Fixed roof tank
Storage tanks

Fixed / Cone Roof Tanks

Top pourers and foam chambers discharging onto the liquid surface.

Discuss this hazard

At a glance

Hazard family
Storage tanks
Protected by
Foam, Hydrant
Designed to
NFPA 11, OISD‑STD‑116, OISD‑STD‑117
Foam application rate
4.1 L/min/m² of liquid surface (minimum)

Design basis

Foam application rate
4.1 L/min/m² of liquid surface (minimum)NFPA 11 — fixed-roof tanks
Discharge time, flash point 37.8–93.3 °C
30 minutesNFPA 11 — fixed-roof tanks
Discharge time, flash point below 37.8 °C, or crude oil
55 minutesNFPA 11 — fixed-roof tanks
Discharge outlets per tank
1 up to 24 m diameter; 2 to 36 m; 3 to 42 m; 4 to 48 m; 5 to 54 m; 6 to 60 mNFPA 11 — fixed-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

Under a fixed roof there is a vapour space above the liquid, and it can hold a flammable mixture. An ignition inside the tank lifts the roof — the roof-to-shell seam is built to fail first so the shell stays standing — and leaves a fire across the whole liquid surface.

A surface fire on a large tank radiates enough heat to threaten every tank around it, and with crude oil or other heavy products it carries the risk of boilover.

The scheme

Put foam onto the surface gently, and keep the neighbours cool:

  • Foam chambers or pourers on the shell, with vapour seals, deflect foam down the inside of the shell so it spreads across the surface without plunging in.
  • Foam solution is supplied from outside the bund, from a bladder tank or inductor.
  • Water spray cools the shell of the tank on fire and the tanks exposed to it.
  • Hydrants and monitors back up the fixed systems, and foam makers on the bund wall deal with any spill.
Section through a fixed cone roof tank showing the foam chamber with vapour seal, deflector, foam line from outside the bund and shell cooling

Systems that apply

Products used

Foam Equipment · 10

Spray Nozzles · 1

Questions we are asked

Why does foam go in through the shell, not over the roof?

Foam chambers on the shell deflect foam down the inside wall so it spreads gently across the surface. Foam dropped onto the liquid would plunge in and break down.

What does the vapour seal do?

It keeps tank vapour out of the foam line while the system is idle, and breaks under foam pressure when the system operates. It should be inspected regularly and replaced after every discharge.

How many foam outlets does a tank need?

It depends on the diameter: under NFPA 11, one outlet up to 24 m, rising to six at 60 m.

Related hazards

Track record

Projects on this hazard

All projects →

Considering us for a turnkey scope?

Send the hazard, the code and the site. Our engineers reply with a scheme and a price.

Contact Us