described in 5,5.4,2 through
the unit force acting in compression does not exceed 5% of 5,5.4.8, These rules do not purport to apply when the cir-
the coexistent tensile unit torce acting perpendicular to it, the cumferential stress on a cylindrical wall is compressive (as
designer has the option of pemlitting a tensile stress of the in a cylinder acted upon by external prcssure). However,
magnitude specified in 5,5,3,2 instead of complying strictly values of Scs computed as in 5,5.4,2, with R equal RJ when
with the provisions of this paragraph, (see F 1 for examples the compressive unit force is latitudinal or to R2 when the
illustrating the detennination of allowable tensile stress val- compressive unit force is meridional, in some degree fom1
ues, Sla, in accordance with this paragraph). In no event shall the basis for the rules given in 5,5.4.3, 5,5.4.4, and 5,5.4,5,
the value of sta exceed the product of the applicable joint e1-ft- which apply to walls of double curvature,
ciency for tension as given in Table 5-2 and the allowable
stress for simple tension shown in Table 5-1, 5.5.4.2 If a cylindrical wall, or a portion thereoC is acted
upon by a longitudinal compressive f()rce with neither a ten-
5.5.4 Maximum Compressive Stresses sile nor a compressive force acting concurrently in a circum-
terential direction, the computed compressive stress, Sec> shall
5.5.4.1 Except as provided in 5.12.4,3 for the compres- not exceed a value, Scs, established for the applicable thick-
sion-ring region, the maximum compressive stresses in the ness-to-radius ratio as follows:
outside walls of a tank, as detennined for any of the load-
ings listed in 5.4 or any concurrent combination of loadings For values of (t -- c)/R less than OJ)0667,
expected to be encountered in the specified operation, shall
not exceed the applicable stress values detem1ined in accor- Scs = 1,800,OOO[(t -- c)/R]

5-4 API STANDARD 620

Table 5-1-Maximum Allowable Stress Values for Simple Tension

Specified Minimum Maximum Allowable
Tensile Stress for
Specification Grade Notes Tensile Strength Yield Point Tension, Sl'
(See Note 1)
(lbG'in.2) (lbti'in. 2) (lbtlin.2, see Notes 2 and 3)
ASTMA 36
ASTMA 131 Plates
ASTMA 131
ASTM AI31 4 58,000 36,000 16,000
ASTMA283
ASTMA283 A 4,5 and 6 58,000 34,000 15,200
ASTMA285
B 4 58,000 34,000 16,000
ASTMA516
ASTMA 516 CS 4 58,000 34,000 16,000
ASTMA516
ASTMA516 C 4 and 5 55,000 30,000 15,200

ASTMA 537 D 4,5 and 6 60,000 33,000 15,200
ASTMA537
ASTM A573 C 5 55,000 30,000 16,500
ASTMA 573
ASTMA 573 55 55,000 30,000 16,500
ASTMA 633
60 60,000 32,000 18,000
ASTMA662
ASTMA662 65 65,000 35,000 19,500
ASTM A678
ASTMA678 70 70,000 38,000 21,000
ASTMA 737
01. ASTMA841 Class I 7 70,000 50,000 21,000
Class 2 7 80,000 60,000 24,000
CSA G40.21-M 58 4 58,000 32,000 16,000
CSA G40.21-M 65 4 65,000 35,000 18,000
CSA G40.21-M 70 4 70,000 42,000 19,300
CSA G40.21-M Cand D 4 and 7 70,000 50,000 19,300
ISO 630
ISO 630 B 7 65,000 40,000 19,500
4 and 8 70,000 43,000 21,000
Seamless C 4 and 7 70,000 50,000 19,300
API Spec 5L 80,000 60,000 22,100
ASTM A 33 A 7 70,000 50,000 21,000
ASTMA 106 B 7 70,000 50,000 21,000
ASTMA 106 B
ASTM A333 Class I
ASTMA333
ASTMA333 260W and 260WT 4 59,500 37,700 16,400
ASTMA524 300Wand 300WT 4 65,300 43,500 IS,OOO
ASTMA 524 4 65,300 50,SOO IS,OOO
350W 4 69,000 50,SOO 19,200
Electric-Fusion 350WT 4 61,900 37,000 17,100
Welded E275 Quality C, D 4 4S,500 19,600
n,ooo
ASTMA 134 E355 Quality c, D
ASTMA 134
ASTMA 139 Pipe
ASTMA671
ASTMA671 B 60,000 35,000 18,000
ASTMA671
ASTM A 671 B 60,000 35,000 18,000
ASTMA671
ASTM A 671 B 60,000 35,000 IS,OOO
ASTMA671
ASTMA671 C 70,000 40,000 21,000

