- This type of seal utilizes a light-gauge metallic band as the sliding contact with the shell and a flexible polymeric vapor barrier to close the annular space between the metallic band and the rim of the floating roof deck. The band is typically formed as a series of sheets (shoes) that are overlapped or joined together to form a ring and are connected to the roof via a scissor-shape mechanism and held against the shell by employing leaf springs in order to maintain complete and unified touch with the tank shell. - On each shoe plate, an array of leaf springs, installed at different heights exert uniform pressure in both vertical and horizontal directions keeping the entire width of the shoe in contact with the tank shell, leading to better and enhanced sealing performance. - This new design is rapidly replacing older types of mechanical shoe seals due to its innovations. - VISTA’s scissor type Shoe plate seals are suitable for tanks with diameters up to 110m, which are designed and constructed in accordance to customer’s requirements considering the regulatory compliance, service life expectancy and other compatibility factors.
Generally speaking, each and every tank has its own particular and unique dimensional and operational parameters; thus, VISTA’s solution covers many different designing aspects considering the Rim Gap space, the particulars of the fluid to be stored, the material of the tank, various geometrical dimensions of the tank, etc. This leads to the construction of a seal according to the customers’ requirements. In VST – 11 design the following points are that of paramount importance: - The leaf spring arrangement ensures the constant and uniform adherence of the shoe plates to the tank wall. - The shoe plates extend downward, into the stored liquid, withholding the vapors and fumes in the tank to the maximum extent. Moreover, extending the upper edge of the shoe plates above the rim of the roof deck, ensures the discharge of the rain water toward the center of the roof rather than the edge of the roof. - The seals are designed in compliance with the relevant international standards such as API 650 & NFPA 780. Obviously, customer’s request to comply with API RP 545 & API 2003 can easily be fulfilled. - VST – 11 seals are suitable for tanks with Rim Gap measuring 100 mm – 400 mm with the life expectancy of 10 – 20 years, considering the operational environment and conditions.
The materials selection is carried out in view of the tank’s operational condition, taking into account the product stored, ozone factor, UV radiation, rainwater downfall and so on to reach the best solution and life expectancy for a long and trouble-free service at the lowest cost. The material used in VST – 11 is as below: Sealing Ring (Shoe Plates): normally galvanized mild steel, but stainless steel alloys can be used upon the customer’s request. Scissor System: Normally galvanized mild steel and stainless steel are also available. The pins in the joints are made of stainless steel. Continuous Seal: Normally PVC/Nitrile, other reinforced polymers are also available. Anti-static Shunts: are normally made out of stainless steel materials. Aromatic Resistance: up to 100% if required.
Wax Scraper: when storing waxy products, in accordance with the seals specification, VISTA would design a Wax Scrapper Device which could be easily installed at the lower sections of the Shoe Plates to prevent the accumulation and deposition of wax on the floating roof deck. Secondary Seal: VST – 21 is recommended as the secondary seal to all sealing solutions. However, all the other secondary seal devices including Shoe Mounted Secondary Seals can be supplied and installed.