Foam concentrate calculator

How do you calculate foam concentrate quantity?

In a low-expansion foam system the stored concentrate must be enough to deliver foam solution to the protected surface at a set application rate for a set minimum discharge time. NFPA 11 tabulates both values by hazard type. The calculation is straightforward: solution flow first, then total solution volume, then the split into concentrate and water.

A handy identity: mm/min × m² = L/min. The standard prints 0.10 gpm/ft² as 4.1 mm/min in its SI column; the calculator uses whichever column matches the selected unit system.

NFPA 11-2024 values by scenario

For floating-roof tanks the protected area is not the whole liquid surface but the annulus between the shell and the foam dam, which must sit 300–600 mm from the shell (5.3.5.4.4). For polar liquids needing alcohol-resistant foam NFPA 11 gives no numeric rate; the value comes from the concentrate listing and is a user input here.

Supplementary foam hose streams

Tank protection also needs foam hose streams for small spill fires (5.9.2.1). Their number follows the diameter of the largest tank: 1 up to 20 m, 2 from 20 to 36 m, 3 above 36 m (Table 5.9.2.2). Each stream delivers at least 190 L/min (50 gpm) of solution (5.9.2.3). Simultaneous operating time is 10 min up to 11 m, 20 min from 11 to 29 m and 30 min above 29 m (Table 5.9.2.4). Hose flow adds to the tank flow, so the pump and proportioner are sized for the sum.

Concentrate, reserve and water

Consumption follows the design proportioning rate — for example 3 or 6 percent, or another rate such as 1% if the concentrate is so listed or the AHJ approves it (4.3.2.5.1) — and the quantity must cover the largest single hazard or group protected simultaneously (4.3.2.2). The tank needing the largest flow is not necessarily the one needing the most concentrate, because discharge times differ. A reserve able to meet the worst case again is also required to put the system back into service; it may be in separate tanks, drums, or available from an approved source within 24 hours (4.3.2.5.2). The standard gives no figure for filling the pipework, so enter your own value from the pipe volume.

Worked example

A 30 m fixed-roof tank storing a product with a flash point below 38 °C, protected by Type II outlets with 3% concentrate:

• Surface: π × 30² / 4 = 706.9 m² → solution 4.1 × 706.9 = 2898 L/min for 55 min → 159,400 L.
• Hose streams: 30 m diameter → 2 × 190 L/min = 380 L/min for 30 min → 11,400 L.
• Total solution ≈ 170,800 L → concentrate at 3% ≈ 5124 L, water ≈ 165.7 m³. Including the reserve doubles the concentrate.
• Outlets: at least 2 fixed outlets for 24–37 m (Table 5.2.5.2.1).

The example in the NFPA 11 annex follows the same logic: 300 gpm of solution for 30 minutes at 3% needs 270 gal of concentrate.

Outside the scope of this calculator

Foam-water sprinkler and spray systems (NFPA 11 Chapter 6), medium- and high-expansion foam, loading racks and primary protection of fixed-roof tanks by monitors or handlines are not calculated here; their rates, times and water demand come from other clauses. When choosing a proportioner, also check its listed minimum and maximum flow range (4.5.1), because the ratio can drift at low flow.

Common mistakes

• Looking only at flow: a small tank at 55 minutes can need more concentrate than a large tank at 30 minutes.
• Forgetting hose streams: supplementary streams raise both flow and concentrate.
• Using hydrocarbon rates on polar liquids: alcohols, ketones and esters need AR foam at the listed rate, and subsurface injection is not allowed for them (5.2.6.1.1).
• Counting the whole surface on a floating roof: a seal-fire design protects the annulus; a full-surface fire is assessed separately.
• Ignoring the seal detail for below-the-seal application: for a tube seal with less than 150 mm between the top of the tube and the top of the pontoon, row C of Table 5.3.5.3.6.1 requires a foam dam; a metal secondary seal counts as a dam (row D). With a combustible secondary seal, below-the-seal application may not be used at all (5.3.5.3.6.4); use the top-of-seal method.
• Unlimited time reduction: a higher rate allows a proportional reduction, but never below 70% of the minimum time.

Frequently Asked Questions

How is foam concentrate quantity calculated?

Solution flow is application rate times protected area (mm/min × m² = L/min). Multiply by the NFPA 11 discharge time, add the supplementary hose stream volume, and take 3%, 6% or the listed rate (e.g. 1%) of the total solution as concentrate.

What is the discharge time for a fixed-roof tank?

With Type II fixed outlets it is 30 minutes for hydrocarbons with a flash point of 38–60 °C and 55 minutes for products below 38 °C, heated products and crude oil, at a minimum of 0.10 gpm/ft² (NFPA 11 Table 5.2.5.2.2).

How many supplementary foam hose streams are needed?

It depends on the largest tank: 1 up to 20 m (65 ft), 2 from 20 to 36 m, 3 above 36 m. Each delivers at least 50 gpm (190 L/min) for 10, 20 or 30 minutes (NFPA 11 Tables 5.9.2.2 and 5.9.2.4).

Is a reserve supply of concentrate required?

Yes. NFPA 11 4.3.2.5.2 requires a reserve that can meet the worst-case scenario again and return the system to service. It can be stored on site or be obtainable from an approved source within 24 hours.

Why is the rate a user input for polar liquids?

Water-soluble liquids such as alcohols, ketones and esters break down ordinary foam, so alcohol-resistant foam is needed and NFPA 11 refers to the concentrate listing instead of a fixed number. Take the rate from the approval of the product and the foam-making device.

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