Changes to API 650, Eleventh Edition
Welded Steel Tanks for Oil Storage
By Vincent A. Carucci
Two addenda have been issued for the Eleventh Edition of API 650 since it
was published in June 2007. Addendum 1 was issued November 2008, and
Addendum 2 was issued November 2009. The following highlights several of
the changes that were made and is not all inclusive. Refer to API 650 for
Section 1: Scope
- Table 1-1 includes the titles of new appendices that have been
added to API 650:
- Appendix EC – Commentary on Appendix E
- Appendix SC – Stainless Steel and Carbon Steel Mixed Material
- Appendix X – Duplex Stainless Steel Tanks
- Appendix AL – Aluminum Storage Tanks
- A new Para. 1.5 requires that consistent units be used where units
are not defined in formulas. I have not checked all the formulas, but
presumably there are cases where this occurs. Perhaps a future
revision will define the units in each case to avoid potential errors?
Section 2: References
The following additional references have been added:
- API RP 582 – Welding Guidelines for the Chemical, Oil and Gas
- API Publ 937 – Evaluation of the Design Criteria for Storage Tanks
with Frangible Roofs
- API Publ 937-A – Study to Establish Relations for the Relative
Strength of API 650 Cone Roof, Roof-to-Shell, and Shell-to-Bottom
Section 4: Materials
- Figure 4-1 that specifies the MDMT curves for materials has been
replaced by Figures 4-1a and 4-1b in SI and US Customary Units,
- New Tables 4-3a and 4-3b provide linear equations for the curves
contained in Figures 4-1a and 4-1b, respectively. The addition of
these tables resulted in renumbering of subsequent tables in this
- New Para. 4.4.4 permits the use of weldable-quality pipe for
structural purposes with its physical properties and allowable
stresses as stated.
- Separate Tables 4-5a and 4-5b are now included that specify impact
test requirements for steel plates in SI and US Customary Units,
Section 5: Design
- Para. 5.2.1b now permits a 0 kPa (0 in. water) design external
pressure for internal floating roof tanks with circulation vents that
meet Appendix H requirements.
- New Para. 5.2.1e specifies internal floating roof design loads.
Subsequent paragraphs in Para. 5.2.1 have been renumbered.
- Para. 5.2.1f permits the minimum roof live load to be determined
per ASCE 7 as an alternative.
- Para. 5.2.1g provides more requirements for determining the
general snow load.
- Table 5-1 that specifies annular bottom-plate thicknesses has been
separated into Table 5-1a and
5-1b for SI and US Customary Units, respectively.
- Para. 220.127.116.11 has been modified to permit shell thicknesses to be
determined by an elastic analysis if the allowable stress for an upper
shell course is less than the allowable stress of the next lower shell
course. This approach may be used as an alternate to the “historical”
requirement that the lower shell course cannot be thinner than the one
- Table 5-2 that specifies permissible plate materials and
corresponding allowable stresses has been separated into Tables 5-2a
and 5-2b for SI and US Customary Units, respectively.
- Para. 18.104.22.168 provides more details covering insert plate
requirements for shell openings.
- Generally speaking, the tables that specify the dimensional
requirements for openings (e.g., 5-3, 5-4, etc.) were separated into
two tables each for SI and US Customary Units, respectively.
- Para. 22.214.171.124 for an opening in the cover plate of flush-type
cleanout fittings specifies requirements for its location and design.
It also points out that the free movement of connected piping and
applied piping loads must be considered.
- Para. 5.8.4 requires that loads applied to roof manholes and
supporting roof structure be considered.
In Tables 5-20a and 5-20b, Paras. 126.96.36.199, 188.8.131.52, etc. (i.e., where
top and intermediate wind girder requirements are specified), the
definition for “shell thickness” was generally changed from
“as-ordered” to “as-built” thickness. Of course, the “as-built” shell
thicknesses represent the actual strength of the shell, and these
should be at least equal to the “as-ordered” thicknesses.
- Para. 184.108.40.206 covering frangible roof joint requirements is now
divided based on tank diameter – tanks 50 ft. (15 m) diameter or
greater, self-anchored tanks with a diameter greater than or equal to
50 ft. (9 m) but less than 50 ft. (15 m), and alternate requirements
for self-anchored tanks less than 50 ft. (15 m) diameter. These
modified requirements recognize the long-known fact that the
“historic” API 650 frangible joint requirements did not ensure
frangible joint behaviors for tanks less than 50 ft. (15 m) in
diameter. The revised requirements are based on a R&D program that was
done on behalf of API.
- Para. 220.127.116.11 has been changed by now using an equation to
determine the maximum permitted roof plate space between rafters for
supported cone roof tanks.
- Para. 18.104.22.168 covering the required roof plate thickness for
self-supporting cone roofs has been changed to also consider two
different ways to consider snow loads. A similar change was made in
Para. 22.214.171.124 for self-supporting dome and umbrella roofs.
- The equation in Para. 126.96.36.199 to calculate the participating area
of the roof-to-shell joint of self-supporting cone roof tanks has been
changed. It now explicitly accounts for the material strength. A
similar change was made in Para. 188.8.131.52 for self-supporting dome and
- Tables 5-21a and 5-21b that specify the design uplift loads for
anchored tanks now include a formula to account for the frangibility
pressure for tanks with frangible roofs. The formulas that account for
wind and seismic loads have also been changed.
