API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks

Last update: July 31, 2006

Standard Edition Section Inquiry # Question Reply

620 9th - Feb. 1.2.1 620-I-10/00 Question 1: Does Section 1.2.1 prevent tanks with umbrella roofs Reply 1: No.
1996 and eccentric cones, from being marked as being in accordance
with API Standard 620?

Question 2: Does Section 1.2.1 prevent tanks with flat bottoms from Reply 2: No.
having a slope?

620 9th AWS 620-I-09/00 1. Does API 620 require that weld procedures be qualified by 1. Yes
standard qualification tests in accordance with ASME Section IX, 2. Yes
using the contractors actual welding procedure, without any all-weld- 3. Yes
tensile tests, except for the additional tests required by Question 2 4. API can only respond to requests for interpretations of the
below? 2. requirements in API standards or consider revisions based on new
Does Api 620 require or permit additional, separate qualification data or techology. Please refer to API 620, APpendix J for more
testsfor proof of weld metal strength assumed for design using the information on technical inquiries.
AWS test-piece as per Figure 9 of AWS 5.11, which will be different
from Reply 1 above (Since single-sided welds with backing strips are
not used for tank construction)? 3. Does API 620 allow the use of
test plates for proof of weld metal strength assumed for design in
accordance with AWS A5-11, Table 3, note 1, which is referred to by
Figure 2, note 1 of AWS5.11, and which specifically permits the use of
buttered carbon steel test plates?
4. Are LNG tank welding procedures and the weld metal
strength qualified for the design normally qualified in the manner
described above?

620 10th - Feb. 4.2 620-I-13/02 Background: We are manufacturing a storage tank according to API Yes. Refer to API 620 Table 4-1 and Section 4.2.3.2.
2002 Table 4-1 620 code, with MDMT 53.6 °F, material specification A 36, thickness
1/2 in. and 3/8 in.

Question: Can 1/2 in. A 36 plate (not A 36 mod 2) be used for the
shell of an API 620 tank with a design metal temperature of 53.6ºF?

620 9th - Feb. 4.2.5 For a tank design to basic API 620 requiring impact testing of plate As stated in API 620, Section 4.2.5, the direction of the axis of
1996 material per Section 4.2.5: (a) Should the long dimension of impact the impact test specimen is dependent upon the direction of
test specimen be parallel to direction of maximum stress (biaxial plate rolling by the mill. The direction of maximum stress when
stresses are not equal) and the minimum absorbed energy value per installed in the tank is not a factor. The impact test energy
Table R-5? or; (b) Should the impact test specimen be taken criteria applicable to either type of specimen is per Table R-2, or
transverse to the final plate rolling and the absorbed energy value per Table R-5 where applicable.
per Table R-2?

Page 1 of 15
 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. 4.2.3.2 620-I-06/02 Background: According to Std 620, Section 4.2.3.2d, ASTM A 283 Yes. This was a typographical error introduced in the previous
2002 plates (Grades C and D) are approved with a maximum nominal edition of the standard. This note will be corrected in the next
thickness of ¾ in. The maximum allowable thickness stated in Table addendum of Std. 620.
5-1, Note 5, is ¼ in. for the same material and grades.

Question: Should the thickness stated in Table 5-1, Note 5, be ¾

in.?

620 9th - Feb. 4.2 620-I-06/00 Question 1: Is it permissible to utilize plate materials at design metal Reply 1: No.
1996 Table 4-1 temperature and thickness combination which are outside the limits
contained in API 620, Table 4-1 if impact testing is performed?

Question 2: Does API 620 allow impact testing to be performed in

lieu of normalizing for combinations of plate material grade, Reply 2: No.
thickness and design metal temperature that require normalizing?

Question 3: May impact testing be utilized in lieu of normalizing for

plate materials where Table 4-1 of API 620 and the ASTM
specification require normalizing e.g., A 516 with thickness greater Reply 3: No.
than 1-1/2 in.?

620 9th - Feb. 5 Is it permissible under API Standard 620 to construct a full- API does not approve, recommend, or endorse any proprietary
1996 containment storage tank for LNG having a high liquid design level or specific design; nor does it limit the method of design or form
(as defined in Appendix A) more than 20 meters in the inner tank? of construction that might be used to conform to an API
Standard. Therefore, we cannot act on your inquiry.

620 10th - Feb. 5.1.1 620-I-05/02 Background: API Standard 620, Tenth Edition, Section 1.2.1, states, Yes.
2002 “This standard does not cover design procedures for tanks that
have walls shaped in such a way that the walls cannot be generated
in their entirety by the rotation of a suitable contour around a single
vertical axis or revolution.” This statement would appear to indicate
that there are no design procedures presented in this standard for
umbrella roofs, eccentric cone roofs and bottoms, and bottoms
sloped to one side. For example, Sections 1.1, 1.2.5, 5.1.1 and 5.1.5,
among others, indicate that subject to stated restrictions, designs
that will be as safe as those provided by the standard may be used.
This inquiry seeks to confirm this interpretation. I am trying to
clarify Inquiry 620-I-10/00.

