Rimseal foam vessel on a floating roof tank

API 2021

Rimseal Fire Protection System

Linear heat detection cable at the rim seal, with AFFF or AR-AFFF foam discharged into the seal area. Applied to floating roof storage tanks in crude and naphtha service.

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At a glance

Governing standard: API 2021
Designed to: NFPA 11, OISD-STD-117
Equipment source: Our own works

How it works

Schematic of a rimseal fire protection system, showing how water, agent or signal flows through it

On a floating roof tank the vulnerable place is the seal between the roof and the shell, where vapour escapes and a lightning strike or a hot work spark can ignite a fire that runs the whole circumference.

A rim seal system detects at the seal itself — usually a linear heat sensing cable laid in the rim space — and discharges foam into the rim area through pourers spaced around the roof, so the fire is fought where it starts rather than being allowed to develop into a full-surface fire. Because the roof moves, the discharge pipework and the detection line have to accommodate travel over the full range of the tank.

A full-surface fire is a different problem needing a different answer, and the two should not be conflated when a scheme is specified.

Main components

Linear heat detection in the rim space, and its control panel.
Foam storage: premixed solution or a bladder tank.
A release valve that opens on detection.
A foam line round the roof, pourers at intervals, and a foam dam to hold the foam over the seal.

Testing and maintenance

Test the detection loop and the release sequence.
Check the foam dam and the pourer outlets.
Check the premixed solution or concentrate at the intervals the code sets.

System arrangements

Arrangement	How it works	Suits
Fixed system	Foam solution piped from outside the bund to pourers round the rim, released on detection	Large tanks with a fire water network at hand
Roof-mounted modular	Premixed foam stored on the roof, released locally by the detection	Tanks without a nearby foam supply

Standards

NFPA 11Foam systems, including rim seal protection on floating roof tanks.
OISD-STD-117Fire protection facilities for petroleum depots and terminals.

Design basis

Parameter	Figure	Source
Application rate, top-of-seal with foam dam	12.2 L/min/m² of annular seal area	NFPA 11 — floating-roof tanks
Discharge time	20 minutes	NFPA 11 — floating-roof tanks
Foam dam height	At least 0.3 m	NFPA 11 — floating-roof tanks
Discharge outlet spacing on the circumference	12.2 m with a 0.3 m dam; 24.4 m with a 0.6 m dam	NFPA 11 — floating-roof tanks

Service and maintenance

A fire protection system only performs if it is kept in the condition it was commissioned in. Inspection, testing and maintenance to the intervals the code sets — under an annual maintenance contract or as a single visit.

What it involves

Task	Interval	Reference
Test the detection loop and the release sequence	Six-monthly	NFPA 72 / NFPA 25
Inspect pourers, foam dam and roof pipework	Six-monthly	NFPA 25
Check the premixed solution or concentrate	Annually	NFPA 11

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Why De’s Technico

Equipment from our own catalogue
Foam pourers and bladder tanks are made in our own works, so supply and execution answer to one schedule.
9 installations on record
Our project record lists 9 rim seal fire protection installations, from design through commissioning.
Built and tested in our works
Manufactured in two factory units of 48,000 and 30,000 sq ft, with in-house testing before despatch.
One contract, design to handover
A lump-sum turnkey scope on single-window responsibility: design, engineering, supply, installation and commissioning answer to one schedule.

Questions we are asked

Why protect the rim seal specifically?

Most floating-roof tank fires start at the seal, where vapour escapes and a lightning strike can ignite it. Put out there, the fire never becomes a full-surface fire.

What detects a rim seal fire?

Usually linear heat detection cable laid in the rim space, which senses heat along its whole length.

What is the foam dam for?

It holds the foam over the seal area so it does not run off across the roof. NFPA 11 sets a minimum height of 0.3 m.

Can it be installed on a tank in service?

Much of the work can, planned around the tank’s operation and its hot-work permits; some roof work may need the tank emptied, which the survey establishes.

How often should it be tested?

The detection and release sequence is tested routinely, and the foam supply checked at the intervals the code sets; a discharge test is run where the operator permits one.