Allentech reprint.qxd - Aluminum Floating Roofs

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HAZARDOUS CARGO BULLETIN May 2006
storage
Hang in there
FLOATING ROOFS The introduction of cable suspension systems has taken aluminium internal floating roofs to a
new level of safety. New technology and subsequent independent testing of bolted seams for certain full contact
honeycomb roofs have also indicated significant reductions in emissions, as Allentech’s Bill Grimes* explains
Safety is the first and highest priority to Allentech
and to all the petrochemical companies we work
for and are associated with. Although we have been
performing this work for many years, entering a
tank while in service to change leg settings is a risky
procedure. And because it takes three well trained
workers, equipped properly, it is a time-consuming
and an expensive job.
In order to enter a tank in service the internal
floating roof (IFR), whether steel or aluminium,
must be floating within fifteen feet of the top of
the fixed roof and be dormant for at least two
hours with all tank vents open to exhaust the contained vapours. The lower explosive limit (LEL)
inside the tank within the space between the floating roof and the fixed roof must not be greater
than 10 per cent, measured comprehensively. The
worker entering the tank must wear a fall and rescue device (full body harness) connected via cable
to a tripod with a rescue winch and fall protection
device mounted over the manway on the fixed
roof. He must be wearing a full face mask and
Scott Air Pack or be connected to forced fresh air.
The second worker, a dedicated standby, must be
equipped with all of the same safety gear and fresh
air equipment. The third worker, a second dedicated standby, must maintain contact with the
worker in the tank and with the terminal via radio.
The worker changing the leg settings enters the
tank and walks all around the roof while it is floating on gasoline, ethanol or a similar product and
removes or installs a bolt and nut, depending on
the support leg design, changing the setting on all
support legs. If a leg is inadvertently missed, the
roof could be damaged when landed. Many petrochemical companies today simply will not allow inservice entries and all of them certainly want to
avoid it because of the risk.
Since most aluminium IFRs weigh less than two
pounds per square foot, new tanks can be designed,
and most existing tanks can be retrofitted to suspend
the AIFR from the fixed roof. Suspending an AIFR
by cable eliminates the need for conventional two
position legs and produces multiple demonstrative
benefits. From a safety point of view it allows the
IFR position to be changed from the top of the fixed
roof, eradicating the requirement to enter the tank
while in service. It becomes a one-man, much lower
risk job that can be accomplished in approximately
four hours by a terminal operator rather than two
days by a three-man crew of specially trained and
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Allentech’s honeycomb seal is at the heart of the system
fully equipped workers.
Eliminating in-service entries to make leg
changes is an enormous safety improvement.
Another safety factor becomes apparent when personnel are in a tank, under a floating roof for cleaning, inspections and repairs. While in the process of
writing this article, a fatal accident occurred in a
Southern California refinery involving personnel
working under a steel floating roof. The initial
report stated that the workers were jacking the support legs in order to replace tank floor plate. One
worker died and four were injured when the 120foot diameter steel IFR collapsed. This is not the
first time an accident of this nature has occurred.
Safety advantages
Conventional legs are either fixed or two-position
types. Fixed legs are generally set at about 4 feet
above the tank bottom. Two-position legs usually
have a low setting at the lowest level above the
highest obstruction at the bottom of the tank, usually the shell manway, and a high setting at the
maintenance level. With a manway plug the low
setting will be about 3 feet (1 m). The high or
maintenance position is usually about 6.5 feet (2
m) to give head clearance and provide a reasonable
assembly height in the case of an AIFR.
A cable suspension system allows multiple
height settings not possible on a steel IFR.
Therefore, in addition to the low and high posi-
tion, our customers often specify a third position.
We call the third position a high/high position, and
it is usually set at around 15 feet (4.5 m). This third
position provides clearance for future maintenance
to the bottom of the tank such as bottom repair or
replacement. The 15-foot position allows the contractor to bring a bobcat and other equipment into
the tank to replace floor plate.
Working on a tank floor without having to deal
with support legs greatly improves safety and efficiency and significantly reduces the repair time and
costs. In any case, the absence of legs is indeed an
additional benefit when tank cleaning occurs. It is
much easier and safer to clean a tank without having to negotiate around legs that can be moved and
or damaged in the process.