Question: Are umbrella roofs, eccentric cone roofs and bottoms,

and bottoms sloped to one side, among those designs for which API
620 does not present design rules as noted in Section 5.1.1?

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. Figure 5-6 620-I-11/00 Is the width of lap of the roof panels, onto or beneath the No. See 5.22.1.5
1996 compression ring plate, prescribed in Figure 5-6 Details e, f, f-1, and
g?

620 9th - Feb. Figure 5-6 620-I-12/00 May the lap welds of the roof panels, onto or beneath the Yes.
1996 compression ring plate, prescribed in Figure 5-6, Details e, f, f-1,
and d be double-lap welds or single-lap welds if the appropriate
joint efficiency and inspection are applied?

620 9th - Feb. Figure 5-6 620-I-13/00 May the roof be butt-welded to the compression ring as an Yes.
1996 alternative to the lap weld shown in Figure 5-6 Details e, f, f-1, and g,
if all requirements for circumferential butt welds are met?

620 9th - Feb. Figure 5-6 620-I-14/00 Question 1: Does the expression, wh – L < 16t, apply to Figure 5-6 Reply 1: Yes.
1996 Detail e as well as to Figure 5-6 Details f and f-1?

Question 2: Does the outward extension limitation at a maximum of Reply 2: Yes. An agenda item will be issued by the committee to
16t of the plate compression ring in Figure 5-6, Detail e apply as well clarify Figure 5-6.
to Figure 5-6 Details f and f-1?

620 9th - Feb. Figure 5-6 620-I-15/00 Question 1: Is the dimension, wh, of Figure 5-6 Details f and f-1, a Reply 1: No
1996 limitation on the greatest inward physical projection that a plate
compression ring may have?

Question 2: Is the dimension, wh, of Figure 5-6 Details f and f-1, a Reply 2: Yes.
limitation on the “effective” portion of the plate compression ring
that may be considered as contributing to the required cross-
section area, Ac?
Reply 3: Yes.
Question 3: Is the dimension, wh, of Figure 5-6 Details f and f-1 also
applicable to Figure 5-6 Details e and g?

620 9th - Feb. Figure 5-6 620-I-16/00 Are the roof plates of Figure 5-6 Detail e not permitted to contribute Yes.
1996 to the required cross-sectional area of the compression region
because of the lap weld between the roof and the compression-ring
as stated in Section 5.12.2 and Note 17?

620 9th - Feb. Table 5-1 Can the allowable stresses for materials not covered in Table 5-1 be No. See Appendix B for the use of non-listed materials
1996 calculated using Footnote 2?
620 9th - Feb. Table 5-1 Can Footnote 2 in Table 5-1 be used to calculate the allowable No.
1996 stresses for stainless steel material?
620 9th - Feb. Table 5-1 Can Footnote 2 in Table 5-1 be used to calculate the allowable No.
1996 stresses at elevated temperatures (say "t") by using the minimum
ultimate tensile strength and the minimum yield point at the
elevated temperature t?

Page 3 of 15
 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. Table 5-2 620-I-03/00 Table 5-2 lists single welded butt joints with backing strip as having Yes, however radiography is not required for E = 0.7 if
1996 75% basic joint efficiency. A spot radiographed joint is also listed at nonmetallic or temporary metallic backing strips are used.
75%. Are all single welded butt joints with permanent metallic
backing strip not more than 1 1/4 in. thick required to be spot
radiographed as a minimum?

620 9th - Feb. Table 5-2 620-I-01/01 Does the magnetic particle examination covered by Note 3 of Table No.
1996 5-2 in API-620 apply to the butt weld of a nozzle neck to a weld neck
flange?

620 10th - Feb. Table 5-2 620-I-10/02 Question 1: Do the specified thicknesses in Table 5-2 refer to the Reply 1: Yes.
2002 as-constructed thickness?

Question 2: Do the specified thicknesses in Table 5-2 refer to the Reply 2: Refer to Question 1.
actual thickness?

620 10th - Feb. 5.3 620-I-03/03 Does a coned roof have a minimum thickness similar to that noted Yes, see 5.3 and 5.10.4.1. Section 5.10.4.1 specifies the
2002 for the sidewall in 5.10.4.1 and bottom in 5.9.4.2? minimum thickness of the tank wall and 5.3 defines “tank wall” to
include the roof. Therefore, the minimum thickness of the roof is
3/16 in. for cone roofs.

