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Permeability of Synthetic
Membranes for the Containment of Petroleum Products
Fiberglass Tank & Pipe Institute
Sullivan D. Curran, P.E. Executive Director
Introduction
Synthetic membranes for the containment of petroleum products are typically made
of thermoplastic or rubber materials and are permeable. While there are accepted
permeability values for petroleum aboveground secondary containment areas, there
are additional considerations when using these materials for underground tank
and piping applications. This paper discusses the historical rationale,
applicable test methods, permeation values and recommendations for the
application of thermoplastic and rubber membranes in the design of petroleum
tank and piping secondary containment systems.
What is permeability? Permeability is defined as the rate of molecular
diffusion of a gas or liquid through another substance or material. It may be
thought of as "slow motion" mixing, with the rate a function of the relative
polarities and sizes of the molecules in the liquids and containment
material.
Permeability should not be confused with the liquid transport mode known as
hydraulic conductivity. For example, soil porosity transports most of the
foreign material by "hydraulic conductivity", whereas membranes transport vapors
and liquids, with this phenomenon called "permeability".
The permeability of a material is affected by the stored liquid’s molecule
polarity and size as discussed below.
Molecule Polarity: Highly polar liquids, such as alcohol-based methanol
and ethanol, will mix more readily with another polar substance, such as nylon
or polyester materials. Non-polar liquids, such as gasoline aromatic components
(e. g., benzene, toluene, xylene), will mix more readily with another non-polar
substance, such as polyethylene.
Hence, both polar and non-polar materials, such as nylon, polyester and
polyethylene, are permeable to a gasoline-alcohol mixture.
Molecule Size: Small-sized molecules, such as methanol, will mix more
readily than medium-sized molecules, such as toluene, or a large molecule, such
as Methyl Tertiary Butyl Ether (MTBE).
Hence, both polar and non-polar materials are more permeable to
gasoline-oxygenates using alcohol, rather than petroleum based MTBE.
With the foregoing as a background, following is a discussion on the design
criteria for the permeability of aboveground and underground tank and piping
containment areas used for petroleum product applications.
Aboveground Containment
There are two nationally-adopted and a proposed permeability requirement for the
lining material used in secondary containment areas. Of the current
requirements, the earliest addresses fire safety for large tank and piping
installations, whereas, more recently, there are local or anticipated
Environmental Protection Agency (EPA) environmental protection rules. In
addition, Underwriters Laboratories Inc. (UL) proposed a standard for
"Excavation Liners for Secondary Containment of Flammable and Combustible
Liquids, Subject 1854."
Fire Safety: The National Fire Protection Association (NFPA) Flammable
and Combustible Liquids Code NFPA 30 states that the "walls of a
diked area shall be of earth, steel, concrete, or solid masonry designed to be
liquid-tight and to withstand full hydrostatic head." Also, the Code states
that, in areas where tanks contain Class I liquids (e. g., gasoline) and are
"located in extremely porous soils [the tanks] may require special treatment to
prevent seepage…." NFPA staff commented on the term "liquid-tight" and stated
that it means the liquid will not soak through the dike wall, at least for some
reasonable length of time (i.e., it is assumed that the spilled liquid will be
removed as soon as the fire emergency is stabilized).
Environmental Protection: In 1973, EPA published their Spill Prevention,
Control and Countermeasures (SPCC) requirements and, in 1991, published proposed
revisions that will likely require containment retention to be "without
substantial loss" for a period of at least 72 hours. This requirement is
consistent with the fire code philosophy that a tank or piping spill cleanup
should be completed within three days. However, several states and local
jurisdictions specify criteria that will not permit the use of impermeable soils
or untreated concrete (e. g., Alaska, New Jersey, New York and Florida).
Underwriters Laboratories: In 1995 UL proposed Subject 1854 which
outlines the investigation of "Excavation Liners for Secondary Containment of
Flammable and Combustible Liquids." The materials may be fabric, reinforced
rigid plastic (i.e., thermo or thermosetting plastic) or non-rigid plastic (i.
e., thermoplastic). Test liquids include gasolines, methanol, ethanol and
alcohol-gasoline mixtures. Test method ASTM E96-94 is conducted over a 28-day
period to determine the amount of permeation.
Permeability Design Criteria for Aboveground Containment: Although today
there isn’t a nationwide permeability standard, where required locally a common
factor is 10-7 cm/sec. This permeability value is often obtained by using
synthetic membranes. The design engineer should also be aware that UL 1854 sets
the rate of permeation at not greater than 0.25 ounces/square foot/24 hours
(79.6 ml/square meters/24 hours). There are a variety of synthetic materials
available including polyethylene, high-density polyethylene (HDPE), polyester,
butyl rubber, neoprene and polyurethane. These materials should be evaluated for
chemical compatibility of the membrane for possible chemical spills in the area;
durability and other forms of deterioration such as that caused by bacteria in
the soil. One alternative to pre-manufactured flexible membranes is a spray-on
application of urethane or polysulfide. A geotextile fabric is often placed
prior to the spray application to increase the durability of the lining. In
conclusion, the design engineer is well positioned to design dike walls and
containment basins with confidence that future retrofitting should not be
required.
Underground Containment
Requirements for underground containment at petroleum facilities are relatively
new when compared to those for aboveground facilities. While EPA included
requirements for Underground Storage Tanks (UST’s) in their 1988 Technical
Requirements, it wasn’t until the 1996 edition of NFPA 30 that a fire code
addressed piping "secondary containment of liquids and associated vapors."
