EN 12845 Water Spray Subsidiary Extensions

Subsidiary water spray extensions off a sprinkler installation: the connection size limit, the hydraulic load, and how they are actually designed around real hazardous equipment.

When the client on a cogeneration plant added a note to our design saying "run sprinklers to the transformer too", the first instinct was to hang a few sprayers off the sprinkler main. This is exactly where the subsidiary water spray clause comes in: connecting a water spray extension to a sprinkler installation is permitted — but it is wrong to assume the job has absorbed the OH or HHS sprinkler design. A subsidiary water spray extension is a sub-system fed from the sprinkler main, controlled by a deluge valve, with all nozzles open — and it adds load to the sprinkler hydraulics.

What the clause requires

The text is short but carries two critical conditions:

• The connection must not exceed 80 mm. The branch taken from the sprinkler distribution pipe to the water spray line has a maximum nominal size of DN80. If more flow is needed, the water spray system must have its own control valve set and, if necessary, its own supply.
• The additional demand must be accounted for in the water supply design. Pumps, tank and distribution hydraulics are sized as if sprinkler flow and water spray flow run simultaneously.

The standard describes the subsidiary extension as being for places with a severe, rapidly spreading fire risk where water must be applied to a whole area at once — transformer bunds, gas turbine enclosures, fuel pump islands, cable galleries, hydraulic power units. It is whole-area wetting, not individual thermal operation.

Sprayers versus sprinklers

The standard defines a sprayer separately: a water spray nozzle producing a downward conical pattern. It differs from a sprinkler in two fundamental ways:

• No thermal trigger — the nozzle is always open. Actuation is the valve's job.
• Droplet size and pattern are selected to the equipment geometry (medium velocity, high velocity, full cone, hollow cone).

So the water spray line is always a dry manifold, with no water reaching the nozzles until the deluge valve opens.

The deluge valve as the single actuation point

A subsidiary water spray extension connects to the sprinkler main through an actuation valve — a deluge valve or multiple control. Two topologies are common on site:

1. A pneumatic pilot line: pilot sprinklers (typically K57 at 68 °C) in the protected area hold a closed pneumatic line. When a pilot opens, air pressure falls and the deluge valve opens mechanically.
2. An electric solenoid: a signal from a linear heat detector, an IR/UV flame detector or a gas detector de-energises the solenoid and the deluge valve opens. In transformer protection, wiring the Buchholz relay into the solenoid is the classic arrangement.

In both cases a false trip soaks the protected equipment: transformer oil is contaminated, and water on hot surfaces in a turbine enclosure causes thermal shock. That is why the actuation line is always designed on at least two independent confirmed signals — cross-zoned detection.

Design density: from the manufacturer and the risk, not the standard

EN 12845 gives no numerical design density for water spray extensions. In practice the values used come from field experience and manufacturers' listed catalogues:

Protected equipment	Typical design density	Wetted surface
Oil-filled power transformer	10.2 mm/min	Tank top surface, radiators and bund
Gas turbine enclosure interior	10.2–20.4 mm/min	Exposed internal surfaces, per the manufacturer
Fuel pump island	10.2 mm/min	Pump core area
Cable gallery	12.2 mm/min	Horizontal projection of the cable ladder
Flammable liquid tank shell (cooling)	2.0–10.2 mm/min	Tank side wall, for exposure protection

These figures come not from the EN 12845 text but from NFPA 15-derived industry practice and nozzle manufacturers' listings. All the standard says is: whatever you apply, provide a water supply capable of feeding it alongside the sprinkler system.

The added hydraulic load — a worked example

A typical OH2 sprinkler installation design point is 144 m² at 5.0 mm/min ≈ 720 L/min plus 250 L/min for hose reels = 970 L/min. Add a water spray extension protecting a 25 m² transformer bund at 10.2 mm/min:

• Net water spray flow: 25 × 10.2 = 255 L/min
• With K80 sprayers at an assumed 1.7 bar nozzle pressure → about 104 L/min each → three nozzles suffice
• Pump selection: the greater of sprinkler alone, water spray alone, or both together → both together = 970 + 255 = 1225 L/min at the critical pressure.