620 9th - Feb. 5.3.3 Are there hydrostatic test guidelines for stresses allowed in API No. Refer to 5.3.3.
1996 620?
620 10th - Feb. 5.5.4.6 620-I-10/02 Question 1: Does the word "local" as used in 5.5.4.6, refer to the Reply 1: Yes.
2002 general bending stress across the whole cross section of the
cylinder (normal to its longitudinal axis) that is compressive on one
half of the cross section and tensile on the other half?

Question 2: In Section 5.5.4.6, can the sentence “For tanks full or Reply 2: Yes.
partially full of liquid and for an earthquake induced compression
stress, the allowable compression stress need not be limited for
biaxial stress as otherwise may be required by Figure 3 1” be
restated as: “For tanks full or partially full of liquid subjected to an
earthquake . . .”?

Question 3: In Section 5.5.4.6, can the sentence “For tanks full or Reply 3: No.
partially full of liquid and for an earthquake induced compression
stress, the allowable compression stress need not be limited for
biaxial stress as otherwise may be required by Figure 3 1” be
restated as: “For tanks full or partially full of liquid subjected to a
local axial compressive buckling stress in a cylindrical shell from a
moment in the cylinder, and also for: tanks subjected to an
earthquake induced compressive stress,
the allowable compressive stress need not . . .”

620 9th - Feb. 5.5.6 Is there a conflict between Section 5.5.6 and Section 5.6.5? No, Section 5.5.6 is in Section 5.5 and applies to walls. Tank
1996 wall is identified in A.3.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. 5.5.6 620-I-09/98 What is the maximum allowable compressive stress for a cylindrical For all combinations of design loadings per Section 3.4,
1996 tank subject to a combination of design loadings, including wind excluding wind and seismic, the maximum allowable
and seismic, both with and without internal gas or liquid contents compressive stress shall be limited to value, S cs, calculated per
per Section 3.5.6? The inquirer proposes that the maximum Section 3.5.4.2. For any combination of design loadings plus
allowable compressive stress be determined per Section 3.5.4.2 wind or seismic, the maximum allowable compressive stress
without any multiplication factor for design loadings, except may be increased to 1.33 times S cs. For seismic loading only,
seismic loadings, and for the test conditions. For seismic the maximum allowable compressive stress shall be per
conditions the inquirer proposes for b/12t greater than S cs that the Appendix L, Sections L.5.2 and L.5.3. For the test condition, API
design is acceptable if b/12t is less than or equal to F a. 620 does not provide specific guidance on the maximum
allowable compressive stress. The committee has an agenda
item to study revising the standard to address the maximum
allowable tensile and compressive stress for the test condition.

620 10th - Feb. 5.5.6 620-I-10/02 Is it necessary to apply an appropriate joint efficiency to the stress No. See Table 5-2 for maximum allowable efficiency for arc
2002 limits specified in 5.5.6, Q.8.1.3, and R.8.1? welded joints.

620 9th - Feb. 5.6.5 Does Section 5.6.5 allow an increase of 25% for seismic loading of No, this section addresses external structural members subject
1996 roof structural members? to stresses produced by combination of wind and other
applicable loads. The API 620 Standard does not address
seismic design of roof framing members, nor does it address
allowable stresses for seismic loading of roof framing.

620 9th - Feb. 5.7 620-I-08/98 Is corrosion allowance an absolute requirement under API 620? No. A corrosion allowance is a requirement only if specified by
1996 the purchaser. It is intended to provide for future corrosion over
the design life of the tank.
620 9th - Feb. 5.9.4.2 Does API 620 have a minimum thickness for the bottom of a flat Yes, see Section 5.9.4.2 and Q.3.4.7.
1996 bottom tank?
620 10th - Feb. 5.10.3.5 620-I-14/02 Does API 620 Section 5.10.3.5 address the requirements for conical Yes. Refer to 5.2.10.5.b and footnote 15 for further information
2002 roofs, since one of the R’s equals infinity? on this subject.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. 5.10.3.5 620-I-17/02 Background: Section 5.10.3.5, Step 1, applied to a no liquid in the
2002 roof, self-supported conical roof indicates an infinite thickness for
expression (18) whenever the negative latitudinal unit force T’ is
greater than the negative meridional unit force T’’ and R’ = R1 is
infinite. This suggests that design-by rule is not intended for
determining the roof thickness for such a geometry.

Question 1: Can the design rules of Section 5.10.3.5 be used to

determine the thickness of a self-supported conical roof whenever Reply 1: No.
the negative latitudinal unit force T’ is greater than the negative
meridional unit force T’’ and R’ = R1 is infinite?

Question 2: If the design rules of 5.10.3.5 cannot be used to

determine the thickness of a self-supported conical roof whenever Reply 2: Yes.
the negative latitudinal unit force T’ is greater than the negative
meridional unit force T’’ and R’ = R1 is infinite, may such a roof be
designed per 1.1 (second paragraph) and 5.1.1 (second sentence)?

