Keeping Fire in Its Place in Data Centers

06/01/2006


The first automatic sprinkler system was patented by Philip W. Pratt of Abington, Mass. in 1872, and sprinklers have since been the global leader among fire extinguishing systems. However, with the arrival of the electronic age, it became apparent that fire suppression techniques needed to change in order to provide ways of suppressing fires without damaging the sensitive equipment found within today's mission-critical facilities.

Today, both NFPA 75 and FM 5-32 still recommend automatic sprinkler systems throughout all data centers. However, both also recommend a gaseous total flooding extinguishing system for data centers that have a critical need to protect data in process, reduce equipment damage and facilitate quick return to service. These systems generally activate much faster than automatic sprinkler systems—i.e., they are activated by detection of smoke, not heat—which increases the likelihood of suppressing the fire in its incipient stage and minimizes the probability of sprinkler system activation.

Total flooding systems and clean agents

NFPA 2001 is the standard that applies to clean agent fire extinguishing systems. According to this standard, a total flooding system is one that consists of an agent supply and distribution network designed to achieve a total flooding condition in a hazard volume. Both NFPA 75 and FM 5-32 recommend that the gaseous total flooding system should be automatically actuated by an approved method of detection. Additionally, all electronic equipment and air-handling systems should be disconnected/shut down upon activation of such a system, and alarms should warn of pending and actual discharges.

Clean agents are defined by NFPA 2001 as electrically nonconducting, volatile or gaseous fire extinguishants that do not leave a residue upon evaporation. For an agent to qualify as a clean agent, it must have no effect on human survival within the enclosure it protects, it must have no known effect on the ozone layer and it must be used in normally occupied areas in a concentration that is less than the "no observed adverse effect" level.

Clean agents can be divided into two groups: halocarbon agents and inert gas agents. Halocarbon agents consist of hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and perfluorocarbons (PFCs). Inert gas agents contain one or more inert gases such as helium, neon and argon, mixed with nitrogen and carbon dioxide.

Halocarbons

Halocarbons extinguish fires by chemical and mechanical mechanisms as opposed to oxygen deprivation. The most well-known halocarbon-extinguishing agent is halon, which was widely specified and installed from the 1940s until the 1980s, when it became apparent that it was harmful to the earth's protective ozone layer. In accordance with the Montreal Protocol, created in 1987, the production of halon was terminated on Jan. 1, 1994. That said, the protocol did not require the elimination of existing halon extinguishing systems. Consequently, owners of halon systems were faced with the prospect of a total loss of fire protection pursuant to an accidental or purposeful discharge, and many of them opted to replace their halon systems with either a substitute gaseous or water-based system. (For more on halon replacement, see "Life after Halon," CSE, 11/2005, p. 58 .)

FM-200 is the most commonly used halocarbon-extinguishing agent today and is a replacement for Halon 1301. It is chemically known as heptafluoropropane, has no ozone depletion potential and is found by leading toxicologists to be safe for use when people are present. FM-200 is effective on Class A surface burning, Class B flammable liquid and Class C electrical fires, and extinguishes fires quickly through a combination of chemical interaction and physical heat removal. It is stored as a liquid in pressurized cylinders and flows through a piping network to a discharge nozzle where it is deployed as a gas. It penetrates with three-dimensional capability and is able to extinguish fires in obscure or difficult-to-reach areas.

Inert gases

Inert gases extinguish fires by lowering the oxygen concentration contained within a room from the normal or ambient condition of 21% to below 15%, which is less than the level required to sustain combustion for most combustibles. Simultaneously the carbon dioxide in the inert gas protects anyone that may be trapped in the fire area from the effects of the lowered oxygen levels.

Inergen. Along with FM-200, Inergen is today one of the most commonly used clean agents. It is an environmentally friendly inert gas and, like FM-200, is also a replacement for Halon 1301. Inergen is electrically nonconductive, safe for use in human-occupied facilities, non-damaging to sensitive electrical equipment and has zero ozone depletion. Inergen is effective against Class A, B and C fires occurring within an enclosure. It's discharged through specially designed nozzles fitted with discharge deflectors to minimize turbulence in the protected area.

Recent newcomers: Novec 1230 and NAF-S-125. Two newcomers on the clean agent market are Novec 1230 and NAF-S-125. Novec 1230 is a clear, colorless and low-odor fluid that extinguishes fires by chemical interaction and heat absorption. The fluid vaporizes at 120°F—92°F below the boiling temperature of water. Novec 1230 looks like water, but does not cause the type of damage associated with water when putting out a fire. Its fire protection performance, environmental profile and low toxicity characteristics makes it an excellent Halon replacement.

NAF-S-125 (HFC 125) is a halocarbon-extinguishing agent intended for total flooding applications. It has physical characteristics that are very similar to those of Halon 1301, but with zero ozone depletion and a low acute toxicity level, which makes it suitable for use in normally occupied areas. The maximum exposure time for NAF-S-125 is set in NFPA 2001 to 11.5% for a maximum period of five minutes.

Carbon dioxide. Carbon dioxide is a colorless, odorless and gaseous fire protection agent. It is incapable of conducting or transmitting electrical charges, which makes it useful for protection of computer equipment. However, its properties can also create a scenario for a potential hazard to human life when high volumes of it are injected into a room. Carbon dioxide is 1.5 times heavier than air, and it extinguishes fires by reducing the oxygen concentration within a room to below the concentration required for sustained combustion. It should not be used for materials containing an oxygen supply or for hazards involving reactive metals and metal hybrids. Additionally, it should not be considered for normally occupied enclosures, enclosures requiring extensive egress times or those where occupants may get trapped. Carbon Dioxide is suitable for Class A, B and C fires.

Water mist. Water mist systems are automatic water-based fire protection systems with nozzles capable of distributing mist to a variety of hazards and have the potential to serve as replacement systems for Halon. These systems can be used for local and total flooding applications. Water mist is natural, non-toxic, efficient and widely applicable.

Preaction sprinkler systems. Automatic sprinkler heads open as a result of elevated temperatures (normally at 165°F) and not upon detection of smoke. Consequently, the size of the fire upon activation of the automatic sprinkler system will normally be so significant that water damage to equipment may be of less concern. Nevertheless, the concern related to overhead water within data centers is still alive today. As a result of this concern, pre-action systems are commonly found in data centers.

Preaction sprinkler system pipes are charged with compressed air rather than water, and the water is held back by means of a preaction valve. A preaction system is equipped with a supplemental detection system, which upon activation permits the preaction valve to automatically open. Only then is water allowed to enter the sprinkler pipes. However, the fire will have to generate enough heat to activate one or more sprinkler heads before water is discharged from the system.

Best fit

Clearly, there are several options for fire suppression, including many replacement alternatives for Halon 1301. Choosing one is simply a matter of comparing their individual characteristics and finding the best fit for your data center's needs.





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