The question is simple, and we are asked it almost immediately by every client: "What kind of fire protection should I use in my data center? Sprinklers or gas?" Actually the question is usually phrased as: "Pre-action or FM-200?", which is not entirely proper as we'll explain, but you get the point. I've previously answered a reader question about this so there will be some repetition, but we'll go into a lot more detail about this critical decision here. It involves philosophy, regulations, corporate policy and, of course, that ever-present factor, money.
We can state the premise in two sentences. Water is intended to protect buildings and people. Inert gas is intended to protect equipment. Either one should be capable of doing both, but we'll see as we go why that's not a generally accepted position.
There is no single "right answer" regarding fire protection in a data center and, as I stated in my answer to the reader's question, I doubt you will find anyone, other than a fire suppression salesperson, who will tell you there is a "correct solution." Unpredictable things happen in fires, and if the "recommended" solution doesn't cover that particular contingency, the liabilities are just too high. What we can do is explain the different systems, discuss the factors you need to understand, investigate and consider, and give the pros and cons of the different approaches. The decision must be, sorry to say, entirely up to you. Let's start by explaining the systems, some of their history and where we are today.
s In the days when large mainframe computers were the norm and virtually everything in the data center relied on one giant machine, the concern about water was enormous and definitely legitimate. These computers were usually the sole processors in data centers, and they ran on high-voltage, high-current power supplies that generated a great deal of heat for the amount of compute power (by today's standards) they offered. If water got into these big machines, the damage could be quick and enormous, and machine repair or replacement could take weeks.
With the growth of business dependency on these computers came a simultaneous paranoia about water in the data center -- or at least about water above the equipment. The largest of these mainframes ultimately utilized water cooling from pipes below the floor, but that was not likely to run into the computers. The concern about overhead water led to the development of the "pre-action" sprinkler system, and ultimately to other forms of fire suppression as well.
The concern about overhead water has not changed today. No IT department wants water pipes running over their technology, out of concern that the "running" part might become literal. This is where we can definitely make a solid recommendation. As noted above, the "pre-action, dry pipe" sprinkler system was developed specifically to alleviate this concern.
In a "dry pipe" system, the overhead pipes are charged with compressed air until fire or smoke is detected, normally by at least two sensing devices. Only then is the valve opened that allows water to enter, and in a good design there's a "override" available to even delay that. Until the valve actually opens, there is no water in the pipes and, therefore, no water over your equipment. Prior to the detection event, the air pressure is constantly monitored so that even a small leak in a pipe or head would be detected and an alarm sounded. Therefore, even after the pipes are "charged," there is still little concern about leakage. And since there is still a fair amount of air ahead of the water, most of the pipes are not literally "wet" until a sprinkler head is actually activated. But that's where concerns develop that are even bigger than having "overhead water."
Sprinkler heads open, not on the detection of smoke, but as a result of elevated temperature -- usually around a factory-set temperature of 165 degrees F, although they can be obtained with higher ratings. You'll have a pretty good fire going by the time this happens, and water damage to equipment may be the least of your concerns. Sprinklers open only where the heat of the fire actually exists, so most fires will open only two to four heads. This will stop the destruction from direct fire or heat, but smoke, which is often the nastiest contaminate, may permeate a much wider area and can be worse than direct damage from water, which is often not as bad as it is always considered. We recommend that a data center sprinkler source be filtered to minimize contamination from any water that might get into the hardware, but we have seen computers get wet from other sources (not from fire systems, but from water leaks above, which shows poor design practice) and be dried out with a hair dryer and put back into operation within a few hours.
The biggest vulnerability of a "dry pipe" sprinkler system in a data center is damage to a sprinkler head. If heads are exposed below a ceiling there is always the potential of breaking one while moving a ladder or handling a tool overhead. A broken head will allow the air to immediately escape, opening valves and bringing water right behind it. Data centers without ceilings (which we recommend for a number of other reasons) offer a certain advantage in this regard, in that sprinkler heads are then turned upward rather than down. This obviously makes them less vulnerable to damage. In data centers where a false ceiling is used, it is best if the heads can be flush with the surface, as they are in most finished office and corridor spaces. But since heads must be arranged in relation to the cabinet layout to be effective, it is often necessary to have them below the ceiling surface in order to achieve the necessary coverage, exposing them to damage.
