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Tank Bladders ~ New or
Not-so-new?
Fiberglass Tank & Pipe Institute
Sully Curran, PE
Introduction
Recently there has been renewed interest in underground tank bladder retrofits.
One such flexible bladder is manufactured from a polyvinyl chloride material
held in place by a pump that draws a vacuum in the interstitial space between
the bladder and the tank wall. While the bladder in itself does not satisfy the
EPA criteria for corrosion protection of an unprotected steel tank, it is
purported to be an alternative to interior lining and it is claimed the vacuum
system may serve as an interstitial leak detection method. While all of this may
sound like a new approach, the petroleum industry has "been there, done that"
and today there is to deal with new customer perception issues.
Historical Perspective
The application of extenders and plastic materials to contain liquids and vapors
is not new to the petroleum industry nor is it new to steel and fiberglass tank
manufacturers. The petroleum industry has a vested interest in the development
of liners manufactured from petrochemical materials and published compatibility
and permeability study results. Tank manufacturers have also conducted paper and
laboratory research in their efforts to develop low-cost retrofit and new
products.
Petroleum Industry Experience
In early 1950 bladders were first installed in the vapor space of
aboveground petroleum storage tanks to reduce evaporative emissions and minimize
product "stock loss". Unfortunately, seam failure experience led to high
maintenance costs and typically a cost-effective long-term solution has been to
retrofitted these tanks with internal floaters.
Then in the 1960’s bladders were used inside vapor collection tanks with
some of the early Stage I tank truck loading vapor recovery systems. While
essentially all of these early systems have been replaced, the industry learned
they were costly to maintain and bladder seam repair experience contributed to
the high maintenance cost.
It was in the 1970’s that the EPA found that licensed hazardous waste
disposal storage pits were leaking from liner seam failures. As a result, the
EPA rule was changed to require complete seam testing rather than spot tests and
a two liner-interstitial system.
And in the 1980’s the retail petroleum industry evaluated the viability
of applying appropriate thermoplastic wraps around underground steel tanks as a
barrier to corrosive soils. While this formed a barrier between the soil and
tank, humidity and condensation between the wrap and the tank was identified as
a problem since it could promote galvanic corrosion of an unprotected steel
tank.
Today military "pillow tanks" constructed from flexible plastic materials
are used for temporary storage. However, they are enclosed in diked areas and
petroleum seepage from these tanks is not uncommon.
In summary, petroleum industry experience with thermoplastic seams and
material exposure recognizes that there will be deterioration over time. Thus,
these materials have been limited to short term applications or where
maintenance repairs are anticipated as part of the operation.
Tank Manufacturer Research
Raw material manufacturers publish chemical resistance information on
thermoplastic materials and compatibility and permeability conduct testing
certain tank manufacturers.
Future Considerations
Material compatibility and permeability became a challenge for the
petroleum industry when they began using additives that were polar rather than
non-polar molecules. For example, nylon hoses were commonly used at a time when
unleaded gasoline incorporated toluene and xylene as octane boosters. This blend
was non-polar and there was little deterioration of the nylon non-polar
material. However, the introduction of polar additives (e. g., ethanol,
methanol) into the gasoline mixture causes the same nylon material to soften and
become permeable to the total gasoline mixture. Another example is the use of
polyethylene, a polar material that is virtually impervious to methanol, but a
poor barrier to non-polar gasoline.
MTBE vapor emissions have become a major issue for the petroleum
industry. The MTBE issue is a human perception problem rather than a health
problem. As such, complaints emanate from smelling the highly volatile ether in
the MTBE compound. Thus the petroleum industry must exercise caution when using
materials that, while resisting leakage, may be permeable and cause customer
perception problems.
Bladders
UL Listing - A UL Listing has been granted for the application of
polyvinyl chloride (PVC) and polyvinyl chloride/polyurethane bladders in tanks
storing diesel and gasoline, respectively. Once positioned inside the tank, a
vacuum is maintained in the interstice by a vacuum-pump system that holds the
bladder in place. The detection of a leak into the interstice is based on
monitoring a constant vacuum.
Permeability – The permeation test used by UL is ASTM E96-94 and not
designed to measure permeation of a material under constant vacuum. Further UL
used a pass-rate criterion of 0.25 ounces/square foot/day when Listing the
bladder for gasoline storage. As a result, the UL test would permit an allowable
leak rate (i.e., permeation) of some 4 gallons per month from the surface area
of a bladder installed in an 8,000 gallon tank.
Corrosion protection – The EPA technical standards require steel tank
upgrading to include corrosion protection for the exterior of an unprotected
steel tank. Thus, the installation of a bladder would need to be supplemented
with a cathodic protection system for compliance with the EPA rules.
Thermoplastic Jackets
Tank Type – Although advertised as "Double Wall Tanks", they are not UL
Listed as such. UL’s definition of a double-wall tank "is a tank within a tank."
By applying this definition, the UL Listing of conventional steel, composite or
fiberglass "double-wall tank" requires that the thickness of the outer wall be
thicker than that of a jacketed tank.
UL Listing – Jacketed tanks are Listed under UL 1746 which provides the
test protocol for "jacketed corrosion protection systems". Under this Listing,
UL specifies that the thermoplastic material be at least 0.100 inches thick high
density polyethylene (HDPE). This material is formed into a jacket to cover the
tank exterior. Since some jacketed tanks have a confined annular space, the UL
leak detection test allows 24 hours to lapse before the leak is detected.
Corrosion Protection – The jacket is intended to form a barrier between
the steel tank and corrosive soil environment. However, the UL Listing does not
address the potential for corrosion of the steel tank shell from the
accumulation of condensation and humidity in the closed interstice space.
Summary
Low cost tank retrofits and certain new tank installations may have applications
for the short term. However, industry experiences and earlier concerns remain
unanswered. In addition, there are future concerns while the retail petroleum
industry deals with public perceptions that are difficult to separate from
perceived health issues. Time will tell. |
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