04 May Controlling Fire Development & Spread with Fire Sprinkler Systems
In the event of a fire in a building, active fire suppression systems can have a major impact in fire development and spread, and they can be one of the crucial factors in affecting occupant safety. Here Mina Abdo, Senior Fire Safety Engineer at Warringtonfire explains what we all need to know about automatic fire sprinkler systems.
The primary purpose of an active fire suppression system is to control or suppress a building fire to limit its development and spread throughout the building in order to allow sufficient time for occupant evacuation and fire services intervention.
Automatic fire sprinkler systems (AFSS) are a widely-used form of active fire suppression system because they use water as the firefighting agent, which is widely available and inexpensive. Water has desirable fire extinguishing characteristics such as a high specific heat and high latent heat of vaporisation, which can absorb heat from the fire so that combustion is not sustained.
AFSS can be easily incorporated in the fire and life safety strategy and building design, and are generally a cost-effective fire safety measure to satisfy the requirements of building regulations.
Benefits of fire sprinkler systems
The successful activation of an active fire sprinkler system is expected to provide the following benefits:
- A reduction in the rate of burning and quantity of smoke produced, subsequently increasing the available safe evacuation time.
- A reduced fire intensity and duration, which in turn reduces the severity of fire exposure to structural and fire separating elements.
- A reduction in the chances of a fire spreading beyond the area of origin or flashover occurring.
- Full scale tests have shown that standard sprinklers can be expected to maintain tenable conditions in relation to temperature and toxicity outside the room where the fire started.
- Data collected in the US demonstrates that, ‘when sprinklers are present, the chances of dying in a fire are reduced by one-half to three-fourths, and the average property loss per fire is cut by one-half to two-thirds, compared to fires where sprinklers are not present.’
The CIBSE Guide E notes the following potential concessions for buildings protected by sprinklers:
- Building compartment areas / volumes may be increased over that for a similar building without sprinklers.
- A structural element is liable to maintain its load-bearing capacity and a separating element will maintain both its integrity and its ability to resist the transfer of heat. The fire resistance levels may therefore be reduced if sprinklers
are fitted. - The distance required to travel to an exit can potentially be increased without reducing the level of safety to people.
- Statistics on US examples of fires that occur in a building with sprinklers show that 88% of fires are controlled by one or two sprinklers when sprinklers were effective. Data provided by Marryatt concludes that 92% of fires are controlled by one to five sprinklers.
- Sprinkler systems have been demonstrated to achieve high operational reliability through numerous statistical studies. Budnik estimated that the mean reliability of sprinkler systems was 93-96%, based on the analysis of 16 separate studies. Reliability is likely to be even higher where sprinkler systems are correctly designed, commissioned and maintained.
Types of fire sprinkler systems
With reference to the SFPE Handbook, there are four basic types of sprinkler systems that can be used for the majority of applications. The difference between these four basic types of systems relates to how water is discharged from the system into the fire protected area. There are other variations of sprinkler system arrangements, however all sprinkler systems can be categorised as per the following basic types.
Wet pipe fire sprinkler system
Wet pipe sprinkler systems comprise a network of piping filled with water under pressure and automatic sprinklers. Automatic sprinklers are provided with heat responsive elements that are generally a heat bulb or fusible link that activates at a specific temperature. In the event of a fire, the heat applied on the sprinkler will activate the sprinkler (i.e. breaks the heat bulb or fusible link), which allows water to discharge from the sprinkler over an assigned floor area. This is the area of protection.
A wet pipe sprinkler system is the simplest and most commonly used system in various building types and applications.
Dry pipe system
Dry pipe fire sprinkler systems are similar to a wet pipe system, though as the name suggests, no water is held in the piping network. Water is held back by a special dry pipe valve, where the piping network is filled with air or nitrogen so that the dry pipe valve is kept closed. Dry pipe systems are provided with automatic sprinklers with heat responsive elements. When sprinklers activate, the pressure in the dry pipe system drops as air escapes from the activated sprinkler, causing the operation of the dry valve by the water flow. Water is then admitted to the piping network and discharged from the activated sprinklers over the assigned area of protection.
A dry pipe system is used in applications where the water in the piping would be subject to freezing.
Deluge system
A deluge system consists of a network of piping connected to open sprinklers rather than automatic sprinklers, meaning that the sprinklers are not provided with heat responsive elements. Water is held behind a special deluge valve that is activated by a separate fire detection system. When the deluge system is activated, water flows through the pipe and discharges simultaneously from all of the sprinklers connected to the
deluge valve.
Deluge systems are used for protection against rapidly spreading, high heat release fires (i.e. tunnels, airport hangars, etc.).
Pre-action fire sprinkler system
A pre-action system is composed of a piping network charged with air or inert gas under pressure so that the system is airtight, while water is held back by a special pre-action valve. Automatic fire sprinklers are used in the pre-action sprinkler system, as well as an independent detection system that is also installed in the same area as the sprinklers.
Similar to the deluge system, the detection system activates the pre-action valve to admit water to flow into the piping network. Since the pre-action sprinkler system is provided with automatic sprinklers with heat responsive elements, water is only discharged from the sprinklers activated by the heat from the fire.
Pre-action systems could also be arranged so that the pre-action valve is activated when both the detection system and the automatic sprinklers are activated (via receiving an alarm from the detection system and a monitored low-pressure alarm from sprinkler operation).
Pre-action sprinkler systems are generally used in applications where there is a concern for accidental water discharge (i.e. data centres, computer rooms, etc.).
Testing fire sprinkler systems
Active fire testing of sprinkler systems is the process of testing fixed fire protection systems for use in residential and domestic buildings. Active fire protection systems need to be reliable, so the testing it undergoes should be comprehensive.
Testing frequently checks the variable factors of a sprinkler, like the deflector patterns and size and shape of the orifice. It also checks the efficacy of the heat sensor and that it is operating against the prescribed response time index.
Applying a testing standard to an active fire testing system saves money in the long run by reducing weekly inspections and testing, and identifying preventative measures that can reduce the likelihood of future expensive repairs.
As well as saving money, comprehensive active fire testing can also reduce environmental impact due to water savings. And reducing maintenance and testing can also impact greenhouse gas emissions.
Completing active fire testing is one of the best ways of demonstrating commitment to duty of care and the ongoing maintenance of fire suppression systems. Bespoke programmes, in conjunction with meeting necessary standards, can move products to market faster while maintaining high quality standards.
Fire sprinkler systems can protect various building types and applications with various hazards as required by building regulations. With flexible design arrangements, relatively low operating cost, and high efficiency in the event of a fire, they are an effective fire safety measure for most fire safety strategies. Importantly, they can also limit property damage and protect life, so should always be considered as part of the fire safety systems in a building’s design.
