Technical·7 min read
Water Spray (Spray Cooling) Systems: What They Are and Where They Are Used — NFPA 15
A water spray (spray cooling) system uses open nozzles to blanket an entire surface with water at once — a deluge system. Its job is usually not to 'extinguish' a fire but to control burning, cool and protect adjacent equipment, and mitigate vapor during a flammable-liquid or gas leak. Using NFPA 15, we cover what water spray does, which fire classes it suits, where it is applied and how it is designed.
A-Pro Engineering
In a transformer, a fuel tank or a cable gallery, the greatest danger in a fire is often that it spreads to a neighbor and travels through radiant heat. For such hazards the objective is not merely to extinguish the flame; it is to continuously cool the surface, keep the burning under control and prevent leaking vapor from igniting. This is exactly what a water spray (spray cooling) system is built for. In this article we cover what water spray is, how it differs from sprinklers, which fire classes and locations it suits, its functions and its design parameters, all based on NFPA 15 and drawing on our water spray system.
What is water spray? The core difference from sprinklers
A water spray system is a fixed system that discharges water through open nozzles aimed at the equipment or surface to be protected. It is often confused with sprinklers, but its logic is different:
- In a sprinkler system each head is closed; it opens individually only when its heat element (glass bulb / fusible link) bursts at a set temperature. Water is applied locally to the heated area.
- In a water spray system the nozzles are open. The system is actuated by a deluge valve controlled by a separate detection system; the instant it operates, all nozzles in that zone discharge at once and blanket the target surface entirely with a film of water.
In short, a sprinkler protects a volume/occupancy, while water spray is designed to cool and blanket the surface of a specific piece of equipment. That is why water spray appears mostly in flammable liquid/gas and exposure protection scenarios.
The four functions of water spray (NFPA 15)
NFPA 15 defines four core objectives for water spray systems. A project targets one or several of these:
- 1) Extinguishment: actually suppressing the fire in suitable hazards.
- 2) Control of burning: where extinguishment is not feasible or safe (e.g. a pressurized gas leak), keeping the fire under control and preventing its growth and spread — controlled burning is allowed until the fuel is shut off.
- 3) Exposure protection / cooling: the most typical role of water spray. The surface of equipment/structure that has not yet ignited is continuously cooled against radiant heat from a burning neighbor, preventing an adjacent tank, pipe or transformer from heating up and becoming involved.
- 4) Prevention of fire (vapor mitigation): during a flammable liquid/gas leak or spill, wetting the area and diluting the flammable vapor to prevent ignition.
These four objectives explain why water spray is usually described not as an “extinguishing” system but as a cooling and protection system.
Which fire classes does it suit?
Water spray’s place across the fire classes can be summarized as follows:
- Class B (flammable/combustible liquids and gases): this is water spray’s core strength. For fuel, oil, solvents and LPG it controls the fire, cools the surface and protects adjacent equipment. Here water’s job is usually not to extinguish the liquid pool directly but to provide control + cooling + vapor mitigation.
- Class A (ordinary combustibles — paper, wood, textiles): water spray can be used for some Class A hazards and applications such as conveyor belts.
- Electrical equipment (transformers, motors, cables): an important nuance applies here. Water is not used to extinguish energized electrical equipment; on oil-filled transformers water spray is applied to control the oil fire and cool the transformer body, and is always designed together with de-energizing (isolation). For energized electronics/switchgear/data-center rooms water is not appropriate; there, gas suppression is preferred.
Where is it used? Typical applications
Water spray stands out at special-hazard points that need “surface cooling and exposure protection”:
- Oil-filled power transformers and large electrical equipment — oil-fire control + body cooling.
- Fuel / chemical storage tanks and tank farms — exposure protection and surface cooling.
- LPG / pressurized-gas spheres and cylinders — cooling a heat-exposed vessel to reduce the risk of BLEVE (boiling liquid expanding vapor explosion).
- Cable galleries, cable trays and runs — limiting fire spread along the cables.
