Computer Fire

Fire-protection design for computer rooms doesn't need to be an expensive guessing game. But it's true that when companies attempt to protect these most valuable electronic assets, the stakes are higher and the fire-protection systems become more complex and costly. For example, in most cases, the conventional wet sprinkler system plays only a secondary role.

By David N. Holst, Department Manager, Plumbing and Fire Protection, Bala Consulting Engineers, Inc., Philadelphia November 1, 2003

Fire-protection design for computer rooms doesn’t need to be an expensive guessing game. But it’s true that when companies attempt to protect these most valuable electronic assets, the stakes are higher and the fire-protection systems become more complex and costly. For example, in most cases, the conventional wet sprinkler system plays only a secondary role. When it comes to protecting electronic equipment, clean agent systems take over, because an owner cannot afford to let fire conditions reach the level where the wet sprinkler system is activated.

With the development of newer, clean agent fire-protection systems (National Fire Protection Assn. 2001), end users no longer need to use the ozone-depleting gas halon, as suggested in NFPA 12a. The new clean agent systems currently available to protect the computer room environment are perfectly suited for the task and do not harm the electronic equipment, disks, tapes or servers.

Clean agents are just that—clean, environmentally-friendly gases. They leave no residue, there is no water and they are very efficient at extinguishing fires in the very early stages of development without environmental damage.

For more than 10 years now, the industry has been using a clean agent gas called HFC-227ea, heptafluoropropane, with excellent results. Similar to halon, this gas requires constructing an airtight seal or box around the computer room. Walls need to be constructed to full height above and below the raised floor. In addition, openings for conduits, pipe, wiring and ductwork must be sealed where they penetrate the wall and floors. Doors at the room perimeter must be gasketed and have automatic closures. Also, automatic dampers need to be provided at both fresh air and exhaust ducts supplying the room.

To determine the necessary quantity of HFC-227ea, the volume of the room is calculated, and using the ambient temperature of 70the rupture disc or similar device on the tank outlet. The piping system, usually schedule-40 galvanized steel pipe, delivers the gas to nozzles that are strategically placed around the room. NFPA requires all of the gas to be discharged to the room within 10 seconds. It’s important to note that thedischarge sound resembles a mild explosion and can be very startling to individuals remaining in the space.

Recently, HFC-125, Pentafluoroethane, has become available for use as a clean agent for data center fire protection. This clean agent is nearly a perfect match and replacement for the old halon systems. From a fire-protection engineering design perspective, the chemical properties are so close to halon that in most retrofit cases, the existing halon piping system can be saved and reused. Only the discharge nozzles and agent tanks need replacement. This simple retrofit is especially beneficial for data centers, since the existing piping system becomes intertwined with the cabling and raised floor equipment.

Equally important to the type of gas employed in a fire-suppression system are the sensors used to detect fire. Since the extinguishing system only takes 10 seconds to fully discharge, the detection system must be equally sensitive to the changes within the data center and signal that a fire condition is brewing. The first products of combustion create smoke. Therefore, smoke detection is one of the most important ingredients in the detection system.

There are two common types of smoke detection systems, passive and active. In the passive system, the smoke must pass through the detector. One type of passive detection is photoelectric. Photoelectric detectors are located in a uniform pattern on the data center ceiling and below the raised floor to sense smoke in the air and sound the alarm. These detectors can be wired and are uniquely identified by a control panel to provide the occupants with the exact location of the origin of the smoke. This may be very beneficial to the manager of the data center as immediate attention to a precise location could head off a more damaging event.

The active type of detection takes samples of the room air, like a vacuum cleaner, at various locations and analyzes the change in density or obscurity of the air. If a variance exists, an alarm is sounded. These air-sampling systems are extremely sensitive, but the exact source of the smoke cannot be pinpointed, so the source could be anywhere in the room.