While on the subject of leg damage, conventional legs are frequently subject to corrosion damage, especially in crude oil service. In tanks with
steel IFRs, it is not uncommon to find tank floor
damage caused by heavy steel roof landings.
Additionally, the absence of legs greatly improves
the ease and speed of personnel evacuation should
it become necessary.
Emissions reduction potential
For the purposes of the rest of this article all references and calculations will be made on a tank that
is 120 ft (36.6 m) in diameter, 48 ft (14.6 m) high,
with a fixed roof with eight roof support columns.
This is a reprint from the May 2006 issue of Hazardous Cargo Bulletin, presented with the compliments of Allentech
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May 2006 HAZARDOUS CARGO BULLETIN
It is equipped with an aluminum IFR and is located in Houston, Texas, US. It is assumed the product service is motor gasoline with a Reid Vapor
Pressure of 10 psi and the tank experiences 36
turnovers per year. All calculations are based on the
US Environmental Protection Agency (EPA)
Tanks 4 program, the standard method for calculating emissions losses for permitting and enforcement purposes in the US. More information can
be found on the US EPA website at
http://www.epa.gov/ttn/chief/ap42/index.html.
In simple terms, using the Tanks 4 program calculations, eliminating conventional two-position
legs and replacing them with a cable suspension system on a new AIFR or by retrofitting an existing
AIFR with a cable system that seals off the leg
sleeves reduces vapour emissions by approximately
30 per cent. The subject 120-foot tank with an aluminium pontoon IFR with 75 legs and a primary
shoe seal emits 17,232 lbs (7.8 t) per year. Replace
the conventional legs with a cable system and it
emits a total of 11,854 lbs (5.4 t) per year. Although
it is not possible to remove the legs and cable suspend a steel IFR because of its weight, it is possible
to retrofit an existing AIFR with a cable suspension
system. Allentech’s retrofit design completely seals
off all leg sleeves, eliminating all leg sleeve emissions.
It is important to mention this because we have seen
storage
cable systems in the field that do not.
The latest technology in bolted seams for
AIFRs has the potential to further reduce emissions. The Tanks 4 program has a default factor to
calculate the emissions losses though the total
number of linear feet of bolted seams on an AIFR.
This factor is 0.14 lbmol/(ft yr). Using this factor
the resultant loss is 5,488 lbs per year in our subject tank. EPA allows for alternative control devices
if the emission factor is less than the standard but
this alternative factor must be the result of independent testing using strict EPA protocol and criteria. The bolted seam illustrated, manufactured in
the US by Allentech and used in its Full Contact
Honeycomb AIFR has a factor of 0.0085 lbmol/(ft
yr). Utilizing this factor in our subject tank results
in an emissions loss of only 333 lbs per year.
Cost savings too
Generally speaking, a new aluminium skin and
pontoon type AIFR will cost one half or less than
the price of a steel pontoon IFR. The more expensive aluminium honeycomb full contact type IFR
will be competitive with and probably in most cases
cost less than a steel pontoon IFR. With the addition of a cable suspension system to either design
operational costs are also reduced due to the time
savings during maintenance discussed at the start of
this article. With mobilisation the leg change job
will probably cost about $9,000. The approximate
cost of the cable suspension system for our subject
tank is $15,000. This gives the purchaser a payout
for the system after less than two leg changes.
In both cases the aluminium IFR can be
installed in as existing tank without a costly door
sheet and subsequent hydro test. Additionally, the
installation time for an AIFR is generally about
one-third that of a steel IFR, saving money on
installation and tank downtime.
When AIFRs were introduced about 40 years
ago they were a very low-cost alternative internal
floating roof with problems. Over the years conscientious manufacturers have solved the problems
and in so doing produced a product that is more
than a low-cost alternative. High quality AIFRs
today have an operational life of 30 years or more.
They are reliable but still economical. And, as
detailed in this article, quality cable-suspended aluminium internal floating roofs greatly improve
safety and dramatically reduce emissions.
*Bill Grimes is a sales engineer for Allentech
(Bethlehem, PA), which designs, manufactures and
installs aluminium internal floating roofs, perimeter
seal systems, and related products for aboveground
petrochemical storage tanks. telephone: (+1 520) 575
1443; e-mail: billg@allentech.com.
This is a reprint from the May 2006 issue of Hazardous Cargo Bulletin, presented with the compliments of Allentech
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