- Process units, pump/compressor areas, conveyor belts and similar industrial hazards.
The common thread is always the same: points with a high heat load, a risk of spread and growth to a neighbor, where a specific surface must be wetted rapidly and completely.
Design parameters
Water spray design is built around the geometry and hazard type of the equipment to be protected. Key parameters:
Design density (flow/area). Density is set by NFPA 15 according to the hazard and applied to the projected area of the surface to be protected:
| Hazard / application | Typical design density |
|---|---|
| Oil-filled transformer, combustible liquid | ≈0.25 gpm/ft² (10.2 L/min/m²) |
| Flammable gas / liquid hazard | ≈0.35 gpm/ft² (14.2 L/min/m²) |
| Cable gallery / cable tray | ≈0.15 gpm/ft² (6.1 L/min/m²) |
Nozzle type and position. Nozzles are open and selected as medium velocity or high velocity depending on the hazard; nozzle angle, flow and layout are set by hydraulic calculation so the water film covers the entire protected surface without gaps.
Deluge valve and detection. The system is controlled by a deluge valve. The valve is opened by a detection system installed in the same zone as the nozzles (heat, flame, smoke or, for flammable-material leaks, gas/vapor detectors) or by remote/manual release. On actuation the deluge valve opens, the pipework fills rapidly and all nozzles operate at once.
Water supply and duration. An open-nozzle deluge system demands a much higher instantaneous total flow than sprinklers (all nozzles in a zone run together). A fire pump and water tank of sufficient capacity is therefore sized to deliver that flow for the required run time, which is set by the type and scenario of the protected hazard.
Zoning. In large facilities each hazard point (each transformer, each tank) is usually designed as a separate deluge zone, so only the relevant zone’s nozzles operate, avoiding unnecessary water and damage.
Difference from sprinklers, water mist and gas
The right system is chosen by hazard. A short frame:
| System | What it does / where |
|---|---|
| Sprinkler | Closed head, opens locally; volume/occupancy protection (warehouse, office, mall) |
| Water spray (deluge) | Open nozzles, all at once; surface cooling + exposure protection (transformer, tank, cable) |
| Water mist | Very fine water mist; machinery/turbine rooms with little water |
| Gas suppression | Residue-free gas; energized electronics, data centers, archives |
Water spray’s distinguishing feature is that it targets not a volume but a specific equipment surface, wetting it completely and simultaneously.
Standard and regulation in Türkiye
The internationally recognized primary design reference for water spray/deluge systems is NFPA 15 — Standard for Water Spray Fixed Systems for Fire Protection. EN 12845, mandatory for sprinkler design in Türkiye, does not cover water spray systems; water spray is a separate class of system whose hydraulic design is carried out to NFPA 15.
In practice the Turkish Fire Code (BYKHY) requires a suitable fixed suppression system for flammable/explosive material facilities, transformer rooms and similar special hazards; the system type and capacity are determined by the risk analysis. When water spray is selected, the design is built to NFPA 15 and the relevant standards of the protected equipment (e.g. IEEE/related specifications for power transformers), and the nozzles and deluge equipment are chosen from approved (UL/FM etc.) products.
Summary
A water spray (spray cooling) system is a fixed system that, through open nozzles, wets the surface of the equipment to be protected completely and simultaneously via a deluge valve and detection. Its difference from sprinklers is that it targets not a volume but a specific surface, with all nozzles operating together. Under NFPA 15 it has four functions: extinguishment, control of burning, exposure protection (cooling) and vapor mitigation. Its core use is Class B flammable liquid/gas hazards and special hazards such as transformers, fuel/LPG tanks and cable galleries; for energized-electronics rooms, gas suppression rather than water is appropriate. The design is built on hazard-based density (10.2 / 14.2 / 6.1 L/min/m²), the correct nozzle type, a deluge valve + detection, and a strong water supply capable of the high instantaneous flow. As A-Pro, we engineer water spray systems for transformer, fuel/LPG and industrial facilities to NFPA 15; review our water spray system page and reach us for a free site survey (within Ankara) for a solution matched to the special hazards at your facility.