The third component of a gas fire-suppression system is the control panel. The control panel must be capable of accepting all of the information and perform the other requirements to satisfy NFPA and the legal fire authorities. The panel ties all of the components together into a complete system.

Clean, environmentally friendly gases are an essential component to protecting valuable computer equipments. To protect these assets, an owner can’t wait for the sprinklers to activate. A fire calls for fast-acting agents that will not harm equipment.

Water Mist still a Niche Technology

In the arena of special suppression systems, water mist technology is one of the most underused options available. Despite the great excitement surrounding this method of fire extinguishment in the mid to late ’90s, its anticipated application in buildings has never come to fruition.

Water, as a fire suppression agent, generally relies on its ability to absorb heat produced during the combustion process. This is accomplished first while the water is heated from its ambient temperature to 100°C, and also while the water absorbs heat during its phase change from liquid to gas. The relatively small size of water droplets associated with water mist results in an increased surface area of water exposed to the fire, thus allowing for a more rapid absorption of heat.

Recent studies have demonstrated that entrainment of atomized water droplets into the fire plume is the predominant contributor to the success of water mist for flame extinguishment. This explains why water mist is a more successful extinguishing agent for fully developed fires than it is for smoldering fires with a lower heat release rate. The stronger entrainment flow of a fully developed fire results in a larger volume of water droplets being entrained into the plume, resulting in a more efficient heat absorption. Additional contributing factors include fuel pre-wetting, and to a lesser extent, flame blow-off and oxygen depletion as a result of steam generation. It is the absence of these mechanics in certain scenarios that have limited water mist’s use to niche areas.

Atomization of water is achieved with specially designed nozzles, as well as either low (175 psi), intermediate (175-500 psi), or high-pressure (500+ psi) single or twin fluid supplies. Single-fluid systems are generally limited to water that may include additives to enhance system performance. Twin-fluid designs rely on water with an additional atomizing agent—such as air or nitrogen—that are stored separately and combined at the nozzle during discharge.

Water mist was initially seen as a candidate for protection of sensitive computers, data equipment and other environmentally sensitive items such as cultural artifacts. These purposes are now almost exclusively served by clean agents and some of the more exotic halon replacements. How-ever, research and testing have demonstrated that water mist can be a significant extinguishing agent in the area of marine applications such as engine turbine rooms, flammable liquid storage compartments and general passenger compartments.

Land-based applications for water mist systems include engine-turbine enclosures and flammable liquid storage and handling rooms—all of which are limited in terms of height and area.

NFPA 13 recognizes similar limited applications such as those for light and ordinary hazards. In these cases, nozzle spacing and ceiling height of the enclosure are greatly limited, but there are real opportunities for this technology.

Order of Events

In a typical fire event, the following occurs:

1. The detection system goes into alarm upon sensing products of combustion, and the building fire alarm system is notified. Two different alarms—room and building—will sound.

2. The external air-supply system shuts down and/or the supply and return dampers close to contain the room for the clean agent.

3. Room occupants have the opportunity to leave the space before the clean agent is discharged.

4. Specific power systems can be shut off to eliminate the propagation of the fire.

5. In a cross-zoned detection system, a second detector goes into alarm. In an air-sampling detection system, the concentration of smoke increases to the next prescribed level of density. The two alarms are still ringing, but the clean agent alarm changes tempo.

6. All specified power systems within the data center are shut down, and the HVAC systems are off.

7. An abort switch can be energized to stop the clean agent discharge. This is a manual switch that must be held continuously.

8. At the end of a predetermined delay, usually not more than 30 seconds, the clean agent is discharged.

The fire has been extinguished, the data center equipment has been spared and the company has saved thousands of dollars. All of the alarms can be returned to the normal condition, the clean fire-suppressant agent is replaced within a reasonable amount of time and the room can be returned to normal operation.

Little or no time, or data, was lost, and the company maintains its position in the competitive world.

Consequently, companies that have experienced a fire event certainly don’t consider the costs of clean agents to be such a big issue.