TYPICALLY, THE GOALS for fire detection in any scenario require the use of a detector that will provide the optimum performance in terms of early detection and freedom from false alarms, and remain operationally reliable during its life cycle. Users of the 2013 edition of NFPA 72®, National Fire Alarm and Signaling Code, know that this document contains no requirements to install automatic detectors in any specific location, except as required in subsection 10.4.4 for fire alarm control units, notification appliance circuit power extenders, and supervising station transmitting equipment.
But users often do not understand that it is the designer who must design detection for installation in a particular space and also meet the three general goals outlined above.
Two recent questions on the NFPA LinkedIn forum demonstrate this issue. In the first case, the questioner asked whether a designer should choose a smoke detector, a heat detector, or a multi-sensor detector for installation in a garbage collection room. The second question asked what type of heat detection a designer should choose for installation in a frozen-food warehouse. The answers to both of these questions will vary somewhat based on the specific fire protection goals of the owner, as well as meeting the general goals stated above. Nevertheless, assessing the impact that the environment will have on the chosen detector presents the major issue important to answering both questions.
Although designers often choose to use smoke detectors, these devices can be negatively affected by the environment. The 2013 edition of NFPA 72 states that in order to prevent nuisance and unintentional alarms, as well as ensure proper operation after installation, the designer must consider the performance characteristics of the detector and the environment of the proposed location of the device. Unless specifically designed and listed for the expected conditions, NFPA 72 does not permit the installation of smoke detectors where any of the following ambient conditions exist: temperatures below 32°F (0°C) or above 100°F (38°C); a relative humidity above 93 percent; or an air velocity greater than 300 ft/min (1.5 m/sec). The code also requires that the designer carefully evaluate the environment of the proposed location for “potential ambient sources of smoke, moisture, dust, or fumes, and electrical or mechanical influences, to minimize nuisance alarms.”
So one of the answers to the first question is that a designer must decide if the installation of smoke detection is appropriate for a garbage collection room. Depending on the temperature fluctuations and the amount of contamination in the air in the collection room, a fixed-temperature, rate-of-rise, or rate-compensated heat detector would likely provide the most appropriate protection. If the room already has protection from some form of automatic sprinkler system, additional heat detection would serve no purpose, unless it provided more accurate identification of the location where the sprinklers had actuated.
The answer to the second question regarding the choice of detection for a frozen-food warehouse can follow the same logic as the first question. Both smoke detection and rate-of-rise heat detection will likely prove inappropriate. Rather, line-type fixed-temperature heat detection would likely serve the detection needs, especially when coupled with a combination dry pipe/pre-action automatic sprinkler system.
In both of cases, the proposed environment will dictate the type of detection that will reliably perform in the space. In neither of these cases does the code dictate which detection method to use—the system designer, as defined in the code, must make this decision.
Wayne D. Moore, P.E., FSFPE, is vice president at Hughes Associates.