Floating-Roof Storage Tank Product

Overview

A fixed-roof tank holding crude oil or gasoline keeps a vapor space above the liquid, and every fill, empty, and warm afternoon pumps hydrocarbon vapor out of it — lost product and an ignitable cloud. The floating-roof tank removes the vapor space entirely: the roof is a steel raft that rests directly on the liquid and travels with it. Tanks of this type store most of the world's crude inventory; the unit described here is an 80 m diameter, 100,000 m³ external floating-roof tank built to API 650.

Shell and bottom

The Tank Shell is eight welded Shell Course rings, each thinner than the one below it because hydrostatic pressure sets plate thickness — roughly 28 mm at the bottom course, 10 mm at the top on a tank this size. With no fixed roof to hold the cylinder round, the open top is stiffened by the Wind Girder, which doubles as the walkway around the rim, with Intermediate Wind Stiffener rings guarding against wind buckling when the tank is empty.

The Tank Bottom sits on a compacted foundation: a butt-welded Annular Ring Plate takes the shell line load, lap-welded Bottom Plates slope toward the Water Draw-Off Sump for water draw-off, and the soil side is defended by Cathodic Protection System with an HDPE Leak Detection Liner beneath to reveal any through-wall leak at perimeter telltales. Bottom corrosion is the life-limiting mechanism of most tanks and the focus of every API 653 out-of-service inspection.

The floating roof

The Floating Roof is a single-deck pontoon design: an annular Pontoon Ring of 20-odd sealed compartments provides buoyancy, sized per API 650 Annex C to keep floating with any two compartments punctured and 250 mm of rainwater on the deck. Inside the ring, the 5 mm Center Deck lies directly on the product. When the tank is emptied, the roof lands on adjustable Roof Support Leg about 1.8 m above the floor, leaving room for maintenance crews. A slotted Guide Pole prevents rotation and provides the gauging path, and Deck Vent bleed air from under the deck during initial filling so it cannot balloon.

Rain is the roof's enemy. The Roof Drain System collects water at the Drain Inlet Sump sump and carries it down through the stored product in an Articulated Drain Pipe with sealed Drain Swivel Joint, exiting through the Drain Shell Valve. Operators open that valve whenever rain threatens; roofs have been sunk by nothing more than a closed drain valve in a storm. Emergency Drain dump extreme rainfall straight into the product as a last resort.

Rim seal

The 200–300 mm gap between roof rim and shell is where nearly all remaining emissions escape, and where almost all floating-roof fires start. The Primary Shoe Seal is a mechanical shoe seal: galvanized Seal Shoe Plates pressed against the shell by Seal Hanger Mechanism, with a fabric membrane closing the space back to the rim, the assembly riding on the liquid so no vapor pocket forms beneath it. A Secondary Rim Seal wiper above it cuts what leaks past by another ~75%, and stainless Shunt Straps every few meters bond roof to shell so a lightning strike finds copper, not a spark gap over a flammable rim space — the ignition source in most recorded rim seal fires, addressed in detail by API 545.

Operations and protection

Access to the moving roof is by the Rolling Ladder, hinged at the Gauger Platform with self-leveling treads and wheels running on a Ladder Track across the deck. Inventory is measured by a Radar Level Gauge to ±1 mm for custody transfer, corrected by the Average Temperature Bulb average temperature; an independent Overfill Protection Switch trips the inlet per API 2350. Side-Entry Mixer keep crude sludge suspended, saving tank bottoms from waxy buildup between cleanings.

Fire protection concentrates on the rim: Rim Fire Detection cable rings the seal space, and Foam Chamber pour expanded foam behind the Foam Dam, flooding the annular seal area without needing to blanket the entire 5,000 m² roof. A Cooling Water Ring sprays the shell when a neighboring tank burns. Domed variants (covered EFRTs) add a geodesic aluminum roof above the floating roof where rain, snow or strict emission rules demand it.