Fire Safety and UL: The 1996 edition of NFPA 30 requires that tanks and
piping, "including those incorporating secondary containment, shall be built in
accordance with recognized standards of design, or approved equivalents" and
identifies appropriate tank standards and UL 971 "Standard for Nonmetallic
Underground Piping for Flammable Liquids." The UL 971 test method consists of
sealing an 18-inch length of pipe with appropriate fittings. The pipe sample is
weighed empty, filled with the test liquid, and weighed monthly over a 180-day
period for primary pipe and twice a week over 30 days for secondary containment
pipe. The differences in weight are used to determine the permeability rate.
Environmental Protection: In 1988, EPA published Technical Standards for
the "secondary barrier" around or beneath the UST system. The barrier is to
"consist of artificially constructed material that is sufficiently thick and
impermeable (at least 10-6 cm/sec. for the regulated substance stored) to direct
a release to the monitoring point and permit its detection." Also, the barrier
is to be "compatible with the regulated substance stored so that a release from
the UST system will not cause a deterioration of the barrier allowing a release
to pass through undetected."
The EPA test criteria is expressed in terms of cm/sec and is the coefficient of
permeability derived from ASTM D 3385-73, the standard test method for
"Infiltration Rate of Soils in Field Using Double-Ring Infilterometers." Thus,
this and similar test methods are designed for soils and not "artificially
constructed materials", such as thermoplastics. The EPA value of 10-6 cm/sec is
the acceptable value for the coefficient of permeability (k) for gasoline
retention in aboveground containment areas. This value is achieved with a
bentonite soil layer and was established in field test work by Gulf Oil,
Imperial Oil and others and was cited in PACE Report No. 79-2, March 1979.
In summary, EPA did not reference an appropriate ASTM test method or permeation
criteria for the design of synthetic materials in UST service. However, it
appears that the intent of the EPA requirement is to limit secondary containment
to artificially-constructed material (e. g., thermoplastic), which is
impermeable to all motor fuels (e. g., gasoline-alcohol mixtures), for the
period of time necessary for a leak to be monitored and mitigated.
UL 971 Allowable Leakage: While there is considerable experience with the
design of aboveground secondary containment areas, there is less experience with
the design of underground containment areas…. And there is a difference!
Leak Detection Frequency: Aboveground containment design assumes that a
spill will be observed and cleaned up within three days. However, it may be 30
days when most prudent UST operators would check for liquid or vapors in sumps
as is done in observation wells.
Underground Leak Detection Design: Systems are available to monitor continuously
dispenser and tank sumps for the accumulation of underground piping/joint leaks
and/or dispenser and tank sump spills into the secondary containment system.
These are costly systems, not required in their entirety by the EPA rule.
However, many operators depend on monthly monitoring and annual tightness
testing and will want to be aware of the permeation levels permitted under UL
971. This test protocol permits secondary containment permeation of 0.079 ounces
per square foot per day and conducts their test for 30 days. The surface area
measurement is made from the inside diameter (ID) of the pipe. For example, if
500 feet of 1 ½ inch underground coaxial secondarily contained pipe is installed
in a typical retail service station, then the allowable leakage from permeation
over a 30 day period is 3.6 gallons.
Conclusions
If the design engineer accepts the UL 971 allowable permeation rate of 3.6
gallons over a 30-day period, then it is important that secondary containment is
designed to drain freely to the tank sump and avoid the retention of product.
Hence, the design and construction should consider the following:
Piping Slope - PEI RP 100 (1994) states that piping "should be sloped 1/8 inch
per foot to the tank"…."And "should be evenly and adequately supported to
prevent creation of traps or sumps." API RP 1615 (1996) states that piping
"should have a uniform slope of not less than 1/8 inch per foot down towards the
tank" and that "traps or sags should be avoided in all piping." Maintaining the
proper slope is relatively easy due to the nature of rigid piping. However to do
the same using flexible piping, care should be taken to first level and then
compact the trench bedding in an effort to maintain a constant slope after the
site is backfilled and settlement occurs.
Dispenser Sumps – The bottom of a dispenser sump must be below the inlet
level of the secondary pipe and will inhibit drainage to and detection in the
tank sump. If the sump material is permeable i. e., thermoplastic (e. g.,
polyethylene) rather than thermo-setting plastic (e. g., fiberglass), then
periodic inspections or product sensors should be employed to ensure product
accumulation is removed to prevent leakage due to permeation.
Corrugated Piping Containment – The corrugations in some secondary
containment piping systems will not drain completely and, thus, retain some
liquid. Vapor rather than liquid monitoring at the tank sump may be employed to
detect when this occurs and the system should be purged.
May, 1997
References
- "State of the Art Review –
Petroleum Product Containment Dyking"
- Report No. 79-2, March 1979,
Petroleum Association for Conversation of the Canadian Environment (PACE)
- "State-of-the Art Review:
Tank Bottom Lining Systems and Impervious Diked Areas" Final Report,
January, 1988, Golder Associates Inc., Atlanta, Georgia
- Standard test Method for
"Infiltration Rate of Soils in Field Using Double-ring Infiltrometers" ASTM
D 3385-75
- State of Maryland Tank Dike
Permeability Recommendation letter written by Mobil Oil Corporation, 1988
- Storage Tank and Piping,
Chapter 35, Secondary Containment, February 12, 1995 Chevron Company, USA
- "UL 971 Nonmetallic
Underground Piping for Flammable Liquids" October 30, 1995, Underwriters
Laboratories Inc.
- "Installation of Underground
Petroleum Storage Systems" American Petroleum Institute Recommended Practice
1615, March 1996
- "Installation of Underground
Liquid Storage Systems" Petroleum Equipment Institute Recommended Practice
100-94
- 40 CFR Parts 280 ;
Underground Storage Tanks; Technical Requirements Federal Register,
September 23, 1988
- "Permeation" paper dated
1/12/95; Ameron, Fiberglass Pipe Group
- "Just the Facts" paper dated
December, 1996; Smith Fiberglass Products Inc.
May, 1997
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