Metallic Seals

MN-1 Metallic Seal

The MN-1 Metallic Seal with Pantagraph Hangers is the most widely used of all floating roof seals.

It consists 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 tank-shell coatings.

The pantagraph 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.

The pantagraph 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 products.

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 It will remain centred, and return into the tank safely and easily when the product is withdrawn.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.

The continuous 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 ignition.

MN-2 Metallic Seal

The 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.

Accessories for Metallic Seals

Wax Scrapers

Wax scrapers 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 roof.

Sole Plates

Sole plates 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.

Wear Plates

Wear Plates 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

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.

They should 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.

Insulated Pantagraphs

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.

Insulated pantagraph hangers and shunts protect the floating roof from damage through lightning.