The fire water tank is the last line of the supply: when the town main is inadequate or interrupted, the system feeds from here. FM Global DS 3-2 brings tank selection, construction detail, freeze protection and corrosion control together in one document.
Capacity Calculation
Tank capacity is the design flow multiplied by the required duration, with the duration set by the hazard:
- HC-1 (light): around 30 minutes at the calculated flow
- HC-2 (ordinary): around 60 minutes
- HC-3 (higher hazard): 90 to 120 minutes
- Storage occupancies: 60 to 180 minutes, with 120 minutes typical for Class 4 and plastics
- Hose stream: added to the sprinkler demand for the same duration
Worked example: a logistics warehouse with an ESFR K360 design at roughly 5,500 L/min for 60 minutes, plus a hose allowance for the same period, points to a tank of around 390 m³.
Tank Type Selection
- Above-ground steel: the most common, quick to erect, with an FM Approved internal lining. Economical across a wide capacity range.
- Below-ground concrete: used where there is no space above ground. Water quality is harder to monitor, but freezing risk is low.
- Below-ground steel: requires external coating and carries a higher corrosion risk.
- Polyethylene: only for small capacities in light hazard applications.
- Break tank: separates the fire water supply from the town main to prevent contamination.
Freeze Protection
- Tank heating: steam coil, electric immersion or gas heater, holding the water at a minimum of 4 °C
- Circulation: forced or gravity, keeping the water moving
- Insulation: at least 50 mm of mineral wool with a weatherproof jacket
- Burial: below-ground tanks placed beneath the frost line
The pump suction pipe must also be protected: external sections insulated and, where necessary, trace heated.
Corrosion and Water Quality
- Internal lining: FM Approved epoxy at a defined dry film thickness
- Cathodic protection: sacrificial anodes required for below-ground steel tanks
- Annual inspection: internal visual check of lining condition and sediment level
- Water renewal: a full change on roughly a five year cycle, coordinated with the corrosion provisions of DS 2-1
- Suction strainer: an FM Approved strainer upstream of the pump
Suction Connection Detail
- Inlet height: at least 150 mm above the tank floor so sediment is not drawn in
- Anti-vortex plate: above the suction to prevent air entrainment
- Pipe fall: the suction line falls from tank to pump so no air pocket forms
- Twin tanks: connected by an equalising line, both feeding the pump in parallel
Quick Checklist
- Capacity equals design flow plus hose, multiplied by duration
- Tank type and internal lining appropriate
- Freeze protection by heating and circulation
- Suction at least 150 mm above the floor with an anti-vortex plate
- Cathodic protection on below-ground steel
- Annual internal inspection recorded
- Water renewed on a five year cycle
Frequently Asked Questions
How is fire water tank capacity calculated?
Multiply the design flow by the required duration and add the hose stream allowance for the same period. Duration follows the hazard: roughly 30 minutes for light hazard, 60 for ordinary, and 60 to 180 minutes for storage occupancies, with 120 minutes typical for Class 4 and plastics.
Why must the suction inlet sit above the tank floor?
Sediment, corrosion products and debris accumulate on the floor over years. Positioning the suction at least 150 mm above it means the pump draws clean water rather than sludge that would block the strainer or damage the impeller at the moment the system is needed.
What is an anti-vortex plate and why does it matter?
As the water level falls, a vortex can form above the suction and draw air into the pump, which causes it to lose prime and cavitate. An anti-vortex plate above the suction breaks that rotation so the pump continues to draw solid water down to the design low level.
How often should fire water be renewed?
A full change on roughly a five year cycle, coordinated with the corrosion provisions of DS 2-1. Stagnant water promotes microbiologically influenced corrosion and sediment build-up, so renewal plus annual internal inspection of the lining is what keeps a tank serviceable over decades.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 3-2 (Water Tanks for Fire Protection); related data sheet DS 2-1 (corrosion).