Shell (tank wall)A cylindrical steel shell stands on the flat tank bottom.
Fixed roof (cone roof)It is permanently fixed to the shell and never moves.
Floating roof (deck)It rises and falls with the liquid level.
Containment dike (bund)Holds the contents in if the tank spills.
Stairway (spiral stairs)Spiral steps climb the shell to the top platform.
Foundation (ringwall)A reinforced concrete ring under the shell carries the tank.
Breather vent (pressure-vacuum vent)Lets vapor out and air in at slight pressure or vacuum.
Rim sealFixed to the deck edge, it touches the shell all around.
Fixed roof tank 9
Handrail (guardrail)Railings guard the stairway and the platforms.
Roof manholeA way in through the fixed roof.
Shell manhole (manway)An opening in the shell for getting inside the empty tank.
Gauge hatch (sample well)Opened to measure the level or take a sample by hand.
Level gauge (float gauge)A float on the liquid moves a tape to show the level.
Tank bottom (floor plates)Steel bottom plates slope so that water drains to a low point.
Outlet line (suction line)The stored liquid is pumped out through it.
Sump (drain sump)Water that settles out of the stored liquid collects here.
Emergency ventGives extra vent flow if pressure climbs too high.
Floating roof tank 9
Gauger's platformIt stands on top of the shell, above the gauging well.
Pontoons (float chambers)Liquid-tight compartments that give the roof its buoyancy.
Deck legs (roof support legs)They hold the landed roof off the tank bottom.
Roof drain (deck drain)Collects the rainwater that falls on the deck.
Roof drain hose (flexible drain line)Carries rainwater through the stored liquid and out of the tank.
Guidepole (gauge pole)Stops the floating roof from turning.
Vacuum breaker (automatic bleeder vent)Opens as the roof lands on its legs or floats off them.
Rim vent (rim space vent)Releases pressure from the vapor pocket behind the seal.
Access hatch (deck manhole)A way through the deck for workers and materials.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against pages read on 2026-09-20. US Environmental Protection Agency, AP-42 chapter 7.1, Organic Liquid Storage Tanks (pages dated 10/2024): https://www.epa.gov/sites/default/files/2020-10/documents/ch07s01.pdf (a fixed roof tank is a cylindrical steel shell with a permanently affixed roof, cone- or dome-shaped or flat; it is freely vented or has a pressure/vacuum vent that lets it run at a slight internal pressure or vacuum; emergency vents give increased vent flow capacity in the event of excessive pressure; figure 7.1-1 names the breather vent, float gauge conduit, tank roof and shell, gauge-hatch/sample well and roof manhole; an external floating roof tank is an open-topped cylindrical steel shell whose roof floats on the liquid; the floating roof is a deck, deck fittings and a rim seal system; decks are of the pontoon or double-deck type; the rim seal system is attached to the deck perimeter and contacts the tank wall; access hatch, gauge-float, gauge-hatch/sample port beneath the gauger's platform mounted on top of the tank shell, rim vent, closed deck drains that carry rainwater through a flexible hose through the stored liquid, deck legs that hold the deck off the tank bottom, the guidepole as an antirotational device also called gauge pole, gauge pipe or stilling well, and the vacuum breaker that equalizes pressure as the deck is landed on or floated off its legs; the tank outlet and outlet line, a sump in the tank bottom, and the shell manhole are named in the tank cleaning section). US Department of Defense, UFC 3-460-01, Design: Petroleum Fuel Facilities, change 3 of 8 June 2023, approved for public release: https://www.wbdg.org/FFC/DOD/UFC/ufc_3_460_01_2019_c3.pdf (table 8-1 Appurtenances: shell manholes, an access manway, roof manhole, a spiral stairway with platforms and handrail, platform with safety railing for gauging, pressure/vacuum vents, emergency relief venting, mechanical tape level gauge, gauge hatch, slotted stilling well, inlet fill connection, main suction and low suction, water draw-off; section 8-8.4 tank bottoms slope downward for drainage; section 8-8.5 a reinforced concrete ringwall foundation beneath the tank shell; section 8-14 diked enclosures; glossary: Dike, a bank or wall, usually earth or concrete, round a storage tank to hold the contents if there is a spill; Pontoon Roof, liquid-tight compartments for positive buoyancy; Sump, a low area which receives drainage; Water Bottom, free water that has settled at a storage tank's bottom). Code of Federal Regulations, 40 CFR 60.112b: https://www.govinfo.gov/content/pkg/CFR-2023-title40-vol7/xml/CFR-2023-title40-vol7-sec60-112b.xml (automatic bleeder vents, also called vacuum breaker vents, closed except when the roof is being floated off or landed on the roof leg supports; rim space vents; roof drains; access hatch; an external floating roof