620 9th - Feb. 5.10.2.5 620-I-02/00 Per API 650 Section 5.10.2.5.1 through 5.10.2.5.3, a tank with a No, the shell to be designed for the maximum liquid level and
1996 frangible roof may fail at shell-to-roof juncture before shell-to-shell internal pressure per Sections 1.1.1, 3.6.3, and F7.1
joint or shell-to-bottom joint. Does this mean that shell should be
able to resist pressure of 0.153 W/D2 (inches of water), resulting
from the equation for A in 5.12.5.3 in any tanks designed by this
section?

620 10th - Feb. 5.10.4.1 620-I-10/02 Question 1: Regarding the least permissible thickness described in Reply 1: Spherical radius if sphere. Maximum radius if unequal
2002 5.10.4.1.c and listed in Table 5-6, although not specified, it seems latitudinal and longitudinal radii.
that these are intended only for a cylindrical shell. This is confirmed
by comparing the above Table 5-6 to Section 3.6.1.1 of API 650 for a
cylindrical shell. If this is not correct then what will be radius of
tank wall for shapes other than cylindrical shell at various
elevations?

Question 2: Does 5.10.4.1c and Table 5-6 refer only to a cylindrical Reply 2: No. The radius for shapes other than cylinders is not
shell? If not, what is the definition of the radius for other than defined in Std. 620.
cylindrical shapes in Table 5-6?

620 10th - Feb. 5.10.5 620-I-04/03 What are the requirements for vacuum testing tanks built to API Tanks built to API 620 cannot be designed for vacuum, refer to
2002 620? API 620 Section 5.10.5.2. Therefore, the tanks are not tested
for vacuum.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. 5.12.5 620-I-10/02 Sections 5.12.5.2, 5.12.5.3, and 5.12.5.8 describe the requirements Yes.
2002 for added material when the code mandates such addition.
However, there may be cases where the additional material is not
required but is specified for safety reasons. Is it required to meet
the requirements in the aforementioned sections for a reinforced
compression area when such reinforcement is not required?

620 10th - Feb. 5.12.5 620-I-10/02 In Figure 5-5, if Wc is greater than the vertical leg length of the angle Yes.
2002 Fig. 5-5 or thickened bar, can the remaining portion of Wc be included as
participating in the included area of the compression ring?

620 9th - Feb. 5.12.5.2 620-I-02/00 Item (c) of API 620, Section 5.12.5.2, states if the compression area The horizontal centroid of the total cross-sectional area of the
1996 required is not provided in the compression-ring region with composite corner compression region shall not be off the
minimum thickness plates, then the additional required horizontal plane formed by the intersection of shell and roof by
compression area provided in the composite corner compression more than 1.5 times the average thickness of the shell (tc) and
region should ideally lie in the horizontal plane of the corner formed roof (th). If an angle is welded to the top of the shell and the roof
by the two members. In no case shall the centroid be off the plane attaches to the angle, than tc shall be equal to the thickness of
by more than 1.5 times the average thickness of the two members the angle and the corner is the intersection of the angle and the
intersecting at the corner. roof.

Does this mean that the horizontal centroid of the additional angle
excluding the effective shell (Wh) and roof (Wc) as per Fig. 5-6 detail
a, b, c, d, h and i, or the horizontal centroid of the total cross-
sectional of composite corner compression region lies ideally in the
horizontal plane of the corner formed by two members? In addition,
shall the centroid of the composite corner compression region not
be off the horizontal plane by more than 1.5 times the average
thickness of the shell (tc) and roof (th) or of the two members of any
additional angle?

620 9th - Feb. 5.12.5.2 620-I-02/00 Is it required to use a top angle for tanks designed per API 620 and No.
1996 API 650 Section F.7.2 and F.7 3 for the case where the effective area
of shell and roof can resist the Q-force and the horizontal projection
of Wh is at least 0.015Rc according to API 620 Section 3.12.5.2?

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. 5.12.5.8 620-I-10/02 Question 1: Is it required to meet L < 16t in Details f and f-1 of Reply 1: Yes.
2002 Fig. 5-6 Figure 5-6 to satisfy 5.12.5.8 when the projecting part is not braced
and I1 is not met?

Question 2: Is it correct for Detail f and f-1 of Figure 5-6 that Wh Reply 2: No.
minus L should not exceed 16t? Is it required to meet this criterion
also for Detail e?

Question 3: In Details e, f, and f-1 of Figure 5-6, should the outer Reply 3: Yes.
extended part be treated as a bar, as defined in item b of 5.12.5.2,
5.12.5.3, and 5.12.5.8?

Question 4: In Details e, f, and f-1 of Figure 5-6, should the outer Reply 4: No.
extended part meet Equation 28 when required by 5.12.5.8?

620 9th - Feb. 5.15 620-I-05/01 Does API 620, Section 5.15, permit the two openings to be one Yes. API 620 does not stipulate the location of the two
1996 opening in the roof and one opening at the bottom of the side wall? openings.

