An array of conditions in industrial occupancies can affect the performance of a fire alarm system
BY WAYNE D. MOORE
THE AMBIENT CONDITIONS PRESENT in industrial occupancies present challenges for the placement of detection devices and audible notification appliances for a fire alarm system. The 2016 edition of NFPA 72®, National Fire Alarm and Signaling Code, states the limits imposed on these devices based on ambient conditions, and designers must carefully consider those conditions when choosing components for a fire alarm system intended to protect an industrial space.
Those spaces can include conditions that can affect detector operation or initiate false alarms: mechanical vibration, electrical interference, process smoke, moisture, particulates, fumes, noise, and radiation, to name a few. Designers must investigate any of these sources of possible degradation and choose other detectors or mitigate the potential for smoke detector failure. While it might not be possible to completely isolate environmental factors, an awareness of these factors during system layout and design can significantly improve detector performance.
Additionally, ceiling height and shape can also affect smoke detector performance. The higher the ceiling, the greater the impact of the environmental effects on the smoke plume and on the ability of a smoke detector to initiate an alarm signal in the early stages of a fire. High ceilings mean designers must consider the possible impact of stratification, where the smoke plume has insufficient energy to drive it upward to the location of the smoke detectors. Altitude is also a factor; the greater the distance above sea level, the thinner the air, which can also affect a smoke detector’s ability to detect smoke particles.
The ambient ceiling temperature also becomes an important factor for heat detector placement. The code requires that the temperature rating of the heat detector exceed the maximum expected temperature at the ceiling by at least 20 degrees F. According to the code’s annex, designers should select heat detectors to reduce this temperature difference to minimize response time. However, a heat detector with a temperature rating greater than the highest normally expected ambient temperature will help avoid the possibility of premature operation of the heat detector to nonfire conditions. (The code limits the installation of smoke detectors to areas where the ambient temperature ranges between 32 and 100 degrees F, the relative humidity does not exceed 93 percent, and the air velocity does not exceed 300 feet per minute.)
Ambient noise levels must also be considered in the placement of audible notification appliances. The code requires visible notification appliances when the average ambient sound level is greater than 105 dBA, regardless of whether the mode of signaling is public or private. As with emergency communications systems, designers may choose to specify multiple audible appliances rather than attempt to provide alarm signaling above the ambient noise levels with just a few appliances. The total sound pressure level produced by combining the ambient sound pressure level with all audible notification appliances operating cannot exceed 110 dBA at the minimum hearing distance. The fire alarm system activation may automatically shut down the ambient noise–generating equipment whenever stakeholders deem it a safe practice.
Finally, designers must conduct frequent follow-ups with industrial clients to ensure that none of the conditions originally observed have changed while the fire alarm system design is completed.