Metallic Seal with Pantagraph Hangers is the most widely used of
all floating roof seals.
of a ring of galvanised steel shoe plates, the bottom of which remains
below the liquid surface. A continuous vapour-tight and weatherproof
synthetic rubber-coated fabric is used to close the space between
the sealing ring and the rim of the floating roof. The lower edge
of the fabric is bolted to the edge of the floating roof. The upper
edge of the fabric is attached to the top of the shoe plates.
The shoe plates
are supported and held firmly, but gently, against the tank shell
by pantagraph hangers which apply a uniform outward radial pressure.
The light pressure minimises abrasion of the shoe plates and of
hangers automatically keep the roof centred in the tank. If the
roof should tend to move off centre, the pressure of the hangers
will increase on the side where the rim space is the narrowest.
This action automatically corrects the roof location.
hangers will accommodate a rim space variation of plus-or-minus
five inches (127mm) from nominal dimensions. Each is ruggedly built
to have sufficient reserve strength to operate even if its section
is greatly reduced by corrosion. Pantagraph hangers are recommended
for use in all types of service, including the storage of corrosive
An MN-1 Metallic
Seal with Pantagraph Hangers will retain its vertical position even
when the shoe plates extend above the top of the tank shell. Thus,
the sealing ring will not be damaged even if the tank is filled
to overflowing. It will remain centred, and return into the tank
safely and easily when the product is withdrawn. The knee of the
pantagraph hanger accounts for this desirable feature. When hangers
without this feature are used, a tank shell extension is required.
In order to
accommodate variations in tank diameter and circumference, there
must be provision for expansion and contraction of the sealing ring.
The MN-1 shoe plates provide for expansion and contraction with
vertical flexures spaced approximately 22 inches (559mm) apart.
These flexures serve both as stiffeners to distribute hanger pressure
over the height of the shoe plates, and as hinges which allow them
to conform to the curvature of the tank shell. Since the flexures
are an integral part of the shoe plates, this construction results
in a one-piece metallic sealing ring around the entire periphery
of the tank without the need for inter-shoe seals.
fabric used to close the rim space is resistant to up to 100% aromatics.
Antistatic and low flame grades are also produced.
Stainless steel shunts above the continuous seal provide sure electrical
bond between the floating roof and the shoe plates which are in
contact with the tank shell. These shunts reduce the chance of lightning
MN-2 Metallic Seal is specially recommended for riveted or lap-welded
tanks. They can sometimes overcome sealing difficulties where tank
shells have become badly deformed.
The MN-2 Metallic
Seal acts in a similar manner to the MN-1 Seal except that additional
provision for expansion and contraction is made with inter-shoe
seals of rubberised fabric. These intershoe seals are spaced from
50 to 60 feet (15.2 to 18.3m) apart and provide extra lateral flexibility
to the sealing ring.
for Metallic Seals
may be fitted to metallic seals on butt-welded tanks handling waxy
crude. They minimise problems resulting from a layer of waxy material
on the tank shell above the floating roof, melting from the sun’s
heat, and flowing down the shell onto the roof.
The wax scrapers
are made from 1.6mm galvanised sheets and are attached to the shoe
plates. The scraper portion is below the main sealing ring. The
reservoir portion above the sealing ring collects the wax which
melts down from the shell and prevents it from flowing onto the
are added when metallic seals are used in riveted or lap-welded
tanks. They assist the shoe plates in riding over rivets and plate
edges. Sole plates are made from 1.6mm galvanised sheets and are
flared at the top and bottom. Since they are bolted only at the
top where they are connected to the shoe plates, they may be removed
easily from above.
are used to extend the life of the shoe plates by adding an additional
wearing layer which may be replaced conveniently when worn out.
Wear plates are similar to sole plates except that the length of
flare is reduced, and the height of the wear plates is approximately
the same as the shoe plates.
Shoe plate secondary
seals can be fitted to increase the efficiency of a metallic seal.
A rubber loop is fitted at the top of the shoe plates to prevent
the wind from sweeping vapour out from between the shoe plates and
the tank shell.
preferentially be used on riveted or lap welded tanks storing gasoline
or other volatile stocks, because rivets and laps hold the shoe
plates some distance from the shell.
In tanks storing
low vapour pressure stocks, floating roofs with metallic seals can
be equipped with insulated pantagraph hangers for added protection,
especially in areas having frequent electrical storms.
hangers and shunts protect the floating roof from damage through