NFPA 11 Floating Roof Seal Protection: Over and Under

In an open-top floating roof tank the roof floats directly on the product. A seal closes the narrow ring between the roof and the shell. Annex A notes that most fires in these tanks are seal fires, not full-surface fires after the roof sinks (A.5.3.3). For that reason NFPA 11-2024 puts the seal area at the centre of floating roof protection (5.3). This article covers the two main methods in Section 5.3, foam over the seal and foam under it. It also covers the foam dam, outlet spacing, handlines and the Section 5.4 rules for internal floating roof tanks. For a scenario walk-through and a full-surface calculation, see refinery floating roof tank and NFPA 11.

Which tank, which protection? (5.3.1)

A tank is protected either by seal area protection only, or by seal area protection together with full-surface protection (5.3.1.1). Seal-only protection is accepted for these roofs (5.3.1.2):

• Steel double-deck roofs
• Steel pontoon roofs
• Other roof types that have passed external tank construction testing

Roofs made from floating diaphragms, roofs that rely on flotation closures that sink easily when damaged, and pan roofs are more likely to sink in a fire. Clause 5.3.1.3 asks for both seal and full-surface protection on them. Clause 5.3.1.4, however, lists the same three roof types as outside the scope of Section 5.3. The practical result is that seal-only protection is not enough for these roofs. Agree the design basis with the authority having jurisdiction.

Annex A says full-surface fires are very infrequent and do not normally justify a fixed system. It recommends a response plan for that scenario based on portable and mobile equipment (A.5.3.3).

Not allowed: subsurface injection

Subsurface and semi-subsurface injection are not used on open-top or covered floating roof tanks. The roof, even partly sunk, can stop foam from spreading properly over the fuel (5.3.3.1). See NFPA 11 subsurface foam injection for the details.

Methods for a seal fire (5.3.4)

The standard recognises three primary methods for seal fires (5.3.4.1). These are fixed discharge outlets, foam handlines and small portable monitors of 1,900 L/min (500 gpm) or less used from the top platform. Annex A adds a warning: overapplying foam to the seal area can tilt the roof and make the fire worse (A.5.3.4.1(3)). Supplementary protection under Section 5.9 is required on top of the primary method (5.3.4.2). The system is sized for the tank that needs the largest solution flow, including hose streams (5.3.4.3).

Fixed outlets can deliver foam in two ways (5.3.5.1):

1. Over the seal: foam is delivered above a mechanical shoe seal, a metal weather shield or a secondary seal.
2. Under the seal: foam goes below a mechanical shoe seal straight onto the liquid. It can also go behind a metal weather shield onto the tube seal envelope, or beneath a secondary seal onto the primary seal.

Top-of-seal application: Table 5.3.5.3.1

Fixed outlets that deliver foam over the seal are always used with a foam dam (5.3.5.2.1). They can be mounted in two ways. The first is above the top of the shell, normally Type II outlets. The second is on the periphery of the floating roof (5.3.5.2.2). In this application the outlets are not fitted with frangible vapour seals (5.3.5.2.3). Annex A explains why: all outlets are fed from a common ring main, so pressure variations at start-up may leave some seals unbroken (A.5.3.5.2.3).

The table title covers internal floating roof tanks as well as open-top tanks. The table applies to hydrocarbons and to liquids needing alcohol-resistant foam (5.3.5.3.2). If a product listing requires a higher rate, that rate applies (5.3.5.3.3). A higher rate allows a proportional cut in time, but not below 70 % of the minimum time (5.3.5.3.4). For a 20-minute time the floor is 14 minutes. The 300 mm and 600 mm in the spacing columns are the dam height. The notes to the typical arrangement in Annex A tie 12 m spacing to a dam at least 300 mm high, and 24 m spacing to a dam at least 600 mm high (Figure A.5.3.5.2(a)). Where outlets are mounted above the top of the shell, a foam splashboard is needed because of wind (A.5.3.5.3.1).

The foam dam (5.3.5.4)

The dam holds foam in the seal area, deep enough to cover the seal. The foam then flows sideways along the seal to the point where the seal has failed (5.3.5.4.3). The rules are:

• The dam is circular and made of steel plate at least 3.4 mm (No. 10 US standard gauge, 0.134 in) thick (5.3.5.4.1). It is welded or otherwise fastened to the floating roof (5.3.5.4.2).
• It is at least 300 mm (12 in) high, measured from the joint between seal and roof (5.3.5.4.3.1).
• It extends at least 50 mm (2 in) above a metal secondary seal, or above a combustible secondary seal that uses a plastic-foam log (5.3.5.4.3.2). It is also at least 50 mm higher than any burnout panels in metal secondary seals (5.3.5.4.3.3).
• At its highest point it is at least 300 mm and no more than 600 mm from the shell (5.3.5.4.4).
• Its bottom is slotted to drain rainwater. Slot area is 278 mm² per m² of dammed area (0.04 in² per ft²), and slots are no more than 10 mm (⅜ in) high (5.3.5.4.5). Annex A warns that too much opening lets foam escape (A.5.3.5.4.5).

Below-the-seal application: Table 5.3.5.3.6.1

In this method foam goes straight into the space under the seal, so the rate is higher and the time shorter. It is not used on tanks with combustible secondary seals (5.3.5.3.6.4).

The table applies to hydrocarbons and to liquids needing alcohol-resistant foam (5.3.5.3.6.2). A higher listed rate overrides it (5.3.5.3.6.3). Clause 5.3.5.3.5.2 also sets out when a dam is needed. It is needed where foam is released above the primary seal or its vapour barrier and the top of the primary seal is less than 152 mm (6 in) below the top of the pontoon.

