Changes to API 650, Twelfth Edition - Welded Tanks for Oil Storage
By Vincent A. Carucci
Three addenda were issued for the Eleventh Edition of API 650 in 2008,
2009, and 2011 since it was first published in June 2007. The Twelfth
Edition of API 650 was published in March 2013. The following highlights
several of the changes that were made and is not all inclusive. Refer to
API 650 for complete information. One general comment is that sections
that were previously called “Appendix” are now called “Annex.”
Section 2 – Normative References
ASCE Standard 7-10, “Minimum Design Loads for Buildings and Other
structures” has been added in addition to ASCE Standard 7-05 of the same
title. Both standards are referenced later as alternatives that may be
used to determine the appropriate design wind speed to use for tank
Section 3 – Terms and Definitions
- A definition for “examiner” has been added and is a person
qualified and certified to perform a specific NDE method. This makes a
distinction between an examiner and an inspector, who is the person
taking responsibility for compliance with API 650 requirements. Word
changes from “inspection” to “examination,” etc. were made throughout
the standard as appropriate, many of which are highlighted below.
- The definition of “inlet diffusers” was modified to better
describe their function.
Section 4 – Materials
- Para. 4.7.b. Additional requirements and another option were added
for anchor bolting.
Section 5 – Design
- Para. 184.108.40.206. Additional situations were added where attachments
to tank exterior surfaces may be attached by intermittent rather than
- Para. 5.2.1b. Clarifies that tanks that meet API 650 requirements
may be subject to a partial vacuum of 0.25 kPa (1 in. of water)
without needing calculations to support this.
- Para. 5.2.1k. Both ASCE 7-05 and ASCE 7-10 are now referenced as
acceptable alternatives for determining the design wind speed.
Explicit reference to a 190 km/hr (120 mph) design wind velocity as a
“base case” is not included. However, 190 km/hr (120 mph) is still
implied when calculating a design wind pressure in this paragraph and
later in wind girder calculation requirements.
- Para. 5.2.2. This is a new paragraph that defines the various load
combinations to be considered in the storage tank design (e.g., dead
load of tank, weight of stored liquid, wind, internal or external
pressure, etc.). These had been in an Appendix R, which has been
- Para. 220.127.116.11. States that inlet diffusers may be specified by
either the purchaser or floating roof manufacturer. It was previously
implied that only the purchaser could do this. It also provides some
information regarding diffusers.
- Para. 18.104.22.168. Explicitly defines the allowable stress basis to
use in the “participating area” equation for self-supporting cone
roofs, rather than just referencing another standard.
- Para. 22.214.171.124. Same approach used as in Para. 126.96.36.199, but this
time for self-supporting dome and umbrella roofs.
- Para. 5.11.2. Additional uplift criteria are specified that must
be satisfied for unanchored tanks.
- Tables 5.21a/b. The allowable anchor bolt stress and shell stress
at the anchor attachment have been changed from explicit values to
functions of the bolt or bottom shell course yield strengths,
Section 7 – Erection
- Para. 188.8.131.52. Clarifies shell-to-bottom weld inspection
requirements (e.g., whether inside or outside weld to be done first
considering inspection method used).
- Para. 7.3. Heading changed to not incorrectly imply that “testing”
requirements are included here.
- Para. 7.3.2. Heading changed to “Examination of Welds” from prior
“Inspection of Welds.” The same word change was also made in later
paragraphs, apparently to make a distinction between an “examiner” and
an “inspector” (consistent with addition of definition for “examiner”
previously noted in Section 3).
Section 8 – Methods of Examining Joints
Section title changed to above from prior “Methods of Inspecting Joints.”
The same word change was made in relevant paragraphs within this section.
Annex AL – Aluminum Storage Tanks
- Paras. AL.5.6.5 and AL.5.6.6. Explicitly define the allowable
stress basis to use for the required roof-to-shell junction area of
self-supporting cone, dome, and umbrella roofs.