All this brings us to the question most asked regarding sprinkler systems: "What is the potential for actual equipment damage if a sprinkler head were to open?" In today's world, the answer is actually "minimal," simply because of the way in which most of our equipment is mounted. Most of today's computing equipment is comprised of literally hundreds of small, discrete "boxes" stacked one above the other in equipment cabinets that are arranged in rows within the data center. Many of these cabinets have, or should have, solid tops to prevent hot discharge air from finding a short path back to what is supposed to be the cold air intake side of the devices. (See blog article #8.) As a result, water from sprinklers is unlikely to even get into most of the computers, let alone damage them. That obviously begs the question as to whether sprinklers will extinguish an in-cabinet computer fire at all. The answer is, they will, eventually. But the greatest server exposure can actually be from water vapor pulled into machines not even directly touched by water as their ventilating fans pull it right through. This concern is addressed by installing an "emergency power off" (EPO) shutdown that activates when the sprinkler pipes are charged. But a good design will also have an "override" button that will prevent EPO activation temporarily, since the sudden shutdown of power in a data center can actually result in more monetary loss than damage from water. When an entire facility must be restarted from a "crash" status, which can take hours or even days.
So if it is unlikely that water will actually get into most of the computers, and if we can't be assured that a fire in a computer cabinet will be extinguished by water before it grows and damages even more hardware from fire, heat or smoke, what can we do? Enter "inert gas" or "clean agent" fire protection systems. These systems were originally developed to provide a machine-safe fire suppression alternative to water. Large mainframe computers, due to their enormous power supplies, had components that caught fire more often than we ever see with today's equipment. Since repair and replacement time were so significant, it became important to extinguish fires with minimal additional damage to the computers or their peripherals. A number of approaches were tried, including CO⊂2; (carbon dioxide), which was bad for the people inside the room when the oxygen suddenly disappeared.
When DuPont developed the chemical compound trade-named "Halon" it seemed the preeminent solution had arrived. Despite its significant cost, Halon was installed almost universally in data centers because the business cost of a mainframe fire far outweighed the cost of the gas.
However, as municipalities started looking carefully at their building codes, many realized that data centers with huge amounts of power usage, and lots of people working 24/7, were installing Halon as the exclusive fire agent and not proving that it actually worked. Testing became required, which meant dumping the expensive gas while the fire department monitored the concentration with strategically placed sensors. Room after room failed, mostly due to construction errors -- another cost factor that will be discussed shortly. This required additional tests until the room passed. Businesses became distressed when tests had to be conducted as many as five, six, seven or eight times at often $50,000 or more a test. When code writers started realizing that most facilities had no secondary method if a Halon dump failed to extinguish the fire, requirements were instituted for backup Halon tanks and/or sprinklers. Some jurisdictions simply mandated sprinklers regardless, as did lots of insurance companies. Regular inspections were also required by some localities to detect leakage problems resulting from small data center modifications that rendered a previously tested Halon system unreliable. As years went by, and mainframes were no longer the primary processors, many people decided the cost was just too high and opted for pre-action sprinkler as the sole method of protection. But for those who continued to install Halon, another problem was yet to come.
As concerns were raised about the effect of various chemical compounds on the earth's ozone layer, one of the product's identified as a problem was Halon. Ultimately, it was "grandfathered out" as of December 31, 1993. Existing installations could remain, but new installations were not allowed and no industrialized country could continue to manufacture it. Halon rooms still exist today, but if a facility is disbanded, the Halon system has to be "decommissioned" (a potentially dangerous job requiring professional handling). The gas can either be disposed of in an environmentally safe manner or re-sold to one member of the small group of approved users for whom there is no other viable alternative. There was, of course, great pressure for alternatives, since concern still existed about sprinklers in the data center, as well as numerous other industries.
Several products have been developed to answer both the environmental and equipment protection problems. The most often heard is "FM-200," which is actually a brand name copyrighted by the originator, Great Lakes Chemical Corporation. A similar product is manufactured by DuPont. FM-200 extinguishes fire primarily by rapidly absorbing its heat. Another product trade, named "Intergen," is a mixture of nitrogen, argon and CO⊂2; that extinguishes fire by rapidly reducing the oxygen level to below 15%. These products are all considered both environmentally and "people" safe and discharge in exactly the same way as the familiar Halon. The construction methods necessary for their use are the same as well, including a sealed room, "clipped" ceiling panels where false ceilings are used, automatic power shutdown to air conditioners and equipment, room testing and, usually, the use of sprinklers in addition to the "clean agent" gas system. Because the quantity of gas required is volumetrically based, a tradeoff must be considered between the advantages of a high space with no ceiling and the cost of the clean agent gas in a room with a large volume. As said initially, there is no single "right answer."