620 9th - Feb. 5.16.5 Does “t” as determined by calculation method detailed in Section Yes.
1996 5.10 apply to roofs and bottoms as well as sidewalls with respect to
calculating nozzle reinforcement as defined in 5.16.5?

620 9th - Feb. 5.17.1 620-I-03/01 Does Section 5.17.1b require the spacing between a fillet weld and The API committee responsible for this standard has issued an
1996 the shell plate fillet weld be spaced some specified distance apart? agenda to address this issue. Any changes resulting from this
agenda item will appear in a future edition or addendum to API
620.
620 9th - Feb. 5.26.2 620-I-03/98 Referring to API 620, if a reinforcement pad covers a vertical or Depending on the thickness of the tank shell, or on the joint
1996 horizontal weld seam of a tank, is radiographic examination or other efficiency of the design, radiographic examinations may be
NDE required at the weld seam before installing the reinforcing required. Refer to 5.26.2. However, API 620 does not specify the
pad? timing of this examination, deferring to the practical aspects of
accessing the weld if a repair is necessary. Note, for tanks built
to Appendices Q or R, reinforcing pads are not permitted to
cross weld seams.

620 9th - Feb. 5.26.2 Does API 620 require full radiography of a butt-welded tank wall a) Yes, full radiography is required, because a joint efficiency of
1996 when the wall thickness is designed using a joint efficiency of 1.0, 1.0 is being used, and, per Section 5.26.2, both the thickness
for a) the case of wall thickness, t, greater than 1-1/4 inch and S t and stress level requirements are met. b) Full radiography is
greater than 0.1 Ts, b) the case of t less than or equal to 1-1/4 inch required, because the joint efficiency of 1.0 is being used, per
and St greater than 0.1 Ts, and c) the case of t greater than 1-1/4 Table 3-2. Otherwise, spot radiography could be used, per Table
3-2 and Section 5.26.2. c) Full radiography is required because
inch and St less than 0.1 Ts? the joint efficiency of 1.0 is being used, per Table 3-2.
Otherwise, spot radiography could be used, per Table 3-2 and
Section 5.26.2.

620 9th - Feb. 5.26.3.2 620-I-02/01 Does Sections 5.26.3.2 require that the butt joints of the dome steel Yes, unless exempted by 5.26.2.
1996 roof compression ring be radiographed?

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. 6.4 620-I-01/00 Does API 620 require that shell plates be rolled prior to erection to No, unless forming impairs the mechanical properties. Refer to
1996 the design radius? Section 6.4 of API 620

620 9th - Feb. 6.4 Does API 620 allow higher allowable stresses for structural Yes, if the bolting material is not specified in API 620 and is of
1996 connections than provided in Table 5-3 (reference Footnote 13 higher tensile and yield strength.
Section 6.4)?

620 9th - Feb. 6.5.8 Referring to the Ninth - Addendum 1 of API 620, Section 6.5.8, is "D" Yes.
1996 intended to be the diameter of the roof (twice the radius of the roof),
when considering the tolerance of a double-curvature roof for a
cylindrical tank?
620 9th - Feb. 6.5.8 Does the term "outside (or inside) the design shape" mean "outside Yes.
1996 (or inside) the theoretical shape"?

620 9th - Feb. 6.13 Regarding API 620, Section 6.13, what is the interpretation of the The smaller undercut criteria would apply for welds
1996 words "longitudinal or meridional" and "circumferential or perpendicular to the direction of the larger primary stress. The
latitudinal" when applied to vertically erected and welded flat larger undercut criteria would apply to the welds perpendicular to
plates? the direction of the smaller primary stress, assuming there is a
2:1 ratio of stress. If not, then the smaller undercut would apply
to all welds.

620 9th - Feb. 7.16 620-I-07/00 Does API 620 address the maximum local deviation/weld distortion No.
1996 on a 6 mm thick bottom plate resulting from the full penetration
butt-welds in transverse and longitudinal butts?

620 9th - Feb. 7.17.1.3 620-I-02/01 If the requirements of magnetic particle testing of Section 7.17.1.3 No.
1996 are met, is additional radiographic examination of these joints
required?

620 9th - Feb. 7.17.3 Does API 620 require compliance with Section 7.17.3.2 or 7.17.3.4 As stated in Section 7.17.3.1, the requirements in 7.17.3.2
1996 for main seam spot radiography? through 7.17.3.4 must be satisfied.

620 10th - Feb. 7.18 620-I-10/02 Question 1: Does API 620 contain rules for allowable stresses Reply 1: No.
2002 during a hydrostatic test?

Question 2: If the reply to Question 1 is no, does that mean that the Reply 2: No, see 5.3.1.2.
unspecified hydrostatic test stresses are not required to be
checked?

