T ERMINALS & S TORAGE

Aluminum Internal Floating Roofs -

A
friend, responsible for a marine petroleum storage have occurred over the years, overcoming the shortcomings
facility in seismically active Southern California, and and failures that plagued their forerunners. It is now the most
I were recently discussing the characteristics of widely used AIFR system.
aluminum and steel internal floating roofs (IFR). He was There are three major manufactures of Skin and Pontoon
concerned about using aluminum for an IFR due to concerns AIFR in the U.S, with several more producers worldwide. The
about its ability to handle turbulence. I suggested facetiously quality of design, fabrication and installation differ greatly
that, in light of his concern, all of his future long distance between manufacturers and these are all factors in longevity.
travel be made by rail since steel is the primary component of However, the better AIFR can be expected to provide service
this method of travel. On the other hand, airplanes are com- lives of twenty-five years or more and can be designed to accom-
posed mostly of aluminum, and it’s common knowledge that modate pigging operations at high pressures and volume. And as
military, commercial and private aircraft regularly encounter a sidelight, my friend would be amazed if he saw what some of
high degrees of turbulence and stress. our products are routinely subjected to by their owners!
Extracted chiefly from bauxite, the element aluminum Here are some key characteristics I recommend the pur-
(Al) is a light, silvery-white, trivalent, ductile and malleable chaser look for when considering a quality aluminum Skin
metal. It is equal in strength to steel and one-third the weight. and Pontoon roof.
It is non-sparking, non-magnetic and corrosion resistant. It is • The process should begin with a roof custom designed
easily worked, and can be rolled, stamped, drawn, ham- and fabricated for a designated tank. A truly custom
mered, forged, formed, extruded, machined, riveted, welded, engineered AIFR with pre-fabricated components sent to
brazed, soldered, polished, sandblasted, wire brushed, and the field to be assembled from engineered drawings will
heat treated. Aluminum is used extensively in the aerospace provide a better product then one that has minimal shop
and automotive Industry and in architecture. It is used to fabrication but instead is largely fabricated in the field.
manufacturer such things as precision forgings and pressur- • A tank shell and column plumbness survey before field
ized containers and is utilized for a multitude of products assembly is imperative to prevent future operational
throughout the industrialized world. problems, and is required by API. Purchasers must
Aluminum internal floating roofs (AIFR) were first verify that the installer is qualified to perform and does
developed in the late 1950’s. There were several original con- accomplish the survey.
cepts; some were short lived, but two of them evolved into • Design for and use custom extrusions rather than
currently used designs. One of the first short-
lived designs was the Pillow floats and skin
deck. These non-contact AIFR were extensive-
ly used but lacked sufficient buoyancy and
strength and thus evolved into the foam-filled
rectangular pontoon design. These non-con-
tact AIFR have not been manufactured since
the early seventies. Most have been removed
from service, because of failed deck seam
sealants and leaking pontoons, leading to
excessive emissions.
The pillow float roof system was replaced
by tubular Skin and Pontoon IFR systems. The
Skin and Pontoon IFR typically consists of
interconnected hollow tubular pontoons sur-
faced with a grid of aluminum beams and
sheets. Many significant changes to this design Figure 1. Aluminum skin and Pontoon floating roof.

