c) Field-erected tanks requiring an internal pan
comply with Standard Design AW 78-24-27. The standard design
includes tanks ranging in capacity from 2,500 barrels (400 m3)
through 50,000 barrels (8000 m3) with internal pan and
requires site-adapting by the design team. For tanks larger
than 50,000 barrels (8000 m3), use the multicolumn API 650
design. Prohibit the use of exterior wind girders, unless
required to meet abnormal local conditions.
Tank Roofs. For tanks with internal floating pans,
design the roofs in conformance with Standard Design AW
78-24-27, which uses truss-supported cone roofs to eliminate
columns in tanks less than 50,000 barrels (8000 m3). For
tanks larger than 50,000 barrels (8000 m3), use an
intermediate multicolumn (steel pipe as columns) roof support
Internal Floating Pans. Provide a honeycomb cell
floating pan constructed of honeycomb aluminum for all
aboveground vertical storage tanks storing Class I liquids or
Class II mission critical fuels (e.g., JP-4, JP-5, JP-8, and
diesel fuels used for shipboard readiness) or fuels with a
true vapor pressure of 0.75 psig (5 kPa) or higher at
operating temperature. Class I and Class II are as defined by
NFPA 30. In the handbook, the term "internal floating pan"
will be used to identify the specific type described herein.
It is acknowledged that in API Std 650 and NFPA 11, "internal
floating roof" is used in a broader sense.
a) Provide a seal between the internal floating pan
and the tank shell. The primary and secondary seals should be
a flexible wiper squeegee sealing device. This device should
be attached in such a manner that they may be replaced
manually from the top of the pan. The seals should be vapor
mounted, function above the liquid level and be free draining
without trapping liquid. Provide seals around all stationary
devices which penetrate the internal floating pan. The type
of seal should not damage the tank shell coating. Ensure the
seal tightness complies with API 650 Appendix H, 40 CFR
Subpart 60Kb, and State and local regulations, whichever is
more stringent. The seal should be designed to accommodate
plus or minus 4 inches (100 mm) of local deviation between the
floating pan and tank shell.