Question 3: If the reply to Question 2 is no, then does that mean Reply 3: No.
that the unspecified hydrostatic test stresses are to be the same as
those during operation?

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. 8.2 620-I-04/01 Question 1: Does the term erector, as used in Section 8, mean the Reply 1: Yes.
1996 party that is responsible for field assembly will complete welds on
the tanks during that assembly?

Question 2: For tanks built to API 620 that are completely welded Reply 2: API 620 covers field-assembled tanks, not shop-built
out in a fabricator’s facility with field installation consisting of tanks. See Section 1.1 of API 620. Appendix J of API 650
another company setting and anchoring without performing any prescribes requirements for shop-built tanks, including testing
welds on the tanks, does the responsibility for the hydrostatic test and division of responsibility for same.
rest with the fabricator or the installer?

620 10th - Feb. F.5.2 620-I-03/04 In Section F.5.2 Example 2, the formula for trn. Should the value Yes. This error will be corrected in a future edition/addendum to
2002 shown of 29.9 lbf/in.2 be 24.9 lbf/in.2? When one solves the formula the standard.
using 29.9 lbf/in.2 they get a result of 0.0215 in., using 24.9 lbf/in. 2
one gets the result shown in the standard of 0.018 in.

620 9th - Feb. L.3.3.1 Can values lower than given for C1 and C2 in Sections L.3.3.1 and No, not if the seismic design is to be in agreement with API 620
1996 L.3.3.2 L.3.3.2 be used by agreement between purchaser and Appendix L.
manufacturer?
620 9th - Feb. Appendix L 620-I-17/00 Question 1: Is vertical acceleration considered in API 620 Appendix Reply 1: No.
1996 L.

Question 2: Please advise how to consider vertical acceleration. Reply 2: API does not provide consulting on specific
engineering problems or on the general understanding of its
standards. We can only provide interpretations of API 650
requirements or consider revisions based on new data or
technology. You may wish to review a procedure in AWWA
D100-96 for further information.

Question 3: If specified, should vertical earthquake force be taken Reply 3: Yes.

into consideration?

620 9th - Feb. Appendix N 620-I-04/02 Background: API 620, Appendix N, states discharge piping on the No, the requirements of both standards must be met.
1996 outlet side from pressure-relieving devices must have a minimum
area equal to the valve outlet. In API Std. 2000, Section 4.5.3, no
mention is made of this constraint on the minimum area required;
only a mention of the backpressure being included in the relief
device sizing calculations.

Question: Does API 2000 allow you not to comply with the
discharge piping area requirements of API 620 Appendix N?

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. Appendix Q 620-I-05/00 Can a tank containing condensate consisting of methanol and other Yes, by agreement between the owner and the manufacturer,
1996 organic compounds (methanol concentration, 20,000 ppm, i.e. not provided the tank is field erected. If the tank is shop fabricated,
flammable) with pressure range of ± 35 in. water column and it should fabricated per API 650 Appendix J, S, and F.1.3. The
fabricated out of SA-240 Type 304I be built per API 620, Appendix Q? minus 35 in. water column design pressure is beyond the scope
of both API 620 and 650. Additional design methods, by
agreement between the owner and manufacturer, must be used
to properly address this external pressure.

620 9th - Feb. Q.1.3 Can Section Q.8.1.3 be used for tanks covered by the basic design API 620 does not address the use of Appendix Q materials for
1996 rules in API 620 but do not require the use of Appendix Q? non-Appendix Q tanks. The committee currently has an agenda
item to study adding rules to permit the use of stainless steels
for tanks designed to the basic rules of API 620. Any changes
resulting from this agenda item will appear in a future edition or
addendum to API 620.

620 10th - Feb. Q.1.4 620-I-16/02 Background: We are constructing two double-walled LNG storage
2002 tank designed per API 620, Q.1.4. Both tanks are full containment
type and the inner tank is made of 9% Ni material with a suspended
deck closing the inner tank made of aluminum. Inside the inner tank
there are several stainless steel structural elements (platforms,
stairs, supports...) submerged in the LNG. As the service
temperature of the suspended deck and of all the elements
(platforms, stairs, supports . . .) into the inner tank in contact with
the LNG or the product vapours is lower that -60F (between -60F
and -270F), my understanding is that the mentioned suspended
deck and structural elements shall be considered as primary
component.

Question 1: Is a roof of an API 620 Appendix Q inner tank that is in Reply 1: Yes. See Q.1.4.1.
contact with LNG and is stressed by liquid and/or vapor pressure
considered to be a primary component?

Question 2: Is a deck of an API 620 Appendix Q inner tank that Reply 2: The component addressed is not specifically covered
is designed to support insulation above the inner tank product within API 620; however, material must be selected for
level and is not stressed by liquid or vapor pressure compatibility with service conditions.
considered a primary component?

