In a deluge system the sprinkler heads have no thermal element; they are all open. When the deluge valve opens, every head sprays at once. The aim is not to suppress at one point but to wet the whole volume simultaneously.
How it differs from a conventional sprinkler system
In a conventional system each head makes its own decision: only the heated one opens. In a deluge system the decision is central: the detection system opens the valve and everything else operates together. That is why a deluge system is less a "sprinkler system" than a "water spray system", and sits under its own NFPA standard (NFPA 15).
Where it is used
- Fast-spreading fires: flammable liquid filling areas, paint shops, aircraft hangars.
- Equipment cooling: transformers, LPG tanks, process columns. The aim is not to extinguish the fire but to keep adjacent equipment cool.
- Battery energy storage (BESS): continuous cooling to stop thermal runaway spreading to neighbouring cells.
- Conveyors and dust handling lines: stopping fire travelling along the line.
The water supply consequence is heavy. In a conventional system the design flow is limited to a defined area; in a deluge system every head counts. Keeping the protected area small, dividing the system into sections and giving each section its own valve is therefore the fundamental design decision.
Design steps
- Define the protection objective. Extinguishment, control, cooling or exposure protection? The flow follows from that decision.
- Choose the density. NFPA 15 gives different densities by application type; for equipment cooling it is calculated over the surface area.
- Choose the nozzle type. Open sprinkler, water spray nozzle or high-velocity nozzle; droplet size and spray angle follow the objective.
- Section it. Each deluge valve should water only its own zone; simultaneous operation of the whole installation is rarely needed.
- Choose the detection. Flame detector, linear heat cable or pilot line. A flame detector is the typical choice for fast-spreading fires.
- Solve the drainage. A deluge system discharges a great deal of water; floor drainage, retention and contaminated water management are part of the design.
The pilot line principle
In some deluge systems detection is done with a pressurised "pilot line" carrying closed sprinklers. When a pilot sprinkler opens, the pressure in the line drops and the deluge valve opens mechanically. Needing no electricity is an advantage, but the response is slower than electronic detection.
Critical maintenance points
- Open nozzles can block with dust and insects; protective caps and periodic inspection are needed.
- The deluge valve needs an annual flow test; a seized valve is the most common failure.
- Because the pipework is dry, condensation and corrosion accumulate; drain points must be checked.
- The link between the detection system and the valve must be tested end to end periodically.
Frequently Asked Questions
Why are the heads open in a deluge system?
There is no thermal element because the decision is central. The detection system opens the deluge valve and every head sprays at once.
Where are deluge systems used?
For fast-spreading fires (flammable liquids, paint shops, hangars), equipment cooling (transformers, LPG tanks), battery storage and conveyor lines.
What is the effect of a deluge system on the water supply?
In a conventional system the flow is limited to a defined area; in a deluge system every head counts. Sectioning and keeping the protected area small are therefore fundamental design decisions.
What is a pilot line?
A pressurised detection line carrying closed sprinklers. When a pilot sprinkler opens the pressure drops and the deluge valve opens mechanically; it needs no electricity but responds more slowly.
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Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 15 · NFPA 855 · BS EN 12845:2015+A1:2019 · FM Global DS 5-33 · FM Global DS 4-1N. Definitions are for information only.