- Para. AL.8. Heading changed from “Inspection of Weld” to
“Examination of Welds.”
Annex C – External Floating Roofs
- Para. C.3.1.6. Deleted NFPA 780 as an additional reference
standard for current carrying capacity of electrical shunts and
maintained reference to API RP 2003.
- Para. C.3.15. New paragraph to supply inlet diffusers, if
required, on the tank data sheet. Renumbered previous C.3.15
paragraphs as C.3.16.
- Paras. C.4.1, C.4.2, and C.4.4. Changed “inspection” or
“inspected” to “examination” or “examined.”
Annex D – Inquiries and Suggestions for Change
- Title changed to also include suggestions for change. Text
rewritten and expanded.
- Specific questions and replies contained in the prior edition were
deleted. Links to sections in the API web site are noted.
Annex E – Seismic Design of Storage Tanks
- Para. E.2.2. New definitions are provided for the convective
design response spectrum acceleration parameter and vertical
earthquake acceleration parameter.
- Para. E.6.1.3. This paragraph covering vertical seismic effects
has been revised.
- Para. E.184.108.40.206. Equation for calculating the total stress when
vertical acceleration is specified was revised.
Annex F – Design of Tanks for Small Internal Pressures
- Para. F.2. This new paragraph requires that shell, manhole, and
flush-type cleanout fitting thicknesses for Annex F tanks account for
the internal design pressure. This is done by increasing the value of
“H” (design liquid height) in the equations. This adjustment is not
required if the design pressure is less than 1 kPa (4 in. of water,
0.14444 psi). A similar requirement had been contained in Para. F.7.1
of the previous standard for anchored tanks. The current Para. F.7.1
is the previous Para. F.7.2.
- Para. F.4.2. Clarification provided that this paragraph and
associated equation only applies for anchored tanks.
Annex G – Structurally-Supported Aluminum Dome Roofs
- Para. G.9. Added statement that if screened peripheral venting is
provided between the roof and shell, its area may be considered to
contribute to the required emergency vent area.
- Para. G.11.3. Changed wording from “… visually inspected by…” to
“… visually examined by…” with respect to aluminum structural welds.
Annex J – Shop Assembled Storage Tanks
- Paras. J.5 and J.5.1. Wording changed from “inspection” or
“inspecting” to “examination” and “examining,” respectively.
Annex S – Austenitic Stainless Steel Storage Tanks
- Paras. 5.4.14, 220.127.116.11, and 18.104.22.168. Wording changed from
“inspection” to “examination.”
Annex SC – Stainless and Carbon Steel Mixed Materials Storage Tanks
- Para. SC.1.2. Maximum design temperature limit increased from 93˚C
(200˚F) to 260˚C (500˚F).
- Para. SC.3. The criteria for when a structural analysis of the
entire tank structure is required have been modified. These are used
when austenitic stainless steel is joined to either carbon steel or
duplex stainless steel, and consider tank design temperature and
Annex V – Design of Storage Tanks for External Premiere
- Para. V.3.1. Correction made to SI units used to calculate the
first moment of area of stiffener, qs - cm3
instead of mm3.
- Para. V.22.214.171.124. Equations used to determine the maximum spacing
between adjacent intermediate stiffeners have been revised.
- Para. V.8.2.4. The equations used to determine the strength of the
stiffener attachment weld have been augmented by adding equations for
SI units (previous equations were apparently only valid for USC
units). The USC equation was also changed in one case for unit
- Para. V.10.2. The example problem calculations were revised as
needed for consistency with changes to calculation details made
elsewhere in the annex.
Annex X – Duplex Stainless Steel Storage Tanks
- Para. X.1, X.2.3.1, Table X.1, and later tables. Material grade
2202 (UNS S32202) has been added.
- Paras. X.4.14 and X.4.14.1. Wording changed from “inspection” to