Question 3: Are structural components, such as ladders and Reply 3: Ladders and platform requirements are not addressed
platforms, exposed to LNG at temperatures less than -60ºF in API 620; however, attachments to walls (reference footnote 1)
considered primary components? must be considered as primary components.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. Q.6.6.4 620-I-01/02 For low-temperature and cryogenic tanks constructed under the No.
2002 provisions of API 620, Appendices Q and R, when circumferential
joints are welded by a technique where the weld is produced by a
machine but it is constantly observed and various adjustments are
periodically made by a welding operator, do sections Q.6.6.4 and
R.6.3.4 require production weld tests for the circumferential welds?

620 10th - Feb. Q.7.3.1 620-I-01/04 Question 1: Per API Standard 620, does stiffening ring angle that is Reply 1: Yes. See Q.7.3.1
2002 cold formed from 9% Nickel flat sheet require postweld heat
treatment when the fiber strain from cold forming exceeds 3%?

piping that is cold formed from 9% Nickel flat sheet require

postweld heat treatment when the fiber strain from cold forming Reply 2: Yes. See Q.7.3.1
exceeds 3%?

620 10th - Feb. Q.7.6.1 620-I-02/02 When the operating membrane stress on both vertical and Item (a) is required by Q.7.6.1.
2002 horizontal weld seams is greater than 0.1 times the specified
minimum tensile strength of the material, which of the following
does Q.7.6.1 of API 620 require:

(a) Complete joint radiography of both vertical and horizontal weld

seams, or;
(b) 100% radiography of the vertical seams, all T-joints, and Spot
radiography of the horizontal seams, per Figure Q-2?

620 10th - Feb. Q.8.1.3 620-I-10/02 Question 1: Section Q.8.1.3 in Appendix Q places a limit on stress Reply 1: Yes.
2002 as “the maximum fill shall not produce a stress in . . .”. Is it correct
to read the “stress” in the above sentence as “membrane tensile
stress” similar to R.8.1 in Appendix R?

Question 2: Is it true that the allowable hydrostatic test stress in Reply 2: Yes.
Q.8.1.3 refers only to tensile stress and not to a compressive
stress?

Question 3: Appendix Q has not defined allowable stresses for Reply 3: No.
hydrostatic test conditions except in Q.8.1.3. Is it intent of code that
the unspecified hydrostatic test stresses are not required to be
checked?

Question 4: If the reply to Question 3 is no, is it intent of code that Reply 4: No.
the unspecified hydrostatic test stresses should be equal to those
for operating conditions?

Page 12 of 15
 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. Q.8.3 620-I-03/02 Question 1: Regarding the quality of test water specified in the Reply 1: API 620 does not define the chloride content for test
2002 section Q.8.3, does the test water in a tank built with 9% Ni steel water in 9% Ni steel tanks.
have to comply with the 50 ppm chloride content limit?

Question 2: Please advise us on what would happen in the tank if Reply 2: API does not provide consulting on specific engineering
we use water having a chloride content of more than 50 ppm. problems or on the general application of its standards.
Technical inquiries submitted to API must be limited requests for
an interpretation of the requirements in a standard, or to the
consideration of revisions based on new data or technology

620 9th - Feb. Table Q-3 Can the allowable stress (test) indicated in Table Q-3 be used for API 620 does not address the use of Appendix Q materials for
1996 tanks that do not require the use of Appendix Q, i.e. for the storage non-Appendix Q tanks. The committee currently has an agenda
of demineralized water, etc.? item to study adding rules to permit the use of stainless steels.
Any changes resulting from this agenda item will appear in a
future edition or addendum to API 620.
620 10th - Feb APP Q 620-I-04/06 Yes
05

Does an all weld metal tensile test defined in API 620 Appendix Q
exclude the heat affected zone?
620 10th - Feb APP Q 620-I-04/06 In the case of a LNG storage tank fabricated in accordance with API No refer to API 620 Section Q.6.1 and Table Q-3-Note a.
05 620 Appendix Q using A553 Type 1 base metal, can the 2 all-weld-
metal specimens required for the welding procedure qualifications
defined in Appendix Q, be omitted when the plate thickness of the
tank shell
Clause is too small?
References: 1) API 620, Appendix Q .5.3 (Minimum Wall
620 10th Feb 05 APP Q 620-I-02/06 Requirements) 2) API 620, Appendix Q.5.3.1 (Outer Tank)

When corrosion allowance is specified, is it required to add the

corrosion allowance to minimum nominal thickness (3/16”)?
Yes, Refer to Section 5.10.4.1
620 9th - Feb. Appendix R Is a tank designed and fabricated in accordance with API 620 Yes.
1996 Appendix R required to be tested per API 620 Section 5.18.25?