Returning to our original premise, inert gas systems are designed to protect equipment. Obviously if the inert gas puts out the fire, then it should also protect the building. If it is also non-toxic (as it must be), it should protect people as well. Most fire codes don't look at it this way, however, and in most jurisdictions sprinklers are going to be required by code whether you have something more sophisticated or not. If you're in a jurisdiction where sprinklers are not also legally required, you're not necessarily home free. Check with your management and your insurer. They may insist that you put in sprinklers anyway, perhaps to avoid potential liability and possibly to reduce monetary exposure if you have a big data center with a huge equipment value. So, you will probably ask, if I'm going to have sprinklers anyway, should I also pay for a second "inert gas" system? The answer usually comes down to cost.
Room size is the first concern, because the amount of gas required is based on the volume of the space. We have indicated several times that inert gas is not inexpensive (although the newer gasses are less than their Halon predecessor), so the amount required has an obvious cost implication. Ceiling height, therefore, requires a real evaluation of tradeoffs. Keeping the ceiling as low as possible obviously reduces room as well as gas volume, but it also imposes a significant limitation on the flexible installation of overhead systems such as cable tray and lighting. It is also the antithesis of what we want for good cooling. (See blog article #3).
But the gas system itself is not the entire cost. The requirements that an inert gas system imposes on room construction can add significantly to the total price. A data center needs to be air-tight in order to contain the gas concentration long enough to extinguish a fire. This means door gaskets, sealing all walls to both the floor and ceiling slabs and closing all wall penetrations (which should be done anyway to maximize cooling performance). It is also necessary to automatically shut-down all air conditioners, close fire dampers on all air ducts entering and leaving the room and shut off all power to computer equipment before "dumping" the gas. This means a significant amount of control hardware and wiring. Lastly, the nearly explosive release of the gas creates such pressures inside the room that ceiling tiles need to be "clipped" into place so they won't blow out, thereby letting gas escape. Getting all this accomplished correctly, particularly under a construction deadline, takes a great deal of on-site supervision.
Both inert gas and good pre-action sprinkler systems will sound an alarm when a condition is detected, will incorporate a graphic enunciator panel to show where the smoke has been detected and will enable you to invoke a short "override" so that gas is not "dumped" or the pipes are not charged with water before you've had the chance to solve the problem differently. The cost of the inert gas is too high to simply dump it at the first sign of a problem if there's another way to quickly resolve it. And draining back a pre-action system that has been filled with water, even if a head never activates, is probably the biggest headache in their maintenance, so it's good to avoid it if reasonably possible.
So what is the advantage of spending all this extra money on an inert gas fire suppressant? It comes down to a matter of how much the total gas system costs versus your potential damage and loss from fire, smoke and water, and your assessment of the risk of having an equipment fire in the first place. Keep in mind that almost everything in your data center today is housed in metal enclosures and, unlike the old mainframes, can be physically replaced virtually overnight. If you have a backup site, with mirrored data, your vulnerability is even further reduced. But if your risk assessment says you need all the protection you can get, an inert gas system will operate much sooner than the sprinklers since it is generally programmed to activate when two detectors alarm. Even if you invoke overrides, the gas system is nearly certain to activate long before sprinklers would open. And you should have a much smaller mess to clean up after the fire is out.
I must mention one other item regarding fire protection, just as a matter of awareness. Most cabling in a data center tends to be under the raised floor. Prior to "low-smoke" or "plenum-rated" cables, certain jurisdictions with fire paranoia (Chicago being the unquestioned leader ever since Mrs. O'Leary's cow kicked over that famous lantern) required fire protection below the floor as well as above it. With the advent of these better cables, the under-floor fire suppression requirement mostly went away. That could be changing again. New code requirements appear likely to impose yet another consideration when the 2008 National Electrical Code is released. Data cabling in an under-floor air plenum space may no longer be acceptable as "plenum-rated." It may need to be "limited combustibility" (LC) or fire protection will be required below the floor as well as above. (Note that code already requires abandoned cable to be removed.) This leads to the recommended practice of installing a permanent LC tie cable infrastructure within the data center that can virtually eliminate the need to install or remove cable, or to even go under the raised floor. Although initially expensive, if installed correctly the long-term savings and operational benefits can be significant, particularly in facilities with a great deal of change.