Fire sprinkler system

Fire sprinkler system

A fire sprinkler system is an active fire protection method, consisting of a water supply system, providing adequate pressure and flow rate to a water distribution piping system, onto which fire sprinklers are connected. Although historically only used in factories and large commercial buildings, systems for homes and small buildings are now available at a cost-effective price.[1] Fire sprinkler systems are extensively used worldwide, with over 40 million sprinkler heads fitted each year. In buildings completely protected by fire sprinkler systems, over 96% of fires were controlled by fire sprinklers alone.

Regulations

United States

The primary fire code writing organization is the private National Fire Protection Association or NFPA. NFPA sets the standards for technical aspects of sprinklers installed in the USA. Building codes, which specify which buildings require sprinklers are generally left to local jurisdictions. However there are some exceptions:

In 1990 the US Congress passed PL-101-391, better known as the Hotel and Motel Fire Safety Act of 1990.[14] This law requires that any hotel, meeting hall, or similar institution that receives federal funds (i.e. for a government traveler’s overnight stay, or a conference, etc.), must meet fire and other safety requirements. The most visible of these conditions is the implementation of sprinklers. As more and more hotels and other public accommodations upgraded their facilities to enable acceptance of government visitors, this type of construction became the de facto industry norm – even when not directly mandated by any local building codes.

If building codes do not explicitly mandate the use of fire sprinklers, the code often makes it highly advantageous to install them as an optional system. Most US building codes allow for less expensive construction materials, larger floor area limitations, longer egress paths, and fewer requirements for fire rated construction in structures protected by fire sprinklers. Consequently, the total building cost is often decreased by installing a sprinkler system and saving money in the other aspects of the project, as compared to building a non-sprinklered structure.

In 2011, Pennsylvania and California became the first US states to require sprinkler systems in all new residential construction.[15] However, Pennsylvania repealed the law later that same year.[16] Many municipalities now require residential sprinklers, even if they are not required at the state level.[17]

Europe

Renewed interest in and support for sprinkler systems in the UK has resulted in sprinkler systems being more widely installed. In schools, for example, the government has issued recommendations through Building Bulletin 100 that most new schools should be constructed with sprinkler protection. In 2011 Wales became the first country in the world where sprinklers are compulsory in all new homes. The law applies to newly built houses and blocks of flats, as well as care homes and university halls of residence.[18][19] In Scotland, all new schools are sprinklered, as are new care homes, sheltered housing and high rise flats.

In the UK, since the 1990s sprinklers have gained recognition within the Building Regulations (England and Wales) and Scottish Building Standards and under certain circumstances, the presence of sprinkler systems is deemed to provide a form of alternative compliance to some parts of the codes. For example, the presence of a sprinkler system will usually permit doubling of compartment sizes and increases in travel distances (to fire exits) as well as allowing a reduction in the fire rating of internal compartment walls.

In Norway as of July 2010, all new housing of more than two storeys, all new hotels, care homes and hospitals must be sprinklered. Other Nordic countries require or soon will require[citation needed] sprinklers in new care homes, and in Finland as of 2010 a third of care homes were retrofitted with sprinklers. A fire in an illegal immigrant detention center at Schiphol airport in The Netherlands on 27 October 2005 killed 11 detainees and led to the retrofitting of sprinklers in all similarly designed prisons in the Netherlands. A fire at Düsseldorf Airport on 11 April 1996 which killed 17 people led to sprinklers being retrofitted in all major German airports. Most European countries also require sprinklers in shopping centers, in large warehouses and in high-rise buildings.[citation needed]

Operation

A sign warns hotel guests not to hang items from fire sprinklers

Each closed-head sprinkler is held closed by either a heat-sensitive glass bulb or a two-part metal link held together with fusible alloy. The glass bulb or link applies pressure to a pipe cap which acts as a plug which prevents water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head. In a standard wet-pipe sprinkler system, each sprinkler activates independently when the predetermined heat level is reached. Thus, only sprinklers near the fire will operate, normally just one or two. This maximizes water pressure over the point of fire origin, and minimizes water damage to the building.[20]

A sprinkler activation will do less water damage than a fire department hose stream, which provide approximately 900 litres/min (250 US gallons/min). A typical sprinkler used for industrial manufacturing occupancies discharge about 75-150 litres/min (20-40 US gallons/min). However, a typical Early Suppression Fast Response (ESFR) sprinkler at a pressure of 50 psi (340 kPa) will discharge approximately 380 litres per minute (100 US gal/min). In addition, a sprinkler will usually activate within one to four minutes of the fire's start, whereas it typically takes at least five minutes for a fire department to register an alarm and drive to the fire site, and an additional ten minutes to set up equipment and apply hose streams to the fire. This additional time can result in a much larger fire, requiring much more water to extinguish.

Types

Wet pipe systems

By a wide margin, wet pipe sprinkler systems are installed more often than all other types of fire sprinkler systems. They also are the most reliable, because they are simple, with the only operating components being the automatic sprinklers and (commonly, but not always) the automatic alarm check valve. An automatic water supply provides water under pressure to the system piping.

