Calculate foam solution flow, foam concentrate and water for storage-tank, rim-seal and dike foam systems with NFPA 11 application rates and discharge times — including supplementary hose streams, pipe fill and reserve.
Preliminary quantities. Rates and times must follow the adopted edition of NFPA 11, the foam manufacturer's listing (which can require higher rates, e.g. for polar solvents) and the authority having jurisdiction. Polar solvents (alcohols, ketones) need alcohol-resistant concentrate at listed rates.
| Hazard / method | Rate | Time |
|---|---|---|
| Fixed-roof tank, fixed outlets Type II — flash point < 37.8 °C | 4.1 L/min·m² (0.10 gpm/ft²) | 55 min |
| Fixed-roof tank, fixed outlets Type II — flash point 37.8–93.3 °C | 4.1 L/min·m² | 30 min |
| Fixed-roof tank, monitors / handlines — flash point < 37.8 °C | 6.5 L/min·m² (0.16 gpm/ft²) | 65 min |
| Fixed-roof tank, monitors / handlines — flash point 37.8–93.3 °C | 6.5 L/min·m² | 50 min |
| Floating roof rim seal, outlets above seal with foam dam | 12.2 L/min·m² (0.30 gpm/ft²) | 20 min |
| Dike, fixed low-level outlets — Class I / Class II | 4.1 L/min·m² | 30 / 20 min |
| Dike, monitors — Class I / Class II | 6.5 L/min·m² | 30 / 20 min |
Confirm every value against the edition of NFPA 11 adopted for your project and the concentrate's listing.
Tank installations with fixed foam systems also need hose streams for spill fires around the tank. NFPA 11 sets the number by the diameter of the largest tank and the operating time by the same diameter:
| Largest tank diameter | Hose streams (189 L/min each) | Operating time |
|---|---|---|
| up to 10.5 m (35 ft) | 1 | 10 min |
| 10.5–19.5 m (35–65 ft) | 1 | 20 min |
| 19.5–28.5 m (65–95 ft) | 2 | 20 min |
| 28.5–36 m (95–120 ft) | 2 | 30 min |
| over 36 m (120 ft) | 3 | 30 min |
30 m fixed-roof gasoline tank (flash point below 37.8 °C), Type II fixed outlets, 3% AFFF.
A 3% concentrate needs half the storage of a 6% one for the same solution, and 1% halves it again — useful where tank space or delivery logistics are tight. The proportioner and the listed application rate must match the concentrate actually purchased; mixing percentages within one system is not allowed.
Multiply the application rate by the area to get solution flow, multiply by the discharge time to get solution volume, then multiply by the concentrate percentage. Add supplementary hose streams, pipe fill and any reserve.
4.1 L/min·m² (0.10 gpm/ft²) for fixed foam discharge outlets, and 6.5 L/min·m² (0.16 gpm/ft²) for monitors and handlines, for hydrocarbons. Polar solvents use the rate in the concentrate's listing.
Gasoline has a flash point below 37.8 °C and keeps producing ignitable vapour, so the foam blanket must be maintained longer — 55 minutes with Type II outlets against 30 minutes for liquids with a flash point between 37.8 and 93.3 °C such as diesel.
NFPA 11 requires a reserve where the authority having jurisdiction or the owner's risk assessment calls for one, typically equal to the primary supply. Many oil and gas specifications require a 100% reserve. Enter it as a percentage.
No. A floating roof only needs foam in the annular area between the shell and the foam dam, which is far smaller than the full tank area. That is why rim-seal systems use a high rate but a small total quantity.
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