FM Global DS 3-2: Fire Water Tanks — Capacity, Construction, Break Tanks and Maintenance

A fire water tank is the water supply that meets the sprinkler and hose demand for the required duration. FM Global DS 3-2 (Water Tanks for Fire Protection, October 2015, Interim Revision April 2026, 105 pages) covers four types: gravity (elevated) tanks, fire pump suction tanks, pressure tanks and break tanks. It covers suction tanks of steel, concrete or wood; embankment-supported fabric tanks are in DS 3-4 and lined earth reservoirs in DS 3-6 (Section 1.0).

Capacity: Duration Comes From the Occupancy Data Sheet (2.1)

DS 3-2 does not give durations. Suction tanks are usually sized for the total sprinkler and hose stream demand for the design duration, and both come from the occupancy-specific data sheet:

• Nonstorage occupancies (DS 3-26): 60 minutes for all hazard categories (2.3.1.13); hose allowance 950 L/min for HC-1 and HC-2 and 1,900 L/min for HC-3 (2.3.1.12).
• Storage (DS 8-9 Table 14): 60, 90 or 120 minutes depending on the number of sprinklers in the ceiling design; hose allowance 950 or 1,900 L/min.
• The rated capacity of a tank is the water between the overflow inlet and the anti-vortex plate (3.6.5).
• A break tank is not a full-capacity tank and is not expected to meet the whole demand for the design duration (2.1).

Example: for the DS 3-26 HC-2 design (8 mm/min over 230 m², 20 sprinklers at 95 L/min) the minimum volume is (1,900 + 950) L/min × 60 min = 171 m³; the actual sprinkler flow at the base of the riser makes it larger. See the DS 3-0 most demanding area article for the calculation.

Tank Type and Design (2.2.1, 2.2.3, 3.1)

• Steel suction and gravity tanks: ground-supported, flat-bottom, cylindrical welded or bolted steel tanks are designed to FM Approvals Standard 4020 (2.2.1.1). FM Approved suction tanks are welded steel, bolted steel, bolted aluminum and embankment-supported fabric; the Approval Guide lists the snow load, wind and FM earthquake zone each tank is approved for (3.1.2).
• Concrete and other materials: designed to the general principles of FM 4020 and the material standards (ACI 350, ACI 350.3, ACI 318), with a roof and for fluid, snow, wind, earthquake and soil loads (2.2.1.2). Reinforced concrete suction tanks are not FM Approved but are expected to perform well if so designed (3.1.8).
• Pressure tanks: to NFPA 22 and ASME Section VIII, Division 1, in a heated area (2.2.1.3).
• Design loads (2.2.3): minimum roof live load 15 psf (0.75 kPa); minimum generic roof snow load 25 psf (1.2 kPa); wind design with a minimum 3-second gust of 90 mph (40.2 m/s) and importance factor 1.15; earthquake design in FM 50- to 500-year zones; soil and groundwater loads, including buoyancy, for buried tanks.
• Use: tanks preferably serve fire protection only; where dual service cannot be avoided, plant service piping is separate and arranged so the fire reserve is retained (2.2.1.11). With multiple supplies, locate the tank at the end of the yard system opposite the other source (2.2.1.5).
• Construction and acceptance: a licensed engineer for the purchaser inspects throughout (2.2.1.16); welded flat bottoms are leak tested before painting (2.2.1.17); the filled tank is confirmed watertight (2.2.1.18).

Break Tanks (2.2.2, 3.2)

• Used where a direct connection between the public supply and a private fire system is prohibited (2.1). Do not use a break tank if the pump’s maximum flow (150% of rated) exceeds the minimum the public supply can provide, allowing for seasonal fluctuation (2.2.2.1).
• One break tank per pump; the water between 0.6 m above the anti-vortex plate and the level where automatic filling starts must supply 150% of rated pump flow for 15 minutes (2.2.2.2).
• At least two automatic and one manual fill outlets, each able to supply 150% of rated flow alone; automatic fill valves open when the level is 150 mm below the full line (2.2.2.3).
• Fill outlets at least 4.6 m from the suction pipe, preferably opposite it; fill velocity no more than 6.1 m/s (Table 1) (2.2.2.6).
• A level indicator visible to the pump operator; a low-level alarm 225 mm below the full line; an overflow at least one size larger than the fill pipe; a fire department connection downstream of the tank and pump (2.2.2.8–2.2.2.11).
• DS 3-2 notes that break tanks are less reliable than an adequately sized suction tank because the automatic fill mechanisms can fail (3.2).