38 • World Refining • June 2003 www.worldrefining.com

standard shapes and bent sheet, thus adding significant to a basic difference in the honeycomb core itself. The company
strength and rigidity to structural components. The perimeter that originally marketed this design originally used a fuel-tank
should be extruded and rolled to accommodate the circum- sealant to join the honeycomb skins to the module framing. The
ference of the tank rather than fabricated from bent sheet and use of this sealant is still the method they use today to assemble
segmented to approximate a circle. their modules. Their design employs a non-perforated honey-
• Pontoon end caps should be flanged and fillet welded by comb core, based on the premise that an accidental puncture to
certified welders. Appropriate reinforcement of pontoon end their 0.016” thick surface skin would isolate the breach to a sin-
caps is achieved by employing brackets the full diameter of gle cell of the honeycomb.
the pontoon, welded full length of the bracket on both sides, The second manufacturer, on the other hand, fabricates their
then gusseted top and bottom again with seal welds. Only roof with a perforated honeycomb core and seal welds their enclo-
these end caps will develop the full compression and bend- sures to 0.040” thick skins employing a test plug in each module.
ing strength of the pontoon. The seal welded perforated design allows for shop pressure test-
• Pontoon spacing is equally critical; don’t be fooled into ing of each module and also enables field testing before mainte-
thinking that a roof with larger diameter pontoons is nance and hot work now required by the API. Without the abili-
better. Quite often, vendors will provide larger diameter pon- ty to test the module, safety is greatly compromised. In fact, some
toons to gain buoyancy but the pontoons rows are 50% to owners of the non-welded design routinely require the IFR be
100% farther apart than those of a well designed IFR. A removed prior to performing any hot work in the tank. This
closely spaced grid of 8” diameter pontoons offers better expenditure of course far exceeds any minimal savings, which
support than 10” diameter pontoons on a wide grid. may have been realized at the time of purchase.
The honeycomb sandwich panel roof is the latest in technol-
The AIFR and its components should be designed and assem- ogy and has its roots in the aerospace industry. It is used in the
bled as a mechanically rigid structure such that it will safely sup- high tech aerospace industry where optimum strength to weight
port the full dead load plus a concentrated load of no less than ratio is required. In fact, it was employed in the landing gear on
500 pounds (two men) on one square foot anywhere on its sur- the lunar lander. It is corrosion, impact and fatigue resistant, and
face without structural damage either while the AIFR is floating or has optimal weight deflection under load and long-term stability
while resting on its supports. All design loads should have appro- at elevated temperatures. So floating roofs are to some
priate factors of safety as specified by the Aluminum Association. degree...Rocket Science.
Insist on stainless steel or aluminum hardware. There have When contemplating the purchase of an internal floating roof
been many instances of aluminum roofs, which have had to have consider that a full-contact honeycomb AIFR is competitively
extensive repairs or have been replaced as a result of galvanic cor- priced with a steel pontoon roof, takes one third of the time to
rosion at the deck sheeting fasteners. Plated fasteners may save the install, does not require a hydro-test after installation and can be
vendor several hundred dollars but they may end up costing you cable suspended from the tank roof so that no legs are required
many thousands. and changing support positions can be accomplished from the
Choose a vendor that stands behind their products and offers outside of the tank. Taking all of these factors into account should
inspection and repair services.
Look for a vendor with a history
of long term support and service.
Refer to American Petroleum
Institute (API) Publication 650,
Appen-dix H for guidance and
require that your prospective sup-
plier verify compliance.
The other AIFR design is the
full-contact honeycomb core roof.
Introduced in the 1970s and still in
use today, the full-contact design
utilizes a series of flat interconnect-
ed composite modules consisting of
a honeycomb hex core sandwiched
between aluminum sheeting.
There are two fabricators of
the honeycomb design in the US,
and both market their products
worldwide. Both companies uti-
lize a honeycomb core but the
products are vastly different down Figure 2. AIFR custom extrusions.
40 • World Refining • June 2003 www.worldrefining.com
 TE R M I NALS & STO RAG E

elevate this design to the top of your list. the standard and optional appurtenances, peripheral seal sys-
To learn about the minimum standards for internal floating tems require serious consideration. Shoe seals are fast becom-
roofs, become familiar with the American Petroleum Institute ing the standard far out living wipers and dangerous liquid
publication, API 650, Appendix H, (10th edition, dated mounted log seals. Here again, API 650, Appendix H is a stan-
November 2001). API recommends that each compartment be dard reference. Seal types are briefly described and minimum
capable of being field-inspected for the presence of combustible requirements discussed. Most importantly the purchaser must
gas and can be resealed to prevent liquid or vapor entry. The take into account the proposed and future product service,
Standard also recommends that all floatation compartments be tank shell construction and condition, maintenance require-
seal-welded. It addresses the use adhesives to seal floatation ments, regulatory compliance, service life expectancy, ambient
panels (in lieu of welding), if allowed by the purchaser, by rec- temperature, design temperature, permeability, abrasion resis-
ommending that the adhesives be compatible with the product tance, as well as aging, flammability, and other compatibility
service, the floatation test water and that the purchaser consid- factors. Regulatory compliance will include Federal, State and
er the future product service, the hydrostatic test condition, and local jurisdictions. Refer to the “Code of Federal Regulations,
design condition changes to specify adhesive compatibility. Title 40 - Protection of environment, Part 60, Subpart Kb for
In paragraph H.4.3.4, of API 650 there are additional state- EPA compliance (CFR40, Part 60, Subpart Kb). ●
ments about the use of any joint sealing compound, insulating
material, elastomer or adhesive. All must be pre-approved by ABOUT THE AUTHOR
the purchaser and the manufacturer must describe completely Bill Grimes is a sales engineer for ALLENTECH (Bethlehem,
the joining procedure along with the test results, demonstrating PA), which designs, manufactures, and installs internal float-
the properties required in H.4.3.4. Where such joints are per- ing roofs, perimeter seal systems and related products for
mitted, any joint sealing compound, insulating material, elas- aboveground petrochemical storage tanks and floating oil-
tomeric or adhesive shall be compatible with the product water separator cover for use at oil refineries and petrochem-
stored, specified service conditions, and with materials joined. ical storage terminals. He has over twenty-five years experi-
API 650, Appendix H is a must read if you are a potential IFR ence in the petroleum industry. Grimes was the author of the
purchaser. It is universally accepted as defining the minimum Feature article in the 2002, ILTA Terminals edition of
standards for IFR taking into consideration product loss, prop- WORLD REFINING, titled “API Revises Floating Roof Guide
erty damage, and most important, safety to human beings. Lines”. Grimes can be contacted at phone (520) 575-1443 or
Of the many decisions associated with IFR selection and e-mail billg@allentech.com.

www.worldrefining.com June 2003 • World Refining • 41