How 3-D glass fabric storage tank lining technology works

By Derek Bolianatz, Parabeam Distribution NA, Mike O’Donoghue, Ph.D., and Ron Garrett, ICI Devoe Coatings

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:

1. over 45,000 vertical pile threads per square foot;
2. capillary forces during and after impregnation with a resin system;
3. firmness with which the vertical piles are woven into the fabric planes;
   and
4. E-glass composition of the fabric.

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:

• The majority of the woven E-glass fabric is at the extreme surfaces of the
  3D-Laminate, which is separated by the capillary support columns, giving the
  laminate high tensile strength and good flexural modulus.
• The interstitial space also helps by safely absorbing impacts that would
  otherwise cause fractures or holidays in standard tank lining, therefore
  protecting the substrate from contact with the corrosive cargoes stored.

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.

This article was published in ES&E Magazine’s January 2000 issue.