Traditional technology cannot be used to protect the area beyond the perimeter of a building from fire for several reasons, the most overriding one being that protecting the outdoors is not the objective of most fire detectors. Normally outdoor fire protection devices are designed to protect products or items being stored in this exterior environment.
The problem with using conventional detectors outdoors is that there is no ceiling to collect the heat and smoke that rises from a fire. (This is why you don't install detectors on the bottom of trusses.) Without some form of a ceiling, traditional spot-type detectors cannot be used. Even some structures like airplane hangers and open parking garages are not well suited for spot-type smoke and heat detectors. Airplanes, being a bit pricy, require that small fires be detected as soon as possible. Waiting for a fire to produce enough thermal lift and buoyancy to drive smoke and heat to detectors mounted at those extreme heights is unacceptable.
Detection in an instant
Storage of oil and gas is another expensive commodity that usually requires near-instant detection. Even if petroleum products were cheap, their volatile nature would still dictate early detection-if for no other reason than for an effective evacuation. A parking garage has ceilings, but cannot be relied upon to always trap smoke and heat since the sides of the parking structures are open. Wind blowing through the open walls dilute the fire's hot air and gases as it sweeps it away, interfering with smoke and heat reaching the ceiling and the area where conventional detection might be placed.
However, there is a principle that works quite nicely in applications like these and that is having a detector that 'looks' directly into the protected area and actually 'sees' the anticipated source of the fire from a distance. (Video smoke detection is one example of this type of device but that topic will be covered separately in an upcoming issue.) In a turn-around, these detectors do not need for the fire/smoke to be transported to where they are installed. These detectors are able to oversee the areas where they are installed and protect by observing that area for a specific wavelength of infrared (IR) and/or ultraviolet (UV) light being produced by the fire.
These detectors may be purchased for wide spectrum coverage or to ascertain if the specific fuel or fuels are present. Natural gas, gasoline, oil and other fuels burn with a distinctive light emission. In fact, our military can determine the type of missile and who fired it (enemy or foe), just by analyzing the light produced by the unique rocket fuel it uses. The light to look for, in both cases, would be in the invisible IR/UV range. A detector that may see gasoline burning from 210 feet away may not be able to see burning metals from only 10 feet away. The imaging software in these devices allows them to ignore most lightning, arcs and natural sunlight. Some optical flame detectors incorporate a feature that verifies that the flame actually "flickers" to prevent normally hot objects, such as vehicles, from causing a nuisance alarm should they enter their field of view.
New turns and twists
Fire alarm systems are not just about fire anymore, they are about life-safety in general. NFPA 72, The National Fire Alarm and Signaling Code, allows you to use the fire alarm system to monitor for the presence of combustible or poisonous gases. There are many types of detectors that sample the air for the presence of hydrocarbon gases or carbon monoxide, but only at the location of the detector. These typically use catalytic or electro-chemical elements that are able to sense dangerous levels of these toxic gases before they poison, ignite or explode. Even gases can be detected from a distance in large outdoor areas by using a 'projected beam' type of gas detector. These devices are similar to the commonly used projected beam smoke detectors and produce an IR light beam that is directed at its companion receiver located up to 110 yards away. The total amount of IR light absorbed by the specific gas being targeted is used as the basis for determining an alarm. Sometimes, spot-type IR gas detectors are often used in conjunction with the IR beam detectors, since the projected beam types (like most outdoor detectors) are subject to obstructions by rain, snowflakes and any dirt and grime on its lens.
There is a new gas detector on the market that may also be used outdoors and in large open spaces. With an FM-approved 65-foot radius detection range, these units actually hear the ultrasonic 'hiss' produced by gas escaping under pressure. According to the manufacturer, these devices work in high ambient noise environments, unaffected by changing wind direction and can detect escaping gas in any direction relative to the microphone--all outdoors, from a distance, with no ceiling required.
Since necessity is the mother of invention, we are now able to provide fire detection in certain outdoor applications where we were unable to in the past.
As electronics and technology evolve, we are applying them in new and creative ways. The combined effect has given us the ability to push out our customer's detection perimeter, by improving their outdoor life-safety protection.
Greg Kessinger SET, CFPS is SD&I's longtime resident fire expert and regular contributor to the magazine. Reach him at firstname.lastname@example.org.