I 55,000 30,000 16,500

3 65,000 35,000 19,500

6

I 60,000 35,000 IS,OOO

II 55,000 30,000 16,500

A 283 Grade C 4,5 and 9 55,000 30,000 12,100
A 2S5 Grade C 5 and 9 55,000 30,000 13,200
B 9 60,000 35,000 14,400
CA55 9 55,000 30,000 13,200
CC60 9 60,000 32,000 14,400
CC65 9 65,000 35,000 15,600
CC70 9 70,000 3S,000 16,SOO
CD70 7 and 9 70,000 50,000 16,SOO
CDSO 7 and 9 SO,OOO 60,000 19,200
CE55 9 55,000 30,000 13,200
CE60 9 60,000 32,000 14,400

DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-5

Table 5-1-Maximum Allowable Stress Values for Simple Tension (Continued)

Specitled Minimum Maximum Allowable
Tensile Stress for
Speci fication Grade Notes Tensile Strength Yield Point
(See Note I) (lbf/in. 2) (lbllin.2) Tension, Sl'
(lbflin.2, see Notes 2 and 3)

Forgings

ASTMA lOS I 60,000 30,000 18,000
ASTMA 181 II 60,000 30,000 18,000
ASTMA 181 LFI 70,000 36,000 21,000
ASTMA 350 LF2 60,000 30,000 IS,OOO
ASTMA 350 LF3 70,000 36,000 21,000
ASTMA 350 70,000 40,000 21,000

ASTMA 27 60-30 10 Castings and bolting 30,000 14,400
ASTMA36 For anchor bolting II 36,000 15,300
ASTMA 193 87 11 60,000 105,000 24,000
ASTMA 307 8 lor flanges 11 and 12 58,000 8,400
and pressure parts 125,000 105,000
ASTMA307 B for structural II 55,000 15,000
parts and anchor
ASTMA 320 bolting 55,000 24,000
L7
II 125,000

Structural shapes Rcsisting Internal Pressure

ASTMA36 A 4 and 6 5S,OOO 36,000 15,200 I 01
ASTMA 131 A 4 and 6 5S,OOO 34,000 15,200
ASTMA 633 4 63,000 42,000 17,400
ASTMA 992 260W and 260WT 4 and 6 65,000 50,000 15,200
CSA G40.21-M 300W and 300WT 4 and 6 59,500 37,700 15,200
CSA G40.21-M 350Wand 300WT 4 and 6 65,300 43,500 15,200
CSA G40.21-M 4 and 6 69,600 50,800 15,200

Notes:

I. All pertinent moditlcations and limitations of specifications required by 4.2. through 4.6 shall be complied with.

2. Except for thosc cases where additional factors or limitations are applied as indicated by references to Notes 4, 6, 10 and 12, the allowable
tensile stress values given in this table for materials other than bolting steel are the lesser of (a) 30% of the specitled minimum ultimate tensile
strength for the material or (b) 60% of the specitled minimum yield point.

3. Except when ajoint efficiency tactor is already retlected in the specitled allowable stress value, as indicated by the reterences to Note 10, or
where the value ofN determined in accordance with 5.5.3.3. is less than the applicable joint elIiciency given in Table 5-2 (and therel~)rc ellects
a greater reduction in allowable stress than would the pertinent join efficiency factor, if applied), the specified stress values li)r welds in tension
shall be multiplied by the applicable joint dIiciency factor, E, given in Table 5-2.

4. Stress values for structural quality steels include a quality factor of 0.92.

5. Plates and pipe shall not be used in thickness greater than 1/4 in.
6. Stress values are limited to those for steel that has an ultimate tensile strength of only 55,000 Ibflin 2 .

7. Less than or equal to 21 h in. thickness.

S. Less than or equal to 11h in. thickness.

9. Stress values lor fusion-welded pipe include a welded-joint elficiency factor ofO.SO (see 5.23.3). Only straight-seam pipe shall be used; the
use of spiral-seam pipe is prohibited.

10. Stress values for castings include a quality factor of 0.80.

11. See 5.6.6.

12. Allowable strcss based on Section VIII of the ASME Boiler and Pressure Vessel Code multiplied by the ratio of the design stress factors in
this standard and Section V III of the ASME Code, namely 0.30/0.25.