Fire -- The Other Data Center Heat Problem
"Mission-critical facilities inherently involve a greater level of risk than most commercial space because of the presence of both a constant ignition source (electricity) and a plentiful supply of fuel (generally plastics as in printed circuit boards)," said Lance Harry, business development manager for Fenwal Protection Systems (Ashland, Mass.), which is part of United Technologies Corporation.
Major fires may be rare, but even minor ones can cause significant disruption.
"It is all about how an individual owner or company views their risk strategy," said Harry. "Most people would agree that while it may not be a common occurrence, the potential impact on the business could be huge."
Reducing the Risk
Thus, crafting a fire protection strategy is critical. The first step in designing a fire protection strategy is to reduce the risk of a fire occurring.
"Fires in the data center typically initiate through electrical equipment or component failure, or some sort of human error, such as poor maintenance practices in various spaces or equipment," said Harry. "It should be noted that in many cases, companies prefer not to publicize even the smallest of events within a mission-critical environment, so data in this area can be difficult to extract."
Simple steps can be taken to reduce the risk of fire. The 2003 edition of the National Fire Protection Association's Standard for the Protection of Electronic Computer/Data Processing Equipment (NFPA) is available for free online, and a new edition is scheduled for release in 2008. Best practices to follow include using fire-resistant cabling and not storing any unnecessary combustible materials in the data center. And keep those fire extinguishers handy; a quick application can prevent a fire in the data center from growing.
Also bear in mind that a the fire doesn't have to start in the data center. It can spread from other areas of the building. The data center's walls and ceiling should therefore be designed to impede the transfer of heat from other parts of the building. If the data center has windows, use the latest type of fire-rated glass products to resist the transfer of heat through the window panes.
"Glass in data centers needs to be able to protect equipment from exposure to extremely high temperatures in some cases, 1,800 degrees Fahrenheit," said Devin Bowman, sales manager at Technical Glass Products (Kirkland, Wash.). "Products such as Pilkington Pyrostop and Heat Barrier frames are classified as transparent wall systems and limit heat transfer in the same manner as a solid fire-rated wall but [also] offer the same visibility as a non-fire-rated window assembly."
Early Warning Systems
The key to limiting the damage of any type of fire is early detection. This requires more sensitive detection equipment than is normally used in fire protection. You want to be able to detect the first smoke particles given off by an overheating power supply rather than waiting until the whole rack is in flames. Data center environments pose a particular challenge. They have a high air flow, which tends to disperse any smoke, so it may not reach concentrations high enough to set off a smoke detector placed on the wall. Further complicating the issue are the air filters many data centers have on their HVAC systems that will remove the smoke particles.
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One option is to place detectors throughout the data center, including above the suspended ceiling and below the raised floor. The other option is to use a system such as Xtralis' (Norwell, Mass.) VESDA, which uses fans and piping to pull air samples from locations throughout the room to a central point for analysis.
Fire Suppression System
Once a fire is detected, a fire suppression system can prevent it from spreading. The National Fire Protection Association (NFPA) reports that when automatic fire suppression equipment is present, 75 percent of fires are confined to the object where the fire originated, and there is only a 5 percent chance the fire will spread outside of the room.
Fire suppression systems use a variety of gases to prevent combustion. The early models used halon 1301, which did an excellent job of putting out fires. However, it also depleted the ozone layer and so was banned. Two main classes of replacements are on the market today:
- Heat Removal Gases: These include FM-200, FE-13 and FE-25, brand names for different hydro-fluorocarbon (HFC) chemicals that extinguish fires by absorbing the heat. They do not leave a residue, lower the oxygen in the room, or harm people or equipment. A new entry is Novec 1230, a fluid developed by 3M that also absorbs heat. Unlike the HFC fire suppression agents, which release greenhouse gases that last for decades in the atmosphere, Novec decomposes in a few days.
- Oxygen Reduction: The other strategy is to lower the oxygen level in the room below the point necessary for combustion but without suffocating any people. This is done by flooding the room with an inert gas, such as argon or nitrogen. At least one such gas blend includes carbon dioxide, which triggers an increased breathing rate in humans to counteract the lower oxygen level.
Forrester Research analyst Galen Schreck said, however, that you can't just select the gas by itself, it must be part of an overall fire suppression strategy designed to meet the needs of a particular data center. The HFC agents have higher chemical costs, but lower installation costs since they can use smaller piping. The inert gases are far less expensive, about one tenth the cost of Novec, but require larger pipes and more room to store the tanks. In addition, since they flood so much gas into the room, the room requires venting so it doesn't blow the doors open.
"Each one of those agents has a particular purpose and place in the toolbox of someone who is installing it," said Schreck. "The only person who can say which is best for your data center is a true professional installer."
Opening the Floodgates
If the clean suppression system fails to put out the fire, the water sprinkler system takes over. There has been some debate as to whether to use water or a chemical fire suppression system in the data center, but the two are actually complementary systems. The fire suppression system is a fast-acting, first line of defense designed to protect equipment in the event of a minor fire.
Water, in contrast, is a building-level system, designed to save lives and keep the building from burning down. Yes, the water will damage the equipment. But to trigger a sprinkler, its sensor (a piece of fusible metal or a glass bulb) must first reach its rated temperature (135-degrees Fahrenheit to 165-degrees Fahrenheit). By the time the sensor reaches its melting point, the surrounding air temperature may be as high as 500 degrees, and smoke and heat have already damaged the servers.
"The hope is that you never get to the point of the building system discharging," said Schreck. "But if the temperature does keep rising, the water will kick in by itself and douse the place."
Branch Office Protection
For mission-critical data centers or equipment rooms, a waterless fire suppression system is a must. Although the risk of a fire is low, the potential impact is very high. However, branch offices, particularly when all the data is replicated elsewhere, may get by with water only.
"Many people just go with the water, but with the understanding that anything there will be destroyed if there is a fire," he said. "It is a business continuity and risk question."
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