Shoe Seal VS Wiper Seal -Tank Internal Floating Roof Rim Seals
The area between an internal floating roof and the tank shell is known as the rim space.
It is important for the operator to look closely at the design features of an IFR when purchasing it. With regards to the rim seal, API 650 states that a gap of 100 mm should exist between the tank wall and rim channel to allow for variations in the tank wall. There are two generic primary rim seal types:
- Liquid mounted seal “Shoe Seal“: A liquid-mounted seal means that a portion of the seal extends into the product.
- Vapour-mounted seal “Wiper Seal“: A vapour mounted seal means that the seal is above the product at all times.
Evaporation loss is a function of the surface area exposed. If a liquid-mounted rim seal is gapping, loss from the shoe seal is emitted from the small annular area between the shoe and the shell. With a vapour mounted rim seal, losses are emitted from the full surface area of the rim space below. Losses from vapour mounted rim seals are significantly higher than that for liquid mounted rim seals.
Life of Wiper Seals for Internal Floating Roofs:
- Light Duty Aluminium Internal Floating Roofs often use a “Single Wiper Seal”. This seal material is generally made from a urethane/Polyethylene type compound. In our experience, Urethane/Polyethylene type wiper seals have generally been found to have a short lifespan, usually between 5 to 10 years depending on tank service & average ambient temperature.
- The simple wiper seals are also not capable of coping well with “out of round” tanks.
- The wiper seal does not have many capabilities to cope with large variations in rim space.
- Urethane/Polyethylene type wiper seals are also limited to the number of liquids they are chemically resistant to.
Life of Shoe Seals for Internal Floating Roofs:
- Internal Floating Roofs that use rim-mounted Shoe Seals have a life expectancy of over 25 years.
- Rim mounted shoe seals are effective with older tanks that are “out-of-round” providing excellent sealing against the tank wall. It is a “tight-fitting-seal”
- When using “Teflon” vapour barrier fabric, a rim-mounted shoe seal is chemically resistant to 100% aromatics and MTBE as well as an extremely large range of other chemicals. (Teflon is virtually resistant to any chemical)
- This is useful if the customer wishes to change the service of the tank and is not sure if the wiper seal is chemically suitable.
- Fabricated from corrosion-resistant materials and fasteners (stainless steel)
- Shoe plates are overlapped to provide effective sealing during expansion
and contraction while the internal floating roof travels up and down