1. What the fire water reserve is and why it exists
The hydrant standard defines the fire reserve in one line: the water volume dedicated exclusively to fire fighting. The market and the fire department Technical Instructions consecrated the name technical fire reserve — RTI. The logic is autonomy: when a hydrant opens or a sprinkler bursts, the system must sustain minimum flow and pressure long enough for first response, until the Fire Department arrives and takes over with the public network, tanker trucks or natural sources.
The reserve is not the building water tank. It may even live in the same tank, but it is a legally and hydraulically separate volume: building consumption cannot reach it. And the reserve effective capacity must be permanently maintained — a half-full fire tank means a system failed at inspection and, ultimately, a building with no real autonomy on the day of the fire.
2. How to calculate: V = Q × t, with the two most unfavorable outlets
For hydrant and hose station systems, the standard determines the minimum volume with a direct formula: V = Q × t. Q is the combined flow of two system outlets — the two hydraulically most unfavorable operating simultaneously, at the minimum per-outlet flows of the standard Table 1.
And t is the operating time: 60 minutes for type 1 and 2 systems, 30 minutes for type 3 — the heaviest type gets less time precisely because its flow is three times higher.
Applying the formula to the three types yields the reference minimum volumes in the table below. These are standard minimums: the state Technical Instruction and the design memorial may require more — never less. And the memorial golden rule applies: every parameter used in the calculation must be documented in it, with no reference to another design.
V [litros] = Q [L/min] × t [min]Minimum fire reserve volume for hydrants and hose stations — Q in L/min (two outlets, standard Table 1), t in minutes (60 min types 1-2; 30 min type 3)
| System type | Flow per outlet (Table 1) | Q (2 outlets) | Time t | Minimum volume V |
|---|---|---|---|---|
| Type 1 (hose stations) | 80 to 100 L/min | 160 to 200 L/min | 60 min | 9,600 to 12,000 L |
| Type 2 (hydrants, 40 mm hose) | 300 L/min | 600 L/min | 60 min | 36,000 L |
| Type 3 (hydrants, 65 mm hose) | 900 L/min | 1,800 L/min | 30 min | 54,000 L |
3. State instructions and sprinklers: when the reserve comes from a table
The V = Q × t formula is the Brazilian standard floor — but who approves the design is the state Fire Department, and Technical Instructions frequently tabulate the reserve directly. São Paulo’s IT-22 (CBPMESP), for example, classifies hydrant systems into five types and sets minimum volumes in a table, by hazard and built area — the range goes from 5 m³ in small low-hazard buildings to 180 m³ in the most demanding cases.
Different states, different tables: the designer always works with the instruction in force where the development is located.
Sprinkler systems have their own reserve, calculated from the design area hydraulic demand multiplied by the hazard operating time — 30 minutes for light hazard, 60 for ordinary and 90 for extra, per fire department technical standards. When the volume is not calculated, tabulated values apply: in the CBMDF standard, from 25,000 liters (light hazard) to 515,000 liters (extra II).
In buildings with both hydrants and sprinklers, the final reserve meets the combined demand defined in the design — and that demand is what reaches the pump.
4. Total volume is not effective capacity: level X, vortex and shared tanks
The most expensive reserve mistake is counting the tank total volume as reserve. The standard works with effective capacity: the water between the normal operating level and level X — the lowest level the main pump can use before forming a vortex at full load. Below level X there is water in the tank, but it does not exist for fire fighting: the vortex drags air into the suction and the pump loses prime in the middle of the fire.
Level X is determined by the suction pit geometry: the standard tabulates the minimum submergence above the pipe mouth as a function of the suction nominal diameter — 250 mm for DN 65 pipe, up to 750 mm for DN 250 — and allows reducing it when an anti-vortex device is installed. That is why the recessed suction pit, with minimum wall and bottom clearances, is standard-drawn geometry, not manufacturer preference: it maximizes the volume fraction that becomes effective capacity.
A tank shared with building consumption is allowed, with one elegant design condition: the other uses’ water outlets sit at a higher level than the reserve volume — the building literally cannot reach the fire water. Two operational requirements complete the picture: tank cleaning cannot interrupt the whole supply (at least 50% of the reserve is kept, with two interconnected cells) and water replenishment at 1 L/min per cubic meter of reserve is recommended.
5. What the reserve requires from the fire pump
The reserve position defines the supply type. An elevated tank with enough water column may operate by gravity — and when the height does not guarantee minimum pressure at the most unfavorable outlets, a booster pump in bypass comes in. A ground-level, semi-buried or underground tank, which is the case for most industrial plants and large buildings, requires fixed automatically-started pumps: the main + jockey set installed in the pump room next to the tank.
The preferred installation condition is positive (flooded) suction: the pump shaft centerline below the water level X. The standard allows tolerance — shaft up to 2 meters above level X, or up to 1/3 of the tank effective capacity, whichever is smaller — and beyond that the suction is considered negative, with mandatory foot valve and vacuum gauge and a permanent priming risk no designer wants to carry.
Completing the suction: water velocity limited to 4 m/s in the suction pipe and the pit sized as seen in the previous section.
The rest of the set comes from the stationary pump standard: sizing per NBR 16704/NFPA 20, automatic start upon any hydrant opening, full duty in about 30 seconds, manual-only shutdown and a pump room with access for complete maintenance. Reserve and pump room are one single design — the tank defines the suction elevation, the suction defines the pump, and the pump defines the room.
6. Common mistakes with the fire water reserve
Four mistakes dominate practice. First: declaring the tank total volume as the reserve, without discounting building consumption and the dead volume below level X — the real reserve ends up 20% to 40% smaller than on paper. Second: calculating the reserve for hydrants only and forgetting sprinkler demand in buildings with both systems. Third: sharing a tank without raising the consumption outlets, letting the building drink the fire water.
Fourth: placing the tank far from or below the pump room for construction convenience, pushing the pump into negative suction.
The approving sequence is direct: system type and reserve per the state instruction and the standard, effective capacity verified against the suction pit geometry, pump elevation guaranteeing flooded suction — and only then the set specification per NBR 16704/NFPA 20. FB Bombas application engineering receives the tank data (elevations, volumes, pump room position) together with the design demand and returns the complete skid specification — main, standby, jockey and panel — within 48-72 business hours.