Pre Action Sprinkler System: How It Works and Where Used

A pre action sprinkler system is a sprinkler installation in which the pipework is kept dry and water is held back at a control valve until a separate fire detection system confirms that a fire is present. Only then does water enter the pipes, and only the heads that have been heated open. The arrangement protects spaces where an accidental discharge from a damaged head or leaking pipe would cause serious loss.

This article explains how pre-action systems are built, the different interlock arrangements, where they are used, and the trade-offs designers weigh against a conventional wet system. It also notes how such systems fit within Singapore’s fire safety framework.

Why pre-action exists

In an ordinary wet fire sprinkler system, the pipes are always full of water under pressure. That makes the response immediate and the design simple, and for most buildings it is the right choice. The weakness is that any mechanical damage to a head, a knock from a ladder or forklift, or a leaking joint releases water straight away, whether or not there is a fire.

For most rooms that is an acceptable risk. For some it is not. A server hall, a telecommunications room, a museum store, a library of irreplaceable records or an electrical switch room can suffer more damage from a false discharge than from a small, quickly controlled fire. Pre-action design reduces that risk by requiring two independent events before water flows, while keeping the reliability of an automatic sprinkler as the final line of defence.

How a pre action sprinkler system works

The system has three working parts. The first is the sprinkler pipework with ordinary closed heads, kept dry and usually filled with low-pressure air or nitrogen so that leaks can be monitored. The second is a pre-action valve, a deluge-type valve that keeps water out of the pipework until it is released. The third is a detection system, typically smoke or heat detectors in the protected space, wired to a releasing control panel.

In a fire, detectors respond first, often well before any sprinkler head would heat to its rating. The releasing panel registers the alarm and, depending on the interlock type, opens the valve. Water fills the pipework and stands ready behind the closed heads. If the fire continues to grow and a head reaches its operating temperature, it opens and water discharges onto the fire. If the alarm was caused by something minor that staff deal with, no head opens and no water is released into the room, though the pipes will then need to be drained and the system reset.

Supervisory air pressure gives a second benefit. If a head is damaged or a pipe leaks, the loss of air pressure raises a trouble signal, so the fault is found before it matters, without any water escaping.

Single, double and non-interlock arrangements

Pre-action systems are described by what must happen before water reaches the heads.

Non-interlock

The valve opens on either detection or the loss of air pressure from an opened head. This behaves much like a dry pipe system with faster detection and is chosen where a quick water supply matters more than preventing accidental filling.

Single interlock

The valve opens only when the detection system operates. Water then fills the pipes, and discharge happens only when a head opens. A damaged head alone does not admit water, which is why this is the most common arrangement for data and archive spaces.

Double interlock

The valve opens only when both the detection system has operated and a head has opened, shown by a drop in supervisory air pressure. This gives the strongest protection against accidental water, which suits freezer stores and very sensitive rooms. The penalty is that water must travel through dry pipes after a head has already opened, so the delay before discharge is longer, and design must account for it.

The choice between them is a balance of water-damage risk against delay. It is set early in design, because it affects the pipe volume, valve arrangement and detection layout.

Where pre-action systems are used

Typical applications include:

Data centres, server rooms and network equipment rooms
Telecommunication exchanges and control rooms
Museums, galleries, archives and library stacks holding irreplaceable material
Electrical switch rooms and other spaces where water on live equipment is a hazard
Cold stores where pipes must stay dry to avoid freezing

In many of these spaces, owners also consider gaseous or other fixed extinguishing systems. Singapore’s Fire Code addresses fixed automatic fire extinguishing systems in clause 6.5, separately from the sprinkler clause 6.4. A pre-action sprinkler is sometimes used together with a clean agent system, with the gas acting first on equipment and the sprinkler retained as a backstop that also protects the building structure.

Design and coordination points

Pre-action design draws on two disciplines at once, so coordination matters. The detection layout must suit the room’s airflow, which in a data hall is dominated by cooling. Detectors may sit at ceiling level, in the floor void and in the return air path, and aspirating smoke detection is common where very early warning is wanted. The releasing panel must be compatible with the main fire alarm system so that alarms, faults and valve status are reported in the right place.

The pipework volume between the valve and the furthest head affects how long it takes water to arrive. Designers often limit the volume of each zone or use accelerators and exhausters, and the system is tested to confirm delivery time. The valve set needs space, drainage and access for testing. Air compressors or nitrogen generators need power and maintenance. All of this should be planned with the architect and structural engineer so that valve rooms, pipe routes and hangers are accommodated rather than squeezed into place late.

Connecting the pre-action release to the building automation system for monitoring is useful, but the fire function must stay with the fire alarm and releasing panels, which are designed and listed for life-safety duty.

Trade-offs and maintenance

Compared with a wet system, pre-action costs more to install, has more components that can fail and needs more maintenance. After any operation, the pipes must be drained and dried before reset. Detection must be tested alongside the sprinkler valve, and supervisory air pressure must be kept within range. A system that is poorly maintained can end up less reliable than the simple wet system it replaced, which defeats the purpose.

The decision should be based on a frank assessment of what water damage would actually cost in that room, set against the added complexity. Where sprinklers are required by the Fire Code, the chosen system still needs SCDF acceptance through the fire safety plans prepared by a Qualified Person, and the installed system will be inspected before the fire safety certificate is issued.

Frequently asked questions

What is the main advantage of a pre action sprinkler system?

It prevents water from entering the pipework unless a fire detection system has first confirmed a fire. A knocked or damaged head does not release water by itself, which greatly reduces the risk of accidental discharge in data rooms, archives and other spaces where water damage would be costly.

Is a pre-action system slower than a wet system?

Yes, somewhat. Water must travel from the valve into dry pipework before it reaches an open head. The delay is limited by design, through pipe volume limits and testing, and early smoke detection usually opens the valve well before any head would operate, which offsets much of the difference.

What is the difference between single and double interlock?

A single interlock system admits water when the detection system operates, and discharge follows when a head opens. A double interlock system admits water only when both detection has operated and a head has opened. Double interlock gives more protection against accidental water but a longer delay.

Is pre-action the same as a deluge system?

No. Both use a valve released by detection, but a deluge system uses open heads, so water discharges through every head at once. A pre-action system uses closed heads, so water discharges only through the heads that have been heated to their operating temperature.

Can a pre-action system replace a gas suppression system?

They do different jobs. Gaseous systems act on the fire quickly without water and are aimed at protecting equipment, while a pre-action sprinkler protects the room and structure as a final line of defence. Many sensitive rooms use both, and the choice should be agreed with the fire engineer and SCDF.

Fitting out a data room or archive and need the structure, plant loads and services coordinated? Talk to our consultancy or get in touch.