Corrosion Protection (2.2.4)

• Interior and exterior steel is protected per AWWA D102 (welded) or AWWA D103 (bolted) as modified by FM 4020 (2.2.4.1), using lead-free systems with at least 18 months of documented satisfactory service (2.2.4.2).
• Bolted tanks: factory-applied hot-dip galvanizing, glass-fused-to-steel, liquid epoxy or powder epoxy (2.2.4.3.1).
• Welded tank interiors (Table 2): two-component epoxy systems and 100% solids polyurethane/polyurea, with total dry film thickness from 0.20 mm (ICS-1) to 0.64–0.66 mm (ICS-4); wet interior surfaces blasted to SSPC-SP10/NACE No. 2 (2.2.4.4.5).
• Under the bottom: if uncoated, an oiled or lime-sand layer of at least 100 mm on compacted grade or 25 mm on a concrete slab (2.2.4.5).
• No wax coatings or coal tar/bituminous coatings thicker than 0.51 mm on wet interior surfaces (2.2.4.6); no coating below 10°C or in damp conditions (2.2.4.7).

Suction Connection and Anti-Vortex Plate (2.2.1.14, 3.6.5)

• Suction piping follows DS 3-7, with DS 2-8 bracing and flexible couplings in FM 50- to 500-year earthquake zones (2.2.1.14); allow for settlement on first filling with flexible couplings or by making rigid connections afterwards (2.2.1.15).
• Side entry: a 90° long-radius elbow turned down, with an anti-vortex plate at least 6 mm thick and at least twice the suction diameter in each plan dimension, located 150 mm or half the suction diameter (whichever is greater) above the tank bottom.
• Bottom entry: the suction pipe extends at least 100 mm above the bottom as a silt stop; the plate sits at least 150 mm or half the diameter above the pipe end, at least 6 mm (preferably 9.5 mm) thick, edge-stiffened with 50 × 50 × 6 mm angles and supported on angle legs.
• Concrete tanks: the suction may drop into a sump at least 1.5 m square and 0.6 m deep, ending at least 0.38 m below the tank bottom in the centre of the sump.

Freeze Protection (2.2.6)

Protection follows the site’s 100-year return period daily minimum temperature (100-year DMT) zone on the FM Worldwide Freeze Map; heating and insulation must keep the water at no less than 42°F (5.6°C) on the coldest day, with a 40°F (4.4°C) low-temperature alarm. See the DS 3-2 tank heating and freeze protection article.

Operation and Maintenance (2.3)

• Keep the tank full at all times (2.3.1); maintain the pump installation per DS 2-81 (2.3.2).
• Annual visual inspection of everything accessible without draining; roof vents cleaned annually (2.3.6).
• Drain sediment at least annually from dual-service tanks and tanks fed from unfiltered sources (2.3.7).
• Examine exterior coatings at least every two years (2.3.8).
• Inspect the interior at intervals not exceeding five years, including interior piping and anti-vortex plates; an underwater evaluation after removing silt is acceptable (2.3.9).
• Test break tank automatic fill valves monthly and verify fill rates annually (2.3.3); check pressure tank level and air pressure weekly, or monthly where supervised alarms are provided (2.3.4).

Quick Checklist

• Capacity: occupancy data sheet sprinkler + hose demand × duration
• Steel tanks to FM 4020; concrete to ACI 350/350.3/318
• Snow, wind and earthquake loads per 2.2.3
• Coating system per Tables 2–3; treated sand under the bottom
• Anti-vortex plate sized and positioned per 3.6.5
• Break tank: 150% × 15 min volume, two automatic plus one manual fill, 225 mm alarm
• Freeze protection by 100-year DMT zone
• Annual visual, two-yearly exterior coating and five-yearly interior inspections recorded

Frequently Asked Questions

How is fire water tank capacity calculated?

FM DS 3-2 does not set durations; a suction tank is sized for the total sprinkler and hose demand for the design duration (2.1), taken from the occupancy data sheet. DS 3-26 uses 60 minutes for all hazard categories with a 950 or 1,900 L/min hose allowance; DS 8-9 Table 14 uses 60, 90 or 120 minutes. For the HC-2 example the minimum is (1,900 + 950) × 60 = 171 m³.

Why does the suction pipe stand above the tank floor?

Under DS 3-2 3.6.5, a bottom-entry suction pipe extends at least 100 mm above the tank bottom to act as a silt stop, and a side-entry suction ends in a downturned elbow with its anti-vortex plate 150 mm or half the pipe diameter above the bottom. Sediment is also removed through periodic draining (2.3.7) and the five-yearly interior inspection (2.3.9).

What is an anti-vortex plate and how is it built?

A steel plate that reduces eddies and stops air being drawn into the suction as the water level approaches the pipe during a large flow (DS 3-2 3.6.5). It is at least 6 mm thick and at least twice the suction diameter in each plan dimension; on bottom-entry suctions it sits at least 150 mm above the pipe end, edge-stiffened with angles and supported on angle legs.

How often should a fire water tank be inspected?

Per DS 3-2 Section 2.3: an annual visual inspection of all equipment accessible without draining, exterior coatings at least every two years, and an interior inspection at intervals not exceeding five years, which may be an underwater evaluation after silt removal. DS 3-2 does not call for periodic renewal of the stored water; dual-service and unfiltered-source tanks are drained of sediment at least annually.

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