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ArcMaster Submerged Grounding System
HMT
has developed a Submerged Grounding System that will
provide multiple conduction paths for lightning induced
electrical charges to be transmitted from the external
floating roof to the tank shell and subsequently to
ground.
The
ARCMASTER™
is designed to carry this charge below
the floating roof and below the stored liquid surface,
thereby confining any subsequent electrical arcs or
sparks to the liquid, below the rim space and below the
combustible air/hydrocarbon vapor mixture. While a
lightning induced charge may produce minor splashing of
the stored liquid, well below the liquid surface, no
sparks will be produced, and there will be no
significant eruption of liquid. Testing has been carried
out by experts in the field, which has confirmed that
this system will significantly reduce the likelihood of
a rim fire caused by a lightning strike.
The
ARCMASTER™ consists
of five separate devices, each designed to function in
concert with one another to protect the AST from a
lightning induced rim fire. It is not possible to
unconditionally state that a rim fire will never occur,
considering the considerable number of variables
contributing to such an occurrence, such as intensity
and type of lightning strike, volatility of stored
liquid product, if tank is being filled or emptied at
time of occurrence, type and condition of primary and
secondary seals, etc. However, this system will
significantly reduce the likelihood of such a lightning
induced rim fire. The system allows pathways for both
the fast rate, high current amplitude lightning
components and the slow lightning components, both of
which are present in a lightning strike.
ARCMASTER™ Components
The five
ARCMASTER™
components include:
1.
Submerged
Lightning Conduction Path Shunts, connecting the
underside of the floating roof at the outer
circumference of the roof to the tank shell, spaced at
approximately 10 foot centers around the perimeter of
the roof.
2.
Submerged
Lightning Conduction Path Cable, connecting the
underside of the floating roof center drain sump to
the shell drain nozzle, located near the base of the
tank shell.
3.
Insulated
Scissor Shoe Primary Mechanical Seal, insulated from the
floating roof to prevent a conduction path from the
floating roof to the tank shell above the liquid
surface.
4.
Insulated
Wiper Type or Seal King Secondary Seal, insulated from
the floating roof to prevent a conduction path from the
floating roof to the tank
shell above
the liquid surface.
5.
Insulated
seals around the Gauge Pole wells and other such
penetrations in the floating roof.
As indicated,
these five components of the
ARCMASTER™
work together to dissipate the lightning
charge. Further, these components will aid in the
dissipation of any static build-up and eliminate
electrical potential differentials between various tank
parts.
Information From The Industry
1.
The AST industry, and API, recognize that the most
effective defense against ignition by lightning is a
tight fitting primary and secondary seal system.
Obviously, the ARCMASTER™
is
most effective with such properly fitting
seal systems.
2.
The AST must be out of
service, clean and gas-free, in order to install the
ARCMASTER™.
The floating roof-to-shell conduction shunts are
attached to brackets which are welded to the underside
of the floating roof.
3.
The floating
roof primary and secondary seals are designed to be
insulated from the floating roof, but will allow any
electrical static potential differentials of their
component parts to “bleed” to the tank shell.
4.
Conventional floating
roof-to-shell static shunts, located above the secondary
seal, although not necessary with the
ARCMASTER™,
may
remain in place, since they are located above the
combustible air/hydrocarbon vapor mixture.
5.
The
conventional Gauger’s Platform-to-Floating Roof
electrical conduction cable may remain in place, as it
is located above the combustible air/hydrocarbon vapor
mixture.
6.
HMT’s
ARCMASTER™
does not employ retractable cable reels
mounted on the top of the shell, which may cause
maintenance and operational problems.
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