Open-top tank with an external floating roof
Storage tanks

Floating Roof Tanks

Rimseal fire and full-surface fire need different answers. Detection at the seal, foam at the roof.

Discuss this hazard

At a glance

Hazard family
Storage tanks
Protected by
Rim seal, Foam, Hydrant
Designed to
NFPA 11, OISD‑STD‑116, OISD‑STD‑117
Rim seal foam, top-of-seal with foam dam
12.2 L/min/m² of annular seal area

Design basis

Rim seal foam, top-of-seal with foam dam
12.2 L/min/m² of annular seal areaNFPA 11 — floating-roof tanks
Rim seal discharge time
20 minutesNFPA 11 — floating-roof tanks
Foam dam height
At least 0.3 mNFPA 11 — floating-roof tanks
Discharge outlet spacing
12.2 m with a 0.3 m dam; 24.4 m with a 0.6 m damNFPA 11 — floating-roof tanks

Standards that apply

  • NFPA 11Low-, medium- and high-expansion foam.
  • OISD-STD-116Fire protection for refineries and oil and gas processing plants.
  • OISD-STD-117Fire protection for petroleum depots and terminals.

The risk

A floating roof rides on the liquid, so there is almost no vapour space and a full-surface fire is rare. The weak point is the rim seal — the gap between the roof and the shell, where vapour escapes. Most floating-roof fires start there, and lightning is the classic ignition source.

Caught early, a rim seal fire stays small. Left to burn, it damages the seal and can sink or tilt the roof, and then it is a full-surface fire — with crude oil, one that can end in boilover. The radiant heat puts neighbouring tanks at risk too.

The scheme

Fight the fire where it starts, and hold the rest in reserve:

  • Detect at the seal: linear heat detection cable in the rim space.
  • Suppress at the seal: a rim seal system that discharges foam through pourers behind a foam dam, automatically.
  • Be ready for a full-surface fire: foam makers on the shell or monitors, fed from the foam system.
  • Cool the shell of the tank on fire and its neighbours with water spray.
  • Back everything with the hydrant ring main and monitors.
Section through a floating roof tank showing the rim seal, foam dam, foam pourers, linear heat cable and shell cooling

Systems that apply

Products used

Foam Equipment · 10

Spray Nozzles · 1

Questions we are asked

Where do floating-roof tank fires start?

Mostly at the rim seal, where vapour escapes between the roof and the shell. Lightning is the most common ignition source.

Is a rim seal system enough on its own?

It deals with the fire that is most likely. A full-surface fire needs foam makers or monitors and cooling as well, which is why tank farms carry both.

What is boilover?

In a long-burning crude oil fire, a heat wave travels down through the oil and reaches water at the bottom of the tank, which turns to steam and throws burning oil out of the tank. Early suppression is what prevents it.

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