Handlines and monitors

For seal fires on open-top floating roof tanks, foam handlines may be used from the wind girder (5.3.6.1). The equipment must be listed or approved (5.3.6.2). Annex A limits this approach to tanks under 76 m (250 ft) in diameter and recommends the following arrangement (A.5.3.6):

• A foam dam is installed in line with 5.3.5.4.
• A single fixed outlet goes at the top of the stairs. It covers about 12 m (40 ft) of seal on each side of the stairs and delivers at least 190 L/min (50 gpm). This gives the crew a safe base on top of the tank.
• Two 40 mm (1.5 in) valved hose connections go at the top of the stairs, and the wind girder gets a railing.

Monitors are not used as the primary means of extinguishing a seal fire. It is hard to direct foam into the annular space, and the roof may sink (5.3.7). Annex A also notes that after a long free burn, foam may fail to seal against the shell. If water supplies allow, shell cooling is recommended (A.5.3.4.3).

Worked example: 60 m open-top tank

The tank has a steel pontoon roof and a mechanical shoe seal, protected over the seal. The foam dam stands 0.6 m in from the shell and is 600 mm high. The concentrate is 3 %.

With a 300 mm high dam, spacing would drop to 12.2 m and the count would rise to 188.5 / 12.2 = 15.5 → 16 outlets. Now take below-the-seal protection on the same tank. If the rim gap is taken as 0.25 m for this example, the area is about 46.9 m². Take the real figure from the tank drawings. The flow is then 46.9 × 20.4 ≈ 957 L/min for 10 minutes (about 9.6 m³). The outlet count is 188.5 / 40 = 4.7 → 5. You can try the figures in the foam concentrate calculator.

Internal (covered) floating roof tanks (5.4)

An internal floating roof tank is a vertical cylindrical tank with a fixed metal roof (cone or geodesic dome) over a floating roof (A.5.4). Section 5.4 does not cover these roof types (5.4.1):

• Floating diaphragm roofs
• Plastic blanket roofs
• Roofs using plastic or other flotation material, even if encapsulated in metal or glass fibre (except as permitted in 5.4.2)
• Roofs relying on flotation closures that sink easily when damaged
• Pan roofs

Seal protection alone is accepted only on these roofs: steel double-deck and steel pontoon roofs, and two kinds of full liquid-surface-contact sandwich roof (5.4.2). One sandwich roof uses metal panels. The other is a composite roof/seal system. The composite system has extra conditions. The top layer has a flame spread index not over 25. The core is closed-cell. The perimeter seal is gas-tight, and noncombustible material covers the rim space. All other roofs need full-surface protection (5.4.2.1).

For a seal fire in an internal floating roof tank, the body text refers only to Table 5.3.5.3.1, which is top-of-seal application (5.4.2.3.2). The time-reduction rule in 5.4.2.3.4.2 again uses the 70 % floor. However, that clause refers to the subsurface table (Table 5.2.6.5.1) for the rule. Settle that reference with the authority having jurisdiction at design stage. For full-surface design, use the tables in NFPA 11 fixed-roof tank foam.

Common mistakes

• Not matching dam height to the spacing column. A 300 mm dam cannot use 24.4 m spacing.
• Using below-the-seal application on a tank with a combustible secondary seal (5.3.5.3.6.4).
• Fitting frangible vapour seals to top-of-seal outlets (5.3.5.2.3, 5.4.2.2.3).
• Cutting drain slots taller than 10 mm or leaving too much open area (5.3.5.4.5).
• Designing monitors as the primary means for a seal fire (5.3.7).

Frequently Asked Questions

What rate and time apply to a floating roof seal fire?

Top-of-seal fixed outlets use 12.2 mm/min (0.3 gpm/ft²) for 20 minutes (Table 5.3.5.3.1). Below-the-seal outlets use 20.4 mm/min (0.5 gpm/ft²) for 10 minutes (Table 5.3.5.3.6.1). A higher rate allows the time to be cut to no less than 70 %.

How high must a foam dam be and how far from the shell?

The dam is at least 300 mm high, measured from the seal-to-roof joint (5.3.5.4.3.1). It stands at least 50 mm above a metal secondary seal. It is between 300 mm and 600 mm from the shell (5.3.5.4.4). It is made of steel plate at least 3.4 mm thick, with drain slots no more than 10 mm high.

What is the maximum spacing between top-of-seal outlets?

Table 5.3.5.3.1 gives 12.2 m (40 ft) with a 300 mm high foam dam and 24.4 m (80 ft) with a 600 mm high dam. For below-the-seal application, spacing is 40 m for a mechanical shoe seal and 18 m for tube seals (Table 5.3.5.3.6.1).

Can subsurface foam injection be used on a floating roof tank?

No. NFPA 11-2024 5.3.3.1 and 5.4.2.2.4 exclude subsurface and semi-subsurface injection on open-top and internal floating roof tanks. The roof can stop foam spreading properly over the surface.

How is an internal floating roof tank protected?

For a seal fire, the tank is treated as an open-top floating roof tank of the same diameter and Table 5.3.5.3.1 applies (5.4.2.3). For a full-surface fire, it is treated as a cone-roof tank of the same diameter under 5.2.5 and 5.9 (5.4.2.2). Many roof types other than steel double-deck and pontoon roofs need full-surface protection (5.4.2.1).

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