This content is for information only. Binding water spray design must be carried out on a project basis by a fire engineer, based on the actual geometry of the protected equipment, the risk analysis and the current editions of NFPA 15 and the relevant equipment standards.
© 2026 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
What is a water spray (spray cooling) system?+
A water spray system is a fixed suppression system designed to NFPA 15 that discharges water through open nozzles aimed at the equipment or surface to be protected. Its main difference from sprinklers is that the nozzles are open: the system is actuated by a deluge valve and a separate detection system, and when it operates, all nozzles in a zone discharge simultaneously and blanket the target surface entirely. The goal is often not to extinguish the fire directly, but to cool the surface, control the burning and protect adjacent equipment.
What is the difference between water spray and sprinklers?+
In sprinkler systems each head is closed and opens individually only when its heat element bursts; water is applied locally to the heated area. In a water spray system the nozzles are open, and when the deluge valve is actuated all nozzles in that zone operate at once. A sprinkler system protects a volume/occupancy, whereas water spray is designed to cool/blanket the surface of a specific piece of equipment (a transformer, tank or cable tray). Water spray is typically used for flammable liquid/gas hazards and exposure protection scenarios.
Which fire classes does water spray suit?+
Water spray is strongest against Class B hazards (flammable/combustible liquids and gases): for materials like fuel, oil and LPG it controls the fire and cools adjacent surfaces to protect them. It can also be used for some Class A hazards (ordinary combustibles such as paper, wood, textiles) and for conveyor belts. On electrical equipment (such as transformers) water is not used to extinguish energized equipment; instead it controls the oil fire and cools the transformer body, and is always designed together with de-energizing (isolation). For energized electronics/switchgear/data-center rooms, gas suppression is the appropriate choice rather than water.
What is the function (purpose) of a water spray system?+
NFPA 15 defines four objectives for water spray: (1) Extinguishment — suppressing the fire; (2) Control of burning — where extinguishment is not feasible, keeping the fire under control and preventing growth; (3) Exposure protection — continuously cooling the surface of equipment/structure to protect it from the heat of a burning neighbor; (4) Prevention of fire — wetting a surface during a flammable liquid/gas leak and diluting the vapor to prevent ignition (vapor mitigation). A given project usually targets one or several of these objectives.
What are the design densities in a water spray system?+
Density is set by NFPA 15 according to the hazard. Typical values: ≈0.25 gpm/ft² (10.2 L/min/m²) for oil-filled transformers and combustible liquid hazards; ≈0.35 gpm/ft² (14.2 L/min/m²) for flammable gas/liquid hazards; ≈0.15 gpm/ft² (6.1 L/min/m²) for cable trays/runs. This density is applied to the projected area of the surface to be protected; nozzle count, type and position are set by hydraulic calculation. Exact values are always selected per the equipment type and the current edition of NFPA 15.
How is a water spray system actuated?+
Water spray is a deluge system: because the nozzles are open, the system is actuated through a deluge valve controlled by a separate detection system. Detection can use heat, flame or smoke detectors, or gas/vapor detectors for flammable-material leaks; some systems also allow remote/manual release. On actuation the deluge valve opens, the pipework fills rapidly and all nozzles in the zone discharge simultaneously.
Which standard governs water spray design?+
The internationally recognized primary design reference for water spray/deluge systems is NFPA 15 (Standard for Water Spray Fixed Systems for Fire Protection). EN 12845 — mandatory for sprinklers in Türkiye — does not cover water spray systems; water spray is a separate class of system. In practice the Turkish Fire Code (BYKHY) requires a suitable fixed suppression system for flammable/explosive material facilities, transformer rooms and similar special hazards; the hydraulic design of water spray is then carried out to NFPA 15 and the requirements of the protected equipment (e.g. IEEE/related transformer standards).
Related systems
Let’s discuss the right solution for your facility
Contact our engineers for a free site survey and quote within Ankara.