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UST Deflection and Striker Plates
~ Past and Present
“Deflection” plates (per Underwriters Laboratories (UL) 1316
Glass-fiber-Reinforced Plastic Underground Storage Tanks for Petroleum Products)
and “striker” plates (per UL 58 Steel Underground Tanks for Flammable and
Combustible Liquids) are the two most commonly used terms for reinforcing or
wear plates installed on the bottom of an underground storage tank (UST)
directly below the openings located on the top of the tank. The steel plate
protects the bottom tank shell from repeated contact with the gauge stick when
it is lowered through the tank opening during manual tank gauging. Often the
gauge stick is dropped into the tank from the ground level, which, for a 12-foot
diameter tank, can be some 18 feet above grade.
Manual tank gauging has become more frequent due to increased regulatory
emphasis on following proper inventory control and tank truck delivery
procedures. Manual stick gauging is performed before and after each delivery,
for daily liquid level measurements and, where automatic tank gauges (ATG) are
installed, for periodic confirmation that the ATG is performing properly.
Although UL 1316 was not revised to require a deflection plate under each
opening or one opening that is so marked until 1983, fiberglass tank
manufacturers made deflection plates an available option starting in 1973. In
1977, one deflection plate was standard and the tank user selected which fill
opening would be so marked. However, tank users experienced field installation
changes when the fill pipe location was relocated to the center or the opposite
end of the UST.
Thus, as early as 1979, certain fiberglass users specified deflection plates
under two openings. By 1983, manufacturers were installing such plates under all
three common openings (i.e., both ends and center) because conditions were not
the same for each user. By 1986, all fiberglass tanks were manufactured with
deflection plates under all openings.
Steel USTs
UL 1746, External Corrosion Protection Systems for Steel Underground Storage
Tanks, addresses three types of steel tanks, namely factory-installed
galvanic-type cathodic protected, fiberglass-clad and HDPE jacketed steel tanks.
In each case, the UL 1746 tanks are fabricated using UL 58 tanks, thus the UL 58
striker plate requirements apply. It should be noted that Steel Tank Institute (STI)
construction specifications sti-P3 and ACT-100 are more stringent than UL 58 or
UL 1746. For example, the May 1, 1987, STI sti-P3 Specification required striker
plates under each opening for tank diameters 64 inches and larger. Further, the
STI ACT-100 External Corrosion Protection of FRP Composite Steel Underground
Storage Tanks required striker plates under all openings.
Thus, post-1990 galvanic-type, fiberglass clad and jacketed steel tanks
manufactured only to the UL 58 standard may contain one striker plate for the
tank gauge opening and, if there was a installation change or a later change in
the opening used for gauging, a striker plate may be absent under the opening
actually used for gauging.
While UL 1316 specifies 0.053-inch thick deflection plates a minimum of 9-inches
wide and one square foot in area, fiberglass manufacturers use nominal 12 to
10-gauge (i.e., 0.105 - 0.1196 inch thick) plates that are 12 x 12 inches under
each single opening. Also, although UL 1316 does not address man ways,
fiberglass tank deflection plates are 12 x 24 inches for 22-inch diameter man
ways and 12 x 36 inches for 36-inch man ways.
Further, UL 1316 does not address plate installation. Manufacturers form the
plate to fit the curvature of the tank where the plate is completely
encapsulated using fiberglass and resin extending at least three inches beyond
the outline of the plate. The resulting thickness of the plate and lay-up
averages form a nominal 0.200 to 0.3125 inches and calibration charts are
compensated for the deflection plate and lay-up thickness.
UL 58 first addressed steel tank striker plates in August 1990 and specified a
minimum 0.240-inch steel striker plate(s) 9-inches wide and one-square foot in
area with the exception that if a fill pipe is used that extends at least 3/4 of
the tank diameter into the tank, the area of the striker plate may be reduced to
64 square inches. Later, on December 13 1996, UL 58 was revised, eliminating the
exception and reducing the minimum striker plate width to 8-inches and 64 square
inches in area.
UL 58 does not address plate installation. The sti-P3 specification requires
8-inch x 8-inch x 1/4-inch minimum size plates. The striker plates may be flat
or rolled to conform to the internal surface of the tank and the specification
states that the effect of a flat striker plate located in the bottom of small
diameter tanks must be considered. Similar requirements are contained in the
ACT-100 standard, but there is an additional requirement that installed flat or
rolled striker plates leave a minimum 1/4-inch gap between the striker plate and
the tank shell. ACT-100 also specifies that striker plates greater than 12
inches in width shall be rolled.
In summary, fiberglass tanks manufactured after 1986 (i.e., 18 years ago) should
have deflection plates under all openings and many manufactured after 1983
should as well. USTs manufactured after 1979 should have deflection plates under
the openings as specified by the user. For the most part, USTs purchased by
major oil companies since 1973 should have at least one deflection plate.
While larger STI sti-P3 tanks manufactured after May 1987 should have striker
plates under all openings, it was not until August 1990 when UL 58 and 1746
tanks were required to install either one or multiple striker plates if the one
striker plate was not so marked for the installer.
Often, the presence of a deflection plate under a tank opening may be checked in
the field by feeling the raised plate using a heavy-duty magnet suspended from
the top of the tank on a cord (for fiberglass tanks) or a gauge stick (for steel
tanks). While a fiberglass tank deflection plate and lay-up may be more
difficult to “feel”, the relatively sharp edges of a typical 1/4- inch flat
steel tank striker plate can be felt. Finally, if there is any doubt, petroleum
equipment suppliers’ market non-intrusive retrofit metal plates that are
inserted into the fill pipe for fiberglass tanks, but they should not be used in
contact with steel tank bottoms.
Sullivan (Sully) Curran P. E., Executive Director
Fiberglass Tank & Pipe Institute
Sullivan D. Curran, P.E., is the executive director of the Fiberglass Tank and
Pipe Institute. This article discusses the application of deflection plates in
fiberglass underground storage tanks manufactured by members of the Institute
and striker plates in published standards for steel underground tanks. The
article is not an analysis of products by other fiberglass underground tank
manufacturers. 4/06/04 |
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