Changes in API 650, Welded Steel Tanks for Oil Storage
By Vincent A. Carucci
The following highlights several changes to API 650 that are contained
in Addendum 4 to the Tenth Edition dated December 2005.
- Para. 184.108.40.206. New paragraph (although not indicated as such)
defines “maximum design temperature.” The paragraph appears to be
misplaced as part of “toughness requirements.”
- Para. 220.127.116.11. Lap-welded bottom plates may now have either
“square cut” or “mill” edges (previously had to be square edged). Mill
edge requirements are specified.
- Para. 18.104.22.168. Requirements added regarding sizing of
shell-to-bottom fillet welds and minimum required distance between the
fillet weld and outside edge of the bottom or annular plate.
- Para. 3.2.1. New paragraph that defines the various loads that a
tank must be designed for (e.g., dead load, stored liquid design,
external pressure, etc.). Note that the design wind speed shall be 120
mph (190 km/hr), the 3-second gust speed determined from ASCE 7, or
the 3-second gust speed specified by the purchaser.
- Para. 3.2.2. Adds that the purchaser must also specify the
- Para. 3.2.5. Refers to a new Appendix V that contains design
requirements for tanks that are subject to partial vacuum conditions
exceeding 0.25 kPa (1 in. of water). Tanks that are designed to API
650 requirements are inherently adequate for 0.25 kPa (1 in. of water)
without the need for supporting calculations.
- Para. 3.4.2. Minimum bottom plate projection beyond the shell
changed from 1 in. (25 mm) to 2 in. (50 mm).
- Para. 22.214.171.124. Deleted requirement that the maximum allowable wind
velocity be calculated and reported to the purchaser if it was not
originally specified by the purchaser.
- Table 3-2. Added A 841M (A 841) Class 2 (Class 2) plate material.
- Para. 126.96.36.199. Clarified that thickness, t1, is defined
as excluding any specified corrosion allowance.
- Para. 188.8.131.52. Changed definition of t1 from
actual to calculated
bottom shell course thickness (less corrosion allowance). Also
clarified that t2a excludes any specified corrosion allowance.
- Para. 184.108.40.206. Clarifies that tu and TL exclude corrosion
- Para. 220.127.116.11. Clarifies that welding, in addition to
reinforcement, of shell openings that meets API 620 requirements is
- Para. 18.104.22.168. New paragraph specifies allowable stresses for the
welds that attach a nozzle and reinforcing pad to the shell.
- Table 3-6. Flanged nozzles NPS 1 and NPS 3/4 have been added.
- Para. 22.214.171.124. New paragraph allows shell nozzles without internal
piping to be either flush with or project beyond the inside shell
surface, unless the purchaser specifies them to be flush, or they are
for a floating roof tank.
- Paras. 3.9.6 and 3.9.7. Wind girder equations are changed to be
consistent with changed required design wind velocity basis
considering 120 mph (190 km/h) rather than 100 mph (160 km/h) and
- Para. 126.96.36.199. Refers to new Appendix R that defines load
combinations that roof and supporting structures must be designed for.
- Para. 188.8.131.52. Modified to permit roof plates of cone roof tanks
to be attached to support members if approved by the purchaser.
However, the roof cannot be considered frangible in that case.
- Para. 184.108.40.206. New paragraph requires that the design of roof
support members consider internal pressure when roof plates are
attached to the support members.
- Paras. 220.127.116.11, 18.104.22.168, 22.214.171.124, and 126.96.36.199. Equations used
to calculate the minimum required thickness of roof plates modified to
consider applied load combinations.
- Para. 188.8.131.52. Maximum internal pressure criteria are specified
that must be met when pressure relief devices are used to satisfy
emergency venting requirements.
- Paras. 3.11.1, 3.11.2 and 3.11.3. Tanks must be designed for
overturning stability (no longer needs to be specified by the
purchaser) and design criteria are specified.
- Para. 3.12.9. Maximum allowable stress basis specified for local
shell stress at anchor attachment point.
- Tables 3-21a and 3-21b. Allowable shell stress at anchor
attachment specified for each load case.
- Para. 4.1.3 and Figure 4-1. Shell plate shaping requirements
specified in a figure rather than a table.
- Para. 184.108.40.206 and Table 5-1. Minimum preheat temperatures
specified based on Material Group and plate thickness.
- Para. 8.1 and Figure 8-1. Revised to require specification of
external design pressure (if appropriate) and maximum design pressure
- Para. C.3.4.1c. Alternate floating roof damage criteria permitted
for floating roofs 20 ft. (6 m) in diameter or less, with purchaser’s
- Para. C.3.5. A pontoon inspection port may substitute for a
manhole for floating roofs 20 ft. (6 m) in diameter or less, with
- Appendix E. Completely rewritten. Note that there appear to be
some typographical errors (e.g., Para. E.220.127.116.11 equations).
- Para. F.4.2. Equations for Pmax slightly modified.
- Para. G.4.2.2. Wind load basis revised.
- Para. H.4.1.7. An inspection port may substitute for a manhole for
floating roofs 20 ft. (6 m) in diameter or less, with purchaser’s
- Para. H.5.7. Requirements specified for gauging and sampling
- Appendix M. Generally changed references from maximum
operating temperature to maximum
- Appendix R. New Appendix R specifies required load combinations to
be used in API 650.
- Para. S.3.3.5. New paragraph specifies allowable weld stresses for
- Appendix V. New appendix contains requirements for tanks designed
to operate with external pressure.