This is the most commonly confused point on site: because the water spray is seen as a "small extension", it is never added to the pump duty. Then when the deluge trips, sprinkler pressure falls and both systems are weakened. The clause explicitly prohibits that error.

Common field errors

• A DN100 connection. Exceeding the 80 mm ceiling "so it flows better" puts you outside the standard; the system is then no longer a subsidiary extension and needs its own control valve set.
• Treating the deluge alarm like a sprinkler flow alarm. Deluge operation is instantaneous and very high flow; a pressure switch rather than a flow switch is preferred, and it must go to the BMS as a separate point.
• Wrong pilot sprinkler temperature. Ambient around a transformer can reach 50 °C, so a 68 °C pilot will trip spuriously. A 79–93 °C pilot is safer.
• No drainage. A water spray system releases over a thousand litres after a test or false trip. A gully of at least 150 mm with a trap and a drainage pit beneath the protected equipment is mandatory alongside the additional supply requirement.
• Assuming the water spray survives a sprinkler isolation. The extension is fed from the sprinkler main; closing the sprinkler stop valve takes the water spray out too. Both must be tagged during maintenance.

Comparison with NFPA 15

In Europe, EN 12845 covers only the "subsidiary extension" framework for water spray; there is no separate EN standard for independent systems, and projects generally adopt NFPA 15, Water Spray Fixed Systems. The two worlds map roughly as follows:

• An EN 12845 subsidiary extension is a small version of an independent NFPA 15 deluge system connected to the sprinkler main.
• The NFPA 15 density tables (2.0 / 6.1 / 10.2 / 20.4 mm/min) are used as the de facto field reference.
• The NFPA 15 zone topology — deluge zone overlapping detection zone — is applied identically in EU projects with EN 54 detectors.

Turkish context

BYKHY treats NFPA 13, EN 12845 and NFPA 15 as equally acceptable reference standards for sprinkler and water spray systems. For high-value equipment such as transformers, generators and gas turbines, field practice combines NFPA 15 densities with the EN 12845 connection conditions. Utility projects typically require 10.2 mm/min over the oil surface plus exposure cooling; that density is a load added on top of the main sprinkler calculation, and tank and pump selection follow accordingly.

Hydraulic calculation check list

1. Sprayer K-factor and the manufacturer's listed minimum nozzle pressure (generally ≥ 1.4 bar).
2. Protected surface area and the design density applied to it (mm/min).
3. Net water spray flow = area × density, divided per nozzle to give the nozzle count.
4. Confirmation that the subsidiary connection is ≤ 80 mm.
5. Pump curve checked with the sprinkler critical design point and the water spray flow running simultaneously.
6. Effective tank volume: sprinkler duration × sprinkler flow plus water spray duration × water spray flow, for the worst combination.
7. Deluge actuation line with cross-zoned detection and a manual release button.
8. Drainage capacity: 1.5 × maximum system flow, with a 10-minute holding volume.

Frequently asked questions

Does a subsidiary water spray need its own alarm valve?

Yes. The deluge valve is itself a separate control valve set with its own flow alarm, monitored independently of the sprinkler alarm valve and listed as a separate BMS point.

How do you take a DN80 branch off a main with no spare capacity?

Redo the hydraulic calculation, and increase that segment's range or distribution size by a step if needed. In OH2 and OH3 systems the usual practice is a DN80 tee off a DN150 main. Assuming "it'll probably fit" is outside the standard.

Do water spray nozzles need caps or guards?

EN 12845 has no specific provision, but because the nozzles are open, blow-off caps or wire guards are used with manufacturer approval where foreign objects or freezing are a risk. Choose a version that does not affect design flow.

Must transformer protection be combined with foam?

No. Plain water spray is sufficient under EN 12845. On transformers with large oil volumes above about 10 m³, field practice adds AFFF or fluorine-free concentrate to make a foam-water spray — at which point the system leaves the EN 12845 framework and is designed to EN 13565-2 together with NFPA 16.

How is a water spray extension tested?

An annual full-flow test is recommended. Cover the protected equipment, confirm plain water discharge and check the pattern visually. On energised equipment such as transformers, testing must always be done with the equipment de-energised.

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