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| A magnified side view of a section of a 3-D laminate. |
By Derek Bolianatz, Parabeam Distribution NA, Mike
O'Donoghue, Ph.D., and
Ron Garrett, ICI Devoe Coatings
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| A magnified side view of a section of a 3-D laminate. |
With today's emphasis on leak detection and spill prevention, there are a growing number of authorities implementing statutes and codes that make it mandatory for tank owners to install some type of secondary containment system. There are products and solutions available that may do the minimum that these laws suggest, but do they really protect the tank owner from the potentially high costs of cleaning up contaminated soils?
Storage tank owners and operators need to be aware of the potential liability that may arise in the course of managing hazardous liquid storage tank facilities. Often, liabilities occur as a result of spills or leaks, particularly if the hazardous cargo escapes off-site, pollutes an aquifer, or causes a fire hazard. When problems like these arise, the liabilities, damages and enforcement penalties can be astonishing.
Until recently, tank owners who opted to install an impermeable liner, coat the interior, or to rework the bottoms rather than replace them with a double-wall tank, had to accept one important fact. They did not have the added assurance that would have come from installing tanks with secondary containment and continuous leak detection that is, double-wall tanks.
With the 3-D glass fabric tank lining system, a relatively new technology, tank owners can transform their existing single-wall tanks into double-wall tanks with secondary containment and continuous leak detection, without reworking or replacing their single-wall storage tanks.
How does this technology work?
This tank conversion technology consists of a composite matrix of 100 percent solids epoxy and 3 mm thick three-dimensional (3-D) glass fabric that is bonded to the inside wall of an underground storage tank or, in the case of an above ground storage tank, the tank floor. The matrix is then top-coated with a layer of 100 percent solids epoxy specifically formulated to provide corrosion resistance against the cargoes stored. The fabric in the matrix, which is constructed from E-glass yarn into the 3-D glass fabric that is trademarked as PARABEAMTM worldwide, was developed in Europe in 1989.
This 3-D glass fabric consists of two identical parallel fabric decks (upper and lower planes) woven integrally and mechanically together by means of vertical pile threads. There is a pre-set interstitial space between the two deck surfaces or planes. The 3-D glass fabric is available in interstitial space thicknesses ranging from three to seventeen millimetres (0.12 to 0.67 inch).
All surfaces of the 3-D glass fabric have a silane sizing that makes them compatible with the specially formulated epoxy resin system and allows the resin system to saturate the fabric. During the wetting-out process, the fabric has an inherent rebounding property, called spring resilience, that forces the upper deck to spring back from the lower deck to a height dictated by the length of the vertical pile threads. The vertical pile threads look like a multiple series of miniature I-beam columns, distributing loads and providing excellent mechanical properties.
The spring resilience and compressive strength of the 3-D glass fabric is derived from four factors:
When the 3-D glass fabric is impregnated with this epoxy it expands and then begins to cure; a continuous cavity is formed between the upper and lower decks of the laminate. A cross-sectional view of this cured fabric/resin matrix looks somewhat like that of the girders of a bridge. While the lower deck is tightly adhered to the tank wall or floor, the upper deck is flooded with an epoxy top-coat. This process leaves an interstitial space, which can be continually monitored by a number of standard leak-detection systems.
The upper and lower decks of this system are impermeable. In the event of a leak in the interior surface, the interstice will contain the product so that a monitor can detect the leak and sound an alarm. Similarly, water entering the interstitial space from outside the tank will be contained inside the space so that it can be detected and appropriate steps taken.
In underground tanks, the technology provides a 360° double wall with an interstitial space that combines high strength and an appropriate degree of bending stiffness. The 3-D glass fabric-epoxy system is non-corrosive.
Leak-detection options
Many permanent leak-detection techniques can be applied with this 3-D lining to provide continuous leak detection. In practice, the most common leak-detection techniques that can be employed include hydrostatic, air pressure, vacuum, or electronic liquid sensing. Continuous air pressure is the most reliable and cost-effective method employed to date.
Mechanical properties
Besides its leak detection capabilities, the fast-curing 3D-Laminate has several properties not found in standard fiberglass mat laminates. These include:
Getting the right resin
PARABEAMTM 3-D glass fabrics are easy to use with most thermosetting resins; particularly good results can be achieved with isophthalic polyester, vinyl ester, or epoxy resins. However, it is widely known and demonstrated by immersion test data, that certain resins deliver better corrosion resistance and structural performance than others; therefore it is important to select an appropriate resin for the application at hand.
Converting to a double-wall tank
Before applying any lining, a tank's condition and integrity must be carefully determined and the tank must be cleaned and prepared according to applicable codes and standards. This work and the lining application itself must be performed only by qualified applicators.
When the new advanced hybrid cycloaliphatic (AHC) epoxies are applied in conjunction with 3-D glass fabric, the initial application of the tinted AHC epoxy is 20 to 30 mils WFT. This is followed immediately by placing the three-millimetre 3-D glass fabric on to this wetted area. The epoxy must be worked up into the 3-D glass fabric and any air trapped beneath the lower deck should be removed and wrinkles eliminated.
The 3-D glass fabric is laid down in parallel courses with edges butted together but not overlapping. After each parallel course is rolled out, a specially-designed seam material known as PHONIX TAPE is applied, uniformly wetted, and rolled out over the butted seams of the 3-D glass fabric. This material is required to seal off the joints of the 3-D glass fabric, creating a homogeneous 3D-lining. Additional epoxy is sprayed on this first layer as required, to ensure proper wet-out.
The application then needs sufficient time to cure so that it can be inspected. Then, any anomalies, protruding strands, rough edges, seams, clips and projections are either ground or sanded smooth, preparing the surface for the next step. The entire surface of the tank is swept clean and vacuumed. After all prep work and touch up has been completed, the topcoat can be applied. The topcoat is also a 100% solids epoxy resin formulated to be compatible with the cargo stored in the tank. This top coat is applied to a nominal thickness of 100 mils.
The final stage of the installation is a pressure test of the interstitial space using the air and soap bubble method. The applicator will then reconnect the piping, install the leak detection system, and perform a precision leak test in order to verify the tank's tightness. Quality assurance guidelines are followed at every step of the installation process and are reviewed completely prior to putting the tank into service again.
Many tanks have already been converted from single-wall to double-wall in North America using this technology. In addition, several major oil companies are finalizing plans for long-term national programs to upgrade their tanks using the 3-D glass fabric/epoxy resin technology.