620 9th - Feb. Appendix R 620-I-01/99 Does API 620, Appendix R permit the use of the local stress-relief No. The PWHT is must be done in a furnace to achieve uniform
1996 method specified in the ASME Section VIII, Div. 1, UW-40(a)(3) for heat treatment. See Section 6.18.2.
the stress-relieving a nozzle-neck-to-shell weld in a shell plate that
has been welded to another shell plate?

620 10th - Feb. Appendix R 620-I-12/02 Question 1: Referring to API 620, Appendix R, do both longitudinal Reply 1: No.
2002 and circumferential joints have to be 100% radiographed?

Question 2: Referring to API 620, Appendix R, do only longitudinal

joints have to be radiographed? Reply 2: Yes.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 9th - Feb. R.3.4.6 620-I-03/99 Question 1: Referring to API 620, Appendix R, Section R.3.4.6, does Reply 1: No.
1996 lapping of two bottom plates on butt-weld annular plates constitute
a three-plate lap weld?

Question 2: If the answer to Question 1 is no, is there a minimum Reply 2: No.

distance a two-plate lap weld must be from the butt welds in an
annular plate?

620 10th - Feb. R.6.3.4 620-I-01/02 For low-temperature and cryogenic tanks constructed under the No.
2002 provisions of API 620, Appendices Q and R, when circumferential
joints are welded by a technique where the weld is produced by a
machine but it is constantly observed and various adjustments are
periodically made by a welding operator, do sections Q.6.6.4 and
R.6.3.4 require production weld tests for the circumferential welds?

620 9th - Feb. R.7.3 620-I-11/98 If a tank complies with R.7.3, parts a and b, but not with part c, is No
1996 PWHT required?

620 9th - Feb. R.7.3.1 Referring to API 620, Section R.7.3.1, is stress relieving of a nozzle No. Section R.7.3.1 requires all nozzle assemblies in primary
1996 assembly prior to installation required only if the nozzle assembly is components to be stress relieved, with specific exceptions. The
fitted with a thickened insert plate? use of a thickened insert plate is not a factor.

620 9th - Feb. R.7.3.1 Is local stress relieving acceptable if the above nozzle is welded No, the rules in Appendix R take precedence over those in the
1996 directly to the shell with a reinforcing pad? basic document. See Section R.7.3.1.

620 10th - Feb. R.7.6.1 620-I-11/02 Does R.7.6.1 apply to horizontal shell joints? Yes.
2002

620 9th - Feb. R.8.4 Is a tank designed and fabricated in accordance with API 620 Testing meeting requirements of both sections is required.
1996 Appendix R to be vacuum tested per Section R.8.4.4 or Section
5.18.5?
620 10th - Feb. Figure R-2 620-I-11/02 Does Figure R-2 of API 620 represent all the requirements of
2002 R.7.6.1?

620 10th Feb 05 App S 620-I-03/06 No. Refer to API 620 Table 5.2
Can a stainless steel tank shell be fabricated in accordance with API
620 Appendix S without radiography and a joint efficiency of 0.70?
620 10th - Feb. 650 APP P 620-I-01/05 Does API 620 have allowable nozzle load guidelines similar to API No
2002 650 Appendix P
620 10th - Feb. Q6.1 620-I-06/03 1. Does API 620 require that weld procedures be qualified by Yes
2005 standard qualification tests in accordance with ASME Section IX,
without any all-weld tensile tests, except for the additional tests
required by Question # 2 below?
620 10th - Feb General 620 620-I-04/06 These terms are defined in API 620 Section 3.4.15 and 3.4.6
05 What is the definition of weld metal and heat affected zone? respectively.

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 API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks
Last update: July 31, 2006
Standard Edition Section Inquiry # Question Reply

620 10th - Feb. General 620 2a. Does API 620 require in addition to the requirements in Section Yes a single test, consisting of two all weld metal specimen, for
2005 6.7, the testing of two all weld metal specimen that conform to the each weld procedure is required when any of the conditions
dimensional standard of Figure 9 of AWS A5.11, to establish the outlined in Section Q.6.1 exist
allowable stress to be used in the design of the tank?

620 10th - Feb. General 620 2b. Does API 620 section Q6.1 require that the welding and testing API Section Q6.1 does not specify the party responsible for the
2005 of the two all weld metal specimen be performed by the fabricator. welding and testing of the all weld metal specimens required in
Section Q.6.1. The only case that would require the fabricator to
perform the test is when the welding procedure qualification test
shows the deposited weld metal tensile test strength is lower
than the specified minimum ultimate tensile strength of the base
metal.

620 10th- Feb. General 620 3. Does API 620 require or permit that only the minimum yield No. Both the minimum yield and tensile strengths shall be
strength of the weld metal be determined when the specified determined. Refer to Q.6.1.
minimum ultimate tensile strength is equal to the specified
minimum UTS of the base metal?
620 10th Feb 05 General 620 620-I12/06 Is the term knuckle region defined in API 620? No, however, refer to Figure 5-9 Panel C

Page 15 of 15