Wet pipe antifreeze

Wet systems may be charged with an antifreeze chemical, for use where pipes can't reliably be kept above 40 degrees Fahrenheit.

While such systems were once common in cold areas, after several fires caused by systems running too high a percentage of antifreeze, the regulatory authority in the United States effectively banned new antifreeze installations. A sunset date of 2022 applies to older antifreeze systems in the USA [21]. This regulatory action has greatly increased costs and reduced options for cold weather sprinkler systems.

Dry pipe systems

Garage sprinkler system in New York City

Dry pipe systems are the second most common sprinkler system type. Dry pipe systems are installed in spaces in which the ambient temperature may be cold enough to freeze the water in a wet pipe system, rendering the system inoperable. Dry pipe systems are most often used in unheated buildings, in parking garages, in outside canopies attached to heated buildings (in which a wet pipe system would be provided), or in refrigerated coolers. In regions using NFPA regulations, wet pipe systems cannot be installed unless the range of ambient temperatures remains above 40 °F (4 °C).[22]

Water is not present in the piping until the system operates; instead, the piping is filled with air at a pressure below the water supply pressure. To prevent the larger water supply pressure from prematurely forcing water into the piping, the design of the dry pipe valve (a specialized type of check valve) results in a greater force on top of the check valve clapper by the use of a larger valve clapper area exposed to the piping air pressure, as compared to the higher water pressure but smaller clapper surface area.

When one or more of the automatic sprinkler heads is triggered, it opens allowing the air in the piping to vent from that sprinkler. Each sprinkler operates independently, as its temperature rises above its triggering threshold. As the air pressure in the piping drops, the pressure differential across the dry pipe valve changes, allowing water to enter the piping system. Water flow from sprinklers, needed to control the fire, is delayed until the air is vented from the sprinklers. In regions using NFPA 13 regulations, the time it takes water to reach the hydraulically remote sprinkler from the time that sprinkler is activated is limited to a maximum of 60 seconds. In industry practice, this is known as the "Maximum Time of Water Delivery". The maximum time of water delivery may be required to be reduced, depending on the hazard classification of the area protected by the sprinkler system.[23]

Some property owners and building occupants may view dry pipe sprinklers as advantageous for protection of valuable collections and other water sensitive areas. This perceived benefit is due to a fear that wet system piping may slowly leak water without attracting notice, while dry pipe systems may not fail in this manner.[citation needed]

Disadvantages of using dry pipe fire sprinkler systems include:

If the sprinklers share the same standpipe system as the standpipe system which supplies fire hoses, then the water supply to the fire hoses would be severely reduced or even curtailed altogether.

Increased complexity: Dry pipe systems require additional control equipment and air pressure supply components which increases system complexity. This puts a premium on proper maintenance, as this increase in system complexity results in an inherently less reliable overall system (i.e. more single failure points) as compared to a wet pipe system.

Higher installation and maintenance costs: The added complexity impacts the overall dry-pipe installation cost, and increases maintenance expenditure primarily due to added service labor costs.

Lower design flexibility: Regulatory requirements limit the maximum permitted size (i.e. 750 gallons) of individual dry-pipe systems, unless additional components and design efforts are provided to limit the time from sprinkler activation to water discharge to under one minute. These limitations may increase the number of individual sprinkler zones (i.e. served from a single riser) that must be provided in the building, and impact the ability of an owner to make system additions.

Increased fire response time: Because the piping is empty at the time the sprinkler operates, there is an inherent time delay in delivering water to the sprinklers which have operated while the water travels from the riser to the sprinkler, partially filling the piping in the process. A maximum of 60 seconds is normally allowed by regulatory requirements from the time a single sprinkler opens until water is discharged onto the fire. This delay in fire suppression results in a larger fire prior to control, increasing property damage.

Dry pipe sprinkler system supply main with corrosion debris caused by oxidation

Increased corrosion potential: Following operation or testing, dry-pipe sprinkler system piping is drained, but residual water collects in piping low spots, and moisture is also retained in the atmosphere within the piping. This moisture, coupled with the oxygen available in the compressed air in the piping, increases internal pipe corrosion, eventually leading to pin-hole leaks or other piping failures. The internal corrosion rate in wet pipe systems (in which the piping is constantly full of water) is much lower, as the amount of oxygen available for the corrosion process is lower.[24] Corrosion can be combated by using copper or stainless steel pipe which is less susceptible to corrosion, or by using dry nitrogen gas to pressurize the system, rather than air. Nitrogen generators can be used as a permanent source of nitrogen gas, which is beneficial because dry pipe sprinkler systems require an uninterrupted supply of supervisory gas. These additional precautions can increase the up-front cost of the system, but will help prevent system failure, increased maintenance costs, and premature need for system replacement in the future.

Deluge systems

"Deluge" systems are systems in which all sprinklers connected to the water piping system are open, in that the heat sensing operating element is removed, or specifically designed as such. These systems are used for special hazards where rapid fire spread is a concern, as they provide a simultaneous application of water over the entire hazard. They are sometimes installed in personnel egress paths or building openings to slow travel of fire (e.g. openings in a fire-rated wall).

