Special Hazard Fire Protection Using FM 200
There are approximately 2,000 telecommunication companies providing service in the United States. They operate networks that may be sized to serve a few thousand customers up to millions of customers. Facilitating the timely transmission of voice and data across a single phone line requires a large network of assets to store, maintain and route information
The Problem: Service DisruptionsTelecommunication facilities typically contain rows of equipment that is fairly standard in appearance and function. Most of this modern equipment is mounted in metal cabinets and looks like ordinary computers. This arrangement, combined with miniaturization of equipment and higher energy densities, requires forced ventilation (internal fans and extensive room air conditioning systems) to dissipate the heat generated during operation. Rapid temperature and humidity changes can also cause equipment problems. The potential for fires is considerable, due to the large quantity of power necessary for operations.
Fire damage in a telecommunications facility can easily reach the multi-million-dollar mark. Loss of service can add immeasurably to the damages (see Table 1).
Providing adequate fire protection requires special consideration, due to the critical nature of the operations and the presence of sensitive electronic equipment. Leaving this responsibility to "ordinary" means of fire protection, such as hand-held portable extinguishers or an automatic sprinkler system, can be a costly mistake. Hand-held extinguishers rely on human response, and even a small fire in a complex electrical system can be virtually inaccessible. An automatic sprinkler system provides adequate structural and back-up protection, but requires that a major heat source must exist in order to actuate the system. By the time a fire reaches this level, serious and costly damage can occur from both the fire itself and the quantity of water required to extinguish it.
To reduce the risks of personnel injury, costly downtime and substantial equipment damage, an automatic fire suppression system is needed. More importantly, the system must be capable of rapidly detecting and extinguishing a fire without leaving damaging residue or creating a life safety hazard.
A Solution: FM 200 Clean Agent Fire Suppression SystemsFM 200 possesses many physical properties that make it desirable as a fire-extinguishing agent. It is odorless, colorless, electrically non-conductive, non-corrosive and leaves no residue when discharged. FM 200 is especially appropriate for protecting telecommunication facilities because it is electrically non-conductive. Since it is discharged as a gaseous vapor, it rapidly penetrates enclosures to get to the source of the fire, reaching areas that water or dry chemical agents cannot.
In a fire suppression system, FM 200 is stored as a liquid in system storage containers. When initiated by an approved detection and control system, the FM 200 will flow through the designed system piping and will immediately flash from a liquid to a vapor as it is released through the discharge nozzle.
To provide proper fire protection for a telecommunication facility, it is imperative that a well-designed, fast response, and trouble-free automatic fire detection system be installed. In most cases, photoelectric smoke detectors will be used due to the high airflow conditions found in telecommunication facilities. Smoke damage is also a concern. Photoelectric detection enables the system to detect and react to a fire condition before there is any smoke damage.
Special attention must be paid to airflow rate and how it impacts detector spacing. A great addition to the detection system is an aspirating (air sampling) smoke detection system. This type of system allows for the fire to be detected in its incipient stages, providing an opportunity to control the spread of a fire. Aspirating fire detection systems offer a wide range of installation options, from ceiling mount and computer rack mount to air handling system placement.
Designing a SystemThis example walks through the design of a fire detection and suppression system in a telecommunication facility. The protected room houses telecommunication switch gear and is equipped with high efficiency cooling and airflow equipment. The room dimensions are 40 feet x 40 feet, with a 10-foot ceiling and a 2-1/2-foot sub-floor.
The first step is to determine the quantity of FM 200 required for the protection of the enclosure. Both the room and the sub-floor area will be protected with FM 200. The designer must consider the sub-floor because most fires start with an overheated wire in equipment in a rack or under the sub-floor. Also, with the leakage that occurs in most sub-floors, it is impossible to contain the agent only in the room space above. The fire hazard consists of Class A combustibles, such as cable insulation and Class C electrical fires. Per NFPA standards and other approval agencies, a design concentration of 7% FM 200 is used to protect this hazard. A flooding factor of .0341 lb/ft3 is multiplied by the room volume in cubic feet to calculate the amount of agent required, per NFPA 2001. The volume of the room is 16,000 ft3 and requires 546 pounds of FM 200 to achieve a 7% design concentration. The volume of the sub-floor is 4,000 ft3 and requires 137 pounds of FM 200.
A very efficient method of protecting this hazard is to use an engineered FM 200 system design. This allows the designer to use one FM 200 container to protect multiple hazard areas and allows for different flow rates at each nozzle in the system. In this example, the two hazard areas can be protected with one 1,000-pound container filled with 683 pounds of FM 200.
The next step is to position the system nozzles in the protected spaces. To minimize turbulence during system discharge, it is recommended that the flow rates of the system nozzles be at or below 20 lbs/sec over a 10-second discharge time. In order to keep a balanced system, we would place four nozzles in the room. Each nozzle will discharge approximately 136.5 pounds of FM 200. The same logic works for the sub-floor. Each nozzle will discharge 34.25 pounds of FM 200. Note of interest: This sub-floor could actually be protected using a single nozzle, but we recommend four nozzles due to the number of obstructions in the sub-floor. It is very common to have sub-floors that contain a large amount of cable, and it is not recommended to push the limits of nozzle coverage in a crowded sub-floor. A diagram of the container and piping isometric is shown in Figure 1 for your reference.
The next step is to layout the detection and control system for this facility. Photoelectric smoke detectors are typically used for the room and sub-floor. The reason for choosing photoelectric detectors is that they perform well in areas of high airflow. In order to determine the proper detector spacing, consult NFPA 72, National Fire Alarm Code. Table 2-188.8.131.52 of NFPA 72 provides guidance for detector spacing in high air movement areas. This hazard has 60 air changes per hour in the room and sub-floor. Therefore the detectors in the room and sub-floor will be spaced at 125 ft2. This determination results in 16 detectors in the room and 16 detectors in the sub-floor.
In order to provide the most time to react to a fire incident, an aspirating smoke detection system may be added in this protection scheme. An aspirating smoke detection system allows for direct smoke detection in a variety of areas within a telecommunication facility. Common areas of protection include the return air duct of the air handling units (AHUs), ceiling spot detection, and equipment such as computer racks. For this facility, a detection unit for spot detection is installed in the telecommunication equipment racks. The aspirating smoke detection system is not used to release the FM 200. It is installed to provide an opportunity to detect and control a potential fire before it does damage to a facility. These units typically communicate with the FM 200 system control panel to display real time information and alert personnel to hazardous conditions.