EN 12845 Sprinkler Storage Tank: Volume, Refilling and Effective Capacity

Tank types, effective capacity, the 36-hour refill rule, anti-vortex plates and suction inlet clearances, from field practice.

At the mechanical handover of a logistics warehouse, the tank plate read "200 m³" — but the suction pipe inlet sat 35 cm above the base slab. That 35 cm of water counts for nothing in the sprinkler calculation; the pump cannot draw it. Effective capacity was not 200 m³ but roughly 188, once the loss from the base to the suction inlet was deducted. That is precisely the concept EN 12845 keeps at the door: not the volume on paper, but the effective volume the pump can actually draw.

This article walks the water storage tank clauses from a site perspective, with a practical decision matrix, refill times, anti-vortex plates and a worked high-hazard storage volume example.

Tank types and general conditions

The standard recognises three categories: pump suction tank, gravity tank and reservoir. Whichever is chosen, it must be either a full capacity tank meeting the whole demand alone, or a reduced capacity tank combining a reduced volume with automatic town main infill.

Where freezing is a risk, the normal water level is raised by at least 1.0 m and ice venting is provided. Covered tanks must be readily accessible; other than open reservoirs, every tank needs an externally readable water level indicator. This is often waved through on site as "there's a plate on it"; the standard says externally readable, meaning an indicator the inspecting engineer can see from outside.

What effective capacity means

Effective capacity is defined as the volume between the normal water level and the low water level. In site language: starting from a full tank down to the last level the pump can draw without taking air. Water below that level — the residue beneath the suction pipe — does not count, because reaching it produces a vortex, air is entrained and the pump's discharge collapses.

The most frequently missed point: the "geometric volume" quoted for a tank and the "effective capacity" the standard asks for are not equal. Effective capacity equals geometric volume less the dead column at the base, less the freezing allowance if the level has been raised. When ordering a tank, always specify effective capacity, never geometric.

Minimum volume for pre-calculated systems

Where no hydraulic calculation is done, the tables bind:

Hazard class	h ≤ 15 m	15 < h ≤ 30 m	30 < h ≤ 45 m
LH (wet / pre-action)	9 m³	10 m³	11 m³
OH1 wet / pre-action	55	70	80
OH2 wet / pre-action	105	125	140
OH3 wet / pre-action	135	160	185
OH4 wet / pre-action	160	185	200

For high hazard, the table reads by density: 7.5 mm/min gives 225 m³ wet and 280 m³ dry; 12.5 mm/min gives 350 and 440; 30 mm/min gives 875 and 1090. Because a dry system loses time filling the pipework, the standard enlarges the dry volume by about 25 % in each band — worth checking against the tank room dimensions once during planning.

The calculated system formula, with a worked example

In a fully calculated system the formula is one line:

V_eff = Q_max × t

Q_max is the maximum system flow from the most remote point calculation, and t the required duration. A worked example: designing an HHS3 logistics aisle, the hydraulic calculation gives Q_max = 1750 L/min, with a 90 minute duration for high-hazard storage.

V_eff = 1750 L/min × 90 min = 157 500 L = 157.5 m³

The pre-calculated table requires 350 m³ for the same HHS3 at 12.5 mm/min, while the calculated route gives 157.5 m³. That 192.5 m³ difference is tank room area, adjacent wall loading and cost. So do not take seriously anyone who says a calculated system is not worth the effort on high-hazard storage projects; the area recovered on the tank usually more than pays for the hours spent on the hydraulics.

Refill times and reduced capacity

On a full capacity tank, the water source — town main, borehole — must be able to refill the tank completely within 36 hours. That limits the infill rate: for a 200 m³ tank, 200/36 ≈ 5.56 m³/h (about 93 L/min). Where the town main cannot deliver that, even the reduced capacity route becomes difficult.

Conditions for a reduced capacity tank are stricter:

• Infill from the town main, and automatic, through at least two mechanical float valves.
• Failure of one float valve must not compromise the required infill rate.
• Effective capacity must not fall below the tabulated minimum (70 m³ for high hazard, and never less than 10 % of full capacity).
• The infill rate must be measurable at site acceptance; "there's no meter" is not an excuse.
• The infill arrangement must be accessible for inspection.

The standard also requires the feed pipe outlet to sit at least 2.0 m horizontally from the suction pipe inlet. Where the two are placed side by side, infill water creates surface turbulence and entrains air into the suction.