Section 10: Marking
Specific markings are now identified to be used depending on where thermal
stress relief has been done.
Appendix AL: Aluminum Storage Tanks
A new Appendix AL has been added covering material, design, fabrication,
erection and testing requirements for tanks constructed of aluminum and
Appendix B: Recommendations for Tank Foundations
A Para. B.6 has been added that identifies foundation considerations for
tanks operating at elevated temperatures [ > 200°F (93°C)].
Appendix E: Seismic Design of Storage Tanks
Several of the formulas contained in this Appendix have been changed.
Among them are those contained in Paras. E.4.5.1, E.4.6.1, E.4.6.2,
E.6.1.4, E.184.108.40.206, and E.7.2.
Appendix EC: Commentary on Appendix E
This new appendix provides background information on the development of
the requirements contained in Appendix E. It also includes several example
problems that illustrate the application of the requirements.
Appendix F: Design of Tanks for Small Internal Pressures
- The formulas in Para. F.4.1 have been changed to explicitly
account for the lowest minimum specified yield strength (modified for
design temperature) of the materials in the roof-to-shell junction.
Prior editions of API 650 implicitly included an assumption of the
material strength. The nominal roof thickness used in the calculation
can also now be adjusted if needed to account for stiffeners welded to
the roof plate.
- Figure F-2 that specifies permissible details of compression rings
has been modified regarding the length of the top angle that may be
considered as contributing to the reinforcement area. It now also
considers the yield strength of the material.
- The formulas in Para. F.5.1 have been changed to explicitly
account for the material strength, comparable to what was done in
- Para. F.7.2 requires that full penetration butt welds shall be
used to connect sections of the compression ring. It also states that
the compression ring material may be selected from Section 4 of API
650 but does not need to meet toughness criteria.
- Para. F.7.3 provides more explicit and additional requirements for
the design and welding of roofs and the design, reinforcement, and
welding of roof manholes and nozzles.
Appendix G: Structurally Supported Aluminum Dome Roofs
- Table G-1, Bolts and Fasteners, has been split into G 1a and G-1b
for SI and US Customary Units, respectively.
- The formula in Para. G.4.1.3 to calculate the allowable general
buckling pressure was changed to explicitly account for the Modulus of
Elasticity of the dome frame members.
- Para. G.4.1.4. Formula for the required net tension ring area was
changed to account for the maximum pressure calculated per Para.
R.1.(e) for gravity loads.
- Para. G.4.2 covering design loads was completely revised.
Appendix H: Internal Floating Roofs
- Para. H.220.127.116.11 has been changed regarding required load
combinations for the design of floating roof supports.
- New Para. H.18.104.22.168 provides allowable load requirements for roof
- New Paras. H.4.6.10 through H.4.6.13 provide additional
requirements for fixed roof supports for the operating roof position
and cable supports.
- Para. H.5.2.1 covering pressure-relieving vent requirements has
been divided into sub-paragraphs with several requirements changed or
Appendix K: Sample Applications of Variable Design Point Method
There is now a second example that provides complete calculations for both
the design and hydrotest calculations.
Appendix P: Allowable External Loads on Tank Shell Openings
Para. P.3 that provided an alternative procedure for evaluating external
loads considering WRC Bulletin 297 was completely deleted.
Appendix S: Austenitic Stainless Steel Storage Tanks
- Tables S-1a and S-1b include additional materials.
- Para. S.3.1.1 now specifies the minimum shell-to-bottom plate size
as a function of nominal shell plate thickness.
- New Para. S.3.1.3 specifies requirements for butt-welded annular
- Para. S.3.4 specifies several additional requirements for roof
design and roof manholes.
- Tables S-2a and S-2b contains the allowable stresses for the
additional materials that were added to Tables
S-1a and S-1b.
- Para. S.4.11 covering welding has more sub-paragraphs with
additional welding requirements.
Appendix SC: Stainless and Carbon Steel Mixed Materials Storage Tanks
This new appendix covers materials, design, fabrication, erection, and
testing requirements for tanks constructed with stainless steel and carbon
Appendix V: Design of Storage Tanks for External Pressure
- Para. V.3.1. The formula to calculate the maximum wind pressure,
W, has been changed but the result has not. The new equation merely
directly incorporates the wind gust and wind height factors which had
been specified as constant values in the prior formula.
- Para. V.7.3.3 and V.7.3.4. The formulas used to determine the
length of roof and shell considered to be within the
tension/compression ring region of self-supporting dome or umbrella
roofs have been changed.
- Paras. V.8.1, V.8.1.2, V.8.1.3. The procedure and formulas used to
determine the allowable external pressure for an unstiffened shell
have been changed.
- Paras. V.22.214.171.124, V.126.96.36.199. The formulas used to determine the
maximum permitted spacing and required number of intermediate shell
stiffeners have been changed.
Appendix X: Duplex Stainless Steel Storage Tanks
A new appendix has been added to cover materials, design, fabrication,
erection, and testing requirements for tanks constructed of duplex
Appendix Y: API Monogram
A new appendix provides information on use of the API Monogram.