How do fire sprinklers work?

Sprinkler systems are one of the most common types of fire protection systems found in buildings today, and they have a history of being the most reliable form of fire protection when it comes to protecting people, controlling fires, and saving lives. Over the past 130 years they have had a better than 99% success rate in controlling fires around the world. The installation of fire sprinklers will virtually eliminate all deaths from fire, reduce injuries by at least 80%, reduce property damage by 90% and substantially reduce damage to the environment from fire. Indeed, there have been no multiple fatality fires in the UK in buildings with a working sprinkler system. But how does a fire sprinkler work? This advice and guidance article will explore how sprinklers work, touching on how they activate, how long they stay active for, how sensitive they are, whether they detect smoke, and finally whether it is possible for them to activate accidentally.

How are fire sprinklers activated?

How your automatic sprinklers activate depends on the type of sprinkler system installed in your premises. There are two types of sprinkler head release mechanisms that are commonly found being used in sprinkler systems - glass bulbs and fusible links.

In a system where your fire sprinkler heads are fitted with a glass bulb, the bulb is filled with a heat sensitive liquid, which is commonly glycerin-based. In a fire, this type of sprinkler will activate when the temperature increases and causes the liquid in the bulb to expand. At a predetermined temperature, which is usually 68°C, the liquid in the bulb will expand to a point where the glass shatters. Consequently, this releases a valve seat which permits the flow of pressurised water through the sprinkler head, onto the sprinkler deflector and spraying towards the seat of the fire.

A fusible link sprinkler head will operate in a very similar way, however the key difference is that the pressurized water is held in the sprinkler pipework by a fusible link made of a low-melting alloy. In a fire, the heat from the fire causes the alloy to melt, which will release water from the pipework through the sprinkler head and towards the seat of the fire.

It is important to note that each sprinkler head operates independently in a fire, meaning that only the sprinkler heads directly in the area of the fire will operate to prevent fire spread and water damage will therefore be minimised to this area.

How long do fire sprinklers stay on?

A sprinkler will remain activated and continue to discharge water over a fire until the fire has been extinguished, or until the water supply is manually shut off or exhausted. Depending upon the risk this can be 30 minutes for Light Hazard occupancies (LH), 60 minutes for Ordinary Hazard occupancies (OH), or 90 minutes for High Hazard (HH) occupancies. Once a sprinkler has been activated during a fire, the delivery of water in the right quantity will control the fire and in some instances extinguish it. Statistics from the US indicate that 94% of all fires in sprinklered building are extinguished with four or less sprinkler heads operating. The active sprinkler system will then be manually stopped only by the fire and rescue service once they have determined that the fire is under control or has been extinguished.

How sensitive are fire sprinklers?

The sensitivity of a sprinkler head will depend on the location in which they are present. In common residential and office settings, residential fire sprinklers together with commercial fire sprinklers will be designed to activate at temperatures circa 68°C. In settings where the temperature will be expected to be higher, such as an industrial setting or near to heat producing equipment, it may be more common to find sprinklers that have a higher operating temperature - between 79oC and 141°C, or even higher for specific risks.

Specific temperature ratings allow for the minimisation of false activations from non-fire related heat sources, such as heat from standard cooking practices in residential occupancies to exhaust or steam pipework for commercial applications.

Do fire sprinklers detect smoke?

No, fire sprinkler systems do not detect smoke. They are designed to only respond to heat in a fire situation, but can be utilised as a heat-detecting system

Commonly, buildings will utilise a sprinkler system in conjunction with smoke detection systems in order to provide a higher level of fire safety. In a fire, automatic fire alarms and smoke detection systems will raise an alarm to alert building occupants of the fire to enable safe evacuation, whereas a sprinkler system will control and prevent the spread of the fire with water.

Can fire sprinklers go off accidentally?

Sprinkler systems are designed in such a way that the accidental discharge of a sprinkler system is a very rare occurrence, however it cannot be ruled out.

There may be reasons that a sprinkler system could accidentally discharge. Examples are:

  • mechanical or physical damage to the sprinkler head
  • exposure to temperature extremes which are not fire related
  • vandalism
  • manufacturing defects
  • chemical corrosion
  • improper service and maintenance.

For these reasons, it is essential that your sprinkler system is regularly inspected by a competent and qualified independent  sprinkler expert with the relevant experience and qualifications at least once a year, and the sprinkler heads are tested at 25 years from system installation to ensure they will operate when required in a fire.

The Fire Protection Association (FPA) is the market leader for sprinkler head testing and has conducted thousands of tests in our purpose-built sprinkler head test laboratory. We also provide sprinkler pipework inspections at our laboratory in accordance with the LPC/BS EN 12845 rules. You can find out more about sprinkler head testing at the FPA here.

The FPA pride ourselves on being independent within the fire protection market and can offer sprinkler system inspection services in accordance Clause 20, 21, Annex Q and TB203 of the LPC/BS EN 12845 Rules.. You can find out more about sprinkler system inspection at the FPA here.

Please be aware that considerable efforts have been made to ensure the accuracy of the information contained within this article at the time of publication, however any legislative (or other) changes that come into effect after this may render the information out of date until it is reviewed and updated as part of the FPA’s content review cycle.