Anti-vortex plates and suction clearances

The lower bound of effective capacity is set by the low water level "X". Where X sits depends on the suction pipe diameter and whether a vortex inhibitor is fitted. A few rows from the clearance table:

Suction pipe d (mm)	B min (m)	A min — without vortex plate (m)	A min — with vortex plate (m)
65	0.08	0.25	0.10
100	0.10	0.37	0.10
150	0.10	0.50	0.10
200	0.15	0.62	0.10
300	0.20	0.90	0.10
500	0.35	1.20	0.10

A is the minimum height from the suction pipe to the low water level. Without a plate, a DN300 pipe leaves 0.90 m of water as dead volume; with a plate, the same pipe's A falls to 0.10 m. On a high-hazard storage tank, a DN200 suction leaves about 0.62 m without a plate and 0.10 m with one. On a tank with a 100 m² footprint that difference is about 52 m³ of effective capacity — enlarging the tank for free.

Field practice: have the anti-vortex plate made in stainless, or at least galvanised, and consider a sump beneath it to pull the low water level lower still. The plate dimension must meet the tabulated minimum; many locally fabricated plates are cut by eye and welded on, and an acceptance inspector will not accept that.

Tank material and strainers

The standard imposes no material requirement; three conventions exist in practice:

• Modular bolted galvanised steel panels: quick to erect, suitable for small and medium volumes, with an internal lining (epoxy or membrane) essential.
• Reinforced concrete fire tank: part of the structure, economical between 200 and 1000 m³, with an internal membrane or epoxy primer.
• GRP: no corrosion and little heat absorption, but needing an external enclosure against UV exposure.

Whatever the material, the strainer must be ahead of the foot valve on suction-lift pumps, or outside the tank on the suction pipe under positive pressure; free area at least 1.5 times the nominal pipe area, with apertures passing no sphere larger than 5 mm. The strainer must be cleanable without draining the tank — never design one without an isolating valve.

Comparison with NFPA

NFPA 13 and NFPA 22 set structurally similar tank requirements, with two important differences. NFPA 22 requires an 8 hour refill time — roughly a quarter of EN 12845's 36 hours — which considerably reduces the town main demand on European projects. Second: NFPA gives suction clearances from a comparable table, but the credit for a vortex plate is more modest. On combined NFPA and EN projects, apply whichever is stricter.

Turkish context

BYKHY ties the sprinkler tank to design flow times duration and references EN 12845. In practice the biggest bottleneck in Turkey is town main size and pressure: in many industrial zones the main is only two inches and delivers under 4 m³/h even at night. In that situation a full capacity tank becomes unavoidable, because the town main cannot even meet the 36 hour refill condition. The routes out: a borehole with a buffer tank, an increased utility connection, or moving from pre-calculated to calculated design to bring the nominal volume down.

Common errors

• Writing geometric volume on the tank plate and presenting it as effective capacity. At acceptance, the inspector measures the suction inlet level.
• Not ordering an anti-vortex plate, then leaving 50–200 m³ of dead water beneath the suction pipe.
• Placing the feed pipe beside the suction pipe and breaching the 2.0 m rule.
• Fitting a single float valve on a reduced capacity tank — at least two are required, so that one failure does not stop infill.
• Mounting the strainer inside the tank so that it cannot be cleaned without draining.
• Failing to raise the normal water level by 1.0 m in a freezing region, so half the effective capacity ends up suspended as ice.
• Applying the pre-calculated table without question in high-hazard storage and installing a 350 m³ tank. The calculated route often gives half that.

Check list

• Tank type chosen: full capacity, reduced capacity or pressure tank?
• Effective capacity calculated: from the table for pre-calculated design, or Q_max × t for calculated.
• 1.0 m freezing allowance added? Ice venting provided?
• Externally readable water level indicator fitted?
• Refill rate: tank divided by 36 hours for full capacity, or the reduced capacity conditions.
• Feed pipe outlet at least 2.0 m horizontally from the suction inlet?
• Anti-vortex plate meeting the tabulated minimum dimension?
• The A clearance between suction pipe and low water level within the table?
• Strainer free area 1.5 times the pipe area, 5 mm apertures?
• Strainer cleanable without draining the tank?

Frequently asked questions

What does effective capacity mean?

The usable volume between the normal water level and the low water level. Water below the suction pipe does not count, because it cannot be delivered to the pump.

How quickly must a full capacity tank refill?

Within 36 hours from the water source. Reduced capacity tanks carry additional conditions, and pressure tanks must refill within 8 hours.

What does an anti-vortex plate achieve?

It breaks the vortex forming at the suction inlet and prevents air entrainment. With a plate, the clearance between suction pipe and low water level can fall to 0.10 m; without one, a 100 mm pipe needs 0.37 m.

What is the minimum volume for high-hazard storage?

In pre-calculated design, the table governs — 350 m³ wet and 440 m³ dry at 12.5 mm/min, for instance. In a calculated system, Q_max × t gives the real demand, which in most storage scenarios comes out lower.

What are the conditions for a reduced capacity tank?

Automatic town main infill through at least two independent mechanical float valves, with one failure not compromising the rate, effective capacity above the tabulated minimum, and a measurable infill rate.

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