Water is not present in the piping until the system operates. Because the sprinkler orifices are open, the piping is at atmospheric pressure. To prevent the water supply pressure from forcing water into the piping, a "deluge valve" is used in the water supply connection, which is a mechanically latched valve. It is a non-resetting valve, and stays open once tripped.

Because the heat sensing elements present in the automatic sprinklers have been removed (resulting in open sprinklers), the deluge valve must be opened as signaled by a fire alarm system. The type of fire alarm initiating device is selected mainly based on the hazard (e.g. pilot sprinklers, smoke detectors, heat detectors, or optical flame detectors). The initiation device signals the fire alarm panel, which in turn signals the deluge valve to open. Activation can also be manual, depending on the system goals. Manual activation is usually via an electric or pneumatic fire alarm pull station, which signals the fire alarm panel, which in turn signals the deluge valve to open.

Operation - Activation of a fire alarm initiating device, or a manual pull station, signals the fire alarm panel, which in turn signals the deluge valve to open, allowing water to enter the piping system. Water flows from all sprinklers simultaneously.

Pre-action systems

Pre-action sprinkler systems are specialized for use in locations where accidental activation is especially undesirable, such as in museums with rare art works, manuscripts, or books; and data centers, for protection of computer equipment from accidental water discharge.

Pre-action systems are hybrids of wet, dry, and deluge systems, depending on the exact system goal. There are two main sub-types of pre-action systems: single interlock, and double interlock.

The operation of single interlock systems are similar to dry systems except that these systems require that a "preceding" fire detection event, typically the activation of a heat or smoke detector, takes place prior to the "action" of water introduction into the system's piping by opening the pre-action valve, which is a mechanically latched valve (i.e. similar to a deluge valve). In this way, the system is essentially converted from a dry system into a wet system. The intent is to reduce the undesirable time delay of water delivery to sprinklers that is inherent in dry systems. Prior to fire detection, if the sprinkler operates, or the piping system develops a leak, loss of air pressure in the piping will activate a trouble alarm. In this case, the pre-action valve will not open due to loss of supervisory pressure, and water will not enter the piping.

The operation of double interlock systems are similar to deluge systems except that automatic sprinklers are used. These systems require that both a "preceding" fire detection event, typically the activation of a heat or smoke detector, and an automatic sprinkler operation take place prior to the "action" of water introduction into the system's piping. Activation of either the fire detectors alone, or sprinklers alone, without the concurrent operation of the other, will not allow water to enter the piping. Because water does not enter the piping until a sprinkler operates, double interlock systems are considered as dry systems in terms of water delivery times, and similarly require a larger design area.

Foam water sprinkler systems

A foam water fire sprinkler system is a special application system, discharging a mixture of water and low expansion foam concentrate, resulting in a foam spray from the sprinkler. These systems are usually used with special hazards occupancies associated with high challenge fires, such as flammable liquids, and airport hangars. Operation is as described above, depending on the system type into which the foam is injected.

Water spray

"Water spray" systems are operationally identical to a deluge system, but the piping and discharge nozzle spray patterns are designed to protect a uniquely configured hazard, usually being three-dimensional components or equipment (i.e. as opposed to a deluge system, which is designed to cover the horizontal floor area of a room). The nozzles used may not be listed fire sprinklers, and are usually selected for a specific spray pattern to conform to the three-dimensional nature of the hazard (e.g. typical spray patterns being oval, fan, full circle, narrow jet). Examples of hazards protected by water spray systems are electrical transformers containing oil for cooling or turbo-generator bearings. Water spray systems can also be used externally on the surfaces of tanks containing flammable liquids or gases (such as hydrogen). Here the water spray is intended to cool the tank and its contents to prevent tank rupture/explosion (BLEVE) and fire spread.

Water mist systems

Water mist systems are used for special applications in which it is decided that creating a heat absorbent vapor is the primary objective. This type of system is typically used where water damage may be a concern, or where water supplies are limited. NFPA 750[25] defines water mist as a water spray with a droplet size of "less than 1000 microns at the minimum operation pressure of the discharge nozzle." The droplet size can be controlled by the adjusting discharge pressure through a nozzle of a fixed orifice size. By creating a mist, an equal volume of water will create a larger total surface area exposed to the fire. The larger total surface area better facilitates the transfer of heat, thus allowing more water droplets to turn to steam more quickly. A water mist, which absorbs more heat than water per unit time, due to exposed surface area, will more effectively cool the room, thus reducing the temperature of the flame.

Operation - Water mist systems can operate with the same functionality as deluge, wet pipe, dry pipe, or pre-action systems. The difference is that a water mist system uses a compressed gas as an atomizing medium, which is pumped through the sprinkler pipe. Instead of compressed gas, some systems use a high-pressure pump to pressurize the water so it atomizes as it exits the sprinkler nozzle.[26] Systems can be applied using local application method or total flooding method, similar to Clean Agent Fire Protection Systems.