Seal guide

Floating roof seal types: a practical guide

A floating roof rises and falls with the liquid, so a ring-shaped gap always remains between the roof edge and the tank shell. The rim seal closes that gap. It limits vapor loss, helps keep the roof centered, and keeps rain and dirt out of the rim space.

The shell is never perfectly round, so the seal must keep contact as the gap changes around the circumference and between fill levels.

Primary and secondary seals

Most floating roofs use two seals, one above the other.

Primary seal: the lower seal. It closes the rim space.
Secondary seal: an added seal on the roof rim above the primary. It catches vapor that gets past the primary, shields the primary from weather and covers small gaps or tears.

Trade-off: a secondary seal can reduce usable tank capacity, because it must stay in contact with the shell or clear the fixed roof when the tank is full.

Primary seal types

Mechanical shoe seal

A vertical metal sheet (the shoe) is pressed against the shell by springs or weighted levers and connected to the roof by braces or hangers. A flexible vapor-barrier fabric runs from the shoe to the roof. The bottom of the shoe reaches into the liquid.

It follows out-of-round shells well and only metal touches the product. Not suited to lined tanks.

Close-up of a mechanical shoe seal pressed against a tank shell

Liquid-mounted seal

A foam-filled or liquid-filled envelope in contact with the liquid surface all the way round. This removes most of the vapor space under the seal. The material must be compatible with the stored product.

Vapor-mounted seal

A resilient seal (foam or a flexible wiper blade) mounted above the liquid surface, so a vapor space remains underneath. Inexpensive and quick to install. Under US rules it is typically allowed only as the lower seal of a two-seal system.

How shoe seals are held against the shell

Pantograph

A hanger linkage that lets the shoe follow changes in the rim space, with springs, weights or both supplying the force. The most widely used shoe design.

Scissor

Scissor-style links plus springs keep the full height of the shoe in contact. Lighter than a weighted design and often chosen for light, volatile or corrosive products.

Secondary seal types

Secondary seal types
Type	What it is
Rim-mounted	Mounted on the roof rim and covers the entire rim space. More effective at cutting losses than a shoe-mounted secondary.
Shoe-mounted	Attached to the primary shoe. Used with mechanical shoe primaries.
Weather shield	A cover over the primary seal that protects it from sun, dust, debris and rain.

Most secondary seals use spring plates that press a wiper tip against the shell.
Double-layer wipers need less tip pressure.
Roller-assisted versions help stop a seal from rolling over on old, out-of-round tanks.

Primary seal types compared

Comparison of mechanical shoe, liquid-mounted and vapor-mounted primary seals
Property	Mechanical shoe	Liquid-mounted	Vapor-mounted
Where it sits	Metal shoe against shell, bottom in liquid	Envelope on the liquid surface	Above the liquid
Vapor space under seal	Small, sealed by fabric	Almost none	Yes
Strengths	Handles shell irregularities; only metal touches the product	Most vapor control of the three	Low cost, fast to install
Watch for	Corrosion, damaged linkage or fabric; not for lined tanks	Material compatibility with the product	Needs a secondary seal under US rules in most cases

Why gaps matter

Under US rules for external floating roofs, the gap between seal and shell is measured and compared to limits.

Seal gap limits for external floating roofs
Seal	Max total gap area	Max single gap width
Primary (shoe or liquid-mounted)	212 cm² per m of tank diameter	3.81 cm
Secondary	21.2 cm² per m of tank diameter	1.27 cm

Gloved hand measuring a rim seal gap with a probe

Gaps are found with probes. Each gap's width is multiplied by its length around the circumference, the areas are added up and divided by the tank diameter.

Holes, tears or openings in seal fabric must be repaired.

Primary seal gaps are measured within 60 days of first fill and at least every 5 years. Secondary seal gaps are measured within 60 days of first fill and at least once a year.

What inspectors look for

Gaps measured and recorded with location, width and length.
Continuous contact between each seal and the shell.
Tears, holes, wear or exposed reinforcement in fabric and wipers.
Corrosion, bending or jamming in shoes, linkages and supports.
Liquid or residue in the rim space.
Intact shunts and continuous electrical bonding.

Inspector in safety gear walking along the rim of a floating roof

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