is a pontoon-type or double-deck type cover; primary and secondary seals; mechanical shoe seal). Wikipedia: https://en.wikipedia.org/wiki/Fixed_roof_tank (a cone- or dome-shaped roof permanently affixed to a cylindrical shell; a breather valve, or pressure-vacuum valve; gauge hatches, sample wells, float gauges and roof manholes); https://en.wikipedia.org/wiki/External_floating_roof_tank (the roof moves up and down with the liquid; a rim seal system between the tank shell and roof; support legs on which the roof lands at low liquid levels; rainwater drained by a flexible hose from the roof's drain sumps, through the stored liquid); https://en.wikipedia.org/wiki/Storage_tank (vertical cylindrical flat-bottom tanks with fixed or floating roofs; foundations that keep water off the tank bottom plates; containment dike) and https://en.wikipedia.org/wiki/Bunding (bund as the British word for the containment around a tank). Two tank builders' pages, for names only: https://www.hmttank.com/products/external-floating-roofs/ (pontoon, roof drain, rim seal, gauge pole, roof support leg) and https://www.fishertank.com/services/fabrication (stairways, platforms, handrails and manways as tank fittings). The plan also names a university chemical engineering department's equipment notes. https://encyclopedia.che.engin.umich.edu/ returned 403 to every request on 2026-09-20, so nothing here rests on it; the Department of Defense criteria took its place as the second public document. The industry's welded tank standard is not free to read and was not used. No agency and no maker is named on the page. Re-checked on 2026-09-30: all 26 names found again in two or more independent sources: the Environmental Protection Agency's AP-42 chapter 7.1, the Department of Defense criteria UFC 3-460-01, 40 CFR 60.112b, the two builders' pages and Wikipedia's wikitext for Fixed roof tank, External floating roof tank, Storage tank and Bunding (all at the addresses above, fetched again). Because the regulation is the same agency's and Wikipedia's two tank articles follow AP-42 closely, the floating roof fittings were also read in an engineering textbook, Aboveground Storage Tanks (McGraw-Hill), https://archive.org/details/abovegroundstora0000myer (rim vent, vacuum breaker, gauger's platform mounted on top of the tank shell, access hatch, roof legs, emergency vent valves, guide pole, roof drain with a hose or flexible pipe, pontoon, rim seal), which gives the vacuum breaker, the rim vent and the gauger's platform a second source. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; own work, nothing traced or copied. Licence: own work
How it works
The parts of a storage tank depend on its roof. A fixed roof tank breathes through a vent on a cone that never moves. A floating roof tank has an open top and a deck that rides on the liquid. This drawing cuts both open.
Read the full guide
Fixed roof tank parts, from the ground up
A concrete ringwall foundation carries the shell. The steel tank bottom slopes, so water that settles out runs to the sump. The stored liquid leaves by the outlet line low on the shell. On the cone roof, the breather vent lets vapor out and air in at slight pressure or vacuum, and the emergency vent adds flow if pressure climbs. The gauge hatch opens for measuring the level by hand.
What a floating roof tank has instead
No fixed roof. The deck floats on pontoons, and a rim seal closes the gap to the shell. A guidepole stops the deck turning. Rain leaves by the roof drain and a hose through the liquid. At low level the deck lands on its legs and the vacuum breaker opens.
Start here: 8 main parts covers the shell, both roofs, the dike, the stairway, the foundation, the breather vent and the rim seal.
Pick the group Floating roof tank for its 11 parts alone. The drawing zooms in on the right-hand tank.
Questions and answers
What are the main parts of a storage tank?
An above-ground vertical tank has a foundation, a tank bottom, a cylindrical steel shell and a roof, fixed or floating. Around them come a stairway and handrail, manholes, vents, a gauge hatch, a level gauge, an outlet line and a containment dike.
What is the vent on top of a storage tank called?
On a fixed roof tank it is the breather vent, also called a pressure-vacuum vent. It lets vapor out and air in, so the tank runs at slight pressure or vacuum. An emergency vent adds vent flow if pressure climbs too high.
What is the difference between a fixed roof and a floating roof tank?
A fixed roof is permanently fixed to the shell, with a vapor space under it. A floating roof rests on the liquid and rises and falls with the level, and a rim seal closes the gap between its edge and the shell.
What is the wall around a storage tank called?
A containment dike, also called a bund. It is a bank or wall, usually earth or concrete, that surrounds the tank and holds the contents in if the tank spills.